Journal of Petrology | Volume 38 | Number 6 | Pages 677-702 | 1997
© Oxford University Press 1997
Cretaceous Basaltic Terranes in Western Columbia: Elemental, Chronological and Sr–Nd Isotopic Constraints on Petrogenesis
1 Department of Geology, University of Leicester University Road, Leicester LE1 7RH, UK
2 Department of Geology, Royal Holloway University of London Egham, London TW20 0EX, UK
3 Ingeominas—Regional Pacifico AA 9724, Cali, Colombia
4 College of Oceanography, Oregon State University Corvallis, OR 97331, USA
Received August 19, 1996; Revised typescript accepted January 10, 1997
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ABSTRACT |
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 TOP ABSTRACT IntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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Accreted terranes comprising Mid to Late Cretaceous picrites, basalts and dolerites occur in three north–south trending belts in western Colombia, in the Central Cordillera, Western Cordillera and along the Pacific coast. The geochemistry of these rocks is consistent with an oceanic plateau (plume-related) origin, and they most probably formed in the Pacific as part of the Caribbean oceanic plateau. These igneous rocks display small but significant inter-cordillera variations, being younger and more depleted in incompatible trace element ratios (and with more positive
Nd values) to the west. The igneous rocks of the Pacific coast (Serranía de Baudó) are dated at 73–78 Ma (40Ar/39Ar), and those of the Western Cordillera at 
90 Ma, whereas the volcanics of the Central Cordillera are believed to be older than 100 Ma. Most of the igneous rocks are basaltic, and it is suggested that they have fractionated from picritic primary magmas, generated by partial melting within a hot mantle plume. Variable and positive Nd values reveal that the plume must have been heterogeneous, originating from a mantle source with a long-term history of depletion. Partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of 20%. Mixing of melts from different depths, either in the mantle melting column or during fractionation in lithospheric magma chambers, can explain the relative homogeneity of basaltic lavas erupted to form this (and other) oceanic plateaux. The Caribbean–Colombian oceanic plateau may have formed at an oceanic spreading centre, and valuable comparisons can be made between Iceland and the Caribbean–Colombian plateau. KEY WORDS: basalt; Colombia; geochemistry; mantle plume; oceanic plateau
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Introduction |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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The relative contribution of lithospheric vs asthenospheric (plume) mantle sources to continental flood basalt volcanism is a contentious subject in the petrological literature (e.g. Gallagher & Hawkesworth, 1992
; Saunders et al., 1992; Arndt et al., 1993; Thirlwall et al., 1994; Gibson et al., 1995). One thing seems relatively clear: only a small percentage of plume-derived continental flood basalts pass cleanly through the continental lithosphere, with the rest being contaminated. The nature of this contamination and the discussions bearing on its relative importance have tended to divert attention away from the primary cause of the volcanism, the plume itself. Increasingly, however, it has been realized that mantle plumes form not only continental flood basalts but also oceanic flood basalts or, more specifically, oceanic plateaux that have thickened oceanic crust (>8 km) (e.g. Carlson et al., 1980; Ben-Avraham et al., 1981; Mahoney, 1987). Such plateaux are only locally exposed at the surface (e.g. Ontong Java in the Pacific and Kerguelen in the Indian Ocean), but they offer us the chance to study what is, in effect, an oceanic analogue of a continental flood basalt province, without the complication of sub-continental lithospheric contamination.
A major problem is that most parts of extant oceanic plateaux are still submerged beneath deep water. To a certain extent, they can be sampled by drilling (e.g. Vallier et al., 1980; Saunders, 1985; Mahoney, 1987; Mahoney & Spencer, 1991; Storey et al., 1992) but this really just 'scratches the surface’ (Coffin & Eldholm, 1993), as the greatest depth to which any of these oceanic large igneous provinces has been drilled is 700 m into the Nauru Basin (Saunders, 1985). Nevertheless, another way in which oceanic plateaux have been made accessible for detailed study stems from the fact that these plateaux (especially if they are young) are more buoyant than normal oceanic crust and will therefore resist subduction (Ben-Avraham et al., 1981; Cloos, 1993; Saunders et al., 1996). Thus the upper reaches of such a plateau may be obducted onto the margin of the overriding plate (see Kimura & Ludden, 1995). Such a situation is believed to have occurred in the Solomon Islands (Tejada et al., 1996) and the Wrangellia terrane (Lassiter et al., 1995). Arguably, the best exposed oceanic plateau available for study is the Late Cretaceous Caribbean oceanic plateau, which makes up most of the Caribbean Plate, and is exposed around its tectonically disturbed margins (Donnelly et al., 1990; Kerr et al., 1996b, c, 1997) (Fig. 1). In this paper we focus on the less well-known parts of the Caribbean plateau which were accreted against northwestern South America, and now form a series of mafic terranes in western Colombia. We present new geochemical and Sr–Nd isotopic data to demonstrate that these Cretaceous igneous rocks are part of the Caribbean oceanic plateau, rather than supra-subduction zone ophiolites. We assess the nature of the mantle plume involved in the petrogenesis of these Colombian volcanics, and present new 40Ar/39Ar radiometric ages for the basalts, so as to estimate the spatial and temporal extent of this plume-related volcanism.
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Regional Tectonic Setting |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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The distribution of volcanic rocks belonging to the Caribbean–Colombian Cretaceous Igneous Province is shown in Fig. 1. These Cretaceous volcanics around the margin of the Caribbean have been discussed by various workers [see Donnelly et al., (1990)
and Kerr et al. (1996c, 1997) for reviews], and indeed the basalts forming the Caribbean sea floor itself have been drilled by DSDP Leg 15 (Donnelly et al., 1973; Bence et al., 1975). The Caribbean ocean floor is composed of anomalously thick crust (up to 20 km; Edgar et al., 1971), which Donnelly, (1973) and Donnelly et al., (1973) considered a flood basalt province. It is now generally accepted that the Caribbean plate was formed in the eastern Pacific as a Large Igneous Province (LIP) in the Late Cretaceous (e.g. Duncan & Hargraves, 1984; Burke, 1988; Pindell & Barrett, 1990; Kerr et al., 1997).
Using a fixed hotspot reference frame, Duncan & Hargraves, (1984) and Hill, (1993) suggested that the magmas of the Caribbean–Colombian Cretaceous Igneous Province were produced by partial melting within the initial ‘plume head’ of the Galápagos hotspot. Eastward movement of the Farallon plate in the Late Cretaceous–Early Tertiary forced the northern half of the plateau into the ocean basin which had been opening between North and South America since the Jurassic. The eastward moving plateau appears to have been too buoyant (owing to its thermal structure and crustal thickness) to be subducted beneath the westward moving American oceanic plate (Burke et al., 1978; Hill, 1993), thus jamming the subduction zone and causing a flip in the direction of subduction from east to west, such that the Atlantic plate was being consumed at the subduction zone, as opposed to the Farallon–Caribbean plate (Pindell & Barrett, 1990; Lebrón & Perfit, 1994). Further south, the Caribbean–Colombian plateau impinged against the northwestern continental margin of South America, leading to imbrication and obduction of the plateau and progressive westward back-stepping of the subduction zone to form the accreted oceanic plateau terranes of northwestern South America (Millward et al., 1984; Kerr et al., 1996c).
The Romeral Fault zone (Fig. 2) represents a major terrane boundary in northwestern South America. To the east of the Romeral Fault, Bouguer gravity anomalies are strongly negative (–220 mgal; Case et al., 1973), confirming geological observations that the basement is composed of continental crust. In contrast, to the west of the Romeral Fault system, anomalies are strongly positive (+135 mgal at the Pacific coast and +75 mgal in the Western Cordillera of Colombia; Case et al., 1971). Therefore, the basement to the west of the Romeral Fault is composed of high-density material, consistent with an oceanic origin.
