Slab break-off model for the Triassic syn-collisional granites in the Qinling orogenic belt, Central China: Zircon U-Pb age and Hf isotope constraints

Numerous Triassic granitoids in the Qinling orogenic belt related to the Late Triassic collision between the North China Craton (NCC) and the Yangtze Block (YB) are important for determining the crustal composition at depth and the geodynamic processes by which the orogen formed. Most of the Triassic plutons in the Qinling orogen were emplaced between 205 and 225 Ma. The granitoid rocks from the southern margin of the NCC, North Qinling, South Qinling, and the northern margin of the YB that were emplaced during this interval have two-stage Hf model ages of 0.60–2.52 Ga (average 2.19 Ga), 0.90–2.66 Ga (average 1.29 Ga), 0.41–3.04 Ga (average 1.48 Ga), and 1.00–1.84 Ga (average 1.34 Ga), respectively, and mean <i>ε</i>_Hf(<i>t</i>) values of −14.5, −0.32, −1.36, and −3.98, respectively. The Hf isotope compositions of the granitoids in different tectonic units differ significantly, mirroring the diverse history of crustal growth of the four units.The temporal and spatial distribution and Hf isotope compositions of the granitoids suggest that there was a unified geodynamic process that triggered the magmatism. Formation of the Triassic granitoid plutons at 225–205 Ma was a consequence of slab break-off or E–W-striking slab tearing, related to slab rollback in the west part of the Qinling orogen and oblique continental collision in the east. Upwelling of the asthenospheric mantle led to partial melting of the subcontinental lithospheric mantle and the lower crust, and mixing and/or mingling of the resulting magmas resulted in the formation of granitoids with diverse geological and geochemical characteristics.

秦岭造山带内发育大量与华北克拉通（North China Craton, NCC）和扬子地块（Yangtze Block, YB）晚三叠世碰撞事件相关的三叠纪花岗岩类岩石，其对于揭示深部地壳组成以及造山带形成的地球动力学过程具有重要科学意义。秦岭造山带内多数三叠纪深成岩体的侵位时代集中于205~225 Ma。该时段内侵位的、分别来自华北克拉通南缘、北秦岭、南秦岭以及扬子地块北缘的花岗岩类岩石，其二阶段Hf模式年龄分别为0.60~2.52 Ga（平均2.19 Ga）、0.90~2.66 Ga（平均1.29 Ga）、0.41~3.04 Ga（平均1.48 Ga）以及1.00~1.84 Ga（平均1.34 Ga），对应的平均ε_Hf(<i>t</i>)值分别为-14.5、-0.32、-1.36与-3.98。不同构造单元内花岗岩类岩石的Hf同位素组成存在显著差异，反映了四个构造单元各自独特的地壳增生历史。花岗岩类岩石的时空分布与Hf同位素组成特征表明，存在统一的地球动力学过程触发了此次岩浆作用。225~205 Ma期间三叠纪深成岩体的形成，是板片折断或东西向板片撕裂作用的结果，该作用与秦岭造山带西部的板片回滚以及东部的斜向陆陆碰撞事件相关。软流圈地幔上涌引发岩石圈下伏地幔与下地壳发生部分熔融，所形成的岩浆发生混合和/或混杂，最终造就了兼具多样地质与地球化学特征的花岗岩类岩石。