东昆仑布尔汗布达早古生代岩浆弧的形成与演化初探

东昆仑布尔汗布达弧位于东昆中断裂南侧,不同于北昆仑晚古生代-中生代岩浆弧,是东昆仑早古生代岩浆岩广泛分布的地区,对研究原特提斯洋俯冲-增生过程具有重要意义.报道了布尔汗布达地区早古生代岩浆岩岩石学、地球化学、锆石U-Pb定年、Lu-Hf和Sr-Nd同位素资料,结合以往研究成果,推断布尔汗布达弧与增生造山作用相关的岩浆活...

Research on Formation and Evolution of Early Paleozoic Bulhanbuda Arc in East Kunlun Orogen

The Bulhanbuda arc is located in the south of the central fault of East Kunlun orogen. Different from the Late Paleozoic-Mesozoic magmatic arc in the North Kunlun, Early Paleozoic magmatic rocks are widely exposed in the Bulhanbuda area and of great significance to the researches of subduction-accretion process of the Proto-Tethyan Ocean. In this study, it reports the petrology, geochemistry, zircon U-Pb ages, Lu-Hf and Sr-Nd isotopic compositions of the Early Paleozoic magmatic rocks from the Bulhanbuda area. This and previous studies indicate that the subduction-related magmatic activities can be divided into three stages: Cambrian (515-482 Ma), Middle Ordovician (465-463 Ma), and Late Ordovician-Early Silurian (454-438 Ma). The Cambrian magmatic rocks contain Nb-enriched mafic-ultramafic rocks and metaluminous high-K calc-alkaline diorite. The Ordovician magmatic rocks are weakly peraluminous high-K calc-alkaline monzogranites. The Late Ordovician magmatic rocks are dominated by weakly peraluminous medium- and high-K calc-alkaline diorite and monzogranite and contain some metaluminous calc-alkaline quartz diorite and tholeiitic dolerite. These three stage intrusive rocks show similar trace element compositions with those of continental arc andesite. The Sr/Y ratios of the Cambrian diorite are low (22.6-30.0), the Sr/Y ratios of the Middle Ordovician monzogranite are medium, and these of Late Ordovician granodiorite and monzogranite are high. All of these intrusive rocks are enriched in radiogenic Sr and Nd isotopes. The Cambrian diorites display high initial 87Sr/86Sr ratios of 0.715 1-0.715 7 and low ε_Nd(t) values of -7.4 to -7.3. The Middle Ordovician monzogranite have initial 87Sr/86Sr ratios of 0.707 6-0.707 7 and ε_Nd(t) values of -2.5 to -2.4. The Late Ordovician granodiorite and monzogranite have low initial 87Sr/86Sr ratios (0.705 9-0.706 5) and negative ε_Nd(t) values (-3.3 to -1.7). However, the diorite shows relatively high initial 87Sr/86Sr ratios (0.706 9-0.708 5) and low ε_Nd(t) values (-6.0 to -5.6). The three stage magmatic rocks show wide range of the zircon Hf isotopes. Cambrian diorites have negative ε_Hf(t) values (-6.8 to -4.4). Middle Ordovician monzogranites display positive ε_Hf(t) values (+0.13 to +2.90). Late Ordovician granidorite and monzogranite show wide range of ε_Hf(t) values (-2.7 to +9.2). The diorites mainly yield negative ε_Hf(t) values (-8.6 to -2.1). In conclusion, the Bulhanbuda arc is an Andean-type continental margin arc formed during the southward subduction of the Proto-Tethyan Ocean. It has undergone three significant evolutionary stages. In the early stage (515-482 Ma), partial melting of the mantle wedge generated the basic magma, which induced remelting of the Paleoproterozoic lower crust to form intermediate-acid magma. In the middle period (465-463 Ma), the mantle derived basic magma underpenetrated and the thickened lower crust was partially melted to form small-scale acid magma. In the late period (454-438 Ma), the mantle derived basic magma underpenetrated and the thickened lower crust was partially melted to form large-scale intermediate-acid magmatic rocks with adakitic geochemical composition.