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Because present-day volcanoes in western Colombia erupt subduction-related lavas, several earlier interpretations of the tectonic setting of the Cretaceous basalts suggested a subduction-related environment of formation (Barrero, 1979
; McCourt et al., 1984; Bourgois et al., 1987; Spadea et al., 1987; Grosser, 1989). Other workers have argued that the basic lavas of western Colombia represent former oceanic crust (Pichler et al., 1974; Mooney, 1980; Bourgois et al., 1982). However, as Nivia, (1987) has pointed out, typical ophiolitic sequences, most of which have sheeted dyke complexes, indicative of spreading ridges, are not found in the Cretaceous accreted terranes of Colombia. Here we evaluate more fully the suggestion by Millward et al., (1984), Nivia, (1987), Storey et al., (1991) and Kerr et al. (1996c) that the Cretaceous Colombian mafic terranes formed as part of an oceanic plateau, resulting from partial melting of a mantle plume, which generated thicker than normal oceanic crust.
Previous studies (e.g. Millward et al., 1984; Spadea et al., 1987; Storey et al., 1991) have tended to focus on the major and trace element composition of basalts from relatively small areas. This study, however, is the first to report comprehensive major and trace element data, Sr–Nd isotopic analyses, and precise 40Ar/39Ar ages, for the Cretaceous basalts throughout a wide area of Colombia. In doing this we hope to shed some new light on the petrogenesis and original tectonic setting of the Cretaceous Colombian basalts.
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Cretaceous Colombian Basaltic Terranes—Field Relations and Sampling |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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The Cretaceous mafic sequences form three main belts (Fig. 2) which trend approximately NNE–SSW, namely the Central Cordillera, the Western Cordillera and the Serranía de Baudó along the Pacific coast.
Central Cordillera
The mafic igneous rocks of the western flank of the Central Cordillera occur in several discontinuous lenses, from Medellín in the north to Pasto in the south. The exposures are bounded to the east by the Romeral Fault (Fig. 2). Cretaceous volcanics of the Central Cordillera also extend along the trace of the Romeral Fault into northern Ecuador (Lebras et al., 1987).
The largest continuous outcrop of mafic volcanics in the Central Cordillera, known as the Amaime Formation (Fig. 3b) (McCourt et al., 1984), is 140 km long, 5–20 km wide and bounded by the Romeral Fault to the east and by the Guabas–Pradera Fault to the west. The formation consists of both massive and pillowed tholeiitic basalts and occasional cumulate picrites which occur in several fault-bounded blocks of 5–10 km width. The occurrence of picrites within the Amaime Formation has also been noted by McCourt et al., (1984) and Spadea et al., (1989).
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To the west of the Guabas–Pradera Fault an elongate (north–south) body of mafic and ultramafic rocks (30 km long x 7 km wide) is exposed, known as the Ginebra Ultramafic Complex (Nivia, 1987
) (Fig. 3b). This body consists of a sequence of dunites, wehrlites and layered and isotropic gabbros, which are overlain by amphibolitized basalts. Aspden & McCourt, (1986) proposed that this sequence represents a lower-crustal level of the Amaime Formation. Other exposures of the igneous rocks of the Central Cordillera shown in Fig. 2 include the pillowed picritic and picritic-basalts and tuff-breccias found at El Encenillo, 20 km SSW of Popayán, and the Los Azules complex, which consists mostly of a series of ultramafic cumulates with some massive and pillowed picritic to basaltic lavas (Spadea et al., 1989). Some 40 km north of Pasto a sequence of pillowed and massive lavas is exposed along a new road cut traversed by the Pan American Highway, near the village of Taminango.
Along the eastern margin of the Romeral Fault a 5–10 km wide belt of high-pressure lawsonite–glaucophane schists and eclogites can be found stretching from southern Ecuador to north–central Colombia. These high-grade metamorphic rocks are associated with highly tectonized and serpentinized ultramafic rocks and gabbros (McCourt & Feininger, 1984), and are known as the Arquía Complex (Fig. 2). Exposed to the east of the Arquía Complex is a discontinuous (5–10 km wide) belt of basalts, andesites and tuffs, tectonically intermixed with Palaeozoic low-grade schists—the Quebradagrande Complex (Fig. 2). Preliminary geochemical studies (A. Nivia et al., unpublished data, 1996) of lavas and tuffs from the Quebradagrande Complex suggests that they have been formed in a subduction-related tectonic setting.
During the present study, basalts and several cumulate picrites were sampled from the main outcrop of the Amaime Formation east of Cali [SW and NE of the town of Sevilla and due west of Buga (AMA1–12), as well as west of Florida (FLO1–4); Fig. 3]. Basaltic samples were collected along the Pan American Highway near Taminango (YAN1–8), and at El Encenillo (Fig. 2) blocks in a picrite breccia (ROM4ii) and a picritic flow (ROM2) were sampled. The volcanic section of the Los Azules complex (Fig. 2) yielded basalts and cumulate and non-cumulate picrites. Only the non-cumulate picrites and basalts (LER4, AZU2 and -3 and ROM5–10) will be discussed in this paper. A basaltic sample (BAR2) was also collected from a minor fault-bounded sliver 30 km SW of Medellín, which is believed to be part of the basaltic sequence of the Central Cordillera.
Western Cordillera
The most extensive outcrops of Cretaceous basalt in Colombia occur in the Western Cordillera (Figs 2 and 3), which is separated from the Central Cordillera by the Cauca–Patia Graben, and from the coastal Serranía de Baudó sequence by the San Juan–Atrato Trough (Fig. 2). The igneous rocks crop out in NNE–SSW trending fault-bounded slices which can be up to 15 km wide, although they are more often <10 km in width. Steeply eastward-dipping fault-bounded lenses of metasediments sometimes separate the volcanic slices (Fig. 3). Sedimentary and structural data indicate that the sequence in each fault block youngs towards the east, with vertical to sub-vertical bedding planes (Millward et al., 1984). The rocks of the Western Cordillera are known by several different formation names depending on their location in Colombia, and these are summarized in Table 1.
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In central and southern Colombia sedimentary rocks comprise
30% of the Western Cordillera and they have been divided into two main formations: the Cisneros and Espinal Formations (Barrero, 1979; Aspden, 1984). The Cisneros Formation may be up to 2000 m thick (Barrero, 1979; Millward et al., 1984); it consists of a sequence of strongly deformed slates and phyllites with thin lenses of meta-limestones, cherts and greywackes, and is in faulted contact with igneous rocks of the Western Cordillera. In contrast, the 700 m thick Espinal Formation is composed of a series of relatively unmetamorphosed cherts, shales and pebbly sandstones, and appears to be conformable with the volcanics of the Western Cordillera (McCourt et al., 1984; Millward et al., 1984). It has recently been suggested by Nivia et al., (1996) that the difference in deformation between the two formations is due to the relative competence of each formation, with the generally finer-grained Cisneros sediments being more deformed than the coarser Espinal Formation sediments. Palaeontological evidence (Barrero, 1979), suggests that the Cisneros Formation is older than the Espinal (and Volcanic) Formation.
The Volcanic Formation (formerly the Diabase Group) consists of a >5 km thick (Barrero, 1979) sequence of both pillowed and massive basaltic lavas, dolerites, local gabbros and rare tuffs. The pillow lavas can be up to 50 m thick, and hyaloclastite breccia is commonly found between the individual pillows, which sometimes preserve chilled skins and occasionally concentrically arranged amygdales. Coarser-grained basalts and dolerites form either massive flows or intrusive sheets (200–300 m thick). The basalts and dolerites are highly sheared in places and have undergone low-grade (zeolite and prehnite–pumpellyite facies) metamorphism (Barrero, 1979). Associated sedimentary lenses (chert and grey shales) are usually <30 m thick. Very few mafic dykes cut the volcanic formation, although there are several small gabbroic plugs.
