Journal of Petrology

Journal of Petrology Advance Access originally published online on March 7, 2006
Journal of Petrology 2006 47(6):1147-1175; doi:10.1093/petrology/egl006

This Article Abstract FREE Full Text (PDF) All Versions of this Article: 47/6/1147    most recent egl006v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (11) Request Permissions Google Scholar Articles by BRYANT, J. A. Articles by BAILEY, D. G. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Geochemical Constraints on the Origin of Volcanic Rocks from the Andean Northern Volcanic Zone, Ecuador

J. A. BRYANT¹, G. M. YOGODZINSKI^1,*, M. L. HALL², J. L. LEWICKI³ and D. G. BAILEY⁴

¹ DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF SOUTH CAROLINA, 701 SUMTER STREET, COLUMBIA, SC 29208, USA
² INSTITUTO GEOFISICO, ESCUELA POLITECNICA NACIONAL (IG-EPN), APARTADO 17-01-2759 QUITO, ECUADOR
³ EARTH SCIENCES DIVISION, LAWRENCE–BERKELEY NATIONAL LABORATORY, 1 CYCLOTRON ROAD, MAIL STOP 90-1116, BERKELEY, CA 94720, USA
⁴ DEPARTMENT OF GEOSCIENCES, HAMILTON COLLEGE, CLINTON, NY 13323, USA

RECEIVED JULY 25, 2004; ACCEPTED JANUARY 27, 2006

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Whole-rock geochemical data on basaltic to rhyolitic samples from 12 volcanic centers are used to constrain the role of continental crust in the genesis of magmas formed beneath the anomalously wide subduction-related volcanic arc in Ecuador. Relatively homogeneous, mantle-like, isotopic compositions across the arc imply that the parental magmas in Ecuador were produced largely within the mantle wedge above the subduction zone and not by extensive melting of crustal rocks similar to those upon which the volcanoes were built. Cross-arc changes in ¹⁴³Nd/¹⁴⁴Nd and 7/4Pb are interpreted to result from assimilation of geochemically mature continental crust, especially in the main arc area, 330–360 km from the trench. Mixing calculations limit the quantity of assimilated crust to less than 10%. Most andesites and dacites in Ecuador have adakite-like trace element characteristics (e.g. Y <18 ppm, Yb <2 ppm, La/Yb >20, Sr/Y >40). Available whole-rock data do not provide a clear basis for distinguishing between slab-melting and deep crustal fractionation models for the genesis of Ecuador adakites; published data highlighting geochemical evolution within individual volcanoes, and in magmatic rocks produced throughout Ecuador since the Eocene, appear to support the deep fractionation model for the genesis of most evolved Ecuadoran lavas. A subset of andesites, which display a combination of high Sr (>900 ppm), Nd >4·1 and 7/4Pb <6·0, appear to be the best candidates among Ecuador lavas for slab-melts associated with the subduction of the relatively young, over-thickened, oceanic crust of the Carnegie Ridge.

KEY WORDS: andesite; Ecuador; trace elements; isotopes; adakite

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
It is widely believed that subduction-related andesites form predominantly through the differentiation of basaltic melts and their interaction with arc crust (e.g. Gill, 1981; James & Murcia, 1984). In oceanic settings, where the arc crust is thin, the contribution of crustal material to the formation of lavas is often minimal, and andesites are viewed as secondary products of basalt evolution by crystal fractionation and magma mixing (Gill, 1981; Conrad & Kay, 1984; Woodhead, 1988; Singer & Myers, 1990). This model for andesite genesis in island arcs is central to our understanding of subduction zone magmatism, and has played a significant role in the development of ideas on the formation and evolution of continental crust in subduction zones (e.g. Kay & Kay, 1991; Kelemen, 1995).

An alternative view of arc magmatism and andesite genesis has recently grown out of the recognition that in some arcs, especially in places where the subducting lithosphere is young and/or warm, calc-alkaline andesites and dacites may form not only through the differentiation of basaltic magmas, but also through partial melting of basaltic rocks at high pressures in subducting oceanic crust (Kay, 1978; Saunders et al., 1987; Defant & Drummond, 1990; Kay et al., 1993; Yogodzinski et al., 1995; Stern & Killian, 1996; Rapp et al., 1999; Kelemen et al., 2003). Andesites and dacites thought to have formed by this mechanism have unusual trace element characteristics, including high Sr/Y (>40) and La/Yb (>20), and low concentrations of Y (<18 ppm) and Yb (<1·9 ppm) compared with common island arc lavas of intermediate composition (Defant & Drummond, 1990). The geochemically distinctive class of andesites and dacites with these features, now commonly termed ‘adakites’ (Defant & Drummond, 1990), was first recognized by Kay (1978), who argued that they were formed through a two-step process that involved melting of the subducting oceanic crust in the presence of garnet (to produce the highly fractionated trace element patterns), followed by the interaction of the resulting silicic melt with peridotite in the hot mantle wedge (to form the andesitic and high Mg-number characteristics).

This model has been relatively successful in explaining the genesis of adakitic lavas in island arcs or other arc settings where the crust is relatively thin (Saunders et al., 1987; Defant et al., 1991; Morris, 1995; Yogodzinski et al., 1995; Stern & Killian, 1996; Abratis & Wörner, 2001). Recently however, the model has been applied in areas of thickened continental crust, where the mineral garnet, which plays the central role in generating the highly fractionated trace element patterns seen in adakites (Kay, 1978; Defant & Drummond, 1990; Rapp et al., 1999), may be stable not only in the subducting oceanic plate, but also in the deep crust (e.g. Gutscher et al., 2000; Beate et al., 2001; Bourdon et al., 2002, 2003). Not surprisingly, studies in this geological setting, in particular in Ecuador, have produced a divergence of opinions, with some researchers focusing on the crust as a major component of northern Andean magmatism (Kilian & Pichler, 1989; Arculus et al., 1999; Monzier et al., 1999), and others advocating processes within the subducting slab and mantle wedge (Barragan et al., 1998; Gutscher et al., 1999; Bourdon et al., 2002, 2003). Reconciliation of this issue will ultimately determine the extent to which we view widespread andesite–dacite volcanism in the northern Andes as juvenile additions to the continental crust, or as recycling of old crust via intra-crustal melting and differentiation.

In this paper we present whole-rock geochemical data, including Pb, Sr, and Nd isotope ratios, for Holocene and Late Pleistocene lavas collected from most of the length and width of the volcanic arc in Ecuador. Because the crust varies greatly in thickness and composition across the arc, from primitive oceanic terranes in the west (basalts and related volcaniclastic rocks) to geochemically mature continental lithologies in the east (granites and metapelites), these data provide a potentially powerful basis for evaluating the role of crustal melting and assimilation in the genesis of adakitic and other volcanic rocks in Ecuador.

	GEOLOGICAL SETTING

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The spatial distribution of active volcanism in the northern Andes, which is controlled by the convergence of the Nazca and South American plates, undergoes a dramatic series of along-strike changes from central Colombia southward into northern Peru (Fig 1). In southern Colombia, the arc is a relatively narrow volcanic chain (40–50 km wide), which is underlain by subducting oceanic lithosphere that dips to the SE at an angle of 30° (Gutscher et al., 1999). In northernmost Ecuador, the volcanic arc widens abruptly to 120 km, but ends a short distance to the south at Sangay Volcano, in central Ecuador (Fig. 1). In southernmost Ecuador and northern Peru, the subduction zone enters the non-volcanic/flat-slab region (Barazangi & Isacks, 1976). The abrupt widening of the volcanic arc and the southward cessation of active volcanism in Ecuador coincides spatially with the impingement of the 14 km thick crust of the Carnegie Ridge (Graindorge et al., 2004) on the South American margin (Fig. 1). Landward of this point, the dip of the subducting plate beneath Ecuador is 25–30°, similar to the dip of the subducting plate beneath southern Colombia to the north (Guillier et al., 2001). These observations suggest that the deep structure of the subducting Carnegie Ridge exerts some control over the surface expression of active volcanism in the Northern Andes (note in particular the widening of the volcanic arc in Ecuador, Fig. 1), even though the dip of the subducting plate appears to change very little from southern Colombia to central and southern Ecuador (Gutscher et al., 1999; Guillier et al., 2001).

View larger version (37K): [in this window] [in a new window]   Fig. 1. Topographic (onshore) and bathymetric (offshore) map of the Northern Andes region, showing the major geographical features mentioned in the text. , locations of Quaternary volcanoes from the Smithsonian Global Volcanism database. Black arrows show the convergence direction and rate of movement of the Nazca Plate relative to stable South America from Trenkamp et al. (2002). Crustal thickness estimate (50 km) is a minimum value based on the work of Prévot et al. (1996) and Guillier et al. (2001). Average dip of 30° for the subducting slab benaeath Ecuador is from Guillier et al. (2001).

 
The north Andean crust that underlies the currently active volcanoes in Colombia and Ecuador varies systematically across the margin with respect to thickness, age, rock type and geochemical maturity (Fig. 2). Estimates based on gravity data indicate that the crustal thickness across Ecuador changes from 25–30 km beneath the volcanic front to >50 km beneath the highest elevations of the Eastern Cordillera, with progressive thinning of the basement eastward toward the craton (Feininger & Seguin, 1983). In Colombia, gravity data indicate that the crust is 45–50 km thick along the axis of the Andes and that it thins to 32–35 km towards both the craton and the coast (Salvador, 1991). Recent seismic data have been interpreted to indicate that the crust is 40–50 km thick beneath western Ecuador, and perhaps 50–75 km thick beneath the arc crest (Prévot et al., 1996; Guillier et al., 2001).

View larger version (37K): [in this window] [in a new window]   Fig. 2. Simplified geological map of Ecuador, modified from Jaillard et al. (1997). Samples for this study include basalts, andesites, dacites and rhyolites from 12 volcanic centers (numbered in boxes) across the 120 km width of the arc.

 
The geological settings of individual volcanoes in Ecuador can be divided into three cross-arc zones, based on their distance from the trench and on the age, composition, and thickness of the crust upon which they were built (Figs 1 and 2). We refer to these zones as (1) the volcanic front, which lies in the Western Cordillera, 260–300 km from the trench, (2) the main arc, which encompasses the Inter-Andean Graben and Eastern Cordillera and lies 300–380 km from the trench, and (3) the back-arc area, which lies in the eastern foothills of the Northern Andes at distances greater than 380 km from the trench (Fig. 1).

Surface exposures of basement rocks at the volcanic front are composed primarily of geochemically primitive oceanic basalts and volcanoclastic rocks that were accreted to the northern Andean margin since the mid-Cretaceous. These oceanic terranes include fragments of the Caribbean Plateau as well as geochemically primitive, island-arc-type crustal blocks (Fig. 2; e.g. Feininger, 1987; Megard, 1989; Jaillard et al., 1990, 1997; Reynaud et al., 1999; Hughes & Pilatasig, 2002; Kerr & Tarney, 2005). The geochemistry of the oceanic basalts is broadly similar to those of the Galapagos and the Carnegie Ridge (Dupré & Echeverria, 1984; Lapierre et al., 2000; Kerr et al., 2002, 2004; Mamberti et al., 2003). To the east, the crust beneath the main arc and back-arc becomes progressively older on average, and geochemically more mature, consisting increasingly of granites, schists, and gneisses of mostly late Mesozoic to Paleozoic age (Fig. 2; Aspden et al., 1992; Aspden & Litherland, 1992; Litherland et al., 1994; Noble et al., 1997). Beneath the back-arc of Ecuador, the volcanoes appear to lie above Precambrian basement. Precambrian rocks are absent from surface exposures, but have been recovered in cuttings from deep oil wells (Feininger, 1987).

	ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Whole-rock major and trace element concentrations were determined from fresh samples that were crushed in a steel jaw crusher and ground in agate or Al-ceramic containers. Major elements and some trace elements (Ni, Cr, Cu, V) were determined by X-ray fluorescence (XRF) at the Washington State Geoanalytical Laboratory on a low dilution Li-tetraborate fused bead (Johnson et al., 1999). Major elements are reported on an anhydrous basis with totals normalized to 100% (Table 1). The remaining trace element concentrations were measured by inductively coupled plasma mass spectrometry (ICP-MS), also at Washington State University. Techniques for ICP-MS sample preparation, instrument operation, and calibrations have been described by Lichte et al. (1987). Major element data for samples from Sumaco, Atacazo, and Antisana are from Barragan et al. (1998). Isotope ratios for Sr, Nd, and Pb were determined by thermal ionization mass spectrometry at the University of North Carolina, Chapel Hill. All samples were dissolved at the University of South Carolina in high-purity HNO₃ and HF for 3 days in steel-jacketed Teflon capsules in an oven at 155°C. Chemical separates of Sr and Nd were made from the aliquoted sample by cation exchange using Sr-spec and HDEP-coated Teflon cation-exchange resins. Lead was separated by anion exchange using HBr. The Sr and Nd data are normalized to ⁸⁶Sr/⁸⁸Sr = 0·1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219, assuming exponential fractionation behavior. The Pb data are corrected for 0·12%/a.m.u. fractionation relative to Pb isotope standard NBS981. Errors on Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb = ±0·023, ²⁰⁷Pb/²⁰⁴Pb = ±0·028, ²⁰⁸Pb/²⁰⁴Pb = ±0·093, 2 absolute on all data in Table 2) are dominated by fractionation uncertainty of 0·06%/a.m.u. Seventy-two runs of the NBS-987 isotope standard yielded an average ⁸⁷Sr/⁸⁶Sr value of 0·710263 ± 0·000009 (2). Data listed in Table 2 are corrected to a value of 0·710240 for NBS-987. Twenty runs of the JNdi-1 isotope standard solution yielded an average ¹⁴³Nd/¹⁴⁴Nd value of 0·512110 ± 0·000016 (2). Data in Table 2 are corrected to a ¹⁴³Nd/¹⁴⁴Nd value of 0·512115 for JNdi-1, consistent with a CHUR value of 0·512635 and an Nd value of –15·15 for the La Jolla Nd standard, and implying an Nd value of –10·13 for the JNdi-1 standard (Lugmair & Carlson, 1978; Wasserburg et al., 1981; Tanaka et al., 2000). Errors on ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr reported in Table 2 are internal-run precision only. Isotopic measurements were collected primarily in the 9 month period that began in January 2002.

View this table: [in this window] [in a new window]   Table 1: Whole-rock major element (wt %) and trace element (ppm) compositions of Ecuador lavas

 

View this table: [in this window] [in a new window]   Table 2: Whole-rock Pb, Sr, and Nd isotope compositions for Ecuador lavas

 

	RESULTS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Basalts and basaltic andesites (Sumaco Volcano)
Basalts and basaltic andesites are rare in the northern Andes, occurring in significant numbers only at Sumaco Volcano (43–54 wt % SiO₂, Table 1, Fig. 3), located in the back-arc of Ecuador (Fig. 2) 400 km from the trench (Beate et al., 2001; Bourdon et al., 2003). Sumaco lavas are shoshonitic (1·05–4·12 wt % K₂O, Fig. 3) and silica-undersaturated (Pichler et al., 1976). They have high abundances of moderately compatible/incompatible elements (e.g. Y >37 ppm, Yb >2·7 ppm, Sr >2500 ppm) and concentrations of highly incompatible elements (e.g. Rb >97, Ba >1400, Ce >180,) that commonly lie well above those of normal subduction-related lavas (Fig. 4; see also Barragan et al., 1998; Bourdon et al., 2003). Basalts and basaltic andesites from Sumaco have strongly fractionated rare-earth element (REE) patterns (La/Yb = 31–43, Fig. 5a) and high Sr/Y despite the fact that Y and heavy REE (HREE) concentrations are also high (Fig. 6a). Concentrations of high field strength elements (HFSE) are high in an absolute sense (Nb >35 ppm, Fig. 4), but are sometimes low compared with similarly incompatible large ion lithophile elements (LILE). This is illustrated in ratios such as Rb/Ta and Pb/Ce, which are higher than for mid-ocean ridge basalt (MORB) at Sumaco but not as high as typical arc lavas (Fig. 7). These incompatible element ratios appear to indicate that Sumaco lavas contain a subduction-related geochemical component that is slightly smaller than that present in the andesites and dacites that typify volcanism throughout Ecuador (Fig. 7; see also Barragan et al., 1998, fig. 4).

View larger version (25K): [in this window] [in a new window]   Fig. 3. Major element variation in Holocene–Late Pleistocene volcanic rocks from Ecuador. Data are from Table 1. Symbols for compositional categories defined here (basalt–basaltic andesite, andesite–dacite, rhyolite) are used throughout this paper. The calc-alkaline–tholeiitic boundary line is from Miyashiro (1974). Boundaries defining low-, medium- and high-K series are from Ewart (1982).

 

View larger version (22K): [in this window] [in a new window]   Fig. 4. Variation in trace element (Rb, Nb, Ba, Y, Pb, Ce, Sr, Ni) concentrations as a function of SiO2 for Ecuador volcanic rocks. All data are from Table 1. Symbols are as in Fig. 3.

 

View larger version (40K): [in this window] [in a new window]   Fig. 5. Chondrite-normalized REE patterns of volcanic rocks from Ecuador (a, b) compared with leucogranites of the Cordillera Blanca, Peru (c) and common island arc lavas from the central Aleutian and Lesser Antilles island arcs (d). Stippled field in (b) and (c) is the field bounded by the Antisana andesite and the Quilotoa dacite in (a). Leucogranite data are from Atherton & Petford (1993). Island arc data are from Davidson (1987) and Kelemen et al. (2003) All other Ecuadoran data are from Table 1.

 

View larger version (20K): [in this window] [in a new window]   Fig. 6. Variation of Sr/Y vs Y (a) and Sr concentration vs Mg/(Mg + Fe) (b). Ecuadoran data (large symbols as in Fig. 3) are from Table 1. Boundaries for ‘adakitic’ and ‘common calc-alkaline’ fields have been approximated from Defant et al. (1991, fig. 15). Central and Southern Andean volcanic zone data are from published sources (Lopez-Escobar et al., 1977, 1985; Deruelle, 1982; Thorpe et al., 1983; Frey et al., 1984; Harmon et al., 1984; Lopez-Escobar, 1984; Hickey et al., 1986; Davidson et al., 1988, 1990; Futa & Stern, 1988; Gerlach et al., 1988; Hildreth & Moorbath, 1988; Wörner, 1989; Walker et al., 1991). Field for Quaternary andesites in Colombia is from Sigurdsson et al. (1990), Droux & Delaloye (1996) and Calvache & Williams (1997).

 

View larger version (28K): [in this window] [in a new window]   Fig. 7. Incompatible element concentrations and ratios in Ecuadoran lavas compared with average values for MORB and OIB: (a) Rb–Ta; (b) La–Nb; (c) Pb–Ce; (d) Ba–La. Ecuadoran data are from Table 1. Symbols used here are as in Fig. 3. Values for MORB and OIB are from Sun & McDonough (1989).

 
Based on the above it is clear that the lavas from Sumaco are readily distinguished from all others in Ecuador with respect to major and trace elements (Figs 3–7). In contrast, the isotopic compositions of the Sumaco samples fall generally within the relatively narrow range of compositions observed in Ecuador (e.g. Harmon et al., 1984; Kilian et al., 1994, 1995; Barragan et al., 1998; Bourdon et al., 2003). The three samples from Sumaco that we have measured have isotope ratios for both Sr (⁸⁷Sr/⁸⁶Sr = 0·7041–0·7044) and Nd (¹⁴³Nd/¹⁴⁴Nd = 0·51289–0·51293) that fall near the middle-to-high end of the range of values that we report for lavas of all compositions (⁸⁷Sr/⁸⁶Sr = 0·7040–0·7045; ¹⁴³Nd/¹⁴⁴Nd = 0·51273–0·51295; Table 2 and Fig. 8). In contrast, the Pb isotope compositions of Sumaco lavas are relatively unradiogenic (²⁰⁶Pb/²⁰⁴Pb = 18·792–18·872; ²⁰⁷Pb/²⁰⁴Pb = 15·581–15·602) and fall within the field of Galapagos basalts, and at the low end of a nearly vertical array of Ecuador data on plots of ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb, that spans the compositional range from the Galapagos Plume to average upper crust and/or local basement rocks in Ecuador (Fig. 9).

View larger version (27K): [in this window] [in a new window]   Fig. 8. Variation in 143Nd/144Nd vs 87Sr/86Sr showing the compositions of lavas from Ecuador compared with data from the Galapagos plume and Carnegie Ridge (White et al., 1993; Hauff et al., 1999; Harpp & White, 2001; Werner et al., 2003), the Andean Central and Southern Volcanic Zones (Frey et al., 1984; Hickey et al., 1986; Davidson et al., 1988, 1990; Futa & Stern, 1988; Hildreth & Moorbath, 1988; Wörner, 1989; Walker et al., 1991), Pacific pelagic sediments (Ben Othman et al., 1989), average upper crust (Zartman & Haines, 1988), Mesozoic oceanic terranes in Colombia and Ecuador (Kerr et al., 1996, 1997, 2002, 2004; Reynaud et al., 1999; Hauff et al., 2000; Lapierre et al., 2000; Mamberti et al., 2003) and continental crust in Ecuador (Bosch et al., 2002). Ecuadoran data are from Table 2. Symbols are as in Fig. 3.

 

View larger version (26K): [in this window] [in a new window]   Fig. 9. Variation in 207Pb/204Pb vs 206Pb/204Pb showing the compositions of lavas from Ecuador compared with data from the Galapagos plume and Carnegie Ridge (White et al., 1993; Hauff et al., 1999; Harpp & White, 2001; Werner et al., 2003), the Andean Central and Southern Volcanic Zones (Hickey et al., 1986; Davidson et al., 1988, 1990; Hildreth & Moorbath, 1988; Wörner, 1989), average upper crust (Zartman & Haines, 1988), Mesozoic oceanic terranes in Colombia and Ecuador (Dupré & Echeverria, 1984; Zartman & Haines, 1988; Kerr et al., 1996, 1997, 2002, 2004; Reynaud et al., 1999; Hauff et al., 2000; Lapierre et al., 2000; Mamberti et al., 2003) and continental crust in Ecuador (Bosch et al., 2002). Northern Hemisphere Reference Line (NHRL) is from Hart (1984). Ecuador data are from Table 2. Symbols are as in Fig. 3.

