GEOCHEMISTRY OF GABBRO AND GRANITOIDS IN THE MAGADAN MASSIF

Abstract

The geochemistry of subduction-related gabbroids and granitoids was studied in the Magadan Massif, a large intrusion in the inner zone of the Okhotsk-Chukotka volcanic belt (OCVB), which is an active continental margin of the Andean type. The massif was produced over a broad time interval (J(3)-K-2) which is commensurable with the development period of the OCVB itself. The Magadan Massif consists of two genetically heterogeneous parts. The early phase of the massif(J(3)-K-1) is composed of a bimodal association (gabbronorites and trondhjemites, combined into the Ventsovyi Complex). The other, volumetrically predominant, part consists of granitoids of a continuous calc-alkaline series. Petrochemically, the Ventsovyi gabbro is classified with island-are tholeiites. whereas the younger granitoids affiliate with the calc-alkaline series. The gabbroids are characterized by relatively low concentrations of alkalis, Ti, P, and Zr; i.e., they show features typical of the derivatives of shallow magmatic chambers, which were formed at upper lithospheric levels, within the stability field of olivine-plagioclase assemblages. The trace-element concentrations of the Magadan Massif granitoids are comparable with those of the intrusions of similar silicity in volcanoplutonic associations of the outer OCVB zone. The K- and Na-enriched granitoids of the Magadan Massif are characterized by elevated concentrations of Au, Ag, Cu, Mo, and W.