THE AGE OF THE ZHEMCHUZHNYI DRUSITE MASSIF, NORTHWESTERN WHITE SEA AREA: U-PB ISOTOPE DATA AND GEOLOGICAL IMPLICATIONS

Abstract

A zircon age was determined for diorite from the Zhemchuzhnyi drusite complex, which was used as a reference complex in recognizing the prefolding and premetamorphic basites. There are two populations of zircon which differ in the presence of fine euhedral zoning, morphology, and geochemistry, and which, by analogy with zircon from other massifs, are thought to be magmatic and metamorphic. The magmatic and metamorphic zircons were dated at 2356 ± 4 Ma and 2354 ± 2 Ma, respectively, which indicates that the diorite was metamorphosed immediately after its crystallization. Our results, along with published data on the U-Pb age of other reference drusite massifs, indicate that nearly all of the drusites, including the gabbroanorthosite, affiliate to a single Early Proterozoic drusite complex. The drusite intrusion lasted at least 100 m.y. during the initial evolution of the main Early Proterozoic rift belts on the Baltic Shield. The differences in the schistosity and migmatization of the drusites are related to the zonation of these processes in the northwestern White Sea area during Early Proterozoic time.