Rajmahal traps petrogenesis

CHEMICALGEOLOGY ISO TOPE GEOSCIENCE ELSEVIER Chemical Geology 121 ( 1995 ) 73-90 Petrogenesis and timing of volcanism in the Rajmahal flood basalt province, northeastern India Ajoy K. Baksi Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA Received 26 April 1994; revision accepted 3 October 1994 Abstract A suite of rocks from the Rajmahal-Bengal-Sylhet Traps of northeastern India has been analyzed to ascertain the timing and duration of volcanism and elucidate their petrogenesis. 4°Ar/39Ar step-heating studies identified specimens that suffered post-crystallization loss of 4°Ar" and indicate the Rajmahal Province was extruded in ~ 2 Ma around 117 Ma ago. Trace- and rare-earth-element data suggest the existence of three different types of magmas. Rajmahal quartz tholeiites were formed from primary melts, following considerable gabbroic fractionation. Bengal Trap olivine tholeiites represent lavas formed by large partial melting of mantle material, leaving garnet in the residue. Alkali basalts in the Bengal Traps appear to represent partial melts of mantle containing LILE-enriched sections, rather than very small ( < 2%) melts of a garnet lherzolite source. Whole-rock ~lSO-values for slightly altered tholeiites fall in the range + 5.9 to +6.6%0, indicating mantle-derived melts that have suffered minor crustal contamination; two alkali basalts, formed following considerable crystal fractionation of primary magmas, yield values of ~ + 7.2%0. Sr-Nd isotopic analyses show two different contamination trends, overlapping those observed in an earlier study of surface Rajmahal quartz tholeiites, with the most primitive material showing S7Sr/S6Sr ~ 0.70400, 143Nd/144Nd ~ 0.51280 at 117 Ma ago. The Bengal Trap olivine tholeiites were formed following assimilation of high-S7Sr/S6Sr (granulitic?) material. The main contamination trend includes quartz tholeiites from the Rajmahal Traps and alkali basalts from the Bengal Traps. Tholeiites, showing considerable isotopic modification, suggest ingestion ofa high-Sr component, unlikely to be upper-crustal material; for the alkali basalts, with high Sr ( ~ 1000 ppm) and Nd ( ~ 55 ppm ) contents, incorporation of a few percent of"exotic" material (in the source region?) is indicated. Carbonatite is the probable contaminant, strengthening the postulated link between flood basalt volcanism and carbonatite-lamproites in this area. The occurrence of two lavas with reversed magnetic polarity, in association with the 4°Ar/39Ar ages reported herein, suggests the ISEA reversed event is displayed in the lavas of the Rajmahal Traps. 1. Introduction T h e Rajmahal Traps o f northeastern India (Fig. 1 ) consist o f at least 25 flows o f quartz tholeiitic and basaltic andesitic composition, with a total thickness o f up to 600 m, containing fEW] intertrappean shales and sandstones, and overlie ( U p p e r Triassic?) sediments and a m e t a m o r phic basement; in the eastern area, the lavas dip gently beneath the Gangetic alluvium (Pascoe, 1950; Klootwijk, 1971; M a h o n e y et al., 1983). T h e recovery o f basaltic material by drilling to depths o f ~ 5 k m in the Gangetic alluvium to the southeast o f the surface Rajmahal exposures (the 0009-2541/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSD10009-2541 (94)00124-3 indicated post-crystallization loss of 4°Ar*. and form a single flood basalt province (Baksi et al. 1991. 1. 7= Debagram.K. studied by the 4°Ar/ 39Ar incremental heating technique. K . 5=Burdwan. to enable comparison with the results of an earlier geochemical study on surface . 1987 ). Herein. together with locations of drillholes from which Bengal Trap samples were recovered (1=B-1038.74 A. 3=A-538. trace. For petrogenetic purposes. (Modified from Baksi et al. 1974). 4°Ar/39Arincremental heating studies led to better constraints on their crystallization ages. Baksi et al. ) Bengal Traps. 6=Galsi.. showing exposures of Rajmahal and Sylhet Traps.N d . 1971 ).6 ":':':•:':':':i :::::::::::::" MIIIIIRImlII]I~ INDIA Calcutta l ] Alluvium -~ Geosynclinol Sediments $h4111Sodimonts Trough SaKG~ts Volconics Prm-Comlxion Fig. These authors suggested "a reliable minimum age" of ~ 103-108 Ma for the outpouring of the lavas [ dates recalculated to the decay constants and isotopic abundances recommended by Steiger and J~iger ( 1977 ) ]. the lava underlying one of these sites showed a reversed overprint on a normal polarity. two sites. showed reversed polarity. Bengal and Sylhet Traps (lying ~ 400 km to the east) initially covered ~2-105 km 2. These results indicate the Rajmahal Traps were formed during the Cretaceous Normal Superchron. A detailed paleomagnetic study confirmed that the Rajmahal Traps show normal magnetic polarity. Baksi / Chemical Geology 121 (1995) 73-90 T 86* 26* 90° 88* 0 t 50 l I i OcahmapotrO 1(30 krns I •" Shillov iiiiiiii!iii!iii!!!:l m=almm'mmmmm~=ml l = t' " ~ . I present the results of further geochemical and geochronological analyses carried out on the same rocks.. 1987 ). A genetic link between the Kerguelen hotspot activity in the southeast Indian Ocean and the formation of the Rajmahal Traps has been debated in the literature (Mahoney et al. 4=Jalangi.A r dates and major-/trace-element chemistry for a suite of rocks from the Rajmahal-Bengal-Sylhet Traps were reported earlier (Baksi et al. Geological map of eastern India. A study of surface Rajmahal material showed the lavas were normally magnetized and gave scattered K-Ar dates (McDougall and McElhinny. indicating that the reversed polarity of the overlying lava is not spurious (Klootwijk. 1983. and an age of > 108 Ma (Dalrymple and Lanphere. out of a total of 27...and rare-earth-element (REE) concentrations and S r . see Fig. 1987. 1987. A section of the Rajmahal Traps displays a short reversed event within the Cretaceous Normal Superchron. 8= Nadia). 1 ) has been detailed elsewhere (Sengupta. it was suggested the Rajmahal.O isotopic composition were determined. A single whole-rock specimen. Curray and Munasinghe.. 2=A-531.. 1993). 1966). Muller et al. Based on K-Ar dates and geochemical similarities. 1970). examined under the microscope at LSU. 1972) were accepted as yielding useful geochronological information.S. California. 1990) as the monitor. since only crushed samples were available at LSU.5 (Brooks et al. 1978 ) and exhibiting MSWD-values of <2. Baksi / Chemical Geology 121 (1995) 73-90 Rajmahal rocks (Mahoney et al. Some of the specimens reported on earlier. For the former set. Thin sections of the rocks were obtained from D. Ontario.. 1969). 1987) are generally in good agreement with ICP-MS values. 1991 ). two alkali basalts and two olivine tholeiites from the Bengal Traps.. and improved the quality of the results obtained (Baksi and Farrar..9 Ma for SB-3 Biotite. 