Journal of Petrology Volume 41 Number 7 Pages 1057-1069 2000
© Oxford University Press 2000
Olivine Compositions in Picrite Basalts and the Deccan Volcanic Cycle
11-10-153 SARDAR PATEL ROAD, XRF & EMP LABORATORY, ATOMIC MINERALS DIRECTORATE FOR EXPLORATION AND RESEARCH, DEPARTMENT OF ATOMIC ENERGY, BEGUMPET, HYDERABAD 500 016, INDIA
2NATIONAL GEOPHYSICAL RESEARCH INSTITUTE, HYDERABAD 500 007, INDIA
3GEOSCIENCES RESEARCH DIVISION, SCRIPPS INSTITUTION OF OCEANOGRAPHY, LA JOLLA, CA 92093-0220, USA
Received September 2, 1999; Revised typescript accepted March 22, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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Olivine phenocryst compositions and whole-rock chemical compositions are used to identify primitive picrite basalts from widely separated parts of the Deccan flood basalt province. Overall, primitive picrites constitute a significant volume of rocks within the province. Most were probably emplaced along deep faults in the Cambay graben and Narmada rift regions. We combine mineral composition data on previously described samples from boreholes at Dhandhuka, Wadhwan and Botad with information on new finds of picritic basalts at Paliad, Anila, Pawagarh, Kawant and Ambadongar to help delineate the petrogenesis of these mafic rocks, and we also examine the nature and probable origin of picrite basalts from other regions of the Deccan, such as the Western Ghats. The combined data suggest that the incidence of high-MgO lavas decreased with time during the Deccan volcanic cycle.
KEY WORDS: Deccan Traps; olivine composition; picrite basalts; volcanic cycle
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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The petrogenesis of flood basalt lavas, particularly their often iron-rich character, was a long-term interest of Keith Cox (e.g. Cox, 1980
; Cox & Hawkesworth, 1985; Cox & Mitchell, 1988; Scarrow & Cox, 1995). Recognition that the earliest liquids produced during partial melting of peridotite are Mg rich, combined with the widespread but volumetrically minor occurrence of picritic rocks in flood basalt provinces, provides an important basis for the discussion of basalt genesis in these settings. In this study we examine picritic rocks from the Deccan flood basalt province from the point of view of their olivine phenocryst compositions, and use these mineralogical data to infer the mode of origin of these rocks and their place in the overall cycle of Deccan volcanism. This work builds on the earlier investigations of Krishnamurthy & Cox (1977), who examined picritic basalts from boreholes in the Cambay graben area (see Fig. 1).
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In the IUGS classification scheme (Le Bas, 1999) a picrite or picrite basalt contains 
12% MgO, <52% SiO2 and >3% Na2O + K2O. Picrobasalts have compositions intermediate between basalts and picrites as defined here. Thus in a suite of mafic rocks, a spectrum of compositions may occur, ranging from primitive picrite basalts to picrobasalts and basalts. Rocks with picritic chemical characteristics can arise in a variety of ways; for example, as primitive picritic liquids that are little-modified melts of upper-mantle peridotite, by accumulation of early formed olivines from such primary picritic liquids, or by accumulation of olivines from ‘normal’ basaltic magmas. Under equilibrium conditions, olivine compositions will reflect the composition of the magma from which they crystallize; thus the composition of olivine phenocrysts in picrites is a valuable clue to their petrogenesis. This is the approach used by Krishnamurthy & Cox (1977), who examined a suite of mafic lavas recovered from boreholes in Western India that were first described by West (1958). The present study is an extension of Krishnamurthy & Cox’s work to samples from additional occurrences of picritic rocks in the Deccan, collected in part during a joint project between the Physical Research Laboratory, Ahmedabad, and the Scripps Institution of Oceanography, to study Deccan basalts.
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FIELD RELATIONS |
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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Known occurrences of flows and dykes with picritic compositions within the Deccan province are shown in Fig. 1. Among these the volumetrically most significant are those encountered in the borehole sequences at Dhandhuka, Wadhwan Junction and Botad (Krishnamurthy & Cox, 1977
), and the surface occurrences in adjoining areas such as Anila and Paliyad. Taken together, these comprise a significant areal extent of picrite basalts. Perhaps of similar magnitude is the Pawagarh occurrence north of the Narmada River, although here the exposed thickness is minor (
75 m). However, a flow forming the base of Pawagarh Hill may be extensive; its dimensions are unknown because of alluvium and soil cover.