McBirney, (1963) has calculated that at hydrostatic pressures >315 bars (3.15 km seawater depth) explosive fragmentation and vesicle formation are not possible. Thus the abundance of pillow lavas with only rare vesicles (now infilled) and the scarcity of intercalated limestones and tuffs, combined with the occurrence of cherts and grey shales within the basaltic succession, imply that the bulk of the lava succession was erupted in a considerable depth of water (>3 km). The presence of a few tuffs within the lava succession of the Western Cordillera suggests that as the lava pile accumulated, eruptions may have occurred in shallower water; nevertheless, no evidence of subaerial eruptions has so far been found
In the eastern part of the Western Cordillera, at 4°15'N (Fig. 3b) a suite of ultramafic–mafic rocks crops out (Barrero, 1979; Nivia, 1996), known as the Bolívar–Río Frio ultramafic complex (Kerr et al., 1997). This complex consists of both layered and isotropic gabbros and norites, structurally underlain by serpentinized dunite containing bands of both clinopyroxenite and olivine gabbro–norite. The composition of the rocks from the Bolívar complex strongly suggests that they are genetically related to the basalts of the Western Cordillera (Nivia, 1996; Kerr et al., in preparation) and that they could represent the lower section of the Caribbean–Colombian oceanic plateau (Kerr et al., 1997).
In the northern section of the Western Cordillera, sediments make up more than half of the outcrop. These sediments, the Penderisco Formation, consist of a sequence of cherts, black micritic limestones and deposits with possible turbiditic characteristics (Alvarez & González, 1978). The associated volcanic rocks, the Barroso Formation, include basalts (with some pillows), dolerites, hyaloclastites, thin tuffs, and volcanic breccias, with occasional lenses of pelites and cherts.
Along the western periphery of the Western Cordillera, a sequence of Early Tertiary subduction-related volcanics crop out, the Dabeiba Volcanic Arc (Tistl & Salazar, 1994). The basalts, andesites and pyroclastic rocks of this arc are best developed to the west of Medellín (Fig. 2), and the rocks appear to be significantly younger than the basalts of the Western Cordillera. 40Ar/39Ar dating of a basalt from the Dabeiba Volcanic Arc (AN1464), using step heating techniques, yielded a plateau age of 43.1±0.4 Ma (i.e. Eocene) (Table 2).
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To the south, in Ecuador, the Macuchi Formation has been widely correlated with the Volcanic Formation (McCourt et al., 1984
; Lebras et al., 1987; Wallrabe-Adams, 1990; Van Thournout et al., 1992). This formation consists of a more westerly subduction-related sequence of lavas and volcaniclastic sediments (Henderson, 1979; Lebras et al., 1987). A scattered discontinuous belt of basalts intercalated with silicified shale, along with peridotites and layered gabbros, crops out along the eastern margin of the belt, and has been termed the ‘mid-ocean ridge basalt’ of the Macuchi Formation (Lebras et al., 1987). Western Cordillera igneous rocks were sampled from new road cuts along the Cali to Buenaventura road (CBU2–17; Fig. 3b). Additionally, volcanic rocks were sampled in the Vijes area, 30 km NNE of Cali (VIJ 1–4, PAN2–11 and 19) and at Calima, 50 km north of Cali (COL-1; Fig. 3b). A suite of lavas have also been collected from the Barroso Formation type-section 40–60 km SW of Medellín (Fig. 2), on the Remolino to El Barroso road, and from the Barroso valley along the road to Jardín (BAR3–11).
Serranía de Baudó
The westernmost belt of Cretaceous volcanic rocks outcrops principally along the Pacific coast of NW Colombia, known as the Serranía de Baudó (Figs 2 and 3a). The coastal exposures in the region consist of pillowed and massive basalts, with some basaltic breccias (with clasts up to 30 cm in diameter), dolerites and gabbros. Although not reported by previous workers (Goossens et al., 1977; Macia, 1985), intercalations of fine-grained sandstone, chert and limestones were found between some of the basalts. The nature of these intercalated sediments implies a shallower eruption depth for these basalts than for those of the Western Cordillera, and this is perhaps a reflection of their younger age (see below). The basalts are unconformably overlain by a poorly exposed sequence of subduction-related basalts which are intercalated with Eocene limestones (Gansser, 1973).
McGeary & Ben-Avraham, (1986) proposed that the volcanic succession on Gorgona Island (Fig. 2) represents a continuation of the basalts of the Serranía de Baudó. Gorgona Island is the location of the world's only known Phanerozoic komatiites (Echeverría, 1980; Kerr et al., 1996a). However, despite extensive sampling along the coast from 7°10'N to 5°30'N (Fig. 3a), no evidence of lavas with spinifex (komatiitic) textures was found. Basalts and some gabbros were collected from the Serranía de Baudó coast (SDB1–25; Fig. 3a). Dense jungle and deep tropical weathering meant that sampling was restricted to coastal exposures.
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Analytical Methods |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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After powdering in an agate Tema® mill, major and trace elements were analysed by X-ray fluorescence (XRF) at Royal Holloway University of London and Leicester University using conventional techniques [see Tarney & Marsh, (1991)
and Kerr et al. (1996b) for further details]. The 2 SD errors are as follows: SiO2, 0.2; TiO2, 0.03; Al2O3, 0.1; Fe2O3, 0.1; CaO, 0.05; MgO, 0.1; Na2O, 0.1; K2O, 0.03; MnO, 0.03; P2O5, 0.03; Ba, 2.0; Cr, 1%; Ga, 1.0; Nb, 0.2; Ni, 1%; Rb, 0.3; Sr, 1%; V, 1.5%; Y, 0.4; Zr, 0.5. The XRF data are reported in Table 3.
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The rare earth elements (REE) and Th, U, Co, Sc, Ta and Hf (Table 4) have been analysed by instrumental neutron activation analysis (INAA) at the University of Leicester [see Fitton et al., (1997)
for analytical details]. Additionally, several samples were analysed for REE by inductively coupled plasma-atomic emission spectroscopy at Royal Holloway [see Walsh et al., (1981) for analytical details].
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Sr and Nd isotope ratios (Table 5) were measured at Royal Holloway, on a VG354 mass spectrometer fitted with five Faraday collectors. The isotopic analysis procedures have been described by Thirlwall (1991a
, b). The powders used for Sr isotope analysis were leached in 6M HCl for 1 h before commencing the chemical procedures. The more altered samples were leached a second time in 6M HCl. During the period of the analyses, international reference material SRM987 averaged 0.710243±17 (2 SD, n=64) and laboratory standard ‘Low Aldrich’ averaged 0.511421±9 (2 SD, n=14). [See Thirlwall (1991a) for comparison of ‘Low Aldrich’ with international standards.]
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Age determinations for whole-rock and plagioclase mineral separates from the least altered samples were performed at Oregon State University using standard 40Ar–39Ar incremental heating techniques (Duncan & Hargraves, 1990
; Duncan & Hogan, 1994) Whole-rock samples were either crushed in bulk in a ceramic jaw crusher (PAN6) and sieved to a uniform 0.5–1.0 mm grain size or made into mini-cores. Plagioclase crystals were magnetically separated, checked for purity under a binocular microscope, briefly washed in 5% HF and ultrasonically cleaned in distilled water. Samples were sealed in evacuated quartz glass vials and irradiated for 6 h at the OSU TRIGA nuclear reactor facility. Neutron flux during irradiation was monitored by FCT-3 biotite (27.7 Ma, Hurfurd & Hammerschmidt, 1985). Ar isotopes of the whole-rock samples (500 mg) were determined using an AEI MS-10S mass spectrometer. The plagioclase separates (100 mg) were analysed using an MAP 215–50 mass spectrometer.
Individual ages for each 40Ar–39Ar temperature step were calculated after corrections for background, mass fractionation, isotopic interferences and atmospheric argon contamination (40Ar/36Ar=295.5). Plateau ages were calculated from consecutive steps that are concordant within 2
error using the procedure described by Dalrymple et al., (1988), in which step ages were weighted by the inverse of their variance (Table 2). Isotope correlation diagrams (36Ar/40Ar vs 39Ar/40Ar) were made for each analysis, in which the slope is proportional to the age (isochron) and the inverse of the y-intercept gives the initial 40Ar/36Ar composition. The isochron ages are calculated using the same steps as in the plateau ages. Plateau ages almost invariably have smaller errors because the weighting procedure emphasizes the most precisely determined age steps, whereas uncertainties in the isochron ages reflect the analytical errors of the more poorly determined step ages and the particular distribution of Ar isotopic compositions in 36Ar/40Ar vs 39Ar/40Ar plots. We will therefore restrict our discussion to the plateau ages.