 
Andesites and dacites
Medium-K, calc-alkaline andesites and dacites (SiO₂ = 55·76–65·98%, FeO^*/MgO = 1·39–1·76) are the predominant magma types erupted in Ecuador (Table 1, Fig. 3). Major element contents of MgO (2·00–6·40 wt %), CaO (4·53–7·80 wt %), FeO^* (3·51–7·75 wt %) and Na₂O (3·18–4·68 wt %) all show coherent linear trends when plotted against SiO₂ (Fig. 3). In contrast, K₂O (0·87–2·99 wt %) varies two-fold across the andesite–dacite compositional range and is not correlated with increasing SiO₂ (Fig. 3).

Highly incompatible trace elements in the andesites and dacites (Rb, Ba, Ta, Nb, La, etc.) are generally correlated with one another (Fig. 7), but like K₂O, are not well correlated with major element differentiation indicators such as SiO₂ or MgO (e.g. Fig. 4). Strontium (Sr) and Y, which typically exhibit moderately incompatible behavior in subduction-related lavas, are highly variable in abundance (Sr = 450–1200 ppm; Y = 9–23 ppm), but show no clear relationship when plotted against strongly incompatible elements such as Cs, Rb or Ba. Strontium is similarly uncorrelated with SiO₂ but does decrease systematically with decreasing Mg-number [Mg/(Mg + Fe), Fig. 6b]. Yttrium (Y), which is relatively low in abundance in most samples (commonly <15 ppm), generally decreases with increasing SiO₂ between 56% and 64% SiO₂, indicating that it has behaved as a moderately compatible element during the evolution of these lavas (see also Monzier et al., 1999, fig. 11). Abundances of strongly compatible trace elements, such as Cr and Ni, are highly variable in the andesites and dacites (Cr up to 240 ppm), and like many of the most incompatible elements, also appear to be largely independent of SiO₂ (e.g. Fig. 4).

Ratios among the incompatible elements in the andesites and dacites are clearly characteristic of a subduction zone setting, and in most respects are typical of andesites and dacites throughout the Andes. Ratios of LILE to HFSE (e.g. Rb/Ta, Ba/Nb) are elevated in the andesites and dacites compared with oceanic basalts (Fig. 7). The REE are highly variable with regard to both elemental abundance and pattern shape (Fig. 5). The total range for La/Yb is high (4·3–38·8; Fig. 10, Table 1), but there is a strong clustering of samples at relatively low La/Yb (6–10) with a few more highly fractionated samples (only four samples with La/Yb >20; Fig. 10). Among the samples with La/Yb >10, some are clearly depleted in the HREE (Yb <1·0) and have fractionated HREE patterns with normalized Dy/Yb >2·0, whereas others are primarily enriched in the light REE (LREE) and have flatter HREE patterns (Fig. 5a). The most common andesites and dacites have straight and modestly fractionated patterns with La/Yb = 6–10 and normalized Dy/Yb <2·0 (Fig. 5b). Overall, these REE characteristics do not stand out as unusual compared with andesites and dacites from other parts of the Andes (Fig. 10). Barium (Ba) concentrations are also variable, and Ba/La ratios are relatively high in the andesites and dacites (Ba/La = 20–80); in this respect the Ecuadoran lavas are similar to lavas in island arcs but are unlike andesites and dacites of the central and southern Andes, which typically have Ba/La <25 (Fig. 10; Hildreth & Moorbath, 1988; Davidson et al., 1990). In addition, nearly all of the andesites and dacites from Ecuador have high Sr/Y (22–70; Figs 6a and 10), and according to the criteria of Defant & Drummond (1990) most would be classified as adakites (see also Beate et al., 2001; Bourdon et al., 2002, 2003; Samaniego et al., 2002). In this regard again, the rocks from Ecuador are also typical of andesites and dacites from throughout the Andes (Figs 6 and 10).

View larger version (18K): [in this window] [in a new window]   Fig. 10. Ba/La (a), La/Yb (b) and Sr/Y (c) vs SiO2 (wt %) comparing lavas from Ecuador with those from the Andean Central and Southern Volcanic Zones. Ecuador data are from Table 1. Symbols are as in Fig. 3. Published data sources for the central and southern Andes are listed in the captions to Figs 6, 8 and 9.

 
Trace element characteristics in the andesites and dacites also change systematically from west to east across the magmatic arc in Ecuador. In general, the average incompatible element concentrations are higher in the main arc and back-arc areas, at greater distances from the trench (Fig. 11a and b). Certain incompatible-element ratios (e.g. Ba/Nb and La/Yb) also change systematically across the arc (Fig. 11c and d; see also Hörmann & Pichler, 1982; Kilian & Pichler, 1989; Vanek et al., 1994; Barragan et al., 1998). These patterns are similar to those seen in magmatic arcs worldwide (Dickinson, 1975).

View larger version (14K): [in this window] [in a new window]   Fig. 11. Trace element concentrations for (a) Ba and (b) La, and ratios (c) La/Yb and (d) Ba/Nb plotted against distance from trench across Ecuador. Data are from Table 1.

 
Isotopically, the andesites and dacites from Ecuador define a relatively narrow range of values that fall within the Sr–Nd mantle array and overlap somewhat with Galapagos plume basalts for each of the isotopic systems studied (Sr, Nd, Pb; Figs 8 and 9). Isotope ratios for Sr and Nd (⁸⁷Sr/⁸⁶Sr = 0·7040–0·7044, ¹⁴³Nd/¹⁴⁴Nd = 0·51273–0·51295; Table 2) lie at the high ¹⁴³Nd/¹⁴⁴Nd end of the field of modern Andean lavas (Fig. 8), and encompass nearly the full range of published values for the Northern Volcanic Zone (Harmon et al., 1984; Kilian et al., 1995; Barragan et al., 1998; Bourdon et al., 2003). For Pb isotopes, the andesites and dacites form a near-vertical trend on plots of ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb, spanning the compositional gap between Galapagos basalts and average upper crust and/or local basement (Fig. 9). The data from Ecuador are broadly like those from Colombia (James & Murcia, 1984), but contrast sharply with the Pb isotope compositions of lavas from the central and southern Andes, which commonly have ²⁰⁶Pb/²⁰⁴Pb values that are relatively low, but ²⁰⁷Pb/²⁰⁴Pb values that are high, resulting in a nearly horizontal data array in ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb (Fig. 9). It is useful to quantify the Pb isotope variation in Andean lavas in terms of the parameter 7/4 Pb, which is a quantitative expression of the deviation in ²⁰⁷Pb/²⁰⁴Pb from the mantle reference line at the ²⁰⁶Pb/²⁰⁴Pb composition of the sample (Hart, 1984). Expressed in this way, Pb isotope compositions in Ecuador andesites and dacites show a crude inverse correlation with Nd isotopes, and appear to fall along mixing lines between Galapagos plume basalts and pelitic metasediments from the local continental crust (Fig. 12).

View larger version (21K): [in this window] [in a new window]   Fig. 12. Variation of 7/4Pb vs Nd for Ecuador lavas. Bulk mixing lines (labeled 1–4) are between estimated Ecuador basalt (3 ppm Pb, 5 ppm Nd, 143Nd/144Nd = 0·512900, Nd = 5·11, 207Pb/204Pb = 15·589, 206Pb/204Pb = 18·820) and putative crustal assimilants with isotopic compositions like those of Ecuador metapelites (143Nd/144Nd = 0·512183, Nd = –8·9, 207Pb/204Pb = 15·700 and 206Pb/204Pb = 18·916). The isotopic composition of the metapelite end-member is from Bosch et al. (2002). Mixing line 1 uses the Pb and Nd concentrations of the average metapelites (Nd = 25 ppm, Pb = 15 ppm). Subsequent mixing lines use higher Pb concentrations for the assimilant (higher Pb/Nd in the metapelite end-member) to produce the curved mixing lines, which bracket the Ecuadoran lavas (Pb/Nd = 1·2 and 4·0). The isotopic composition for the basalt end-member is the average of the Sumaco lavas (Table 2), which appear to be relatively unaffected by assimilation. Concentrations for Nd and Pb in the basalt end-member were estimated by extrapolating andesite compositions for these elements back to 52% SiO2 (on graphs of Nd and Pb vs SiO2). Values for 7/4Pb were calculated according to Hart (1984). Sources of published data from the Carnegie Ridge and Galapagos Islands are as in Figs 8 and 9.

 
The isotopic compositions of the andesites and dacites change both east-to-west across the magmatic arc in Ecuador, and with changes in certain trace element parameters. Specifically, ⁸⁷Sr/⁸⁶Sr and 7/4 Pb in the Ecuadoran lavas reach their highest values in lavas erupted in the main-arc region (330–360 km from the trench), along the crest of the Andes, where the crust is likely to be thickest (Fig. 13). Conversely, ¹⁴³Nd/¹⁴⁴Nd (Nd) is lowest across this region. It should be noted, however, that the alkali basalts and basaltic andesites from Sumaco Volcano in the back-arc area do not follow the cross-arc isotopic trends of the andesites and dacites (Fig. 13). The isotopic compositions of the andesites and dacites in Ecuador also change systematically with increasing incompatible element concentrations (e.g. decreasing ¹⁴³Nd/¹⁴⁴Nd with increasing Nb, Ba, Rb, La; Fig. 14), and with changes in certain trace element ratios, especially Ba/Nb and La/Yb, and to a lesser extent Ba/La (Fig. 15). In general, the trace element–isotope relationships are clearest for Nd and Pb isotopes, but are absent or less clear when they involve either Sr or ⁸⁷Sr/⁸⁶Sr. Finally, there appears to be no clear systematic relationship between any of the isotopic systems and any major element parameter (e.g. SiO₂) in the andesite–dacite compositional range (Fig. 16).

View larger version (14K): [in this window] [in a new window]   Fig. 13. Isotope ratios plotted against distance from trench across Ecuador, showing cross-arc changes in 87Sr/86Sr (a), Nd (b), and 7/4 Pb (c) in lavas produced at different positions across the arc. Data plotted here are from Table 2. Symbols used here are as in Fig 3.

 

View larger version (23K): [in this window] [in a new window]   Fig. 14. 143Nd/144Nd vs Nb (a), Zr (b), Rb, (c), Ba (d), Sr (e) and Y (f) in Ecuadoran lavas. Data are from Tables 1 and 2. Symbols are as in Fig. 3.

 

View larger version (25K): [in this window] [in a new window]   Fig. 15. 143Nd/144Nd and 7/4Pb vs La/Yb (a, d), Ba/Nb (b, e) and Sr/Y (c, f) for Ecuadoran lavas. Large white arrows on Ba/Nb plots provide an interpretation of source components in the mantle wedge (low 7/4Pb and Ba/Nb; high 143Nd/144Nd), fluids and/or melts from the subducting oceanic crust (low 7/4Pb; high Ba/Nb and 143Nd/144Nd) and continental crust (high 7/4Pb; low Ba/Nb and 143Nd/144Nd). Data are from Tables 1 and 2. Symbols are as in Fig. 3.