Picrite basalts and picrobasalts also occur within the thick Deccan basalt sequences of the Western Ghats. They have been reported from the Igatpuri, Neral, Thakurvadi, Bushe and Poladpur formations (see Fig. 1 for locations), constituting somewhat less than 10% of the total volume (Beane & Hooper, 1988). Some of these are porphyritic units in compound flow sequences showing clear evidence of crystal fractionation and accumulation (Mishra, 1971; Beane & Hooper, 1988; Deshmukh, 1988).
Picrodolerite dykes and picrite basalts also occur in southern Kathiawar. The dykes at Dedan (Fig. 1) are 10–15 m thick and can be traced over distances of >2 km. They exhibit clear flow differentiation features (Krishnamacharlu, 1972). The picrite basalt flows of southern Kathiawar (SK in Fig. 1) have only recently been described; they further enlarge the known area of occurrence of such rock types in the northern Deccan (Melluso et al., 1995). Picritic dykes, including ankaramites, are also found together with rocks of the carbonatite–alkaline complex in the Ambadongar and Kawant areas (Nageshwara Rao, 1975; Simonetti et al., 1995; present work).
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METHODS |
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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Olivine analyses were obtained using an automated CAMEBAX electron microprobe at the Scripps Institution of Oceanography. Standards included Smithsonian mineral standards and pure metal oxides. Based on repeated analyses of the Smithsonian San Carlos olivine standard (Jarosewich et al., 1980
), relative analytical uncertainty is <1% for the major elements and <16% for minor elements. Further details of the experimental methods have been given by Bloomer et al. (1982). |
RESULTS |
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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Olivine compositions and variations
Representative olivine compositions for samples from the various picrite localities are given in Table 1, and the ranges of MgO concentrations at each location are summarized in Fig. 2. Figures 3 and 4 provide further details of the range of compositions encountered, including CaO and NiO concentrations.
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The primitive picrite basalts sampled from the borehole cores at Dhanduka, Wadhwan Junction and Botad, as well as the surface samples at Anila, Paliyad (Saurashtra region), Pawagarh, Ambadongar and Kawant (Narmada region), all contain forsteritic olivine phenocrysts. The cores of these olivines exhibit limited chemical variability (Fo92–86; see Fig. 3) although the phenocryst rims are generally more iron rich. In a few cases the core-to-rim variability is large, such as in the sample from Kawant (ANK), where rim compositions reach Fo60. The compositions of groundmass olivines that we analysed from this primitive picrite basalt range from Fo84 to Fo72, which is within the range of phenocryst rim compositions observed.
The picritic flow from Botad quarry (sample SK/5/83 in Table 1) lacks the forsteritic olivines found in the Botad borehole sequence (e.g. sample 110 in Table 1) but exhibits considerable variation between phenocrysts (Fo83) and groundmass olivines (Fo64). Given the close proximity, this flow may be derivative from the more primitive lavas sampled from the borehole. A picritic dyke at Dedan shows very limited variation in olivine composition (Fo80–77) and appears to be an intrusion of a crystal-laden magma with clear flow differentiation features (Krishnamacharlu, 1972).
The picrite basalts and picrobasalts from the Western Ghats also lack forsteritic olivines, and they exhibit the maximum chemical variation observed among the analysed olivines. Compositions range from Fo84 to Fo43, with virtually this entire range observed in a single crystal from a flow at Igatpuri (sample IG-68, Table 1). Olivines from Kalsubai samples (e.g. KB-88, Table 1) show the least variation (Fo80–78) whereas those from Triambak (e.g. TRB-5, Table 1) have a somewhat larger range (Fo80–67). The data in Fig. 3 suggest that there are minor compositional gaps among the analysed samples, but these may simply be the result of the relatively small number of grains analysed. The wide range of olivine compositions observed cannot be in equilibrium with a single magma type. They probably reflect the enrichment of Fe in residual melts during crystal fractionation, or mixing of evolved and less-evolved magmas, or both. Intra-flow differentiation within a compound lava flow, as observed by Deshmukh (1988), might also be responsible for some of the large compositional variations.
Variations in minor element compositions
Minor and trace elements such as Ca, Mn and Ni show significant variations among the analysed olivines. There is a general positive correlation between Ni and forsterite contents (Fig. 4a), with NiO reaching as high as 0·4 wt % for the most forsteritic olivines, although there is a significant range in Ni at a given olivine composition.