Samples CBU11 and CBU12 from the Western Cordillera displayed erratic step ages such that no age plateaux or isochrons could be discerned. This can be attributed to a combination of alteration, the low K2O contents of the rocks and interferences from hydrocarbons.
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Geochronology |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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Different environments of formation as well as different relative ages have been proposed for the three belts of Cretaceous igneous rocks:
- The igneous rocks of the three belts form part of one near-synchronous province (Goossens & Rose, 1973; Goossens et al., 1977; Marriner & Millward, 1984).
- The volcanic and intrusive rocks of the Western and Central Cordillera form one province, whereas those of the Serranía de Baudó constitute a younger province (Barrero, 1979; Feininger & Bristow, 1980).
- The Cretaceous volcanic belts young westwards from the Central Cordillera, through the Western Cordillera, to the Serranía de Baudó (McCourt et al., 1984; Aspden et al., 1987; Lebras et al., 1987).
These previous interpretations are unfortunately heavily dependent on unreliable (owing to Ar and K loss, or K gain) K/Ar ages, and on palaeontological evidence from associated sediments that are only in tectonic contact with the volcanic rocks. We have dated two basalts from the Serranía de Baudó, one from the Western Cordillera, and one from the Dabeiba Volcanic Arc by 40Ar/39Ar step heating. We have also restricted palaeontological evidence to those fossils that are derived from sediments which are clearly stratigraphically intercalated (rather than tectonically intercalated) with the volcanic rocks.
The Serranía de Baudó rocks yield 40Ar/39Ar step-heating plateaux which range from 72.5±0.4 to 77.9±1.0 Ma (Table 2), consistent with the occurrence of Upper Cretaceous bivalves in intercalated sediments (Gansser, 1973). 40Ar/39Ar dates for the basalts and gabbros of Gorgona Island, the proposed offshore continuation of the Serranía de Baudó (Kerr et al., 1996a), have yielded significantly older ages (86.0±4.6 to 88.3±1.9 Ma) than the basalts of the Serranía de Baudó. The implications of this will be discussed below.
The basalts of the Western Cordillera have proved more problematical to date, owing to both their altered nature and low K2O contents. Nevertheless, one sample (PAN6) has been successfully dated (Table 2) and has yielded a plateau age of 91.7±2.7 Ma. This age is also consistent with palaeontological evidence from the intercalated sediments. For example, Bourgois et al., (1987) noted the presence of Cenomanian to Turonian (97–88 Ma) microfaunas in limestones and cherts interbedded with basalts, 50 km NW of Medellín, whereas Barrero, (1979) reported the occurrence of Turonian–Coniacian (91–87 Ma) microfossils and ammonites from the Espinal Formation.
As was noted above, the Macuchi Formation in Ecuador can be divided into two petrological provinces: an eastern, more basaltic province, and a western tuff-rich andesitic province. The only palaeontological ages come from intercalated sediments in the western arc-derived province, where foraminifera of Eocene age have been reported (Henderson, 1979). This suggests that the more andesitic portion of the Macuchi Formation, like the Dabeiba Volcanic Arc sequence (sample AN1464 dated at 43.1±0.4 Ma; see above), is significantly younger than, and should not be correlated with, the basalts of the Western Cordillera. The tectonic significance of these Eocene subduction-related volcanics along the western periphery of the Western Colombian and Ecuadorian Cordillera, and the Quebradagrande Complex, the metasediments of which have yielded poorly preserved fossils ranging from Hauterivian to Albian [135–97 Ma; summarized by Nivia, (1987)] will be discussed in a later section.
Unfortunately, no Central Cordillera basalts or dolerites were fresh enough or contained enough K2O (>0.1 wt %) to yield reliable 40Ar/39Ar ages. Additionally, no fossils have been found within the intercalated sediments (Aspden & McCourt, 1986). One of the few constraints on the age of the Amaime Formation comes from the fact that it is intruded by the Buga tonalitic batholith. This tonalite has yielded an Rb/Sr mineral isochron (on biotite and hornblende) of 99±4 Ma (McCourt et al., 1984), suggesting that the formation of the Amaime basaltic crust and its accretion onto the margin of NW Colombia must have occurred in the Early Cretaceous, well before 100 Ma. Thus the igneous rocks of the Central Cordillera would appear to be appreciably older than those of the Western Cordillera, which are in turn younger than the volcanic and intrusive rocks of the Serranía de Baudó. Nevertheless, the compositions of basalts from the Central Cordillera are similar to those of basalts from the other two Cordilleras, thus implying a similar petrogenetic history.
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Petrography and Mineral Chemistry |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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Central Cordillera
The basalts and dolerites consist of plagioclase, clinopyroxene and Fe–Ti oxides, with secondary chlorite, iron oxides, zeolites, pumpellyite and quartz. Euhedral to subhedral olivine phenocrysts, which are altered to serpentine, are more common than in the Western Cordillera (see below), but aphyric basalts still predominate. The clinopyroxenes are relatively fresh and range in composition from diopside to augite (Nivia, 1987
), whereas the fresher plagioclase crystals vary between An67 and An50 (Nivia, 1987). Textures range from plagioclase laths and euhedral clinopyroxene microphenocrysts in a groundmass of plagioclase and clinopyroxene spherulites, to ophitic to subophitic clinopyroxene enclosing or partially enclosing plagioclase laths, or occasionally a very fine-grained granular texture. An excellent summary of the petrography and mineral chemistry of the picrites and basalts from El Encenillo and Los Azules has been given by Spadea et al., (1989).
Western Cordillera
Most of the basalts and dolerites are fine- to medium-grained holocrystalline rocks, with altered glass only being found in the outer skin of pillows. Textures vary from ophitic to subophitic with plagioclase laths poikilitically enclosed within anhedral clinopyroxenes, to intergranular and intersertal. The basalts and dolerites are generally aphyric but occasionally microphenocrysts of clinopyroxene can be found, set in a groundmass of variably altered plagioclase laths (An72–50; Nivia, 1987) and anhedral to occasionally euhedral diopside and augite (En50–35Wo44–35Fs6–30; Nivia, 1987), with <5% Fe–Ti oxides. Olivine phenocrysts (altered to chlorite and iddingsite) are occasionally found as pseudomorphs, but olivine is more common as an altered groundmass mineral. Other minor minerals are mostly secondary in origin, and include chloritized glass, chlorite, zeolites, pumpellyite, quartz and calcite.
Serranía de Baudó
The basalts and dolerites are mostly holocrystalline, fine to medium grained, and contain augite and variably altered plagioclase laths (labradorite to andesine) with <5% anhedral Fe–Ti oxides as the principal minerals. Secondary minerals such as chlorite, albite, quartz and zeolites occur in small amounts within the groundmass and also in amygdales and sometimes in veinlets. In several samples altered olivine has been recognized in the groundmass, and occasionally altered (to chlorite and iddingsite) subhedral olivine microphenocrysts can be found. Ophitic textures are common, and these lavas can contain pyroxene and plagioclase phenocrysts, within a granular groundmass of plagioclase, pyroxene and secondary minerals. Occasionally, plagioclase and pyroxene form glomeroporphyritic clusters.
