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Petrology and Geochemistry of Metamorphosed Basaltic Pillow Lava

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Petrology and Geochemistry of Metamorphosed Basaltic Pillow Lava JournalBasaltic of Mineralogical pillow lava and and basaltic Petrological komatiite Sciences, in the ArcheanVolume 101Bundelkhand, page 199 craton─ 217, 2006 199 Petrology and geochemistry of metamorphosed basaltic pillow lava and basaltic komatiite in the Mauranipur area: subduction related volcanism in the Archean Bundelkhand craton, Central India *,** * *** * Vivek P. MALVIYA , Makoto ARIMA , Jayanta K. PATI and Yoshiyuki KANEKO *Geological Institute, Graduate School of Environmental and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya－ku, Yokohama 240－8501, Japan **Present address: School of Environmental Sciences, National Facility for Geochemical Research, Jawaharlal Nehru University, New Delhi-67, India *** Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad 211002, India The occurrence of metamorphosed basaltic pillow lava in close association with serpentinized ultramafic rock, metamorphosed basaltic komatiite, volcaniclastic metasediment, and banded iron formation (BIF) in the Mauranipur area is the first explicit evidence for subduction related submarine volcanism in the Archean Bundelkhand craton, Central India. The Mauranipur pillow lava underwent greenschist to amphibolite facies metamorphism while retaining a geochemical signature of its igneous protolith. The pillow lava and associated massive volcanic rock is subalkalic, low−K tholeiitic basalt to basaltic andesite with SiO2 = 51.9−55.9 wt% and Mg/(Mg + Fetotal) × 100 = 63.4−67.2. The rock shows depletion in HFS elements with Nb/Nb* (0.07−0.17), and LIL elements are relatively enriched compared with present−day N−MORB compositions. Chondrite normalized REE patterns for the Mauranipur pillow lava are nearly flat with (La/Sm)N = 0.9−1.5, (Gd/Yb)N = 1.0−1.1, and Eu/Eu* = 0.81−1.10. The geochemical characteristics of the pillow lava are similar to those of low−K tholeiitic basalt reported from modern intra−oceanic arcs. The associated high−Mg andesite is compositionally similar to basaltic komatiite, enriched in LREE, with (La/Sm)N = 2.95−6.44, and depicts a nearly flat chondrite normalized HREE pattern with a low MREE/HREE ratio (Gd/Yb)N = 1.24−1.58. The basaltic komatiite displays remarkably similar geochemical characteristics to modern boninite. The present study, combined with available geological data, suggests that the supracrustal rocks of the Mauranipur area represent an Archean ophiolite sequence formed in a plate convergent setting. Keywords: Pillow lava, Basaltic komatiite, Bundelkhand craton, Subduction zone, Ophiolite, Late Archean INTRODUCTION tectonothermal models have been proposed for this unit and these have been based on geochemical and geochro- Cratonic segments of Archean－Proterozoic age in the nological studies of its constituent tonalite－trondhjemite－ Indian peninsular shield were formed by collision and granodiorite (TTG) gneiss, early Proterozoic granite intru- accretion of the Bundelkhand, Dharwar, Aravalli, Baster, sions, and mafic dikes (Sarkar et al., 1984, 1994, 1996; and Singhbhum cratons. These cratons evolved from indi- Basu, 1986; Rahman and Zainuddin, 1993; Sharma and vidual continental nuclei that merged along deep－seated Rahman, 1995; Mondal et al., 1998; Mondal and Ahmad, lineaments (Radhakrishna and Naqvi, 1986). The Bundelk- 2001; Mondal et al., 2002; Rao, 2004; Rao et al., 2005). hand craton was once considered as the Archean to Pro- Supracrustal rocks have been identified as linear terozoic, predominantly granitic complex occurring in the slivers along shear zones in younger granitoids. These are northern part of the Indian peninsular (Fig. 1). Various seen as the Karera－Jhansi, Babina－Kuraicha－Kabrai, and － － － doi:10.2465/jmps.101.199 Madaura Rajaula Girar Baraitha belts, which consist of M. Arima, [email protected] Corresponding author banded iron formation (BIF), interbedded quartzite, schist, V.P. Malviya, [email protected] amphibolite, and/or calc－silicate intruded by TTG. Shar- 200 V.P. Malviya, M. Arima, J.K. Pati and Y. Kaneko Figure 1. (A) Tectonic map of the peninsular Indian shield showing various tectonic elements and the locations of cratonic blocks (after Naqvi and Rogers, 1987). (B) Simplified geological map of the Bundelkhand craton modified after Basu (1986), illustrating prominent boundaries, marginal basins, marked linear features such as giant quartz veins and mafic dikes, and the location of the “Bundelkhand Tectonic Zone”. ma (1998) suggested that these slivers represent Archean the Bundelkhand craton (Malviya et al., 2004). The lithol- greenstone belts and predate the granitic gneisses of TTG ogies encountered comprised basaltic to basaltic andesitic