CaO also shows wide variations. Forsterites from the primitive picritic basalts of Dhanduka, Wadhwan, Botad, Paliyad, Anila and Kawant in Saurashtra contain distinctly higher CaO (0·4–0·5% CaO) than those from Pawagarh and Ambadongar along the Narmada rift (0·25%; see Table 1). The Saurashtra flows are mildly alkalic and have higher CaO, TiO2, K2O and P2O5 contents than those from the Narmada rift (Table 2). There is also a negative correlation between CaO and forsterite content for the Saurashtra samples. In general, phenocryst rim and groundmass olivine CaO contents are higher for olivines from samples with higher whole-rock CaO contents. There is a conspicuous absence of olivines with <0·1% CaO, perhaps not surprising as such low CaO contents are characteristic of olivines from deep-seated plutonic complexes and ultramafic inclusions (Simkin & Smith, 1970). However, taken together with the lack of features such as kink bands, this suggests that the phenocrysts in the Deccan picrite basalts are mainly of low-pressure origin and are not xenocrysts.
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MnO contents in general are consistently low in forsteritic olivines (<0·1%), except for samples from Paliyad, Anila and Botad quarry (see Table 1). There is a general positive correlation between MnO and iron content.
Whole-rock chemistry and olivine compositions
Representative whole-rock analyses of picrite basalts from the Deccan province are given in Table 2. Computed equilibrium olivine compositions and the observed (maximum) forsterite contents within the studied samples or associated lavas are also listed.
Figure 5 shows mole percent forsterite in the analysed olivines vs mg-number of the host rock. These data can be used to examine the mode of origin of these picrites. A number of the samples with forsteritic olivine, such as those of Dhandhuka (D-12), Botad (B-6), Ambadongar and Kawant, show a fairly close correspondence between estimated equilibrium olivine compositions and those observed in the samples. This suggests a primitive status for these picrites. As noted above, phenocryst rim compositions are often much more Fe rich and therefore not in equilibrium, indicating evolution of the liquid compositions during petrogenesis of these rocks. In a few of the picrite basalts, olivine compositions show relatively minor deviations from the equilibrium field; for example, the reader is referred to the data for samples from Pawagarh (PB-39 and PB-52) and Wadhwan (W-1). The latter may have accumulated some forsteritic olivine as suggested by its high MgO content. Thus, broadly, these picrites must have formed from little-modified magma compositions, similar to those observed in anhydrous melting studies of upper-mantle peridotite.
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In contrast to those just described, the picrite basalts of the Western Ghats contain relatively iron-rich olivine phenocrysts, typically Fo80 ± 2. In some samples (e.g. IG-68 and TRB-5) the phenocryst compositions are close to estimated equilibrium olivine compositions, but in others there are large deviations (e.g. samples JEB 291, SAM-017 and JEB-116; see Table 2 and Fig. 5). In most cases these rocks probably acquired their high MgO contents by olivine accumulation.
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DISCUSSION AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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Until recently, known Deccan province localities with primitive picrite basalts were confined to the borehole sequence in Saurashtra described by Krishnamurthy & Cox (1977)
. Here we have described additional occurrences at Pawagarh, Ambadongar and Kawant, all to the north of the Narmada rift zone (see Fig. 1). In addition, the presence of surface outcrops in the vicinity of the borehole localities, for example, at Anila and Paliyad, as well as those reported by Melluso et al. (1995) in southern Kathiawar suggest that picrite basalt flows may originally have had a large areal extent in this part of the Deccan province. The geographic and stratigraphic positions of these rocks may be related to the overall operation of the Deccan volcanic cycle, similar to the situation outlined by Cox (1972) for the Karroo province.
Compositional diversity among the Deccan picrite basalts
The data in Table 2 suggest that, in terms of chemical composition, there are at least two groups of Deccan picrite basalts, and that there is some overlap between them. They include (1) a mildly alkalic or transitional type rich in TiO2 (>1·8% TiO2), K2O and P2O5, and (2) a tholeiitic type, poorer in TiO2 (<1·8%), K2O and P2O5. We will refer to these subsequently as Type 1 and Type 2. The former is mainly confined to areas north of the Narmada rift and west of the Cambay graben, e.g. Dhandhuka, Wadhwan, the Botad borehole sequence, Anila, Paliyad and Ambadongar. The second group occurs primarily in southern Kathiawar and the Western Ghats. The sample from Kawant is somewhat anomalous, having low TiO2 but high K2O and P2O5. Lavas of both types, as well as some that appear to be transitional, occur at Pawagarh (see Tables 2 and 3).