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Geochemistry |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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Alteration and elemental mobility
The altered nature (up to prehnite–pumpellyite grade) of the rocks from all three cordilleras means that before any petrological inferences can be drawn from the chemistry of the rocks, the possible chemical effects of subsolidus mobility of elements must be considered. Zirconium is widely regarded as being essentially immobile during low-grade alteration of basaltic rocks by hydrothermal fluids (e.g. Humphris & Thompson, 1978
; Gibson et al., 1982; Kerr, 1995) and so has been plotted against all the other minor and trace elements. A selection of these diagrams is shown in Fig. 4. Nb, Y and TiO2, which are also believed to be relatively immobile, produce good correlations against Zr. The minor differences in ratio seen in the Y and Nb vs Zr plots are significant, and are due either to variable degrees of partial mantle melting or to a heterogeneous mantle source region. In contrast, Ba, Sr, Rb and K2O display virtually no correlation with Zr, which strongly implies that, as in other altered basalts, these large ion lithophile elements have been extensively mobilized. Accordingly, variation in these elements will not be discussed further. All the other minor and trace elements are, like TiO2, Y and Nb, well correlated with Zr contents, implying relative immobility.
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Analytical results
Serranía de Baudó
The volcanic and intrusive rocks of the Serranía de Baudó fall into two main groups. The first group is represented by three samples (SDB23–25) which come from the southernmost exposure of basalts on the Pacific coast of Colombia (Fig. 3a). The rocks of this small group, comprising one basaltic lava and two volcanic breccia blocks, are mildly alkaline, having more enriched incompatible trace element contents than the rest of the Serranía de Baudó rocks at similar MgO (5–8 wt %) levels (Figs 4 and 6). Figure 5 shows that the primitive mantle-normalized pattern of SDB25 is relatively steep, and that the La/Y and Nb/Zr ratios of this group are consistently higher (Fig. 7) in comparison with the rest of the Serranía de Baudó rocks. It is interesting to note that the more enriched (e-)basalts from Gorgona Island (Fig. 2), also part of the Caribbean–Colombian oceanic plateau (Kerr et al., 1996a
), possess similar incompatible trace element contents and ratios to SDB23–25 (Fig. 7).
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The second group comprises the remaining volcanics and intrusives from the Serranía de Baudó. These tholeiitic basalts and gabbros exhibit a moderate range in major element composition [48.0–51.7 wt % SiO2; 6.2–10.1 wt % MgO (Fig. 6); 9.2–14.6 wt % Fe2O3(total)]. In terms of trace element contents, two basalts (SDB10 and -13) and one gabbro (SDB15) have slightly lower Zr, Y, Nb and TiO2 values than the rest of the basalts and gabbros, and lie slightly below the main fractionation trend (Fig. 6). This depletion is characteristic of all the other incompatible trace elements, as the primitive mantle-normalized pattern of SDB13 shows (Fig. 5a). These three samples (SDB10, -13 and -15) have lower Nb/Zr ratios (Fig. 7b) and higher 147Sm/144Nd (Fig. 8) than the rest of the Serranía de Baudó rocks. The rest of the basalts and gabbros of the Serranía de Baudó possess relatively flat primitive mantle-normalized patterns (Fig. 5) ranging from 2 to 6.5 times primitive mantle, with the more evolved basalts having generally higher levels of incompatible trace elements. All the rocks (except SDB22–25) are slightly depleted in light REE (LREE) [(La/Nd)pn 0.75–1.0; Fig. 7a] and have (Sm/Yb)pn ratios ranging from 1.0 to 1.2 (pn denotes primitive mantle-normalized; Sun & McDonough, 1989
).
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The isotopic data for the Serranía de Baudó samples are given in Table 5 and plotted in Fig. 8. (
Nd)i (where i=75 Ma age corrected value) span a narrow range from +8.0 to +8.7. Four of the samples also display a restricted range in (87Sr/86Sr)i from 0.70292 to 0.70307; however, SDB18 and SDB11 have elevated (87Sr/86Sr)i of 0.70338 and 0.70369, respectively.
Western Cordillera
The lavas and intrusives of the Western Cordillera are tholeiitic in character. Most of the rocks are basaltic in composition, and range from 48 to 53 wt % SiO2, 10.1 to 14.7 wt % Fe2O3(total) (Table 3) and 6.0 to 10.4 wt % MgO (Fig. 6). In addition, two basaltic andesites (PAN2 and -3) and an andesite PAN19, occur near the town of Vijes (Fig. 3b) in an area where rhyolitic dykes of uncertain origin cut the lava succession (Kerr et al., 1996c). These more evolved lavas have SiO2>53 wt % and MgO<5.1 wt %. A diorite (CBU13; Cali–Buenaventura road) intruding the basaltic pile has only 1.5 wt % MgO (Fig. 6) and higher levels of incompatible trace elements than the basalts of the Western Cordillera (Figs 4, 5 and 6), and exhibits marked troughs in P and Ti on a primitive mantle-normalized diagram (Fig. 5).
Plots of incompatible trace elements vs MgO (Fig. 6) show a typical negative correlation. This feature is also observed in Fig. 5, in which basalts with >6.0 wt % MgO (VIJ1, CBU11 and CBU12) have higher abundances of incompatible trace elements. All the basalts shown in Fig. 5 have essentially parallel and flat primitive mantle-normalized patterns, and thus have relatively small ranges in incompatible trace element ratios (Figs 7 and 8c). As Fig. 7a shows, the basalts of the Western Cordillera are moderately depleted in the LREE [(La/Nd)pn 0.75–0.95], with essentially flat heavy REE (HREE) patterns [(Sm/Yb)pn 1.0–1.1].
The rocks of the Western Cordillera have (Nd)i (i=90 Ma) in the range +7.5 to +8.1, whereas the diorite is slightly higher at +8.3. Three of the samples have (87Sr/86Sr)i in the range 0.7032–0.7033, the diorite being distinctly higher at 0.7036 (Fig. 8a). However, like the basalts of the Serranía de Baudó, three of the basalts have elevated (87Sr/86Sr)i (>0.7045) with only a small concomitant decrease in (Nd)i values.
Central Cordillera
The igneous rocks of the Central Cordillera are the most compositionally diverse of the three cordilleras, ranging from moderately enriched (relative to proposed primitive mantle compositions) in incompatible trace elements to relatively depleted (Fig. 5). The rocks can be divided into two broad groups: the moderately incompatible trace element-enriched basalts and picrites from Los Azules and El Encenillo, and the mostly less enriched basalts and dolerites of the Amaime Formation and the Yana area.
Lavas from Los Azules are tholeiitic, whereas those from El Encenillo are more alkaline. The non-cumulate lavas range from 7.1 to 16.6 wt % MgO (Fig. 6), the higher values falling within the range of calculated MgO content (12.8–17.3 wt %) (Spadea et al., 1989) for a liquid in equilibrium with Fo89 olivine (maximum in Los Azules picrites). Therefore ROM7, the sample containing 16.6 wt % MgO, may represent a potential primary magma for the Los Azules complex. In terms of trace elements, the two analysed lavas from El Encenillo are significantly more enriched in incompatible trace elements (at equivalent MgO content) than the lavas from Los Azules (Fig. 6). This is also reflected in the steeper primitive mantle-normalized trace element patterns for the El Encenillo lavas (Fig. 5) and incompatible trace element ratio plots (Fig. 7), with the El Encenillo lavas having, for example, higher La/Y and Nb/Zr than the Los Azules lavas.
The tholeiitic basalts and dolerites of the Amaime Formation and Yana area range in MgO content from 5.0 to 10.3 wt % (Fig. 6). Most of the lavas possess relatively flat primitive mantle-normalized trace element patterns (Fig. 5), and their incompatible trace element ratios are similar to those from the Western Cordillera and from Serranía de Baudó (Fig. 7). An interesting exception to this is a group of basalts from the more southerly parts of the Amaime Formation (AMA3–6 and FLO1–4), which possess higher Nb/Zr ratios than the rest of the Amaime and Yana basalts (Fig. 7). This enrichment seems only to affect Nb contents, as ratios not involving Nb (e.g. Ti/Zr) are the same as for the rest of the basalts. Two of the basalts from the Amaime Formation (AMA8 and -11) are relatively enriched in incompatible trace element contents and ratios (Figs 5, 6 and 7), whereas one basalt (AMA12) has been found which possesses more depleted levels of incompatible trace elements (Figs 5, 6 and 7).
