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Mengmeng Li,Jingdang Liu,Tianyi Liang,Chen Li,Dejun Bao 한국지질과학협의회 2022 Geosciences Journal Vol.26 No.1
The Mesozoic granodiorite and their diorite enclaves in the Shulan area have been investigated to understand the petrographic characteristics and elemental geochemistry and to further interpret the petrogenesis and paleotectonic environment. The petrography, mineral component, zircon U-Pb age, and major and trace element contents of 9 granodiorite host rock samples and 9 diorite enclave samples were analyzed. Results show host rock and the enclave both present of medium-fine subhedral and porphyritic. The main rock-forming minerals of host rocks are plagioclase (57–60%), quartz (25–30%), and K-feldspar (6–13%), whereas the main rock-forming minerals of enclaves are plagioclase (65–83%), amphibole (3–20%), and biotite (5–25%). Zircon U-Pb dating results show that the weighted-mean 206Pb/238U ages of the granodiorite host rock and the diorite enclave are 168.1 ± 1.6 Ma and 181.0 ± 1.0 Ma, respectively. The SiO2 contents of the host rocks and enclaves are 61.95–64.86% and 51.12–59.32%, respectively. Their K2O/Na2O ratios are 0.51–1.24 and 0.41–1.02; aluminum saturation index A/CNK [Al2O3/(CaO + Na2O + K2O) molar ratio] are between 0.80–1.14 and 0.70–0.98 and A/NK [Al2O3/(Na2O + K2O) molar ratio] are 1.48–1.97 and 1.47–2.11, indicating a medium-high-K calc-alkaline series and a metaluminous series. The chondrite-normalized rare earth element (REE) patterns of both host rock and enclave samples show a right-dip model, but the Eu ratios of the host rock samples show weak negative anomalies, whereas the Eu ratios of the enclave samples show distinctive negative anomalies. The δEu [δEu = EuN/sqrt(SmN × GdN)] averages of both host rock and enclave samples are 0.75 and 0.63, respectively. The primary mantle normalized trace element patterns shows the enrichment of Rb, Th, U, La, Zr, Hf and Li and the depletion of Nb and Sm in the granodiorite host rocks, whereas enriched Rb, U, La, Pr, Nd, Zr, Hf, and Li and depleting Ba, Nb, Sr, Sm, and Eu are presented in the diorite enclaves. The REE, trace element patterns and the other geochemical indices indicate both granodiorite and diorite were derived from partial melting of pre-existent basaltic materials with a minor contribution of mantle materials. Host rocks and enclaves were formed in the same subduction-related paleotectonic environment, but in different stages of magma evolution. The subduction triggered the delamination of the lithosphere, causing upwelling of the asthenosphere and partial melting of the crust, forming the magmatic precursors of both kinds of rocks.
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