JOURNAL OF PETROLOGY
VOLUME 41
NUMBER 7
PAGES 1099–1120
2000
Geochemistry of Flood Basalts of the Toranmal Section, Northern Deccan Traps, India: Implications for Regional Deccan Stratigraphy J. J. MAHONEY1∗, H. C. SHETH2†, D. CHANDRASEKHARAM2 AND Z. X. PENG1 1
SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA
2
DEPARTMENT OF EARTH SCIENCES, INDIAN INSTITUTE OF TECHNOLOGY, POWAI, BOMBAY 400 076, INDIA
RECEIVED AUGUST 20, 1999; REVISED TYPESCRIPT ACCEPTED FEBRUARY 18, 2000 Tholeiitic lavas forming a flood basalt sequence of 870 m thickness at Toranmal in the northern Deccan Traps have a large range in isotopic ratios [Nd(t) = +2·1 to –15·7, ( 87Sr/86Sr)t = 0·70467–0·71416, 206Pb/204Pb = 16·699–20·246], similar to that of lavas in the well-studied southwestern part of the province. The basalts with the lowest Nd(t) values display distinctive lows at Nb, Ta, P and Ti, and large positive Pb spikes in their primitivemantle-normalized element patterns, indicative of significant continental lithospheric influence in their petrogenesis. As in much of the southwestern Deccan, Nd(t) exhibits a rough negative correlation with Mg/Fe and SiO2 and a positive correlation with Fe, consistent with temperature-controlled assimilation. Overall, the Toranmal section appears distinct from sections in the northwestern sector of the province; however, some Toranmal basalts are isotopically and chemically similar to flows in the northeastern Deccan, and a thick pile of lavas resembling the Poladpur Fm of the southwestern Deccan, the closest type-sections of which lie >380 km to the south, is present. If it indeed represents a northern remnant of this formation, the Poladpur Fm, which also extends far into the central and southeastern parts of the province, is one of the most widespread of Deccan formations, with a possible original extent [3 × 105 km2. The Ambenali Fm, which forms a thick sequence lying above the Poladpur in the southwestern Deccan, is not present at Toranmal. Several flows have broad geochemical affinities with the southwestern Bushe and Mahabaleshwar formations, which respectively lie below the Poladpur and above the Ambenali; however, these flows are not in the southwestern stratigraphic order and are probably of relatively local origin as dikes compositionally very similar to these flows are present at Toranmal and elsewhere in the vicinity.
An understanding of the nature and magnitude of magma generation, storage, and transport in flood basalt provinces requires knowledge of the stratigraphic and geochemical relationships between lava sequences in different regions of any given province. For most provinces, including the Deccan Traps of India (Fig. 1), such knowledge is still fragmentary. Although much reduced by erosion and, in the west, subsidence below sea level, the on-land extent of the Deccan remains vast, at 500 000 km2. Recent 40Ar–39Ar ages and Re–Os isotopic data for basalts from different parts of the province indicate that most of it formed in a geologically brief period around 67–64 Ma (e.g. Courtillot et al., 1988; Duncan & Pyle, 1988; Venkatesan et al., 1993; Baksi, 1994; Baksi et al., 1994; Bhattacharji et al., 1996; Sheth et al., 1997; Alle`gre et al., 1999). Combined field and geochemical work in the 1980s produced a stratigraphic framework for the well-exposed southwestern sector (boxed area in Fig. 1) that divides the lava sequence in this region into three subgroups and 11 formations with a maximum stratigraphic thickness of >3·4 km (Table 1; e.g. Cox & Hawkesworth, 1985; Beane et al., 1986;
∗Corresponding author. Telephone: +1-808-956-8705; e-mail:
[email protected] †Present address: Centro de Investigacion en Energia, Universidad Nacional Autonoma de Mexico, Apartado Postal 34, Temixco, Morelos 62580, Mexico.
Oxford University Press 2000
KEY WORDS: Deccan
Traps; geochemical stratigraphy; flood basalts; large
igneous provinces
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 41
Khadri et al., 1988; Subbarao et al., 1988; Lightfoot et al., 1990). More recent work has shown that several of these formations extend into the central Deccan (particularly the Khandala and Poladpur formations; Peng, 1998; K. V. Subbarao et al., unpublished data, 1998) and the southeastern Deccan (particularly the Poladpur and Ambenali formations; Mitchell & Widdowson, 1991; Bilgrami, 1999) for as much as 350 km from their southwestern type-sections. In addition, lavas isotopically and chemically equivalent to those of the Ambenali Formation (Fm) are present in the northeastern sector of the province as far as 900 km from their southwesternmost counterparts (Mahoney, 1988; Deshmukh et al., 1996; Peng et al., 1998). However, many of the flows in the northeastern region (Mhow–Chikaldara–Jabalpur area), although similar to the southwestern Poladpur Fm in their chemical and Nd–Sr isotopic characteristics, possess Pb isotopic ratios higher than those of Poladpur lavas and thus must have taken somewhat different pathways through the crust on their way to the surface (Peng et al., 1998). Lavas in the northwestern sector of the province (from approximately Rajpipla westward) differ markedly as a group from those in the other regions studied. In particular, whereas the latter are almost exclusively tholeiitic, many northwestern lavas are relatively alkalic (e.g. Krishnamurthy & Cox, 1977; Mahoney et al., 1985; Melluso et al., 1995; Peng & Mahoney, 1995). The relationships between the northwestern, northeastern and southern lava sequences—the extent to which they are interdigitated, or stratigraphically or structurally isolated from one another—are unknown. In part, this uncertainty reflects a lack of combined Nd–Sr–Pb isotopic and chemical studies in the area between these three major regions of the province; that is, roughly in the area between Rajpipla, Igatpuri, Buldana and Mhow. Here, we discuss the elemental and isotopic geochemistry and stratigraphy of the lava pile at Toranmal, the thickest in this area (Fig. 1).
NUMBER 7
JULY 2000
Table 1: Stratigraphic summary of the southwestern Deccan formations
FIELD GEOLOGY, PETROGRAPHY AND ANALYTICAL METHODS Toranmal (1152 m; 21°53′N, 74°28′E) forms a prominent peak of the Satpura range, which constitutes a horst separating the Narmada and Tapi grabens (along which flow rivers of the same names; Fig. 1). The Tapi graben is downfaulted along the Satpura Foothill Fault (Guha, 1995; Sheth, 1998), which has produced a steep scarp in the vicinity of Toranmal; however, we have found no evidence of faulting within the Toranmal section itself. The section exposes a basalt sequence of 870 m thickness, intruded by several approximately east–west-trending basaltic and doleritic dikes, none of which are obvious
feeders to the flows. To our knowledge, the only previous geochemical work on this section was carried out by Nair et al. (1996), who analyzed major elements and a suite of trace elements by X-ray fluorescence spectrometry (XRFS). We have identified, conservatively, 26 flows and two interspersed bole horizons on the basis of exposed flow boundaries, the presence of major vesicular–
1100
MAHONEY et al.
