Journal of Petrology Volume 41 Number 7 Pages 1177-1194 2000
© Oxford University Press 2000
A Post K–T Boundary (Early Palaeocene) Age for Deccan-type Feeder Dykes, Goa, India
1DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK
2SURRC, EAST KILBRIDE, GLASGOW G75 0QU, UK
3DHEMPE COLLEGE, PANJIM, GOA, 403 001, INDIA
Received November 1, 1999; Revised typescript accepted March 29, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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Basaltic dykes exposed along the coast of Goa represent the youngest phase of a number of different mafic suites intruding the complex Precambrian terrain that forms the pre-Deccan basement of peninsular India. These dykes crop out
50–80 km beyond the southern limits of the Deccan continental flood basalt (CFB) province. Four of seven sampled dykes were dated using the whole-rock 40Ar/39Ar step-heating technique. Three 40Ar/39Ar determinations yield Palaeocene plateau and isochron ages that clearly indicate an early post-Cretaceous (i.e. Danian) age and, as a weighted mean average of all four samples yields an age of 62·8 ± 0·2 Ma, these intrusions are clearly significantly younger than the current estimates for the age range of Deccan Traps volcanism (65–69 Ma). The trace element signatures, rare earth element patterns and 87Sr/86Sr ratios of these dykes are consistent with those of the nearby Deccan basalts. More specifically, their geochemical signatures are similar to those of the basalt flows composing the volumetrically important Ambenali–Mahabaleshwar Formations, which form the uppermost succession (Wai Subgroup) of the SW Deccan. These geochemical and age data demonstrate that Deccan-type magmatism continued to affect western India for at least 1–2 my after the K–T boundary (65·0 ± 0·1 Ma) and, therefore, their emplacement places important constraints upon the timing, duration and evolution of this much-debated CFB province. The importance of these dykes is examined within the context of the Deccan eruptions and the associated rifting of the western Indian margin. KEY WORDS: K–T boundary; Deccan Traps; 40Ar/39Ar dating; dykes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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Basic dyke swarms are common in western peninsular India and their existence has long been documented [see Auden (1949)
, and Murthy (1995) for a review]. The Precambrian terrain of peninsular India has been subject to a series of dyke injection phases which, although widely separated in time, have nevertheless utilized inherent structural and compositional heterogeneities developed within the Archaean and Proterozoic rocks that together comprise the pre-Deccan ‘basement’. Application of palaeomagnetic and radiometric dating has resolved some of the questions regarding age and timing of different dyke swarms in SW peninsular India, indicating that the older dyke swarms are of Precambrian age whereas the younger are related to late Mesozoic tectonism. The current work concentrates on examples of dykes exposed along the coast of Goa between 15 and 16°N (Figs 1 and 2), where it is also clear from cross-cutting field relationships that there have been several phases of dyke injection. Here, the younger cross-cutting dyke suites have been loosely attributed to the complex Mesozoic rift history of western India (e.g. Sinha-Roy & Radhakrishna, 1983); this involved primarily the splitting of India–Madagascar at 89 Ma (Storey et al., 1995) and the subsequent late Cretaceous–early Tertiary separation from India of the Seychelles microcontinent (Devey & Stephens, 1991).
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During the Mesozoic the western part of the Indian craton was subject to two major rifting events beginning with the splitting of Madagascar and, 20–25 my later, the separation of the Seychelles–Mascarene microcontinent. The latter was intimately associated with the arrival of the Reunion plume beneath the northwestern margin of the Indian continent, and the associated eruption of the Deccan continental flood basalt (CFB) province (65–67 Ma). On the basis of geochemical, structural and stratigraphical arguments, the earliest basalt eruptions are considered to have occurred in the northwestern part of the main province (i.e. Nasik–Narmada region), with later lava successions building on the southern flank of the volcanic edifice as India moved northward over the hotspot (Fig. 3; Cox, 1983; Mitchell & Widdowson, 1991). In post K–T times the hotspot, much reduced in its activity, emerged from beneath the attenuated and rifted western continental margin (15°N), manifesting itself as a chain of progressively younger minor volcanic edifices forming the Maldive–Laccadive ridge, and culminating in the currently active volcano of Piton de la Fournaise (Reunion Island) located south of the Indian Ocean spreading ridge (Duncan, 1990).
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Dyke systems with a coast-parallel NNW–SSE strike are well documented intruding the Deccan basalts in the Bombay (Mumbai) region (Deshmukh & Sehgal, 1988; Hooper, 1990). In the northern Deccan an extensive ENE–WSW trending swarm occurs in the Narmada and Tapti river valleys (Sethna et al., 1996; Melluso et al., 1999), and a swarm of randomly orientated dykes occurring in the Nasik region is thought to represent the focus of a major vent system (Deshmukh, 1988). Many of these mafic dykes are considered coeval with emplacement of Deccan lavas as they are similar in composition to the adjacent lava sequences (e.g. Devey & Stephens, 1991; Bhattacharji et al., 1994; Melluso et al., 1999). By contrast, dykes are very uncommon in the SW Deccan region and, with the exception of a Poladpur-type feeder-dyke (Devey, 1986) identified near Chiplun, few have been documented and none of these have been radiometrically dated. The current work provides 40Ar/39Ar age determinations for the youngest of the Goan suites and provides detailed geochemical characterization of these dykes, together with a comparison with the nearby SW Deccan basalt succession (Widdowson et al., 1998).
