Late Archaean granites: a typology based on the Dharwar craton (India)

Abstract

Extensive field work in the Eastern Dharwar Craton, associated with petrographic and geochemical (major and trace elements) investigations, allows four main types of Late Archaean granitoids to be distinguished. (1) Na-rich granitoids of trondhjemitic, tonalitic and granodioritic composition (TTG) that are characterised by strongly fractionated REE patterns and low HREE contents and generally interpreted as “slab melts” generated by partial melting of metamorphosed hydrated basalt, most likely in a subduction environment. (2) Sanukitoids, which are K- and Mg-rich monzonites and granodiorites with TTG-like REE patterns associated with marked LILE-enrichment, and considered to result from the reaction of slab melts generated in a subduction environment with, and assimilation of, mantle wedge peridotite. (3) Uncommon high-HFSE, Mg and K granites with strongly REE and LILE-enrichment that, probably formed by partial melting of an enriched mantle source; unlike in the genesis of sanukitoids, in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals, resulting in mantle-enrichment. Subsequent melting of this enriched mantle (probably in a post-subduction setting) gives rise to high-HFSE, Mg and K magmas. As demonstrated for the Closepet Granite (Dharwar Craton), the hot mantle-derived magma can induce melting of continental crust and then mix with the anatectic products. (4) K-rich, Mg-poor anatectic biotite-granites with REE patterns that are less fractionated and show negative Eu anomalies. These granites result from the remelting of old basement or recently accreted plutons, both with TTG compositions. Such anatexis can occur either in a subduction or in a post-subduction environment. This typology, based on the well-exposed Dharwar Craton, can be extended to Late Archaean granitoids from all over the world. About such 500 analysis were compiled, and a number of discriminant diagrams, based on both major and trace elements, are proposed. The most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system, followed by thermal reworking of the newly accreted continental material. However, it has been found that Archaean "subduction-related" granitoids are significantly different from their modern counterparts, implying progressive changes in the modes of magma generation at convergent margins from the Archaean to the present.