FLOOD BASALTS OF TORANMAL, DECCAN TRAPS
Fig. 1. Map of the Deccan Traps showing locations of towns near field areas discussed in the text [modified from Peng et al. (1994)]. The box delineates the well-studied area of the southwestern Deccan where the formational stratigraphy of Table 1 was worked out.
amygdular zones interpreted as flow tops, simple or compound nature of outcrops, and macroscopic and microscopic petrography (see Fig. 2). The flows have northerly dips of 2–3° (see Nair et al., 1996) and thicknesses of 10–120 m. Except for two compound flows at about 450 m and 710 m, the lava pile consists of simple flows, most of which exhibit large cooling columns (lower, and sometimes also upper, colonnades with middle entablatures). The thickest flows identified in our field section are thicker than normal for simple flood basalt flows, possibly indicating the presence of one or more unexposed flow boundaries. Nair et al. (1996) reported 41 flows in this section (see fig. 1 of their paper) but relied on criteria that we consider unreliable; specifically, they inferred flow boundaries between amygdaloidal zones and underlying massive zones, and between outcrops having different weathering patterns, including the presence or absence of spheroidal weathering (K. K. K. Nair, personal communication, 1999). Spheroidal weathering is common, and soil development and extensive weathering significantly restrict the number of outcrops suitable for sampling. From examination of samples with the naked eye and hand
lens, we classified the basalts into aphyric, plagioclasephyric and giant plagioclase (GPB) types, the last having plagioclase phenocrysts >2 cm in length. Nair et al. (1996) reported no GPBs, but we encountered one at >700 m elevation (represented by sample SH105). A roughly equal number of macroscopically aphyric and plagioclasephyric flows are present (Fig. 2), although most of the ‘aphyric’ flows are seen to be microphyric in thin section. The phenocryst assemblage of the Toranmal lavas is not notably different from that in the southwestern and northeastern Deccan (see Nair et al., 1996); however, weathering alteration tends to be more extensive than we have seen in most samples that we have collected from those regions (see also below), although primary textures remain easily recognizable. Plagioclase is the most abundant microphenocryst and is often accompanied by augite and altered olivine; the phenocrysts commonly form clots, producing a cumulophyric texture. Groundmasses are fine grained with intersertal or intergranular textures, and consist of plagioclase and augite with minor iron oxides. The dikes are texturally similar to the flows, although slightly coarser grained, and the boles are composed of altered tuffaceous material.
1101
JOURNAL OF PETROLOGY
VOLUME 41
NUMBER 7
JULY 2000
abundances were measured by inductively coupled plasma–atomic emission spectrometry at the Indian Institute of Technology, Bombay. Two dike samples and 17 samples from flows were analyzed for Nd, Sr and Pb isotopic ratios and abundances of Nd, Sm, Sr, Rb and Pb by isotope dilution at the University of Hawaii [Table 3; see Mahoney et al. (1991) for methods]; as in our previous studies, isotopic work was carried out on small, hand-picked rock chips cleaned briefly in weak acid to avoid possible Pb contamination (e.g. Peng & Mahoney, 1995). These small, picked chips may not always be representative of the bulk-rock mineralogy, particularly in coarser-grained or patchily altered samples; thus, although the isotope-dilution and ICP-MS values are generally close, we use the isotope-dilution abundance data here only for age-correcting Sr and Nd isotopic ratios. For direct comparison with the Pb isotope data in the southwestern Deccan dataset, very few of which are age corrected, Pb isotope ratios are present-day values (note, however, that age corrections would generally be very small relative to the variation seen within individual Deccan formations; e.g. 238U/204Pb estimated from wholerock ICP-MS data ranges between six and 20, corresponding to a 66 Ma age correction of only –0·06 to –0·2 in 206Pb/204Pb).
RESULTS Isotopic data
Fig. 2. Stratigraphic column of flows, dikes and sample locations in the Toranmal section. Sample numbers are on right side of each column. Flow boundaries shown as continuous lines were observed in outcrop; those shown as dashed lines are not exposed but inferred from the presence of features indicating proximity to flow boundaries (e.g. vesicular or amygdaloidal zones, breccias), significant petrographic differences above and below the boundary, and/or geochemical differences. Dikes are indicated by black bars. Sm. pl., small plagioclase; lg. pl., large plagioclase.
After rejecting the most altered samples, we chose 27 for geochemical work (23 flows, three dikes and a sample from a boulder that probably fell from a nearby flow outcrop; see Fig. 2). Major elements were analyzed by XRFS and trace elements by inductively coupled plasma– mass spectrometry (ICP-MS) on agate- and aluminaground powders at the University of Hawaii (Table 2) following the methods of Norrish & Chapell (1977) and Jain & Neal (1996), respectively. In addition, some Sr
For correlation of physically separated lava piles, elemental, isotopic, stratigraphic and absolute-age relationships must all be evaluated. In general, assuming flow ages allow a correlation, Nd–Sr–Pb isotopic ratios can be critical because, unlike elemental abundances, they are not modified significantly by post-contamination differentiation during flowage or in magma chambers, local-scale crystal accumulation or moderate amounts of subaerial weathering alteration (see below), and tend to be more sensitive to differences in amount of contamination and end-member composition. Combined Nd–Pb–Sr isotopic data are a powerful tool for regional stratigraphic correlation in the southwestern, southeastern and central Deccan, because the data fields of the different southwestern formations display only limited overlap with each other in one or more of the isotope diagrams of Fig. 3; the Khandala, Bushe, Poladpur, Ambenali and Mahabaleshwar formations are particularly well characterized isotopically. The Toranmal samples encompass a wide range of isotopic values: e.g. Nd(t) = +2·1 to –15·7 (t = 66 Ma), ( 87Sr/86Sr)t = 0·70467–0·71416, and present-day 206Pb/204Pb = 16·699–20·246. However, most of the data lie in or rather close to the Poladpur, Bushe or Mahabaleshwar Fm fields in Fig. 3, indicating the operation of similar