In terms of (Nd)i (i=120 Ma), the lavas span a comparatively wide range from +6.0 in AMA8, one of the most enriched Amaime basalts, to +8.1 in YAN-8 (Fig. 8). The most trace element depleted Amaime basalt (AMA12) has an (Nd)i of +7.7. As with the basalts of the Western Cordillera, (87Sr/86Sr)i appears be decoupled from (Nd)i, with several of the basalts having (87Sr/86Sr)i>0.7043, but most lie in the range 0.7032–0.7035.
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Discussion |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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Original tectonic setting and relative ages of the Colombian basalts
Tectonic setting
The chemical data presented can help to resolve the long-standing controversy regarding the tectonic setting of the Colombian mafic volcanic terranes, namely, whether they represent a subduction zone environment, ocean floor or oceanic plateau. This problem can be addressed in two ways:
- Mature subduction zone volcanic sequences are characterized by abundant explosion-derived tuffs and lavas of andesitic composition, as seen in recent Colombian subduction-related volcanoes (Marriner & Millward, 1984). However, the Cretaceous volcanic terranes are predominantly basaltic, and although pyroclastic deposits do occur, they are relatively rare and are mostly basaltic in composition.
- As Fig. 5 shows, these Cretaceous volcanic rocks do not possess the characteristic Nb depletion (with corresponding LREE enrichment), which results in a negative Nb anomaly on multi-element normalized plots of subduction-related lavas. Figure 7c confirms that the Cretaceous Colombian basalts and picrites have consistently higher Nb/La ratios (relative to Y) than both Recent Colombian volcanic arc rocks (Marriner & Millward, 1984) and Cretaceous subduction-related lavas from Bonaire in the southern Caribbean (G. Klaver, unpublished data, 1979).
Therefore, both field and geochemical evidence militate strongly against a subduction-related origin for the Cretaceous Colombian volcanic terranes. Moreover, the predominance of basalts with relatively flat primitive normalized trace element patterns, and the not uncommon occurrence of lavas with more picritic compositions, are more consistent with an origin in plume-derived oceanic plateau, rather than in normal ocean floor. This is further supported by Figs 7 and 8, which reveal that lavas drilled from the Ontong Java plateau (Mahoney et al., 1993) possess similar trace element and isotopic ratios to most of the basalts and picrites from Colombia. However, there is a greater chemical variability within the Colombian lavas, and possible explanations for this will be discussed in a later section. The 90 Ma basalts and picrites from Curaçao are part of the Caribbean–Colombian oceanic plateau (Kerr et al., 1996b), and the close similarity in isotopic and trace element ratios between these Curaçao lavas and the Colombian basalts and picrites (Figs 7 and 8) supports an origin within the Caribbean oceanic plateau.
Timing of volcanism
The volcanism associated with the Colombian portion of the Caribbean oceanic plateau appears to be of three distinct ages: two well-dated events (40Ar/39Ar; fossils from intercalated sediments), one of Late Cenomanian–Turonian (88–92 Ma) and another of Late Campanian–Early Maastrichtian (72–78 Ma), plus another eruptive episode older than 100 Ma. Other fragments of Early Cretaceous oceanic plateau material have been reported from elsewhere in the Caribbean region, for example, in the Duarte Complex of Hispaniola (Lapierre et al., 1997) and Cuba (Iturralde-Vinent, 1994). Similarly, more evidence for the younger plateau eruptive event within the Caribbean region (possibly contemporaneous with the formation of the Serranía de Baudó rocks) is also beginning to accumulate. The westernmost Caribbean site drilled during DSDP Leg 15, Site 152 (Fig. 1) encountered basalt with the chemical characteristics of an oceanic plateau which contained fragments of Campanian (83–74 Ma) limestone (Donnelly et al., 1973). Recent Ocean Drilling Program (ODP) drilling in the Caribbean during Leg 165, at Site 1001 (40 km WSW of Site 152), encountered Middle Campanian limestone mixed with clay and basaltic ash–lapilli overlying, and grading down into, plateau basalt (Pearce & Pearson, 1996). Thus the presence of Campanian limestone intimately associated with the uppermost volcanics at both these sites places a maximum age of 83 Ma on the underlying basalt. Additionally, a dolerite sill intruding the upper part of the Curaçao lava succession has recently been dated, using 40Ar/39Ar step-heating, at 75.8±1.9 Ma (C. Sinton, unpublished data, 1996).
It is interesting to note that, as in Colombia, where the westernmost basalts (Serranía de Baudó) are the youngest (72–78 Ma), so it is the westernmost drilled holes in the Caribbean Sea that have also produced the youngest (Campanian) ages. These observations support a younger episode of Caribbean–Colombian plateau volcanism mostly in the west of the province, consistent with eastward movement of the plate(s) above a stationary plume (possibly Galápagos).
The Caribbean–Colombian oceanic plateau is not unique in this regard, because many of the world's plume-related LIPs display at least two distinct periods of major eruptive eruptions, separated by between 20 and 90 m.y. (Bercovici & Mahoney, 1994). The Ontong Java plateau, like the Caribbean–Colombian oceanic plateau, appears to have peaks of volcanic activity at about 120 and 90 Ma (Mahoney et al., 1993). However, perhaps a more suitable analogue for the Caribbean–Colombian Cretaceous oceanic plateau—which appears to have been volcanically active over 40 Ma—is the long-lived volcanism associated with the Icelandic plume. Interestingly, there are other similarities in both chemistry and tectonic setting between the Caribbean–Colombian and Icelandic basalts, which will be explored in a later section.
Finally, whereas it is possible that the earlier (>100 Ma) oceanic plateau forming event in the Caribbean–Colombian province is linked with the same plume as that which produced the 90 and 75 Ma events, we caution that there is no way of discounting the possibility that the earlier event was linked to an entirely different plume.
Petrogenetic aspects
The high 87Sr/86Sr puzzle
Arguably, the most enigmatic feature of the chemistry of these lavas is their tendency to high (87Sr/86Sr)i values relative to (Nd)i (Fig. 8a), which do not appear to correlate with any other chemical parameter. Repeated leaching of powders from several of these samples failed to significantly reduce the high 87Sr/86Sr more than just a single leaching. Within the Caribbean–Colombian Cretaceous igneous province these high 87Sr/86Sr values are not unique, and they have been found in well-leached komatiites and picrites from Gorgona Island (Aitken & Echeverría, 1984), basalts from Nicoya in Costa Rica (F. Hauff, personal communication, 1996) and basalts from Curaçao (Kerr et al., 1996b). The last leaching experiments support the view that these high 87Sr/86Sr values are magmatic in origin and are not caused by sub-solidus hydrothermal alteration (Kerr et al., 1996b). Accordingly, Kerr et al. (1996b) proposed that the high 87Sr/86Sr values could result from assimilation of altered oceanic crust, a process which has also been proposed to account for the moderately elevated 87Sr/86Sr values in Icelandic lavas (e.g. Hémond et al., 1993).
Fractional crystallization
Although the vast majority of rocks from the three volcanic terranes are basaltic in composition, these may not be primary mantle melts. Basalts from the Ontong Java plateau not only have similar trace element ratios to the Cretaceous Colombian basalts, but display a similar range in MgO content (Mahoney et al., 1993). Farnetani et al., (1996) recently proposed that high seismic velocities near Moho levels within the Ontong Java plateau represent crystal cumulates (chiefly olivine) from the large-scale fractionation of primary picritic magmas to produce the erupted basalts. The Cretaceous igneous association in both the Western and Central Cordilleras is in part composed of mafic and ultramafic cumulates (Spadea et al., 1987; Nivia, 1996). The chemistry of these rocks reveals that they are genetically related to the basalts of their respective cordillera (A. C. Kerr et al., unpublished data, 1996). These mafic and ultramafic cumulates may be akin to the high seismic velocity layers found at the base of the crust in the Ontong Java plateau, and so may represent the lower-crustal levels of the 87–92 Ma phase of Caribbean–Colombian oceanic plateau. It is likely that the Cretaceous Colombian basalts were mostly derived from more picritic melts which ponded and fractionated deep within the plateau to produce the ultramafic cumulates and lower-MgO basaltic liquids. The relatively widespread occurrence of 87–92 Ma high-MgO lavas (picrites and komatiites) throughout the Caribbean–Colombian province (Kerr et al., 1997) also implies that high-MgO melts were produced by a large proportion of the proposed plume head. Thus it is not unreasonable to consider that the Colombian basalts are derived from more picritic parental magmas.
