Journal of Petrology | Volume 45 | Number 1 | Pages 107-138 | 2004
© Oxford University Press 2004; all rights reserved
Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 2: Characteristics and Petrogenesis of the High-Ti Latite and High-Ti and Low-Ti Voluminous Quartz Latite Eruptives
1 ADVANCED CENTER FOR QUEENSLAND UNIVERSITY ISOTOPE RESEARCH EXCELLENCE (ACQUIRE), THE UNIVERSITY OF QUEENSLAND, ST. LUCIA, QLD. 4072, AUSTRALIA
2 DEPARTMENT OF GEOLOGY, RHODES UNIVERSITY, GRAHAMSTOWN 6140, SOUTH AFRICA
3 PANALYTICAL, LELYWEG 1, 7602EA ALMELO, THE NETHERLANDS
4 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
5 RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
* Corresponding author. E-mail: ewart{at}cust.caloundra.net
RECEIVED DECEMBER 3, 2001; ACCEPTED JULY 1, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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As a result of their relative concentration towards the respective Atlantic margins, the silicic eruptives of the Paraná (Brazil)–Etendeka large igneous province are disproportionately abundant in the Etendeka of Namibia. The NW Etendeka silicic units, dated at
132 Ma, occupy the upper stratigraphic levels of the volcanic sequences, restricted to the coastal zone, and comprise three latites and five quartz latites (QL). The large-volume Fria QL is the only low-Ti type. Its trace element and isotopic signatures indicate massive crustal input. The remaining NW Etendeka silicic units are enigmatic high-Ti types, geochemically different from low-Ti types. They exhibit chemical affinities with the temporally overlapping Khumib high-Ti basalt (see Ewart et al. Part 1) and high crystallization temperatures (
980 to 1120°C) inferred from augite and pigeonite phenocrysts, both consistent with their evolution from a mafic source. Geochemically, the high-Ti units define three groups, thought genetically related. We test whether these represent independent liquid lines of descent from a common high-Ti mafic parent. Although the recognition of latites reduces the apparent silica gap, difficulty is encountered in fractional crystallization models by the large volumes of two QL units. Numerical modelling does, however, support large-scale open-system fractional crystallization, assimilation of silicic to basaltic materials, and magma mixing, but cannot entirely exclude partial melting processes within the temporally active extensional environment. The fractional crystallization and mixing signatures add to the complexity of these enigmatic and controversial silicic magmas. The existence, however, of temporally and spatially overlapping high-Ti basalts is, in our view, not coincidental and the high-Ti character of the silicic magmas ultimately reflects a mantle signature. KEY WORDS: large-volume quartz latites; magma mixing; open-system fractional crystallization; crustal assimilation; high-Ti and low-Ti provinciality
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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As described in the accompanying paper (Part 1, Ewart et al., 2004
), the Etendeka of NW Namibia represents the eroded remnant of the early Cretaceous Paraná (Brazil)–Etendeka continental large igneous province. In fact, the Etendeka comprises only the relatively small southeasterly fragment of this once continuous bimodal volcanic province that existed prior to the opening of the Atlantic Ocean. The Etendeka covers an area of 78 000 km2 (Erlank et al., 1984), with a maximum preserved thickness of 1 km. This compares with the area of exposed volcanic sequences in the Paraná of 1·2 x 106 km2, and maximum thickness (in the northern region) of 1·7 km (Melfi et al., 1988; Peate et al., 1990). The silicic volcanics, however, are asymmetrically distributed within the volcanic province, being disproportionally concentrated within the southeastern and central Paraná Basin and the Etendeka and thereby broadly focused in the coastal regions of southeastern Brazil and NW Namibia, which are now bisected by the Atlantic Ocean (Turner et al., 1994; Peate 1997).
Previous research dealing with the Etendeka silicic volcanism has focused on the southern Etendeka (Fig. 1; Erlank et al., 1984; Milner 1988; Milner et al., 1992; Ewart et al., 1998b; Schmitt et al., 2000), with a regional overview and synthesis, including northern and southern silicic eruptives, presented by Marsh et al. (2001). This account specifically concentrated on the silicic volcanic sequences that crop out in the two coastal domains of the northern Etendeka (Fig. 1; see Part 1, Ewart et al., 2004). Chemical similarities of certain of the silicic units between the southeastern Paraná and the southern Etendeka were initially described by Erlank et al. (1984). These observations were extended by Milner et al. (1995b), who established trans-Atlantic correlations between specific quartz latite units cropping out in the Paraná basin and the southern Etendeka. Estimated eruptive volumes for three of the major correlated eruptive units range between 3320 and 6340 km3 (Milner et al., 1995b). Marsh et al. (2001) further recognized correlations, also based on chemical criteria, between specific quartz latite units that crop out in the coastal northern Etendeka and the central Paraná basin, the areas of which are estimated to cover between 130 000 and 170 000 km2. The scale of Paraná–Etendeka silicic volcanism is incredibly large and the volumes of the larger units must rank amongst the largest terrestrial silicic eruptive units so far recognized. Estimated temperatures of the silicic magmas are also high, being consistently >1000°C (see below). Volcanological aspects of the emplacement mechanisms of the southern Etendeka quartz latites have been discussed by Milner et al. (1992).
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In the northern Etendeka, eight high-Ti and one low-Ti silicic eruptive units occupy the higher exposed stratigraphic levels of the northern coastal subdomain (Fig. 1), locally interbedded with basalts. The origins and affinities of the silicic eruptive units of the Paraná–Etendeka igneous province have generated an extensive literature proposing rather diverse petrogenetic interpretations (e.g. Erlank et al., 1984
; Bellieni et al., 1986; Milner, 1988; Piccirillo et al., 1988a; Garland et al., 1995; Ewart et al., 1998b). These interpretations have centred on the geochemical affinities of the various silicic units with the spatially associated but diverse basaltic magma types; the relative importance of the processes of fractional crystallization, assimilation fractional crystallization (AFC), and/or partial melting; and the possible contributions of continental crust and underplated mafic crust to the silicic magmas. One aim of this paper is to evaluate these processes (including also magma mixing), magmatic affinities, and the role of the continental crust in magma genesis, in the light of the more extensive field and stratigraphic relationships that are now available for the northern Etendeka. This is made possible by the excellent outcrops available in the Namib Desert. The mechanisms of generation of such diverse, high-temperature and large-volume silicic magmas remain fundamental problems in igneous petrology, which clearly also impinge on the processes operating during granitoid petrogenesis. |
MAGMA TYPES AND THEIR STRATIGRAPHY |
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Nomenclature of the silicic rocks is problematic, but the terms latite (59–65 wt % SiO2) and quartz latite (65–72 wt % SiO2) have been used historically in the Etendeka and continue to be adopted here. In total alkalis–silica (TAS) nomenclature, the latites vary continuously through trachyandesite, andesite, trachydacite and dacite, whereas the quartz latites (QLs) span the fields of trachydacite, dacite and rhyolite (Marsh et al., 2001
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The Etendeka latite–QL volcanic suites comprise two distinct chemical types, high-Ti and low-Ti, the recognition of which was developed in the Paraná (Bellieni et al., 1986; Peate, 1997). Piccirillo et al. (1988b) noted that higher Ti is correlated with higher Ba, light rare earth elements (LREE), P and Zr concentrations. Defining high- and low-Ti types on the basis of TiO2 can be done in conjunction with some additional elements (e.g. FeOtot), but not others (e.g. SiO2), as a result of continuum of compositions (Marsh et al., 2001). Eighteen distinct chemical types, ranging from latite to QL, occur within the Etendeka. The high-Ti and low-Ti suites exhibit marked geochemical provinciality. A complete listing of the mean compositions of all the Etendeka latites and QLs has been presented by Marsh et al. (2001, noting the inadvertent transposition of the Ventura and Elliott QL compositions in this reference). Updated mean compositions of the northern Etendeka units are presented in Table 1. The Etendeka high-Ti and low-Ti suites correlate with the Chapecó and Palmas acid volcanic subgroups (CAV and PAV), respectively, in the Paraná (Marsh et al., 2001).