 
Rhyolites
Rhyolites reported here are from Chacana, Chalupas, and Cotopaxi volcanoes, which are all located in the main-arc area, along the crest of the Andes in northern Ecuador (Table 1, Figs 1 and 2). Apart from the obvious gap in silica between approximately 66% and 71% SiO₂ (Fig. 3), the major and trace element compositions of the rhyolites from Ecuador appear, in a broad sense, to be extensions of the andesite–dacite compositional trends (Figs 3–5). One exception is in the REE patterns, which are clearly much flatter in the HREE (normalized Dy/Yb <1·50) and have 18–57% negative Eu anomalies (i.e. Eu/Eu^* = 0·43–0·82; Fig. 5c). The broad similarity with respect to trace elements does not extend to the isotopic compositions, which on average, are more radiogenic in Pb and Sr and less radiogenic in Nd in the rhyolites than in the andesites and dacites (Figs 8, 9 and 13–15). This is to be expected in high-SiO₂ melts containing a relatively high proportion of geochemically evolved crustal rock relative to mantle components.

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Isotopic constraints on crustal components in Ecuadoran lavas
Compared with the central and southern Andes, where the case for substantial melting and assimilation of geochemically evolved crustal rocks has been well documented (e.g. Hildreth & Moorbath, 1988; Davidson et al., 1990; Kay & Kay, 1991), lavas in Ecuador fall within a narrow range of relatively primitive isotopic compositions, partially overlapping the range of Galapagos Plume basalts for Pb and Nd isotopes (Figs 8 and 9). The isotopic compositions in Ecuadoran lavas also appear to be largely independent of their major element compositions (Fig. 16) and of the age or composition of the crust through which they were erupted (e.g. compare back-arc and volcanic front in Fig. 13). This is particularly evident for the lavas from Sumaco, which are mafic and alkaline in composition (nepheline-normative), and appear to have erupted through geochemically mature Precambrian crust, but have Sr and Nd isotope ratios that overlap broadly with the common sub-alkaline andesites and dacites at the volcanic front, where the volcanoes are underlain by geochemically immature oceanic terranes of Mesozoic age (Fig. 13). These observations, which indicate a limited role for geochemically mature continental crust in the genesis of modern lavas in Ecuador (see also Harmon et al., 1984; James & Murcia, 1984), are supported by certain trace element ratios. In particular, Ba/La is variable and high in Ecuadoran lavas (similar to island arc basalts), but thus unlike lavas of the central and southern Andes, which have consistently low Ba/La, apparently as a result of assimilation of large quantities of granitic crust (Fig. 11; Hildreth & Moorbath, 1988; Davidson et al., 1990).

View larger version (16K): [in this window] [in a new window]   Fig. 16. Variation in 87Sr/86Sr (a, b), 143Nd/144Nd (c) and 7/4Pb (d) vs SiO2 for Ecuadoran lavas (large symbols), and other areas within the Andes (small symbols). Ecuadoran data are from Tables 1 and 2. Sources for other Andean data are cited in the caption to Fig. 8.

 
Arculus et al. (1999) pointed out that partial melting of a garnet-bearing mafic lower crust might explain the adakitic trace element patterns and relatively non-radiogenic isotope compositions observed in Ecuador andesites and dacites (see also Kilian et al., 1994, 1995; Monzier et al., 1999). Xenolith studies, combined with more recent work on the Raspas Metamorphic Complex, suggest that the deep crust beneath the volcanic front and main-arc areas should be a lithological mixture that spans a broad isotopic range from geochemically primitive/oceanic to geochemically mature and continental (Bosch et al., 2002; Weber et al., 2002). In particular, Weber et al. (2002) showed that deep crustal xenoliths from southern Colombia are unusually variable isotopically, with relatively radiogenic Pb (high ²⁰⁷Pb/²⁰⁴Pb) and a wide range in ¹⁴³Nd/¹⁴⁴Nd compared with ⁸⁷Sr/⁸⁶Sr (Figs 8 and 9). Similarly, Bosch et al. (2002) showed that the Raspas Metamorphic Complex includes eclogite-grade oceanic and continental lithologies which also span a large isotopic range (e.g. ⁸⁷Sr/⁸⁶Sr = 0·7039–0·7226). The highly variable isotopic composition of the deep crustal rocks (revealed in the Colombian xenoliths and Raspas Metamorphic Complex) is unlike that of the Ecuadoran lavas, which fall within an unusually narrow range, especially for ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr where they are offset only slightly (to more radiogenic ⁸⁷Sr/⁸⁶Sr) from the Galapagos Plume lavas (i.e. offset slightly from anticipated subducting plate compositions; Figs 8 and 9). It is likely that the andesites, dacites and rhyolites in Ecuador have assimilated some crust (e.g. Harmon et al., 1984; James & Murcia, 1984; Bourdon et al., 2003), but the highly variable and often radiogenic isotopic composition of the North Andean crust (based on the composition of the oceanic terranes, Colombian xenoliths and high-grade metamorphic terranes) suggests that it is not a major contributor to the source of Ecuadoran lavas. Again, this conclusion is reinforced by the cross-arc changes in isotopic compositions, which are small compared with the isotopic variability of the Ecuador crust, which changes from geochemically primitive oceanic terranes in the west to geochemically mature continental rock in the east.

It remains possible, of course, based on isotopic data alone, that partial melts of a newly underplated mafic layer at Moho depths (formed from basalts generated in the modern subduction zone and, therefore, isotopically similar to the modern arc lavas), could have contributed significantly to the source of the Ecuadoran lavas. Such an origin, which involves rapid remelting of a recently created (i.e. newly underplated) basaltic layer, has been proposed for Ecuador (Monzier et al., 1999) and for other areas of thickened crust in the Andes, in particular in the case of the Miocene Cordillera Blanca Batholith in Peru (Petford & Atherton, 1996). At some level, this process seems inevitable in any vigorously active magmatic arc. It is unlikely, however, that this process has exerted a major influence on magma genesis in the modern volcanic arc in Ecuador, where the lavas are mostly medium-K, calc-alkaline andesites whose major element compositions are unlike those of the silicic, alkali-rich, leucocratic granites that dominate the Cordillera Blanca in Peru (SiO₂ >70%, Na₂O >4% K₂O >3%, MgO <2%, FeO^* <2%). The latter appear to have formed by regional-scale melting of underplated basalt during the Miocene (Petford & Atherton, 1996). The rhyolites of Ecuador have major element compositions that are similar to the Cordillera Blana leucogranites, but they constitute only a small proportion of the surface eruptive rocks, and unlike leucogranites from the Cordillera Blanca, the Ecuadoran rhyolites have relatively flat HREE patterns, which also contrast strongly with Cordillera Blanca mafic and intermediate rocks (e.g. compare Fig. 5 with Petford & Atherton, 1996, fig. 9).

Having emphasized that the crustal rocks of the northern Andes have played a limited role in the genesis of Ecuadoran lavas, compared with other parts of Andes where assimilation and wholesale melting of the crust appears to be common, it is important to point out that the Sr–Nd–Pb isotope systematics clearly indicate that, similar to the case in Colombia (James & Murcia, 1984), at least some assimilation of continental crust has affected many of the samples from Ecuador that we have analyzed. This is particularly clear from Pb and Nd isotope data, which are well correlated and appear to define mixing lines between Galapagos Plume–Carnegie Ridge basalts (i.e. probable subducting plate and mantle wedge compositions) and the local, geochemically mature continental crustal rocks (probable assimilants) that underlie many of the volcanoes in central Ecuador (Fig. 12). Bulk mixing calculations based on this Pb–Nd correlation indicate that most lavas in Ecuador contain no more than 5–10% of the local crust (Fig. 12). Assimilation of geochemically mature continental crustal material by the Ecuadoran lavas is well supported by the observed spatial–geochemical shifts in isotopic compositions across the arc, which reach maximum values for ⁸⁷Sr/⁸⁶Sr and 7/4Pb, and minimum values for ¹⁴³Nd/¹⁴⁴Nd, over the crest of the northern Andes in the main arc area at distances of 330–360 km from the trench (Fig. 13). Based on the high elevations and the presence of rhyolitic volcanism at Cotapaxi, Chalupas and Chacana, this area around the Inter-Andean depression appears to be the part of the Northern Andes where the crust is thickest, warmest, and where it is therefore most likely to contribute an assimilant to melts that are in transit from the subduction zone to the surface. Our estimate of the extent of the crustal contamination in Ecuadoran lavas, which is somewhat lower than some previous estimates (e.g. Kilian et al., 1995; Barragan et al., 1998), does not constrain the possible role of geochemically primitive accreted oceanic terranes as a crustal assimilant, because those terranes are isotopically similar to the anticipated subducting plate and mantle wedge composition (Carnegie Ridge, Galapagos Plume, Figs 8 and 9).

The above interpretation implies that a significant proportion of the Ecuadoran lavas have remained unaffected or minimally affected by assimilation of geochemically mature continental crust (e.g. samples with 7/4Pb <6·80 and Nd >4·0, Figs 8 and 9). We cannot, of course, rule out the possibility that all of the lavas have been modified by assimilation of isotopically primtive crustal rocks, or that isotopically more primitive parental magmas were never erupted at the surface and, therefore, have not been sampled. We do not believe this is the case for two reasons. First, a large number of the Ecuadoran andesites and dacites have Pb, Sr and Nd isotopic compositions that are tightly clustered and partially overlap with (in Nd and Pb isotopes) likely subducting plate compositions in the form of Galapagos Plume and Carnegie Ridge basalts. This is similar to what is observed in many island arcs where the Pb, Nd and Sr isotopic compositions of the lavas approach those of MORB and are therefore widely believed to be controlled largely by geochemical components that originate in subducted MORB, marine sediment and seawater (Kay, 1980; Plank & Langmuir, 1993; Elliott et al., 1997; Class et al., 2000). Second, the alkali basalts and basaltic andesites at Sumaco, which appear, based on major and trace element compositions, to be low-percentage melts of mantle peridotite that has been less affected by subduction metasomatism than the associated andesites and dacites (Barragan et al., 1998; Bourdon et al., 2003), have extraordinarily high concentrations of Pb (12–29 ppm), Sr (>2000 ppm) and Nd (>80 ppm). Such high concentrations would have caused these melts to be much more resistant to isotopic modification during crustal assimilation compared with the andesites and dacites, which have lower concentrations of these elements, are more evolved (lower Mg-number) and were erupted at lower temperatures. Assuming that the more silicic and cooler lavas are more likely to have assimilated crust on their way to the surface (DePaolo, 1981), the broad isotopic overlap between the mafic alkaline lavas at Sumaco and the andesites and dacites implies that control over the composition of most Ecuadoran lavas lies primarily in the subduction zone, and not in the continental crust through which the melts passed (see also Barragan et al., 1998; Bourdon et al., 2003).

