Southward subduction of the Paleo-Asian Ocean : records from Paleozoic rocks of the northern margin of the North China Craton

Abstract

The Paleo–Asian Ocean, originated from the breakup of the Rodinia supercontinent, had undergone a long and complex process of expansion and closure. The oceanic lithosphere of the Paleo–Asian Ocean subducted beneath the surrounding continents, including Siberia and East–Europe to the north, and Tarim and North China to the south, leading to the accretion and collage of a series of micro–terrains, island arcs, and seamounts in the interior. The subduction and closure of the Paleo–Asian Ocean along the Tianshan-Beishan-Solonker Suture Zone led to the formation of the Central Asian Orogenic Belt, one of the largest accretionary orogenic belts in Earth’s history. However, controversy has long surrounded issues of when and where subduction and closure of the Paleo–Asian Ocean occurred. The Bainaimiao Arc Belt, situated at the northern margin of the North China Craton, is considered to have resulted from the southward subduction of the Paleo–Asian oceanic lithosphere in the Paleozoic, and thus can help to constrain the subduction process and final closure of the Paleo–Asian Ocean. An integrated sedimentary provenance analysis from different Paleozoic strata across the Bainaimiao Arc Belt was used to constrain the evolution of the belt in spatial and temporal simultaneously. Detrital zircon U–Pb–Hf isotopic analyses were utilized to trace the diagenetic time and to constrain the origin of the parental magma. Neo–proterozoic zircons were found in the Silurian sedimentary rocks, implying that the Bainaimiao Arc Belt may have represented an exotic block, which was rifted away from some other blocks between circa 590 and 520 Ma, as constrained by the oldest arc–related rocks and youngest Neo–proterozoic zircons detected in the Bainaimiao Arc Belt. The increasing positive εHf(t) values of Early Paleozoic detrital zircons indicate that the southward subduction of the Paleo–Asian Ocean started and predominately contributed to the formation of the Bainaimiao Arc Belt. Relatively lower εHf(t) values from Late Paleozoic detrital zircons are indicative of the reworking of an old continent, implying the southward subduction of the Paleo-Asian Ocean beneath the northern margin of the North China Craton. Differences in the “fingerprint” distribution of Paleozoic strata between the Bainaimiao Arc Belt and the Mongolian Accretion Terrane suggest that the material exchange was limited, and the Paleo–Asian Ocean remained open in the Paleozoic. Late Paleozoic and Mesozoic magmatic rocks are widespread along the northern margin of the North China Craton. Geochronologic and geochemical analyses from Late Paleozoic to Early Mesozoic intrusions were conducted to constrain on the timing of final closure of the Paleo–Asian Ocean. On the basis of these data, two stages of magmatism were recognized, one in the Late Carboniferous–Middle Permian (318–268 Ma) and another in the Late–Middle Triassic (250–240 Ma), with the former showing arc affinities and the latter reflecting an extensional setting. It is the same with the easternmost segment of the Central Asian Orogenic Bel, where the analogous two stages of magmatism developed but were later than those in the middle segment, implying the scissor–like suturing from west to east. In conclusion, an integrated sedimentary analysis suggests southward subduction of the Paleo–Asian Ocean in the Paleozoic, which remained opening till the Early Permian. Moreover, geochronologic and geochemical data for the Late Paleozoic to Early Mesozoic intrusions restrict the final closure of the Paleo–Asian Ocean to the Late Permian–Early Triassic.