Felsic Magmatism through Intracrustal Melting of Previously Formed Volcanic-Arc Crust: Implications for Differentiation and Secular Evolution of the Continental Crust

Abstract:

 

The fundamental challenge in understanding the origin and evolution of the continental crust is to recognize how primary mantle source, and oceanic crust, which are essentially mafic to ultramafic in composition, could differentiate into a more or less felsic compositions. It is possible to understand growth and differentiation of the continental crust by constraining the interplay of magmatism, deformation, and high-grade metamorphism in the lower crust. Here, we apply this knowledge on the lower crustal granitoids of southern India and speculate on the variations in geochemistry as a consequence of differentiation and secular evolution of the continental crust.

The major groups of granitoids of southern India are classified as metatonalites, comparable to typical Archaean TTGs with pronounced calc-alkaline affinity, and metagranites which are magmatic fractionation produced by reworking of early crust. Metatonalites are sodic-trondhjemites with slightly magnesian, moderate LREE (average La_N = 103) and low HREE (average Yb_N = 2) characerestics, where as metagranites are calc-alkaline ferroan types with enriched LREE (average La_N = 427) and HREE (average Yb_N = 23). Petrogenetic characteristics of granitoids illustrate continuous evolution of a primary crust into diverse magmatic units by multiple stages of intracrustal differentiation processes attributed to following tectonic scenarios: (1) formation of tonalitic magma by low- to moderate-degree partial melting of hydrated basaltic crust at pressures high enough to stabilize garnet-amphibole residue and (2) genesis of granite in a continental arc-accretion setting by an episode of crustal remelting of the tonalitic crust, within plagioclase stability field. The first-stage formed in a flat-subduction setting of an volcanic-arc, leading to the formation of tonalites. The heat budget required is ascribed to the upwelling of the mantle and/or basaltic underplating. Progressive decline in mantle potential temperature over geological time triggered the second-stage by flat-to-steep transition in subduction style. Granite so formed with its comparatively low-density induced rheological perturbations in the lower continental crust, thereby initiating foundering of the residual materials and effective differentiation of the lower crust.