1102
Rb Ba Th U Nb Ta La Ce Pr Pb Sr Nd Zr Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu V Co Ni
SiO2 Al2O3 TiO2 Fe2O3∗ MnO MgO CaO Na2O K2O P2O5 Total mg-no. LOI
56 162 2·8 0·63 5·2 0·37 11·1 21·6 2·82 3·3 186 11·7 86 3·00 0·90 3·62 0·60 3·78 24·8 0·80 2·21 0·36 2·34 0·31 268 97 84
52·31 15·21 1·00 11·43 0·16 6·66 9·62 2·18 1·07 0·10 99·74 57·6 2·92
dike
SH89
Sample:
280
dike:
Flow or
Elev. (m):
24 199 2·2 0·47 12 0·75 16·7 35·9 5·15 2·3 304 22·2 162 5·20 1·68 5·92 0·93 5·05 28·0 1·01 2·46 0·35 2·16 0·32 300 39 52
51·10 15·45 2·33 13·82 0·15 4·11 9·73 1·96 0·89 0·24 99·78 40·9 2·69
SH88
flow
310
1103 79
83 332 2·4 0·65 6·8 0·46 7·96 14·9 1·85 5·3 151 7·69 102 2·08 0·87 2·51 0·44 3·16 19·4 0·69 2·01 0·29 1·65 0·23
52·80 15·13 1·12 13·32 0·12 6·47 3·62 4·31 2·12 0·11 99·12 53·1 4·01
SH90
flow
330
12 87 0·42 0·18 7·3 0·40 7·30 19·8 2·88 1·3 323 15·1 131 4·01 1·49 5·30 0·81 4·70 26·0 0·96 2·29 0·35 1·96 0·29 351 55 106
48·68 14·09 2·25 14·34 0·22 6·71 10·26 2·66 0·31 0·18 99·70 52·1 2·40
SH91
dike
330
24 549 5·2 1·1 33 1·9 35·9 70·5 8·39 5·3 307 35·5 227 7·68 2·34 8·44 1·36 7·62 48·0 1·57 4·23 0·62 3·97 0·60 334 36 44
50·47 14·00 2·64 14·56 0·18 4·85 8·49 2·32 1·32 0·59 99·42 43·7 3·92
SH92
flow
340
1·0 94 1·9 0·37 10 0·56 15·2 35·1 4·45 3·2 187 19·3 139 5·06 1·54 5·80 0·89 5·16 30·0 1·09 2·75 0·44 2·54 0·36 325 43 49
50·12 14·37 2·21 13·35 0·18 6·20 11·02 1·78 0·17 0·21 99·61 51·9 2·67
SH93
flow
350
38 415 5·5 1·1 34 2·1 36·5 69·8 9·17 5·1 203 37·1 229 8·14 2·52 9·37 1·39 8·44 52·8 1·85 4·75 0·68 4·46 0·66 349 37 46
52·50 13·85 2·76 13·72 0·13 4·06 8·52 2·30 1·42 0·61 99·87 40·8 3·39
SH94
flow
380
5·0 121 0·66 0·19 8·1 0·40 11·2 27·2 3·76 2·0 253 17·6 116 4·75 1·55 5·11 0·85 5·14 28·3 1·01 2·60 0·35 2·05 0·35 336 52 79
49·72 14·34 2·15 14·00 0·19 6·07 10·62 2·06 0·25 0·20 99·60 50·2 2·18
SH95
flow
420
7·0 121 1·0 0·25 7·7 0·47 10·9 23·3 3·66 1·8 235 17·1 118 4·60 1·51 5·20 0·86 4·83 26·9 0·96 2·46 0·35 2·10 0·31 321 45 82
50·15 14·73 2·10 13·95 0·18 6·37 10·63 1·86 0·33 0·20 100·50 51·5 2·13
SH96
dike
420
83
9·4 102 1·3 0·30 12 0·79 13·5 32·7 4·76 1·9 218 24·0 171 6·23 1·95 7·13 1·12 6·35 36·4 1·27 3·20 0·48 2·80 0·43
48·22 13·31 2·78 15·89 0·21 5·72 10·14 1·90 0·38 0·20 98·75 45·6 1·03
SH97
boulder
450
Table 2: Major element (wt %) and trace element (ppm) compositions of the Toranmal basalts
110
1·2 15 1·4 0·37 12 0·86 10·7 21·6 3·19 2·0 70·2 14·1 161 3·88 1·45 4·70 0·74 4·39 25·8 0·93 2·43 0·35 2·15 0·32
53·22 12·44 2·50 14·62 0·17 5·13 7·41 3·41 0·03 0·11 99·04 45·0 6·68
SH99
flow
460
12 101 1·4 0·39 9·1 0·48 12·3 30·2 3·89 1·6 207 18·5 129 4·61 1·62 5·90 0·94 5·45 31·4 1·15 2·71 0·40 2·80 0·40 330 51 70
49·04 13·70 2·20 15·70 0·21 6·20 10·90 1·85 0·35 0·20 100·35 47·9 1·65
SH100
flow
480
11 60 1·6 0·34 8·9 0·63 10·8 24·5 3·79 1·3 179 17·7 130 4·66 1·50 5·54 0·85 5·32 29·5 1·05 2·64 0·35 2·47 0·36 326 48 75
49·79 13·71 2·16 13·53 0·20 6·42 11·04 1·74 0·38 0·20 99·17 52·5 3·42
SH101
flow
510
0·49 80 1·5 0·49 9·7 0·55 11·8 29·7 4·00 1·6 158 18·5 137 5·22 1·61 6·05 0·93 5·70 34·0 1·13 2·97 0·46 2·69 0·40 349 54 71
47·56 14·13 2·30 15·57 0·18 6·61 11·47 1·95 0·12 0·21 100·10 49·7 3·28
SH102
flow
540
78
0·80 81 1·8 0·48 10 0·68 12·9 30·1 4·40 1·5 188 20·2 158 5·42 1·72 6·60 1·03 5·75 34·0 1·21 3·04 0·45 2·81 0·41
47·73 13·46 2·32 15·59 0·24 6·30 11·26 1·75 0·14 0·21 98·98 48·5 1·56
SH103
flow
600
MAHONEY et al. FLOOD BASALTS OF TORANMAL, DECCAN TRAPS
6·0 141 1·8 0·49 18 1·1 17·0 40·4 5·18 2·2 178 23·6 155 5·87 1·98 7·02 1·15 6·90 40·2 1·37 3·48 0·54 3·49 0·52 367 49 65
11 138 1·8 0·35 13 0·85 14·8 34·1 4·73 2·4 197 22·0 142 5·10 1·59 6·10 0·94 5·46 30·6 1·12 2·77 0·42 2·76 0·42
68
23
50·75 13·44 2·33 14·91 0·19 5·89 9·84 1·76 0·52 0·24 99·87 47·9 0·84
flow SH107
740
25 253 3·8 0·76 19 1·2 25·2 55·3 7·50 6·7 219 34·5 238 7·99 2·28 9·17 1·35 8·05 46·9 1·65 4·07 0·61 3·75 0·59
51·77 14·27 2·61 15·04 0·19 2·98 8·53 2·29 0·99 0·39 99·06 31·6 3·12
flow SH106
730
25 444 4·3 1·1 32 1·7 32·5 65·9 8·08 4·4 231 33·2 169 7·22 2·35 8·44 1·31 7·59 44·5 1·55 4·07 0·63 3·75 0·55 333 36 39
53·09 13·29 2·71 13·89 0·21 4·02 8·73 2·21 1·00 0·60 99·75 40·2 6·36
flow SH109
790
5·0 81 1·8 0·41 7·9 0·42 13·2 30·8 3·91 2·3 304 18·4 122 4·85 1·49 5·30 0·81 5·00 28·3 1·02 2·71 0·37 2·57 0·35 298 47 65
50·09 14·70 1·94 13·38 0·19 5·81 11·31 1·63 0·21 0·19 99·45 50·3 1·99
flow SH110
840
1104
2·0 109 1·2 0·35 11 0·59 11·3 25·3 3·23 1·7 166 14·2 98 3·76 1·25 4·86 0·86 5·56 36·3 1·23 3·44 0·55 3·61 0·52 343 46 59
50·13 14·00 1·67 14·94 0·21 5·72 10·55 1·85 0·26 0·17 99·50 47·1 1·82
flow SH112
930
1·4 84 2·6 0·67 5·1 0·37 11·1 20·7 2·71 3·4 294 11·1 82 2·49 0·92 3·47 0·62 3·87 23·5 0·88 2·14 0·39 2·39 0·39 253 41 84
51·18 15·46 1·00 11·41 0·17 7·08 11·72 1·13 0·11 0·10 99·36 59·1 5·06
flow SH113
960
10 136 2·0 0·47 4·2 0·22 9·70 19·9 2·37 3·1 168 10·0 66 2·64 0·82 3·15 0·54 3·70 22·8 0·79 2·15 0·37 2·33 0·33 239 45 87
52·12 14·92 0·83 10·91 0·16 7·11 11·32 1·59 0·47 0·08 99·51 60·3 1·53
flow SH114
990
7·0 92 0·95 0·32 12 0·67 12·3 31·7 4·28 2·0 211 20·4 152 5·40 1·85 6·94 1·04 6·65 35·9 1·24 3·07 0·48 3·10 0·39 351 232 72
48·94 13·20 2·74 16·05 0·22 5·40 10·38 2·00 0·21 0·26 99·40 43·9 1·29
flow SH115
1050
13 210 3·8 0·81 8·5 0·60 15·6 29·9 3·85 3·7 154 16·4 120 3·98 1·23 5·25 0·82 5·19 33·7 1·18 3·19 0·43 3·16 0·51 302 40 50
52·39 14·68 1·54 12·87 0·17 5·06 9·93 1·91 0·75 0·16 99·46 47·8 1·54
flow SH116
1100
8·6 137 1·1 0·39 17 1·1 15·9 38·3 5·38 2·1 396 24·6 179 5·72 1·89 6·06 0·89 5·12 24·0 0·91 2·36 0·29 1·89 0·26 317 46 123
49·89 13·71 2·74 12·44 0·17 7·26 11·42 2·24 0·51 0·27 100·65
Meas. BHVO-1
9·7 133 1·08 0·41 19 1·16 15·8 37·8 5·40 2·05 370 24·8 168 6·10 1·98 6·40 0·96 5·20 27·0 0·99 2·40 0·33 2·02 0·29 317 45 121
49·59 13·70 2·69 12·39 0·17 7·22 11·32 2·24 0·52 0·27 100·11
Recomm. BHVO-1
NUMBER 7
67
3·0 160 2·7 0·61 19 1·0 18·4 38·7 5·14 2·6 194 22·7 154 5·30 1·68 6·30 1·01 6·10 35·4 1·28 3·34 0·50 3·34 0·50
48·59 13·72 2·26 15·30 0·21 5·94 11·02 1·72 0·30 0·30 99·36 47·5 2·61
flow SH111
880
VOLUME 41
Relative uncertainties on major and minor elements are >1%; for SiO2, >0·5%. For trace elements, at the 0·4–10 ppm abundance level in the rock, the relative uncertainty for most elements is 3·5% or better (range is 1–5%); for Ta 20%. An indication of accuracy is given by measured and recommended (Govindaraju, 1989) values for standard rock BHVO-1. Fe2O3∗ is total iron as Fe2O3. mg-number = [atomic Mg/( Mg + Fe2+)] × 100, assuming Fe3+/Fe2+ = 0·15. LOI, weight loss on ignition.
