Accepted Manuscript Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar craton, India: Implications for early mantle heterogeneity J.M. Maya, Rajneesh Bhutani, S. Balakrishnan, S. Rajee Sandhya PII: S1674-9871(16)30025-1 DOI: 10.1016/j.gsf.2016.03.007 Reference: GSF 443 To appear in: Geoscience Frontiers Received Date: 5 November 2015 Revised Date: 1 March 2016 Accepted Date: 3 March 2016 Please cite this article as: Maya, J.M., Bhutani, R., Balakrishnan, S., Sandhya, S.R., Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar craton, India: Implications for early mantle heterogeneity, Geoscience Frontiers (2016), doi: 10.1016/j.gsf.2016.03.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT MANUSCRIPT ACCEPTED ACCEPTED MANUSCRIPT 1 Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar 2 craton, India: Implications for early mantle heterogeneity * 3 J. M. Maya, Rajneesh Bhutani , S. Balakrishnan, S. Rajee Sandhya 4 Department of Earth Sciences, Pondicherry University, Puducherry-605014, India 5 6 *Corresponding author. Phone: +919443636422; 7 E-mail address: [email protected] 8 9 MANUSCRIPT ACCEPTED ACCEPTED MANUSCRIPT 10 Abstract 11 Spinifex-textured, magnesian (MgO >25 wt.%) komatiites from Mesoarchean 12 Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were 13 analysed for major and trace elements and 147,146 Sm-143,142 Nd systematics to constrain 14 age, petrogenesis and to understand the evolution of Archean mantle. Major and trace 15 element ratios such as CaO/Al 2O3, Al 2O3/TiO 2, Gd/Yb, La/Nb and Nb/Y suggest 16 aluminium undepleted to enriched compositional range for these komatiites. The depth of 17 melting is estimated to be varying from 120 to 240 km and trace-element modelling 18 indicates that the mantle source would have undergone multiple episodes of melting prior 19 to the generation of magmas parental to these komatiites. Ten samples of these komatiites 20 together with the published results of four samples from the same belt yield 147 Sm-143 Nd 21 isochron age of ca. 3.14 Ga with an initial εNd (t) value of +3.5. High precision 22 measurements of 142 Nd/ 144 Nd ratios were carried out for six komatiite samples along with 23 standards AMES and La Jolla. All results are within uncertainties of the terrestrial 24 samples. The absence of 142 Nd/ 144 Nd anomaly indicates that the source of these 25 komatiites formed after the extinction of 146 Sm, i.e. 4.3 Ga ago. In order to evolve to the 147 144 26 high εNd (t) value of + 3.5 by 3.14 Ga the time-integratedMANUSCRIPT ratio of Sm/ Nd should be 27 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during 28 that time. These results indicate at least two events of mantle differentiation starting with 29 the chondritic composition of the mantle. The first event occurred very early at ~4.53 Ga 30 to create a global early depleted reservoir with superchondritic Sm/Nd ratio. The source 31 of Isua greenstone rocks with positive 142 Nd anomaly was depleted during a second 32 differentiation within the life time of 146 Sm, i.e. prior to 4.46 Ga. The source mantle of 33 the Bansandra komatiite was a result of a differentiation event that occurred after the 34 extinction of the 146 Sm, i.e. at 4.3 Ga and prior to 3.14 Ga. Banasandra komatiites 35 therefore provide evidence for preservation of heterogeneities generated during mantle 36 differentiationACCEPTED at 4.3 Ga. 37 Keywords: Komatiite; Dharwar craton; Early mantle differentiation; 142 Nd/144 Nd; Sm-Nd 38 geochronology 39 2 ACCEPTED MANUSCRIPT 40 1. Introduction 41 Komatiites since their discovery have provided useful constraints on the nature of 42 Archean mantle (Viljoen and Viljoen, 1969a, b; Arndt, 1975; Nesbitt et al., 1979; 43 Blichert-Toft and Puchtel, 2010). There is now a broad framework on genesis of 44 komatiites, though some details vary from region to region. Particularly, there have been 45 contrasting views on the extent of melting required to generate komatiite magmas 46 (Takahashi and Scarfe, 1985; Rajamani et al., 1985; McKenzie and Bickle, 1988). 47 Komatiites from Archean greenstone belts particularly proved useful in providing 48 information on extent of depletion of incompatible elements in their source regions and 49 also inheritance from the older events of mantle differentiation. Touboul et al. (2012) 50 reported excess in 182 W from 2.8 Ga old Kostomuksha komatiites indicating preservation 51 of heterogeneities generated during the early differentiation events. On the other hand 52 Blichert-Toft and Puchtel (2010) showed remarkable consistence of initial Sm-Nd and 53 Lu-Hf isotope ratios in komatiites over a wide spatial scale. They also showed that the 54 initial ratios decreased with time indicating progressive mixing of depleted and enriched 55 reservoirs which were created during the early diffMANUSCRIPTerentiation of the bulk silicate earth. 