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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The small coastal state of Goa (
3700 km2) lies between the states of Maharashtra and Karnataka on the west coast of India (Fig. 1). Some tens of kilometres to the north and east lies the southern margin of the immense Deccan Traps CFB province (500 000 km2), the main body of which (Fig. 1) extends from the Arabian Sea coastline toward Hyderabad and Nagpur (78–79°E) in the east, and northward from Belgaum to the Namada valley (22–23°N). The succession of the Deccan basalt lavas of this region has been the subject of much detailed chemostratigraphical study (e.g. Cox & Hawkesworth, 1985; Beane et al., 1986; Devey & Lightfoot, 1986; Khadri et al., 1988; Subbarao et al., 1994; Mahoney et al., 2000), and the structure and stratigraphy of the province is now well established (Table 1). The stratigraphy of the southern region is based upon basalt chemical characteristics, themselves resulting from a combination of source variation, fractional crystallization and degree of crustal contamination between successive magma batches during Deccan CFB activity (Table 2). These secular changes in magma chemistry provide the basis for a stratigraphical and structural interpretation, and have been independently verified by subsequent palaeomagnetic studies (Vandamme & Courtillot, 1992).
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In the southern Deccan the basalt succession can be considered essentially lensoid in character with the greatest thicknesses exposed inland of the coast along the Western Ghats escarpment. The succession thins noticeably eastward and southward and displays a southerly overstep of older formations (Watts & Cox, 1989; Mitchell & Widdowson, 1991), with the stratigraphically youngest basalts occurring in the region of Belgaum (Fig. 3). These latter flows cap the Western Ghats escarpment and elevated plateau region inland of Goa.
The geology of Goa itself is complicated and consists of a wide range of lithologies that are often loosely termed ‘pre-Deccan basement’. This consists of a complex combining Archaean Peninsular gneiss and the Dharwar supracrustals of the Goa Group. The latter comprises metabasalts (Barcem Fm); metagreywackes (Sanvordem Fm); phyllites with banded haematite quartzites and Mn-rich clays (Bicholim Fm); and metagreywackes with metavolcanics (Vageri Fm). This Precambrian complex is, in turn, intruded by Early Proterozoic granitic masses (Dhoundial et al., 1987) derived from the partial melting of the gneisses, and also by a number of smaller basic and ultrabasic bodies. Inland of the coast, field relationships are often obscured by Tertiary laterites and modern weathering products (Widdowson & Gunnell, 1999) or, most commonly, by thick scrub jungle. As the dykes intrude this Precambrian basement, all previous estimations of their actual ages have necessarily been speculative in the absence of any other constraining field relationships or radiometric dating.
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SAMPLING, PETROGRAPHY AND ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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Dykes are best exposed along the coast at the rocky headlands dotted along the length of Goa (Fig. 2). Inland, examples may be found in roadstone quarries and road cuts, and additional exposures have become available as a result of the recent construction of the Konkan Railway. Dyke samples IND2, -3, -5, -125 and ORL2, -4, -5 (Table 3) were identified as belonging to the ‘younger’ suite from the cross-cutting relationships observed in the field, and are intruded into the metagreywackes and argillites of the Sanvordem Fm along the coast between 15°30' and 15°45'N. These dykes are 5–12 m thick and, with the exception of ORL2, follow the well-defined NW–SE trend typical of the many dykes that crop out further north along the Arabian Sea coastline. The more southerly examples, from near Aguada Fort headland and Baga headland, cut across a suite of Proterozoic age dykes (M. Widdowson & M. S. Pringle, unpublished 40Ar/39Ar data, 1996) at a very low angle, and the two suites run in contact for some distance, indicating the long-term control exerted by the NNW–SSE regional Dharwar structure (Widdowson & Mitchell, 1999
). Field relationships and apatite fission-track analysis (AFTA) modelling (M. Widdowson & A. J. Hurford, unpublished data, 1996) indicate that these dykes were injected at shallow crustal levels, probably just beneath an original basalt lava–basement contact.
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Sampling of coastal dykes was performed on cliff outcrop above the limit of high tide, thus minimizing possible marine alteration effects. Importantly, the Goan coast cliff sections permit sampling beneath the weathering front and so exposure is largely free from the alteration products characteristic of many inland outcrops. Care was taken to extract dyke material from within the body of intrusion, thus minimizing any surficial subaerial alteration effects, and, following visual inspection, any material displaying weathering discoloration was rejected.