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Only the low-Ti silicic types occur in the southern Etendeka (Erlank et al., 1984
; Milner, 1988; Milner et al., 1992, 1995a; Ewart et al., 1998b). Eight high-Ti silicic volcanic units, together with the newly recognized low-Ti Fria QL, dominate the higher exposed stratigraphy of the northern coastal subdomain in the northern Etendeka (Fig. 1; see also fig. 2 in Part 1, Ewart et al., 2004). These comprise the Nadas (lowest), Sechomib and Hoarusib latites, followed by the Ventura, Khoraseb, Fria, Sarusas, Elliott and Naude QLs. The Elliott QL is compositionally intermediate between high- and low-Ti types, but as it defines one end of the high-Ti spectrum in FeOtot–TiO2 space, it is grouped within this suite. The southern coastal subdomain between Möwe and Terrace Bays (Fig. 1; see also fig. 3 in Part 1, Ewart et al., 2004) marks the transition between the northern and southern Etendeka provinces, exposing the low-Ti Hoas latite and the low-Ti Hoanib, Lower Coastal and Terrace QLs, plus the high-Ti Sarusas QL. This last unit provides a critical marker horizon common to the two coastal subdomains, thereby linking the stratigraphy of the silicic eruptives between the northern and southern Etendeka. Figure 2 summarizes the stratigraphy of the silicic volcanic sequence exposed along the coastal domains, and its relationship to the interbedded basalts. The latter are not differentiated because of the complex interlayering of Khumib, Tafelberg, Esmeralda and minor Kuidas type basalts, together with coast-parallel faulting (Marsh et al., 2001; Part 1, Ewart et al., 2004).
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Figure 3 summarizes two examples of key geochemical signatures of latite and QL types currently recognized in the northern Etendeka. Combinations of the elements, specifically SiO2, P2O5, TiO2, FeOtot, Zr, V, Nb, Rb and Y, as well as isotopic compositions, characterize the different types (Marsh et al., 2001
). The eruptive units occur both as single flow and multiple eruptive (cooling) units, each defining distinct stratigraphic entities. The units are inferred to represent rapidly erupted packages of discrete cooling units; each cooling unit exhibits characteristic lithological vertical zonation, similar to the patterns observed in QL units in the southern Etendeka (Milner et al., 1992). Each eruptive package has been given informal member status within the Skeleton Coast Formation (Marsh et al., 2001; Fig. 2), which is underlain by the thick sequence of low-Ti and high-Ti basalts (Part 1, Ewart et al., 2004). It is nevertheless important to note that no single section exposes all the latite and QL units. The Sarusas and Khoraseb QLs crop out in greatest volumes, whereas the Hoas and Nadas latites, and the Elliott and Naude QLs, comprise small-volume units. The latites occur at the base of the Skeleton Coast Formation, there being a general upward increase in SiO2, and transition to QLs. The relative stratigraphic relation between the Fria and Khoraseb QLs is still uncertain.
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Marsh et al. (2001)
demonstrated that the Sarusas, Khoraseb and Fria QLs have equivalents in the Guarapuava, Ourinhos and Santa Maria ‘rhyolites’ of the Paraná, respectively, and are representatives of giant silicic systems associated with continental rifting, with volumes comparable with those of the largest known terrestrial silicic eruptive systems. Although no latites have been formally described from the Paraná, we note that two samples listed as Urubici basalts by Peate et al. (1999; samples DSM04 and DSM08) are chemically similar to (but not identical with) the Etendeka Nadas latite. Piccirillo & Melfi (1988) also listed an analysis (B457) of a sample from the central Paraná basin that is similar to the Nadas latite. It is believed that the relatively small volumes of the latites (see below) make it unlikely that they extend continuously between the Paraná and NW Etendeka.
Alberti et al. (1992) described three Lower Cretaceous quartz latite samples, from southern Angola, one sample of which was dated at 132 Ma (Renne et al., 1996). These are clearly of high-Ti affinities, and chemical data (Figs 3 and 4) suggest that they correlate with the Khoraseb and/or Sarusas QLs of the NW Etendeka.
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PETROGRAPHY AND MINERALOGY |
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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Key field, petrographic and mineralogical features of the latites and quartz latites are summarized in Table 2. Individual flows vary from nearly aphyric (Naude QL) to strongly phyric (some Khoraseb QL samples) with up to 20 % (vol.) phenocrysts, the overall average being
10%. Phenocryst assemblages are relatively simple: (1) the volumetrically minor Naude and Elliott QL types with plagioclase + augite + Ti-magnetite; (2) the dominant assemblage of plagioclase + augite + pigeonite + Ti-magnetite + ilmenite found in the bulk of the high-Ti latites and QLs and some low-Ti Terrace QL units; (3) plagioclase + orthopyroxene ± pigeonite + Ti-magnetite of the low-Ti Hoanib, Lower Coastal and some Terrace QL units. Groundmass textures in all rocks vary from vitric through hyalopilitic to granophyric. Quench textures are developed within groundmass and microphenocryst plagioclase in the Fria, Hoanib, Terrace and Lower Coastal QL units. Groundmass phases are dominantly plagioclase, potassium feldspar, quartz and magnetite.
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Plagioclase
Phenocryst compositions (Fig. 5) fall into two main groupings: (1) those within the high-Ti, augite + pigeonite-bearing latites and QLs in the range of An40–50, averaging An43–46; (2) those within the low-Ti, orthopyroxene-bearing QLs with more calcic compositions, averaging An57–62. Plagioclase within the Elliott QL and the Fria QL outside the Cape Fria ponded lava complex are compositionally intermediate. The Cape Fria ponded facies of the Fria QL contain more sodic (and less potassic) plagioclase than the non-ponded facies, the result of re-equilibration. Phenocrysts in all silicic units are characterized by normal oscillatory zoning, and rare reverse zoning. Compositional ranges within individual phenocrysts are typically <6 mol % An, excluding the rims, which overlap groundmass compositions. Proportions of the Or component vary between 2 and 7 mol %, being negatively correlated with per cent An. In the high-Ti types, Or solid solution increases from quartz latites to latites, suggesting temperature control (e.g. Fuhrman & Lindsley, 1988
).
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Pyroxenes
Euhedral pyroxene phenocrysts, typically between 0·4 and 2 mm size, occur as discrete crystals and glomerocrysts. Augite is rarely altered, but pigeonite is altered in the most coarsely devitrified samples. The pyroxenes within each eruptive member have relatively uniform and distinctive compositions (Table 2), with Fe–Mg variation
2%. The Khoraseb QL and the Cape Fria ponded facies of the Fria QL are exceptions. The Khoraseb QL contains bimodal pyroxene compositions, each type occurring within separate rock specimens (the whole-rock compositions showing no complementary variation). The Fe-rich assemblages exhibit greater Fe–Mg variation (5%) than the Mg-rich pyroxene assemblages (2%). Augite and pigeonite within the Fria QL ponded facies vary by 10% in their Fe–Mg ratios, thought to reflect mixing with basaltic lavas during surface ponding (see Part 1, Ewart et al., 2004).
Pyroxene geothermometry (Lindsley, 1983; Table 3) indicates an overall temperature fall from the latites (1030–1120°C) to the Fria and Elliott QLs (875–980°C), noting the latter to be minimum temperatures. The relatively high temperatures are consistent with existing estimates for QLs in the southern Etendeka (Milner, 1988; Ewart et al., 1998b) and Paraná Basin (Bellieni et al., 1986; Garland et al., 1995), and imply volatile-undersaturated melts. The coexisting augite and pigeonite temperature estimates are mostly concordant, except again for the ponded facies of the Fria QL. Comparison with calculated anhydrous liquidus 1 bar and 2 kbar temperatures (Ghiorso & Sack, 1995) show generally good agreement with the 1 bar liquidus estimates, but are mostly lower than the 2 kbar liquidus temperatures.
