Geological and 40Ar/39Ar age constraints on late-stage Deccan rhyolitic volcanism, inter-volcanic sedimentation, and the Panvel flexure from the Dongri area, Mumbai (2014)

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Geological and 40Ar/39Ar age constraints on late-stage Deccan rhyolitic volcanism, inter-volcanic sedimentation, and the Panvel flexure from the Dongri area, Mumbai (2014)
  Geological and  40 Ar/ 39 Ar age constraints on late-stage Deccan rhyoliticvolcanism, inter-volcanic sedimentation, and the Panvel flexurefrom the Dongri area, Mumbai Hetu C. Sheth ⇑ , Kanchan Pande Department of Earth Sciences, Indian Institute of Technology Bombay (IITB), Powai, Mumbai 400076, India a r t i c l e i n f o  Article history: Available online 19 August 2013 Keywords: Flood volcanismDeccan TrapsPanvel flexureMumbaiRhyolite a b s t r a c t Post-K–PgBoundaryDeccanmagmatismiswellknownfromtheMumbai areainthePanvel flexurezone.Represented by the Salsette Subgroup, it shows characters atypical of much of the Deccan Traps,including rhyolite lavas and tuffs, mafic tuffs and breccias, spilitic pillow basalts, and ‘‘intertrappean’’sedimentary or volcanosedimentary deposits, with mafic intrusions as well as trachyte intrusionscontainingbasalticenclaves.Theintertrappeandepositshavebeeninterpretedasformedinshallowmar-ine or lagoonal environments in small fault-bounded basins due to syn-volcanic subsidence. We report apreviously unknown sedimentary deposit underlying the Dongri rhyolite flowfromthe upper part of theSalsette Subgroup, with a westerly tectonic dip due to the Panvel flexure. We have obtained concordant 40 Ar/ 39 Ar ages of 62.6±0.6Ma (2 r ) and62.9±0.2Ma (2 r ) for samples takenfromtwo separate outcropsof this rhyolite. The results are significant in showing that (i) Danian inter-volcanic sedimentary depositsformed throughout Mumbai, (ii) the rock units are consistent with the stratigraphy postulated earlier forMumbai, (iii) shale fragments known in some Dongri tuffs were likely derived from the sedimentarydeposit under the Dongri rhyolite, (iv) the total duration of extrusive and intrusive Deccan magmatismwas at least 8–9million years, and (v) Panvel flexure formed, or continued to form, after 63Ma, possiblyeven 62Ma, and could not have formed by 65–64Ma as concluded in a recent study.   2013 Elsevier Ltd. All rights reserved. 1. Flood basalts, rifted continental margins, and monoclinalflexures Continental flood basalt (CFB) provinces are frequentlyassociated with rifted continental margins, and typically exhibitmonoclinal flexures at the rifted margins. Prominent examplesare the Karoo province of southern Africa, the Paraná provinceof South America, the East Greenland province, and the Deccanprovince of India. In a monoclinal flexure zone, the otherwiseessentially flat-lying CFB lava pile shows significant seaward dips(asmuchas45  intheKaroo),andthisiswheresignificantvolumesof evolved magmas like rhyolites and trachytes, scarce over therest of the province, are concentrated (e.g., Nielsen and Brooks,1981; Lightfoot et al., 1987; Cox, 1988; Peate, 1997; Klausen andLarsen, 2002; Klausen, 2009).The Panvel flexure of the Deccan province runs parallel to theNNW–SSE-trending western Indian rifted margin for >150km,and has a width of    30km (see Fig. 1b of  Sheth et al., 2014). It has been suggested to have formed due to simple monoclinalbending of the basalt pile (Blanford, 1867; Wynne, 1886; Auden,1949), as an extensional fault structure (Dessai and Bertrand,1995), and as a reverse drag structure on an east-dipping listricmaster fault (Sheth, 1998). These models are not completelymutually exclusive, but another important issue is the timing of flexure formation relative to flood