The lithospheric architecture of two subterranes in the eastern Yidun Terrane, East Tethys: Insights from Hf–Nd isotopic mapping

Abstract

A long-standing controversy exists regarding the tectonic division, lithospheric architecture and evolution of the eastern Yidun Terrane in the Late Triassic. A compilation of geochronological, geochemical and isotopic data (91 whole-rock Nd and 413 laser points of zircon Hf) for volcanic and intrusive rocks across the entire eastern Yidun Terrane has allowed for a detailed investigation into its lithospheric architecture. Two subterranes were identified using Hf–Nd isotopic mapping. They include a high ƐHf(t) (>−3.0) and ƐNd(t) (>−3.5) domain constrained in the Southern Yidun Terrane (SYT), and a low ƐHf(t) (<−3.0) and ƐNd(t) (<−3.5) domain constrained in the Northern Yidun Terrane (NYT). The NYT and SYT are characterized by distinctive arc-related volcanic and plutonic rocks (NYT: 235–230Ma basalt, andesite, dacite and rhyolite, as well as a 225–215Ma granite batholith; SYT: 228–215Ma adakite-like andesite, as well as diorite to monzonite porphyry), detrital zircon populations (NYT: ~2.50–2.45Ga, ~980–880Ma and ~480–400Ma; SYT: ~2.50–2.40Ga, ~1.90–1.75Ga, ~1000–720Ma, ~480–400Ma and ~240–220Ma) and mineralization styles (NYT: volcanic massive sulfide Ag–Cu–Pb–Zn and epithermal Ag–Hg deposits hosted in the ~230Ma rhyolites; SYT: porphyry–skarn Cu–Mo–Fe deposits genetically related to the ~216Ma dioritic to monzonitic porphyries). This dataset collectively shows that the NYT magmas were likely derived from a Paleoproterozoic or older mafic to intermediate lower crust with a variably minor addition of Triassic juvenile mantle melts, whereas the magmas for the SYT magmatic rocks were dominated by the arc juvenile mantle wedge melts with subordinate input of Late Mesoproterozoic or older crustal materials. The different melting processes between the NYT and SYT were attributed to changing subduction dip in space and time, with earlier steeper subduction at ~235–230Ma, and later shallow-dip subduction from ~228–220Ma. During the steeper dip subduction phase, it is likely that the NYT experienced a slightly greater degree of extension than the SYT, raising the possibility of a slab segmented by a major transform fault resulting in slightly steeper subduction beneath the NYT.