The growth of continental crust during the Late Proterozoic: geochemical evidence from the Arabian Shield

Abstract

Abstract Proterozoic magmatic provinces that result from active subduction provide a critical link between contrasting styles of crustal growth during the Archæan and Phanerozoic. The Arabian Shield is an excellent example of Late Proterozoic (860–560 Ma ago) felsic magmatism that is largely calc-alkaline in composition and displays the selective enrichment of LIL elements indicative of subduction. Juvenile Nd and Sr isotope ratios confirm that the Arabian Shield represents a collage of island arcs that were responsible for a significant proportion of global crustal growth (as measured by volume of magma extracted from the mantle per unit time) during the Pan-African tectonothermal event. The high Rb/Sr ratios and low Sr concentrations of the majority of granitoids from the Arabian Shield suggest that they may be modelled as the products of intra-crustal feldspar fractionation from a basaltic source. However, about 250f Arabian granitoids, mostly formed between 860 and 820 Ma ago, are characterized by high Sr and depleted Y abundances, indicative of melts which have not undergone fractional crystallization of feldspar, derived from a garnet-rich source. In the absence of tectonic thickening this suggests that these magmas were generated by melting the subducted slab, a process observed in contemporary arcs where unusually young, hot crust has been subducted. The average composition of the Arabian crust at its present level of exposure is highly silica rich (SiO 2 = 66%), and mass balance constraints on the composition of the continental crust of the Arabian Shield together with seismic evidence for a lower crust of gabbroic composition, require that either delamination of a mafic lower crust has occurred or the average composition of the mantle-to-crust flux was intermediate rather than basic in composition. The latter explanation is preferred since crustal delamination implies unrealistically high crustal growth rates during the Pan-African event. A flux of intermediate composition is indicative of slab melting during subduction. Thus trace element and mass balance considerations support the hypothesis that a significant proportion of Arabian granitoids formed during the early stages of the Pan-African event were derived from partial melting of the subducted slab, indicating that subduction of young, hot oceanic lithosphere was widespread at that time.