1981 ). Hence. Paul (India). A plateau is defined as three or more contiguous steps. 4°Ar/39Arage determinations were carried out at the U. thus.. plateau sections for intermediate-high . details of the techniques used are available elsewhere (Baksi.S.K.R.A. the values of the following elements are suspect due to contamination [ see Frey et al.. corrections for interfering reactions were applied following Dalrymple et al. Lanphere et al. 1987 ) and Ta (this study). 4°Ar]39Ar dating results The data were evaluated on age spectra and isochron plots.. Paul. except for Rb and Nb ( < 10 ppm). carrying > 50% of the total 39Ar. pers. 2. Canada (Q) using coarse-crushed whole-rock basalts. O'Neil. commun. U. In an earlier report (Baksi et al.J. thinsection descriptions were obtained from coauthors in India. These new results place tighter constraints on both the timing of volcanism and the petrogenesis of this province. 1969 ).. For the analyses at Queen's University. whole-rock oxygen isotopic values were determined by J. involving 20 min of ultrasonic cleaning of powdered material in dilute nitric acid. All 4°Ar/ 39Ar ages reported herein are relative to this standard. Step-heating was done by radio-fre- 75 quency induction heating and the gas samples analyzed on a multi-collector mass spectrometer (Stacey et al. neutron irradiations were achieved at the McMaster University Reactor.. Baksi et al. 1992). basalt standard BHVO-1 was analyzed to verify the accuracy of these results. In particular. Kingston.9 Ma. and further studied herein. Sr-Nd isotopic compositions were measured at The Open University (U. ( 1981 ). 1992).K.yielding ages that agree within 95% confidence limits using a standard confidence value test (Dalrymple and Lanphere. using SB-3 Biotite (age= 162. Steps defining plateau sections were examined on isochron plots (York. Some of the rocks were analyzed following acid washing. a split of one of their uncontaminated rocks was also analyzed. A summary of the geochronological results is presented in Table 2. 3.K.K. 1 for sample location sites). the TRIGA reactor was used for neutron irradiation. 1990). 1987). ( 1991 ) for further discussion ]: Co (Baksi et al. using Fish Canyon Tuff-3 Biotite (age=27. 1983). straight lines yielding initial argon ratios not significantly different from the atmospheric argon value (Lanphere and Dalrymple. and a single quartz tholeiite from each of the Sylhet and Bengal Traps were studied (see Fig.A. 4°Ar/39Ar dating and geochemical studies indicated that some of the whole-rock specimens had suffered sufficient alteration to be discernible in petrographic examination. N b / T a ratios for surface Rajmahal rocks fall in the range 4-7. 1994).. were crushed in a tungsten carbide mill prior to shipment to LSU (D.. Trace elements and REE were determined by inductively coupled plasma mass spectrometry (ICP-MS) at Washington State University. Geological Survey at Menlo Park.95 Ma calibrated to 162. confirmed visible alteration in some of the rocks (see the Appendix). followed by repeated washing in distilled water. Analytical techniques Three surface Rajmahal Trap quartz tholeiites. as compared to ~ 20 for the other rocks (see Table 1 ). Hawkesworth following standard techniques (Lightfoot et al.) by C. (MP) and at Queen's University. earlier ICP determination of major and trace elements (Baksi et al. This procedure removes alteration products from the rocks. Analytical data are listed in Table 1. 7 25.19 10.4 0.9 59.d.7 0.5 5.0 0.6 5.1 0.70543 -0.62 7.7 1.1 0.7 0.50 3.A.70804 n.7 4.95 0.1 0.2 6.2 5.3 51.0 6.8 0.9 Major and minor elements analyzed by XRF for RM82-8 and BHVO.3a 0.38 3.2 0.0%. end{77 presumes present-day CHUR 143Nd/~*4Nd= 0.3 2.3 0.3 0.1 14.9 13.62 10.7 4.9 94 12.8 15. n.7 0.32 2.34 755 3.2 1.d.2 2.2 16.8 31 34 7. rare-earth and trace elements by ICP-MS at Washington State University.29 0. "Ta values in error (see text ).1 4.5 51.7 4.36 625 2.d.9 13.8 15.29 1.5 0.6 0. Baksi / Chemical Geology 121 (1995) 73-90 76 Table 1 Results of geochemical analyses on basalts from the Rajmahal Province Rock type RM82-8 A-531 A-538 B-1038 QT QT QT QT Nadia QT Burdwan OT Jalangi OT 49.23 10.7 6.8 7.5 0.60 6.56 0.9 2.15 2.6 1.85 16.34 10.41 2.92 2.30 130 1.5 0.69 0.23 2.70439 +2.1 10.15 0.00010 for interlaboratory difference in analyses reported for NBS 987 Sr.2 2.1 36 2.7 2.29 9.99 13.d.75 0.94 9.86 8.3 1.19 0. Sr isotopic value lowered by 0.61 8.5 2.4 4.70544 +0.9 14.4 0.38 1.5 0.6 5.J.93 8. bResults from Mahoney et al.6 111 13.59 13.53 2.3 20 28 4.70394 +2.87 5.2 15.6 2. Minor elements (ppm ): Ni Cr V Sr Zr Zn 50 195 270 230 100 95 55 240 310 215 95 95 45 1 l0 330 225 115 110 55 75 330 260 120 110 35 45 355 280 215 115 210 790 215 350 90 85 190 690 225 355 90 80 220 360 165 970 390 120 200 280 160 1.3 3.0 37. 0.512638.3 2.95 6. c o r ~ e d to 117 Ma. Hawkesworth.02 10.6 1.5 7. +6.8 +4.0 2.0 1.2 9.9 15.4 7.1 3. Major-element totals normalized to 100.4 0.50 10.62 3.7 0.1 38 2.75 0.7 14. Rare.0 0.1 1.26 2.2 0.50 0.0 4.6 0.91 2.8 5.6 1.1 0. .9 Debagram AB Galsi AB BHVO-I Standard Major elements (%): SiO: A1203 TiO2 FeO* MnO CaO MgO K20 Na20 P205 LOI 52.9 50.85 44.29 0.1 b n.23 54.3 0.3 0.5 2.9 0.7 0. = not determined.71 4.9 1.45 2.17 10.3 0.7 0.29 1.9 1.2 40 35 8.5 8.90 10.43 70 0.70403 n.2 1.9 0.3 14.3 1. ( 1983 ). O-isotopic compositions determined by J.96 6. OT = olivine tholeiite.30 1.0 0.28 2.3 8.4 0.8 3.92 0.4 7.2 0.6 0.6 0.9 0.3 0.17 10.44 3.4 +6.5 31 2.5 1.86 5.4 5.K.5 23.9 1.1 5.0 14.7 0.7 0.R.4 9 n.17 I 1.41 2.4 0.39 5.3 9.9 9.0 54.d. 5.4 0.23 0.6 0.51 9.59 165 2.35 2.7 6.4 7.8 1.27 160 0.4 45 4.00 3.0 0.06 12.14 6.1 9.5 51.1.1 1.5 51.5 4.75 4.70405 b +3.2 0.1 5.3 0.3 1.3 3.7 1.7 40 0.4 3.5 3.2 4.d.0 0.8 1.7 5.5 1.4 1.28 n.4 1.4 0. LOI = loss of weight on firing at 1000°C for 20 rain.4 2.4 51.8 46.4 12.08 1.3 1.09 2. Sr and Nd isotopic compositions determined by C.7 0.8 13.9 0.70606 .9 23.8 0.50 3.1 6.16 2. 1 6.20 10.83 2.4 0. Rock types: QT = quartz tholeiite.4 0.13 1.9" 0.8 0.95 2.72 0.02 0.3 2.7 1.17 2.6 4.13 22.4 5.47 365 0.20 11.2 23.0 3.3 3.77 3.0 14.45 100 0.5 1.3 24.35 0.5 21.41 60 0. +5. n.100 440 115 105 260 310 390 165 95 9.5 1.13 10.29 2.5 49.1 3.2 0.6 0.2 0.8 2.1 +7.1 22 2.19 1. O'Neil.d.6 30 2. AB = alkali basalt.28 2.9 15.03 0.1 2.d.30 165 0.70828 +0.2 13.7 0.9 23 2..d.1 0.1.0 n.9 0.1 1.3 0.76 12.3 0.5 22.9 0.75 10.earth and trace elements (ppm): La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ba Th Nb Y Hf Ta U Pb Rb Cs (S~Sr/~TSr)r ~rqd~r~ (%o) 6180 (%0) 7. primarily by ICP for other rocks (see Baksi et al.0 +7.9 I.10 2.5 10.55 3.1 +6.2 0.5 50.3 3.6 4.6 0.1 0.12 3. 1987).0 12.12 5.5a 0.3 +6.7 2.71 8. I 33 1.1 1.9 1. n.8 0.1 0.7 15.28 1.4 0.44 2.9 3. 10 g. ( 4 ° A r / 3 9 A r ) i = 386 ± 30. M S W D = 5. 