AUTHOR=Gong Xiangkuan , Abuduxun Nijiati , Jia Xiaoliang , Cheng Yuanfeng , Cai Hongming , Wu Xueqian , Yang Haodong 

TITLE=Early Jurassic A-type granite and monzodiorite from the Baoji batholith: Implication for tectonic transition from post-collision to post-orogenic extension in the Qinling Orogenic Belt, China

JOURNAL=Frontiers in Earth Science

VOLUME=12

YEAR=2024

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2024.1428055

DOI=10.3389/feart.2024.1428055

ISSN=2296-6463

ABSTRACT=<p><bold>Introduction:</bold> The early Jurassic granitoids in the Qinling Orogenic Belt (QOB) play a crucial role in understanding the tectonic implications for the geological evolution of China. To elucidate the early Jurassic tectonic setting of QOB, we performed a comprehensive analysis of zircon U-Pb ages, whole-rock geochemistry, and <italic>in situ</italic> zircon Lu-Hf isotopes from early Jurassic monzodiorite and Kfeldspar granite within the Baoji batholith in western QOB.</p><p><bold>Geochronology Method and Results:</bold> The intrusions yielded zircon U-Pb ages of 186 ± 2 Ma and 188 ± 2 Ma, respectively.</p><p><bold>Geochemistry Results:</bold> The monzodiorites are characterized by relatively high MgO, Rb, Th, U, and LREE contents, as well as low P, Ti, and HREE contents. They also exhibit high Nb/Ta ratios (20.6–23.4). The zircon <italic>ε</italic><sub>Hf</sub>(<italic>t</italic>) values for the monzodiorite sample range from −4.36 to 6.47, indicating significant contributions from a fertile continental lithospheric mantle with the involvement of crustal components. The K-feldspar granites are enriched in K<sub>2</sub>O+ Na<sub>2</sub>O, Rb, Zr, Hf, and Nb, and lower Ba, Sr, Ti, and P. They exhibit high Nb/Ta and Ga/Al ratios but low Y/Nb and Yb/Ta ratios. Their geochemical characteristics reveal an A-type granite affinity with elevated zircon saturation temperatures (848°C–900 °C). Additionally, the K-feldspar granite exhibits REE and trace element patterns similar to those observed in the monzodiorite. However, a wide range of zircon <italic>ε</italic><sub>Hf</sub>(<italic>t</italic>) values (−4.72 to 3.98), differing from those of the monzodiorite, indicate that the parental magma of the K-feldspar granite experienced magma mixing between a monzodioritic magma and a crustal-derived felsic magma.</p><p><bold>Discussion:</bold> These findings suggest that both A-type K-feldspar granite and monzodiorite likely formed during post-orogenic processes. Additionally, the QOB commenced its postorogenic evolution as an extensional tectonic environment during the early Jurassic period.</p>