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Fe–Ti oxides
Phenocrystal titaniferous magnetite and ilmenite coexist in the latites, but in the QLs, ilmenite is uncommon to absent (Table 2). Most oxides are modified by subsolidus re-equilibration, leading to contrasting oxide compositions and poor stoichiometry, especially within magnetites. A small number of coexisting Fe–Ti oxides within the latites give temperature estimates >1050°C [using procedures of Sack & Ghiorso (1991)
], but even these are suspected to have re-equilibrated during cooling. The coexisting oxide data indicate equilibration between the fayalite–magnetite–quartz (FMQ) and wüstite–magnetite (WM) buffer curves, consistent with equivalent Paraná data (Bellieni et al.,1986).
Amphibole and apatite
Calcic amphibole varying in composition between actinolite and edenite (Mg number = 0·51–0·59) occurs in the Fria QL ponded facies as acicular and granular groundmass crystals pseudomorphing original pyroxenes. Apatite is a ubiquitous microphenocryst phase in the latites, but deceases in abundance and size in the QLs.
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CHEMISTRY |
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Quartz latite alteration
A potentially complicating factor in interpreting the chemistry of the silicic rocks is differences of K2O, CaO and, to a lesser extent, Na2O between pitchstone and coexisting devitrified lithologies within the same units, documented by Milner & Duncan (1987)
and Milner (1988) in southern Etendeka QLs. Those workers demonstrated K2O loss within the pitchstones to be significant, related to hydration as indicated by negative correlation between K2O and ignition loss. In contrast, CaO, Na2O and Cs were found to be lower in the devitrified facies relative to the pitchstones, with variable behaviour of Sr. Other elements were reported to be unaffected, including the diagnostic elements used in defining the quartz latite magma types and which demonstrate their compositional homogeneity over large areas (e.g. Ti, Fe, P, Ba, Zr, V, Y and Rb). Milner (1988), Harris (1989, 1995) and Harris et al. (1989) showed that 18O/16O disequilibrium exists between phenocrysts and coexisting glass within samples of southern Etendeka quartz latites, consistent with low-temperature (50–65°C) hydration. The decrease of CaO and Na2O in the devitrified facies was attributed to alteration of plagioclase and pyroxene during initial devitrification and cooling of the main body of the quartz latites (Milner & Duncan, 1987; Milner, 1988), and they argued that pitchstone CaO and Na2O values were better estimates of original magma compositions.
In the NW Etendeka, pitchstone lithologies occur within quartz latites but are rare to absent in the latites. With the possible exception of the Elliott QL, the devitrified lithologies greatly dominate. K2O, Na2O and CaO concentrations within individual samples from the four quartz latites representing pitchstone, cryptocrystalline (inferred to represent the earliest stage of devitrification and quenching) and fully devitrified lithologies exhibit behaviour similar to that observed in the southern Etendeka quartz latites. This is most pronounced in the Sarusas and Elliott QL samples. As the Sarusas QL is the only unit for which a significant number of samples of each lithological type are available, the means and standard deviations of the compositions of these three sets of lithological types are tabulated (Table 4). The trends are clear, with the cryptocrystalline compositions being intermediate between the fully devitrified and the pitchstone compositions. Nevertheless, the differences appear to be less strongly developed than reported for the southern Etendeka equivalents, especially when inter-sample variability (
) is considered.
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Although pitchstones typically preserve unaltered plagioclase, pyroxene and least altered Fe–Ti oxide phases, routine electron microprobe analyses of devitrified quartz latites show unaltered plagioclase and pyroxenes to be common, with plagioclase and pigeonite alteration occurring only in the most coarsely recrystallized samples. We suggest that the alkali and CaO variations observed are the result of localized element mobility occurring internally within flow units during initial cooling and devitrification, as indicated by the intermediate cryptocrystalline compositions. Pitchstone hydration may further modify K2O. Trace elements appear unaffected, with the possible exception of Cs, for which insufficient data exist. We recognize that a significant proportion of samples analysed do not fully represent pristine magma compositions with respect to CaO, Na2O and K2O. As the extent of modification of these three elements cannot be quantified, we use all analytical data in the following discussion.
Major and trace elements
Analytical methods and sampling strategy are as described in Part 1 (Ewart et al., 2004), with new analytical data presented in Table 5. Additional data can be accessed from the Electronic Appendix, which may be downloaded from the Journal of Petrology web site at http://www.petrology.oupjournals.org.
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Fundamental geochemical differences exist between the low-Ti and high-Ti latite–QL series. The low-Ti types are relatively enriched in Cs, Rb, Th, U, Pb and Li, but lower in Ba, high field strength elements (HFSE), REE and especially Sr, as illustrated by upper crust normalized trace element variation diagrams (Fig. 6c). The low-Ti QLs also have stronger negative Eu/Eu* anomalies (Fig. 7b, and fig. 11b in Part 1, Ewart et al., 2004
), and lower Nb/Ta, Nd/Pb, Ce/Pb, Nb/U and La/Yb ratios (Table 6). Trace element plots suggest overlaps with model upper-crustal compositions (e.g. figs 10, 11 and 15 in Part 1, Ewart et al., 2004). The northern Etendeka low-Ti QL compositions are closely comparable with those of the southern Etendeka (Milner, 1988; Ewart et al. 1998b). In contrast, the normalized high-Ti latite and QL data (Fig. 6b) show marked departures from the model upper-crustal compositions, with relative depletions of Cs, Th, U, Ta, Pb, Sr and Li.
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Within the high-Ti series extending from the Sechomib latite to the Khoraseb QL, trends of decreasing TiO2, Al2O3, FeO, MgO, CaO, P2O5, Nb, Sr, Ba, LREE and V, and increasing Rb correlate with increasing SiO2 (e.g. Fig. 4a and b). Comparable trends incorporating Th, U, Pb, K, Na, Zr, Y and heavy REE (HREE) are more complex (Fig. 4c). Evidence of plagioclase fractionation is suggested by Ba and Sr depletions in the Khumib–Urubici normalized multi-element plot (Fig. 6a), and negative Eu anomalies, the magnitudes of which increase from latites to QLs, and correlate with decreasing Sr and Ba abundances (Fig. 7b, and fig. 11b in Part 1, Ewart et al., 2004
). As noted above, however, even these may be potentially inherited during upper-crustal assimilation. The overall data, nevertheless, suggest crystal fractionation control by plagioclase, pyroxenes, apatite and Fe–Ti oxides. Within individual magma types, certain element concentrations exhibit little variation (e.g. P, Ti, Zr, LREE; see examples in Fig. 3), providing the basis of the geochemical recognition of the magma types. Plots incorporating MgO, Mg number and compatible elements such as Ni, Cr, Sc, V and Co, however, exhibit small but significant compositional heterogeneity within individual units, suggesting localized or small-scale crystal fractionation or magma mixing (Fig. 4d) operating within individual magma systems. The overall compositional variation within the high-Ti suite suggests that the different magma types fall into three suites: (1) the minor Nadas latite and Elliott QL, which in most plots are separated from the other latite and QL members; (2) the Sechomib and Hoarusib latites and Ventura QL, which in plots of SiO2 vs Na, K, Rb, Zr Th, Pb, Cu, HREE and Y are displaced relative to the third suite; (3) the Sarusas, Naude and Khoraseb QLs.
Isotopic compositions
In conformity with equivalent Paraná data (Bellieni et al., 1986; Garland et al., 1995), the silicic eruptives isotopically define two clear groupings, corresponding to the high-Ti and low-Ti magma types (Fig. 8; Table 5). The former has initial 87Sr/86Sr ratios between 0·706 and 0·71 and 143Nd/144Nd between 0·5121 and 0·51227, consistent with the Paraná CAV isotopic data (Garland et al., 1995). The low-Ti QLs, including the southern Etendeka Beacon, Grootberg, Wereldsend and Awahab QLs (equivalent to the Paraná PAV) have overlapping 143Nd/144Nd ratios (0·5120–0·5121), but strongly radiogenic initial Sr isotopic ratios (0·717–0·725), characteristic of crustal compositions (Marsh et al., 2001). The Elliott QL possesses the most radiogenic Sr within the high-Ti QL types, being intermediate, in Sr–Nd isotope space, between the high- and low-Ti QL types, consistent with trace element data. Comparable relationships are evident within 206Pb/204Pb–87Sr/86Sr isotope plots (Fig. 8c), the high-Ti and low-Ti latites and QLs apparently defining a mixing hyperbola, illustrated by the mixing curve between average Khumib–Urubici basalt and Fria QL.
