Controls of variable crustal thicknesses on Late Triassic mineralization in the Yidun Arc, Eastern Tibet

Abstract

Modern arc-related granitic magmas have variable Sr/Y ratios and metal contents, depending on the thickness of the arc crust on the global scale. However, whether this is true of ancient arcs on a regional scale is still unclear. The Yidun Arc is a Triassic continental arc in Eastern Tibet that has magmatic and mineralization characteristics varying between the northern and southern Yidun Arcs during the Triassic. Several models have been proposed to explain these differences, however, these models are still controversial. To this end, we investigated the petrogenesis of the Late Triassic Xiuwacu intrusion, which is located between the northern and southern Yidun Arcs. The zircon U-Pb dating and Sr-Nd-Hf-O isotopes indicate that the Late Triassic Xiuwacu intrusion was emplaced at 215.6 ± 2.2 Ma and was predominately derived from partial melting of an enriched metastasized mantle combined with amphibole dominated fractionation and a small amount of crustal assimilation. Comparing the Late Triassic Xiuwacu intrusion with other volcanic rocks and intrusions in the Yidun Arc indicates that the Late Triassic Xiuwacu intrusion has a source similar to that of igneous rocks in the southern Yidun Arc. In addition, volcanic rocks and porphyries in the southern Yidun Arc have high Dy/Yb (1.8–2.6, mostly > 2.0) and Sr/Y (24–120) ratios, lack or have slightly negative Eu anomalies, and exhibit calc-alkaline trends, indicating that garnet, amphibole, and magnetite crystalized from their magma source under high pressure. The volcanic rocks in the northern Yidun Arc have low Dy/Yb (1.3–2.2, mostly < 2) and Sr/Y (4–43, mostly < 20) ratios with high negative Eu anomalies and tholeiitic trends, indicating that amphibole and feldspar crystalized predominantly from the source under relatively low pressure. These geochemical features suggest that the crust under the northern Yidun Arc is thin, the crust under the southern Yidun Arc is thick, and the thickness of the crust in the Xiuwacu area is intermediate between them. Several large scale porphyry Cu deposits formed only in the southern Yidun Arc is probably due to the crust of the southern Yidun Arc is thick. The crystallization of amphibole and magnetite reduced the magmas and led to the accumulation of Cu sulfides at the base of the thick southern Yidun Arc at an early stage. Late re-melting of these accumulated Cu sulfides produced these Cu mineralization. The distinct difference in crustal thickness between the northern and southern Yidun arcs is most likely due to the differences in the dip angle of the subducting slab during the Triassic, with these variations controlling magmatism and mineralization.