10 g.43 g. M S W D = 1.4 _+0.5 Ma (degassed at 450°C for 25 min ) no plateau. 1. steps 3-5 ( 7 2 0 . 53% gas): age = 105 -+ 15 Ma. 7 steps. 740-1060°C.64. M S W D = 0. ascending step ages isochron (steps 4-6. 8 steps.30 g.3 _+ 1. (4°Ar/39Ar)i= 297 _+ 11. All errors listed at the 16 level. .1 _+0. 1.3 Ma isochron result: age = 114. Canada.03%.3 Ma. 1. Bengal Traps: K = 3.A r date = 108 ± 4 Ma Q (AW). 9 steps.04 g.5 Ma. 9 steps.A r date = 117 _+2 Ma a MP. 48% gas ) = 117.5 -+ 2.21%. Bengal Traps: K = 0. K . 46% gas). (4°Ar/39Ar)L= 238 ± 37.3 Ma isochron result: a g e = 116.1 Ma plateau age (steps 4-8. .8 _+2. 1. total gas a g e = 118. olivine tholeiite. (4°Ar/39Ar)~= 290 ± 35. "total gas" a g e = 117.0 _+0. 8 steps. total gas age = 114. Sylhet Traps: K = 0.5 ± 1.1 _+ 1. age = 116.9-+ 1. 650-790 ° C. total gas a g e = 85.7 _+2.3 _+2.7.A r date = 88 -+ 2 Ma MP.5 Ma no plateau Q. 7 steps.A r date = 107 _+3 Ma Q ( A W ) .9 Ma marginal plateau age ( steps 5 .85%.12%.8 Ma (degassed at 450°C for 25 min ) no plateau. 5 steps.5 _+ 1.8 9 0 ° C . M S W D = 0 . Nadia.12 g.A r date = 128 _+7 Ma M P. m i n i m u m step age ( 710 ° C) ~ 120 Ma M P = d e t e r m i n e d at U.A.8 B.A r data from Baksi et al. Baksi / Chemical Geology 121 (1995) 73-90 77 Table 2 S u m m a r y o f 4°Ar/39Ar step-heating analyses on whole-rock basalts from the Rajmahal Province Debagram. "total gas" age=91.8-+ 1. no K . 6 4 0 . 7 (4°Ar/39Ar)i = 420 -+ 220.2 _+0.3 Ma (degassed at 450°C for 25 rain ) plateau age (steps 2-4. saddle-shaped age spectrum. 6 steps.. 53% gas): a g e = 110. "total gas" age = 121.12%.9 ± 2. Rajmahal Traps: K = 0. K . I. (4°Ar/39Ar).64 A-531. M S W D = 0. 0. total gas a g e = 117. (4°Ar/39Ar)i = 430 -+ 140.A t d a t e = 118 _+2 Ma Q. K . "'total gas" age = 103. steps 2-3 ( 7 1 0 .0 _+ 1.3 _+ 1. Rajmahal Traps: K = 0. 7 steps. Bengal Traps: K = 2.2 Ma (degassed at 480 ° C for 25 min) no plateau. (1987).0 _+0.22%. 780-930 ° C. 800-1080°C. Menlo Park.06 g. total gas a g e = 118.9 5 5 ° C . age ~ 111 Ma isochron (steps 2-4. total gas age = 216. 1. 63% gas).0 Ma plateau age (steps 4 .5 ± 0. quartz tholeiite. California. 6 5 Q ( A W ) . M S W D = 2 .A r date Q (AW).3 Ma (degassed at 400°C for 25 min) no plateau. (4°Ar/39Ar)i=290 ± 65. K .A.50 g. 7 steps. A W = r o c k s washed in dilute nitric acid prior to fast neutron irradiation (see text).14%.56 RM82-8. I M a no plateau. U. a K . 1. quartz tholeiite.1 Ma. Bengal Traps: K = 0.1038. K . steps 3-5.6 _+2. 7 steps.2 -+ 2.0 Jalangi.0 Burdwan. 1.5 Ma i sochron result: age = 114. M S W D = 0. 43% gas). quartz tholeiite.A r date = 113 -+ 4 Ma MP. "total gas" age = 125 -+ 9 Ma (degassed at 550 °C for 30 min ) no plateau. Kingston.02 g.06 g.3 Ma isochron result: a g e = 118. 88% gas) = 117.5 Ma no plateau age: m a x i m u m step age 640°C ~ 110 Ma isochron (steps 4-8.40 g.A r date = 122 _+3 Ma Q. 0.6 Ma. "total gas" age = 158.7 _+3. quartz tholeiite. olivine tholeiite.2 -+ 2.23%. (4°Ar/39Ar)~= 405 -+ 50. Ontario. age ~ I 11 Ma isochron result: a g e = 111. M S W D = 0 . 70% gas) = 117. 640-890 ° C. 65% gas ) = 117.7-+ 1.7. 0.K.9 _+ 1. Bengal Traps: K = 0. 1. 40% gas): a g e = 123_+6 Ma. "total gas" age = 118. 5 Q ( A W ) . Rajmahal Traps: K = 0.5 Ma. m i n i m u m step age (710 °C) ~ 116 Ma Q ( A W ) .6 Ma (degassed at 500 °C for 15 m in ) plateau age (steps 3-6. K . 9 steps.S. K . M SWD = 1. Geological Survey.05 g. M S W D = 0. alkali basalt. K . (4°Ar/39Ar)i = 329 _+30. Q = d e t e r m i n e d at Queen's University.45 g.7 6 0 ° C .55 g.0 Ma Q. 66% gas ) = 115. 1 9 Galsi.9 _+ 1.=264_+ 9. age ~ 122 Ma isochron result: a g e = 116. 7 steps.18%. 7 steps. alkali basalt.6 _+ 1.8 M a . 33% gas. 0.2 Ma.S.3 Ma c o m b i n e d isochron result: age = 116. quartz tholeiite.05 A-538. 1.0 Ma (degassed at 450°C for 25 min) plateau age: steps 2-4 (640-810°C. "total gas" a g e = 113.2 Ma. 1±0. was calculated from steps 47 only. Baksi / Chemical Geology 121 (1995) 73-90 78 indicates no measurable amount of 39Ar has recoiled out of the sample. . . a 140 140 140 Pllle|u :E Plsteau Ages Age 117. 2S0" 140' :E O iii 60 n n 60 No plstssu Age .... 60 %39Ar .£_ . Bengal Traps Results of an analysis at MP... . 20 A tel -- ~-~ ..0 117.l S S Ms ? 300 :. .5±1..lo3e ..A t value ( 117 _+2 Ma.... an isochron plot for these steps yields essentially the same age. .. . respectively. Age spectra for 'mAr/39Ar incremental heating studies on whole-rock basalts from the Rajmahal Province. with an acceptable initial argon ratio and MSWD-value (see Table 2). exhibit features resulting from 39AFrecoil for finegrained whole-rock basalts (Turner and Cadsgas. J I r ... "L. .. 40 Cumulative . l JALANGI . "''+"''"1.i'[ . . 1974.5±0. .. . 00 O < ...A. t. B-I038: Sylhet Trap q u a r t z t h o l e i i t e .. 2. . The total gas age.. 2. A-531.. Nadia: Bengal Trap quartz tholeiite.. .Alkali basalt.J :~4 110" 80 100 50 20 40 Cumulative 60 80 %39Ar 100 . .S Ms ( 0 ) 12O C ~ 110 110 < DEBAGRAM 20 40 100 60 80 20 100 140 40 60 80 100 100 20 140 140 No plotssu Age • 111 Me A 110 GALSI 12o 120 130 100 120 r" ~ m . internal precision. l + 0. . 40 60 Plateau Age . This flow was erupted at 117 Ma.3 Ma..K. 117. : . ..9±2.. 1990. 170 12o < .... ..3±1.. shown in Fig.1 2 0 Ms No Plslsou Age > 111 Me ID 0 4[ tO0 40 * . "L. in good agreement with the earlier K . All errors shown and listed at the 1drlevel. 2) recoiled into contiguous K-poor phases degassed in the fusion step.. carrying 48% of the total 39Ar. 2O . . D. temperature steps were taken to reflect argon release from a mineral assemblage that has remained closed to K and Ar loss or gain since crystallization.. ......1. a "plateau" age.. 110 O0 r----... .3 Ms (MP) 117. Baksi et al. 80 el. .. the 39Ar lost from the K-rich phases degassed in steps 2 and 3 (see Fig. . Debagram . All errors are reported at the 1tr level. i f ....I 1~ L! .. 80 ~J::::::Ni'O:~ 100 .. Debagram and Galsi: Bengal Trap alkali basalt..3 130 40 Ms 210' No plateau age -105 leochron . . Ages 115.J I :::: BURD~NAN i 20 . Therefore. i O0 .. 20 . . 4 Io " . 1O 100 No pistesu Age ..+ < ::I . .i ~.3 Me 130 130 120 120 i i PIItIIU Me Ms 130 :. 100 20 40 Cumulative 60 80 100 %39Ar Fig. internal precision. A538 a n d RM82-8: Rajmahal Trap quartz tholeiites.. Solid and dotted lines denote runs at Menlo Park and Queen's University. 1987 ). 60 100 .. --- . 116. . Burdwan and Jalangi: Bengal Trap olivine tholeiites. . 3. .1±0..3 117. Baksi..6_+ 1.1 Ma. 1994). I 25O 20O i i .-. ... 150 e... : r .. 60 . . . 117.. It 35O Ms 130 I0 I:: .. • ~===:V-'~"~:~ .. 1009O . 2. serves as an older limit for the age of this rock (cf.5___0. a substantial fraction of the K within this rock resides in one of the alteration phases (smectite?). This lava flow was extruded at ~ 116 Ma. Rajmahal Traps This specimen. Steps 4-8 define an "isochron" (Table 2) with a low initial argon ratio and MSWD = 2.4 + 0. 