Rapid emplacement of the Kerguelen plume–related Sylhet Traps, eastern India: Evidence from 40 Ar 39 Ar geochronology

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  Rapid emplacement of the Kerguelen plume–related Sylhet Traps, eastern India: Evidence from 40 Ar 39 Ar geochronology
  Rapid emplacement of the Kerguelen plume–related Sylhet Traps,eastern India: Evidence from  40 Ar- 39 Ar geochronology Jyotiranjan S. Ray, S. K. Pattanayak, and Kanchan Pande 1 Physical Research Laboratory, Navrangpura, Ahmedabad, IndiaReceived 31 January 2005; revised 8 April 2005; accepted 21 April 2005; published 20 May 2005. [ 1 ] We report for the first time  40 Ar- 39 Ar plateau ages for the Sylhet Traps of eastern India. Our results provideconcordant ages for two samples, vertically separated by  200 m, from a tholeiite lava flow sequence. The ages areindistinguishable at 2 s  confidence level indicating a rapidemplacement of these lavas. The weighted mean of the plateau ages associated with least errors, 116.0 ± 3.5 Ma,most likely represents the age of eruption. Clearly, theSylhet Traps are contemporaneous with the Kerguelen plume generated Rajmahal and Bengal Traps. Our results inconjunction with the existing age data in the Rajmahal-Bengal-Sylhet igneous province suggest that the latter experienced widespread, and rapid emplacement of flood basalts at   118 ± 2 Ma. Such a large-scale volcanism wouldhave required a direct involvement of the Kerguelen plume,suggesting that the Kerguelen hotspot was located closeto the eastern Indian margin during its initiation. Citation:  Ray, J. S., S. K. Pattanayak, and K. Pande (2005),Rapid emplacement of the Kerguelen plume–related SylhetTraps, eastern India: Evidence from  40 Ar- 39 Ar geochronology, Geophys. Res. Lett. ,  32 , L10303, doi:10.1029/2005GL022586. 1. Introduction [ 2 ] Large Igneous Provinces (LIPs) are voluminousoccurrences of predominantly basaltic extrusive and intru-sive rocks. The srcin of LIPs is usually assigned to within- plate tectonic processes, mostly to the deep mantle plumes[e.g.,  White and McKenzie , 1995]. Variations in the rate andduration of lithospheric extension, mantle temperature,mantle flow rates within the plume conduit and the easewith which melt is intruded laterally in the crust are oftenreflected in the composition, thickness and ages of igneouscrust across volcanic rifted margins [e.g.,  Keen et al. , 1994].To model these variations quantitatively the prerequisite parameters include the availability of good field exposuresin the volcanic rifted margin, the age and chemical data of the basaltic sequence, and the seismic and borehole data toestimate the timing of lithospheric extension, uplift andsubsidence [e.g.,  Kent et al. , 1997]. The Rajmahal-BengalTraps of eastern India form part of a LIP that includes riftedmargin basalts in southwest Australia (Bunbury, NaturalistePlateau), and lavas forming the central and southernKerguelen Plateau [e.g.,  Kent et al. , 2002]. This LIP isgenerally believed to be related to mantle plume activity below the Kerguelen hotspot that began during the earlyCretaceous [e.g.,  Coffin et al. , 2002;  Kent et al. , 2002]. Theearliest manifestations attributed to the Kerguelen plume arethe 132 and 123 Ma old Bunbury Basalts [ Coffin et al. ,2002] and the   118 Ma old Rajmahal-Bengal Traps [  Kent et al. , 2002]. Although the extent of the latter in space andtime is unknown, it has been postulated that the magmaticactivity in eastern India was contemporaneous with theoldest activity on the southern Kerguelen Plateau [  Kent et  Figure 1.  (a) Geological map of Eastern India showing the present outcrops of Rajmahal and Sylhet Traps together withthe locations of exploration wells drilled in the westernBengal basin [after   Kent et al. , 1997]. The minimumthickness of the lava flows in meters is indicated for eachdrillsite.M-B:Mawsynram-Balotsection;C-S:Cherrapunji-Shella Bazaar section; SV: Sung Valley carbonatites.