In light of these field observations, we have attempted to reproduce the composition of the Colombian basalts using the TRACE3 computer program of Nielsen, (1988), which, given a starting magma composition, calculates mineral and residual magma compositions during fractional crystallization. The residual magma compositions obtained from this modelling are presented in Fig. 6. We have used three different starting magma compositions: a picrite (CUR7) containing 20% MgO from Curaçao (Fig. 1); a slightly more enriched picrite (ROM7) from Los Azules (16.6% MgO); and one of the more depleted, higher-MgO basalts from the Serranía de Baudó (SDB13; 10.1 wt % MgO). Not surprisingly, the first phase to crystallize in all three models is olivine (±Cr-spinel); then, when 8–9 wt % MgO is reached, plagioclase and clinopyroxene join the fractionating assemblage, to be followed by Fe–Ti oxide at <5 wt % MgO. The results (Fig. 6) show that, between them, the three different ‘parental magmas’ could fractionate to produce the incompatible trace element compositions of most of the Cretaceous Colombian basalts. However, some of the basalts are too enriched or too depleted in incompatible trace elements to be fractionates of any of the three magma starting compositions. As will be discussed in the following section, this wide range in incompatible trace element contents at a given MgO (Fig. 6) reflects variable degrees of partial mantle melting and/or a heterogeneous mantle source region.
The composition of the diorite (CBU13) from the Western Cordillera appears to be modelled fairly well by the TRACE3 program. More than 80% fractionation from a picritic parent is required to reproduce the composition of this diorite, which in addition to olivine, plagioclase, clinopyroxene and Fe–Ti oxide, seems to have fractionated minor apatite (as evidenced by the negative phosphorus anomaly in Fig. 3b). Additionally, it should be noted that the marked positive Nb–Ta spike in Fig. 3b militates strongly against an arc-derived origin for this intrusion.
Mantle sources and melting
The variations in Nd isotope and incompatible trace element ratios (Figs 7 and 8) unequivocally demonstrate that the plume source region of the Cretaceous Colombian basalts was heterogeneous. Although some lavas are more enriched in incompatible trace elements compared with others, the positive Nd values mean that all the lavas have been derived from a mantle source region with a long-term depletion in the LREE. Thus both the enriched and chondritic LREE patterns (Fig. 5) of some of the Colombian basalts (and for that matter, most Ontong Java plateau basalts) must represent a relatively recent enrichment, probably during the melting process. The range of (La/Nd)pn (
1) found in the majority of the Colombian basalts (Fig. 7a) obviously reflects the depleted nature of their source regions. Nevertheless, this depletion in incompatible trace elements is subtly different in character from that observed in present-day East Pacific Rise mid-ocean ridge basalt (MORB), and Fig. 9a elegantly reinforces this point. This diagram was originally used by Fitton et al., (1997) to distinguish incompatible element-depleted, plume-derived Icelandic basalts from North Atlantic MORB. In Fig. 9a, our new data from Colombia are plotted along with the ‘tramlines’ of Fitton et al., (1997) (between which all the plume-derived Icelandic lavas plot), and fields for East Pacific Rise MORB and the Ontong Java plateau (Mahoney et al., 1993, 1994). Virtually all the Colombian basalts fall within the Icelandic tramlines, strongly implying derivation from a plume source region. Even the most depleted Colombian rocks (SDB13 and AMA12) do not appear to originate from a depleted upper-mantle (MORB-source) region; rather, they seem to be derived from a depleted (relative to Bulk Earth) source region within the plume.
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In modelling the mantle melting processes several factors need to be considered:
- The plume source region was isotopically heterogeneous, comprising at least two components which both appear to have a long-term history of depletion, with one component being slightly more enriched (lower Nd) than the other.
- The Colombian basalts are not primary melts from the mantle, but may have been trapped and fractionated from picritic melts in magma chambers en route to the surface. However, as well as fractionation, the entrapment of picritic liquids in magma chambers will also serve to homogenize individual heterogeneous melt fractions. This suggestion is reinforced by the fact that the Colombian-Caribbean Cretaceous high-MgO lavas are, as a group, significantly more heterogeneous than their associated basalts (Kerr et al., 1996a, c). Thus, although many of the Cretaceous basalts in Colombia (and in other oceanic plateaux) are relatively homogeneous, this could just reflect mixing of compositionally distinct magmas derived from a heterogeneous plume source region by variable degrees of melting.
An LREE-depleted mantle source composition was used in the modelling of pooled fractional melting. Three different mineral assemblages were used—a garnet lherzolite, a spinel lherzolite and a 50:50 mixture of these two mineralogies—to simulate melting in the garnet-spinel transition zone [source compositions, mineral proportions and partition coefficients are from McKenzie & O'Nions, (1991)].
The results of the mantle melt modelling are presented in Fig. 9b, along with fractional crystallization trends that show a slight increase in Nb/Y ratios as the magmas fractionate. Thus, when fractional crystallization has been accounted for, it can be inferred that many of the Colombian basalts result from fairly extensive degrees (20%) of partial mantle melting, or that they were derived from a more depleted source than that used in the melt modelling. As Fig. 9b shows, at 20% melting the differences in incompatible trace element contents and ratios between the three mantle mineral assemblages are significantly less than at smaller degrees of melting. It thus becomes more difficult to assess the approximate depth of melting of the erupted lavas, a difficulty which is compounded by the possibility of mixing melts derived from both the garnet and spinel lherzolite stability fields. However, melting seems to have occurred over a rather wide depth range (polybaric melting) from relatively deep, mostly within the garnet lherzolite field, to shallower melting of spinel lherzolite.
Figure 9b shows that basalts with Nb/Y>0.2 probably had a melt input from a mantle source region containing some garnet. These more incompatible element-enriched basalts also generally have lower (143Nd/144Nd)i values (Fig. 8b and c), implying that they are derived from a more enriched source region, or contain a higher proportion of enriched component. The negative correlation between (143Nd/144Nd)i and (Sm/Yb)pn displayed by the samples from the Western and Central Cordilleras reveals that the more enriched melts are derived from deeper source region and vice versa. This source heterogeneity and negative correlation between the degree of enrichment and depth of melting support the recent models of Kerr et al., (1995) and Arndt et al., (1997). These models have proposed that most mantle plumes are heterogeneous and consist predominantly of a relatively refractory depleted matrix component, with a small proportion (10%) of more enriched and more fusible streaks or blobs. Thus deeper, and therefore smaller, extents of mantle melting will preferentially sample these more enriched streaks, whereas at shallower depths, more extensive melting means that a greater proportion of the magma produced will be derived from the more depleted matrix.
A feature worth stressing is that many of the Colombian basalts, and indeed basalts from many other oceanic plateaux, particularly Ontong Java, possess positive Nd values but paradoxically, have near-chondritic ratios of incompatible trace elements. This problem has become known as the ‘plateau paradox’ (Babbs et al., 1996). One possible explanation for this paradox may be that mixing of small degree (<2%) relatively LREE-enriched melts (with Nd>0) from the garnet lherzolite stability field, with larger degree (20%) LREE-depleted melts, predominantly derived from the spinel lherzolite field, has occurred. Such mixing of different degree melts could occur either in the mantle plume source region, or perhaps more likely within crustal or lithospheric magma chambers en route to the surface, and has the potential to produce magmas with near-chondritic incompatible trace element ratios but with positive Nd values.
