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


Hu, Fangyang
Liu, Shuwen
Ducea, Mihai N.
Zhang, Wanyi
Chapman, James B.; Fu, Jinghao and Wang; Maojiang

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 207–202 Ma medium to fine-grained granite, coarse to medium-grained granite of the same age, 214–207 Ma tonalite and granodiorite, and rare 218–213 Ma mafic xenoliths. The maficxenoliths are characterized by enriched large ion lithophile elements, with zircon eHf(t) values of - 68 to.41 and average zircon d18O of 61&, 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 -67 to.1.9 and zircon d18O values of 53& to 90&. We suggest that they were derived from partial melting of Neoproterozoic, low d18O basaltic rocks with aminor input ofmaficmagma. These melts underwent fractional crystallization and assimilated high d18O crustal materials during magma ascent and emplacement. The coarse to medium-grained granitic rocks have zircon eHf(t) values of -73 to.15, with low zircon d18O values (average 57&). The medium to fine-grained granitic rocks have zircon eHf(t) values of -147 to .11, with high zircon d18O values (average 84&). 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 tomedium-grained granites originated from partial melting of low d18O Neoproterozoic metabasaltic to metatonalitic rocks, and that the medium to fine-grained granites were derived from high d18O 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 d18O Neoproterozoic metabasaltic to tonalitic rocks, and 3) high d18O 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.

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Fig. 1. Simplified geological map showing the distribution of tectonic units, sedimentary sequences, and the early Mesozoic granitoid rocks in the Qinling Orogenic Belt (modified from Dong et al., 2011; Hu et al., 2017b). The blue dashed line (Baoji–Chengdu railway) separates the eastern and western Qinling Orogenic Belt (Zhang et al., 2001). The inset at the bottom left corner is a tectonicmap showing the North China Craton, South China Craton, Tarim Craton, and major orogenic belts in China (modified from Hu et al., 2017b). The study area is indicated by the dashed rectangle. Abbreviations for intrusions (from west to east): ZC,Zhongchuan; WQ, Wenquan; MSL, Mishuling; MB, Miba; HJZ, Hejiazhuang; BJ, Baoji; XY, Xinyuan; JJP, Jiangjiaping; ZJB, Zhangjiaba; GTS, Guangtoushan; HSD, Huoshaodian; LB, Liuba; GQP, Gaoqiaopu; XB, Xiba; HY, Huayang; TB, Taibai; XCH, Xichahe; LCP, Longcaoping; WL, Wulong; LC, Laocheng; YZB, Yanzhiba; DJK, Dongjiangkou; CHS, Cuihuashan; ZS, Zhashui; CP, Caoping; SHW, Shahewan; LNS, Laoniushan.

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Journal of Petrology, 2018, Vol. 59, No. 4, 735–770 doi: 10.1093/petrology/egy042