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Pyroxenite dykes with petrological and geochemical affinities to the Alaskan-type ultramafics at the northwestern margin of the Cuddapah basin, Dharwar craton, southern India: Tectonomagmatic implications

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Pyroxenite dykes with petrological and geochemical affinities to the Alaskan-type ultramafics at the

northwestern margin of the Cuddapah basin, Dharwar craton, southern India: Tectonomagmatic implications

Rohit Kumar Giri1, Praveer Pankaj1, Dinesh Pandit1, Samarendra Sahoo1, Ramananda Chakrabarti2 and N V Chalapathi Rao1,*

1Mantle Petrology Laboratory, Department of Geology, Centre of Advanced Study, Institute of Science, Banaras Hindu University, Varanasi 221 005, India.

2Centre for Earth Sciences, Indian Institute of Science, Bengaluru 560 012, India.

*Corresponding author. e-mail: nvcrao@bhu.ac.in

MS received 17 October 2018; revised 2 December 2018; accepted 6 December 2018; published online 8 May 2019 Two previously reported lamprophyre dykes from the Kalwakurthy area, at the northwestern margin of the Cuddapah basin, Dharwar craton, southern India, are reinvestigated. Petrography reveals that they have an overall cumulate texture and comprise clinopyroxene (dominant phase), amphibole (mostly secondary), magnetite, ilmenite and chromite and are reclassified as clinopyroxenites. The chemistry of clinopyroxene and chromite, bulk-rock major and trace element composition and the Sr–Nd isotopic systematics of the Kalwakurthy dykes strongly favour the involvement of subduction-related processes in their genesis and are strikingly similar to those of the continental arc-cumulates and Alaskan- type ultramafics reported from the supra-subduction type of tectonic settings. Incompatible trace element ratios, involving high field strength elements, of these clinopyroxenites are also suggestive of the fluid-related metasomatism influencing their source regions. Petrogenetic modelling reveals that 10–20% partial melting of the fertile lithospheric mantle source was involved in their genesis. The tectonomagmatic significance of the studied clinopyroxenites is evaluated in light of the existing models invoking a Neoarchaean subduction in the evolution of the Dharwar craton.

Keywords. Pyroxenite; petrology; geochemistry; Alaskan-type ultramafics; Dharwar craton; southern India.

1. Introduction

Pyroxenites are coarse-grained ultramafic igneous rocks known to possess the cumulate texture and dominated by pyroxenes with accessory phases such as olivine, feldspar, chromite, rutile and magnetite–ilmenite pairs. The mineralogical and geochemical diversity of pyroxenites provides insight into the petrological processes contribut-

ing to the evolution of the lithospheric mantle (see Downes 2007). Pyroxenites also occur as mantle- derived xenoliths in a variety of alkaline magmas and serve as a window to understand the nature of the subcontinental lithospheric mantle (SCLM) (e.g., Karmalkar et al. 2009; Aulbach and Jacob 2016). Pyroxenites are generated by a variety of processes that include: (i) in-situ partial melting, (ii) metamorphic segregation of peridotites, (iii) 1 0123456789().,--: vol V

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metamorphosed product of the recycled oceanic lithosphere and (iv) high-pressure crystal cumu- lates from the migrating mantle magma (see Dick and Sinton 1979;All´egre and Turcotte 1986; Bod- inieret al. 1987;Sobolev 2007;Tilhacet al. 2017).

Alaskan-type ultramafic rocks were first recorded from southeastern Alaska (Taylor 1967) and sub- sequently many such rock types/complexes have been reported from convergent margins (Su et al.

2012; Yuan et al. 2017). Alaskan-type ultramafic rocks are known to span from the Neoarchaean to the Cenozoic era (e.g.,Taylor 1967;Caiet al.2012;

Sappin et al. 2012; Helmy et al. 2014; Tessalina et al. 2016) and are intimately associated with convergent plate margins and hence are of unique geodynamic significance (e.g.,Su et al.2014;Yuan et al.2017). A dominance of clinopyroxene coupled with the paucity of orthopyroxene and plagioclase together with the enrichment in light rare earth element (LREE) and the concomitant depletion in high field strength element (HFSE) in the present study are well-known characteristic features of the Alaskan-type ultramafics (Himmelberg and Loney 1995;Pettigrew and Hattori 2006;Eyubogluet al.

