The Petrology And Petrogenetic Study Of Late Jurassic Volcanic Rocks In TaiPuSiQi, Inner Mongolia

Late Jurassic Stratum of Volcanic rocks layers occur as the sequence of Manketoue’bo, Manitu and Baiyingaolao. These volcanic rocks are enriched in alkali but are low in titanium. The shapes of chondrite-normalized rare earth element (REE) patterns slope to the right showing that light REE enrichment. The shapes of primitive mantle-normalized rare element patterns also slope to the right indicating incompatibility element enrichment. The sequence of Manketoue’bo mainly consists of the high-K calc-alkaline (HKCA) rhyolites. The Manitu sequence includes the trachytes, the K-rich rhyolites and the latites. The K-rich rhyolites are typical alkalescent rock series, with high potassium, low silicon content; and the trachytes and the latites exhibit shoshonitic characters. Baiyingaolao sequence has been divided into four members. They are from bottom to top the rhyolitic ignimbrites, the trachydacites, rhyolites and comendites, the latites, respectively. The rhyolitic ignimbrites, the rhyolites and the latites define a HKCA series; the trachydacites are shoshonitic rock series; and the comendites are alkaline series. The diversity of rock associations and series show the complexity of volcanic magmas and tectonic settings.The different rock types were the products of distinct magma source regions. The mantle accessory minerals, as chromite and spinel et al., and petrologic significance of the coexisting alkali feldspars in the K-rich rhyolites, combining with Harker diagrams and trace elements covariant diagrams, which provide evidence that trachydacites and the K-rich rhyolites were produced by mixing/mingling of rhyolitic magma and trachytic magma. And the latites were produced by mixing/mingling of basaltic magma and trachytic magma, which have supported by initial 87Sr/86Sr values (0.709064±0.00692) locating between the continental growth curve and mantle growth curve. The significances of phenocrystic sanidine and a slight or no Eu anomalies indicate that trachytic magma was partial melting of mantle -crust boundary layer. A considerable volume of rhyolitic rocks deposits documented that rhyolitic magma was contributed to middle-low crust. Magma mixing/mingling is one of primary magma evolutions, which not only induces the diversity of igneous rocks, but also has significant relationship with mineralization. The multi-metal mineralization is directly bound up with the mixing/mingling of rhyolitic magma and trachytic magma in TaiPuSiQi.The region of TaiPuSiQi is located at northern margin of Northern China block, where considerable calderas and voluminous ignimbrites occurred. What’s more, the features of petrological geochemistry - strong depletion of Ba, P and Ti in rhyolitic rocks and trace element tectonic discrimination diagrams, show that geodynamic settings were the post-collision of Northern China block and Siberian block.