In thin section, all the dykes are medium-grained dolerites typically displaying sub-ophitic glomerocrysts of plagioclase and augite, minor olivine, and an absence of any glassy mesostasis. IND5 is a coarser-grained example with phenocrysts of plagioclase, olivine and subordinate augite. Higher iron contents of IND2, -3, -5 and ORL2 indicate these to have been tapping a more fractionated magma, extrusive Deccan analogues of which were discussed by Cox & Mitchell (1988). ORL4 and -5 are less evolved, more Mg rich, and relatively low in opaque phases. In all samples petrographic study indicates alteration is minimal, an observation supported by the low loss on ignition (LOI) values (Table 3). The plagioclase feldspar is clear and without any turbid patches or alteration along the lamellar twinning, and careful inspection of the other major phases reveals that only olivine phenocrysts display minor alteration (i.e. iddingsite) along crystal fractures.
Geochemical preparation involved cutting slabs 1 cm thick from each sample block, followed by a further check of the freshly cut faces for any evidence of alteration. Selected material was then prepared, using an agate mill, for both X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) analysis. Major and trace element analyses were carried out at Oxford and the Open Universities by XRF on fused glass discs and pellets, respectively, and according to standard methods outlined by Potts (1987). All XRF analyses were performed incorporating USGS standards AGV-1, BCR-1 and BHVO-1 as unknowns in the sample runs and, in addition, previously characterized Ambenali, Mahabaleshwar and Panhala basalt chemotypes were similarly included. This latter precaution provided a cross-check with previously classified Deccan basalt sample suites, thus ensuring that the trace element abundances and ratios identified for the Goan dykes were analytically consistent with those original analyses used in the determination of the stratigraphical chemotypes. Error in reproducibility, based upon these replicate analyses, was <5% for most elements and all the key chemostratigraphical ratios [further details of these cross-check procedures have been given by Devey (1986), Mitchell (1988) and Widdowson (1990)].
A suite of trace element concentrations, including rare earth elements (REE), were determined by ICP-MS at Durham University. Powders were dissolved using a standard HF–HNO3 technique, with care taken to avoid production of any fluoride residue. Samples were then spiked with Rh, In and Bi before dilution to 3·5% HNO3 to monitor internal drift, and the solutions analysed on a Perkin–Elmer–SCIEX Elan 6000 instrument using a cross-flow nebulizer. Calibration was achieved using both matrix-matched international (i.e. USGS standards AGV-1, BCR-1 and BHVO-1) and in-house reference materials. Total procedural blanks for all elements were negligible for all analyses. Reproducibility, based upon replicate digestions of standards and samples, varied between 1 and 3% for most analyses. In addition, typical Ambenali and Panhala basalt chemotypes S5 and KP13 (Widdowson, 1990) were prepared, together with the dyke materials. These were then included in the same sample run to act as an additional analytical datum against which the abundances and detailed REE fingerprints of the dykes could be directly compared (Table 3).
87Sr/86Sr ratios were determined in static multicollector mode on a Finnegan MAT 261. Sr fractionation was corrected to 86Sr/88Sr = 0·1194, and the measured isotope ratios were normalized with respect to internally determined values for 86Sr/88Sr in NBS 987 = 0·710170 (± 21). Quoted errors are 1 SD, and errors on individual runs are significantly less than the quoted reproducibility, whereas blanks were typically <0·5 ng. Sr isotope ratios were age corrected to 60 Ma to permit direct comparison with previous analyses used for determining the stratigraphical criteria. In addition, previously characterized basalt samples KP2, -4 (Ambenali Fm) and KP5 (Mahabaleleshwar Fm) were prepared and run in the same analytical batch to ensure consistency with previous analyses conducted at Oxford University (Mitchell, 1988). These replicate analyses were within <0·00012 of previously determined values.
The 87Sr/86Sr results for the dykes are entirely consistent with the more radiogenic examples falling within the ranges observed for Deccan basalts (Table 4). However, Price et al. (1997) noted a coastal sill exposure in East Greenland having elevated 87Sr/86Sr (by 0·00030) through sea-water alteration. Accepting the possibility that undetected marine alteration might have affected the Goan coastal dyke exposures despite our stringent sampling procedure, shifts of such a small magnitude would not materially affect the stratigraphical comparisons and conclusions presented here, as these are based upon a much larger natural range of 87Sr/86Sr values observed in Deccan basalts (Table 4).
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GEOCHEMICAL COMPARISONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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Along the length of the Deccan coastal plain (i.e. Konkan) and in Goa state, the eastward recession of the Western Ghats scarp has effectively removed any original basalt overburden that may have existed, together with some of the basement rock from beneath the basalt–basement unconformity (Widdowson & Cox, 1996
; Widdowson, 1997). Consequently, any physical links between dykes and lavas that may originally have existed in Goa can no longer be demonstrated in the field. However, chemostratigraphical studies (Devey & Lightfoot, 1986; Mitchell & Widdowson, 1991) demonstrate that within the SW Deccan the basalt flows occurring closest to these dykes belong to the upper part of the Wai Subgroup (Table 1) and, more specifically, those exposures neighbouring Goa consist of successions of the Ambenali, Mahabaleshwar and Panhala Formations, and are exposed on the coastal plain north of Goa, or cap the Ghats escarpment to the east. The trace element and isotopic criteria commonly used to define these different Deccan basalt formations (Table 2), together with REE patterns, provide a method of comparing the composition of the Goan dykes with that of these geographically adjacent basalt formations.