2010;Helmy et al.2014;Suet al.2014).

From the Indian shield, pyroxenites have been reported from various cratons and mobile belts (e.g., Rao and Raman 1979; Kutty et al. 1986;

Natarajan et al. 1994; Le Bas et al. 2002; Srivas- tava and Sinha 2007;Samuelet al.2018). Some of the well-studied Indian occurrences of pyroxenites include (i) Jasra complex, Shillong plateau (Sri- vastava and Sinha 2007); (ii) Kondapalle-layered complex (Dharma Rao and Santosh 2011 and references therein) and the Chimakurti gabbro- anorthosite-ultramafic complex at the eastern mar- gin of the Cuddapah basin (Rao et al. 1988) in the Dharwar craton; (iii) Puttetti complex, Trivandrum block (Rajesh 2003) and (iv) Borra complex, Eastern Ghats (Le Bas et al. 2002). In this paper, we reinterpret the recently reported

‘Kalwakurthy lamprophyre’ (from the Polkam- palli and Potepalli areas) dykes (Meshram and Venkateswara Rao 2009) from the northwestern margin of the Cuddapah basin in the Dharwar craton, southern India, as pyroxenites. We demon- strate, based on the petrography, mineral chem- istry and bulk-rock geochemistry (including Sr and Nd isotopes) that these dykes have charac- teristics of continental arc-related clinopyroxenites.

We also propose their petrogenetic model and explore the tectonomagmatic significance of these dykes.

2. Geological background

The Dharwar craton of southern India is a clas- sical granite–greenstone terrain and is tradition- ally divided into the western Dharwar craton and eastern Dharwar craton (Naqvi and Rogers 1987;

Ramakrishnan and Vaidyanadhan 2008). However, recent studies favour the Dharwar craton to be a composite mosaic of micro-blocks (Peucat et al.

2013;Jayanandaet al.2018; figure1a). Three such blocks – western, central and eastern – are recog- nised based on multiple criteria such as the (i) abundance of greenstone belts, (ii) thickness of the crust, (iii) metamorphism and (iv) melting of the basement (seeJayanandaet al.2018and references therein). Based on published geochronological data there are five major periods of crust formation in the Dharwar craton at ca. 3450–3300, 3230–3150, 3000–2960, 2700–2600 and 2560–2520 Ma, which are subcontemporaneous with greenstone volcan- ism (Jayanandaet al.2018). A NNW–SSE trending shear zone along the eastern margin of the Chi- tradurga greenstone belt is considered to separate the western block from the central block. On the other hand, the Chitradurga shear zone and the western margin of the Kolar–Kadiri schist belt comprise the western and eastern limits, respec- tively, of the central block. The eastern block extends beyond the Kolar–Kadiri belt (Jayananda et al.2018).

The western block is dominated by the tonalite–

trondhjemite–granodiorite (TTG) gneisses, two generations of greenstone sequences and high potas- sic granites. The central block comprises abundant granitoids, including the N–S trending 500-km long Closepet granite, banded tonalitic to granodioritic gneisses, migmatitic TTG and greenstone belts.