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Each high-Ti latite and QL member has a distinct Sr–Nd–Pb isotope composition, with overlap only between the Hoarusib latite and Ventura QL members (consistent with their trace element similarities). The Sarusas QL is the least radiogenic with respect to 87Sr/86 and 206Pb/204Pb ratios, whereas the Nadas latite projects in Nd–Sr and Sr–Pb isotope space away from the other units, again consistent with chemical data. The three high-Ti suites suggested by major and trace element data can be identified in the isotopic data only in the case of the Sechomib–Hoarusib–Ventura latite–QL grouping. The other two suites are not so clearly defined by either Sr or Pb isotope compositions. The isotopic data therefore preclude a single simple closed- or open-system magmatic line of descent linking the high-Ti latites and QLs.
If the high-Ti silicic melts are petrogenetically related to the Khumib basaltic magma type (as we argue below), then crustal input is implied for at least some latites and QLs by their shifts to more radiogenic Sr and Pb isotopic compositions. This is most pronounced for the Elliott and least for the Sarusas QL. Systematic changes in element ratios (Table 6) characterize the high-Ti latites and QLs and these extend relatively smoothly into the low-Ti QL compositions. Included are those ratios indicative of possible crustal–sediment affinities, such as Nd/Pb, Nb/Nb*, Ce/Pb and Nb/Ta. As shown previously in generalized field outlines (Figs. 11a, and fig. 15 in Part 1, Ewart et al., 2004), such ratios correlate with the Sr, Pb and Nd isotopic compositions of the silicic units. A more detailed and specific example is shown (Fig. 9b) in terms of Ce/Pb–87Sr/86Sr, in which decreasing Ce/Pb correlates with increasingly radiogenic 87Sr/86Sr, suggesting increasing crustal input through the high-Ti to low-Ti latite and QL series. The Sarusas QL overlaps in both of these ratios with the Khumib basalts (and a representative Paraná Pitanga basalt composition; Peate, 1997), consistent with the derivation of this QL from such mafic sources with minimal crustal input.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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Previous interpretations
Fundamental geochemical differences clearly exist between the high-Ti and low-Ti Etendeka silicic rocks. The low-Ti magmas have an essentially crustal character. The high-Ti types have clear geochemical affinities to the high-Ti basalt types (e.g. Fig. 6), implying that they have evolved from a mafic precursor through melting or fractional crystallization with relatively small or minor crustal involvement. They therefore possess incompatible element abundances, and abundance ratios (Table 6), more closely reflecting their inferred parental mafic source(s).
Existing petrogenetic models proposed for the low-Ti silicic rocks vary in detail. Garland et al. (1995) proposed that the Paraná PAV resulted from protracted AFC processes on Gramado low-Ti basalts. Ewart et al. (1998b) similarly modelled the southern Etendeka Awahab quartz latites (interpreted to have erupted from the Messum igneous complex; Milner & Ewart, 1989; Ewart et al., 2002), through large-scale AFC-style processes, involving high-degree melting of lower and upper crust with thermal and mass input from plume (Tafelkop type) and hybrid low-Ti Tafelberg-type magmas. Milner (1988), Harris et al. (1990) and Harris & Milner (1997) argued that the low-Ti QLs represent partial melts of fertile crustal anhydrous granulite. Bellieni et al. (1986) preferred a model of melting of either lower-crustal mafic granulites or underplated basalts corresponding in composition to the low-Ti basalt magmas. These differing views reflect the unusual high-temperature, C-type (Kilpatrick & Ellis, 1992) compositional character of the silicic rocks for which few relevant experimental data exist. Our data for the low-Ti Fria QL, which is compositionally very similar to the southern Etendeka low-Ti QLs, add nothing new to this debate. As a result, much of the following discussion is focused towards the high-Ti silicic suite.
In their preferred petrogenetic model for the high-Ti Paraná CAV ‘rhyolites’, Garland et al. (1995) advocated partial melting of underplated high-Ti Pitanga (Paraná) basalt during the final stage of continental rifting. Derivation from Urubici (Khumib-type) basalt was also modelled but was considered to yield a less satisfactory solution. Garland et al. (1995) therefore effectively advocated a two-process model for the origin of the Paraná silicic rocks, in which they proposed that the low-Ti silicic rocks pre-date the high-Ti types and argue for differing depths of generation of the two types. Garland et al. (1995) favoured a high-Ti basalt source partly because of the perceived time gap between the eruptions of the two silicic series. In the Etendeka, Marsh et al. (2001) have shown that the high- and low-Ti silicic suites are coeval, a feature that therefore undermines, at least in part, the Garland et al. (1995) proposal.
Bellieni et al. (1986) proposed two models to account for the Paraná CAV, recognizing that crystal fractionation (from mafic parental magmas) and crustal contamination are consistent with their data. Noting the apparent silica gap (53–63 wt % SiO2) and the localization of the silicic volcanic suites towards the continental margin, their preferred model advocates partial melting of lower-crustal, high-Ti mafic granulites or underplated high-Ti basalts. This differs from their PAV model (see above) only in respect to the inferred parental source compositions. Piccirillo et al. (1988a) supported the partial melting model, inferring 10–20% melting of low-Ti and high-Ti mafic sources, with subsequent crustal contamination modifying the magmas.
The petrogeneses of the low-Ti and high-Ti latites and QLs remain controversial. The following discussion concentrates on three aspects: (1) relationship of the high-Ti latite–QL series to the Khumib (Urubici) and Paraná Pitanga basalt types; (2) variations within the three high-Ti latite–QL suites; (3) possible origin of the contrasting compositions of the low-Ti and high-Ti silicic suites.
Relationships between high-Ti silicic and mafic magma types
Bellieni et al. (1986) noted that evidence supporting a genetic link between the Paraná high-Ti CAV and high-Ti basalts includes their respective trace element distribution patterns. Likewise, within the Etendeka, there is a close geographical association of the high-Ti silicic rocks with the Khumib lavas. In the Paraná, four main high-Ti basalt types crop out: the Urubici, Pitanga, Paranapanema and Ribeira types. Although Ar–Ar dating by Turner et al. (1994) showed that the apparent stratigraphy of the various Paraná basalt types does not necessarily have regional chronostratigraphic significance, the stratigraphic sections of Peate et al. (1992) and Peate (1997) show the sequence: Ribeira (early, but localized), followed by the Pitanga and, in turn, by Paranapanema basalts. Ar–Ar ages (Turner et al., 1994) are in the range of 130·4–137·2 Ma (Paranapanema), 134·1–137·8 Ma (Pitanga) and 131·2–132·3 Ma (Urubici).
The significance of the above stratigraphy and ages relates to their possible petrogenetic relationships to the Paraná and Etendeka high-Ti silicic rocks. Available dates for the Ourinhos and Chapecó types are 128·7–131·7 Ma and 131·8 Ma, respectively (Turner et al., 1994) and 131·9–132·4 Ma for the Chapecó samples of Guarapuava type (Renne et al., 1993; Garland et al., 1995). Noting the correlations of the Ourinhos with the Khoraseb QL and the Guarapuava with the overlying Sarusas QL, the 128·7 Ma date appears anomalously young. The Garland et al. (1995) petrogenetic model for the Chapecó rhyolites is, in part, based on the time gap of 2–4 Myr between the eruptions of the Pitanga basalts and the Chapecó suite.