2) show descending staircase patterns. the resulting age of 123 + 6 Ma is an overestimate of the correct value (cf.3 Ma.0+0. and the pooled isochron plot for the two runs yields essentially the same age. Burdwan .2. the second run resulted in more precise step ages. 2 ) indicative of postcrystallization loss of 4°Ar*. An isochron age of I 18. Plateau ages are 117. 4°Ar/39Ar total gas and plateau ages for the alkali basalts suggests no discernible post-crystallization movement of K by alteration. Rajmahal Traps An analysis at MP yielded an inverted Ushaped age spectrum (Fig. analyzed at Q following acid washing. A-531 .. define a "plateau" age of 115. yielding incorrect results (Baksi and Farrar. RM82-8 ~ Quartz tholeiite. The age spectra (Fig. The Q run.3.9 Ma at M P ) and the K .6. yields a three-step plateau of 117.3 Ma. 1987).5 + 1. indicates an age in the range 110120 Ma..A. Splits of this rock were analyzed twice at Q. since it was degassed at 480°C prior to incremental heating. and along with the age spectrum.1 -+ 0. part of an earlier geochemical study of the Rajmahal Traps (Mahoney et al. Lanphere and Dalrymple. This flow was formed contemporaneously with Debagram at 117 Ma. 3. and is supported by isochron analysis (see Table 2). and yields an inverted U-shaped age spectrum with maximum step ages of ~ 110 Ma (see Table 2). 1978 ). Gaisi . This rock degassed readily at low temperatures (75% of the total 39At released by 700°C. with no proper . Lanphere and Dalrymple.K.5 Ma for steps carrying only 47% of the total 39Ar (Baksi.6 Ma) is in agreement with the K-Ar date and indicates no 39Ar recoil out of the specimen.A r date ( 1 1 3 + 4 Ma. A second split.3 -+ 1.5 Ma.Quartz tholeiite. The agreement between K .5 3 8 . with an acceptable initial argon ratio and MSWDvalue (see Table 2 ).5. Rajmahal Traps Splits of this rock were analyzed both at MP and at Q.0_+0. 1987) indicates no overall recoil loss of 39Ar. since this can result in pressure scattering during mass spectrometry.. the results are shown in Fig. Isochron analysis yields the same age with a high MSWD-value.. The total gas age ( 118. compared to 25% for the geochemically similar A-531).Alkali basalt..Quartz tholeiite. This sets a lower limit for the age of crystallization. since the total gas age (85 _+3 Ma) is in agreement with the K-Ar date (88___2 Ma. 3. Three steps (5-7). Bengal Traps A preliminary run at MP yielded a plateau age of 117.A r dates. a degassing step at 500°C was used (see Table 2) to avoid over-loading the gas clean-up system. There is no overall loss of 39Ar.. 3. 2).5. carrying 41% of the total 39mr. The agreement between the total gas age ( 114. was analyzed after acid washing at Q.4. Baksi et al. Baksi / Chemical Geology 121 (1995) 73-90 3. 1978 ). A . though the age spectra show features resulting from 39Ar recoil redistribution within the rock (see Fig. shows the same type of age spectrum.5 Ma. 1983 ). The highest-step age (110 Ma) is regarded as a minimum value for the time of crystallization. Baksi et al. 1991). with maximum step ages of I 11 Ma (see Table 2 ). on an acid-washed split. although the age spectrum shows a pattern typical of fine-grained basalts exhibiting 39mr recoil redistribution. with an initial argon ratio lower than that of atmospheric argon and an acceptable MSWDvalue (Table 2). 3. the lowest-temperature-step age in the MP run indi- 79 cates a small amount of 4°Ar* loss due to alteration. and serves as a minimum value for the time of extrusion of this lava. for one of the runs. 1988).0 and 117.Olivine tholeiite. Bengal Traps Splits of the rock were analyzed twice at Q.8 + 1. Within the limited quality of the results obtained on these fine-grained. Nadia .. The low-temperature steps yield ages greater than the crystallization value due to: (a) possible recoil loss of 39mr out of the K-rich sites. 2 ) and is supported by isochron analysis (Table 2 ).. together with unu- sual geochemical signatures. Though no plateau section is recovered (Fig. the total gas age of 118. K-Ar results on flows and dykes from the Sylhet Traps (Sarkar et al.3 Ma is obtained (see Fig. Bengal Traps This specimen gave scattered K-Ar dates ( ~ 120-135 Ma. a run was attempted at Q. 2). Following acid washing to remove these alteration products. Jalangi . the lowest-step age is ~ 120 Ma. If a crystallization age of ~ 155 Ma is assumed.A r dates (Table 2). A three-step plateau of 116. 1987). drop monotonically to ~ 116 Ma at ~ 700 ° C and then increase steadily to ~ 215 Ma. 3. Sylhet Traps An acid-washed split was analyzed at Q following a degassing step at 550°C. altered rocks. define an isochron with an age of 116. whereas the highest-temperature fractions reveal ages younger than crystallization value resulting from 39Ar gain (by recoil) in these K-poor phases. their results suggest that the highest ages in inverted U-shaped age spectra for matrix and pyroxene fractions should closely approximate the crystallization age. It is unclear whether the minimum step ages carry geologically useful information. ( 1993 ) carried out 4°Ar/39Ar dating of fine-grained basalts from Antarctica with massive argon loss. 3. 1987 ). 2 ). 2).. and the total gas age ( ~ 217 Ma) is considerably older than the K .9 _+ 1. Analysis at MP indicates a refractory rock. carrying 46% of the total 39Ar. a n d / o r (b) excess argon contained in these phases. Baksi / Chemical Geology 121 (1995) 73-90 plateaux. Apparent ages > 1 Ga for the first few percent of gas. and most intermediate-high temperature steps yield ages of ~ 155 Ma. the Bengal Trap olivine tholeiites were formed contemporaneously with the alkali basalts (Debagram and Galsi) and Rajmahal Trap quartz tholeiite A-531. 9. Foland et al.5 km (see Appendix). The crystallization age is tentatively taken to be 116 Ma. releasing only 30% of the total 39Ar at temperatures < 925 °C. The very high initial step ages reflect gross 39Ar recoil loss out of clay minerals.2 Ma and an initial argon ratio considerably higher than the atmospheric value (Table 2 ). based on the 4°Ar/ 39Ar dating studies and earlier K-Ar results (Baksi et al.. Steps 3-5 of the second run. at ~ 117 Ma.9_2. recovered from a drillhole in the Gangetic alluvium from a depth of 4..Quartz tholeiite.5 _+2. A-538 and RM82-8) show very similar geochemical and isotopic signatures (see below). is inhomogeneous. The age spectrum shows (low precision) step ages increasing with extraction temperature.. This specimen. 1987). The age spectrum is unusual (Fig. 1976). and the minimum step age may be close to the crystallization age (Lanphere and Dalrymple.K. Saddle-shaped age spectra are common for igneous rocks containing excess 4°Ar. Thin-section examination.80 A. carrying 53% of the total gas ) yields an age of ~ 105 Ma with a high initial argon ratio (see Table 2). 