Magmatic response to slab tearing and resultant crustal evolution during scissor-like oblique continental collision: Insights from Triassic mafic and felsic intrusions in the Qinling orogen, China

摘要

Characterization of the magmatic response to scissor-like oblique continental collision and of the associated geodynamic mechanisms and crustal evolution is crucial to understanding the formation of collisional orogens. This paper presents geochronological, elemental, and isotopic data of three Triassic mafic-felsic intrusions formed during scissor-like continental collision in the Qinling orogenic belt (QOB), China. The biotite pyroxenite, gabbro, quartz monzonite, and syenite of the Jiuzigou (JZG) mafic-felsic intrusive complex were emplaced at 233-231 Ma and have extremely high Ba and Sr contents, fractionated rare-earth elements, insignificant Eu anomalies, and enriched Sr-Nd isotopes. A negative relationship between epsilon(Nd)(t) values and SiO2 content suggests that the JZG complex was formed by assimilation and fractional crystallization (AFC) processes of enriched-mantle -derived melts. The Pangjiahe (PJH) and Laowangou (LWG) granite porphyries were emplaced at 224 Ma and 215 Ma, respectively, and are characterized by enriched large-ion lithophile elements, depleted high-field-strength elements, and enriched Sr and Nd isotopic compositions (0.70565-0.70648 and - 3.22 to - 5.33 respectively), which indicate that these granite porphyries were sourced from mafic lower crust of the South Qinling terrane. The relatively high Mg numbers (45.8-49.4) might indicate mixing of crustal melt and minor mafic magma for the PJH granite porphyries. In contrast, the low Mg numbers (24.8-41.4) and obvious negative Eu anomalies of the LWG granite porphyries suggest an origin by fractional crystallization of crustal melts. The spatial-temporal distribution of the Triassic intrusions emplaced during the continental collision is proposed to have been produced by southeastward slab tearing in the QOB. Slab tearing may be a common geodynamic process during oblique collision generally. The origin and evolution of Triassic mafic and felsic intrusions in the QOB reveal that this oblique-collisional orogenic belt likely underwent a transition from net crustal growth to crustal reworking as collision progressed. (C) 2019 Published by Elsevier B.V.