(b) Schematic map of the Indian Ocean, bordering land-masses and hot-spot traces. The numbers alongside solidcircles are measured ages (Ma) of volcanic activity along theReunion and Kerguelen hotspot tracks [after   Duncan , 1991].Stars are present day hotspot locations. DT: Deccan Traps;RT: Rajmahal Traps; ST: Sylhet Traps. The age ranges in MaforDTandRTarealsoshown.(c)LithologoftheCherapunji-Shellabajar section showing twenty lava flows and three tuff horizons. The samples analyzed in this work were collectedfrom flow 4 and flow 19 (shaded). GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L10303, doi:10.1029/2005GL022586, 2005 1  Now at Department of Earth Sciences, Indian Institute of TechnologyBombay, Mumbai, India.Copyright 2005 by the American Geophysical Union.0094-8276/05/2005GL022586$05.00 L10303  1 of 4  al. , 2002]. The Sylhet Traps of eastern Indian state of Meghalaya, exposed   350 km east of the Rajmahal Traps(25.5   N, 91.8  E; Figures 1a and 1b), are also considered to be a part of the above-mentioned LIP [e.g.,  Baksi , 1995;  Kent et al. , 1997] despite the fact that there is no physicalcontinuity between the two traps. The Rajmahal-Bengal-Sylhet (RBS) lava flow sequences are one of the least studied rifted margin volcanism in terms of geochronology.Here we present the first   40 Ar- 39 Ar age of the Sylhet Traps,and discuss its implications for the timing of Kerguelen plume  –  related magmatism in eastern India. 2. Geology and Evolution [ 3 ] The Sylhet lavas occupy a 60 km    4 km east-west trending corridor to the southwest of Shillong [  Baksi , 1995].These lavas underlie Upper Cretaceous rocks and overlie agranitic Archean basement unconformably. Most part of thetraps are either covered with thick vegetation or are weath-ered due to very high rainfall in this region. The best-exposed flows are found only at two roadside exposuresalong the Cherrapunji-Shella Bazaar and Mawsynram-Ballot sections. Twenty continuous lava flows and threetuff beds with a total thickness of   259 m are exposed as avertical sequence in the former section. Apart from tholeiiticlava flows, numerous ultrapotassic, alkaline and carbonatiteintrusives, similar to those found in the Deccan and SiberiaContinental Flood Basalt (CFB) Provinces are also present in the RBS province [e.g.,  Kent et al. , 2004;  Kumar et al. ,2003;  Ray et al. , 1999]. The Sung Valley complex, thenearest alkaline-carbonatite intrusion, is located within the basement rocks at    50 km northeast of the Sylhet Traps(Figure 1a).[ 4 ] Even though the RBS igneous province can be linkedto the Kerguelen hotspot via the Ninety-east Ridge(Figure 1b) based on geochronology [  Duncan , 1991],the geochemical and Sr-Nd-Pb isotopic signatures of theRajmahal basalts do not exactly compare with those of the Kerguelen plateau basalts [e.g.,  Mahoney et al. , 1983;  Kent et al. , 1997]. The normal-Mid Oceanic Ridge Basalt (N-MORB) like Sr-Nd-Pb isotopic ratios of some of these basalts led these authors to suggest that the Kerguelen plume probably furnished the heat, but not material to the Rajmahalvolcanism. However, recent findings of geochemical simi-larity of Cretaceous basalts recovered from Ocean DrillingProgram Leg 183 at site 1137 on the Kerguelen Plateau toRajmahal tholeiites point to a genetic link  [  Ingle et al. ,2002]. Hf and Os isotope ratios of Rajmahal tholeiitessuggest that these basalts were derived from a large ionlithophile enriched plume source, and subsequently evolvedthrough fractional crystallization combined with crustalcontamination [  Ingle et al. , 2004]. Radiogenic isotope ratiosof   115 Ma old kimberlites from the Bengal basin also tell asimilar story supporting the Kerguelen plume srcin for theRBS igneous province [  Kumar et al. , 2003;  Kent et al. ,2004].  