3.5 Ma is in agreement with the K-Ar date of 122 + 3 Ma (Baksi et al. Bengal Traps An acid-washed split was analyzed at Q following a degassing step at 450°C. for the highertemperature steps are indicative of an age greater than ( ~ I 17 Ma) for other specimens in this study. The age spectrum displays features caused by recoil loss of 39Ar for a specimen showing partial loss of 4°Ar*. All three surface Rajmahal quartz tholeiites (A-531. it is noted that the 4°At* "lost" from the second step is balanced by the occurrence of "excess" 4°Ar in step 4 (see Fig. In the . This high-temperature result suggests the Sylhet Traps may be coeval with lava flows in the Rajmahal and Bengal Traps.Olivine tholeiite.8. an isochron plot (steps 5-7. the relevant section of the age spectrum shows features typical of minor recoil loss of 39Ar. The minimum step ages may indicate an older flow reheated by nearby Rajmahal volcanism at ~ 117 Ma. B-I038 ~ Quartz tholeiite. 1992) support this contention.7. suggest B-1038 may have been altered by reaction with groundwater. Baksi et al. "o to tO o IZ 10 i i i i i La C e P r Nd i i i i i t i f i i SmEu GdTb Dy Ho ErTmYb Lu 1000 ' OT k.4%0. see Table 1 ) and suggests interaction with groundwater (see below). I suggest the quartz tholeiites of the Rajmahal Traps and the alkali basalts and olivine tholeiites of the Bengal Traps were all formed within a short interval of time ( < 2 Ma) around 117 Ma ago. K and Ba contents in some instances. La C e P r Nd t i i i l t ~ i i i i SmEu GdTb Dy Ho ErTmYb Lu Fig. its 81SO-value is unusual ( + 4. it would be highly fortuitous if rocks as (geochemically) similar as A-531. 4. Rare-earth-elementconcentrations in Rajmahal Province basalts normalized to chondritic values: (a) all quartz tholeiites. in Fig.. The effects of alteration (on the alkali elements) should be most apparent in the 4°mF/39Ar age spectra. this may be reflected in Rb. 1992) and appears to postdate the volcanic episode forming the Rajmahal Province. 3 and trace-element data are presented in the form of PMN plots [abundances normalized to the primordial mantle values of Wood et al. 1987). . Modelling calculations were carded out for partial (batch)melting and crystal (Rayleigh) fractionation using the distribution coefficients of Hanson (1980) for REE and Henderson (1982) for other elements. 4. In the absence of stratigraphic control for the Bengal Trap material (see Appendix). .K. A-538 and RM82-8 were formed with an intervening period of ~ 6 Ma (117-111 Ma ago). the quartz tholeiite from the Sylhet Traps (B-1038) shows gross enrichment in Ba. In particular. Baksi / Chemical Geology 121 (1995) 73-90 Rajmahal Traps.A. The latter event is dated at 111 + 1 Ma (Obradovich. and (b) Bengal Trap alkali basalts (AB= average of Dcbagram and Galsi) and olivine tholeiites ( O T = average of Burdwan and Jalangi) compared to least contaminated Rajmahal quartz tholeiite. (1983).1. (1981 )]. REE concentrations are shown normalized to chondritic values in Fig. o o tr A-531 . whereas the deepest sample recovered from the Bengal Traps (Nadia) is considerably older. Neglecting the artifact of 39Ar recoil resulting from fast neutron irradiation of finegrained rocks. Thin-section examination (see Appendix) indicates all samples have suffered alteration. together with one of the least contaminated samples (RM82-8) from the collection of Mahoney et al. Loss on ignition values (see Table 1 ) are low for the surface Rajmahal tholeiites and generally higher for all drillhole recovered samples from the Bengal Traps. . Effects of alteration Table l presents the geochemical data for the rocks studied earlier (Baksi et al. 4. . 2 ). the alkali basalts and A-531 show minimal effects of alteration (see Fig. 3. it is not possible to evaluate the temporal relationship between the different rock types in this study. The Sylhet Traps may be of similar age. "o 100 to tO 10 (t3) 1 ' ' i . . It has been suggested that the extrusion of the Rajmahal Traps is genetically linked to the faunal extinction event at the Albian-Aptian stage boundary (Rampino and Stothers. The estimated precision ( _+5%) of the REE analyses reported herein indicates that only > 10% crystal fractionation of plagioclase would have led to detectable Eu anomalies in the REE plots. Trace-/rare-earth-element geochemistry 81 100 b. and their position relative to the Rajmahal Traps. 1988 ). Sr .. 3. strong enrichment of elements Ba through Y.1 R b B a T h i K . Weaver and Tarney.. 1983 ). suggest derivation from a depleted mantle source. although A-538 shows higher amounts of elements Rb through K (Fig. Mahoney et al. slight La enrichment and (La/Yb)N ~ 2 (see Fig. 4b) shows enrichment by factors of 50-100% over A531 for elements Rb through P and is satisfactorily modelled by 10% batch melting of a MORBtype source. These rocks. and show . Nd i P i Zr i Sm i Y TI 1000 g ~. FeO* contents and low C a O / A I z O 3 ratio. similar PMN patterns suggest A-531 and A-538 have suffered little or no contamination. by contrast. followed by fractionation of 1-2% olivine and titanomagnetite. Symbols as for Fig. 4a). (1983) suggested that RM82-8 was an uncontaminated sample. The Sylhet Trap rock (B-1038) shows a similar Based on low Y values. 4. most elemental abundances in these rocks are satisfactorily modelled by assuming 20% batch melting followed by fractionation of 10% ol + 10% cpx + 5% plag. for such rocks (Hergt et al. a relatively unaltered sample based on 4°Ar/39Ar dating results.. 4.. . Based on Sr-Nd isotopic data. Alkali basalts . Mg numbers ( ~ 68. with low levels of Rb and K. 100 10 REE pattern at slightly elevated levels . Baksi / Chemical Geology 121 (1995) 73-90 82 100 c lO O E el o er 4~ i i . A-538 and RM82-8) show humped REE patterns. J AB OT A-531 4. Olivine tholeiites ~ Bengal Traps l (b) E i .Bengal Traps Debagram and Galsi. are higher than the cut-off values of ~ 410 and ~ 6. and together with the geochronological results.e.(La/ Yb)N ~ 2.K. 4. 1981 ) for basalts from the Rajmahal Province. Baksi et al. Incompatible elements normalized to primordial mantle (Wood et al. which can be termed high-Ti basalts since Ti/Y ~ 500 and Z r / Y ~ 12. Ce .3. Low large-ion lithophile element (LILE) concentrations for A-531. whose age spectra argue for considerable alteration.A.. ( 1987 ) argued that Burdwan and Jalangi were formed by partial melting of a mantle source leaving garnet in the residue. The PMN plot (Fig..(La/Yb)N ~4..2. as the N b / T a ratio is ~ 20). not caused by laboratory contamination during grinding. Nb . with low MgO. the high values of Ta and Nb appear to be primary (i. This rock is chemically very different from the other quartz tholeiites. A-538 and RM82-8. 