Abstract

The initiation time and tectonic responses of paleo–Pacific plate subduction beneath the Eurasian continent remain controversial. In this paper we report on Early Jurassic (201–198 Ma) monzogranite-tonalite association from the southern Zhangguangcai Range, northeastern China. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating indicates that the monzogranite and tonalite have identical 206Pb/208U ages of 201 ± 2 (mean square of weighted deviates, MSWD = 1.2, 2σ) and 198 ± 3 Ma (MSWD = 3.2, 2σ), respectively. The monzogranite and tonalite display different geochemical features, suggesting that they were derived from two distinct source regions. The monzogranite displays high SiO2, K2O, and Rb contents, as well as depleted whole-rock Sr-Nd-Pb isotopic compositions, i.e., εNd(t) = +8.3 to +11.7, with single-stage Nd model ages of 0.30–0.05 Ga. Zircons from the monzogranite also have depleted Lu-Hf isotopic compositions, and these contradictory geochemical features suggest that the monzogranite may be derived from melting of mid-oceanic ridge basalt (MORB)–sediment mélanges in subduction zone. The Na-rich tonalite has lower SiO2 and higher TiO2 contents. In combination with their relatively evolved Sr-Nd-Pb isotopic compositions and zircon Lu-Hf isotopic compositions, it can be considered that the tonalite was derived from juvenile basaltic crust in an active continental margin. Considering other Triassic to Jurassic mafic intrusive rocks and I-type granites in the Zhangguangcai Range, we propose that the Early Jurassic monzogranite-tonalite association in the Shihe area was caused by the westward subduction of the paleo–Pacific plate beneath northeastern China.

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