volcanism and continentalbreakup. Understanding this requires, besides careful field work,accurate and precise radio-isotopic dating of fresh, alteration-freevolcanic units from key stratigraphic positions. Because all theseconditions rarely occur together, and because geochronologicalstudieshavefocussedonthethickestCFBsectionstoevaluatetheirlinks to mass extinctions (e.g., Baksi, 2014 and references therein),critical age data on key eruptive units in flexure zones are oftenscarce.Hooper et al. (2010) have argued, based on geochemical and 40 Ar/ 39 Ar age data for mafic lavas and dykes in the Panvel flexurezone, that the flexure formed by 65–64Ma, soon after the DeccanCFB eruptions. Here, we present two  40 Ar/ 39 Ar ages on a keyrhyolite unit from the Dongri area of Mumbai, also in the Panvelflexure zone. Based on geological considerations which wedescribe in detail, and the  40 Ar/ 39 Ar ages, we conclude that thePanvel flexure formed as late as 63Ma, possibly even 62Ma, andcould not have formed by 65–64Ma. This result is significant forunderstanding the tectonic evolution of the western Indian riftedmargin. 1367-9120/$ - see front matter    2013 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +91 22 25767264; fax: +91 22 25767253. E-mail address: (H.C. Sheth). Journal of Asian Earth Sciences 84 (2014) 167–175 Contents lists available at ScienceDirect  Journal of Asian Earth Sciences journal homepage:  2. Deccan geology of Mumbai, Panvel flexure zone Deccanfloodbasaltvolcanism(Fig.1a)overlappedwith,andhasbeen directly implicated in, the major Cretaceous–Palaeogene(K–Pg) Boundary mass extinctions at   65.5Ma (e.g., Keller et al.,2008). Though dominated by tholeiitic flood basalts, the Deccanprovince shows alkalic and silicic rocks concentrated in regionssuch as Mumbai and the Panvel flexure zone, and the Saurashtrapeninsula, both on the western Indian rifted margin (e.g.,Sukheswala and Poldervaart, 1958; Sukheswala, 1974; Sethna andBattiwala, 1977; Godbole and Ray, 1996; Sheth et al., 2011, 2012).The Ghatkopar–Powai area of Mumbai (Fig. 1b) exposes prom-inently seaward-dipping (17  ) tholeiitic basalt flows intruded bymany tholeiitic dykes (Sheth, 1998; Sheth et al., 2014). The south-ern and western parts of Mumbai show volcanic and volcanosedi-mentary deposits as well as intrusions of considerablecompositional diversity, all belonging to a post-K–Pg Boundaryphase of Deccan magmatism (Sethna and Battiwala, 1977, 1980;Sethna,1999;Shethetal.,2001a,b;Crippsetal.,2005).ThisDaniansequence also prominently dips west. Sethna (1999) named it theSalsette Subgroup, and considered it to be younger than the entireWestern Ghats stratigraphic sequence (Table 1). He divided theSalsette Subgroup into a Mumbai Island Formation, made up of subaqueous lavasincludingspiliticpillowbasalts, tuffs, andshales,followed by a Madh–Uttan Formation made up of rhyolite lavaflows, followed by a Manori Formation, comprising trachyteintrusions(Table1).The‘‘intertrappean’’sedimentarybedsoftuffs,clays and shales have yielded fossil frogs, ostracods, turtle skullsand crocodilian eggshell fragments (e.g., Owen, 1847; Carter,1852a,b; Blanford, 1867, 1872; Chiplonker, 1940; Cripps et al.,2005). The shales contain considerable volcanic ash input, andare often carbonaceous (Singh, 2000).Sheth et al. (2001a,b) dated two distinctly west-dippingtrachyte units from Manori and Saki Naka (Fig. 1b) at60.4±0.6Ma (2 r ) and 61.8±0.6Ma (2 r ) respectively ( 40 Ar/ 39 Ar),and the Gilbert Hill basalt intrusion near Andheri (Fig. 1b) at60.5±1.2Ma(2 r ).Interpretingthetrachytesasdippinglavaflows,they suggested that the Panvel flexure had formed after 60Ma.Careful reading of earlier work on these trachytes (Sethna andBattiwala,1974,1976)aswellasmoredetailedfieldandgeochem-ical work (Zellmer et al., 2012) shows that these trachytes areexhumed subvolcanic intrusions, and their 60–62Ma ages cannot ab Fig. 1.  (a) Map of India and the Deccan Traps (gray), showing the Western Ghats type section, Mumbai, and other localities of Deccan intertrappean and infratrappeansedimentarydeposits(stars), someofwhicharenamed(basedonShekhawatandSharma, 1996;JayandWiddowson,2008).(b)MapofMumbai,showingthelocalitiesoftheintertrappean deposits (stars), including the newly discovered one at Dongri (based onSethna, 1999; Singh, 2000; Cripps et al., 2005; this study). Curved arrows indicate thePanvelflexure.(c)GeologicalmapoftheUttan–Dongriarea(basedonZellmeretal.,2012andreferencestherein),showingthelocationsoftheoutcropsstudiedinthepresentwork.168  H.C. Sheth, K. Pande/Journal of Asian Earth Sciences 84 (2014) 167–175  constrain the age of the Panvel flexure. Ages obtained on  eruptive units can, but no radio-isotopic ages have been obtained on anyof the Mumbai rhyolites with the exception of a Rb–Sr isochronage of 61.5±1.9Ma (Lightfoot et al., 1987). Sheth and Ray (2002) questioned this age on several grounds including possible mixingrelationships.In this study, we have obtained two  40 Ar/ 39 Ar ages on theDongri rhyolite flow from the Madh–Uttan Formation of theSalsetteSubgroup.Wehavealsofound,underthisrhyolite,ahithertounknown sedimentary deposit, as well as a tuff deposit nearbywhich contains shale fragments. We now describe the geology of these rock units with some interpretations about their formationenvironment, and follow with details of the  40 Ar/ 39 Ar dating of the Dongri rhyolite and its bearing on the question of the age of the Panvel flexure. 3. Geology of the Dongri rhyolite and associated rock units  3.1. The rhyolite A detailed geochemical study of the Mumbai rhyolites waspresented by Lightfoot et al. (1987) who mentioned up to fiverhyolite units 20–100m thick and with a strike length of 20km.The Dongri rhyolite is a thick (>70m), prominently columnar- jointed lava flow exposed in the Darkhan quarry just to the southof Dongri village, east of the Dongri–Gorai road (Fig. 1c). Therhyolite is light brown, fine-grained, essentially aphyric and non-vesicular, and made up of quartz and K-feldspar. We sampled itsexposed base in the quarry (sample UTRH). Owing to the westerlydip the Dongri rhyolite flow is encountered at a lower level in avalley on the western side of the Dongri–Gorai road, 1.7kmsouth–southwest of the Darkhan quarry and 800m north of the Judicial Institute (Fig. 1c).Here the rhyolite shows well-developed columns which dipsteeply east, suggesting that the flow dips   10   west (Fig. 2a andb), based on the general principle that columnar joints in a solidi-fyingtabularigneousbody(whetheralavaflow,sill,ordyke)prop-agate perpendicular to the isotherms (surfaces of equaltemperature), which in an undisturbed magma body are parallelto its margins (e.g., Spry, 1962; DeGraff and Aydin, 1987; Lyle,2000). We are aware of many possible complications, as when col-umns form in highly random orientations in the ‘‘entablature’’zonesofsolidifyinglavaflows,typicallyduetoingressofrainwater(e.g., Tomkeieff, 1940; Long and Wood, 1986; De, 1996). There arealso cases where stacks of parallel columns may be inclined to theflow margins, owing to late-stage flow (Waters, 1960). In thepresent case, westward dip of the Dongri rhyolite flow is visiblein an oblique view afforded by the exposure (Fig. 2b). Besides, allMumbai rhyolites including those at Madh (Fig. 1b), as well asthe other Salsette Subgroup units, have westward dips, as do alsothe Ghatkopar–Powai tholeiites (Sheth et al., 2014). For the latterreason, the westward dip of the Dongri rhyolite cannot be ex-plained in an isolated manner by invoking eruption of the Dongrirhyolite on an already inclined surface. The consistent westwarddips shown by all these rock units are tectonic dips produced bythe Panvel flexure.The Dongri rhyolite is underlain by a sedimentary deposit(Fig. 2a and c). We sampled the base of the rhyolite flow (sampleUTRH-1) just above the sedimentary deposit, and this ispetrographically similar to the Darkhan quarry rhyolite. WeconsiderthetwosamplesUTRHandUTRH-1torepresentthesamelava flow.  