U-PB AND SR ISOTOPIC STUDIES ON GRANITOIDS FROM TAIWAN AND CHINMEN-LIEYU AND TECTONIC IMPLICATIONS

U-Pb zircon/monazite dating and Sr-isotope results for granitoids from Taiwan and Chinmen-Lieyii, SE China place important constraints for t he tectonic evolution and the timing of felsic magmatism associated wi th mid-Cretaceous subduction process. The Chinmen gneiss (139.4 Ma) an d granite (100.9 Ma) represent the oldest granitic units and belong to the previously recognized late Yanshanian phase of magmatism. A young er suite of granitoids occurs in the Tananao Metamorphic Complex of Ta iwan and includes the Yuantoushan and Tachoshui granites (87.3 Ma) and the Kanagan gneiss (90.2 Ma). These mid-Cretaceous ages for the young er granitoid suite mitigate against the existence of the hypothetical late Paleozoic ''Dongnanya'' microcontinent in the study area as propo sed by other investigators. U-Pb zircon inheritance ages, ranging betw een 1.67 and 2.09 Ga indicate the presence of a Precambrian basement b eneath much of coastal SE China and Taiwan, however, there is no evide nce of Archean crust as indicated by zircon inheritance patterns for r ocks west of the present study area. On a Sr-87/Sr-86 vs. Rb-87/Sr-86 plot, the available data for the granitoids in Chinmen and coastal Fuj ian, SE China, however, define pseudoisochrons with apparent ages of 1 58-167 Ma. Although the pseudoisochron could be interpreted as a resul t of a two-end-member mixing process with subsequent fractional crysta llization, the apparent age of the pseudoisochron, substantiated by th e circumstantial evidence, is postulated as the approximate time of Sr -isotope homogenization of the crustal protoliths of the granitoids du ring the previous (early Yanshanian) tectonic event in this region. Su ch a Sr-isotope homogenization process must have occurred over a large area, possibly facilitated by a metamorphic fluid phase or by large-s cale anatexis. The Cretaceous granitoids in this area were probably pr oduced by partial melting of this crust in a more restricted scale. Th e Cretaceous calc-alkaline granitoids in coastal Fujian and Taiwan are of I-type or contaminated I-type. A subduction component is inferred during the magma genesis. Although the Taiwan granitoids are suggested to be subduction related, the possible tectonic environment for the g enesis of coastal Fujian granitoids is yet to be solved.