11·8 27·2 4·14 1·8 286 19·2 141 5·36 1·67 6·03 0·89 5·52 32·3 1·18 3·03 0·42 2·76 0·39 347 52 82
4·0 66 1·7 0·41 9·7
Rb Ba Th U Nb Ta La Ce Pr Pb Sr Nd Zr Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu V Co Ni
48·72 13·74 2·95 15·63 0·23 5·43 10·55 1·82 0·45 0·33 99·85 44·7 1·74
flow SH105
flow SH104
49·09 13·70 2·24 14·72 0·19 5·94 11·74 1·71 0·18 0·21 99·72 48·4 2·74
700
660
SiO2 Al2O3 TiO2 Fe2O3∗ MnO MgO CaO Na2O K2O P2O5 Total mg-no. LOI
Elev. (m): Flow or dike: Sample:
Table 2: continued
JOURNAL OF PETROLOGY JULY 2000
2·21
1105
280
SH89
4·91
4·73
2·86
5·21
1·94
2·05
2·02
1·61
1·62
3·00
2·50
4·50
1·66
12·80
7·720
13·45
33·31
13·75
17·28
11·80
16·52
14·15
20·00
22·10
26·27
16·13
22·45
11·53
10·09
8·860
16·85
13·00
Nd (ppm)
3·176
2·028
3·578
7·190
3·851
4·644
3·137
4·704
3·931
5·541
5·886
5·843
4·333
5·528
3·203
2·609
2·275
4·755
3·422
Sm (ppm)
177·6
132·3
187·4
203·2
252·9
234·8
47·74
207·4
157·9
286·3
178·5
231·2
304·3
209·7
166·4
294·4
167·8
211·5
153·7
Sr (ppm)
49·9
39·6
0·27
24·4
4·01
8·05
0·37
6·16
0·18
1·20
7·89
24·9
2·88
2·65
2·34
1·52
11·0
5·45
10·3
Rb (ppm)
0·70764 0·71068 0·71199
−3·1 −12·6 −12·8
0·70593 0·70657
−0·6 −0·9
0·70659 0·70583
+0·2 −0·7
0·70579 0·70582
+1·3 +1·1
0·70513 0·70599
+1·8 +1·0
0·70737 0·70675
−3·5 −0·9
0·70660 0·70571
+0·4 +1·1
0·71416 0·71228
−15·7 −12·6
0·71199 0·70467
−9·2
( 87Sr/86Sr)t
+2·1
Nd(t) Pb/204Pb
19·584
19·586
17·478
18·974
16·837
16·699
18·218
19·147
19·523
20·085
18·400
18·981
18·042
18·849
18·965
19·597
18·880
17·393
20·246
206
Pb/204Pb
15·820
15·819
15·442
15·737
15·327
15·281
15·552
15·571
15·708
15·678
15·535
15·718
15·505
15·661
15·651
15·823
15·714
15·384
15·854
207
Pb/204Pb
39·977
39·972
39·001
39·884
37·675
37·477
38·879
39·384
39·819
39·648
38·890
39·829
39·558
39·534
39·664
39·985
39·505
37·980
41·227
208
The Nd and 87Sr/86Sr values have been age-corrected to t = 66 Ma; Pb isotope ratios are present-day values, as in earlier Deccan studies. Data are reported relative to values of 87Sr/86Sr = 0·71024 for NBS 987 Sr, 143Nd/144Nd = 0·511845 for La Jolla Nd, and the Pb isotope values of Todt et al. (1996) for NBS 981 Pb. The total range measured for NBS 987 Sr over a 2 year period was ±0·00002; for La Jolla Nd it was ±0·000011 (0·2 Nd units); for NBS 981 Pb it was ±0·011 for 206Pb/204Pb, ±0·010 for 207Pb/204Pb and ±0·031 for 208Pb/204Pb. Within-run errors on the isotopic data above are less than or equal to the external uncertainties on these standards. Uncertainties on the isotope-dilution abundance data are 0·2% for Sm and Nd, 0·4% for Sr, and 1% for Pb and Rb. Total procedural blanks are 20–25% bulk contamination of suitable magma compositions, similar to estimates for the Bushe Fm (e.g. Lightfoot, 1985; Mahoney, 1988). Figure 9b shows an example for SH114 (the lava with the highest mg-number), assuming a purely felsic average Archean crustal end-member and no post-contamination fractionation. Of course, given the wide range of rock compositions present in Archean and early Proterozoic terrains, such averages represent at best only crude approximations to actual contaminant compositions; also, bulk assimilation is unlikely to be the main process operating at large scales (e.g. Sinigoi et al., 1996). Nevertheless, in Fig. 8, schematic mixing curves are included to illustrate that key features of the low-Nd basalts are consistent with contamination by broadly similar material, followed by variable fractionation. For these curves, the high-Nd mantle end-member is assumed, as in the southwestern and northeastern Deccan (e.g. Peng et al., 1994, 1998, and references therein), to be isotopically equivalent to the least-contaminated Ambenali Fm basalts. However, because the Ambenali basalts themselves are differentiated (most are ferrobasalts) and also preserve
a record of progressively decreasing degree of partial melting in the source (e.g. Mahoney et al., 1982; Cox & Hawkesworth, 1985; Devey, 1986; Devey & Cox, 1987; Lightfoot et al., 1990), they are not a suitable chemical end-member for the magmas inferred in the genesis of the low-Nd Toranmal (or Bushe Fm) basalts. For the illustrative mixes in Figs 8 and 9b we assumed an unfractionated, high-MgO basaltic composition and an average transitional-MORB (mid-ocean ridge basalt) incompatible element pattern, flatter than the Ambenali pattern (i.e. corresponding to a higher degree of partial melting) yet consistent with the required Ambenali source characteristics (e.g. Lightfoot & Hawkesworth, 1988; Mahoney, 1988). Evidence for the existence of magmas with relatively flat patterns before contamination is provided at Toranmal by the isotopically Poladpur-like sample SH112 [Nd(t) = +0·4], which reflects a much smaller amount of contamination than the low-Nd basalts yet, like them, has a relatively flat pattern from Nd to Lu (Fig. 6e); the SH112 flow lies just beneath the upper group of low-Nd lavas (Figs 2 and 4d). Also, littlecontaminated lavas with rather flat incompatible element patterns and even higher Nd(t) (to +2·5, only slightly lower than the Ambenali Fm range) are present west of Toranmal in the northwestern sector of the Deccan (see next section).
