Geochemistry Of The Late Mesoproterozoic-Early Neoproterozoic Volcanic Rocks In The Middle Of South Qinling, And Its Tectonic Implications

A series of Late Mesoproterozoic-Early Neoproterozoic volcanic rocks outcropped in the middle of the South Qinling, which have the key significance for our understanding of the paleo-tectonic frame and the evolution process of the Qinling orogen in Precambrian. The Longtian volcanic rocks located in Chengkou county and Xiaomoling volcanic rocks in Zhashui county were choosen for geochronological and geochemical research, we use these data to trace the tectonic evolution of the South Qinling in Precambrain.The Longtian volcanic rocks mainly consisted of basalt-andesites and andesites. They are enriched in LREE and LILE , depleted in Nb,Ta,Ti, that similar to the typic volcanic arc rocks; Their La/Nb,Th/Yb,Hf/Ta,Zr/Y, Ta/Yb and low Zr/Nb ratios are similar to continent island arc volcanics, but clearly distinct from the ocean island volcanics , which suggest that the magma formed in an active continental border arc. The LA-ICP-MS U-Pb dating of zircon yield an age of 712±3Ma, which is regarded as the crystallization of the Chengkou volcanic rocks, and demonstrates that there had been a plate subduction events in South Qinling belong to northern margin of Yangtze plate in Late Neoproterozoic.The Xiaomoling complex outcropped in Mogouxia area of Zhashui county and Tangfanggou area of Zhen'an county, the basic rocks mainly consisted of meta-basalt and diabase. The meta-basalts include two types: one type has high TiO₂ content, enriched in LREE and LILE, and slightly depleted in Nb and Ta . The high La/Nb, Th/Yb and Hf/Ta ratios suggest that the magma were mantle-derived and had been contaminated by continental crust; Another type is characterized with low TiO₂ content, flat or slightly enriched in LREE, and the content of trace elements are mostly like the typic N-MORB, all above show that was formed in a mature continental rift. But the Nb/La(0.57～0.78),Hf/Ta(8.4～9.2),La/Ta(15.6～20.2),Th/Yb (0.11～0.16),Hf/Th (5.7～7.2) ratios show some features of island arc volcanic, these characteristics may be attributed to the contamination of the composition from pre-existed island arc reserved in the lithosphere mantle. These two types of rocks were co-genetic, but experience different path of AFC process, and recorded the tectonic evolution of the rift.The Neoproterozoic basement rocks in South Qinling, including Wudang group,Yaolinghe group and the basic rocks of Xiaomoling were all formed in continental rift tectonic setting, their island arc geochemical characteristics may be attribute to the continental material contamination from the pre-existed island arc reserved in the lithosphere mantle.Geochronology and geochemistry of Xixiang group and Bikou group that located in norwestern margin of Yangtze plate suggest that the Late Mesoproterozoic-Early Neoproterozoic basement volcanic rocks were not formed in a simple extension setting, but experienced a few stages from the continent break-up,ocean basin forming and subduction events after the union continental palte formed during the Jinning era, and last to the Late Neoproterozoic. It can be concluded that the South Qinling belong to northern margin of Yangtze plate was dominated by extensional setting dynamics background by spreading with local subducting setting.