Middle Permian high Sr/Y monzogranites in central Inner Mongolia: reworking of the juvenile lower crust of Bainaimiao arc belt during slab break-off of the Palaeo-Asian oceanic lithosphere

The high Sr/Y geochemical feature of granitoids can be attributed to various mechanisms, and elucidating genesis of high Sr/Y granitoids provides insights into the material recycling and magmatic processes at depth. In southeastern Central Asian Orogenic Belt (CAOB), many Middle Permian granitoids exhibit high Sr/Y ratios, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Palaeo-Asian ocean. Here we present new zircon U-Pb ages, Lu-Hf isotopes and whole-rock geochemical data for the Middle Permian high Sr/Y monzogranites from central Inner Mongolia, southeastern CAOB. LA-ICP-MS zircon U-Pb data shows that these high Sr/Y rocks were emplaced during 273–261 Ma. They are calc-alkaline, sodium-rich and metaluminous to weakly peraluminous, with enriched large-ion lithophile elements (Rb, Th, K and Pb) and depleted high field strength elements (Nb, Ta, P and Ti), suggesting a mafic lower crustal source rather than evolved potassic crustal materials. Their relatively low (Gd/Yb)N (1.1–2.0), (Dy/Yb)N (1.0–1.3), Nb/Ta (7.9–10.9) ratios and flat heavy rare earth element patterns are characteristics of derivation from a relatively shallow depth with amphibolite as dominant residue. They also have highly variable εHf(t) values (−8.2 to +10.0) and TDMC (1814 to 649 Ma), similar to those of the Early Palaeozoic high Sr/Y intrusions along the Bainaimiao arc belt. Combined with data from literatures, we suggest that the high Sr/Y monzogranites in this study were probably generated by reworking of the newly underplated juvenile high Sr/Y lower crust of the Bainaimiao arc belt. Moreover, taking into account the regional investigations, the sublinear distributed Middle Permian magmatic rocks in the southeastern CAOB were likely associated with the incipient slab break-off of the Palaeo-Asian oceanic lithosphere following initial collision between the North China craton and the South Mongolia terranes.

花岗质岩石的高Sr/Y地球化学特征可归因于多种成因机制，阐明高Sr/Y花岗质岩石的成因，可为深部物质循环与岩浆作用过程提供关键认识。在中亚造山带（Central Asian Orogenic Belt, CAOB）东南部，发育大量具有高Sr/Y比值的中二叠世花岗质岩石，但其成因仍不明确，这阻碍了对古亚洲洋最终闭合的岩浆响应的全面认知。本文针对中亚造山带东南部内蒙古中部地区的中二叠世高Sr/Y二长花岗岩，报道了新的锆石U-Pb年代学、Lu-Hf同位素以及全岩地球化学数据。激光剥蚀-电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb定年数据显示，这些高Sr/Y岩石形成于273~261 Ma。它们属于钙碱性系列、富钠，为准铝质-弱过铝质岩石，大离子亲石元素（large-ion lithophile elements, LILE）（Rb、Th、K、Pb）富集，而高场强元素（high field strength elements, HFSE）（Nb、Ta、P、Ti）亏损，指示其源区为镁铁质下地壳，而非演化的富钾地壳物质。这些岩石具有较低的标准化（Gd/Yb）N值（1.1~2.0）、（Dy/Yb）N值（1.0~1.3）以及Nb/Ta值（7.9~10.9），且重稀土元素配分模式平坦，这是源区相对较浅、以角闪岩为主要残留相的典型特征。它们的εHf(t)值变化范围极大（-8.2~+10.0），地壳模式年龄TDMC为1814~649 Ma，与白乃庙弧带沿线早古生代高Sr/Y侵入体的同位素特征相似。结合前人文献数据，本文认为本研究中的高Sr/Y二长花岗岩，可能是由白乃庙弧带新底侵的新生高Sr/Y下地壳经重熔改造形成的。此外，结合区域地质研究，中亚造山带东南部呈近线性分布的中二叠世岩浆岩，可能与华北克拉通与南蒙古地体初始碰撞后，古亚洲洋大洋岩石圈的初始板片断离作用相关。