The lithology of the eastern block includes sev- eral calc-alkaline intrusions (collectively termed the Dharwar batholith), banded gneisses and green- stone belts containing lesser volcanics (Swami Nath et al. 1976; Radhakrishna and Naqvi 1986;Chad- wicket al.2000;Ramakrishnan and Vaidyanadhan 2008; Jayananda et al. 2018). These volcanics are associated with the convergent margins and comprise basalts, boninites, tholeiitic to calc- alkaline basalts, Mg-andesites and adakites. Rela- tively younger Palaeo-to-Mesoproterozoic platform type Purana sedimentary sequences in the Cudda- pah, Kaladgi and Bhima basins are also present within the Dharwar craton. Several Mesoprotero- zoic kimberlites (1.1 Ga) and lamproites (1.4–

1.25 Ga) are also documented from the Dharwar

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Figure 1. (a) A generalised geological map showing a part of the eastern Dharwar craton, Cuddapah basin and its sur- rounding areas showing the location of the study area along with the disposition of various alkaline and subalkaline rocks (after Naqvi 2005). (b) A generalised geological map of the study area around Kalwakurthy town showing the location of pyroxenite bodies (after Meshram and Venkateswara Rao 2009). Abbreviations: MUD, Mudigubba lamprophyre; UK, Udiripikonda lamprophyre; WKF, Wajrakarur kimberlite field; NKF, Narayanpet kimberlite field; RKF, Raichur kimberlite field; VLF, Vattikod lamproite field; RLF, Ramadugu lamproite field.

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Figure 2. Field photographs of Kalwakurthy pyroxenites. (a) Polkampalli dyke, (b) Potepalli dyke, (c) close view of Polkam- palli dyke showing brown coloured weathered surface and shining fresh surface, (d) pits in the Potepalli dyke.

craton (Chalapathi Rao et al. 2013, 2014). The end-Cretaceous Deccan trap lavas and black soil and alluvium of the recent age constitute the youngest geological units.

The pyroxenites of this study occur as dykes and are located 2 km south of Polkampalli (163405.4N; 783346.2E), and 2 km WSW of Potepalli (164211.7N; 783415.4E) villages near the Kalwakurthy area (figure 1b). As pre- viously mentioned, these dykes were reported by the Geological Survey of India a decade ago and identified as ‘lamprophyres’ (see Meshram and Venkateswara Rao 2009). The Polkampalli pyroxenite has a width of 5 m and a length of 50 m and strikes N–S to N10W–S10E (figure2a and b). The Potepalli pyroxenite trends NW–SE with a width of 20 m and length of 150–200 m (figure 2c and d). Both these dykes intrude the coarse-grained grey biotite granite of granodiorite–

adamellite–granite suite of Archaean age (Gopal Reddy et al. 1992). Pyroxenite dykes are dark- coloured and dominated by pyroxenes and their weathered surfaces have a pale brown colour

whereas the fresh surface is dark-greenish and meso-melanocratic in appearance (figure2).

3. Analytical techniques

The mineral chemistry of various phases in the studied samples was determined by CAMECA SX- Five electron microprobe microanalysis (EPMA) at the department of geology, Banaras Hindu Univer- sity, Varanasi. Wavelength-dispersive spectrometry and a LaB6 filament have been deployed for quan- titative analyses. For X-ray ‘dot’ mapping, an acceleration voltage of 15 kV and a beam cur- rent of 100 nA were used, whereas 15 kV and 10 nA were deployed for BSE imaging. An accel- erating voltage of 15 kV, a beam current of 10 nA and a beam diameter of 1 µm along with TAP, LPET and LLIF crystals were employed for mea- surement. Several natural and synthetic standards were used for calibration. Based on multiple anal- yses, it was found that the error on major element concentrations is<1%, whereas the error on trace

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elements varied between 3% and 5%. The represen- tative microprobe data are presented in tables1–4.

Rock chips of the samples were powdered using a Retsch BB50 jaw crusher and Retsch RM100 motor grinder at the department of geology, Banaras Hindu University, and subjected to whole-rock geo- chemical analysis and isotopic study. The whole- rock geochemical analysis was carried out at Activation Laboratories, Ancaster, Canada.

Alkaline fusion and ICP-OES analysis (Thermo- Jarrell-Ash ENVIRO II model) were used to anal- yse major elements and a few trace elements (V, Sc, Sr, Ba, Zr and Y), and multi-acid digestion and Inductively coupled plasma mass spectrome- try (ICP-MS) analysis (Model: Perkin Elmer Sciex ELAN 6000) were used for the measurement of trace and rare earth element (REE) concentrations.

