Shuguang Song

Tectonic transitions from compression to extension at an active continental margin provide clues for a better understanding of the geodynamics of plate subduction. Here, we present detailed investigations of Jurassic to Cretaceous dike swarms in the east margin of the North China Craton (NCC). It is demonstrated that the Jurassic dikes are distinctly different from the Cretaceous dikes, in terms of both occurrence and geochemistry. The Jurassic dike swarms, which include both mafic and felsic types, all strike in NW-dominated directions. Most Jurassic felsic dike swarms display adakitic geochemical characteristics, interpreted to be formed by melting of the ancient lower crust of the NCC. Jurassic mafic dike swarms originated from the metasomatic enriched lithospheric mantle. Thus, the Jurassic dike swarms are indicative of a compressional environment. However, the Cretaceous dike swarms strike in NE-dominated directions. The Cretaceous felsic dike swarms are characterized by geochemical features of A-type granites. The Cretaceous mafic dike swarms have originated from the asthenospheric mantle during back-arc extension. We propose that the Jurassic crustal thickening, lower crustal reworking, and the melting of ancient lithospheric mantle are the result of the Paleo-Pacific subduction and related compression. In contrast, crustal thinning indicated by the orientation of Cretaceous dike swarms and their high-temperature features, as well as the reworking of the upper and middle crust and the appearance of depleted mantle, are the result of extension caused by the rollback of the Paleo-Pacific Plate. Variations of composition and orientation of dike swarms effectively reflect the tectonic transition from compression in the Jurassic to extension in the Cretaceous along the east margin of the NCC.

The petrogenesis of late Ordovician–early Silurian adakitic plutons in the North Qilian suture zone (NQSZ) and their copper mineralization potential remain poorly understood. Here we present a detailed study of the Heishishan (HSS) granodiorite–granite pluton, spatially associated with Cu–Au mineralization in the eastern section of the NQSZ. Zircon U–Pb dating confirms that the granodiorite–granite were formed at ca. 438–435 Ma, in association with a continental collision. Geochemically, the granitoids resemble low-Mg adakitic rocks featured by elevated Sr/Y and (La/Yb)_N ratios with depleted MgO, Cr, and Ni concentrations, suggesting minimal mantle contribution. They are sodium rich with K₂O/Na₂O < 1, and have higher and more varied Sr/Y, but lower La/Yb than those from the continental lower crust. The ε_Hf(t) values of zircon grains are positive and vary in a wide range of +2.0–12.7, indicating a heterogeneous source rather than a single arc basaltic source. They show moderately radiogenic Sr and Nd isotope compositions with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.705101–0.706312 and ε_Nd(t) values of +0.5–1.0, most likely a mixed source of the oceanic basaltic crust plus ca. 15–20% overlying sediments. The magmatic oxygen fugacity was relatively low as indicated by zircon Ce(IV)/Ce(III) ratios of 32–156, which is unfavorable for a large copper mineralization.