Table 1 Mumbai volcanic stratigraphy relative to the Western Ghats lava stratigraphy. Group Subgroup Formation Rock typesSalsette Manori Trachyte intrusions with mingled basalt enclavesMadh–Uttan Rhyolite lava flowsMumbai Island Hyaloclastites, spilites, basalts and shalesDeccan Basalt Group Wai Poladpur, Ambenali, Mahabaleshwar, Panhala, and Desur Subaerial tholeiitic flood lavasLonavala Khandala and Bushe Subaerial tholeiitic flood lavasKalsubai Jawhar, Igatpuri, Neral, Thakurvadi, and Bhimashankar Subaerial tholeiitic flood lavas Notes : The Salsette Subgroup (Sethna, 1999) has been placed by him above the three stratigraphic subgroups of the Deccan Basalt Group in the Western Ghats sequence(Subbarao and Hooper, 1988 and references therein). Cripps et al. (2005) have considered the Salsette Subgroup to be contemporaneous with the last eruptions of the Wai Subgroup lavas. Fig. 2.  (a–c) Field photographs of the Dongri rhyolite and sedimentary deposit.Students provide a scale. H.C. Sheth, K. Pande/Journal of Asian Earth Sciences 84 (2014) 167–175  169   3.2. The sedimentary deposit  This deposit, discovered by us in March 2011, was not exposedduring many prior visits to the area over several years, but hasbecomeavailableduetorecentexcavationforclayundertherhyoliteby local villagers. The outcrop becomes inundatedby water duringthe vigorous Mumbai monsoons due to its low elevation (only afew meters above sea level) and proximity to the coastline(1km). The exposed thickness of the sedimentary deposit is2.5–3m,andtheexposedlateralextent  20m(Fig.2aandc). Darkgrayshales inthelowerpartof theDongrisedimentarydepositareoverlain by light gray, patchy yellow–gray, dark brown, and lightgray shales and clays (Fig. 2c). They are all laminated on the milli-meter-scale,softandfragile.Intertrappeansedimentarydepositsinthe Deccan Traps are generally only a few meters thick, an excep-tion being a 150-m-thick black carbonaceous shale encountered ina construction tunnel at Bandra (Fig. 1b), indicating long local vol-canic quiescence there (Sethna, 1999).  3.3. The Dongri tuff  We have found a tuff deposit on the 96m hill exactly west of the Darkhan rhyolite quarry (Fig. 1c), in a residential propertyundergoing construction work in 2007. Because the volcanicsequence becomes younger westwards, we consider this tuff tooverlie the Dongri rhyolite. The tuff has a light gray ash matrixwith many dark gray shale fragments, a few reaching 2cm(Fig. 3a). The tuff also shows occasional subangular to subroundedfragments 5–6cm in diameter, made up of very fine, vesicularmaterial and lacking internal structure (Fig. 3b). These are proba-bly the same as the ‘‘coalesced ash bombs’’ described by Sukhesw-ala (1956) from Mumbai intertrappean volcanosedimentarydeposits, and by Cripps et al. (2005, sample 3/99) from the Amboliquarry at Jogeshwari (Fig. 1b).Photomicrographs of the Dongri tuff are given in Fig. 3c and d.In thin section this tuff shows shows shale fragments (isotropic),basalt fragments, as well as clinopyroxene crystals. The tuff has Fig. 3.  (a and b) Hand specimens of the Dongri tuff. (b) Shows a large fragment. Ruler is in centimeters. (c–f) Photomicrographs of the Dongri tuff. The abbreviations are: bf,basaltic fragment (shown by white dashed lines); cpx, clinopyroxene; pl, plagioclase; qz, quartz; sh, shale; sp, spherulitic quartz.170  H.C. Sheth, K. Pande/Journal of Asian Earth Sciences 84 (2014) 167–175  experienced much silicification, with veins of quartz andspherulitic quartz infilling shale fragments. Singh (2000) classifiedMumbai intertrappean tuffs into vitric and lithic tuffs from petro-graphic studies, though these tuffs also contained crystals of pyroxene and feldspars. He observed much devitrification of glass,and diagenesis. Sukheswala (1956) also identified pyroxenes andfeldsparsintheWorliashbeds.BasedontheDongrituff’scompon-entry we term it a mixed lithic–crystal–vitric tuff (following theterminology of  Schmid, 1981).  3.4. Interpretation The sedimentary deposit underlying the Dongri rhyolite isconsistent with the Mumbai volcanic stratigraphy proposed bySethna (1999) (Table 1), in which the Madh–Uttan–Dongri area rhyolite lavas overlie the intertrappean shales (and older lavas)of the Mumbai Island Formation. The Dongri area shows thatvolcanism and sedimentation succeeded each other during thedepositionof the SalsetteGroup, as theydidover muchof Mumbai(e.g., Sethna, 1999; Cripps et al., 2005). Detailed studies of theSalsette Group intertrappean deposits by Singh (2000) and Cripps et al. (2005) have indicated deposition in shallow marine orlagoonal environments, in small fault-founded basins, due to syn-volcanic subsidence.The Dongri sedimentary deposit explains the occurrence of fragments of baked carboaceous shale in tuffs of the Uttan–Dongri area as reported by Sethna and Mousavi (1994). Shalebaking may have been caused by intrusions, as at the Amboliquarry (Singh, 2000), or by incorporation of the shale fragmentsin hot erupting ash as in the Uttan–Dongri area. The angularityof the volcanic and shale fragments and crystal shapes in theDongri tuff suggests minimum transport (cf. Singh, 2000). Thelack of bedding or laminations suggests rapid deposition, possiblyfrom vents nearby. The pale color of the tuff despite the maficcontent may be due to bleaching by vapors. The ash aggregates(Fig. 3b) appear to be of the nature of large accretionary lapilli(see also Cripps et al., 2005), though they lack the typical concen-tric structure, and would then indicate wet explosive eruptions(e.g., Brown et al., 2010). The shale fragments in the Dongri tuff do not appear to have come from explosions under the exposed,undisturbed sedimentary deposit. The explosions probablyoccurred under the current outcrop of the tuff, and given the1km distance between the sedimentary outcrop and the tuff,the lateral subsurface extent of the former is at least that much.Carter (1852b) has mentioned that the small rocky islets west of Uttan (Fig. 1c) are also made up of tuffs. 4.  40  Ar/ 39  Ar dating of the Dongri rhyolite A key point we make is that the westward dips of the Dongrirhyolite flow, the rest of the Salsette Subgroup, and theGhatkopar–Powai tholeiites, are tectonic, and were acquired aftereruption when the Panvel flexure formed. The crystallization age(=eruption age) of the Dongri rhyolite flow, from the uppermostlevels of the Salsette Subgroup, should therefore provide an  upper  limit onthe formationof the Panvel flexure. Withthis understand-ing, we carried out  40 Ar/ 39 Ar dating of the two samples of theDongri rhyolite flow. 4.1. Analytical methods Rock chips of the Dongri rhyolite (samples UTRH and UTRH-1)were crushed and sieved and the 120–180 l m size-fraction wasleached with a 1% HCl solution to eliminate secondary carbonates.The sample material was cleaned in deionised water in anultrasonic bath and about 0.2g of each was packed in aluminumcapsules. The