STM1 MRG1, DNC1, W2 and SY3 were used as internal standards and the precision is 5% and 5–10% for the major oxides and trace elements, respectively. Bulk-rock major and trace element data are presented in table 5.

Strontium and neodymium isotope ratio mea- surements of the two samples, one each from the two dykes, were obtained using a Inductively cou- pled plasma mass spectrometry (ICP-MS) at the Centre for Earth Sciences, Indian Institute of Sci- ence following the same protocol as described by Banerjee et al. (2016). The concentrations of Rb, Sr, Sm and Nd measured by ICP-MS were used.

Samples were digested using ultra-pure acid mix- tures of HF, HNO3and HCl. Analyses of standards and repeat sample were performed to illustrate the accuracy and precision of both major and trace element analyses. The measured 87Sr/86Sr and

143Nd/144Nd ratios were normalised to86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively, to correct for instrumental mass fractionation. The JNdi-1 Nd isotopic standard and SRM-987 Sr isotopic standard were analysed along with the samples and yielded values of 143Nd/144Nd = 0.512120 ± 7 (2SD, n = 3) and 87Sr/86Sr = 0.710260±9 (2SD,n= 3), respectively. The results obtained are listed in table6.

4. Petrography and mineral chemistry The petrographic study of the Kalwakurthy pyroxenites (from Potepalli and Polkampalli) reveal that they consist of clinopyroxene (dominant phase), amphibole along with interstitial oxides of magnetite–ilmenite and chromite (figure 3a). The

oxide phases are present as coarser grains and dusty granules along cleavages of silicate minerals (figure 3b). An overall igneous cumulate texture is present, where amphibole and clinopyroxene are present as cumulates. Amphiboles are of both pri- mary and secondary types, with the secondary amphiboles derived from the clinopyroxenes as evident from their relict cleavage (figure 3c–f).

Whereas augite is the sole clinopyroxene in the Potepalli dyke, the Polkampalli dyke also con- tains a smaller proportion of pigeonite besides augite (figure 4a). Most of the pyroxenes are also altered to chlorite and serpentine; however, pristine clinopyroxene grains are also present. It is well known that a porphyritic–panidiomorphic texture, hydrous mafic silicates as phenocrysts and felsic groundmass are essential attributes of lamprophyres (see Rock 1991) which are lacking in the dykes under study. Olivine pseudomorphs are also noticed at some places. It should be pointed out that chromite is present only in the Polkampalli pyroxenite, whereas the pyroxenite of Potepalli is entirely devoid of it. Chromites, where present, are well-preserved with crystal shapes and euhedral form and display marked zoning (figure3g and h).

4.1 Pyroxene

The mineral chemistry data of the clinopyroxenes is provided in table 1. Clinopyroxene displays a substantial variation in its composition between the Polkampalli (Wo18.23−27.81 En42.54−59.31

Fs14.66−40.07) and the Potepalli (Wo26.09−28.04

En52.59−58.22 Fs15.17−19.83) dykes. Clinopyroxenes from both the dykes are rich in calcium (8.98–

12.83 wt%) but depleted in Na2O (0.18–0.36 wt%), Al2O3(0.84–3.81 wt%) and TiO2 (0.05–0.87 wt%) (see table 1). A slight enrichment of Ca together with lower Al, Ti and Na contents are similar to those reported from Alaskan-type complexes (Snokeet al.1981;Helmy and El Mahallawi 2003), whereas high MgO (15.79–19.79 wt%), high Mg#

(57–74) and variable alumina and TiO2 are simi- lar to arc-related cumulates (Yuan et al. 2017). In the Ca (a.p.f.u.) vs. Ti (a.p.f.u.) space (figure4b), clinopyroxenes from the Pokampalli dyke display a strong orogenic character, whereas those from the Potepalli dyke portray a mixed orogenic to non-orogenic nature. TiO2 vs. Alz in the clinopy- roxene plot further confirms their arc cumulate type nature (figure4c).

