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Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from ⁴⁰Ar/³⁹Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton

¹ State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
² Department of Geosciences, National Taiwan University, Taipei, Taiwan
³ Department of Applied Geology, Curtin University of Technology, Perth, Australia
⁴ Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula, eastern North China craton, and consists of the Jinzhou detachment fault, Proterozoic–Paleozoic sedimentary rocks in its upper plate, and exhumed high-grade Archean metamorphic rocks and Early Cretaceous granitic plutons in the lower plate. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. The footwall mylonite zone is 2.5–3.5 km thick and includes Early Cretaceous (128–118 Ma) granitic rocks, together with older metamorphic rocks. The ⁴⁰Ar/³⁹Ar ages of muscovite, hornblende, biotite, and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120 and 107 Ma, from the time of initial crystallization of zircons (122–118 Ma) at 700–800 °C in syntectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at 500 to 200 °C. Throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity, reflects regional extensional tectonics in the Early Cretaceous. This accompanied lithospheric thinning, possibly resulting from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous.

Key Words: argon thermochronology • metamorphic core complex • crustal extension • Early Cretaceous • North China Craton • Liaodong Peninsula • Liaonan metamorphic core complex

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