Effects of crustal assimilation on trace elements
Ecuador andesites and dacites that have relatively high 7/4Pb and ⁸⁷Sr/⁸⁶Sr, and low ¹⁴³Nd/¹⁴⁴Nd (i.e. those that appear to have assimilated a significant quantity of continental crust), generally have more variable and higher concentrations of the most incompatible trace elements, including Rb, Cs, Nb, Zr, Ba, Pb, and La. For example, as seen in Fig. 14, among the andesites and dacites with ¹⁴³Nd/¹⁴⁴Nd <0·51285 (Nd <4·1), four samples out of five have Zr >100 ppm, whereas no sample with ¹⁴³Nd/¹⁴⁴Nd >0·51285 has Zr >100 ppm. A similar relationship can be seen in the Nb concentrations, which show a clear inverse correlation with ¹⁴³Nd/¹⁴⁴Nd, but not for Y or Yb, where the elemental concentrations are not correlated with isotope ratios (Fig. 14). Not surprisingly, the rhyolites generally also have elevated concentrations of the highly incompatible elements and concomitantly higher ⁸⁷Sr/⁸⁶Sr and 7/4Pb, and lower ¹⁴³Nd/¹⁴⁴Nd, consistent with relatively higher degrees of crustal contamination (Fig. 14). For most incompatible elements, however, the correlation with isotopic composition is relatively weak.

Isotopic shifts attributable to crustal contamination are also accompanied by shifts in certain incompatible element ratios (Fig. 15). This is particularly clear for La/Yb, which is generally higher, and for Ba/La and Ba/Nb, which are generally lower in samples with relatively low ¹⁴³Nd/¹⁴⁴Nd and high ⁸⁷Sr/⁸⁶Sr (i.e. those that appear to have assimilated continental crust). These relationships suggest that cross-arc changes in La/Yb, Ba/Nb, etc., for the andesites and dacites are produced at least in part, by combined fractional crystallization and assimilation (AFC) of thickened continental crust, and not exclusively (as has been consistently argued in the literature) by increased depth and/or decreased percentage melting or by a decrease in the slab–fluid input to the mantle wedge. It is an important point, however, that the back-arc mafic and alkaline lavas at Sumaco (which anchor the cross-arc trends with high La/Yb and low Ba/Nb) retain the high ¹⁴³Nd/¹⁴⁴Nd and low 7/4Pb that is characteristic of lavas at the volcanic front, and so do not appear to have been significantly modified by AFC processes (Fig. 13). We agree with the interpretation of the Sumaco lavas as deep, low-percentage, alkaline-series melts of mantle peridotite (Barragan et al., 1998; Bourdon et al., 2003), and emphasize that, as such, they represent relatively hot and dry melts with low viscosities, that are physically and chemically distinct from the felsic and intermediate-composition calc-alkaline lavas (andesite–dacite–rhyolite) that dominate volcanism in Ecuador.

In contrast to La/Yb and Ba/Nb, which appear to be significantly affected by crustal assimilation, the full range of Sr/Y observed in the Ecuador andesites and dacites appears to be present among samples with relatively primitive isotopic composition (e.g. ¹⁴³Nd/¹⁴⁴Nd >0·51285, 7/4 Pb <7·0; Fig. 15). This suggests that Sr/Y, a key ratio in the adakite characterization, is not substantially affected by crustal assimilation in andesite and dacite lavas. In fact, for individual volcanoes where significant crustal assimilation has occurred, and where several andesite and dacite samples have been analyzed (e.g. Antisana, Imbabura), Sr/Y and Sr concentration appear to be inversely correlated with 7/4Pb, suggesting that, at some volcanoes, these quantities are highest in the samples that have assimilated the least amount of continental crust (Fig. 17).

View larger version (27K): [in this window] [in a new window]   Fig. 17. (a) Sr concentration vs 7/4Pb showing trends at the Ecuador volcanic centers Imbabura, Antisana and Chacana; (b) Sr concentration vs Mg-number for Imbabura and Antisana. Data from Imbabura and Chacana are from Tables 1 and 2. Data from Antisana are from Tables 1 and 2 and from Bourdon et al. (2003). Slab melts (cross-hatched box) are presumed to contain high concentrations of Sr but relatively non-radiogenic Pb (e.g. Kay, 1978). Deep crustal melts and products of AFC (assimilation–fractional crystallization) in the Andes are presumed to contain relatively radiogenic Pb and possibly high concentrations of Sr (e.g. Garrison & Davidson, 2003).

 
We conclude that crustal assimilation has significantly changed the trace element abundances of some Ecuadoran lavas, but that assimilation does not play a clear and consistent role in the genesis of the adakitic trace element characteristics, which are ubiquitous in the andesites and dacites in Ecuador.

Origin of the ‘adakitic’ trace element characteristics in Ecuadoran lavas
Most andesites and dacites in Ecuador have REE characteristics that are adakitic; that is, they are enriched in the LREE, lack significant Eu anomalies, and are strongly fractionated between the middle REE (MREE) and HREE (Fig. 5). Unusually high concentrations of LREE in some Ecuadoran lavas may have been produced by assimilation of crustal rocks (as discussed above); however, assimilation alone cannot explain the low concentrations of Yb and Lu and the strongly fractionated MREE to HREE patterns (normalized Gd/Yb >> 1·0) that distinguish andesites and dacites in Ecuador from those in island arcs (Fig. 5d). Assuming that the melt source beneath the Northern Andes is approximately chondritic with respect to the HREE (i.e. normalized Gd/Yb 1·0), it is clear that garnet must have played a major role in the genesis of the Ecuadoran lavas. This role for garnet could be as a residual phase left after melt extraction or through removal by crystal fractionation under high pressures.

Several studies have concluded that garnet is involved in magma genesis beneath Ecuador when melting occurs at the surface of the subducting slab (e.g. Beate et al., 2001; Bourdon et al., 2002, 2003; Samaniego et al., 2005). This is essentially an application of the Defant & Drummond (1990) adakite model in Ecuador. Among the lavas in Ecuador, those that appear to present the strongest case for the slab-melt interpretation have >700 ppm Sr and relatively MORB-like isotopic compositions (Fig. 17). Lavas with these characteristics are well illustrated by our data from Imbabura Volcano, where there is a high-Sr liquid line of descent separate from the evolutionary pathway defined by lavas with 400–600 ppm Sr over the same range of SiO₂ and Mg-number (Fig. 17). High-Sr lavas of this type appear to be present at Antisana Volcano (Fig. 17) and at Puñalica Volcano, where high Sr (>850 ppm) and unradiogenic Sr isotopes (⁸⁷Sr/⁸⁷Sr <0·7039) define an end-member composition among the 23 samples from the Northern Andes studied by Harmon et al. (1984). Rocks of this geochemical type do not appear to exist among lavas of the central and southern Andes (Fig. 17) or in Colombia (James, 1982). In combination, these observations support the idea that melts from oceanic crust of the subducting Carnegie Ridge play a significant roll in the genesis of at least some Ecuador lavas (Bourdon et al., 2003, and references therein).

Despite the apparent control of the Carnegie Ridge on the surface expression of active volcanism in Ecuador (Fig. 1), and the clear presence of lavas with trace element and isotopic compositions appropriate to the slab-melt interpretation (Fig. 17), the existence of thickened continental crust beneath Ecuador (at least 50 km; Prévot et al., 1996; Guillier et al., 2001) allows an alternative scenario by which adakite geochemistry may be produced, involving mantle melting and high-pressure crystal fractionation.

The presence of thick crust allows only a short column of mantle rock above the subduction zone (Fig. 1). This in turn means that melting within the mantle wedge will be relatively low percentage beneath Ecuador, compared with arcs in which the crust is thin and the melting column is long (Plank & Langmuir, 1988). In addition, we know from Sm/Nd and Lu/Hf systematics that melting in all of the major tectonic settings involves the stabilization of at least a small quantity of residual garnet (Salters, 1991; Hirschmann & Stolper, 1996). With these considerations in mind, it is reasonable to anticipate that Ecuador basalts, which are only rarely erupted in the volcanic front or main-arc areas, will generally have HREE patterns that are more strongly fractionated than those of common island arc basalts, which typically have Gd/Yb = 1·5–1·9 (e.g. Elliott et al., 1997; George et al., 2003). This expectation is confirmed by the few available analyses of basaltic rocks from Ecuador's main arc at Sangay Volcano, where four samples with SiO₂ <54% have Gd/Yb = 2·6–3·2 (Monzier et al., 1999), well within the range of the common Ecuadoran andesites and dacites, which typically have Gd/Yb = 2·0–3·7 (Table 1 and Samaniego et al., 2005). These observations provide clear support for the idea that the physical conditions of melting within the mantle wedge beneath Ecuador may be conducive to the stabilization of small quantities of residual garnet and the formation of basaltic melts that have strongly fractionated trace element patterns with low Y and Yb.

Subsequent cooling and fractional crystallization in the deep crust beneath Ecuador are also likely to contribute to depletion of Y and HREE and enrichment in Sr to produce adakitic melts. This is true because basaltic to andesitic magmas with high H₂O contents (5–6%) at pressures of 15–20 kbar (45–60 km depths) and at temperatures below 1100°C, will crystallize a mineral assemblage that is free of plagioclase and dominated by clinopyroxene and garnet ± amphibole (Green, 1972, 1982). Simple calculations using typical partition coefficients (McKenzie & O'Nions, 1991) demonstrate that if garnet constitutes 15–20% of the mineral assemblage, fractional crystallization will result in strongly compatible behavior for Y, Yb and Lu (D_BULK = 1·5–2·0), progressively less compatible behavior for the MREE, and strongly incompatible behavior for La and Ce (D_BULK < 0·20). This mechanism is clearly capable of producing the low concentrations of MREE to HREE that distinguish andesites and dacites in Ecuador from lavas of similar major element compositions produced in island arcs (Fig. 5d). These physical conditions for melt evolution (high-pressure and hydrous) will also suppress crystallization of plagioclase (Green, 1982), which in turn will cause Sr to behave as an incompatible element that can be enriched to high concentration as the melts evolve by crystal fractionation. The experimental studies of Green (1972, 1982) have similarly demonstrated that the major element character of the calc-alkaline series (basalt, andesite, dacite) can be explained by crystal fractionation processes under high-pressure and hydrous condition (see also Hildreth & Moorbath, 1988; Garrison & Davidson, 2003).

More broadly, the deep crustal fractionation mechanism is consistent with the basic observation that the trace element characteristics of Ecuadoran lavas are highly variable, even though their Pb, Sr and Nd isotopic compositions are relatively primitive and homogeneous. This is true for lavas throughout Ecuador, and within individual volcanoes (Samaniego et al., 2002, 2005), and is consistent with the idea that melt batches may evolve by crystal fractionation to create either ‘adakitic’ or ‘classic calc-alkaline’ trace element characteristics, dependent on the depth/pressure of fractionation, and consequently on the presence or absence of garnet in the fractionating assemblage.