Trace element and Sr-isotope criteria
Within the basalt succession the geochemical characteristics of individual flows toward formation boundaries, as defined on trace element criteria, are commonly transitional between those of the two bracketing chemotypes. For instance, Mitchell & Widdowson (1991) noted that Ba concentrations begin to rise 300 m below the upper boundary of the Ambenali Fm, and that nearer the boundary the observed sharp changes in Zr/Nb and Ba/Y ratios, Sr concentration and 87Sr/86Sr ratio do not all occur at precisely the same elevation (i.e. flow unit). This may be the result of interdigitation of flows during the changeover between eruption of different magma batches, or else may represent a more fundamental mixing of magma at depth during this transitional phase. To simplify matters, Devey & Lightfoot (1986) took 87Sr/86Sr values as the principal criterion for defining formation boundaries in the southern Deccan (Table 2). The relatively uncontaminated Ambenali Fm has 87Sr/86Sr values that fall within the narrow range of 0·7038–0·7056 (the majority of analyses falling within the 0·7040–0·7045 range; Lightfoot, 1985). The more radiogenic end-members that occur at the base of the Ambenali succession reflect the changeover from elevated Poladpur values and, to a lesser extent, those at the top of the formation indicate the increasing influence of crustal contamination within the overlying Mahabaleshwar Fm (i.e. 0·7045–0·7060; Cox & Hawkesworth, 1984). The Panhala Fm caps the inland succession at its southernmost edge, but its characteristically high Zr/Nb ratio (typically 14–18), low Ba, Sr (Table 4), and low 87Sr/86Sr (0·7045–0·7055; Lightfoot, 1985) are not comparable with those values determined for the dyke data. This suggests that if possible matches between dykes and basalt flows are to be investigated, they should be sought lower in the basalt (i.e. Wai Subgroup) succession.
Table 3 shows that the IND samples are characterized by intermediate Sr content (203–205 ppm) and elevated Ba (100–115 ppm), which are consistent with concentrations found in the Poladpur, Ambenali (Sr), and the Poladpur, Ambenali and Mahabaleshwar Fms (Ba). Relatively low Ba/Y ratios (2·8–3·2) are more typical of the Ambenali chemotype, although IND2 displays both elevated Ba and Ba/Y ratio more consistent with that of the Mahabaleshwar Fm, an interpretation supported by its elevated K, Rb and Zr contents. By contrast, IND3 has a less contaminated composition, with lower K, Rb and Zr more consistent with typical Ambenali Fm characteristics, but displays relatively high 87Sr/86Sr. IND5 displays a typically Ambenali Fm Zr/Nb ratio and, in common with IND125, Ambenali-like Ba/Y and Sr values, but more elevated Mahabaleshwar-like K and Rb contents. Samples ORL4 and -5 both display trace element and trace element ratios typical of the Ambenali Fm, which, with the exception of a relatively low Sr (128 ppm) for ORL5, fall within the analysed ranges for this formation (Table 4). By contrast, ORL2 has Ba, Sr and K contents elevated beyond those of the Poladpur, Ambenali and Panhala Fms, which, together with a high Ba/Y ratio, are most consistent with the more enriched examples seen in the Mahabaleshwar Fm analytical range (Tables 3 and 4). However, this sample displays a Zr/Nb ratio (14·0) that is elevated beyond that observed in Mahabaleshwar Fm basalt samples and is more typical of Ambenali Fm values. These dyke data, together with those of the basalts S5 (Ambenali Fm) and KP13 (Panhala Fm), are summarized in Fig. 4a and b, together with the characteristic chemostratigraphical criteria.
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REE patterns
By contrast with the extensive major and trace element (XRF) datasets available for the SW Deccan region, REE data are comparatively sparse. Furthermore, where available, these REE data have largely been employed to help determine petrogenetic models for Deccan basalts, rather than as a means of distinguishing stratigraphical characteristics (Hooper, 1994
). However, as REEs are sensitive indicators of petrogenetic processes, and given that the trace element stratigraphical criteria (Table 2) are themselves based upon secular variations in these processes, differences between the REE patterns of successive chemostratigraphical formations might be expected.
Deccan basalts typically display no Eu anomaly and, in general, the REE patterns of those formations forming the Wai Subgroup are similar, reflecting broadly comparable degrees of melt fraction. Moreover, whereas there can be considerable variation in the ranges of REE concentrations between different basalts of the same formation, the Poladpur, Ambenali and Mahabaleshwar are all characterized by a light rare earth element (LREE) enrichment with relatively flat heavy rare earth element (HREE) patterns. Nevertheless, subtle variations relating to source composition and differing degrees of crustal contamination may also be identified within these general REE patterns (Lightfoot, 1985).