1987 ) also indicate these rocks were formed from primary melts with little crystal fractionation. The quartz tholeiite from the Bengal Traps (Nadia) shows higher amounts of REE and moderate to strong enrichment of the light REE (LREE) . respectively. Quartz t h o l e i i t e .. 3b) for comparable MgO contents. 4a). indicate Nadia is not comagmatic with the other rocks in this study. This is supported by the REE patterns.:1 . such magmas with relatively low LILE levels would be sensitive to crustal contamination (cf.3. were formed from primary melts following fractional crystallization of 15% cpx+ 5% plag. show significantly higher amounts of LILE. showing some enrichment in the LREE [ (La/Yb)N ~ 4 ] and the lowest values of the heavy REE (HREE) (see Fig.. 0 E .1 R b i i Ba Th i i i i K Nb La Ca I St i i i Nd P Zr t Sm i i TI Y Fig. 1991 ).4. La . Baksi et al.. It shows a low ( ~ 10) Sr/Nd ratio and displays a very different PMN plot (Fig.Rajmahal. Using a mid-ocean ridge basalt (MORB) -type source. 3a).. They exhibit steeply dipping REE patterns [ (La/Yb)N ~ 15 ]. Sylhet and Bengal Traps The surface Rajmahal rocks (A-531. indicating derivation as small partial melts.A. The quartz and olivine tholeiites lie close to the trend for partial melting of a primitive mantle.-. Low-degree melts of depleted sections of the upper mantle containing metasomatized sec- 83 tions could have led to the composition exhibited by Debagram and Galsi. (1993) suggested Antarctic Peninsula alkali basalts. serious divergences occur for K. melts derived from regions containing amphibole (K-richterite). Ti/Y plot (Fig. Hole et al. and some Formation of the Siberian Traps (Russia). but exhibit considerably lower amounts of LILE. whilst leaving the K content undisturbed. Trace-element ratio plots. amphiboles derived from the upper mantle typically contain < 2% K (Boettcher and O'Neil. Ba/Yb plot (Fig. Ba (and Nb?). the olivine tholeiites show higher ( ~ 85 ) values. 1992a) may have relevance for the chemical diversity of the lavas noted in this study. Karoo picrites. Cameroon Line (West Africa). In general. they appear to be primary. 1980). since undisturbed age spectra argue against loss of ~ 50% of Rb. Menzies and Murthy. An alternate mode of derivation would be higher ( > 10%) partial melting of metasomatized sections of the mantle (cf.'s ( 1983 ) data.K. 1980. relative depletion in HREE (see Fig. all tholeiitic samples define a trend that could represent mixing between a depleted source and an enriched component. chondritic Sr/Nd values ( ~ 18 ) and high levels of Sr and Nd ( ~ 1000 and . These rocks show high K/Rb ratios. and overlap with the range for Deccan basalts. Ratios such as Ce/Pb and N b / U should be useful petrogenetic indicators. and may reflect addition of continental material. 1993 and references therein).g. including Mahoney et al. which shows N b / U ratios of .. Though the quartz tholeiites and the alkali basalts show Nb/ U ratios close to oceanic magmas ( ~ 50. but with considerably higher Zr/Y values (see Lightfoot . Menzies and Murthy. and below the Antarctic Peninsula ~ 100 Ma later. the plume incubation model for large (continental) igneous provinces (Kent et al. highlight some petrogenetic features. as they are largely unaffected by partial melting processes. A source mixing-melting relation for the Rajmahal and Bengal Trap rocks is indicated on a La/ Yb vs. Karoo (South Africa) and South Australia. Though many continental alkali basalts show strong compositional similarities to ocean island basalts (see Hole et al.. 1980). For the alkali basalts.55 ppm. Alternatively. this cannot be due to incorporation of typical continental crust. the Sylhet Trap rock (B-1038) shows anomalous enrichment in Ba. exhibiting some unusual chemical signatures. Baksi / Chemical Geology 121 (1995) 73-90 enrichment of elements K through Sm (see Fig. Debagram and Galsi show unusual characteristics (high La/Nb. and Nadia departs significantly from the trend. lower Ba/K. This value is different from alkali basalts from other localities ( ~ 300-500). The alkali basalts show Ti/Y ratios similar to Paran~i high-Ti basalts (Brazil). Ce/ Pb ratios for samples herein fall in the range 813. Evaluation on a Zr/Y vs. On a Zr/Nb vs. unlike oceanic lavas ( ~ 25 ). however. respectively) reflect primary features. 4b). this may indicate differences in the mode of producing depleted upper-mantle sections below eastern India during early Cretaceous time. K/Nb and Ba/Th ratios) and these continental basalts appear to be derived from different mantle sources. could lead to high K/Rb ratios. were derived as lowdegree melts of sections of the upper mantle that had undergone a series of melt-extraction episodes. 10-15 (Hofmann. which is unlikely to be a residue in the presence of garnet. The K/Rb ratios of Debagram and Galsi ( ~ 750) are unusual for high-K basalts. surface Rajmahal specimens and quartz and olivine tholeiites from the Bengal and Sylhet Traps form a coherent group. which call for ~3 enrichment. such as the Deccan Traps (India).. REE) can be patterned by very small ( ~ 2%) partial melts of a garnet lherzolite source. 5a). U / N b and Ba/Zr ratios than Debagram and Galsi. If the Rajmahal Province is genetically linked to hotspot activity. which is too low. Hofmann. 1988 ).-. but are very different in oceanic magmas and continental crust. 3b) suggests garnet was left as a residue. typical crustal contaminants would lead to a subparaUel trend. 5c) permits comparison with rocks from other Mesozoic flood basalt provinces. Y/Nb plot (Fig. 1988). Most of the elemental concentrations (e. Ba/Nb. however. 5b). Jalangi. PM=primordial mantle. (1983). Sr and Nd isotopic ratios (Fig.0 to + 7. Field for Post-Archaean Terrestrial Shale of Taylor and McLennan ( 1985 ) shown in (c). fresh glass in this lava flow. . resulted from alteration. B-1038 (B). 5. Estimated analytical errors shown by bars" in (a).see Appendix). Washington. 5d). Fig. 5. 6). average oceanic basalt.9%0 for A-531 is close to that of uncontaminated mantle-derived melts (Kyser et al.N d .84 A. Mahoney et al. Baksi / Chemical Geology 121 (1995) 73-90 10 e j ¢ " [] . this is supported by a Rb/Ba vs.0 i 0. Muehlenbachs and Clayton. Carlson (1984) argued that a Columbia River basalt (ORGR N2-3. this suggests differences in their mantle source regions.. 20 La/Yb : ~ LAMPRC~TE • mm C~B Nh . (b) O .9%0. 1982). fig. -12 and R-20.5%0 higher than A-531. Three rocks (RM8211. U. / O n n Z~ + >- ~A 11111 (a) L 15 B O ~ ~ . (b) and (c). A-538 shows a slightly higher value of + 6. The olivine tholeiites show values ~ 0. typical of partial melts leaving garnet in the residue. wherein most samples show MORB like T i / Y ratios. 