Regional geochemical comparisons Comparison with the northwestern Deccan Geochemical stratigraphic work in the northwestern Deccan is severely hindered by a lack of good exposures. Relatively few multielement isotopic and comprehensive trace element data are available, and fewer 40Ar–39Ar ages and paleomagnetic measurements; nevertheless, the existing data permit some first-order conclusions to be made. Geochemical studies of this region have focused on cores from drillholes (penetrating lava sequences 150–420 m thick) near Dhandhuka, Wadhwan and Botad (Fig. 1), on outcrops in the Rajpipla–Navagam area 60–100 km west of Toranmal in the Narmada graben, and on relatively thin lava sections and intrusions in several other areas (Alexander & Gibson, 1977; Krishnamurthy & Cox, 1977, 1980; Mahoney et al., 1985; Melluso et al., 1995; Peng & Mahoney, 1995). As noted in the Introduction, many of the northwestern lavas are rather alkalic, unlike the Toranmal samples (except SH90, and its high alkali element content appears likely to be a result of alteration). Also, a number of the lavas in the drillholes (West, 1958; Krishnamurthy & Cox, 1977) and elsewhere (Melluso et al., 1995) are picritic or high-MgO basalts, which are not present at Toranmal. Further, the drillhole lavas define very different Nd–Pb and Sr–Pb isotopic trends from any of the southwestern or Toranmal
1113
JOURNAL OF PETROLOGY
VOLUME 41
basalts, such that in Fig. 3a and b their data fields (not shown) would be nearly vertical (Peng & Mahoney, 1995). In addition to relatively alkalic lavas similar to those in the drillholes, Melluso et al. (1995) described several areas where tholeiitic flows with fairly flat primitivemantle-normalized Nd-to-Lu patterns and variable enrichment in the highly incompatible elements are exposed. Unlike the low-Nd Toranmal basalts, these lavas range from picritic basalts to rather low-MgO basalts. Isotopic data for these lavas ( J. Mahoney & L. Melluso, unpublished data, 1998) reveal a range of values [e.g. Nd(t) = +2·5 to –6·0] very different from those of the low-Nd Toranmal basalts. The only hints of a possible petrogenetic link between these flows and the low-Nd basalts are provided by (1) SH112 [with Nd(t) = +0·4], which lies immediately beneath the upper three low-Nd flows, and (2) the relatively high-MgO pre-contamination magmas inferred for these Toranmal flows (see the previous section); it may be that similar high-MgO magmas with relatively flat incompatible element patterns were involved in the generation of both the low-Nd Toranmal basalts and these northwestern lavas. If so, the contamination processes and/or pathways involved appear to have led to different end-products, in general. A possible exception is a flow with broadly Bushe-like Sr and Nd isotopic ratios reported from the extreme northwestern corner of the Deccan (Krishnamurthy et al., 1988). Comparatively close to Toranmal, the flows in the Rajpipla–Navagam area consist of interbedded, isotopically similar alkalic and tholeiitic basalts, which define a Nd–Sr isotopic array that coincides with the Poladpur Fm field at positive values of Nd(t) but, unlike the Poladpur-type Toranmal lavas, diverges from it at lower values toward lower ( 87Sr/86Sr)t (Mahoney et al., 1985). Pb isotopes have not been measured for these lavas, and trace element data are limited. However, all of the lavas analyzed for Nb, for example, have high Nb contents; among the tholeiites, Nb varies from 27 to 52 ppm. Only the low-MgO (4·02–4·85 wt %) Toranmal flows SH92, -94 and -109 have Nb abundances in this range, but the Navagam tholeiites all have higher MgO, mostly in the 6·7–8·6 wt % range. The Navagam tholeiites also have Sr contents between 343 and 592 ppm, whereas the Toranmal flows all have values 380 km from Toranmal, somewhat south of the latitude of Bombay (e.g. Beane, 1988), and the southernmost known occurrences in the southeastern Deccan (Mitchell & Widdowson, 1991; Bilgrami, 1999) are >580 km from Toranmal. The only other formation that may have a comparable extent is the Ambenali (Peng et al., 1998). The existence of sections containing Poladpur-type basalts in the corridor between Toranmal and the southwestern type-sections would effectively settle the issue. As noted earlier, broadly Poladpur-like flows are exposed in the Shahada–Shirpur area east of Kondaibar in thin (450–470 m; inclusion of the isotopically Poladpur-like low-MgO flows (SH92, -94, -106 and -109) increases this value to >550–570 m in at least 16 flows. If the Toranmal lavas indeed represent the Poladpur Fm, this formation thus would be of comparable thickness or even thicker far to the north than to the south of Bombay. If so, only a fraction of the geochemical diversity in the formation as a whole is seen in either the northern or the southern
1115
JOURNAL OF PETROLOGY
VOLUME 41
Deccan. For example, several lavas in the southwest have Nd(t) < –4 (see Poladpur field in Fig. 3c); values this low have so far not been found among the northern or northeastern Poladpur-type basalts. Likewise, as noted earlier, the abundant Poladpur-like flows in the northeastern Deccan (with a thickness of >340 m at Chikaldara) possess similar elemental and Nd–Sr isotopic characteristics to Poladpur flows in the south but generally have higher 206Pb/204Pb (Peng et al., 1998). At Toranmal, lavas with both southern-type and northeastern-type (i.e. SH102 and -104) isotopic characteristics are intercalated, providing an important link to the northeastern sections. Interestingly, however, despite the relative proximity of the Toranmal and Mhow sections (Fig. 1), the detailed stratigraphic–geochemical correspondence between them is poor. The main similarity is that several low-Nd, broadly Bushe-like flows are present in both areas. No isotopically Mahabaleshwar-Fm-like basalts (like SH95 and -115) have been found in the Mhow area, and all of the lavas with Poladpur-like Nd–Sr isotopic signatures have high 206Pb/204Pb values, placing them to the right of the Poladpur Fm field in Fig. 3a and b, like only SH102 and -104 (of the samples analyzed) at Toranmal. Moreover, Toranmal lacks the chemically KhandalaFm-like lavas that are abundant in the Mhow section (Peng et al., 1998). Thus, most flows reaching Toranmal apparently did not reach the Mhow area, and vice versa, or the two sections expose somewhat different stratigraphic levels (see below). Presumably, most flows sampled in the two areas were fed by different feeder dikes. The Mhow section is on the north side of the Narmada graben, which was tectonically active before, during and after the Deccan event (e.g. Sheth & Chandrasekharam, 1997, and references therein), and structures associated with this feature may have effectively hindered most flows traveling in a direction between Toranmal and Mhow.
