Geochemistry and zircon U-Pb-Hf isotopes of Paleozoic intrusive rocks in the Damao area in Inner Mongolia, northern China: Implications for the tectonic evolution of the Bainaimiao arc

摘要

The tectonic evolution of the Central Asian Orogenic Belt (CAOB) is related to the subduction and closure of the Paleo-Asian Ocean (PAO). Considerable controversy still surrounds whether, when and how the PAO subducted underneath the northern margin of the North China Craton (NCC). This study carries out whole rock geochemical and zircon U-Pb-Hf isotopic investigations on Paleozoic intrusive rocks in both the Bainaimiao arc and the northern margin of the NCC. Zircon UPb dating of gabbroic diorite and diorite in the Bainaimiao arc and the granite and diorite at the northern margin of the NCC yielded crystallization ages of 458?±?2?Ma, 428?±?2?Ma, 326?±?2?Ma, and 258?±?2?Ma respectively. In the Bainaimiao arc, the middle Silurian diorite as the main body of the arc-related intrusive rocks there show geochemical characteristics of continental margin arc magma with negartive ?Hf(t) values and old Hf model ages. On the other hand, the middle Ordovician gabbroic diorite mainly shows typical calc-alkaline continental arc geochemical characteristics with positive ?Hf(t) values and relative young Hf model ages, indicative of the southward-propagating reworking and arc-accretion processes. Thus, we interpret these rocks as a consequence of southward subduction of the PAO beneath the Bainaimiao arc, here termed as Bainaimiao Arc terrane, BAT. The early Carboniferous granite and late Permian diorite at the northern margin of the NCC show geochemical characteristics similar to post-collisional adakites and typical continental arc rocks, respectively. Combined with previous work, the early Carboniferous post-collisional adakite shows limited distribution and corresponds to the subsequent effect of the amalgamation between the BAT and the NCC. Whereas most of the Carboniferous-Permian magmatic rocks (ca. 342–258?Ma) at the northern margin of the NCC are arc-related and resulted from the continued effect of the southward subduction of the PAO. Moreover, the southward subduction in the early Paleozoic was transformed from relative high angle subduction beneath the BAT to shallow subduction beneath the unity of the NCC and the BAT in the late Paleozoic, which lasted until the late Permian (ca. 258?Ma).