Lithospheric Dripping Triggered by Slab Break-Off: A Possible Mechanism for Late Carboniferous Magmatism in the Eastern Central Asian Orogenic Belt

Abstract

In this paper, we present zircon U–Pb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for the Late Carboniferous intrusive rocks in the northern Great Xing’an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon U–Pb dating indicates that the Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300±2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb, suggesting they originated from the partial melting of depleted lithospheric mantle (whole rock εNd(t) = +1.88 to +2.00, zircon εHf(t) = +6.6 to +10.7), which was previously modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted Sr–Nd–Hf isotopic compositions (zircon εHf(t)=+12.5 to +18.9, whole rock εNd(t)=+8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate they were derived from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions with similar petrological and geochemical characteristics to I-type granites. These granites have positive εHf(t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing’an Range, suggest that the amalgamation between the Erguna–Xing’an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing’an Block can be attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off.

Keywords: Late Paleozoic, Central Asian Orogenic Belt, Erguna-Xing'an Block, post-collisional magmatism, lithospheric dripping