The history of magmatism in Ecuador since the Eocene appears most consistent with the deep crustal fractionation model. Chiaradia et al. (2004) recently showed that Eocene to Late Miocene andesites in Ecuador (50–9 Ma) have relatively flat REE patterns that contrast with the younger, Miocene to Recent lavas, which are adakitic (see Chiaradia et al., 2004, fig. 8). The differences in REE characteristics imply that different processes operated in the genesis of these lavas of different ages. Available Pb isotope data, however, indicate that the magma source region beneath Ecuador has remained largely unchanged since the Eocene. Chiaradia et al. (2004) argued that these observations imply that the magmatic plumbing systems beneath Ecuador shifted to depths within the stability field of garnet after 9 Ma, probably in response to a crustal thickening event, possibly caused by the initial collision and subduction of the Carnegie Ridge. The alternative interpretation involves a shift in source from the mantle wedge to the subducting plate at the point in time when subduction of the Carnegie Ridge began (i.e. Bourdon et al., 2003, and references therein). This alternative remains possible, but appears less likely in the light of the new isotopic data, which suggest that the melt source beneath Ecuador has remained approximately constant since the Eocene (Chiaradia et al., 2004).

One weakness of the deep crustal fractionation model described here for adakite genesis is that it requires fractional crystallization of garnet, even though garnet phenocrysts are not observed in Ecuadoran lavas. Garnets are not commonly found as phenocrysts in calc-alkaline lavas; they are, however, known from several localities and their potential role in the evolution of andesitic and dacitic lavas is well documented experimentally (Green & Ringwood, 1968; Fitton, 1972; Green, 1972, 1982; Hamer & Moyes, 1982; Day et al., 1992). If the deep crustal fractionation model is correct, this means simply that the plagioclase–pyroxene–amphibole phenocryst assemblage in the Ecuadoran lavas is produced late in the magmatic evolution, during magma storage in shallow-level systems where garnet is unstable (e.g. Fitton, 1972).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
(i) Primitive and relatively homogeneous Sr, Nd and Pb isotopic compositions indicate that lavas throughout Ecuador are produced largely within the supra-subduction zone mantle wedge and not by extensive melting of the North Andean crust.

(ii) Changes in ¹⁴³Nd/¹⁴⁴Nd, 7/4Pb and La/Yb across the arc in Ecuador are interpreted to result from assimilation of geochemically mature continental crust, especially in the main-arc area, at distances of 330–360 km from the trench. Mixing calculations limit the quantity of assimilated continental crust to less than 10%.

(iii) Possible assimilation of geochemically primitive crustal rock beneath the Western Cordillera remains poorly constrained; however, assimilation of crustal rocks does not appear to play a clear and consistent role in the genesis of the adakitic trace element patterns that are common in andesites and dacites in Ecuador.

(iv) Available whole-rock data do not provide a clear basis for distinguishing between slab-melting and deep crustal fractionation models for the genesis of adakitic lavas in Ecuador. Both models derive adakites from basalt, either by partial melting or fractional crystallization, and both explain the observed fractionation of REE patterns with garnet. None the less, the geochemical evolution within individual volcanoes (Samaniego et al., 2005), and in magmatic rocks produced throughout Ecuador since the Eocene (Chiaradia et al., 2004), appears to favor the deep crustal fractionation model for the genesis of most post-Miocene lavas in Ecuador.

	ACKNOWLEDGEMENTS

 
This paper benefited substantially from thoughtful reviews by Henrietta Lapierre, Erwan Bourdon, Pablo Samaniego and Jennifer Garrison. We thank Nick Arndt and Marjorie Wilson for their thoughtful, thorough and timely handling of this work. Thanks also go to Drew Coleman for his assistance in acquiring the isotopic data. This paper also benefited from discussions with members of the University of South Carolina Solid-Earth Group. We thank in particular Jim Kellogg, Matt Kohn, Jim Knapp, and Bob Trenkamp. Thanks also go to Dennis Geist and Roberto Barragan, who generously provided samples from Sumaco, Antisana and Atacazo for this study, and to Sue Kay for many helpful discussions regarding Andean petrology and geochemistry.

---

^* Corresponding author. Telephone: 001-803-777-9524. Fax: 001-803-777-6610. E-mail: gyogodzin{at}geol.sc.edu

	REFERENCES

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Abratis, M. & Wörner, G. (2001). Ridge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm. Geology 29, 127–130.[Abstract/Free Full Text]

Arculus, R. J., Lapierre, H. & Jaillard, E. (1999). Geochemical window into subduction and accretion processes: Raspas metamorphic complex, Ecuador. Geology 27, 547–550.[Abstract/Free Full Text]

Aspden, J. A. & Litherland, M. (1992). The geology and Mesozoic collisional history of the Cordillera Real, Ecuador. Tectonophysics 205, 187–204.[CrossRef][Web of Science]

Aspden, J. A., Harrison, S. H. & Rundle, C. C. (1992). New geochronological control for the tectono-magmatic evoution of the metamorphic basement, Cordillera Real and El Oro Province of Ecuador. Journal of South American Earth Sciences 6, 77–96.[CrossRef][Web of Science]

Atherton, M. P. & Petford, N. (1993). Generation of sodium-rich magmas from newly underplated basaltic crust. Nature 362, 144–146.[CrossRef]

Barazangi, M. & Isacks, B. (1976). Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America. Geology 4, 686–692.[Abstract/Free Full Text]

Barragan, R., Geist, D., Hall, M., Larsen, P. & Kurz, M. (1998). Subduction controls on the compositions of lavas from the Ecuadorian Andes. Earth and Planetary Science Letters 154, 153–166.[CrossRef][Web of Science]

Beate, B., Monzier, M., Spikings, R., Cotten, J., Silva, J., Bourdon, E. & Eissen, J. P. (2001). Mio-Pliocene adakite generation related to flat subduction in southern Ecuador: the Quimsacocha volcanic center. Earth and Planetary Science Letters 192, 561–570.[CrossRef][Web of Science]

Ben Othman, D., White, W. M. & Patchett, J. (1989). The geochemistry of marine sediments, island arc magma genesis, and crust–mantle recycling. Earth and Planetary Science Letters 94, 1–21.[CrossRef][Web of Science]

Bosch, D., Piercarlo, G., Lapierre, H., Malfere, J. & Jaillard, E. (2002). Geodynamic significance of the Raspas Metamorphic Complex (SW Ecuador): geochemical and isotopic constraints. Tectonophysics 345, 83–102.[CrossRef][Web of Science]

Bourdon, E., Eissen, J. P., Monzier, M., Robin, C., Martin, H., Cotten, J. & Hall, M. L. (2002). Adakite-like lavas from Antisana Volcano (Ecuador): evidence for slab melt metasomatism beneath Andean Northern Volcanic Zone. Journal of Petrology 43, 199–217.[Abstract/Free Full Text]

Bourdon, E., Eissen, J. P., Gutscher, M. A., Monzier, M., Hall, M. L. & Cotten, J. (2003). Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America). Earth and Planetary Science Letters 205, 123–138.[CrossRef][Web of Science]

Calvache, M. L. & Williams, S. N. (1997). Geochemistry and petrology of the Galeras Volcanic Complex, Colombia. Journal of Volcanology and Geothermal Research 77, 21–38.[CrossRef][Web of Science]

Chiaradia, M., Fontboté, L. & Beate, B. (2004). Cenozoic continental arc magmatism and associated mineralization in Ecuador. Mineralium Deposita 39, 204–222.[CrossRef][Web of Science]

Class, C., Miller, D. M., Goldstein, S. L. & Langmuir, C. H. (2000). Distinguishing melt and fluid subduction components in Umnak Volcanics, Aleutian Arc. Geochemistry, Geophysics, Geosystems 1999GC000010.

Conrad, W. K. & Kay, R. W. (1984). Ultramafic and mafic inclusions from Adak Island: crystallization history, and implications for the nature of primary magmas and crustal evolution in the Aleutian arc. Journal of Petrology 25, 88–125.[Abstract/Free Full Text]

Davidson, J. P. (1987). Crustal contamination versus subduction zone enrichment: examples from the Lesser Antilles and implications for mantle source compositions of island arc volcanic rocks. Geochimica et Cosmochimica Acta 51, 2185–2198.[CrossRef][Web of Science]

Davidson, J. P., Ferguson, K. M., Colucci, M. T. & Dungan, M. A. (1988). The origin and evolution of magmas from the San Pedro–Pellado Volcanic Complex, S. Chile: multicomponent sources and open system evolution. Contributions to Mineralogy and Petrology 100, 425–445.

Davidson, J. P., McMillan, N. J., Moorbath, S., Wörner, G., Harmon, R. S. & Lopez-Escobar, L. (1990). The Nevados de Payachata volcanic region (18°S/69°W, N. Chile); II, Evidence for widespread crustal involvement in Andean magmatism. Contributions to Mineralogy and Petrology 105, 412–432.[CrossRef][Web of Science]

Day, R. A., Green, T. H. & Smith, I. E. M. (1992). The origin and significance of garnet phenocrysts and garnet-bearing xenoliths in Miocene calc-alkaline volcanics from Northland, New Zealand. Journal of Petrology 33, 125–161.[Abstract/Free Full Text]

Defant, M. J. & Drummond, M. S. (1990). Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665.[CrossRef]

Defant, M. J., Richerson, P. M., De Boer, J. Z., Stewaart, R. H., Maury, R. C., Bellon, H., Drummond, M. S., Feigenson, M. D. & Jackson, T. E. (1991). Dacite genesis via both slab melting and differentiation: petrogenesis of La Yeguada Volcanic Complex, Panama. Journal of Petrology 32, 1101–1142.[Abstract/Free Full Text]

DePaolo, D. J. (1981). Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189–202.[CrossRef][Web of Science]

Deruelle, B. (1982). Petrology of the Plio-Quaternary volcanism of the south–central and meridional Andes. Journal of Volcanology and Geothermal Research 14, 77–124.[CrossRef][Web of Science]

Dickinson, W. R. (1975). Potash–depth (K–h) relations in continental margin and intra-oceanic magmatic arcs. Geology 3, 53–56.[Abstract/Free Full Text]

Droux, A. & Delaloye, M. (1996). Petrography and geochemistry of Plio-Quaternary calc-alkaline volcanoes of southwestern Colombia. Journal of South American Earth Sciences 9, 27–41.[CrossRef][Web of Science]

Dupré, B. & Echeverria, L. M. (1984). Pb isotopes of Gorgona Island (Colombia): isotopic variations correlated with magma type. Earth and Planetary Science Letters 67, 186–190.[CrossRef][Web of Science]

Elliott, T., Plank, T., Zindler, A., White, W. & Bourdon, B. (1997). Element transport from slab to volcanic front at the Mariana arc. Journal of Geophysical Research 102, p. 14991–15091.[CrossRef]

Ewart, E. (1982). The mineralogy and petrology of Tertiary–Recent orogenic volcanic rocks with special reference to the andesite–basaltic composition range. In: Thorpe, R. S. (ed.) Orogenic Andesites and Related Rocks, New York: John Wiley, pp. 25–87.

Feininger, T. (1987). Allochthonous terranes in the Andes of Ecuador and northwestern Peru. Canadian Journal of Earth Science 24, 266–278.