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Primitive picrites and derivative basalts
The search for parental magmas and an understanding of Deccan basalt petrogenesis has been a continuing task for many investigators (e.g. Krishnamurthy & Cox, 1977, 1980; Krishnamurthy & Udas, 1981; Beane et al., 1986; Lightfoot & Hawkesworth, 1988; Lightfoot et al., 1990; Melluso et al., 1995; Greenough et al., 1998). These studies have identified a variety of magma types, but among them tholeiitic and mildly alkalic lavas are the predominant groups. Distinct from these, and not considered here, are strongly alkalic types, which include the spinel peridotite bearing basanites and olivine nephelinites of Kutch [Krishnamurthy et al. (1999) and references therein], and the phono-nephelinites and carbonatites of Ambadongar (Simonetti et al., 1995).
Krishnamurthy & Cox (1977) showed that the bulk compositional characteristics and variability of the primitive picrite basalts from the borehole sequence at Botad, Dhandhuka and Wadhwan could be generated by fractionation of olivine and chromite from an Mg-rich parent. Fractionation of olivine + clinopyroxene from evolved picrite basalts could give rise to basalts of the mildly alkalic type. Krishnamurthy & Cox (1977) interpreted the significant degree of equilibrium crystallization inferred for many of the primitive picrite basalts in terms of compensated crystal settling. However, more recent work suggests that magmatic turbulence in hot, MgO-rich magmas during rapid emplacement may be a better explanation (Huppert & Sparks, 1985). The estimated MgO content for the magmas parental to the picrite basalts of the borehole sequence is at least 16%.
Picrite basalts and picrobasalts of the Western Ghats and olivine compositions
The nature of the parental magmas for the more common tholeiitic lavas that occur in the thick flow sequences of the Western Ghats and elsewhere in the Deccan is more problematic. However, picrite basalts from these sequences, and their olivine compositions, can provide some clues. Such rocks are found in several formations of the Kalsubai and Lonavala subgroups. These include, from base upwards, the Igatpuri, Neral, Thakurvadi, Bushe and Poladpur formations. Representative whole-rock and mineral analyses are given in Tables 1 and 2, and Figs 3 and 5. According to Beane & Hooper (1988), the olivine-rich flows constitute <10% of the total exposed volume of the lava sequence. Two important features emerge from the olivine composition and olivine–whole-rock data. The first is the absence of any samples bearing equilibrium forsteritic olivines, and the second is that for those few samples that do contain equilibrium olivines, even the most magnesian phenocrysts are relatively iron rich (Fo80 ± 2). The large differences in compositions between phenocryst cores and rims, or groundmass, are noteworthy.
Beane & Hooper (1988) used the compositions of olivine from mafic lavas of the Western Ghats to infer parental magma compositions. Some of their data are included in Tables 2 and 3. On the basis of the most magnesian olivine cores (Fo84) they analysed from a phenocryst-poor flow from the Thakurwadi Formation (see data for sample SAM-018 in Table 3), Beane & Hooper estimated the MgO content of the equilibrium liquid to be 10·3 ± 0·2%. According to those workers, such a liquid probably represents the most mafic primitive liquid supplied to the volcanic substratum in the Western Ghats. However, the most iron-rich olivine phenocryst cores from some Type I picrite basalts of the Narmada and Saurashtra regions reach Fo86, and thus are little different from the most magnesian values in the Type 2 samples from the Western Ghats, such as SAM-018. Olivine composition alone may not be a sufficient indicator of parental liquid composition, as source peridotite composition, as well as magmatic processes during fractionation and differentiation, also play a role in determining phenocryst compositions. Other mineralogical evidence may also be important; for example, the presence of magnesian ilmenite (Murari et al., 1993), or chrome–spinel inclusions in olivine (Krishnamurthy & Cox, 1977; Bell & Williamson, 1994). Chromite inclusions are present in some of the olivines analysed in this work. Sample KB-88 from Kalsubai has phenocryst olivines containing chromite with the approximate composition (in weight percent): Cr2O3, 44·11; Al2O3, 17·30; MgO, 10·96; FeO, 25·34; TiO2, 1·55.