Figure 5a–c shows typical REE patterns of a series of Poladpur, Ambenali, Mahabaleshwar and Panhala basalts from the Wai Subgroup of the SW Deccan. The steeper patterns correspond to Deccan chemotypes such as the Mahabaleshwar Fm (Ce/Yb 10–19) and Poladpur (Ce/Yb 9–12), both of which have experienced an increased degree of crustal contamination relative to the Ambenali Fm. In turn, the less steep (Ce/Yb 7–11) Ambenali patterns (e.g. S5) can be readily distinguished from near-flat REE patterns (Ce/Yb 6–8) belonging to Panhala Fm basalts (e.g. KP13), which are the flattest of all of the formation chemotypes (the unusual negative Ce anomaly in this Panhala sample is due to subaerial alteration). The Poladpur REE pattern is LREE enriched to a level intermediate between those of the Ambenali and Mahabaleshwar Fms, but can be readily distinguished from the Ambenali REE pattern as the latter commonly displays slightly elevated middle rare earth element (MREE) patterns together with slightly lower Ce and Pr and an inflection at Nd giving a distinctive ‘hump-backed’ pattern.
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).It is not the purpose here to discuss in depth the origin of the variations within Deccan REE patterns, but the patterns may be used in addition to the established trace element and isotopic criteria (Table 2) as a means of further determining whether the Goan dykes are chemically and petrogenetically related to the adjacent lava fields. Mahoney et al. (2000)
have successfully employed a database combining both trace element and REE variations as a method of determining stratigraphical characteristics of dykes and basalt flows in the northern Deccan region. Such an approach would clearly be of use in the current study area, but first requires development of a more extensive REE database for the SW Deccan basalts than currently exists. Nevertheless, much can be achieved by empirical comparison of the Goan dykes’ REE patterns (Fig. 6) with those of the basalts (Fig. 5a–c).
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, respectively) shown for comparison. Key to dykes: unshaded symbols; 
, IND2; 
, IND3; 
, IND5; 
, IND125. Shaded symbols: square, ORL2; diamond, ORL4; triangle, ORL5.
First, a broad comparison reveals that the dyke REE patterns are consistent with those determined for the basalts of the SW Deccan. In detail, the dyke analyses produce a series of sub-parallel LREE-enriched plots indicating a potential petrogenetic relationship to one another. ORL2 is clearly different, as its pattern is steeper and crosses those of the other dykes. Interestingly, although steeper, this pattern displays a low La/Nd ratio indicative of relatively low degree of contamination, which contrasts with its elevated Ba and Sr contents, together with a relatively elevated Dy/Yb ratio, which is often considered indicative of a higher degree of melting. Despite these differences, however, ORL2 also displays the inflection at Nd that is more typical of an Ambenali pattern. Visual comparison of the dykes’ REE patterns with Fig. 5a–c suggests that the dyke REE patterns, and particularly those of ORL4 and -5, are very similar to Ambenali basalt (e.g. S5), and hence those of the Ambenali Fm. However, further inspection reveals that IND2, -3, -5 and -125 all display a slight LREE enrichment compared with the Ambenali chemotype, but are less LREE enriched than those of the Poladpur and Mahabaleshwar Fms. Lightfoot (1985) noted that the Ce/Yb ratios are similar within each formation, and using the determined ranges (see above). It is clear that samples IND2, -3, -5 and ORL4 are consistent with Ambenali Ce/Yb ratios whereas the elevated Ce/Yb ratio of IND125, and especially ORL2 and ORL5, are more typical of ratios characteristic of the Mahabaleshwar Fm (Table 3 and 4). Importantly, any link between the dykes and the Poladpur basalts can effectively be discounted on the basis of the relatively low Zr/Nb and Ba/Y ratios discussed above.
A detailed comparison of dyke and basalt REE patterns is best summarized in Fig. 7a and b, where it becomes apparent that the greatest similarity is achieved between the dyke signatures and those of the Ambenali Fm. By contrast, a similar comparison between dykes and basalts of the Mahabaleshwar Fm reveals an increasing relative depletion toward the LREE. Nevertheless, these dykes cannot be considered pure Ambenali-type patterns, as they display REE concentrations that are generally slightly lower in the MREE, and more elevated in the HREE, both of which are characteristics more typical of the Mahabaleshwar signature. Importantly, Lightfoot (1985) noted that whereas the switch from Ambenali to Mahabaleshwar magma types within the basalt sequence is sharp in terms of 87Sr/86Sr ratios, there is a continuum in trace element and REE variation within the Mahabaleshwar, which may reflect mixing between Ambenali and Mahabaleshwar magma types. This latter observation may aid in explaining the nature of the dyke chemistry.