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Table1.Representativeanalysis(wt%)ofclinopyroxenefromthePolkampallidyke(sample:PKP/2012/2)andPotepalli(sample:PP/POT/1). PolkampalliPotepalli Oxide(wt%)1234567891012345678910 SiO255.0956.3955.3957.8454.7653.6245.5447.4745.6945.5257.8656.6455.3156.0658.4256.2255.1454.9255.9655.43 TiO20.120.050.080.120.870.080.080.090.070.090.170.090.590.760.650.150.220.210.390.20 Al2O30.960.841.991.571.703.811.602.021.271.040.240.290.590.640.460.491.360.911.061.04 Cr2O30.780.490.630.580.080.570.050.061.820.040.510.510.090.000.040.670.410.070.030.28 FeO9.999.589.188.059.9410.9922.9420.9723.3427.178.529.269.8810.968.628.8810.3611.7910.5310.93 MnO0.300.160.270.270.230.210.300.330.280.350.240.140.100.150.120.100.000.240.190.10 MgO19.7919.7118.3319.3718.7517.5717.6117.5617.4815.7919.2019.6018.6218.6618.8218.7718.4318.1018.2918.71 CaO12.4012.4012.8311.8312.6511.6210.4610.659.548.9812.5212.6513.0512.9611.7413.0013.0613.2112.6112.36 Na2O0.230.210.470.220.340.640.420.460.410.240.000.000.000.000.000.000.000.000.000.00 K2O0.000.000.010.010.010.010.030.030.030.050.030.010.030.010.000.010.030.070.050.00 Total99.6699.8499.1799.8599.3299.1299.0499.6399.9499.2699.2999.1998.27100.2298.8798.3099.0199.5099.1099.05 Cationsfor6oxygenatoms Si2.0082.0392.0192.0652.0001.9691.8051.8421.7991.8272.0862.0582.0392.0332.1042.0602.0292.0212.0452.032 Al0.0410.0360.0850.0660.0730.1650.0750.0920.0590.0490.0100.0120.0260.0270.0190.0210.0590.0390.0460.045 Fe(iii) 0.0000.0000.0000.0000.0000.0000.4940.3650.4520.4500.0000.0000.0000.0000.0000.0000.0000.0000.0000.000 Cr0.0230.0140.0180.0160.0020.0170.0020.0020.0570.0010.0140.0150.0030.0000.0010.0190.0120.0020.0010.008 Ti0.0030.0010.0020.0030.0240.0020.0030.0030.0020.0030.0050.0020.0160.0210.0180.0040.0060.0060.0110.006 Fe(ii) 0.3070.2940.2840.2470.3070.3410.2350.2950.2880.4280.2640.2860.3100.3380.2680.2780.3240.3670.3280.341 Mn0.0090.0050.0080.0080.0070.0070.0100.0110.0090.0120.0070.0040.0030.0050.0040.0030.0000.0070.0060.003 Mg1.0751.0620.9961.0311.0210.9621.0401.0161.0260.9451.0321.0621.0231.0091.0101.0250.9890.9930.9971.022 Ca0.4840.4800.5010.4530.4950.4570.4440.4430.4030.3860.4840.4920.5150.5040.4530.5100.5150.5210.4940.485 Na0.0160.0150.0330.0150.0240.0450.0330.0340.0310.0190.0000.0000.0000.0000.0000.0000.0000.0000.0000.000 K0.0000.0000.0000.0000.0000.0000.0010.0010.0010.0030.0010.0010.0010.0000.0000.0010.0010.0030.0020.000 Total3.9683.9473.9483.9043.9543.9654.1404.1054.1284.1223.9043.9323.9373.9383.8773.9223.9363.9593.9293.942 Wo25.5825.9427.8125.8627.0325.7021.2521.7719.1018.2326.9226.5427.8127.1426.0927.8828.0427.5827.0626.13 En57.3257.6955.6659.3155.7754.4746.8047.7147.1142.5457.7657.5555.2654.3858.2256.4454.0952.5954.6355.22 Fs16.8716.2316.3314.6617.1819.6633.2331.5034.4040.0715.1715.7616.9118.4815.6815.4717.7419.8318.3118.57

