geochemical and isotopic constraints on mantle sources, crustal evolution and implications for supercontinent formation and ore mineralization
The Nuggihalli greenstone belt is part of the older greenstone belts (3.4 - 3.0 Ga) in the Western Dharwar Craton, southern India. This greenstone sequence consists of conformable metavolcanic and metasedimentary supracrustal rock assemblages that belong to the Sargur Group. Sill-like ultramafic-mafic plutonic bodies are present within these supracrustal rocks (schist rocks) which are in turn enclosed by tonalite-trondhjemite-granodiorite gneiss (TTG). The sill-like ultramafic-mafic rocks are cumulates derived from a high-Mg parental magma that are represented by chromitite-hosted serpentinite and tremolite-chlorite-actinolite- schist (altered peridotite), anorthosite, pyroxenite, and gabbro hosting magnetite bands. The first whole-rock Sm-Nd data for the peridotite anorthosite- pyroxenite-gabbro unit has been obtained yielding an age of 3125 ± 120 Ma (MSWD = 1.3) which is similar to reported ages for the komatiitic suite of the associated schist belts in the craton within the limits of errors. A whole rock Pb-Pb errorchron age of 2801 ± 110 Ma (MSWD = 102) has been obtained for the entire plutonic ultramafic-mafic suite; this represents a younger metamorphic age and is in agreement with previous geochronological studies. The magmatic activity of the younger greenstone belts in the Western Dharwar Craton has reset the Pb isotope systematics, which thus yield younger ages. The whole rock Rb-Sr data has been perturbed such that no age data is obtainable. Whole-rock major and trace element geochemistry indicates that the sill-like ultramafic-mafic rocks are cogenetic and related by fractional crystallization. The metavolcanic schist rocks are inferred to be cogenetic with the plutonic suite, and are also fractionated. The subchondritic to chondritic REE abundances of the chromitites, serpentinites, peridotites, and anorthosites, along with the flat heavy REE patterns, resemble the pattern of Al-undepleted komatiites and indicate high degrees of partial melting of the mantle source. The LREE depleted pattern of peridotites reflects derivation from depleted mantle. The pyroxenite and gabbros show superchondritic REE abundances, and their coherent patterns indicate that they are more fractionated differentiates of the ultramafic cumulates. Overall positive eNd values (+1.7 to +3.4) of the ultramafic-mafic rocks, and very low initial 87Sr/86Sr (at 3.1 Ga) of the gabbros (0.70097 - 0.70111) implies a depleted mantle source for the Nuggihalli rocks. Extreme enrichment of the high field strength elements (HFSE) Nb, Ta, Zr, Hf and Ti in the Nuggihalli rocks suggests the presence of a residual subducted oceanic crust (eclogite) in the source. Eclogite contains accessory rutile that acts as the HFSE sink, therefore melting and mixing of the eclogite component with depleted mantle melts resulted in distinct HFSE enrichment in the Nuggihalli rocks. Alternatively, melting of a HFSE-enriched eclogitic slab and the surrounding depleted mantle within an active subduction zone is another possible mechanism, however it is difficult to distinguish between the two models. Comparison of our age data with global ultramafic-mafic rock occurrences in greenstone belts supports an increase in high-Mg magmatism or komatiitic activity from 3.5 Ga to 2.7 Ga. Such a global surge of komatiitic magmatism spanning nearly 800 Ma is most likely related to a supercontinent cycle. High-Mg magmas such as komatiites host important metal deposits like chromite, Ni-sulfide and associated PGE mineralization. The uneven distribution of metal deposits over time can be explained by plate tectonic cycles that included repeated periods of aggregation and breakup of supercontinents. The 3.1 Ga chromite deposits of the Nuggihalli greenstone belt are part of the global komatiitic surge and are perhaps related to the amalgamation stage of the supercontinent 'Ur' in the southern hemisphere, which is believed to have been stable at ˜ 3.0 Ga.