Magma chamber processes and their influence on olivine compositions
It has been claimed by a number of workers that in the Western Ghats extensive gabbroic fractionation produced basalts with MgO contents in the range of 7% or less, and that an initial stage of olivine ± aluminous clinopyroxene accumulation resulted in samples with >7·5% MgO (Sen, 1988, 1995; Cox & Mitchell, 1988; Lightfoot et al., 1990). However, Cox & Hawkesworth (1985) questioned an accumulative origin (which had been based solely on olivine compositions) for some mafic lavas, such as the Khamshedi picrite basalts. According to Cox & Hawkesworth, the Khamshedi rocks could be primitive picrites erupted at the beginning of a new compositional cycle. They overlie the Fe-rich flows of the Lower Poladpur Formation, and although the magma may initially have been trapped in a magma chamber because of its high density, it could have mixed with evolved, Fe-rich magmas of the previous cycle before eruption. Hybrid magmas of this sort would be anomalously rich in iron and Mg poor compared with the original picrite liquid (note that the Khamshedi lavas contain 15% Fe2O3). Crystallizing olivines would also be relatively Fe rich. Thus picritic basalts of the Khamshedi type may derive from true picrite magmas, in spite of the presence of low-Mg olivine phenocrysts. The establishment of well-developed magma chambers, and the operation of RTF (replenished, trapped, fractionated) type processes (O’Hara & Mathews, 1981) may ensure that unmodified primitive picrite basalts are not erupted directly (Huppert & Sparks, 1980, 1985). In the Deccan, the restriction of most primitive picritic lavas to the early parts of the sequence may at least in part be related to the fact that such magma chambers were not established until later in the volcanic cycle.
The importance of mixing processes in the petrogenesis of Western Ghats basalts is corroborated by the mineral and melt inclusions present in zoned plagioclase phenocrysts from the ‘giant phenocryst’ basalts of the region (Pankov et al., 1994). These flows are typically relatively Fe rich and occur at the close of recognizable cycles, particularly in the Kalsubai subgroup, which contains numerous picrite and picrobasalt horizons (see Beane et al., 1986). In some giant phenocrysts, pyroxenes and/or olivines (Fo40–32) of varying compositions are included within the same plagioclase zone, and in addition, Mg-rich olivines have been observed in the rim zones of plagioclases that contain Fe-rich olivines in their cores. Geochemical data for lavas from the Western Ghats also provide evidence for mixing of primitive and evolved magmas (Cox & Hawkesworth, 1985; Mahoney, 1988).
Primitive picrite basalts and stratigraphic considerations
Volumetrically significant occurrences of primitive picrite basalts in the Deccan appear to be confined to areas north of the Narmada rift, west of the Cambay graben, or southern Kathiawar (Fig. 1). This spatial distribution may be related to stratigraphic position within the Deccan flood basalt sequence. If the Reunion-plume model for Deccan volcanism is valid, areas of northwest India along the plume-axis trace should have encountered the plume first, and should therefore contain the oldest members of the sequence (Cox, 1983). That this is the case is supported by several pieces of evidence. First, to the north of the Narmada rift the flow sequence exhibits a normal–reversed–normal magnetic stratigraphy, with the lower normal representing the older Narmada Formation (Sreenivasa Rao et al., 1985). The Western Ghats sequences to the south reveal only a single reversal. Second, detailed stratigraphic correlations in the Western Ghats indicate that the flow sequence there exhibits a very low (<0·5°) southerly dip, so that the oldest formations occur to the north (Beane et al., 1986). Third, older plutonic and sub-volcanic complexes related to the Deccan, such as Mundwara (68·5 Ma; Basu et al., 1993) occur in the north. The Pawagarh hill flow sequence is an erosional outlier. Thus the picrite-containing effusive sequences in Saurashtra and north of the Narmada appear to be early members of the Deccan volcanic sequence.
The primitive picrite localities are also in close proximity to the Narmada and Cambay rift zones (Fig. 1), which apparently provided pathways for rapid transport and eruption. Lacking such conduits, such dense magmas may pond at the crust–mantle boundary (Herzberg et al., 1983). Eruption of fairly large volumes of high-Mg primitive picrites may also have been facilitated early in the Deccan cycle by the high temperatures of parts of the plume head (Campbell & Griffith, 1990). The paucity of such primitive picrite basalts in the stratigraphically younger sequence of the Western Ghats may be attributed to the general decrease of thermal energy later in the volcanic cycle, and the development of steady-state magma chambers and magmatic plumbing systems in which ponding and mixing of dense picritic magmas occurred, as described above.