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Goan dykes as feeders to Deccan basalts
The extrusive products of CFB volcanism are thought to be supplied at the surface by fissure eruptions, which, in turn, are fed at depth by dyke complexes. However, dyke injection can also occur during the later stages of continental rifting, such as the separation of the Seychelles from India at
65 Ma (Hooper, 1990), and may not necessarily be directly associated with earlier, largely plume-related volcanism preceding the rifting event. However, the geographical proximity of the Goan dykes and Deccan basalts, the large dyke width at shallow crustal levels and, most crucially, the obvious chemical similarity between basalts and dykes make a compelling argument for interpreting these dykes as part of the feeder system that originally supplied the lavas of the Wai Subgroup. On the other hand, the alternative interpretation of passive, rift-related emplacement would require the production of temporally discrete, but otherwise chemically and isotopically indistinguishable magmas to generate lavas and dykes separately (Widdowson & Mitchell, 1992).
Thus, accepting that dykes and basalts are petrogenetically related on the basis of their trace element, REE data and 87Sr/86Sr ratio similarities, then the ‘hybrid’ nature of the IND samples and ORL2 requires further explanation. It could have arisen by two different processes. First, the dykes may represent relatively short-lived feeders that supplied only a discrete part of the extrusive succession; in this case, that part characterized by the changeover of one formation chemotype to the next (i.e. Ambenali to Mahabaleshwar Fms). Alternatively, they could represent long-lived conduits, which display the Ambenali to Mahabaleshwar ‘hybrid’ characteristics by virtue of the fact that they tapped different magma chemotypes during their lifetime (i.e. they provided conduits for both Ambenali and Mahabaleshwar magmas). In this latter case, the dyke material would represent a ‘within-conduit’ mixture of the two chemotypes. However, given that mixing trends and changes from one formation chemotype to another are thought to have their origin in deep-seated processes (Lightfoot, 1985; Lightfoot & Hawkesworth, 1988) rather than during very shallow-level dyke–conduit interaction, the short-lived conduit model seems more appropriate.
To summarize, the major and trace element, REE and 87Sr/86Sr data for the Goan dykes are chemically indistinguishable from those of the main extrusive Deccan CFB succession (Table 2). It is therefore reasonable to assume that these dykes were indeed conduits that originally supplied the Deccan basalts, and the more proximal outcrops of the basalt succession are the remains of the lavas they once fed. Therefore, the most probable match for ORL4 and -5 is with the Ambenali chemotype. The IND samples and ORL5 would then be compatible with an Ambenali–Mahabaleshwar transitional chemotype, given that they all display both Ambenali and Mahabaleshwar Fm characteristics. On the basis of 87Sr/86Sr ratios IND5 is most consistent with the uppermost Ambenali, and IND2, -3 and -125 with the lower flow packages of the Mahabaleshwar Fm. However, IND2, and especially IND3, display particularly radiogenic 87Sr/86Sr ratios which, although within the range of Mahabaleshwar values, are more typical of those characteristic of the Desur unit (87Sr/86Sr 0·7073–0·7081) described locally as part of the Mahabaleshwar-type succession of the Belgaum region by Lightfoot & Hawkesworth (1988). These workers also suggested that variations in magmatic plumbing may be responsible for the subtle changes in Mahabaleshwar-type isotope and trace element ratio characteristics observed between the southernmost Deccan basalts (i.e. those exposed inland of Goa), and the stratigraphically equivalent successions exposed farther northward.
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40Ar/39Ar AGE DETERMINATIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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Whole-rock 40Ar/39Ar age determinations were conducted on four of the dykes (IND2, -3, -5 and -125). By comparison with lavas, which may have lain exposed to weathering after eruption, the intrusive nature of dykes makes them less susceptible to alteration. Samples for dating were extracted from the central, coarser-grained core region of dykes, avoiding the finer-grained chilled margins to minimize the effects of excess argon resulting from incomplete degassing of the magma and interaction with the host wall-rock.
During preparation, slabs of 1 cm thickness (IND2A, -3A, -5A, -125A) were cut from selected sample blocks. Thin-section analysis confirmed negligible alteration, and provided further assistance in selecting the freshest areas of the slabs. Small cores 5·5 mm in diameter were drilled with a water-cooled diamond drill bit, and thin wafers less than 1 mm thick (80 mg each) were taken from the cores. These wafers were sealed in air in quartz vials and irradiated for 16 h in the CLICIT facility of the Oregon State University (USA) TRIGA reactor. The neutron flux was monitored with the 27·92 Ma USGS standard sanidine 85G003, equivalent to an age of 27·62 ± 0·02 Ma for Fish Canyon Tuff sanidine (Renne et al., 1998; M. S. Pringle, unpublished data, 1998). 40Ar/39Ar step-heating analyses were performed at the NERC Argon Isotope Facility at SURRC (Scottish Universities Research and Reactor Centre) using a very low blank double vacuum resistance furnace for the whole-rock step-heating experiments and a CW Nd–YAG laser for the sanidine total-fusion analysis. Extraction techniques employed, and subsequent data reduction, follow those outlined by Pringle (1993) and Singer et al. (1999). Ages were calculated using age spectra and isochron analyses (Figs 8 and 9, respectively); errors are reported as 1 SD of analytical precision. Results are summarized in Table 5.
