EVIDENCE FOR EXTENSIVE PROTEROZOIC REMOBILIZATION OF THE ALDAN SHIELDAND IMPLICATIONS FOR PROTEROZOIC PLATE TECTONIC RECONSTRUCTIONS OF SIBERIA AND LAURENTIA

A geological traverse across the Aldan shield along the Aldan River sh ows that the area is underlain by two distinct rock associations. The Middle Aldan association consists of metasedimentary rocks, mainly qua rtzite, that have been intruded by a potassic biotite granite. Downstr eam, north of the Middle Aldan association, lies the Lower Aldan assoc iation, which consists mostly of charnockite with rafts of older granu lite gneiss that contains abundant metasedimentary layers. U-Pb dating of zircons from the Middle Aldan association granite (1900 Ma) and Lo wer Aldan association charnockite (maximum age 1918 Ma) and granitic g neiss (maximum age 2230 Ma) shows that the majority of the rocks expos ed along the Aldan River are Proterozoic in age. Although they have Pr oterozic crystallization ages, the granite, granite gneiss and charnoc kite yield Archean Nd model ages, suggesting that they formed by remob ilization - that is, partial or complete remelting - of earlier Archea n crust. In contrast, pelitic gneisses included within the charnockite s of the Lower Aldan association give Proterozoic Nd model ages, indic ating that a substantial amount of Proterozoic rock is incorporated wi thin the Lower Aldan association. The present results show that the ro cks along the Aldan River, which are part of the Aldan block, display a significantly different history from those in the Olekma block to th e west. The Olekma block contains ca 3.0 Ga greenstone belts Late Arch ean amphibolite-grade granitic gneisses; the Proterozoic remobilizatio n that typifies the Aldan terrane is absent. The Olekma block was thru st under the Aldan block during the ca 1.9 Ga orogeny contemporaneous with, or slightly after the 1.9 Ga charnockitic event in the Aldan. Th e 1.9 Ga magmatic and granulite event seen in the Aldan is similar in age and character to the Thelon magmatic zone of northern Canada. This correlation allows the development of a preferred construction for th e Precambrian Laurentia-Siberia connection in which the present day so uthern portion of the Siberia platform was connected to the northern m argin of Laurentia. (C) 1998 Elsevier Science B.V.