First Paleoproterozoic ophiolite from Gondwana: Geochronologic–geochemical documentation of ancient oceanic crust from Kandra, SE India

Abstract

SHRIMP-RG zircon U–Pb ages confirm the 1.85 Ga age of oceanic crust generated along the SE margin of India. This Paleoproterozoic ophiolite was accreted along a NE-trending suture that juxtaposes the outboard Proterozoic Eastern Ghats Granulite Belt (EGGB) against the inboard Archean Nellore Schist Belt of the Dharwar craton. Collision between the EGGB arc crust and India apparently was highly oblique, involving SW thrusting of oceanic crust and a trailing arc onto the Dharwar craton. Although deformed and dismembered, the original lithologic sequence of the Kandra Ophiolite Complex (KOC) has been largely retained. From SW to NE, the complex consists of layered + isotropic gabbros, sheeted dolerite dikes and amygdaloidal pillow basalts. Ultramafic units are intercalated within the gabbroic rocks, and plagiogranite veins + patches occur within the dolerites. Metacherty layers cap the basalts. The KOC exhibits EMORB geochemistry overprinted by subduction-zone metasomatism. Mafic magmas show high LILE/HFSE, positive Ba and Pb anomalies and negative anomalies for Nb, Zr and Hf in spidergrams — typical of a suprasubduction-zone setting. Its structure and geochemistry suggest that the KOC represents a Chilean-type continental backarc ophiolite. This is the first unequivocal Paleoproterozoic ophiolite reported from India, and probably the first from Gondwana. The 1.85 Ga KOC represents an important Gondwana example in the cascade of arc–continent collisions that assembled the Paleoproterozoic supercontinent Columbia.

Introduction

Net crustal growth along active continental margins and island arcs generally involves episodic accretion of oceanic crust, emplacement of felsic arc batholiths + coeval volcanic rocks, and/or addition of mantle-derived magmas during subsequent arc rifting (Cawood et al., 2009 and references therein). Such a continental growth model involving horizontal lithospheric displacements is well-documented in post-1.0 Ga settings, but is vigorously debated for the early stages of Earth history (Stern, 2005, Brown, 2006). However, recent studies illustrate that the Phanerozoic-style sea-floor spreading and subduction processes were operating in the Archean (Dilek and Polat, 2008 and references therein). Differences in the internal structure, lithology and geochemistry between Phanerozoic and Archean ophiolites, and rare occurrence or lack of sheeted dikes in the latter may reflect thicker oceanic plates, higher spreading rates and higher geothermal gradients in the Archean (Dilek and Polat, 2008). Nevertheless, trace element concentrations of many Archean greenstone belts are similar to Phanerozoic ophiolite suites. Documenting the mechanism of sialic growth over time, and the onset of horizontal motion of lithospheric plates is fundamental for testing models of continental crust evolution. Identification of juvenile oceanic crustal fragments accreted to continental margins provides important proof of the existence of lithospheric plates and lateral growth of the sialic crust (Cawood et al., 2009). Reported occurrences of ophiolites demonstrate the operation of lithosphere-scale plate tectonics in the Paleoproterozoic (Scott et al., 1992, Dann, 1997, Peltonen and Kontinen, 2004) and, albeit controversial, possibly during the Archean (Komiya et al., 1999, Kusky et al., 2001, Dilek and Polat, 2008, Furnes et al., 2009). The oceanic crust that constitutes the ophiolite protolith is generated by sea-floor spreading at mid-ocean ridges, in suprasubduction-zone environments (island arcs and backarcs), and beneath oceanic plateaus (Dilek, 2003a, Dilek, 2003b, Harper, 2003). Rifting, massive sialic crust production, and orogenesis was widespread at 1.9–1.6 Ga (O'Neill et al., 2007), and has been attributed to Paleoproterozoic assembly of the supercontinent Columbia (Rogers and Santosh, 2002, Zhao et al., 2004).

The SE margin of India experienced extensive rift-related magmatism around 1.9 Ga (French et al., 2008). Subsequently, it was affected by the addition of new continental crust, apparently by subduction-related processes (Vijaya Kumar and Leelanandam, 2008). This Paleoproterozoic crustal growth along the SE margin of India is similar to that of the Trans-Hudson orogen (St-Onge et al., 2009) in that it involved ocean closure, arc accretion, and final continent–continent collision, and is considered as a transition from the Pacific to the Alpine style (Ernst, 2005) of mountain building (Vijaya Kumar and Leelanandam, 2008). This continuum, from accretion of island arc crust through continental arc formation to terminal continent–continent collision-type orogeny, similar to that of Phanerozoic continent formation (Lee et al., 2007), generated and refined the Paleoproterozoic continental crust along the SE margin of India (Vijaya Kumar and Leelanandam, 2008). However, the precise timing of accretion and the nature of accreted oceanic crust, prior to the present work, were not well constrained. In this work, we provide geochronological and geochemical documentation of the formation of the Kandra Ophiolite Complex (KOC), in order to evaluate the occurrence of Paleoproterozoic accretionary tectonics and its relevance to the assembly of Columbia.