A-type granite belts of two chemical subgroups in central eastern China: Indication of ridge subduction

Abstract

Early Cretaceous A-type granites in the Lower Yangtze River belt, central eastern China, with both A 1 and A 2 chemical subgroups, formed at 125 ± 2 Ma, after a Cretaceous ridge subduction. Remarkably, A 1 and A 2 group granites are distributed in three zones, roughly parallel to each other and to a slightly older adakite belt. In general, A 1 granites form in intraplate settings, whereas A 2 granites near paleo-convergent margins. The alternate distribution of these two subgroup A-type granites is compatible with a proposed Cretaceous ridge subduction in the region. The subduction of a dry and hot spreading ridge may have only released small amount of fluids, so that metasomatism on the overriding lithosphere was undetectable, correspondingly resulted in A 1 granites later on. In contrast, wetter and colder oceanic crust away from the spreading ridge was responsible for mantle metasomatism and consequently the formation of A 2 granites. Further away from the ridge, the subduction angle was much steeper, and dehydration of the slab had occurred earlier during the subduction, and thus dramatically reduced mantle metasomatism, corresponding to A 1 granites again. Both A 1 and A 2 granites formed within a short period of time due to slab window/rollback, after the ridge subduction. The distribution of the A 1 and A 2 granites together with the adakite belt may be taken as discrimination indice for ancient ridge subduction. © 2011 Elsevier B.V. All rights reserved.