Feininger, T. & Seguin, M. K. (1983). Bouguer gravity anomaly field and inferred crustal structure of continental Ecuador. Geology 11, 40–44.[Abstract/Free Full Text]

Fitton, J. G. (1972). The genetic significance of almandine–pyrope phenocrysts in the calc-alkaline Borrowdale Volcanic Group, Northern England. Contributions to Mineralogy and Petrology 36, 231–248.[CrossRef][Web of Science]

Frey, F. A., Gerlach, D. C., Hickey, R. L., Lopez-Escobar, L. & Munizaga-Villavicencio, F. (1984). Petrogenesis of the Laguna del Maule volcanic complex, Chile (36°S). Contributions to Mineralogy and Petrology 88, 133–149.[CrossRef][Web of Science]

Futa, K. & Stern, C. R. (1988). Sr and Nd isotopic and trace element compositions of Quaternary volcanic centers of the southern Andes. Earth and Planetary Science Letters 88, 253–262.[CrossRef][Web of Science]

Garrison, J. M. & Davidson, J. P. (2003). Dubious case for slab melting in the Northern volcanic zone of the Andes. Geology 31, 565–568.[Abstract/Free Full Text]

George, R., Turner, S., Hawkesworth, C. J., Morris, J., Nye, C., Ryan, J. G. & Zheng, S.-H. (2003). Melting processes and fluid and sediment transport rates along the Alaska–Aleutian arc from an integrated U–Th–Ra–Be isotope study. Journal of Geophysical Research 108, 1–25.

Gerlach, D. C., Frey, F. A., Moreno-Roa, H. & Lopez-Escobar, L. (1988). Recent volcanism in the Puyehue–Cordon Daulle region, Southern Andes, Chile (40·5°S); petrogenesis of evolved lavas. Journal of Petrology 29, 333–382.[Abstract/Free Full Text]

Gill, J. (1981). Orogenic Andesites and Plate Tectonics. New York: Springer.

Graindorge, D., Calahorrano, A., Charvis, P., Collot, J. Y. & Bethoux, N. (2004). Deep structure of the Ecuador convergent margin and the Carnegie Ridge, possible consequences on great earthquakes recurrence interval. Geophysical Research Letters 31, 10.1029/2003GL018803.

Green, T. H. (1972). Crystallization of calc-alkaline andesite under controlled high-pressure hydrous conditions. Contributions to Mineralogy and Petrology 34, 150–166.[CrossRef][Web of Science]

Green, T. H. (1982). Anatexis of mafic crust and high pressure crystallization of andesite. In: Thorpe, R. S. (ed.) Orogenic Andesites and Related Rocks, New York: John Wiley, pp. 465–487.

Green, T. H. & Ringwood, A. E. (1968). Origin of garnet phenocrysts in calc-alkaline rocks. Contributions to Mineralogy and Petrology 18, 163–174.[CrossRef]

Guillier, B., Chatelain, J. L., Jaillard, E., Yepes, H., Poupinet, G. & Fels, J. F. (2001). Seismological evidence on the geometry of the orogenic system in central–northern Ecuador (South America). Geophysical Research Letters 28, 3749–3752.[CrossRef][Web of Science]

Gutscher, M. A., Malavieille, J., Lallemand, S. & Collot, J. Y. (1999). Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision. Earth and Planetary Science Letters 168, 255–270.[CrossRef][Web of Science]

Gutscher, M. A., Maury, R. C., Eissen, J. P. & Bourdon, E. (2000). Can slab melt be caused by flat slab? Geology 28, 535–538.[Abstract/Free Full Text]

Hamer, R. D. & Moyes, A. B. (1982). Composition and origin of garnet from the Antarctic Peninsula Volcanic Group of Trinity Peninsula. Journal of the Geological Society, London 139, 713–720.[Abstract/Free Full Text]

Harmon, R. S., Barreiro, B., Moorbath, S., Hoefs, J., Francis, P. W., Thorpe, R. S., Deruelle, B., McHugh, J. & Viglino, J. A. (1984). Regional O-, Sr-, and Pb-isotope relationships in the late Cenozoic calc-alkaline lavas of the Andean Cordillera. Journal of the Geological Society, London 141, 803–822.[Abstract/Free Full Text]

Harpp, K. S. & White, W. M. (2001). Tracing a mantle plume: isotopic and trace element variations of Galapágos seamounts. Geochemistry, Geophysics, Geosystems 2000GC000137.

Hart, S. R. (1984). A large-scale isotope anomaly in the Southern Hemisphere mantle. Nature 309, 753–757.[CrossRef]

Hauff, F., Hoernle, K., van den Bogaard, P., Alvarado, G. & Garbe-Schönberg, D. (1999). Age and geochemistry of basaltic complexes in western Costa Rica: contributions to the geotectonic evolution of Central America. Geochemistry, Geophysics, Geosystems 199GC000020.

Hauff, F., Hoernle, K., Tilton, G., Graham, D. W. & Kerr, A. C. (2000). Large volume recycling of oceanic lithosphere over short time scales: geochemical constraints from the Caribbean Large Igneous Province. Earth and Planetary Science Letters 174, 247–263.[CrossRef][Web of Science]

Hickey, R. L., Frey, F. & Gerlach, D. C. (1986). Multiple sources for basaltic arc rocks from the southern volcanic zone of the Andes (34–41S): trace element and isotopic evidence for contributions from subducted oceanic crust, mantle, and continental crust. Journal of Geophysical Research 91, 5963–5983.

Hildreth, W. & Moorbath, S. (1988). Crustal contributions to arc magmatism in the Andes of central Chile. Contributions to Mineralogy and Petrology 98, 455–489.[CrossRef][Web of Science]

Hirschmann, M. M. & Stolper, E. M. (1996). A possible role for garnet pyroxenite in the origin of the ‘garnet signature’ in MORB. Contributions to Mineralogy and Petrology 124, 185–208.[CrossRef][Web of Science]

Hörmann, P. K. & Pichler, H. (1982). Geochemistry, petrology and origin of the Cenozoic volcanic rocks of the Northern Andes in Ecuador. Journal of Volcanology and Geothermal Research 12, 259–282.[CrossRef][Web of Science]

Hughes, R. A. & Pilatasig, L. F. (2002). Cretaceous and Tertiary terrane accretion in the Cordillera Occidental of the Andes of Ecuador. Tectonophysics 345, p. 29–48.[CrossRef][Web of Science]

Jaillard, E., Soler, P., Carlier, G. & Mourier, T. (1990). Geodynamic evolution of the northern and central Andes during early-to-middle Mesozoic times: a Tethyan model. Journal of the Geological Society, London 147, 1009–1022.[Abstract/Free Full Text]

Jaillard, E., Benítez, S. & Mascle, G. H. (1997). Les déformations paléogenes de la zone d'avant-arc sud-équatorienne en relation avec l'évolution géodynamique. Bulletin de la Société Géologique de France 168, 403–412.[Abstract]

James, D. E. (1982). A combined O, Sr, Nd, and Pb isotopic and trace element study of crustal contamination in central Andean lavas, I. Local geochemical variations. Earth and Planetary Science Letters 57, 47–62.[CrossRef][Web of Science]

James, D. E. & Murcia, L. A. (1984). Crustal contamination in northern Andean volcanics. Journal of the Geological Society, London 141, 823–830.[Abstract/Free Full Text]

Johnson, D. M., Hooper, P. R. & Conrey, R. M. (1999). XRF analysis of rocks and minerals for major and trace elements on a single low-dilution Li-tetraborate fused bead. Advances in X-ray Analysis 41, 117–132.

Kay, R. W. (1978). Aleutian magnesian andesites: melts from subducted Pacific ocean crust. Journal of Volcanology and Geothermal Research 4, 117–132.[CrossRef][Web of Science]

Kay, R. W. (1980). Volcanic arc magma genesis: implications for element recycling in the crust–upper mantle system. Journal of Geology 88, 497–522.[Web of Science]

Kay, R. W. & Kay, S. M. (1991). Creation and destruction of lower continental crust. Geologische Rundschau 80, 259–278.

Kay, S. M., Ramos, V. A. & Marquez, Y. M. (1993). Evidence in Cerro Pampa volcanic rocks for slab-melting prior to ridge–trench collision in southern South America. Journal of Geology 101, 703–714.[Web of Science]

Kelemen, P. B. (1995). Genesis of high Mg# andesites and the continental crust. Contributions to Mineralogy and Petrology 120, 1–19.[Web of Science]

Kelemen, P. B., Yogodzinski, G. M. & Scholl, D. W. (2003). Along-strike variation in lavas of the Aleutian Island Arc: implications for the genesis of high Mg# andesite and the continental crust. In: Eiler, J. (ed.) Inside the Subduction Factory, Geophysical Monograph 138, Washington, DC: American Geophysical Union, pp. 223–276

Kerr, A. C. & Tarney, J. (2005). Tectonic evolution of the Caribbean and northwestern South America: the case for accretion of two Late Cretaceous oceanic plateaus. Geology 33, 269–272.[Abstract/Free Full Text]

Kerr, A. C., Tarney, J., Marriner, G. F., Nivia, A. & Saunders, A. D. (1996). The geochemistry and tectonic setting of Late Cretaceous Caribbean and Colombian volcanism. Journal of South American Earth Sciences 9, 111–120.[CrossRef][Web of Science]

Kerr, A. C., Marriner, G. F., Tarney, J., Nivia, A., Saunders, A. D. & Thirlwall, M. F. (1997). Cretaceous basaltic terranes in western Colombia; elemental, chronological and Sr–Nd isotopic constraints on petrogenesis. Journal of Petrology 38, 677–702.[CrossRef][Web of Science]

Kerr, A. C., Tarney, J., Kempton, P. D., Spadea, P., Nivia, A., Marriner, G. F. & Duncan, R. A. (2002). Pervasive mantle plume head heterogeneity: evidence from the late Cretaceous Caribbean–Colombian oceanic plateau. Journal of Geophysical Research 107(B7), 10.1029/2001JB000790.

Kerr, A. C., Tarney, J., Kempton, P. D., Pringle, M. S. & Nivia, A. (2004). Mafic pegmatites intruding oceanic plateau gabbros and ultramafic cumulates from Bolivar, Colombia: evidence for a ‘wet’ mantle plume? Journal of Petrology 45, 1877–1906.[Abstract/Free Full Text]

Kilian, R. & Pichler, H. (1989). The North Andean volcanic zone. Zentralblatt für Geologie und Paläontologie, Teil I 5/6, 1075–1085.

Kilian, R., Hegner, E., Fortier, S. & Satir, M. (1994). The genesis of Quaternary volcanic rocks in contrasting lithospheric settings of Ecuador. 7th Chilean Geologic Congress Abstracts 2, 1378–1382.

