Late Carboniferous to Early Permian ridge subduction identified in the southeastern Central Asian Orogenic Belt: Implications for the architecture and growth of continental crust in accretionary orogens

Abstract

The Central Asian Orogenic Belt (CAOB) was established in response to multiple subduction-accretion-collision processes. However, the origin and evolution of the accreted arc terranes remain unclear, as exemplified by the question how ancient and juvenile materials were accreted and constructed in the accretionary belts. This question is addressed here by presenting in-situ zircon UPb and Hf isotopic, whole-rock elemental and SrNd isotopic analyses of the four Late Paleozoic igneous rock units from Xilinhot in the southeastern CAOB. The new geochronological and geochemical data support the existence of two Late Carboniferous to Permian magmatic arc systems — Ulistai active continental margin and Baolidao arc belt. These arc systems are characterized by magmatic activity extending from 330 to 250 Ma, with two magmatic “flare-ups” (325–295 Ma vs. 295–270 Ma). The first magmatic “flare-up” (325–295 Ma) includes high-K calc-alkaline rhyolites, dacites and alkali-feldspar granites, which can be interpreted to have originated from re-melting of the juvenile lower crust beneath a continental arc induced by the Paleo-Asian oceanic ridge subduction. The second magmatic “flare-up” (295–270 Ma) reveals a seaward migration of magmatism due to slab rollback subsequent to ridge subduction. The Middle to Late Permian shoshonitic andesites witnessed the closure of the Hegenshan ocean during this period. In combination with previously reported Sr-Nd-Hf isotopic data, we suggest that the Baolidao arc belt and some other arc terranes (e.g., Bainaimiao arc) in the southeastern CAOB were built upon Precambrian microcontinents that have been largely fragmented and significantly reworked. In summary, ridge subduction accompanied by slab rollback played an important role in the rejuvenation of microcontinents and accretion of arc terranes in the southeastern CAOB.