Minnesota hornblende reference material (MMhb-1) of age 523.1±2.6Ma (Renne et al., 1998) and high purity CaF 2 and K 2 SO 4  salts were used as monitor samples. High purity nickelwireswereplacedinbothsampleandmonitorcapsulestomonitorthe neutron fluence variation, which was typically about 5%. Thealuminumcapsules werekept ina0.5mmthickcadmiumcylinderand irradiated, in two separate batches, in the light-water moder-ated CIRUS reactor at the Bhabha Atomic Research Centre (BARC),Mumbai, for   100h. The irradiated samples were repacked inaluminumfoil and loaded on the extraction unit of a Thermo-FisherScientific noble gas preparation system. Argon was extracted in aseries of steps up to 1400  C in an electrically heated ultra-highvacuum furnace. After purification using Ti–Zr getters, the argonreleased in each step was measured with a Thermo-Fisher ARGUSmass spectrometer located at the National Facility for  40 Ar– 39 ArGeo-thermochronology in the Department of Earth Sciences, IITBombay. The mass spectrometer is equipped with five Faradaycups fitted with 10 11 ohm resistors.Interference corrections for Ca- and K-produced Ar isotopesbased on analysis of pure CaF 2  and K 2 SO 4  salts were ( 36 Ar/ 37 Ar) Ca ,( 39 Ar/ 37 Ar) Ca  and ( 40 Ar/ 39 Ar) K  =0.000438, 0.000921 and 0.004451,respectively, for sample UTRH. The same parameters were0.000334, 0.000762, and 0.000808, respectively, for sampleUTRH-1.  40 Ar blank contributions were 1–2% or less for alltemperature steps. The irradiation parameter  J   for the samplewas correctedforneutronfluxvariationusingtheactivityof nickelwires irradiated with each sample. Value of fluence-corrected  J   is0.001545±0.000006 for UTRH, and 0.002317±0.000009 forUTRH-1. 4.2. Results The  40 Ar/ 39 Ar step heating data were plottedusing the programISOPLOTv.3.75(Ludwig,2012) andaretabulatedinOnlineAppen- dixI. Wedefineaplateauinanargonreleasespectrumascompris-ing a minimum of 60% of the total  39 Ar released and four or moresuccessive degassing steps with mean ages overlapping at the 2 r level including the error contribution from the  J   value (e.g., Senet al., 2012).Sample UTRH yielded a 18-step plateau age of 62.6±0.6Ma(2 r ),withtheagespectrumcomprising69.0%oftotal 39 Arreleased(Fig. 4a). Higher-temperature steps than the plateau spectrumyielded progressively increasing apparent ages, which we ascribeto excess argon (see e.g., Lanphere and Dalrymple 1971, 1976;Kaneoka, 1974, 1980; Balasubrahmanyan and Snelling, 1981;Iwata and Kaneoka, 2000; Kelley, 2002). This excess argon mayreside in fluid inclusions in minerals (e.g., Kelley, 2002). The sam-ple UTRH’s isochronage of 62.9±0.7Ma (2 r ) and inverse isochronage of 62.9±0.6Ma (2 r ) are statistically indistinguishable fromthe plateau age (Fig. 4b and c).Sample UTRH-1 also yielded a 18-step plateau age of 62.9±0.2Ma (2 r ), with the age spectrum comprising 90.1% of total  39 Ar released (Fig. 5a). Its isochron age of 62.9±0.6Ma(2 r ) and inverse isochron age of 62.9±0.3Ma (2 r ) are statisti-cally indistinguishable from the plateau age (Fig. 5b and c). Theconcordant plateau, isochron and inverse isochron ages of bothrhyolite samples, the large amount of total released  39 Ar for theplateau steps, the acceptable MSWD values of the isochron andinverse isochron, as well as the atmospheric value (295.5) of the 40 Ar/ 36 Ar ratio of trapped argon given by their intercepts, suggestthat these ages represent crystallization ages. We take the identi-cal  40 Ar– 39 Ar plateau ages of 62.6±0.6Ma (2 r ) and 62.9±0.2Ma(2 r ) as the crystallization and eruption age of the Dongri rhyoliteflow. H.C. Sheth, K. Pande/Journal of Asian Earth Sciences 84 (2014) 167–175  171
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