Kilian, R., Hegner, E., Fortier, S. & Satir, M. (1995). Magma evolution within the accretionary mafic basement of Quaternary Chimborazo and associated volcanos (Western Ecuador). Revista Geologica de Chile 22, 203–208.[Web of Science]

Lapierre, H., Bosch, D., Dupuis, V., Polvé, M., Maury, R. C., Hernandez, J., Monié, P., Yeghicheyan, D., Jaillard, E., Tardy, M., de Lépinay, B. M., Mamberti, M., Desmet, A., Keller, F. & Sénebier, F. (2000). Multiple plume events in the genesis of the peri-Caribbean Cretaceous oceanic plateau province. Journal of Geophysical Research 105, 8403–8421.[CrossRef]

Lichte, F. E., Meier, A. L. & Crock, J. G. (1987). Determination of rare-earth elements in geological materials by inductively coupled plasma mass spectrometry. Analytical Chemistry 59, 1150–1157.

Litherland, M., Aspden, J. A. & Jemielita, R. A. (1994). The Metamorphic Belts of Ecuador. Keyworth: British Geological Survey.

Lopez-Escobar, L. (1984). Andean Magmatism: Chemical and Isotopic Constraints, Shiva Publishing, Nantwich, UK. Petrology and Chemistry of Volcanic Rocks of the Southern Andes, pp. 47–71.

Lopez-Escobar, L., Frey, F. A. & Vergara, M. (1977). Andesites and high-alumina basalts from the central–south Chile High Andes; geochemical evidence bearing on their petrogenesis. Contributions to Mineralogy and Petrology 63, 199–228.[CrossRef][Web of Science]

Lopez-Escobar, L., Moreno, H., Tagiri, M., Notsu, K. & Onuma, N. (1985). Geochemistry and petrology of lavas from San Jose Volcano, Southern Andes (33°45'S). Geochemical Journal 19, 209–222.[Web of Science]

Lugmair, G. W. & Carlson, R. W. (1978). The Sm–Nd history of KREEP. Proceedings of the 9th Lunar and Planetary Science Conference. Geochimica et Cosmochimica Acta Supplement, 689–704.

Mamberti, M., Lapierre, H., Bosch, D., Jaillard, E., Ethien, R., Hernandez, J. & Polvé, M. (2003). Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador. Lithos 66, 173–199.[CrossRef][Web of Science]

McKenzie, D. & O'Nions, R. K. (1991). Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091.[Abstract/Free Full Text]

Megard, F. (1989). The evolution of the Pacific Ocean margin in South America north of Arica elbow (18°S). In: Ben Avraham, Z. (ed.) Oxford Monographs on Geology and Geophysics, No. 8. New York: Oxford University Press, pp. 208–230.

Miyashiro, A. (1974). Volcanic rock series in island arcs and active continental margins. American Journal of Science 274, 321–355.[Abstract]

Monzier, M., Robin, C., Samaniego, P., Hall, M. L., Cotten, J., Mothes, P. & Arnaud, N. (1999). Sangay Volcano, Ecuador: structural development, present activity and petrology. Journal of Volcanology and Geothermal Research 90, 49–79.[CrossRef][Web of Science]

Morris, P. A. (1995). Slab melting as an explanation of Quaternary volcanism and aseismicity in southwest Japan. Geology 23, 395–398.[Abstract/Free Full Text]

Noble, S. R., Aspden, J. A. & Jemielita, R. A. (1997). Northern Andean crustal evolution: new U–Pb geochronological constraints from Ecuador. Geological Society of America Bulletin 109, 789–798.[Abstract/Free Full Text]

Petford, N. & Atherton, M. (1996). Na-rich partial melts from newly underplated basaltic crust: the Cordillera Blanca Batholith, Peru. Journal of Petrology 37, 1491–1521.[Abstract/Free Full Text]

Pichler, J., Hörmann, P. K. & Braun, A. F. (1976). First petrologic data on lavas of the Volcano El Reventador (Eastern Ecuador). Münstersche Forschungen zur Geologie und Paläontologie 38, 129–141.

Plank, T. & Langmuir, C. H. (1988). An evaluation of the global variations in the major element chemistry of arc basalts. Earth and Planetary Science Letters 90, 349–370.[CrossRef][Web of Science]

Plank, T. & Langmuir, C. H. (1993). Tracing trace elements from sediment input to volcanic output at subduction zones. Nature 362, 739–742.[CrossRef]

Prévot, R., Chatelain, J.-L., Guillier, B. & Yepes, H. (1996). Tomographie des Andes Equatoriennes: évidence d'une continuité des Andes Centrales. Comptes Rendus de l'Académie des Sciences, Série II. Sciences de la Terre et des Planètes 323, 833–840.

Rapp, R. P., Shimizu, N., Norman, M. D. & Applegate, G. S. (1999). Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3·8 GPa. Chemical Geology 160, 335–356.[CrossRef][Web of Science]

Reynaud, C., Jaillard, E. & Lapierre, H. (1999). Oceanic plateau and island arcs of SW Ecuador: their place in the geodynamic evolution of northwestern South America. Tectonophysics 307, 235–254.[CrossRef][Web of Science]

Salters, V. J. M. (1991). The mantle sources of ocean ridges, islands and arcs: the Hf-isotope connection. Earth and Planetary Science Letters 104, 364–380.[CrossRef][Web of Science]

Salvador, M.E. (1991), Gravity Field, Topography and Crustal Structure of the North Andes, University of South Carolina Department of Geological Sciences Masters Thesis, Columbia, SC, 100 pp.

Samaniego, P., Martin, H., Robin, C. & Monzier, M. (2002). Transition from calc-alkalic to adakitic magmatism at Cayambe volcano, Ecuador: insights into slab melts and mantle wedge interactions. Geology 30, 967–970.[Abstract/Free Full Text]

Samaniego, P., Martin, H., Monzier, M., Robin, C., Fornari, M., Eissen, J. P. & Cotten, J. (2005). Temporal evolution of magmatism in the Northern Volcanic Zone of the Andes: the geology and petrology of Cayambe Volcanic Complex (Ecuador). Journal of Petrology 46, 2225–2252.[Abstract/Free Full Text]

Saunders, A. D., Rogers, G., Marriner, G. F., Terrell, D. J. & Verma, S. P. (1987). Geochemistry of Cenozoic volcanic rocks, Baja California, Mexico: implications for the petrogenesis of post-subduction magmas. Journal of Volcanology and Geothermal Research 32, 223–245.[CrossRef][Web of Science]

Sigurdsson, H., Carey, S., Palais, J. M. & Devine, J. (1990). Pre-eruption compositional gradients and mixing of andesite and dacite magma erupted from Nevado del Ruiz Volcano, Colombia in 1985. Journal of Volcanology and Geothermal Research 41, 127–151.[CrossRef][Web of Science]

Singer, B. S. & Myers, J. D. (1990). Intra-arc extension and magmatic evolution in the central Aleutian arc, Alaska. Geology 18, 1050–1053.[Abstract/Free Full Text]

Stern, C. R. & Killian, R. (1996). Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone. Contributions to Mineralogy and Petrology 123, 263–281.[CrossRef][Web of Science]

Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–346.

Tanaka, T., Togashi, S., Kamioka, H., Amakawa, H., Kagami, H., Hamamoto, T., Yuhara, M., Orihashi, Y., Yoneda, S., Shimizu, H., Kunimaru, T., Takahashi, K., Yanagi, Y., Nakano, T., Fujimaki, H., Shinjo, R., Asahara, Y., Tanimizu, M. & Dragusanu, C. (2000). JNdi-1: a neodymium isotopic reference in consistency with La Jolla neodymium. Chemical Geology 168, 279–281.[CrossRef][Web of Science]

Thorpe, R. S., Francis, P. W. & O'Callaghan, L. (1983). Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks. Philosophical Transactions of the Royal Society of London, Series A 310, 675–692.

Trenkamp, R., Kellogg, J. N., Freymueller, J. T. & Mora, H. P. (2002). Wide plate margin deformation, southern Central America and northwestern South America, CASA GPS observations. Journal of South American Earth Sciences 15, 157–171.[CrossRef][Web of Science]

Vanek, J., Vankova, V. & Hanus, V. (1994). Geochemical zonation of volcanic rocks and deep structure of Ecuador and southern Colombia. Journal of South American Earth Sciences 7, 57–67.[CrossRef][Web of Science]

Walker, J. A., Moulds, T. N., Zentilli, M. & Feigenson, M. D. (1991). Spatial and temporal variations in volcanics of the Andean central volcanic zone (26 to 28°S). In: Harmon, R. S. & Rapela, C. W. (eds) Andean Magmatism and its Tectonic Setting, Geological Society of America Special Paper 265, Geological Society of America, Boulder, CO, U.S.A: pp. 139–155.

Wasserburg, G. J., Jacobsen, S. B., DePaolo, D. J., McCulloch, M. T. & Wen, T. (1981). Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochimica et Cosmochimica Acta 45, 2311–2323.[CrossRef][Web of Science]

Weber, M. B. I., Tarney, J., Kempton, P. D. & Kent, R. W. (2002). Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia. Tectonophysics 345, 49–82.[CrossRef][Web of Science]

Werner, R., Hoernle, K., Barckhausen, U. & Hauff, F. (2003). Geodynamic evolution of the Galapagos hot spot system (Central East Pacific) over the past 20 m.y.: constraints from morphology, geochemistry and magnetic anomalies. Geochemistry, Geophysics, Geosystems 2003GC000576.

White, W. M., McBirney, A. R. & Duncan, R. A. (1993). Petrology and geochemistry of the Galapagos Islands: portrait of a pathological mantle plume. Journal of Geophysical Research 98, 19533–19563.[CrossRef]

Woodhead, J. D. (1988). The origin of geochemical variations in Mariana lavas: a general model for petrogenesis in intra-oceanic island arcs? Journal of Petrology 29, 805–830.[Abstract/Free Full Text]

Wörner, G. (1989). The Nevados de Payachata volcanic region (18°S/69°W, N. Chile). Bulletin of Volcanology 50, 287–303.

Yogodzinski, G. M., Kay, R. W., Volynets, O. N., Koloskov, A. V. & Kay, S. M. (1995). Magnesian andesite in the western Aleutian Komandorsky region: implications for slab melting and processes in the mantle wedge. Geological Society of America Bulletin 107,505–519.[Abstract/Free Full Text]

Zartman, R. E. & Haines, S. M. (1988). The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs—a case for bi-directional transport. Geochimica et Cosmochimica Acta 52, 1327–1339.[CrossRef][Web of Science]

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				G. F. Zellmer Petrogenesis of Sr-rich adakitic rocks at volcanic arcs: insights from global variations of eruptive style with plate convergence rates and surface heat flux Journal of the Geological Society, July 1, 2009; 166(4): 725 - 734. [Abstract] [Full Text] [PDF]

				O. Bachmann and G. W. Bergantz Rhyolites and their Source Mushes across Tectonic Settings J. Petrology, January 7, 2009; (2009) egn068v1. [Abstract] [Full Text] [PDF]

				J. P. Richards and R. Kerrich Special Paper: Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis Economic Geology, June 1, 2007; 102(4): 537 - 576. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) All Versions of this Article: 47/6/1147    most recent egl006v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (11) Request Permissions Google Scholar Articles by BRYANT, J. A. Articles by BAILEY, D. G. Search for Related Content GeoRef GeoRef Citation Social Bookmarking          What's this?

Online ISSN 1460-2415 - Print ISSN 0022-3530

Copyright © 2010 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics