Taiwan as a witness of repeated mantle inputs — SrNdO isotopic geochemistry of Taiwan granitoids and metapelites

Abstract

To aid a better understanding of the crustal evolution of SE China, isotopic studies of mid-Cretaceous granitoids and associated late Paleozoic metapelites of the basement complex of Taiwan have been undertaken. Initial ϵ Nd-values and 87 Sr 86 Sr 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 initial Nd and Sr isotopic ratios define a covariant array lying between those of nearby metabasites and metapelites. This indicates that the sources of granitoids could well be two-component mixtures of a depleted component of mantle origin (fresh to altered oceanic crust with ϵ Nd = + 11 and 87 Sr 86 Sr = 0.703−0.708 , represented bythe adjacent amphibolite) and an enriched continental component ( ϵ Nd = −12 and 87 Sr 86 Sr = 0.712 , respresented by the metapelites), with the latter making up from 15% to 60%. Oxygen isotope data are also consistent with this mixing model. Nd model ages with respect to the depleted mantle reservoir ( T DM) for Taiwan granitoids range from 0.9 to 1.5 Ga. These model ages are slighlty younger than those of the S-type granitoids of SE China and world-wide Phanerozoic sedimentary rocks, implying a significant contribution from juvenile material. Similar to the granitoids, Taiwan metapelites are also heterogeneous in Nd and Sr isotopic compositions with each body possessing its own characteristic ranges. 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 Paleozoic sediments of South China. Thus, the oldest sediments in the Taiwan basement received not only recycled continental crustal material but also fresh mantle input. Therefore, Taiwan is a good witness of at least three episodes of fresh mantle inputs to the crust of the Eurasian continent: the ongoing accretion of the intra-oceanic Luzon Arc, the Mesozoic event recorded in granitoids and the Paleozoic event recorded in metapelites.