Picrite basalts and the Reunion plume
Campbell & Griffith (1990) have argued that primary picritic lavas represent high-degree melts from the high-temperature part of a plume head. Combined Sr–Nd–Pb systematics for the primitive picrite basalts of the Dhandhuka–Wadhwan–Botad borehole sequence support this idea, as they suggest a present-day Reunion type plume source for some of the early picrite basalts (Peng & Mahoney, 1995), although other isotopic components are also present. The situation for other Deccan picrite basalts is less clear. On geochemical grounds, a deep ocean-island basalt type mantle source has been suggested for the high-TiO2, large ion lithophile element (LILE)-enriched suite at Pawagarh and Rajpipla (Melluso et al., 1995; Greenough et al., 1998), and Melluso et al. (1995) also postulated a high field strength element depleted mantle source for the low-TiO2 suites in southern Kathiawar.
The depths and extents of melting necessary to generate some of the primitive picrite basalt compositions have been dealt with in two recent publications. Sen (1995) postulated a depth range of 100–60 km (garnet to spinel lherzolite field) with 11–18% melting for the primary magmas. For the low-TiO2 picrite basalts of south Kathiawar, Melluso et al. (1995) found a slightly shallower but overlapping depth range (80–40 km). Peng & Mahoney (1995) used LILE abundances to argue that the picrite basalts result from smaller degrees of melting than the Ambenali type basalts of the Western Ghats. However, especially in a plume-influenced environment, LILE abundances may be affected by a variety of processes.
Mantle metasomatism by plume-derived or plume-related fluids can provide a rich source of LILE. In the Deccan region, alkaline and ultra-alkaline rocks such as the carbonatites of the Narmada zone have been attributed to mantle metasomatism and enriched mantle sources (Krishnamurthy & Udas, 1981; Simonetti et al., 1995). Metasomatism has also been implicated in the genesis of the potassium-rich alkaline suite of the Rajpipla area, on the basis of Sr and Nd data and geochemical decoupling between tholeiitic and alkali basalts (Mahoney et al., 1985). Mantle enrichment has also been suggested to explain some aspects of the Mahabaleshwar lavas of the Western Ghats (Cox & Hawkesworth, 1984). Cryptic metasomatism of the sub-Deccan mantle has been inferred from Sr and Nd isotopic studies of spinel peridotite nodules from alkali basalts in Kutch (Krishnamurthy et al., 1988; Pande, 1988), and from the presence of sodian–ferrian in diopsides in the same nodules (Murari, 1993). Recently the low-pressure, low-temperature, orthopyroxene–rutile–spinel intergrowths that occur in spinel peridotite xenoliths from Mt Sayala Devi in Kutch have been ascribed to mantle metasomatism (Karmalkar et al., 1999). Thus throughout the northern parts of the Deccan a variety of mineralogical and geochemical features point to mantle metasomatism, probably the result of complex interaction between the Reunion plume and the lithospheric mantle.
An additional aspect of this discussion concerns the Fe content of the source as inferred from primitive picrite basalts. It has been postulated (e.g. Francis, 1985; Melluso et al., 1995; Scarrow & Cox, 1995; Francis et al., 1999) that intraplate or plume-related picrites show an inverse relation between Si and Fe that indicates Fe-rich sources. As has already been discussed, the Fe content of the source influences both initial magma compositions and the compositions of early formed olivines. Thus source composition is important for understanding picrite basalt petrogenesis.
Primitive picrite basalts and the volcanic cycle
The primitive picrite basalts identified in the Deccan can be examined in terms of Cox’s volcanic cycle model for the Karoo (Cox, 1972). According to this model, the volcanic cycle has a very early low-degree-of-melting stage, characterized by alkaline and ultra-alkaline rocks, followed by a thermal peak (the culmination stage) that is represented by high-Mg primitive picritic basalts. The thermal peak is followed by a steady-state stage during which basalts constituting the bulk of the province erupt. These typically have fairly uniform major element chemical compositions. A crustal stage, dominated by rhyolitic rocks, may closely follow or be coeval with the basalts. The end of the cycle is again characterized by alkaline and acidic rocks produced as low-degree melts.
