Late Carboniferous to early Permian subduction-related intrusive rocks from the Huolongmen region in the Xing’an Block, NE China: new insight into evolution of the Nenjiang–Heihe suture
The Hegenshan–Nenjiang–Heihe suture is a key record of the Palaeo-Asian Ocean and provides critical evidence regarding termination of the eastern Central Asian Orogenic Belt. Here we determine the age, petrogenesis, and tectonic setting of the late Palaeozoic intrusive rocks from the Huolongmen region, northeastern margin of the Xing’an Block, and propose a new Ordovician to Triassic evolution model for the Nenjiang–Heihe suture. The late Palaeozoic Huolongmen intrusive rocks consist of the 317 million year gabbro diorite, the 323–312 million year I-type monzogranite, and the 291 million year A2-type alkali feldspar granite, as indicated by the laser ablation inductively coupled plasma mass spectrometry zircon U–Pb ages and petrographic–geochemical signatures. Zircon Hf isotopic, geochemical, and regional geological data suggest that the 323–312 million year gabbro diorite and I-type monzogranite were both emplaced in a continental arc setting, with the former generated by partial melting of a depleted rutile-bearing lithospheric mantle wedge that was modified by slab-derived fluids, and the latter formed by partial melting of a juvenile lower crust that involved contamination of pre-existing arc materials. The 291 million year A2-type granite was emplaced in a subduction-related extension and generated from anataxis of intermediate to acid igneous rocks of the middle to upper crust at relatively low-pressure conditions. In our new model, the evolution of the Nenjiang–Heihe suture could be roughly divided into three stages, including (1) the episodic northwestward subduction of the Palaeo-Asian oceanic plate beneath the Xing’an Block changing gradually from a high angle to a low angle at 480–290 Ma; (2) the coexistence of the slab break-off and the northwestward subduction at 290–260 Ma; and (3) the soft collision between the Xing’an and Songliao blocks at 260–244 Ma.