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40Ar/39Ar incremental heating experiments are a particularly powerful method of deciphering reliable crystallization ages from basaltic rocks, and many of the associated pitfalls in interpretation can be avoided by following the criteria outlined by Pringle (1993) and Singer et al. (1999). ‘Plateau’ ages for the four samples studied here included 38–90% of the argon released (Fig. 8), with the majority of these incremental-heating steps revealing ages of <65 Ma. These ‘plateaux’ do not necessarily meet all the criteria described by Pringle (1993) and Singer et al. (1999), and it has long been known that mafic dykes intruded into significantly older country rock may inherit excess argon from the magma and radiogenic argon sweated from the country rock (Dalrymple et al., 1975; Lanphere & Dalrymple, 1976). Thus, it is not surprising that all four dyke samples show some indication of excess argon, with 40Ar/36Ar intercepts significantly greater than the atmospheric ratio of 295·5 (Fig. 9). For three of the samples, IND2A, -3A and -125, this excess argon component is detectable only in the non-‘plateau’ steps (open squares, italic type, Fig. 9), and it appears that the detailed step-heating experiments have, therefore, successfully separated the ‘excess’ and atmospheric components. For one sample, IND5A, the plateau steps (filled squares, bold type, Fig. 9) also have an isochron intercept significantly greater than 295·5.
The individual ages reported here are not as robust as experiments on fresh, subaerial lavas, but such unaltered materials are rare in the Deccan. However, we contend that the major conclusion reached herein, that the Goan dykes are significantly younger than the main phase of Deccan volcanism, is nevertheless robust for the following reasons: (1) for igneous samples with demonstrable excess argon, Lanphere & Dalrymple (1976) concluded that both the age spectra saddle and isochron analysis represent a maximum age estimate; (2) moreover, even with the excess argon demonstrated here, the total gas ages of three of the four dykes are still no older than 65 Ma; (3) whereas it is theoretically possible for a combination of low-temperature alteration, argon recoil and excess argon to rotate an isochron to a younger apparent age and higher 40Ar/36Ar intercept, the range in K/Ca seen in different steps on the same ‘plateau’ would make such a scenario highly unlikely. The best estimate of the crystallization age of these Goan dykes is then the weighted mean of all four samples, 62·8 ± 0·2 Ma. Importantly, both the isochron and plateau ages indicate that emplacement of these dykes occurred during the early Tertiary (Danian), at least 1–2 my after the main phase of extrusive Deccan volcanism [65·0 ± 0·2 Ma relative to the standard values used here (Duncan & Pringle, 1991; Baksi, 1994)] and the K–T boundary at 65·0 ± 0·1 (Gradstein & Ogg, 1996), or 64·81 ± 0·03 Ma based on K–T boundary tektites analysed during the course of this study.
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A COMPARISON OF DECCAN 40Ar/39Ar DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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40Ar/39Ar data for basalt samples from previous stratigraphically constrained studies (Duncan & Pyle, 1988
; Duncan & Pringle, 1991; Venkatesan et al., 1993; Baksi, 1994) and those from the current work are compared in Fig. 10. The 40Ar/39Ar data for the Goan dykes are shown as stratigraphically analogous to flows displaying Ambenali–Mahabaleshwar transitional characteristics.
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Duncan & Pyle (1988) reported no significant difference in age between samples from the lower and upper parts of a composite Deccan section, thus lending support to the rapid eruption rates advocated on the basis of palaeomagnetic data (Courtillot et al., 1986). Consequently, the main volcanic pulse is considered to have occurred during chron 29R (Courtillot et al., 1988; Gallet et al., 1989), with the transition to normally magnetized directions occurring within the upper part of the Ambenali Fm in the SW Deccan. In recent years there have been significant reductions in 40Ar/39Ar analytical error as a result of improvement in technique and equipment, and the later work of Venkatesan et al. (1993) using a similar composite section did yield older ages at the base (67–68 Ma) and, significantly, post K–T ages toward the top (62–63 Ma). The youngest ages were given by two samples from stratigraphically higher parts (i.e. Mahabaleshwar–lowermost Panhala Fms; Fig. 10), although this particular interpretation was challenged by Feraud & Courtillot (1994). Interestingly, Venkatesan et al. (1993) also suggested that younger ages may also be typical of the succession from the topmost Poladpur Fm upwards, and that part of the overlying Ambenali and Mahabaleshwar Fms may significantly post-date the early Deccan eruptions exposed in the north of the province. Younging in the uppermost basalt succession is also supported by a single 40Ar/39Ar age determination reported by Duncan & Pringle (1991) from the SW Deccan. In their stratigraphical context, the current dyke data clearly support this latter view and, together with the smaller reported errors, demonstrate that Ambenali–Mahabaleshwar-type magmatism continued into the early Tertiary. Moreover, recent palaeomagnetic data (Rao & Patil, 1998) for Goan dykes exposed south of the current study area yield a reversed polarity consistent with the Deccan ‘super pole’, whereas the Aguada Fort dykes (see IND125) yield opposite polarity, which would be consistent with the documented transition to normal polarity recorded within the upper Ambenali and lower Mahabaleshwar Fm flows of the nearby SW Deccan succession (Wensink, 1973).
