Nd-Sr Isotopic and Geochemical Evidence on the Protoliths of Exotic Blocks in the Juisui Area, Yuli Belt, Taiwan

Abstract

The Yuli belt of eastern Taiwan comprises a Late Cretaceous subduction complex of exotic blocks enclosed within blackschist. The latter yield TDM model ages of 1500 to 1700 Ma identical to Taiwan sediments, indicating a derivation from basement strata of the Eurasian continental margin. Of the three exotic blocks in the Juisui area, the Tamayen and Tsunkuanshan represent fragments of island-arc assemblages. Meta-igneous rocks in these blocks have compositions from basalt to andesite and have ϵNd(100 Ma) values ranging from +1.0 to +10.4, and in the Tamayen block are associated with a deep-sea sediment assemblage. The latter has the chemical and isotopic characteristics of oceanic red clays and tuffaceous material, and has been metamorphosed to glaucophane schist on accretion of the arc material to the Eurasian continental margin. In the Tsunkuanshan and Wuho blocks, the sediment assemblage is blackschist similar to that in the Yuli belt, suggesting that these blocks either originated close to the continental margin or underwent severe disruption on accretion. Meta-igenous rocks in the Wuho block are tholeiites of probable ocean-island affinity, and have ϵNd(100 Ma) values of + 5.5 to + 6.0. In contrast to the Sm-Nd system, Rb-Sr isotype systematics have been variably reset in all three blocks and the surrounding Yuli blackschists during crustal accretion and subsequent metamorphism. The volume of juvenile relative to continental-derived sediment in the Yuli belt is consistent with the notion that much young continental crust is recycled material. Of the juvenile material, approximately 75% is derived from island arcs, with the remaining percentage comprising ocean islands and other ocean-floor material.