56 We, here report new elemental and isotopic results of coupled 147,146 Sm-143,142 Nd 57 system from Banasandra komatiites of Sargur group of western Dharwar craton to 58 constrain the age and petrogenesis of these rocks. With the help of coupled 147,146 Sm- 59 143,142 Nd system we also demonstrate the preservation of early mantle heterogeneities up 60 to Mesoarchean. 61 2. Geological framework 62 2.1. Geology of Dharwar craton 63 The DharwarACCEPTED craton (Fig. 1) dominantly comprises 3.36–2.9 Ga old Tonalitic- 64 Trondhjemitic-Granodioritic (TTG) gneisses referred to as Peninsular gneisses, 65 greenstone belts and 2.6–2.5 Ga old K-rich granitoid rocks (Beckinsale et al., 1980; 66 Peucat et al., 1993; Balakrishnan et al., 1999; Jayananda et al., 2008; Chardon et al., 67 2011; Hokada et al., 2013; Mazumder and Eriksson, 2015). An increase in the grade of 3 ACCEPTED MANUSCRIPT 68 metamorphism from greenschist to granulite facies reported from the north to south of 69 the craton is interpreted to be the result of tilting of the craton northwards and post 70 Archean differential denudation, exposing deeper level of continental crust in the 71 south relative to the north (Chardon et al., 2008). Archean greenstone-granite terranes 72 are well exposed in northern block of the Dharwar craton which is considered to be 73 made up of eastern and western parts (Fig. 1). N–S trending, mylonitized zone 74 proximal to the eastern margin of the Chitradurga greenstone belt was suggested as the 75 boundary between western and eastern parts of Dharwar craton (Chadwick et al., 76 1992). 77 Western Dharwar craton is characterized by larger, 2.9–2.6 Ga old greenstone 78 belts (Anil Kumar et al., 1996; Nutman et al., 1996; Trendall et al., 1997), belonging to 79 the Dharwar Supergroup and smaller, 3–3.58 Ga old Sargur group of greenstone belts 80 (Nutman et al., 1992; Peucat et al., 1995; Jayananda et al., 2008) mostly occurring as 81 variably sized enclaves within Peninsular gneisses (Fig. 1). Greenstone belts of the 82 Dharwar Supergroup consist of relatively subordinate quantities of mafic-felsic volcanic 83 rocks, voluminous pile of sediments with conglomeraMANUSCRIPTte unit at their base. The Sargur 84 greenstone belts are characterized by ultramafic-ma fic rocks, including komatiites, minor 85 felsic volcanic rocks, quartzites and meta-pelitic and carbonate sediments with rare barite 86 bands and the prominent ones are Ghattihosahalli, Jayachamarajapura, Banasandra, 87 Kalyadi, and Nuggihalli (Jayananda et al., 2008) (Fig. 1). 88 Komatiites in the Dharwar craton were first identified by Viswanathan (1974) in 89 Kolar greenstone belt and were studied in detail by Rajamani et al. (1985) and 90 Balakrishnan et al. (1990). Geological setting, structural and geochemical aspects of 91 komatiites of Sargur group were reported by Radhakrishna and Sreenivasaiah (1974), 92 Viswanatha et al. (1977); Seshadri et al. (1981); Srikantia and Bose (1985); Jaffri et al. 93 (1997) and ChardonACCEPTED et al. (2008). Jayananda et al. (2008) carried out geochemical and 94 Sm-Nd isotope studies on komatiites of Ghattihosahalli, J.C. Pura, Banasandra, Kalyadi 95 and Nuggihalli greenstone belts and reported whole-rock Sm-Nd isochron age of 3352 ± 96 119 Ma. 4 ACCEPTED MANUSCRIPT 97 2.2. Geology of Banasandra area 98 Sparse outcrops of komatiites were found around Banasandra, Birasandra, 99 Kodihalli and Kunikenahalli villages (Fig. 2), between N13°14.32 ′ to N13°17.77 ′ 100 latitudes and E76°39.57 ′ to E76°42.6 ′ longitudes and described in detail by Srikantia and 101 Bose (1985). Representative samples of komatiites were collected from locations 102 indicated in the Fig. 2. 103 Komatiites occur as elliptical, ~200 m wide bodies enveloped within talc-tremolite- 104 chlorite schist (Fig. 2) and are invariably serpentinized. Pillow structures are well 105 preserved at several localities such as to the east of Birasandra and to the south of 106 Kodihalli indicating formation of komatiites under subaqueous conditions. The pillows 107 are 20 to 60 cm long and 15 to 20 cm wide enclosed by cherty layer (Fig. 3b) which, at 108 places, are rich in magnetite formed by metamorphism of the ferruginous chert. 109 Carbonate veins, few mm thick, cut komatiites.