Venkatesan et al. (1993) also speculated that unknown thicknesses of the volcanic succession may have been eroded since volcanism ceased and, therefore, that the duration of Deccan eruption may have been even longer than previously supposed. More recently, Widdowson & Cox (1996) have demonstrated that the topmost basalts exposed in the SW Deccan do represent those lavas that lay at, or close to, the top of the original volcanic pile, and so arguments regarding any ‘lost’ levels of the southern Deccan succession should now be discounted. Moreover, continued chemostratigraphical mapping of the province has revealed that the chemotypes belonging to the Wai Subgroup (i.e. Poladpur, Ambenali and Mahabaleshwar Fms) are particularly widespread (Fig. 11), and not simply restricted to the SW Deccan region where they were first defined. For instance, Ambenali- and Poladpur-type flows occur across to the eastern edges of the Deccan lava field beyond Gulbarga (Mitchell & Widdowson, 1991) and Nagpur (Bilgrami, 1999), and are documented as far north as Buldana, Jabalpur and Mandla (Peng & Mahoney, 1995; Peng et al., 1998) and the southern edge of the Narmada valley (e.g. Toranmal section–Mahoney et al., 2000). Similarly, Mahabaleshwar-type basalts are found NE of the central Deccan near Buldana (Subbarao et al., 1994), and also capping the successions in the extreme SE near Vikarabad (M. Widdowson & T. F. D. Mason, unpublished data, 1999). In effect, the lavas of the Wai Subgroup represent the most widespread and volumetrically significant episode of Deccan eruptive history. In this context, the current 40Ar/39Ar data place important constraints on the timing and duration of the Deccan activity because they extend the pulse of the Ambenali–Mahabaleshwar magma-type beyond the K–T boundary.
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Hooper (1990, 1999) argued that much of the Deccan eruption occurred before rifting. This interpretation is consistent with the Goan dykes, as these appear to be supply conduits for the upper Wai lava sequence, rather than a later stage, rift-related passive emplacement. Nevertheless, the basalts forming the later stages (i.e. Wai Subgroup) of the Deccan CFB may have resulted from an increasing decompressive melt contribution concomitant with the crustal attenuation that was occurring immediately before rifting (White & McKenzie, 1989), and this may explain why such immense volumes of melt are represented by the lavas of Poladpur–Ambenali–Mahabaleshwar succession.
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SUMMARY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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The main points may be summarized as follows:
- the 40Ar/39Ar age data of the Goan dykes reveal significantly younger (Palaeocene) ages when compared with those currently accepted for the nearby Deccan basalt succession.
- The dyke geochemistry is consistent with Deccan-type magmatic products and, specifically, with a transitional Ambenali–Mahabaleshwar magma type.
- The basalts of the Wai Subgroup represent the thickest and most geographically widespread lavas of the entire Deccan succession, and therefore represent a major eruptive pulse during the lifetime of the Deccan CFB province. If part of the Ambenali Fm, together with the ensuing Mahabaleshwar and Panhala Fms were erupted after the K–T boundary, then current thinking regarding the timing and duration of the Deccan episode requires reappraisal.
- The geochemical and 40Ar/39Ar data provide important constraints regarding the timing of Deccan eruptions, the associated crustal attenuation, rifting and separation of the Seychelles microcontinent.
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ACKNOWLEDGEMENTS |
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Fieldwork and isotope analyses for this research were funded by NERC grants GR9/963 and IP/428/0994. We also thank Keith Parish and John Watson for assisting with XRF analyses, Jo Rhodes for assisting with 87Sr/86Sr analysis, and Chris Ottley for helping generate the ICP-MS data. We are grateful for the encouragement and positive comment from Nick Rogers and Ian Parkinson during writing of this paper, and to Simon Kelley, Peter Hooper, Marjorie Wilson and Ray Kent for their constructive reviews. This research owes much to Keith Cox’s continuing fascination with the Deccan CFB province. The implications of the ‘young’ Deccan ages presented here were discussed with him over a beer or two during our last meeting in July 1998. As a result, the preliminary results were given at Goldschmidt ’98. His enthusiasm and encouragement for all aspects of Deccan study are now greatly missed.
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FOOTNOTES |
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*Corresponding author. Telephone: +44-1908-652986. Fax: +44-1908-655151. e-mail: m.widdowson{at}open.ac.uk
Extended data set can be found at: http://www.petrology.oupjournals.org
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDSAMPLING, PETROGRAPHY AND...GEOCHEMICAL COMPARISONS40Ar/39Ar AGE DETERMINATIONSA COMPARISON OF DECCAN...SUMMARYREFERENCES |
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