A key issue is whether the sequence of late Cretaceous komatiites, picrites and basalts on Gorgona are not only an integral part of the Caribbean–Colombian Cretaceous oceanic plateau (Storey et al., 1991; Kerr et al., 1996a) but can be correlated with, and are also a southward continuation of, the basaltic sequence of the Serranía de Baudó (McGeary & Ben-Avraham, 1986). The new chemical data presented above support this latter proposal, in that the three more enriched basalts (from the southernmost exposure of the Serranía de Baudó) are very similar to the most enriched e-basalts (Kerr et al., 1996a) from Gorgona, whereas the rest of the Serranía de Baudó samples possess incompatible trace element ratios which are within the range displayed by the rest of the Gorgona basalts. However, the older ages (86–88 Ma; Kerr et al., 1996a) for the Gorgona lavas and intrusives, compared with the Serranía de Baudó (72–78 Ma) appear to rule out the idea that the basalts of the Serranía de Baudó are a continuation of the sequence found on Gorgona.
As well as becoming younger towards the west, the lavas and intrusives of the three cordilleras also generally become more depleted; for example, the average Nd for the Central Cordillera is +7.2, for the Western Cordillera +7.8 and for the Serranía de Baudó +8.3. In addition to this, the younger lavas drilled by DSDP Leg 15 at Site 152 are similarly the most elementally and isotopically depleted lavas sampled during Leg 15 (Kerr et al., 1997; G. F. Marriner & A. D. Saunders, unpublished data,1987). Thus if one single plume is responsible for the volcanism in the Caribbean–Colombian oceanic plateau, then it appears to have become more depleted with time.
Is the Caribbean Plateau an Icelandic analogue?
Recently, linear NE–SW and east–west long-wavelength magnetic anomalies have been discovered over the Venezuelan and Colombian Basins (Fig. 1) in the Caribbean (Hall, 1995). It has been suggested by Hall, (1995) that these anomalies may have resulted from an Early Cretaceous phase of seafloor spreading at the Farallon–Pacific–Phoenix triple junction, at or near which the Caribbean–Colombian Oceanic Plateau formed in the Late Cretaceous. It is interesting to note that the Galápagos plume, which may represent the present-day expression of the plume responsible for the Caribbean–Colombian oceanic plateau, is currently situated close to the Galápagos Spreading Centre. Indeed, the occurrence of a plume track on both the Cocos and Nazca Plates suggests that it was only within the last 5 m.y. that the ridge moved northwards away from the Galápagos hotspot (Hay, 1977), so for a large proportion of its history the Galápagos plume may have impinged on the base of the lithosphere at, or close to, an oceanic spreading centre.
Thus, as well as both the Galápagos and Icelandic plumes having a long history of activity, it also seems that the two plumes may have been situated below a mid-ocean ridge for a considerable part of their histories. It is therefore a constructive exercise to amplify the initial studies of Nivia, (1987) and compare the chemistry of the Caribbean–Colombian oceanic plateau with that of lavas produced by the Icelandic plume. In terms of trace element and radiogenic ratios, the Icelandic lavas [from Hémond et al., (1993)] span a very similar range to the Cretaceous Colombian lavas and intrusives reported in this paper (Figs 7, 8 and 9). The data in Fig. 8b and c show that, at a given incompatible trace element ratio, the Icelandic lavas possess slightly higher (Nd)i. Nevertheless, the basic similarity between the Icelandic and Colombian lavas adds further weight to the supposition that the Caribbean–Colombian oceanic plateau formed at, or near an oceanic spreading centre. Like present-day Iceland, the more enriched lavas in the Caribbean–Cretaceous oceanic plateau could have been produced by smaller degrees of melting below thicker lithosphere, further away from the spreading centre (see Hards et al., 1995). Similarly, the more depleted basalts found in Colombia could represent the equivalent of those depleted Icelandic picrites and basalts which are produced by more extensive shallower melting close to the ridge.
The significance of Early Tertiary and Early Cretaceous arc volcanism in Colombia
Although not considered in detail in this paper, it is nevertheless important to assess the tectonic significance of the subduction-related Early Cretaceous Quebradagrande Complex and the Eocene Dabeiba Volcanic Arc, along with the Eocene arc lavas and pyroclastics of the Macuchi Formation in Ecuador. It is probable that the Quebradagrande complex formed in either a marginal basin or a continental volcanic arc setting (A. Nivia, in preparation) before 120 Ma. The radiometric age constraints imposed by the Buga batholith mean that the Early Cretaceous Amaime Formation must have been obducted onto the Colombian margin before 100 Ma. The Quebradagrande Complex may thus represent Early Cretaceous pre-collision magmatism on the Colombian continental margin. The obduction of the 75–90 Ma lavas and intrusives of the Cordillera and the Serranía de Baudó appears to have occurred in the Late Cretaceous to Early Tertiary (Kerr et al., 1997). It is possible that the volcanics of the Dabeiba arc were formed during the obduction of the 75–90 Ma portion of the Caribbean–Colombian oceanic plateau onto the Colombian continental margin. If this is the case, then the date of the obduction may well be late Eocene. However, more 40Ar/39Ar step heating work on other possible obduction-related pegmatite veins intruding the Bolívar–Río Frio ultramafic complex is currently under way, and these dates may help to better constrain the possible age of the obduction event.
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Conclusions |
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TOPABSTRACTIntroductionRegional Tectonic SettingCretaceous Colombian Basaltic...Analytical MethodsGeochronologyPetrography and Mineral...GeochemistryDiscussionConclusionsReferences |
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- The accreted Cretaceous basaltic and picritic terranes found in western Colombia appear to have formed part of an oceanic plateau linked to a mantle plume. These terranes represent the southern portion of the large late-Cretaceous Caribbean oceanic plateau, which was obducted and imbricated against the western margin of the northern South American continent.
- It is unlikely that the Cretaceous plateau-derived basalts of western Colombia are primary mantle melts. Rather, they may have fractionated from more picritic parental magmas. These picritic partial mantle melts must have been derived from a heterogeneous mantle plume source region containing at least two components. Both components display time integrated depletion (positive Nd), and both appear to have been an integral part of the plume. Importantly, neither depleted plume component can be attributed to entrainment of depleted upper asthenospheric (MORB-source) mantle.
- Fractional melt modelling using a depleted mantle source composition suggests that most of the Western Colombian lavas require a small melt input from a mantle source region containing garnet as a residual phase, along with more extensive melting in the spinel lherzolite stability field. After allowing for fractional crystallization it appears that these Cretaceous Colombian lavas and intrusives are the result of 20% partial mantle melting. However, mixing of magmas generated at different depths possibly in Moho-level magma chambers during fractionation will also result in mixing, thus masking mantle-derived heterogeneities.
- Three distinct ages of volcanic activity can be identified in the Colombian portion of the Caribbean–Colombian oceanic plateau (and also in the Caribbean part of the province). The westernmost accreted belt, the Serranía de Baudó, is the youngest at 73–78 Ma, the Western Cordillera rocks are 90 Ma old, whereas the volcanic sequence of the easternmost Central Cordillera is Early Cretaceous in age. The geochemical signatures of the igneous rocks of the three cordilleras also become more depleted from east to west.
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Acknowledgements |
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We are grateful to the Natural Environmental Research Council (UK) for supporting this work through Grants GR9/583A and GR3/8984 to J.T. and A.D.S., and to the Leverhulme Trust for a fellowship to A.C.K. We also thank INGEOMINAS for invaluable logistic support in Colombia, Nick Marsh for assistance with XRF analysis at Leicester, and Marion Weber for her help with Spanish spelling. Ray MacDonald, Henriette Lapierre and Christian Coulon are thanked for their constructive reviews of the manuscript. XRF and radiogenic isotope laboratories at Royal Holloway are London University intercollegiate facilities.
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FOOTNOTES |
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Present address: Graduate School of Oceanography, Rhode Island University, Narragansett, RI 02882, USA 
* Corresponding author. Telephone: +44 116 2523639. Personal fax: +44 116 2523639. Department fax: +44 116 2523918. e-mail: ack2{at}le.ac.uk
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