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Table2.Representativeanalysis(wt%)ofamphibolefromthePolkampallidyke(sample:PKP/2012/2)andPotepalli(sample:PP/POT/1). PolkampalliPotepalli Oxide(wt%)1234567891012345678910 SiO255.1554.6355.7355.7154.8354.7156.1756.0155.4755.3353.5253.6150.4256.8954.4954.2855.5755.2154.3454.17 TiO20.060.590.030.020.100.080.050.090.100.070.140.263.970.310.090.430.250.170.090.20 Al2O31.581.411.011.061.181.330.961.031.621.042.510.791.141.431.061.030.720.710.361.64 Cr2O30.000.290.800.630.481.050.600.420.000.580.040.990.200.700.250.240.280.111.150.20 FeO9.809.457.307.889.158.666.568.618.557.5610.548.7312.029.4910.349.859.009.459.839.79 MnO0.200.280.110.220.220.300.080.430.470.230.240.260.420.240.140.320.030.380.030.22 MgO17.9218.0819.3718.6518.3018.5320.1818.7218.3219.4117.2817.7916.9216.2018.0017.9618.4418.2918.3818.15 CaO11.7212.0012.3712.7412.3311.9712.4511.3612.1312.5412.1413.1211.4211.4412.7112.9712.9612.9812.4912.40 Na2O0.360.280.260.200.310.350.180.230.360.240.000.000.000.000.000.000.000.000.000.00 K2O0.010.010.000.000.000.000.000.000.010.010.490.010.010.150.040.020.010.010.020.02 Total96.8097.0196.9797.1196.9096.9997.2396.9197.0397.0196.9295.5696.5196.8597.1297.0997.2797.3096.7096.78 Cationsfor23oxygenatoms Si7.8707.7947.8737.8857.8287.7957.8797.9357.8687.8357.6897.7617.3598.0657.7687.7417.8817.8397.7807.730 Ti0.0070.0630.0030.0020.0100.0090.0060.0090.0110.0080.0160.0280.4360.0330.0100.0460.0270.0180.0090.021 Al0.2660.2370.1680.1770.1990.2230.1590.1720.2700.1730.4260.1360.1960.2390.1780.1730.1200.1180.0610.276 Cr0.0000.0320.0900.0700.0540.1180.0660.0480.0000.0650.0050.1130.0230.0790.0280.0270.0310.0120.1310.023 Fe(iii) 0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0100.0700.1710.1920.0000.2300.2220.0310.1530.2250.196 Fe(ii) 1.1691.1270.8630.9331.0931.0320.7701.0191.0140.8841.1960.8861.2751.1251.0030.9521.0370.9690.9520.972 Mn0.0240.0340.0130.0270.0260.0360.0090.0520.0560.0280.0290.0310.0520.0290.0160.0390.0040.0450.0040.027 Mg3.8133.8464.0803.9363.8953.9364.2193.9553.8734.0963.7013.8403.6823.4233.8253.8183.8983.8713.9223.861 Ca1.7911.8341.8721.9321.8871.8281.8711.7241.8441.9021.8692.0341.7861.7371.9421.9811.9701.9751.9151.896 Na0.0980.0770.0700.0560.0850.0950.0480.0640.1000.0660.0000.0000.0000.0000.0000.0000.0000.0000.0000.000 K0.0010.0020.0000.0000.0000.0000.0000.0000.0010.0020.0900.0020.0020.0280.0080.0040.0020.0030.0040.004 Zr0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.000 Total15.04015.04815.03115.01715.07815.07315.02814.97815.03715.06815.09015.00215.00214.75715.00815.00415.00215.00315.00415.004

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