TAIWAN AS A WITNESS OF REPEATED MANTLE INPUTS - SR-ND-O ISOTOPIC GEOCHEMISTRY OF TAIWAN GRANITOIDS AND METAPELITES

To aid a better understanding of the crustal evolution of SE China, is otopic studies of mid-Cretaceous granitoids and associated late Paleoz oic metapelites of the basement complex of Taiwan have been undertaken . Initial epsilon(Nd)-values and Sr-87/Sr-86 ratios for the granitoids range from - 12 to + 1 and 0.7051 to 0.7108, respectively, suggesting a heterogeneous nature of their protoliths. Furthermore, these initia l Nd and Sr isotopic ratios define a covariant array lying between tho se of nearby metabasites and metapelites. This indicates that the sour ces of granitoids could well be two-component mixtures of a depleted c omponent of mantle origin (fresh to altered oceanic crust with epsilon (Nd) = + 11 and Sr-87/Sr-86 = 0.703-0.708, represented by the adjacent amphibolite)and an enriched continental component (epsilon(Nd) = - 12 and Sr-87/Sr-86 = 0.712, represented by the metapelites), with the la tter making up from 15% to 60%. Oxygen isotope data are also consisten t with this mixing model. Nd model ages with respect to the depleted m antle reservoir (TDM) for Taiwan granitoids range from 0.9 to 1.5 Ga. These model ages are slightly younger than those of the S-type granito ids of SE China and world-wide Phanerozoic sedimentary rocks, implying a significant contribution from juvenile material. Similar to the gra nitoids, Taiwan metapelites are also heterogeneous in Nd and Sr isotop ic compositions with each body possessing its own characteristic range s. Nd crustal residence ages (T-DM) for Taiwan metapelites range from 1.1 to 1.6 Ga which are younger (1.5-2.3 Ga) than those of the Paleozo ic sediments of South China. Thus, the oldest sediments in the Taiwan basement received not only recycled continental crustal material but a lso fresh mantle input. Therefore, Taiwan is a good witness of at leas t three episodes of fresh mantle inputs to the crust of the Eurasian c ontinent: the ongoing accretion of the intra-oceanic Luzon Are, the Me sozoic event recorded in granitoids and the Paleozoic event recorded i n metapelites.