affinity (~ 3.3 Ga). The geological significance of the pillow and massive lava, serpentinized ultramafic rock, supracrustal rocks in the tectonothermal history of the volcaniclastic sediment, tuff, and BIF similar to ophiolite Bundelkhand craton is unresolved. reported from Archean cratons worldwide (e.g., Paris, We mapped metamorphosed supracrustal rocks over 1987; Kusky and Polat, 1999; Cousens, 2000; Kusky et a region of 3 km2 in the Mauranipur area in the center of al., 2001; Smithies, 2002; Feri and Jensen, 2003; Polat et Basaltic pillow lava and basaltic komatiite in the Archean Bundelkhand craton 201 al., 2003; Smithies et al., 2004, 2005). gneisses (3.5－3.3 Ga) (Sarkar et al., 1984; Mondal et al., This paper introduces geological and geochemical 2002), metamorphosed supracrustal rocks (BIF, interbed- evidence for the existence of supra－subduction ophiolite, ded quartzite, schist, amphibolite and/or calc silicate), and which allows the proposal of a new model for the geody- early Proterozoic granite intrusions (2.5－2.1 Ga) (Craw- namical development of the Bundelkhand craton during ford, 1970; Sarkar et al., 1984; Mondal et al., 2002). Meta- Archean time. The geochemical characteristics of meta- morphosed supracrustal rocks are exposed along the E－W morphosed mafic to ultramafic volcanic rock and associ- striking, crustal scale Bundelkhand Tectonic Zone, which ated metasediment in the Mauranipur greenstone belt pro- is a major shear structure orientated N25°15′ (Pati, 1999). vide important information on the early evolution of the The Bundelkhand Tectonic Zone can be traced along mantle－crust system of the craton. a strike length of around 200 km extending from Mahoba through Mauranipur to Babina (Fig. 1). It exhibits sigmoi- GEOLOGICAL SETTING dal schistosity and a protomylonite－mylonite－ultramy- lonite fabric, which is clearly observed in both granitic The Bundelkhand craton has a semicircular shape in plan and supracrustal rocks. The virtually E－W orientation of and extends over about 29,000 km2. It is bounded by the mylonitic foliation has a largely north subvertical dip. The Son－Narmada lineament to the south and the Great boun- stretched lineations measured from the mylonitic foliation dary fault of Aravalli on its west and is composed of three show a subhorizontal plunge. The sense of shear mea- main lithological－tectonic units. These are Archean TTG sured in mesoscopic as well as microscopic scale indi- Figure 2. Geological map of the Mauranipur greenstone belt. 202 V.P. Malviya, M. Arima, J.K. Pati and Y. Kaneko cates pronounced sinistral and weak dextral movement. serpentinized komatiite and basaltic komatiite, volcaniclas- The western, eastern, and southern margins of the tic metasediment, and BIF in the Mauranipur area (Fig. 2). Bundelkhand craton are overlain by the Vindhyan Super- The supracrustal rocks exposed along the highly sheared, group and the north lies under Indo－Gangetic alluvium. E－W trending Bundelkhand Tectonic Zone (Pati, 1999) Three marginal rift basins are the Hirapur and Sonrai have undergone greenschist to lower－amphibolite facies basin to the south and the Gwalior basin on the northwest- metamorphism and extensive deformation (Fig. 1). In ern fringe. Volcano－sedimentary units of Late Archean to some places, however, the amphibolites have well－pre- Early Proterozoic age occur in these boundary structural served pillow structures (Fig. 3A, B). The pillow lava is units. made up of numbers of elongated flow lobes. A few pil- Numerous “giant quartz veins” (2.0－1.8 Ga; Pati et lows reach 100 cm in length, but the average is about 50 al., 1997) up to 1 km thick in places and striking NE－SW cm with a variable aspect ratio ranging from 3.0 to 4.5. are a characteristic of the entire cratonic area (Fig. 1). Each pillow has a 1－3 cm thick fine－grained, pale brown There are also mafic to ultramafic dikes, which strike rim with a massive, coarser－grained central region. There NW－SE (2.0－0.9 Ga; Sarkar et al., 1997; Rao et al., 2005) is little interpillow material and the pillows fit closely and intrude across a wide area. These dikes cut the quartz together. Some pillows are asymmetric with a convex veins and granites and the linear features extend over tens upward top and a cusped bottom, suggesting a “right side of kilometers. The presence of brittle fractures along some up” deposition. Some pillow lavas contain cooling col- of the giant quartz veins is seen as a large－scale, domi- lapse cavities filled by quartz (Fig. 3A). Mylonitized nantly sinistral displacement. granitoids occur in the vicinity of the pillow basalt. The ultramafic rocks have experienced extensive ser- Field occurrence pentinization and deformation in which none of the origi- nal mineralogy is preserved. However, ultramafic rocks We observed metamorphosed pillow
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