CONCLUDING REMARKS Our results, together with those of Peng et al. (1998), Chandrasekharam et al. (1999) and K. V. Subbarao et al. (unpublished data, 1998), indicate the extent of broadly Poladpur-type basalts to be considerably greater than recognized previously. Consideration of all the documented occurrences suggests that such basalts originally may have covered an area of 300 000 km2 or more; in comparison, the largest units in the Columbia River province have an areal extent roughly a third this size (Tolan et al., 1989). However, determining whether the northern and northeastern Poladpur-type flows and the southern Poladpur lavas belong to a single volcanostratigraphic formation, in the normal sense of a packet
NUMBER 7
JULY 2000
of flows emplaced everywhere in the same relatively brief interval of time, remains a critical problem. The thick southwestern Deccan lava pile contains only one magnetic polarity reversal, R (reversed) to N (normal), with formations below the Mahabaleshwar Fm all having R polarity (e.g. Vandamme et al., 1991, and references therein). In contrast, Venkata Rao et al. (1996) reported a lower R interval in the Toranmal section between 295 and 445 m [elevations are those in the stratigraphic section of Nair et al. (1996); see above], a very thin N polarity zone consisting of two inferred flows between 445 and 490 m, an upper R interval from 490 to 930 m, and discordant directions above 930 m, which they suggested recorded a transition to the N epoch seen in the southwest. In the Narmada graben to the southwest of Mhow, Sreenivasa Rao et al. (1985) and Dhandapani & Subbarao (1992) reported an N–R–N sequence and concluded that the lower N lavas at the bases of their sections were older than the entire southwestern Deccan stratigraphy. If the thin lower N interval at Toranmal represents a true N polarity epoch, then the R sequence below it would be even older. Yet four of the six flows below >500 m that we analyzed isotopically are Poladpur-type (SH100, -99, -94 and -93). It is difficult to evaluate the conclusions of Venkata Rao et al. (1996) because their data are presented only in the form of a brief summary. However, because the thin N interval is recorded by only two inferred flows and is not bounded on either side by a bole or other evidence of a protracted period between eruptions, we question whether this interval represents a true N polarity epoch. It may record a short-lived ‘cryptochron’ (such as those documented recently on the sea floor; e.g. Cande & Kent, 1992) within chron 29R, the polarity epoch to which most workers assign the main Deccan eruptions (e.g. Vandamme et al., 1991). Alternatively, it might be an artifact of sampling in physically disturbed zones; for example, in the entablature zone, where blocks of rock are often tilted or overturned, which may not be evident in areas of limited exposure like Toranmal. A third possibility, that the normally magnetized interval consists of a sill or sills, seems less likely as neither we nor Nair et al. (1996) observed any evidence of sills. In view of these uncertainties, two very different interpretations of the geochemical data are possible. If a true, early N polarity epoch is recorded at Toranmal, a petrogenetic connection with the southwestern Poladpur basalts remains plausible, but Poladpur-type lavas in the Deccan would not constitute a volcanostratigraphic formation in the usual sense. Rather, Poladpur-type magmas must be distinctly time-transgressive, having been produced in one or more places starting before the eruption of the stratigraphically lowest exposed lavas of the Jawhar Fm in the southwest until at least the emplacement of the uppermost southwestern Poladpur
1116
MAHONEY et al.
FLOOD BASALTS OF TORANMAL, DECCAN TRAPS
Fig. 10. Two possible relationships between the Poladpur-like portion of the Toranmal section and the southwestern (SW) Deccan formational stratigraphy. That on the right assumes that the thin N interval at 445–490 m reported by Venkata Rao et al. (1996) represents a true epoch of normal polarity; that on the left assumes that it does not. Tr, transitional polarity.
lavas (Fig. 10, right). Poladpur-type flows may have been confined to a relatively small part of the northern Deccan during the lower R and N polarity intervals, for example, becoming widespread to the south and east only during the upper R polarity epoch (note that the SH102 and -104 flows at Toranmal, which provide a link with the northeastern high-206Pb/204Pb Poladpur-type sequences, are at elevations above the purported N interval). In this case, future work is likely to discover Poladpur-type lavas interfingered with those of the lower southwestern formations in places in the region between Toranmal– Shirpur and Igatpuri. A somewhat analogous situation may be found in the Parana´ province of South America, where several spatially extensive lava types appear to span a nearly 10 my interval (e.g. Peate, 1997). Alternatively, if the thin lower N interval at Toranmal does not represent an epoch of N polarity, then the entire Poladpur-type sequence was formed during an epoch of R polarity, consistent with the magnetostratigraphy of the southern and central Deccan. The Poladpur-type lavas at Toranmal are then most likely to represent a northern remnant of a vast Poladpur Fm (Fig. 10, left), which, however, is geochemically more diverse than indicated by the flows exposed in any one region. We
note that the Poladpur-like lavas with high 206Pb/204Pb in the Chikaldara and Jabalpur areas are also in R polarity sequences, as are the Ambenali-like basalts above them [see Peng et al. (1998) and references therein]. Most of the lavas in the sections south of Mhow studied by Sreenivasa Rao et al. (1985) also have R polarity. Beyond confirmation and clarification of the reported N interval at Toranmal, systematic field, geochemical and paleomagnetic study of additional flow sections in the northern and central Deccan and of dikes in the western, northern and northeastern areas of the province should reveal how the various Poladpur-type lavas are related. The answer is vital for understanding the large-scale volcanic structure of the Deccan Traps, whether volcanism migrated with time (e.g. Beane et al., 1986; Devey & Lightfoot, 1986; Widdowson & Cox, 1996) and geochemical evolution on a province-wide, rather than just regional, basis.
ACKNOWLEDGEMENTS Keith Cox’s keen and abiding interest in Deccan geochemical stratigraphy was long a source of inspiration for our own efforts. We thank Peter Hooper for kindly
1117
JOURNAL OF PETROLOGY
VOLUME 41
making his ICP-MS data for the southwestern Deccan available, and Denys VonderHaar, Khal Spencer and Nancy Hulbirt for help with various aspects of the project. Ray Kent, Gautam Sen and Mike Widdowson provided constructive critical reviews. This work was supported by NSF Grant EAR-9418168 to J.M. and a Department of Science and Technology, Government of India, grant to D.C.