6 presents the Sr-Nd isotopic composition for rocks from the Rajmahal Province. et al. 6a . 1991 ).O isotopic data Whole-rock ~80-values for the slightly altered quartz tholeiites (Table 1 ) are typical for basic lavas. 1972). and reflect admixture of a crustal component to mantle-derived melts. The Sylhet Trap rock shows a very low value ( + 4.S.A. S r . 1983 ) suggest contamination of mantle-derived melts by sediments. Results of + 7. (1982) for oceanic alkali basalts.. was probably altered to smectite at low temperature. ) showing high Ba content and 8~80-value of + 4.. • --o 7- Sediment (d) 2 7 12 Zr/Y 100 0. OT=Burdwan. Plots of Ba/Zr vs. a value of + 5.6%o. ~ . Nadia (N). leading to an increase of ~ 1%o in the 3~80-value (cf. ( 1991 ). AB=Debagram and Galsi (all this study). Selected trace-element ratio plots for Rajmahal Province flood basalts.1 0. Legend: M83= samples from Mahoney et al.B l.. they plot between MORB and the lamproite field. QT=A-531.3%0 for the alkali basalts are within the range observed by Kyser et al. 1993. A-538..N 30 10 Zr/Nb .K.4%o) and possibly reflects post-crystallization reheating and recrystallization (in the presence of low-~80 groundwater? . Rb/Ba Fig. lamproite and sediment shown in (d) after Hergt et al. Since mantle melting processes do not readily affect Rb/Ba ratios (Hergt et al. Fields for MORB. Ti/Y plot (Fig. but with slightly low Rb/Ba values. a similar contaminant is in line with the ~ ]SO-values for Burdwan and Jalangi (see Table 1 ). Based on these data.5%o..7o9 ( 8 7 S r / 8 8 S r)T Fig. Z • "~. n 3 " "% Ba/Zr ' ~ * " 2 Ba/Zr " J ~ * o. 1983 ). 6c) noted by Mahoney et al. 1983).. (1983).. 6. 1985 ).K. with 878r/86Sr . continental shield areas with S r / N d < 15.1 1 . Nadia and B-1038 showanomalous positions (see text). 5. ( 1983 ). 1 ) and granulites are found in the Raniganj coalfields ~ 100 km to the southwest (Pascoe. Symbols as for Fig. Isotopic data corrected to 117 Ma. Jalangi.. c. and b) show linear arrays. -12 and R . Rocks R M l l . specimens Nadia and B-1038 are discordant. The Sr-Nd isotopic plot (Fig.F. 6c) includes the Bengal Trap olivine tholeiite(s) and a single surface Rajmahal rock (RM82-5. considerably enriched in 878r/86Sr (dotted line in Fig. A-538 (and A-531?) represents the least contaminated material. a and b.7o6 °6 o. The high-aTSr/a6Sr contamination trend (dotted line in Fig. All three rocks show higher ( ~ 25 ) S r / N d ratios than uncontaminated material ( ~ 18 ). Nd. A section of the Rajmahal Traps rests unconformably on metamorphic basement (see Fig. Mahoney et al. 6c) remains problematic.706 0 N • CO r. and supports the contention that the Rajmahal.. 6c). # • . A granulitic component is possible based on high ( ~ 3 0 ) S r / N d ratio (Taylor and McLennan... The contaminant controlling the main trend (solid line in Fig.. 1984). 1950). Granulites from various terrains show wide scatter on a Sr-Nd isotopic plot. Ba/Zr.-.5 (0. Taylor and McLennan. Sr-Nd isotopic composition plot.. the least contaminated material contains ~ 230 ppm of Sr. 6c) fits well with the results of Mahoney et al.3 ° • .. surface Rajmahal rocks showing significant contamination ( R M 8 2 . together with the high ( ~ 85 ) N b / U ratios of the olivine tholeiites (depletion of U?). As noted earlier (Mahoney et al.2 0 from the Mahoney et al.5 ppm. straight lines denote main contamination trends (see text). 1985). Sr and Nd isotopic composition for rocks from the Rajmahal Province vs. Baksi / Chemical Geology 121 (1995) 73-90 85 0.. (1983) collection appear to have assimilated upper-crustal material (sediment? .70400 and ~Nd(r) ~ + 2.. 1985).2%00 (A. shale. Wilson and Baksi. 1983 ). The former is not com- agmatic with the other specimens and the latter has suffered gross Ba enrichment. Bengal and Sylhet Traps are genetically linked. Mahoney et al. ( 1983 ) study. and a single specimen from the Mahoney et al.~ B -3 • e ~ oQ • (c) 3 " ' " " ' " ' " ' " . their low Rb contents (see Table 1 ) and that o f R M 8 2 . Most samples define mixing arrays.. represent contamination by different material. 1983 ) contain ~ 50% more Sr. Zr and Nb contents) than . atypical of upper-crustal material (granites.• ~+ ~ • ~7°3o (b) • o o B o eo.7o3 I i * o. These rocks contain ~ 50% more Sr (and higher Ba. Mahoney et al. o ~ N I ~ A o ~-- o r. 0.. often in the direction of high 87Sr/86Sr ratios (Taylor and McLennan. Most of the other specimens fall on the trend (solid line in Fig. n 2 . -12 and R-20.. 5c and d). Evaluation of the nature and source of contaminants reflected in the Sr-Nd isotopic plot is required.A.see Fig..'/~ (a) ~a I- "o oo '~ (1) CO 3 o • 0. O-isotopic ratios for non-quartzose mafic granulites from Western Australia average ~ + 7. 878r/86 and l i N d vs.... 87Sr/86 ~ 0.squares BengalTrap alkali basalts): ( a ) 1/Sr vs..... Galsi shows considerable enrichment in Ba.. • • .. .. i " i- .... ... :.08 (ppm " 1) Fig. . ..... Taylor and McLennan. The Bengal Trap alkali basalts lie on the same trend....- I-- 0.... 1985 ) seems to be required.... A contaminant with higher Sr content than upper-crustal material (Post-Archean Shale.... this is unlikely. ... end-Mixing lines suggest a contaminant with very high Sr and Nd contents (see text).- /_i /~Qi ~/ "=0 . 0. I I 0 ... as would result from crustal contamination.. ... table 1 )........ 0..--". Incorporation of upper-crustal material and considerable ( ~ 30%) crystal fractionation could produce these signatures... Sr-Nd isotopicdata evaluatedfor mixingtrends (circlesdenote Rajmahal Trap rocks.... ... 7) were utilized to gain insight into the nature of this additional isotopic component... 7.. together with ~ 120 ppm Nd.. the additional component could be upper-crustal material. .... . • I • • 0........ ...... i...04 liNd ...... Baksi I Chemical Geology 121 (1995) 73-90 86 positions for the two alkali basalts (see Table 1 ). -: • ...... 0 0 2 0 . • . Galsi shows higher C e / P b and N b / U ratios (enrichment in LREE and N b ) than Debagram..i.. .. whereas Galsi has suffered considerable isotopic modification.... where incorporation of a few percent of material with unusually high Sr and Nd contents would alter trace-element and isotopic compositions of the resulting magma more readily than comparatively larger amounts of material at the crustal level.0 I .. 0 0 4 l/Sr (ppm " 1) 5 .... Sr and Nb over the "uncontaminated" Debagram.. the Sr-Nd isotopic ratios of Debagram and the uncontaminated quartz tholeiites are similar.A. . STSr/~6Sr... '""i• .....K.. not higher U and Pb contents..s .708 I.... ... ... "o Z ¢0 i.. .. Further. .... these elemental enrich- the uncontaminated tholeiites.. . Iand S-type granites. -15 ..... ..