Geochemical and Nd isotope constraints on petrogenesis of granitoids from NW part of the eastern Dharwar craton: Possible implications for late Archaean crustal accretion

Abstract

Geochemical and Nd isotope data on granitoids of the NW part of the late Archaean eastern Dharwar craton are presented to elucidate their petrogenesis and role in crust formation. The granitoids are divided into three suites viz. trondhjemite–granodiorite gneisses, biotite monzogranites and porphyritic biotite granodiorites. The gneisses are pre- to syn-kinematic (with respect to deformation in the adjacent Hungund–Kushtagi schist belt), which show variable SiO₂ and Al₂O₃, enriched LREE and depleted HREE with slightly negative to no Eu anomalies. They display unusual chemistry in having higher FeO(T), K₂O, Ba, Cr and Ni compared to the typical Archaean tonalite–trondhjemite–granodiorite (TTG). The biotite monzogranites are mostly syn- to late-kinematic and exhibit evolved calc-alkaline compositions with high SiO₂, K₂O, LILE and LREE, depleted to undepleted HREE and strongly negative to no Eu anomalies. The porphyritic granodiorites show syn- to late-kinematic calc-alkaline, sanukitoid-like character with a wide range of SiO₂, higher TiO₂, P₂O₅, Sr, Ba, Cr and Ni, and lower Rb. They, however, uniquely display higher K₂O, ΣREE and Th than typical sanukitoids. The trondhjemite–granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge. Subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites. Intrusion of the latter triggered melting of the TTG crust accreted earlier and generated the evolved monzogranites. The monzogranites occurring east of the Hungund–Kushtagi schist belt show higher εNd but lower T_DM ages than those occurring to the west, indicating that terranes with different histories were juxtaposed by lateral accretion.

Introduction

Archaean granite–greenstone belts provide vital clues of crust formation events, although the precise mechanism and geodynamic setting are still topics of intense debate (Rollinson, 2006). Models suggested for late Archaean crust formation can be broadly divided into two groups viz. subduction-related accretion (Card, 1990, de Wit, 1998, Dirks and Jelsma, 1998, Kusky, 1989, Stevensen et al., 2009) and mantle plume-related growth (Chardon et al., 1998, Hill et al., 1992, Jayananda et al., 2000, Stein and Hofmann, 1994). Similar divergence of views exists for the late Archaean crust formation of the Dharwar craton, southern India.

The Dharwar craton is subdivided into eastern and western Dharwar cratons (EDC and WDC) (Fig. 1a). The main event of granitoid magmatism and crust formation in the WDC is 3.4–3.0 Ga, whereas EDC is dominated by 2.7–2.5 Ga crust. The whole Dharwar craton shows a N–S to NNW trending structural fabric. Chadwick et al., 2000, Chadwick et al., 2007 have argued for late Archaean accretion of the EDC in a Phanerozoic-style subduction environment, where an oceanic plate subducted below a middle Archaean foreland continental margin (the WDC). They have reported WNW-directed oblique convergence or transpression which was partitioned into arc-parallel NW–SE to N–S sinistral displacements and arc-normal NE–SW shortening. Schist belts in the EDC are thought to represent intra-arc basins, whereas those in the WDC marginal or back-arc basins. It is suggested that the late Archaean plutonic rocks of the EDC were emplaced in linear belts bounded by the NW–SE to N–S sinistral shear zones. On the basis of U–Pb zircon and titanite ages, and initial Nd, Sr and Pb isotope compositions, Balakrishnan et al., 1999, Krogstad et al., 1995 have identified 2.65–2.52 Ga discrete granitic terranes on both sides of the N–S trending Ramagiri and Kolar schist belts in the southern part of the EDC. These workers have suggested that terranes of different history were amalgamated by horizontal accretion in the EDC. Chardon et al., 1998, Chardon et al., 2002, however, identified late Archaean centripetal collapse of greenstone basins in the Dharwar craton. A mantle plume is believed to have supplied heat followed by softening of the crust, which facilitated sinking of greenstone belts (sagduction) and rising of gneiss domes. Low-degree melting of such mantle plume provided juvenile magma of intermediate composition, which variably interacted with and triggered partial melting of the crust in the eastern part of the Dharwar craton (Jayananda et al., 1995, Jayananda et al., 2000). Harish Kumar et al., 2003, Moyen et al., 2003a proposed models involving initial subduction followed by closure of oceanic domain and arc-continent collision. Subsequent arrival of mantle plume caused crustal reworking and metamorphism.

In this paper, we report geochemical and Nd isotope data on different types of granitoids from the poorly studied NW part of the EDC (Fig. 1). The petrogenesis of these granitoids is discussed. Their geochemical signatures are interpreted in terms of plausible geodynamic scenario taking into consideration of regional structural and metamorphic patterns. This allows us to address the following highly-debated issues with implications for the late Archaean growth of the EDC:

• What was the mechanism of crust formation?

• What was the tectonic setting of granitoid generation?

• What was the nature of crust–mantle interaction and crustal recycling?