Triassic ore-bearing and barren porphyries in the Zhongdian Arc of SW China: implications for the subduction of the Palaeo-Tethys Ocean

The NS-treading Zhongdian Arc located in the southern part of the Yidun Arc is an important region to address the evolution and reconstruction of the Palaeo-Tethys Ocean and related mineralization. In this study, we investigate three barren intrusions in the Zhongdian Arc and present geochemical compositions, zircon U–Pb dating and Hf isotopic compositions. Zircons from the three intrusions yielded U–Pb ages of ~227.5, ~222.5, and ~230 Ma, with highly variable εHf(t) values (‒20.5 to 4.3). These quartz monzonite porphyries show typical adakitic affinity, and it is inferred that these intrusions in the Zhongdian Arc, together with those in the northern Yidun Arc, were derived from the partial melting of mantle wedge and contaminated by minor lower crustal components during the westward subduction of the Ganzi-litang Ocean, which probably resulted from the Triassic continental collision between the south China and the north China blocks. In the Yidun Arc, the Triassic ore-bearing intrusions have εHf(t) values that cluster around zero, while the barren intrusions possess negative εHf(t) values, suggesting that the mantle lithospheric components played an important role in the Triassic ore-bearing porphyries.

位于义敦弧（Yidun Arc）南部的南北向延伸中甸弧（Zhongdian Arc），是探究古特提斯洋（Palaeo-Tethys Ocean）演化与改造及相关成矿作用的关键区域。本研究针对中甸弧内3个无矿侵入体开展研究，获取了其地球化学组成、锆石U-Pb定年结果及铪同位素组成数据。3个侵入体的锆石U-Pb年龄分别约为227.5 Ma、222.5 Ma与230 Ma，且εHf(t)值变化范围宽泛（-20.5至4.3）。这些石英二长斑岩具有典型的埃达克岩属性（adakitic affinity），研究推断：在甘孜-理塘洋（Ganzi-litang Ocean）向西俯冲过程中，中甸弧内的上述侵入体与义敦弧北部侵入体均起源于地幔楔（mantle wedge）的部分熔融，并受到少量下地壳组分的混染；该俯冲作用可能源于华南地块与华北地块的三叠纪大陆碰撞。在义敦弧中，三叠纪含矿侵入体的εHf(t)值集中于0附近，而无矿侵入体的εHf(t)值为负值，这表明岩石圈地幔组分在三叠纪含矿斑岩的形成过程中发挥了重要作用。