Broadly speaking, volcanism in the Deccan appears to have followed a similar pattern. Among the oldest known Deccan rocks are alkaline complexes at Sarnu (68·5 Ma) and Mundwara (68·5 Ma) in the north, representing the very early, low partial melt stages (Basu et al., 1993). This is probably followed by the sequences found in Kutch, Saurashtra and Narmada. In Kutch, tholeiitic and ultra-alkaline rocks are almost synchronous, giving ages of 67–64 Ma (Pande et al., 1988). Field evidence appears to place the primitive picrite basalts of the borehole sequence, as well as those of Pawagarh, below the tholeiites of the Western Ghats in the stratigraphic sequence. The primitive picrites would thus represent the culmination stage in Cox’s model, and probably originated in the high-temperature Reunion plume head. Deep-seated faults along the Narmada rift and Cambay graben aided rapid transport of these dense magmas to the surface. The predominant evolved tholeiitic basalts of the Deccan province, well represented in the Western Ghats, were erupted during the steady-state phase of the volcanic cycle. Cox (1980) envisioned sill-like complexes of ponded, picritic magmas in the lower crust, or at the crust–mantle boundary, from which such lavas evolved. The bulk of the tholeiitic lavas probably erupted near 65 Ma (Duncan & Pyle, 1988; Venkatesan & Pande, 1996). The Deccan volcanic cycle apparently closed with late-stage alkaline complexes such as Phenaimata (65·0 Ma; Basu et al. 1993) and Ambadongar (65·0 Ma; Ray & Pande, 1999).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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On the basis of olivine phenocryst compositions (Fo92–86) and whole-rock chemical compositions, primitive picrite basalt flows and dykes have been identified from widely separated parts of the Deccan flood basalt province. These include localities in Saurashtra (Anila and Paliyad) and north of the Narmada rift (Pawagarh, Ambadongar and Kawant). These occurrences, together with those reported earlier from borehole cores of Dhandhuka, Wadhwan and Botad, constitute significant quantities of primitive picrite basalts. They were probably emplaced early in the Deccan volcanic cycle, along deep faults in the Cambay graben and Narmada rift regions.
Picrite dykes from South Kathiawar (near Dedan) and flows at Botad, contain less magnesian olivine phenocrysts (Fo83–77). The former is olivine rich, probably as a result of flow differentiation, and the latter may be derivative from the geographically adjacent primitive picrite flows sampled at the Botad borehole locality.
Picrite basalts of the Western Ghats lack forsteritic olivines and exhibit a large range of olivine compositions. They apparently originated from more primitive picritic magmas through fractionation and mixing processes within the crust.
We suggest that the volumetrically significant primitive picrite basalts of the Narmada and Saurashtra regions can be linked to an early stage of Deccan volcanism and thus conform to the type of volcanic cycle envisioned by Cox (1972) for the Karroo flood basalts. Similarly, the less primitive picrite basalts of the Western Ghats appear to fit the ‘steady-state’ stage of Cox’s ideal volcanic cycle.
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ACKNOWLEDGEMENTS |
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The late Keith Cox, the first author’s Guru for basalts, reviewed an earlier version of this paper and suggested that we consider the role of RTF magma chambers in picrite genesis, and also that we examine the relationship between picrites and stratigraphic position in the Deccan. As usual, his suggestions were insightful. We would like to acknowledge critical reviews and suggestions by Peter Hooper, Claude Herzberg and an anonymous reviewer, and also comments by Marjorie Wilson, Executive Editor of the journal. All helped to improve the paper. Roy Fujita at the Scripps Institution of Oceanography is thanked for tutoring and assistance with the probe work. We are grateful to S. S. Deshmukh of the Geological Survey of India, Nagpur, and John Mahoney of the University of Hawaii, for supplying various samples. We would like to acknowledge the help in manuscript preparation rendered by E. V. S. S. K. Babu, Y. V. S. S. Rao and Ms Latha Vamadevan. P.K. would like to record his gratitude to Raja Ramanna, former Chairman, Atomic Energy Commission, India, and A. V. Phadke, Director (Retired), Atomic Minerals Division, for granting him the deputation to participate in this Indo-US endeavour. This work was supported in part by grants from the US National Science Foundation.
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FOOTNOTES |
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*Corresponding author. Telephone: +91-40-7767101, ext. 250. Fax: +91-40-7762940. e-mail: amdhyd{at}ap.nic.in
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TOPABSTRACTINTRODUCTIONFIELD RELATIONSMETHODSRESULTSDISCUSSION AND CONCLUSIONSCONCLUSIONSREFERENCES |
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