REFERENCES Alexander, P. O. & Gibson, I. L. (1977). Rare earth abundances in Deccan Trap basalts. Lithos 10, 143–147. Alle`gre, C. J., Birck, J. L., Capmas, F. & Courtillot, V. (1999). Age of the Deccan Traps using 187Re–187Os systematics. Earth and Planetary Science Letters 170, 197–204. Arndt, N. T. & Christensen, U. (1992). Role of lithospheric mantle in continental volcanism: thermal and geochemical constraints. Journal of Geophysical Research 97, 10967–10981. Baksi, A. J. (1994). Geochronological studies on whole-rock basalts, Deccan Traps, India: evaluation of the timing of volcanism relative to the K–T boundary. Earth and Planetary Science Letters 121, 43–56. Baksi, A. J., Byerly, G. R., Chan, L.-H. & Farrar, E. (1994). Intracanyon flows in the Deccan province, India? Case history of the Rajahmundry Traps. Geology 22, 605–608. Beane, J. E. (1988). Flow stratigraphy, chemical variation and petrogenesis of Deccan flood basalts from the Western Ghats, India. Ph.D. Dissertation, Washington State University, Pullman. Beane, J. E., Turner, C. A., Hooper, P. R., Subbarao, K. V. & Walsh, J. N. (1986). Stratigraphy, composition and form of the Deccan basalts, Western Ghats, India. Bulletin of Volcanology 48, 61–83. Bhattacharji, S., Chatterjee, N., Wampler, J. M., Nayak, P. N. & Deshmukh, S. S. (1996). Indian intraplate and continental margin rifting, lithospheric extension, and mantle upwelling in Deccan flood basalt volcanism near the K/T boundary: evidence from mafic dike swarms. Journal of Geology 104, 379–398. Bilgrami, S. Z. (1999). A reconnaissance geological map of the eastern part of the Deccan Traps (Bidar–Nagpur). In: Subbarao, K. V. (ed.) Deccan Volcanic Province. Geological Society of India Memoir 43(1), 219–232. Cande, S. C. & Kent, D. V. (1992). A geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 97, 13917–13951. Chandrasekharam, D., Mahoney, J. J., Sheth, H. C. & Duncan, R. A. (1999). Elemental and Nd–Sr–Pb geochemistry of flows and dikes from the Tapi rift, Deccan flood basalt province, India. In: Verma, S. P. (ed.) Rift-Related Volcanism: Geology, Geochemistry and Geophysics. Journal of Volcanology and Geothermal Research 93, 111–123. Courtillot, V., Fe´ raud, H., Maluski, D., Moreau, M. G. & Besse, J. (1988). Deccan flood basalts and the Cretaceous/Tertiary boundary. Nature 333, 843–846. Cox, K. G. & Hawkesworth, C. J. (1985). Geochemical stratigraphy of the Deccan Traps at Mahabaleshwar, Western Ghats, India, with implications for open system magmatic processes. Journal of Petrology 26, 355–377. Deshmukh, S. S., Sano, T., Fujii, T., Nair, K. K. K. & Yedekar, D. B. (1996). Chemical stratigraphy and geochemistry of basalt flows from the central and eastern parts of the Deccan volcanic province of India. In: Deshmukh, S. S. & Nair, K. K. K. (eds) Deccan Basalts. Gondwana Geological Magazine, Special Volume 2, 145–170. Devey, C. W. (1986). Stratigraphy and geochemistry of the Deccan Trap lavas, Western India. Ph.D. Dissertation, University of Oxford.
NUMBER 7
JULY 2000
Devey, C. W. & Cox, K. G. (1987). Relationship between crustal contamination and crystallisation in continental flood basalt magmas with special reference to the Deccan Traps of the Western Ghats, India. Earth and Planetary Science Letters 84, 59–68. Devey, C. W. & Lightfoot, P. C. (1986). Volcanological and tectonic control of stratigraphy and structure in the western Deccan Traps. Bulletin of Volcanology 48, 195–207. Dhandapani, R. & Subbarao, K. V. (1992). Magnetostratigraphy of the Deccan lavas, south of the Narmada River. Journal of the Geological Society of India 24, 63–79. Duncan, R. A. & Pyle, D. G. (1988). Rapid eruption of the Deccan flood basalts at the Cretaceous/Tertiary boundary. Nature 333, 841–843. Fodor, R. V., Frey, F. A., Bauer, G. R. & Clague, D. A. (1992). Ages, rare-earth element enrichment, and petrogenesis of tholeiitic and alkalic basalts from Kahoolawe Island, Hawaii. Contributions to Mineralogy and Petrology 110, 442–462. Govindaraju, K. (1989). 1989 compilation of working values and sample descriptions for 272 geostandards. Geostandards Newsletter 13, 1–113. Guha, S. (1995). Tectonic framework and evolution of the Tapi basin—an intracratonic half-graben in west–central India. Indian Minerals 49, 61–78. Hooper, P. R. (1990). The timing of crustal extension and the eruption of continental flood basalts. Nature 349, 246–249. Hooper, P. R. (1994). Sources of continental flood basalts: the lithospheric component. In: Subbarao, K. V. (ed.) Volcanism. New Delhi: Wiley Eastern, pp. 29–53. Hooper, P. R. (1997). The Columbia River province: current status. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. Geophysical Monograph, American Geophysical Union 100, 1–27. Jain, J. C. & Neal, C. R. (1996). Report of the ICP-MS facility, 1993–1996. Notre Dame University Open File Report, 30 pp. Kent, R. W., Saunders, A. D., Kempton, P. D. & Ghose, N. C. (1997). Rajmahal basalts, eastern India: mantle sources and melt distribution at a volcanic rifted margin. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic and Planetary Flood Volcanism. Geophysical Monograph, American Geophysical Union 100, 145–182. Khadri, S. F. R., Subbarao, K. V., Hooper, P. R. & Walsh, J. N. (1988). Stratigraphy of Thakurvadi Formation, western Deccan basalt province, India. In: Subbarao, K. V. (ed.) Deccan Flood Basalts. Geological Society of India Memoir 10, 281–304. Knaack, C., Cornelius, S. & Hooper, P. R. (1991). Trace element analysis of rocks and minerals. Washington State University 1991 Open File Report, 36 pp. Krishnamurthy, P. & Cox, K. G. (1977). Picrite basalts and related lavas from the Deccan Traps of western India. Contributions to Mineralogy and Petrology 62, 53–75. Krishnamurthy, P. & Cox, K. G. (1980). A potassium-rich alkalic suite from the Deccan Traps, Rajpipla, India. Contributions to Mineralogy and Petrology 73, 179–189. Krishnamurthy, P., Pande, K., Gopalan, K. & Macdougall, J. D. (1988). Upper mantle xenoliths in alkalic basalts related to Deccan Trap volcanism. In: Subbarao, K. V. (ed.) Deccan Flood Basalts. Geological Society of India Memoir 10, 53–69. Lassiter, J. C. & DePaolo, D. J. (1997). Plume–lithosphere interaction in the generation of continental and oceanic flood basalts: chemical and isotopic constraints. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. Geophysical Monograph, American Geophysical Union 100, 335–355. Lightfoot, P. C. (1985). Isotope and trace element geochemistry of the south Deccan lavas, India. Ph.D. Dissertation, The Open University, Milton Keynes, UK.
1118
MAHONEY et al.