Geographically, the Pitanga basalt crops out in the west and north of the Paraná Basin, and the Paranapanema unit lies to the SE. Both units have significantly greater outcrop areas than the Urubici–Khumib type, which extends from the southeastern Paraná into the northern Etendeka (Peate et al., 1999). Significantly, however, the distribution of Urubici–Khumib outcrop areas is bisected by the Proto-Atlantic rift. The correlations between the Ourinhos–Khoreseb and Guarapuava–Sarusas units show the scale of two of these high-Ti QL units, their outcrop areas extending beyond the northwestern limits of the Urubici basalts. The similarity of the Giraul Volcanics (Angola) with the Khoraseb and/or Sarusas QLs indicates that the areas covered by these units are probably even larger. Although the eruptive sources of high-Ti silicic lavas are poorly known, some Ourinhos and Guarapuava dykes have been recognized within the mafic dyke swarms of the Ponte Grosso, north of the known Urubici outcrop region (Garland et al., 1995; Peate, 1997). Paranapanema (dominant) and Pitanga dykes are identified within this swarm, confirming spatial overlap of their eruptive sources with at least some Chapecó eruptive sources. No high-Ti silicic vents have been recognized in the Etendeka. On balance, therefore, the outcrop distributions of the various high-Ti basaltic units favour the Paranparema or Pitanga, rather than the Urubici, as potential parental sources of the CAV and high-Ti Etendeka silicic units (Garland et al., 1995). We note, however, that outcrop patterns do not necessarily reflect possible subsurface extents of the respective basaltic magma systems.
Garland et al. (1995) rejected a melting model involving an Urubici–Khumib source for the Chapecó suite on the basis of mismatches between calculated and observed concentrations of Ba, Sr and Tb/Yb. We believe, however, that a petrogenetic link between Khumib basalts and the high-Ti silicic suite needs to be re-examined, and we note the following.
(1) Within the NW Etendeka, a Khumib lava flow (KLS 540) immediately underlies the Nadas latite, with no evidence of a time break. Additional Khumib flows also crop out between the Nadas and Sechomib latites (KLS 594) and between the Sechomib and Hoarusib latites (KLS 660). This clearly indicates the temporal overlap between the latites and the Khumib basalts, consistent with the Paraná Ar–Ar dates (see above). We also note the extensive spatial and temporal overlap between the Etendeka high-Ti silicic rocks and the low-Ti Tafelberg and Esmeralda basalts.
(2) The recognition of latites in the northern Etendeka adds additional perspective to the geochemistry of the high-Ti eruptives, for example, the reduced significance of the apparent ‘silica gap’ (e.g. Fig. 4, and fig. 4 in Part 1, Ewart et al., 2004), emphasizing the more complete continuity of element abundances between the Khumib basalts and the high-Ti QLs. Specifically, the latites reduce the abundance gaps of Zr, Hf, Sr and Ba. Element ratios (Table 6), notably Ti/Y, Nd/Pb, Ce/Pb and Tb/Yb, exhibit systematic changes from the Khumib lavas through the latites to the QLs. Tb/Yb ratios (e.g. Fig. 7a) were specifically considered by Garland et al. (1995) to be independent of partial melting or fractionation, and thus to provide a reliable indicator of parental chemistry. This interpretation is negated by data presented here, including the use of this ratio to reject a Khumib–Urubici parental source.
(3) In trace element (e.g. Figs. 4a–c, 7–9; and figs 10, 11 and 17 in Part 1, Ewart et al., 2004), and trace element-isotope plots (e.g. Fig. 9, and figs 12–15 in Part 1, Ewart et al., 2004), the high-Ti silicic volcanics (a) define arrays that extend from, and partially overlap the Khumib data fields and (b) possess trace element variations consistent with fractional crystallization, mixing and/or assimilation controls. Evidence for mixing and/or assimilation is supported by decreasing Nb/Nb*, Nd/Pb and Ce/Pb ratios from the Khumib basalts to the QLs. Further support comes from Sr–Nd–Pd isotopic differences between the basalts and the silicic volcanic types, and also between the various latite–QL types (e.g. Fig. 8, and figs 12 and 13 in Part 1, Ewart et al., 2004). The isotope data show that the silicic magmas did not evolve within closed systems. Crystal fractionation is suggested by systematic changes of La/Yb, Tb/Yb and Eu/Eu* ratios (Table 6).
We see no evidence from the mineralogy, major or trace element geochemistry, or isotope data to suggest that the high-Ti latites and QLs have evolved through fundamentally differing processes or parental sources. It is nevertheless clear that each eruptive unit has followed a distinct liquid line of descent. In spite of this, as shown above, several natural groupings of the silicic eruptives are geochemically recognizable, implying a genetic link within each.
In following sections, we consider the possible genetic link between the Khumib magmas and the NW Etendeka high-Ti silicic series more closely. Apart from chemical arguments, significance is placed on the occurrence of the Urubici–Khumib lavas and silicic eruptives across the Atlantic proto-rift, where Tristan plume impact probably was maximized, further supported by the location of the Walvis Ridge intersection with the Namibian coast near Cape Fria (fig. 1 in Part 1, Ewart et al., 2004). In Part 1 (Ewart et al., 2004), it is argued that the Khumib–Urubici lavas contain a significant Tristan plume component.
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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Introduction
In the following section, the possible causes of the observed variations within the three latite–QL groupings, and differences between the three groups, are explored in terms of fractional crystallization (FC), assimilation–fractional crystallization (AFC), and mixing processes. Procedures follow Erlank et al. (1984)
and Ewart et al. (1998a) with major element data fitted by least squares to defined target compositions, using a range of mineral phases (taken from analysed northern Etendeka volcanics). Potential assimilants include specific basement types from the Paraná and Namibia [listed by Ewart et al. (1998a)], northern Etendeka mafic volcanic phases, and various model crustal compositions (Rudnick & Fountain, 1995; Taylor & McLennan, 1995). Trace element and isotopic data are calculated from ‘best fit’ (i.e. lowest residuals, R2) major element models. Only the main high-Ti latite and QL types are considered. Plagioclase compositions of An43–46 are frequently required to satisfy least-squares models presented. This is believed to reflect higher-pressure crystallization of the high-Ti magmas, possibly in the lower crust (Garland et al., 1995), but the effects of devitrification on Na2O concentrations (see above) cannot be entirely discounted as a factor.
Unlike a monogenetic volcanic system, for which modelling can be definitive, the Paraná–Etendeka represents a giant, multimagma system spanning a wide range of crustal provinces. There is therefore a high degree of uncertainty with regard to the exact compositions and types of parental magmas and their potential assimilants, a problem most severe in AFC and mixing models. The range of values and choice of trace element partition coefficients (table A2 in Part 1, Ewart et al., 2004), together with their sensitivity to magma compositions (e.g. Sr, Y, V, Cr and Sc, especially within the more SiO2-rich range, e.g. Ewart & Griffin, 1994), are further variables. In the selected solutions presented below (Fig. 10), trace element results using basaltic (or andesitic) and dacitic partition coefficients are plotted, together with mean calculated compositions. Notwithstanding the uncertainties, the calculation procedures are sensitive and do allow constraints to be placed on the interrelationships between the various silicic units and their possible parent magmas. Results are especially sensitive to the parental and assimilant end-member compositions chosen. The mean Fria QL composition is used in a number of calculations as it represents a large-volume, silicic QL with strongly radiogenic Sr and Pb compositions and with strong upper-crustal trace element similarities.
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Within-suite variations
Sechomib–Hoarusib–Ventura suite
This is a relatively small-volume suite. The Sechomib latite has lower SiO2, Rb, Ba, U and K2O, less radiogenic 87Sr/86Sr and 206Pb/204Pb ratios, and higher Ti, Fe, Mg and Sr concentrations, indicating a possible parental composition within this suite. Geochemical differences between members of this suite are relatively small.
