SOURCES OF PHANEROZOIC GRANITOIDS IN CENT RAL-ASIA - SM-ND ISOTOPE DATA

Model Sm-Nd ages clearly indicate the occurrence of Caledonian, Hercyn ian, and pre-Riphean isotope provinces in Central Asia. The provinces coincide with tectonic zones of the same age. In each of the provinces , whatever the actual age of its granitoids, the model age of granite sources remains essentially constant: T-DM = 700-950 Ma for the Caledo nian province, T-DM = 550-750 Ma for the Hercynian province, and T-DM = 850-2100 Ma for the pre-Riphean province. In terms of isotope compos ition, the granites can be classified into epsilon(+)- and epsilon(-) types. In a epsilon(Nd)-T plot, data points of the epsilon(+) granites generally plot within fields along the evolution trajectories for the host Caledonian and Hercynian crust. The epsilon(-)-granites show wid e variations in the isotope composition of coeval rocks. The model iso tope age of their sources is usually much younger than the geologic ag e of the host rocks. The epsilon(+)granites tend to be restricted to t he Caledonian and Hercynian isotope provinces, whereas the epsilon(-)- granites are more common in the pre-Riphean isotope province. The epsi lon(+)-granites were derived from the young Caledonian and Hercynian c rust, which contained basites (melting products of the depleted mantle ) and pelites (which corresponded to the average compositions of the e rosion areas around the sea basins). The pelites caused the older mode l age of the crust compared to its geologic age, which was determined using the ophiolites (530-570 Ma). The Caledonian and Hercynian crust was isotopically homogeneous. The source of the epsilon(-)-granites wa s the ancient continental crust in blocks of the pre-Riphean isotope p rovince hosted by the Caledonides and Hercynides. ?he isotopic heterog eneity of these granites was predetermined by the isotopic heterogenei ty of their sources. Geological interpretation of the isotopic data im plies that blocks of the consolidated pre-Riphean crust were overthrus ted, during the accretionary-collision formation of the foldbelts, ont o the younger crustal complexes of sea basins between these blocks. Th e resultant two-layer crust structure caused the isotopic heterogeneit y of the epsilon(-)-granite sources.