北山造山带南部晚古生代花岗岩?闪长岩的成因与构造意义

北山造山带位于中亚造山带南部，是中亚造山带的重要组成部分.为了进一步深入认识北山造山带晚古生代的构造?岩浆演化过程，选择北山造山带南部石板墩?白墩子地区的晚古生代花岗岩?闪长岩开展了岩石学、锆石U-Pb定年、Hf同位素、微量元素及岩石地球化学研究.LA-ICP-MS锆石U-Pb年代学研究限定了石板墩花岗岩形成于~304~ 302 Ma，石板墩闪长岩形成于~291 Ma，白墩子石英闪长岩形成于~270 Ma.它们的锆石Hf同位素均呈现较亏损的特征（ε_Hf（t）=-2.0~+15.7），且由老到新，亏损程度依次增加.岩石学和地球化学特征暗示了亏损地幔来源岩浆在北山造山带晚古生代岩浆活动中的主导作用，亏损地幔来源岩浆与古老地壳部分熔融形成的岩浆以不同比例混合，形成了复杂的岩石组合.因此，晚石炭世?早二叠世花岗岩?闪长岩可能形成于后撤式增生造山作用导致的弧后伸展构造环境. 

Petrogenesis and Tectonic Implications of Late Paleozoic Granite-Diorite from the Southern Beishan Orogen

The Beishan orogen occupies the southernmost part of the Central Asian Orogenic Belt (CAOB) and it is an important part of the CAOB. In this paper, we presents petrographical, zircon U-Pb, Hf isotopic, and trace element and whole rock geochemical studies on a late Paleozoic granite-diorite association from the Shibandun-Baidunzi area of the southern Beishan, in order to better constrain the late Paleozoic tectono-magmatic evolution of the Beishan orogen. LA-ICP-MS zircon U-Pb dating yields crystallization ages of ~304-302 Ma for the Shibandun granite, ~291 Ma for the Shibandun diorite, and ~270 Ma for the Baidunzi diorite. These rocks are characterized by variable radiogenic zircon Hf isotopic compositions with ε_Hf(t) values of -2.0-+15.7. Furthermore, they show a more depleted trend with decreasing ages. The petrographical and geochemical features indicate that the Late Paleozoic magmatisms in the southern Beishan was dominated by depleted mantle-derived magmas, and the complex Late Paleozoic rock assemblages of southern Beishan were likely produced by mixing of different proportions of juvenile mantle-derived magma with the ancient crust-derived magma. We further suggest that the Late Carboniferous-Early Permian granite-diorite were formed in a back-arc extension setting related to the retreating accretionary orogeny.