~--- .... (a) :0I 0. ... which had ~ 2500 ppm Sr.. as it would have led to measurably different major-element com- 0.703 "....... Plots of 1/Sr vs. .. 1983.. This places severe petrologic constraints on the material... .. .. ii . ~NO (Fig..0 .. '.710... since the major-element composition of these rocks are similar to the uncontaminated ones (Mahoney et al... and (b) 1/Nd vs..11%0.. U) r~ . . this is unlikely.713 ~.. but requires > 10% crystal fractionation to produce the higher Sr and Nd contents of Galsi. Presuming Debagram was similar to the primary alkaline magma that was contaminated to produce Galsi... with carbonatite as the most likely candidate. eNd ~ ..... The composition of the alkali basalts may reflect contamination in the source region.. A lamproite dyke in the Jharia coalfield. pyroxene.. Elliott Kollman provided considerable help in the laboratory. 87 The data reported herein suggest the Rajmahal Traps were formed at ~ 117 Ma. The existence of a brief interval of reversed polarity ( < 100 ka) during this superchron. Baksi / Chemical Geology 121 (1995) 73-90 ments. These types of isotopic ratios occur dominantly in continental rift environments . have been noted for Proterozoic age material from Algeria (Bernard-Griffiths et al. 1992b.. would result from contamination by carbonatitic material (cf. Appendix . along with the elevated Z r / H f and La/ Sm ratios noted for the alkali basalts. This contamination hypothesis should be evaluated in light of the postulated link between the Rajmahal volcanism and carbonatite-alkaline suites (Kent et al. and supplying a split of the SB3 Biotite standard. Paul of the Geological Survey of India and a split of RM82-8 from Doug Macdougall. Chris Hawkesworth supplied the Sr-Nd isotopic data and Jim O'Neil the O-isotopic values. Acknowledgements This work was supported by a grant from the Ardhendu-Roma Foundation. Thus.8 Ma for the M0 anomaly.5%0 (Conway and Taylor. and two lavas exhibiting reversed magnetic polarity (Klootwijk. 1971 ).K. and is beyond the scope of this paper. 1992) in the rift dominated tectonic regime existing in northeast India at ~ 120 Ma ago. the Ninetyeast Ridge and the Rajmahal flood basalt province.. Fred Frey.K. 1982 ). Wilson (1989). contamination of the primary magma by carbonatitic material would therefore appear to have occurred in the upper mantle. Edward Farrar permitted use of the facilities at Queen's University (supported by grants from NSERC) and arranged for the fast neutron irradiations at McMaster University. Constraints on the age of I S E A reversed event The Rajmahal Traps were formed during the Cretaceous Normal Polarity Superchron ( ~ 11983 Ma). 1988). and a minimum age of 115 Ma for the ISEA reversed event. 1971 ) were apparently erupted during the ISEA reversed polarity interval. postdates Rajmahal volcanism. Such rocks commonly display ~180-values of +6. respectively. 1990).710 and -8%0. Gary Byerly examined thin sections of the rocks and provided detailed comments. opaques . A possible genetic link between the Kerguelen hotspot. 1970. 1969) and the oxygen isotopic composition of the contaminated material would not be radically altered (note ~taO-values of Debagram and Galsi in Table 1 ). 4°Ar/ 39Ar data indicate that the ages for the magnetic reversals close to the long Cretaceous Normal Polarity (K-N) Superchron on earlier timescales are incorrect. has been postulated to occur during Aptian time (VandenBerg et al. Sarkar et al.. compared to ~ 119 Ma in an earlier compilation (Harland et al..Thin-section descriptions A-531 andA-538. Dupuy et al. Pringle et al. designated as the ISEA interval.5 to +7. Tarduno. (1992) suggest a minimum age of 121. The suite of samples from the Rajmahal Province was obtained through D. Medium-grained rocks with fresh feldspar and pyroxene phenoerysts set in a groundmass of plagioclase. Paleomagnetic studies place northeastern India at ~ 50°S latitude at the time of eruption of the Rajmahal lavas (McDougall and McElhinny. ~ 100 km southwest of the Rajmahal Traps. I am indebted to Brent Dalrymple for providing access to the argon dating laboratory at Menlo Park. 1978. with an Rb-Sr age of 110 Ma (Dayal et al. 1992). arranging the fast neutron irradiation at the TRIGA Reactor.. though values of 0. Alan Brandon and an anonymous reviewer made numerous constructive comments on earlier versions of the manuscript.the Group II potassic rocks of M. 1993). Klootwijk. Surface exposures of Rajmahal Traps.A.. would involve critical evaluation of geochronological and geochemical data for rocks from these areas.. 6. The Sr and Nd isotopic ratios of the postulated contaminant is unusual for carbonatites.. Foland. 48: 2357-2372. 55 pp. D.M.. G. pyroxene. and Dostal. K. Fresh feldspar phenocrysts set in groundmass of augite. Prof.A. Earth Planet.. Eos (Trans. and Lanphere. EOS (Trans. Cosmochim. Preprint Vol. T. and Taylor.G. Eos (Trans. 1981. M. Contrib. 1840 (abstract).V. B-1038. 1992. taken from borehole depths of ~ 2850 and ~ 2690 m.. Geophys. Annu..R. A. J.. Geochemical and petrologic data for basalts from Sites 756. pp. J. Isotopic constraints on Columbia River flood basalt genesis and the nature of the subcontinental mantle. Chem. Realistic use of two-error regression treatments as applied to Rb-Sr data. 1969. Origin of the Rajmahal Traps and the 85°E Ridge: preliminary reconstructions of the trace of the Crozet Hotspot. 73:328 (abstract). Medium-grained rocks. Rare earth elements in petrogenetic studies of igneous systems. and Kraker.. Kumar.A.B. W. Baksi.P. Harland. A. 107:173-190. 1992. Lanphere. chemical and petrographic studies of high-pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites. Geol..B.B. and Ouzegone. 258 pp... Dupuy. Geophys.. 38:715-738. Dalrymple. Space Phys. Sci. Curray. Geochim. Bengal Traps. San Francisco. D. Union). 506-507. Calif. Jalangi displays some fine-grained secondary mineral (epidote?) and a few veins filled with zeolite (?).. Llewellyn. A. 121: 43-56.epidote (?). Peucat. . Hart. 280A: 594-621. and Wendt. Cambridge University Press. 1987. Pap. A. A. pyroxene and opaques. tsO/160 and 13C/12C ratios of coexisting minerals in the Oka and Magnet Cove carbonatite bodies. A. T.W.. E. Rev. and Elliot.R.. 1969. Cox.N. Frey. Algeria): Nd and Sr isotopic evidence. Baksi / Chemical Geology 121 (1995) 73-90 and glassy material. J. Jones.88 A. 1993. Rev. Earth Sci... Potassium-argon dating of fine-grained basalts with massive Ar loss: Application of the 4°Ar/39 technique to plagioclase and glass from the Kirkpatrick Basalt.. Debagram displays minor ( < 1%) amount of (primary) biotite in the groundmass. H. 77:618-626. Padmakumari. Bengal Traps... Acta. G. E. V. Prog. altered olivine and opaques. K. (Isot. A. E. E. Fourcade.M. USSR.. 1994. 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