Curray and Munasinghe  [1991] hypothesized that the Figure 2.  Step-heating age spectra and inverse isochron plots for the Sylhet Trap samples: (a) ST94-1 and (b) ST94-2 plotted using ISOPLOT 3.0 [  Ludwig  , 2003]. Vertical width of individual bars in the spectra represents ±1 s  analytical error.The ages are calculated using MMhb-1, with an age 523.2 ± 1.8 Ma, as a fluence monitor. Table 1.  Summary of Results of   40 Ar- 39 Ar Dating of Sylhet Traps SamplePlateau Isochron Inverse IsochronSteps %  39 Ar Age, Ma Age, Ma Trap a  MSWD Age, Ma Trap a  MSWDST94-1 13 100 116.3 ± 4.4 118.4 ± 4.8 294.4 ± 1.7 0.34 118.6 ± 4.5 294.4 ± 1.7 0.36ST94-2 9 100 115.6 ± 5.6 120 ± 10 294.3 ± 2.7 0.26 119.8 ± 9.9 294.2 ± 2.6 0.28Mean 116.0 ± 3.5 118.7 ± 4.3 118.8 ± 4.1 a  Trap is the initial  40 Ar/  36 Ar ratio (trapped argon); MSWD is Mean Square Weighted Deviate. Errors are 2 s , and the monitor standard is MMhb-1 (523.2 ± 1.8 Ma). Mean = Weighted mean. L10303  RAY ET AL.: KERGUELEN PLUME–RELATED SYLHET TRAPS  L10303 2 of 4  Rajmahal volcanism was not related to Kerguelen hotspot  but rather to the Crozet hotspot via the Eighty-five-east Ridge (Figure 1b). However, this hypothesis was deemedto be unrealistic based on revised model calculations for  plate motions by  Mu¨ller et al.  [1993]. Interestingly, theSylhet Traps (Figures 1a and 1b), with greater cumulativethickness (  300 m) than the Rajmahal Traps, lie exactly inthe location predicted by northward extrapolation of theKerguelen plume track  [  Kent et al. , 1997] (Figure 1b). 3. Results of   40 Ar- 39 Ar Dating [ 5 ] Samples were collected from the Cherrapunji-ShellaBazaar section, where lava flows are easily accessible,undeformed, and least weathered. Of several samples se-lected only two least altered samples, of fine to mediumgrained tholeiitic basalt from flow 4 (ST94-1) and flow 19(ST94-2) (Figure 1c), were successfully analyzed by theconventional  40 Ar- 39 Ar step heating method [  Pande et al. ,2001]. About 500 mg each of ultrasonically cleaned wholerock sample powder was irradiated along with the fluxmonitor standard Minnesota Hornblende (523.2 ± 1.8 (2 s )Ma old [ Spell and McDougall  , 2003]). The mean values for the correction factors ( 36 Ar/  37 Ar) Ca  , ( 39 Ar/  37 Ar) Ca  , and( 40 Ar/  39 Ar) K   are 0.0003112, 0.0006827, and 0.079, respec-tively. We have used the criteria outlined in ISOPLOT 3.0[  Ludwig  , 2003] to define a plateau in apparent age spectra(Figure 2). According to this a plateau comprises three or more contiguous steps that include at least 60% of the  39 Ar released, with apparent ages that overlap with the mean at the 2 s  confidence level excluding the contribution from theerror on the  J   value. The plateau age and the associatederror, however, are calculated by weighting each step age bythe inverse of its variance that includes the error on  J  [  Pande et al. , 2001]. The isochron and inverse isochronages were determined, and apparent age spectra and isotopecorrelation diagrams of plateau steps were plotted usingISOPLOT 3.0 (Figure 2).[ 6 ] Table 1 shows the analytical results with the errorsquoted at 2 s  confidence level. Both the samples yield good plateaus and inverse isochrons (Figure 2). The sampleST94-1 yields a 13-step plateau age of 116.1 ± 4.2 Mawith 100% of   39 Ar released (Figure 2a), and the secondsample ST94-2 yields a 9-step plateau age of 115.6 ± 5.4with 100% of   39 Ar released (Figure 2b). The samples do not show any indication of   39 Ar recoil redistribution. For boththe samples the plateau, isochron and inverse isochron agesagree within the limits of analytical uncertainty at 2 s confidence level (Table 1). The concordant