FLOOD BASALTS OF TORANMAL, DECCAN TRAPS
Lightfoot, P. C. & Hawkesworth, C. J. (1988). Origin of Deccan Trap lavas: evidence from combined trace element and Sr-, Nd- and Pbisotopic studies. Earth and Planetary Science Letters 91, 89–104. Lightfoot, P. C., Hawkesworth, C. J., Devey, C. W., Rogers, N. W. & van Calsteren, P. W. C. (1990). Source and differentiation of Deccan Trap lavas: implications of geochemical and mineral chemical variations. Journal of Petrology 31, 1165–1200. Mahoney, J. J. (1988). Deccan Traps. In: Macdougall, J. D. (ed.) Continental Flood Basalts. Dordrecht: Kluwer Academic, pp. 151–194. Mahoney, J. J., Macdougall, J. D., Lugmair, G. W., Murali, A. V. & Gopalan, K. (1982). Origin of the Deccan Trap flows at Mahabaleshwar inferred from Nd and Sr isotopic and chemical evidence. Earth and Planetary Science Letters 60, 47–60. Mahoney, J. J., Macdougall, J. D., Lugmair, G. W., Gopalan, K. & Krishnamurthy, P. (1985). Origin of contemporaneous tholeiitic and K-rich alkalic lavas: a case study from the northern Deccan Plateau, India. Earth and Planetary Science Letters 73, 39–53. Mahoney, J. J., Nicollet, C. & Dupuy, C. (1991). Madagascar basalts: tracking oceanic and continental sources. Earth and Planetary Science Letters 104, 350–363. Melluso, L., Beccaluva, L., Brotzu, R., Gregnanin, A., Gupta, A. K., Morbidelli, L. & Traversa, G. (1995). Constraints on the mantle sources of the Deccan Traps from the petrology and geochemistry of the basalts of Gujarat state (western India). Journal of Petrology 36, 1393–1432. Melluso, L., Sethna, S. F., Morra, V., Khateeb, A. & Javeri, P. (1999). Petrology of the mafic dyke swarm of the Tapti River in the Nandurbar area (Deccan Volcanic Province). In: Subbarao, K. V. (ed.) Deccan Volcanic Province. Geological Society of India Memoir 43(2), 735–755. Mitchell, C. H. & Widdowson, M. (1991). A geological map of the southern Deccan Traps, India and its structural implications. Journal of the Geological Society, London 148, 495–505. Moore, C. L. (1997). Evaluation of regolith development and element mobility during weathering using the isocon technique. In: Eggleton, R. A. (ed.) The State of the Regolith. Geological Society of Australia, Special Publication 20, 141–147. Nair, K. K. K., Chatterjee, A. K. & Sano, T. (1996). Stratigraphy and geochemistry of the Deccan basalts along Toranmal section, western Satpura region. In: Deshmukh, S. S. & Nair, K. K. K. (eds) Deccan Basalts. Gondwana Geological Magazine, Special Volume 2, 23–48. Norrish, K. & Chapell, B. W. (1977). X-ray fluorescence spectrometry. In: Zussman, J. (ed.) Physical Methods in Determinative Mineralogy, 2nd edn. New York: Academic Press, pp. 201–272. Peate, D. (1997). The Parana´ –Etendeka province. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic and Planetary Flood Volcanism. Geophysical Monograph, American Geophysical Union 100, 217–246. Peng, Z. X. (1998). Geochemical studies of the northwestern, central and northeastern Deccan Traps, India: sources, petrogenesis, and relations to southwestern Deccan stratigraphy. Ph.D. Dissertation, University of Hawaii, Honolulu. Peng, Z. X. & Mahoney, J. J. (1995). Drillhole lavas from the northwestern Deccan Traps, and the evolution of Re´ union hotspot mantle. Earth and Planetary Science Letters 134, 169–185. Peng, Z. X., Mahoney, J. J., Hooper, P. R., Harris, C. & Beane, J. E. (1994). A role for lower continental crust in flood basalt genesis? Isotopic and incompatible element study of the lower six formations of the western Deccan Traps. Geochimica et Cosmochimica Acta 58, 267–288. Peng, Z. X., Mahoney, J. J., Hooper, P. R., Macdougall, J. D. & Krishnamurthy, P. (1998). Basalts of the northeastern Deccan Traps,
India: isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy. Journal of Geophysical Research 103, 29843–29865. Price, R. C., Gray, C. M., Wilson, R. E., Frey, F. A. & Taylor, S. R. (1991). The effects of weathering on rare earth elements, Y, and Ba in Tertiary basalts from southeastern Australia. Chemical Geology 93, 245–265. Rudnick, R. & Fountain, D. M. (1995). Nature and composition of the continental crust: a lower crustal perspective. Reviews of Geophysics 33, 267–309. Sen, G. & Cohen, T. H. (1994). Deccan intrusion, crustal extension, doming and the size of the Deccan–Re´ union plume head. In: Subbarao, K. V. (ed.) Volcanism. New Delhi: Wiley Eastern, pp. 201–216. Sheth, H. C. (1998). Geochemistry, petrogenesis, stratigraphy and structure of Deccan flood basalts of the western Satpura–Tapi region, India. Ph.D. Dissertation, Indian Institute of Technology, Bombay. Sheth, H. C. & Chandrasekharam, D. (1997). Early alkaline magmatism in the Deccan Traps: implications for plume incubation and lithospheric rifting. Physics of the Earth and Planetary Interiors 104, 371–376. Sheth, H. C., Duncan, R. A., Chandrasekharam, D. & Mahoney, J. J. (1997). Deccan Trap dioritic gabbros from the western Satpura–Tapi region. Current Science 72, 755–757. Sinigoi, S., Quick, J. E., Mayer, A. & Budahn, J. (1996). Influence of stretching and density contrasts on the chemical evolution of continental magmas: an example from the Ivrea–Verbano Zone. Contributions to Mineralogy and Petrology 123, 238–250. Sreenivasa Rao, M., Ramasubba Reddy, N., Subbarao, K. V., Prasad, C. V. R. K. & Radhakrishnamurty, C. (1985). Chemical and magnetic stratigraphy of parts of Narmada region, Deccan basalt province. Journal of the Geological Society of India 26, 617–639. Subbarao, K. V., Bodas, M. S., Hooper, P. R. & Walsh, J. N. (1988). Petrogenesis of Jawhar and Igatpuri formations, western Deccan basalt province, India. In: Subbarao, K. V. (ed.) Deccan Flood Basalts. Geological Society of India Memoir 10, 253–280. Subbarao, K. V., Chandrasekharam, D., Navaneethakrishnan, P. & Hooper, P. R. (1994). Stratigraphy and structure of parts of the central Deccan basalt province: eruptive models. In: Subbarao, K. V. (ed.) Volcanism. New Delhi: Wiley Eastern, pp. 321–332. Sun, S.-s. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publication 42, 313–345. Todt, W., Cliff, R. A., Hanser, A. & Hofmann, A. W. (1996). Evaluation of a 202Pb + 205Pb double spike for high-precision lead isotopic analyses. In: Basu, A. & Hart, S. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph, American Geophysical Union 95, 429–437. Tolan, T. L., Reidel, S. P., Beeson, M. H., Anderson, J. L., Fecht, K. R. & Swanson, D. A. (1989). Revisions to the estimates of the areal extent and volume of the Columbia River Basalt Group. In: Reidel, S. P. & Hooper, P. R. (eds) Volcanism and Tectonism in the Columbia River Flood-Basalt Province. Geological Society of America, Special Paper 239, 1–20. Turner, S. & Hawkesworth, C. J. (1995). The nature of the subcontinental mantle: constraints from the major-element composition of continental flood basalts. In: McDonough, W. F., Arndt, N. & Shirey, S. (eds) Chemical Evolution of the Mantle. Chemical Geology 120, 295–314. Vandamme, D., Courtillot, V., Besse, J. & Montigny, R. (1991).
1119
JOURNAL OF PETROLOGY
VOLUME 41
Paleomagnetism and age determinations of the Deccan Traps (India): results of a Nagpur–Bombay traverse and review of earlier work. Reviews of Geophysics 29, 159–190. Venkata Rao, K., Nair, K. K. K. & Padhi, R. N. (1996). Magnetostratigraphy of the Deccan basalts of the western Satpura region. In: Deshmukh, S. S. & Nair, K. K. K. (eds) Deccan Basalts. Gondwana Geological Magazine, Special Volume 2, 431–438. Venkatesan, T. R., Pande, K. & Gopalan, G. (1993). Did Deccan volcanism pre-date the Cretaceous/Tertiary transition? Earth and Planetary Science Letters 119, 181–189.
NUMBER 7
JULY 2000
West, W. D. (1958). The petrography and petrogenesis of forty-eight flows of Deccan Traps penetrated by borings in western India. Transactions of the Indian National Institute of Science 4, 1–56. Widdowson, M. & Cox, K. G. (1996). Uplift and erosional history of the Deccan Traps, India: evidence from laterites and drainage pattern of the Western Ghats and Konkan coast. Earth and Planetary Science Letters 137, 57–69. Wilkins, A., Subbarao, K. V., Ingram, G. & Walsh, J. N. (1994). Weathering regimes within the Deccan basalts. In: Subbarao, K. V. (ed.) Volcanism. New Delhi: Wiley Eastern, pp. 217–231.
1120
