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

A zircon age was determined for diorite from the Zhemchuzhnyi drusite complex, which was used as a reference complex in recognizing the pref olding and premetamorphic basites. The zircon comprises two population s, which differ in the presence of fine euhedral zoning, morphology, a nd geochemistry, and which, by analogy with zircon from other massifs, are thought to be magmatic and metamorphic. The magmatic and metamorp hic zircons were dated at 2356 +/- 4 Ma and 2354 +/- 2 Ma, respectivel y, which indicates that the diorite was metamorphosed immediately afte r its crystallization. Our results, along with published data on the U -Pb age of other reference drusite massifs, indicate that nearly all o f the drusites, including the gabbroanorthosite, affiliate to a single Early Proterozoic drusite complex. The drusite entrusion lasted at le ast 100 m.y. during the initial evolution of the main Early Proterozoi c 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.