晚中生代沂沭断裂带左旋韧性剪切与岩浆迁移规律

沂沭断裂带内部的岩浆侵入与断裂活动关系密切,开展沂沭断裂带内部岩浆年代学研究,对于沂沭断裂带构造背景研究具有积极的参考意义。选取沂沭断裂带内部的莒县浮来山正长岩体、边部的莒南官坊石英正长岩体及侵入该岩体的闪长玢岩脉,开展岩石学及年代学研究。首次发现晚中生代沂沭断裂带左旋韧性剪切,获得浮来山岩体的LA-ICP-MS锆石U-Pb年龄为(125.1±1.7) Ma,官坊岩体年龄为(125.6±2.0) Ma, 发生韧性变形的闪长玢岩脉年龄为(122.8±2.1) Ma。获得的125 Ma岩浆活动年龄与沂沭断裂带东侧的五莲变质核杂岩快速剥露(122~128 Ma)、诸城断陷盆地快速沉降的时间相吻合,该时期沂沭断裂带处于岩石圈减薄与破坏强烈的时期;闪长玢岩脉的122.8 Ma结晶年龄,与五莲变质核杂岩后构造岩体(石场、坊子岩体等)结晶年龄一致,该时期沂沭断裂带处于弱伸展状态,同时也说明胶东半岛岩石圈减薄具有时间上的一致性。晚中生代时期,沂沭断裂带两侧岩浆活动具有逆时针迁移的规律,这一规律可能与伊佐奈岐板块向欧亚板块俯冲的方向、角度、速度的变换有关。

Sinistral Ductile Shear and Magmatic Migration Pattern in the Late Mesozoic Yishu Fault Zone

Magma intrusion in the Yishu fault zone (YFZ) is closely related to fault activity, and the magmatic geochronology in the Yishu fault zone is of major significance for revealing the tectonic setting of the YFZ. Petrological and LA-ICP MS zircon U-Pb geochronological analyses were conducted on the Fulaishan syenite (in Juxian) in the Yishu fault zone, as well as the Guanfang quartz syenite (in Junan) and the intruding diorite porphyrite dykes. The sinistral ductile shearing of the Late Mesozoic YFZ was first recognized. Zircon U-Pb dating yielded (125.1±1.7) Ma (Fulaishan pluton), (125.6±2.0) Ma (Guanfang pluton), and (122.8±2.1) Ma (ductile-deformed diorite porphyrite dyke). The 125 Ma magmatic age obtained is consistent with the rapid exhumation (122-128 Ma) of the Wulian metamorphic core complex (MCC) and the rapid sedimentation in the Zhucheng fault-bounded basin on the eastern side of the YFZ. This indicates that the YFZ was in a period of intense lithospheric thinning and destruction. The 122.8 Ma age of the diorite porphyrite dike is consistent with the crystallization age of the post-Wulian MCC (e.g., Shichang and Fangzi plutons), indicating that the extension of the YFZ was significantly weakened during this period. It also shows the similar timing of lithospheric thinning across the Jiaodong Peninsula. During the Late Mesozoic, magmatism on both sides of the YFZ had a counterclockwise migration pattern, which may have related to the changing direction, angle, and/or velocity of the Izanagi plate subduction beneath the Eurasian plate.