Interaction Among Magmas from Various Sources and Crustal Melting Processes During Continental Collision: Insights from the Huayang Intrusive Complex of the South Qinling Belt, China

Abstract

The Qinling Orogenic Belt in central China, which resulted from continent-continent collision, is an excellent area for the study of collision-related magmatism. An integrated study including detailed field investigations, petrography, mineral and whole-rock geochemistry, zircon U-Pb-Hf-O isotopes, and geochemical modeling was carried out on the Huayang intrusive complex-a key magmatic intrusion in the South Qinling Belt-in order to understand the nature and melt source regions of magmatism associated with continental collisional orogenesis. The Huayang intrusive complex is composed of similar to 207-202 Ma medium to fine-grained granite, coarse to medium-grained granite of the same age, similar to 214-207 Ma tonalite and granodiorite, and rare similar to 218-213 Ma mafic xenoliths. The mafic xenoliths are characterized by enriched large ion lithophile elements, with zircon eHf(t) values of -6.8 to+4.1 and average zircon delta O-18 of 6.1 parts per thousand, which suggests that the xenoliths may represent melts derived from phlogopite-bearing lithospheric mantle. The tonalites and granodiorites exhibit high Sr/Y and La/Yb, but low Rb/Sr, with variable zircon eHf(t) values of - 6.7 to +1..9 and zircon delta O-18 values of 5.3 parts per thousand to 9.0 parts per thousand. We suggest that they were derived from partial melting of Neoproterozoic, low delta O-18 basaltic rocks with a minor input of mafic magma. These melts underwent fractional crystallization and assimilated high delta O-18 crustal materials during magma ascent and emplacement. The coarse to medium-grained granitic rocks have zircon epsilon Hf(t) values of -7.3 to +1.5, with low zircon delta O-18 values (average 5.7 parts per thousand). The medium to fine-grained granitic rocks have zircon eHf(t) values of -14.7 to +1.1, with high zircon delta O-18 values (average 8.4 parts per thousand). Both of these granitic rock types show similar whole-rock geochemistry, with metaluminous to strongly peraluminous compositions, and are characterized by intermediate to low Sr/Y values. We propose that the coarse to medium-grained granites originated from partial melting of low delta O-18 Neoproterozoic metabasaltic to metatonalitic rocks, and that the medium to fine-grained granites were derived from high delta O-18 Neoproterozoic metagreywackes. Both granitic magma types experienced plagioclase-dominated fractional crystallization during magma ascent and emplacement. The data suggest that three different source materials were involved in magmatism in the South Qinling Belt: 1) the lithospheric mantle; 2) low delta O-18 Neoproterozoic metabasaltic to tonalitic rocks, and 3) high delta O-18 Neoproterozoic metagreywackes. Slab break-off and/or dehydration of the subducted slab may have induced the melting of the sub-continental lithospheric mantle and caused subsequent crustal melting by heating the base of the crust. The results of this study suggest that magmatism in continental collisional orogens is not only generated by heating from radioactive element decay during crustal thickening.