plateau, iso-chron and inverse isochron ages, the large amounts of released  39 Ar  K   for the plateau steps, atmospheric value of the trapped  40 Ar/  36 Ar component, and acceptable MSWDvalues for the isochrons suggest that these plateau agesrepresent crystallization ages. More importantly, the agesfor the two samples are indistinguishable at 2 s  confidencelevel implying rapid emplacement of the entire Sylhet lavasequence at 116.0 ± 3.5 Ma, the weighted mean of the plateau ages. 4. Discussion and Conclusions [ 7 ] Previous K-Ar and  40 Ar- 39 Ar age data, and paleo-magnetic work on the RBS province has been reviewed by  Baksi  [1995] and  Kent et al.  [1997, 2002]. A major shortcoming with the available age data for RBS basalts isthe lack of information on the relative stratigraphic positionsof samples. The available K-Ar whole rock ages for theRajmahal and Bengal lavas show wide range from 88 ±4 Ma to 133 ± 6 (2 s ) Ma [  Baksi et al. , 1987;  Sarkar et al. ,1996], and the only available date for the Sylhet lavas is aK-Ar age of 108 ± 8 Ma [  Baksi et al. , 1987]. The morereliable  40 Ar- 39 Ar age data for the Rajmahal and BengalTraps, normalized to an age of 523.2 Ma for MMhb-1standard, vary from 118 Ma to 115 Ma [e.g.,  Baksi , 1995; Coffin et al. , 2002;  Kent et al. , 2002;  Pringle et al. , 1994].The alkaline magmatism at Sung Valley (107.8 ± 0.8 Ma[  Ray et al. , 1999]) and Jasra (105 ± 0.5 Ma [  Heaman et al. ,2002]) on the Shillong Plateau is probably the youngest activity in the Indian continent that can be related to theKerguelen plume.[ 8 ] The indistinguishable plateau (116.0 ± 3.5 Ma),isochron (118.7 ± 4.3 Ma) and inverse isochron (118.8 ±4.1 Ma) ages of the Sylhet Traps agree with the peak of Rajmahal volcanism at    118 Ma [  Baksi , 1995], as well aswith the beginning of the activity on the southern KerguelenPlateau at 119 Ma [ Coffin et al. , 2002;  Duncan , 2002]. Thecontemporaneity of the Sylhet and Rajmahal Traps, their similar chemical compositions [e.g.,  Pantulu et al. , 1992;  Baksi , 1995], and the recovery of Rajmahal type basalticflows from numerous boreholes drilled in the Bengal basin[ Sengupta , 1966] can be regarded as evidence for physicalcontinuity between them. These observations argue for rapid and extensive (in excess of 2    10 5 km 2 ) flood basaltic eruptions in the RBS province at    118 Ma. Sucha scenario would require that the material for this CFB was being supplied by a large-scale mantle melting, suggesting adirect involvement of the Kerguelen plume head. Thisindicates that the Kerguelen hotspot was located close tothe Indian eastern margin during its initiation. However, thelarge uncertainties in the ages cannot preclude the possibil-ity of a larger duration (  10 Ma) of volcanism in the RBS province. The younger alkaline intrusions on the ShillongPlateau clearly represent the post flood basalt magmatism.These activities appear to have remained confined to theouter margins of a hypothetical plume head, and their  positions can easily be explained using the moving hotspot model of   Kent et al.  [2002].[ 9 ]  Acknowledgment.  We thank two anonymous reviewers for their constructive reviews. References Baksi, A. K. (1995), Petrogenesis and timing of volcanism in the Rajmahalflood basalt province, northeastern India,  Chem. Geol. ,  121 , 73–90.Baksi, A. K., T. R. Barman, D. K. Paul, and E. Farrar (1987), WidespreadEarly Cretaceous flood basalt volcanism in eastern India: Geochemicaldata from the Rajmahal-Bengal-Sylhet Traps,  Chem. Geol. ,  63 , 131– 141.Coffin, M. F., M. S. Pringle, R. A. Duncan, T. P. Gladczenko, M. Storey,R. D. Mu¨ller, and L. A. Gahagan (2002), Kerguelen hotspot magmaoutput since 130 Ma,  J. Petrol. ,  43 , 1121–1139.Curray, J. R., and T. 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