Hoarusib latite
Compared with the Sechomib latite, the derivation of the Hoarusib latite must account for slightly higher Ba, similar Zr, Nb and LREE, and slightly lower Y in the latter. Closed-system FC models yield unsatisfactory results as well as being inconsistent with isotope data, as are mixing models of Sechomib with a variety of silicic assimilants. AFC models have proved successful only with mafic assimilants such as model lower crust and low-Ti basalts, the latter preferred over high-Ti basaltic assimilants because of Sr isotope constraints. The model illustrated (Fig. 10b) shows good agreement between trace and major elements, with the apparent exceptions of Sr, Ni and Cr. These discrepancies are common to a number of solutions illustrated. Sr is especially sensitive to the relatively high and compositionally sensitive partition coefficients, but Sr also shows significant inter-sample variation in the Hoarusib. The discrepancies of Ni and Cr need to be viewed in relation to the low absolute abundances of these elements in the latites and QLs (the values of which are listed in Table 7). Two apparently anomalous features of the model presented are the requirement of a mafic assimilant and the relatively high R values (0·70; ratio of mass assimilated/mass crystallized), the latter correlated with a high F value (0·90; fraction of liquid remaining). This is considered further below.
Ventura QL
This is similar compositionally to the Hoarusib latite, containing higher SiO2 and Y, slightly lower Ba, Ti, Fe, Sr and LREE, and similar Zr, Nb and heavy REE (HREE) concentrations. Closed-system FC and mixing models again proved unsatisfactory. Modelling the derivation of the Ventura latite, assuming a Sechomib parent, by AFC resulted in an acceptable solution, apart from a Ni discrepancy, only with the model middle crust (Rudnick & Fountain, 1995) as an assimilant (Fig. 10c). This assimilant is of intermediate composition and, from a chemical perspective, could equally represent a magma of similar composition.
Summary
The modelling supports a petrogenetic link between members of this suite, but also suggests the unusual process of assimilation of mafic to intermediate compositions by silicic magmas at high R and F values, i.e. the amount of crystallization is small and the amount of assimilation is therefore also small in absolute terms. The thermal difficulties of these models can be overcome if the assimilants are magmas (noting the high temperatures inferred for the silicic magmas; Table 3). This may be explained if following the Sechomib latite eruption, the unerupted remnant magma underwent mixing and partial crystallization with low-Ti basaltic magma, which subsequently produced the Hoarusib latite. A repeat of this process may have been a precursor to the Ventura QL eruption.
Khoraseb–Sarusas QL suite
This suite represents very large-volume silicic magma systems, also present in the Paraná and probably southern Angola (see above). The question addressed here is the possible relationship of the Sarusas to the preceding Khoraseb magma, noting their chemical affinities (see above). Compared with the Khoraseb, the Sarusas QL is slightly lower in SiO2, K2O, Rb, less radiogenic 87Sr/86Sr and 143Nd/144Nd, and slightly higher in Ti, Fe, Ca, Na, P, Sr, Zr, Y and HREE concentrations, and also exhibits fractionally smaller Eu depletion (Fig. 7b). Other element abundances are similar. The Sarusas QL is therefore slightly more ‘mafic’, and most significantly, has the least radiogenic Sr and Pb isotopic composition of any of the high-Ti silicic units.
The Sarusas QL has proved especially difficult to model by least-squares methods, the variability of CaO and alkalis (Table 4) possibly adding to the difficulties. No acceptable results were obtained using FC or AFC models with Khoraseb QL (or Sechomib latite) as parental compositions. Using a range of Khumib basalt parent compositions, only a relatively evolved basalt (KLS 671; see table 1 in Part 1, Ewart et al., 2004) gives acceptable major element solutions, using FC (with plagioclase, augite, pigeonite, magnetite, ilmenite, apatite; Fig. 10g and Table 7), or AFC models, in conjunction with basaltic or intermediate assimilants. These models are commonly unsuccessful in terms of trace element (specifically Zr, Y, HREE) and isotope matches. One partial exception is represented by the AFC model (Fig. 10f) involving fractionation from evolved Khumib basalt (KLS 671) with crystallization of plagioclase, augite, magnetite, ilmenite and apatite (F = 0·46) and mixing with a low-Ti Tafelberg basalt (modelled on KLS 548; see table 1 in Part 1, Ewart et al., 2004), with R = 0·18. This model provides acceptable matches to trace element and isotopic compositions, but only with the use of basaltic partition coefficients.
Although derivation of Sarusas magmas by mixing between Khumib-type basalts and silicic compositions (e.g. Fria QL) has proved inappropriate, mixing of low-Ti or high-Ti basalts with the Khoraseb QL results in acceptable levels of agreement between trace and major element data. An example is illustrated (Fig. 10h), calculated by mixing Khoraseb QL with 5·6 wt % Khumib (or averaged Khumib–Urubici) basalt. Although some trace element discrepancies are apparent, mainly in Ni and V (consistent with magnetite control), the general agreement is encouraging. The major problem, however, is that these mixing models predict more radiogenic Sr and Pb isotope compositions than observed, owing to the relatively radiogenic Sr and Pb isotope compositions in the parental Khoraseb QL. To circumvent this problem requires that either the basaltic assimilant had higher Pb and Sr abundances (not observed in any of the low-Ti and high-Ti basalts), or that the Khoraseb magma reservoir was compositionally zoned. The latter possibility is consistent with the occurrence of bimodal pyroxene compositions within the Khoraseb QL (Table 2).
Summary
Significantly, the above AFC and mixing models both require input of a component of basaltic composition into silicic magma following the Khoraseb QL eruptive event. Although we have considered the Sarusas and Khoraseb QLs to be magmatically related, the remnant outcrop distributions of the correlatives of these two units (the Ourinhos and Guarapuava types) in the Paraná do not coincide (Peate et al., 1992; Garland et al., 1995; Peate, 1997). It is not known whether these represent eruptions from separate sources, or perhaps separate eruptive lobes from the same general eruptive centre or system. The patterns do, however, point to the possibility that the Sarusas and Khoraseb QLs originated as separate (albeit large-scale) magma systems. This would require that they followed similar petrogenetic paths, from similar parent sources, with variations in the extent of the proposed AFC or mixing processes, or assimilant compositions.
The major question raised, however, by AFC and FC models presented here is whether they can account for the large volume of the erupted Sarusas QL (and Khoraseb QL; see below).
Nadas and Sechomib latites
In the previous section, we modelled relationships between individual silicic magma types within two groups of latites and quartz latites. We identified certain magmas (Sechomib latite and Khoraseb QL) that we infer were parental to their associated latites and/or quartz latites. In the following sections, we consider the possible origins of the parental silicic magmas.
Nadas latite
In terms of its low SiO2, this is the most primitive of the latites, with lower SiO2, Na2O, K2O, Zr, Nb, Ba and LREE, but higher P2O5, TiO2, Fe and V concentrations compared with the Hoarusib and Sechomib latites. The relatively radiogenic initial 87Sr/86Sr and unradiogenic 143Nd/144Nd and 206Pb/204Pb ratios (Fig. 8) are notable. In bivariate variation diagrams, the Nadas latites plot mostly on the extension to more primitive compositions of the main high-Ti latite–QL trends. In those plots incorporating Nb, Zr, LREE and Ba (e.g. Fig. 4c), the Nadas latites are either offset from the main trends or define separate sub-trends. Additionally, the plagioclase and pyroxene phenocrysts are more sodic and Fe-rich, respectively (Table 2) than in the other latites. The combined geochemical and mineralogical features preclude a simple genetic relationship between the Nadas and other silicic units.
Derivation of Nadas latite from Khumib basalt (MgO 6–7 wt %) by AFC is suggested by good least-squares solutions using a range of mafic and silicic assimilants. Trace element agreement is poor in models with mafic assimilants, the calculated Ba, Zr, Nb, Sc and Ni abundances being too high. Better overall agreement is obtained with silicic assimilants. Our modelling indicates that AFC (F = 0·52; R = 0·25) with a composition of the Fria QL, but with less radiogenic Pb and Nd isotopic compositions, can account for the derivation of the Nadas latite from Khumib magma (Fig. 10a).
Sechomib latite
This latite represents a more evolved composition than the Nadas latite, with higher concentration of SiO2, K2O, Ba, Sr, Zr, Nb and REE, lower Ti, Fe and Mg, and less radiogenic Sr but more radiogenic Nd and Pb. Major element least-squares models for the derivation of this latite type from a variety of Khumib basalts (MgO 6–7 wt %), produce acceptable fits for a range of mafic to silicic end-member assimilants. Trace element solutions are superior for the silicic or intermediate assimilants (with small differences between the two), especially for Sr, Rb, Zr, Y and REE concentrations. The example illustrated (Fig. 10d) is for an assimilant with the composition of the Fria QL, with R = 0·04 and F = 0·29. The calculated Sr isotopic ratio for this model is lower than observed, because of the low R value, but is highly sensitive to assimilant and parental magma isotopic compositions. Simple mixing, as for example between Khumib basalt and Fria QL, does not provide acceptable results, and FC calculations do not satisfactorily replicate observed trace element concentration and are excluded by isotopic data.
Summary
The most primitive latite compositions, which are the earliest eruptives of the high-Ti silicic suite, appear to have a similar origin, explicable as derived from Khumib basalt by AFC processes differing in R and F values. In both cases, modelling favours a silicic assimilant with a composition similar to the Fria QL, but differing isotopic character (see Peate et al., 1999).
Khoraseb QL
This high-SiO2 unit is similar to the Sarusas QL (see above), differing in respect to its more radiogenic initial Sr and Pb isotopic compositions, but with similar initial Nd isotopic ratios (Fig. 8). Simple mixing between Khumib basalt and silicic assimilants, and also FC from Khumib basalt have proved unsuccessful. Successful AFC major element models were obtained with parental Khumib basalt in conjunction with various mafic assimilants (model lower crust and various Tafelberg lavas), model middle crust, and specific silicic assimilants (e.g. Damaran Salem granite; Ewart et al. 1998a). The silicic and middle-crustal assimilants result in superior trace element fits, the latter giving the closest fit to the observed concentrations (Fig. 10e). This model requires only R = 0·08, with F = 0·26, i.e. a relatively low volume of final liquid, although with only minor assimilation to produce the Khoraseb QL composition. The middle-crust assimilant model, however, has some problems with Ba, Zr and Nb (all slightly high), plus the general problem with Ni and Cr. The wide variation in calculated abundances from use of the basaltic and dacitic partition coefficients is attributable to the relatively large degree of fractionation calculated in one step.
The silicic crustal assimilant model (not shown) requires very high R and F values (i.e. massive crustal input), with resulting calculated Sr and Pb isotopic composition that are far too high and more appropriate to the low-Ti QL models (see below).
Elliott QL
As noted above, this QL has a composition that is intermediate between those of the high-Ti and low-Ti QLs (e.g. Figs 4 and 6). Specifically, compositions project between the Sarusas and Fria QL arrays on most variation diagrams, suggesting that the Elliott QL may represent mixing Sarusas and Fria magmas. Simple mixing between these two compositions (Fig. 10i) in wt % proportions 44:56, respectively, produces good concordance between calculated and observed trace element abundances, the main discrepancies being for Fe, Ni and V, suggesting possible magnetite fractionation during mixing. Least-squares testing of this possibility gave an acceptable solution as follows: 45% Sarusas + 56% Fria - 1% Mt = Elliott; R2 = 0·20. Isotopic compositions calculated for this model exhibit very good agreement for the initial Nd (Table 7) and Pb isotopic compositions (the latter within 0·5% of observed values), but give slightly higher initial Sr isotopic ratios than observed. This suggests that the exposed Fria QL may not represent the actual mixing component, but that either another low-Ti magma of very similar geochemistry, or even a slightly modified derivative of the Fria QL magma, was the mixing component. The mixing scenario for the Elliott QL is strengthened by its stratigraphic position interbedded within the Sarusas QL units, these preceded by the Fria QL (Fig. 2).
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SYNTHESIS |
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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Apart from the Elliott QL, geochemical data and modelling supports a three-fold grouping of the latites and QLs, each with independent liquid lines of descent. These comprise the early Nadas latite, the Sechomib–Hoarusib–Ventura latite and QL suite, and the Naude–Khoraseb–Sarusas QL suite. The first two groups comprise relatively small-volume units (exact volumes unknown). Although the Naude QL is a thin unit of very limited extent, the Khoraseb and Sarusas QLs represent very large-volume eruptions (Marsh et al., 2001
; see above). There is an increase in both SiO2 contents and overall volumes of the silicic magmas with time.
The ‘best-fit’ results described in the previous section arise from extensive testing of many hundreds of models. They demonstrate that within-group relationships are characterized by open-system fractional crystallization (AFC). The general inability of closed-system fractional crystallization models to reproduce the geochemical characteristics of the various silicic eruptive units is further confirmed by application of the MELTS software (Ghiorso & Sack, 1995) from both evolved and more primitive Khumib basalt compositions, under variable fO2 constraints [WM to HM (haematite–magnetite)].
Where silicic assimilants are required in the AFC models, these are most plausibly explained as partial melts of crustal rocks. In the cases where mafic assimilants are indicated, this can most realistically be interpreted as magma mixing (with crystallization) involving basaltic magma. Our results also indicate that the two important parental magmas to the main silicic groups, the Sechomib and Khoraseb types, can be derived from a Khumib–Urubici source with minimal crustal assimilation or mixing with other magmas. This result stands in contrast to that of Garland et al. (1995), who rejected the Urubici in favour of the Pitanga as a source for the high-Ti Chapecó silicic rocks of the Paraná. We also note that Garland et al. (1995) favoured partial melting of underplated Pitanga basalt for the origin of the high-Ti silicic rocks whereas our modelling focuses on fractional crystallization. These differences draw attention to the necessity, in whatever model is being advocated, of trying to place geochemical inferences in the context of an overall plausible physical model, which ideally should encompass tectonic environment, as well as spatial and temporal relationships of the volcanic sequences.
In the Etendeka, the end-stage of Khumib basalt eruptions overlaps those of the latites, and had ceased by the time the quartz latites were emplaced. Low-Ti Tafelberg basalts are interbedded with Khumib basalts and were also erupted in association with the latites and quartz latites, but give way with time to the low-Ti Esmeralda basalts, which are the dominant type associated with the quartz latites. The NW Etendeka low-Ti Fria QL is temporally associated with the high-Ti quartz latites. The latites represent relatively small-volume eruptions, but both the low- and high-Ti quartz latites represent giant silicic systems. These silicic systems developed in close spatial relationship to the proto-Atlantic rift. The ponding of the Fria QL and the presence of very wide Esmeralda dykes cutting the silicic sequences are indicative of associated crustal extension.
Fractional and combined assimilation–fractional crystallization
The cessation of Khumib eruptions heralded the closing down of the Khumib volcanic system leading to ponding of Khumib magmas deep within the crust. Associated with these Khumib magma bodies are intrusions of Tafelberg type low-Ti basalt. We propose that it is in these Khumib and Tafelberg magma chambers that fractional crystallization took place, with crustal assimilation, to produce the Sechomib latite and Khoraseb and Fria QLs, respectively. The chemical and isotopic similarities of the Fria QL to the voluminous low-Ti QLs of the southern Etendeka and Paraná have been emphasized. This implies that the processes envisaged in Fria QL petrogenesis were very similar to the extensive AFC processes involving massive assimilation of crust proposed by Garland et al. (1995) and Ewart et al. (1998b) for the low-Ti quartz latites. These models have been discussed in detail and are not considered further other than to note that they are consistent with the parameters obtained in our AFC models for the Fria QL (not shown).
The inferred derivation of the Sechomib and Khoraseb compositions from parental Khumib magma is dominated by fractional crystallization with a silicic contaminant for the Sechomib and an intermediate composition (mid-crust or magma of similar composition) assimilant for the Khoraseb. This could represent a function of the positioning of the Khumib magma chambers in the crust. For the Sechomib–Hoarusib–Ventura suite, it is plausible that the Sechomib magma was generated in relatively small-volume higher-level chambers (consistent with the smaller volumes of the latite). Plausible physical processes relating the Sechomib to the Ventura and Hoarusib types are constrained by the thermal implications of high R values and rather mafic assimilants indicated by the AFC models. As suggested previously, the thermal problem can be minimized if the assimilants are magmas. We therefore propose that after eruption of some of the Sechomib magma, the residual unerupted magma underwent mixing and partial crystallization with low-Ti basaltic and intermediate magmas. This physical model implies derivation of these latites from the same magma system, but this is consistent with their overall geochemical similarities.
For the Khoraseb and Sarusas QLs, a plausible physical model is one where fractional crystallization with minor assimilation takes place in larger, deeper Khumib magma chambers. Our geochemical modelling indicates that the Sarusas QL is feasibly derived from the Khoraseb QL by contamination by, or mixing with, a small amount of basaltic material. Although assimilation of basaltic rock by silicic magma is problematical, the mixing of basaltic magma injected into the Khoraseb magma pool is plausible. Alternatively, the Sarusas may represent a continuation of the AFC process that generates the Khoraseb magma. In this scenario the Sarusas is produced from the unerupted Khoraseb by assimilation of ‘hot’, more mafic restite of the Khoraseb-producing assimilant. The Khoraseb and Sarusas magmas are geochemically very similar and evidently have evolved in the same or closely related magmatic system(s). It remains problematic whether the processes advocated here can account for the giant volumes of both of these types.
Given the observed interlayering between both high-Ti and low-Ti basalt types and the latites, and the scale of the basaltic volcanism (see Part 1, Ewart et al., 2004), contemporaneous and spatially overlapping mafic and silicic magma bodies must have existed within the crustal and upper-mantle column beneath the region. Influx of replenishing basaltic magmas is predicted to occur in the various dynamically evolving magma systems. Large-scale mixing and AFC processes should, therefore, become increasingly important, together with other potential open-system (periodically replenished) fractional crystallization (O'Hara, 1977) and continuous fractional crystallization (Cox & Bell, 1972) processes. The high thermal input from the Tristan plume, together with minimal heat loss because of the absence of massive crustal melting in the development of the high-Ti silicic magmas (compare the low-Ti QLs), may facilitate the maintenance of relatively stabilized, large-scale magmatic systems. These would seem to be a requirement if the larger volume silicic magmas were developed through predominantly AFC, mixing and related fractionation processes.
Partial melting
Here we review the feasibility of partial melting of underplated mafic rocks in generating the parental magmas of the two most important suites. Once these parental magma compositions exist then the evolution of the related types can follow the processes outlined above. Herein lies an enigma, as superimposed fractional crystallization, AFC and mixing processes, described above, could plausibly have overprinted geochemical signatures inherited during a prior phase involving partial melting. Relevant points are as follows.
(1) The Khoraseb–Sarusas and Sechomib magmas are unlikely to represent closed-system progressive melting of the same mafic source because of decoupling of some incompatible trace elements from major element compositions. For example, the less evolved Sechomib latite is enriched in a range of incompatible trace elements (e.g. Ba, Nb, LREE) compared with the more ‘evolved’ Khoraseb–Sarusas QLs. Decoupling between some trace elements is also apparent (e.g. Rb from Ba). These aspects are also implicit in the recognition of three separate latite–QL suites within the NW Etendeka, and the differing isotopic compositions within the various latite–QL units. If partial melting has taken place then compositionally different sources are required and there has been some open-system modification of the magmas prior to eruption.
(2) For major elements at least, our least-squares results (which indicate very low R values), can be interpreted, as a first approximation, in terms of partial melting models for the Sechomib and Khoraseb–Sarusas magmas. For a Khumib source, residual phases include plagioclase, apatite and especially pyroxenes and abundant Fe–Ti oxides. The degree of melting of the Khumib source is about 25–30%, but this is dependent on the exact Khumib composition involved. Interestingly, this is identical to the results obtained by Garland et al. (1995) for melting of a Pitanga basalt source.
(3) Progressive melting of the mafic source should result in the earlier melting products being more siliceous and lower in Fe and Mg (i.e. quartz latitic), the reverse of the observed order of eruptions.
(4) For decompression melting of hot underplated basalt to yield silicic melts, a time gap between the eruptions of the relevant mafic and silicic compositions is expected. The presence of a time gap underpins the melting model proposed by Garland et al. (1995) for the high-Ti rocks of the Paraná, as well as the Jurassic rhyolites of the Lebombo rift margin of the Karoo (Harris & Erlank, 1992). We have noted that stratigraphic data indicate the lack of a time gap between Khumib basalts and the latites in the NW Etendeka.
(5) One line of evidence that may point towards a prior phase of partial melting relates to the difficulty of accurately modelling the relatively high observed Y and HREE concentrations in the Sarusas QL, in terms of FC, AFC and magma mixing processes (Fig. 10f–h). The divergence of these elements suggests possible garnet control. Application of the MELTS software (Ghiorso & Sack, 1995) to model the crystallization of both relatively evolved and more primitive Khumib basalts indicates the possible stabilization of garnet at pressures 14 kbar (anhydrous), in relatively silicic (>59 wt % SiO2) derivative liquids (between WM and HM fO2 buffer constraints). Fractional crystallization models including a garnet phase will cause Y and HREE to become further depleted. The discrepancies could, in principle, be explained by partial melting of a mafic to intermediate composition parent (e.g. underplated magmas), at pressures 14 kbar, provided garnet was an early melting phase.
The relative merits of the partial melting and fractional crystallization scenarios need also to be considered within the overall tectonic environment. The rift environment that characterizes the Etendeka region is precisely the tectonic environment where decompression melting of hot underplated basalt during subsequent extension and rifting is expected (Cox, 1993). The close spatial relationship between silicic volcanic rocks and the rift margins in the Etendeka and the Paraná is strongly suggestive in this regard. Allied to this is the problem of stabilizing very large mafic magma pools in the crust in an active extension environment, allowing them to undergo extensive fractionation. However, the evolution of rifts is complex and periods conducive to stabilizing large magma ponds may alternate with stages of significant extension.
It is important to note that the occurrence of large-volume, high-temperature, anhydrous, high-Ti and low-Ti silicic suites with substantially very different compositional attributes in the Paraná–Etendeka province provides additional constraints on petrogenetic processes. These different silicic systems evolved contemporaneously and with a considerable degree of spatial overlap. It is conceivable but improbable that they originated through different physical processes operating in an extensional tectonic environment. Thus, we find it problematic that Garland et al. (1995) proposed a two-process scenario for the Paraná ‘rhyolites’. They invoked large-scale AFC processes in large low-Ti basaltic magma chambers for the origin of low-Ti ‘rhyolites’, but melting of underplated Pitanga basalt is favoured for the high-Ti basalts. Our AFC models for the Etendeka high-Ti latites and quartz latites are consistent with similar models proposed for the low-Ti quartz latites of the southern Etendeka (Ewart et al., 1988b) and conform to a more satisfying ‘one-process’ model for the silicic volcanism.
The origin of the silicic magmas associated with the development of the Atlantic rift in the Paraná–Etendeka province remains problematic. A wealth of geochemical data has established the most likely petrogenetic relationships between silicic types and between the silicic rocks and associated mafic magmas. Future research should be directed at integrating these magmatic relationships with more refined models of tectonic evolution. This seems essential for understanding the development of these giant silicic magmatic systems in the rift environment.
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available on Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We wish to acknowledge the logistic support, over some six years, of the Namibian Geological Survey that allowed the mapping programme to be undertaken. A.E. wishes to thank the FRD, South Africa, for some financial support, and R. Rasch of the University of Queensland Centre for Microscopy and Microanalysis for technical support whilst undertaking microprobe analyses. We are also indebted to Drs D. Peate, A. Le Roex, M. Piccirillo and M. Wilson for very detailed and constructive comments on versions of the manuscript.
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TOPABSTRACTINTRODUCTIONMAGMA TYPES AND THEIR...PETROGRAPHY AND MINERALOGYCHEMISTRYDISCUSSIONMODELLING OF GEOCHEMICAL...SYNTHESISSUPPLEMENTARY DATAREFERENCES |
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