Carboniferous Volcanic Rocks In Southwestern Central Asian Orogenic Belt: Petrogenesis, Temporal And Spatial Eevolution And Associated Geodynamic Processes

The Central Asian Orogenic Belt (CAOB) is one of the largest Phanerozoic accretionary collages worldwide. It has experienced a complex tectonic and geodynamic evolution and is generally recognized as the one of the most striking issues of earth sciences. As an important part of the CAOB, the Northeast Xinjiang has experienced such complex geodynamic evolution including the subduction of oceanic plates, continental collision, crustal thickness and post-orogenic extension. The Carboniferous is a key satge of ocean-continent transformation in this area. The Carboniferous strata with voluminous volcanic rocks are widely distributed in the Northeast Xinjiang area. These volcanic rocks provide important opportunity to study the timing of oceanic closure and the tectonic transition process. An understanding of the timing of oceanic closure and tectonic transition is critical to deciphering the geological history of the Central Asian Orogenic Belt.Data on major and trace elements and Sr-Nd isotopes of whole rocks, and in-situ U-Pb age of zircons are reported for Carboniferous volcanic rocks in the Northeast Xinjiang, in order to investigate their sources, petrogenesis and implications for the Carboniferous evolution of Northeast Xinjiang area. The main results are obtained as follows:(1) Early Carboniferous volcanic rocks from Songkaersu Foamation, mianly basalts, basaltic andesites, andesites, dacites and volcaniclastics, are found in Kalamaili orogenic belt of Northeast Xinjiang area. The zircon U-Pb age of the Songkaersu rhyolitic tuff and basaltic andesite yield a Early Carboniferous age (346 ± 3 Ma and 338 ± 5 Ma respectively). The Songkaersu volcanic rocks with high Al2O3, TiO2 and high Th/Yb, Ba/Nb ratio also show enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). They belong to medium-high K2O and even shoshonitic series. Concordant ages of ～410 Ma, ～432 Ma and ～500 Ma suggest that Early Paleozoic materials may have been involved in their petrogenesis. Combined with the unconformably relationship of terrestrial molasse deposits from the lower part of Songkaersu Foamation with the ophiolitic melanges, the relatively low initial 87Sr/86Sr (0.703440-0.704220) and positive εNd(t) (+4.91-+7.29) of the these rocks, suggest that the magmas were derived by partial melting (1%-5%) of lithospheric mantle which were modified by slab fluids under an initial post-collisional evolution stage after the closure of the Junggar Ocean. Late Carboniferous volcanic rocks from Batamayineishan Foamation, mianly (alkaline) basalts, trachybasalts, basaltic-trachyandesites, trachytes and rhyolitics, are also found in Kalamaili orogenic belt of Northeast Xinjiang area. They show bimodal features. The zircon U-Pb age of the Batamayineishan basaltic-trachyandesite and rhyolite porphyry which intruded in Batamayineishan strata yield a Late Carboniferous age (320 ± 4 Ma and 309 ± 3 Ma respectively). The Batamayineishan basaltic volcanic rocks with high Na2O and TiO2, low Th/Yb, Ba/Nb, Nb/Zr ratio also show slightly enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and slightly depletion in high-field-strength elements (e.g., Nb, Ta). They belong to alkaline and low K2O calc-alkaline series. The acidic volcanic rocks from Batamayineishan Foamation show A-type granite features and have different magma source with the basaltic rocks. The relatively low initial 87Sr/86Sr (0.703275-0.703987) and positive εNd(t) (+4.53-+6.87) of the these rocks, suggest that the magmas were derived by partial melting (1%-5%) of asthenosphere mantle which were not only modified by slab fluids but also slab melts under an maturity post-collisional evolution stage. The acidic rocks were generated from crustal anatexis cause by the underplating of the continental crust by mantle-derived melts.(2) Late Carboniferous volcanic rocks from Erdaogou Foamation, mianly basalts, trachybasalts, basaltic-trachyandesite, trachyte rhyolitic and volcaniclastics, are found in Bogeda-Haerlike orogenic belt of Northeast Xinjiang area. They also show bimodal features. The zircon U-Pb age of the Erdaogou basaltic-trachyandesite and trachybasalt and rhyolite yield a Late Carboniferous age (312 ± 4 Ma,312 ± 3 Ma and 308 ± 3 Ma respectively). The Erdaogou basaltic volcanic rocks with high NaaO, K2O, and TiO2, high Th/Yb, Ba/Nb ratio also show slightly enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and slightly depletion in high-field-strength elements (e.g., Nb, Ta). The acidic volcanic rocks from Erdaogou Foamation show same magma source with the basaltic rocks. The Early Carboniferous volcanic rocks in this area were derived from asthenosphere mantle which were slightly affected by subduction fluids under an initial back of back arc basin rift stage whereas the Erdaogou basaltic volcanic rocks were derived from mantle which were modified by slab fluids under an rifting stage. The relatively low initial 87Sr/86Sr (0.703275-0.703987) and positive εNd(t) (+4.53-+6.87) of the these rocks, suggest that the magmas were derived by partial melting (5%-25%) of asthenosphere mantle. The acid rocks were generated from the direct fractional crystallization of basaltic magmas, possibly with crustal contamination.(3) Early Carboniferous volcanic rocks from Yamansu Foamation, mianly basalts and basaltic andesite and volcaniclastics, are found in Jueluotage orogenic belt of Northeast Xinjiang area. They belong to calc-alkalic in composition. The zircon U-Pb age of the Yamansu basaltic andesite yield a Early Carboniferous age (336 ± 2Ma). The Yamansu volcanic rocks with high Al2O3, TiO2 and high Th/Yb, Ba/Yb, Ba/Nb ratio and low Nb/Zr ratio also show enrichment in light rare earth elements and large-ion lithophile elements (e.g., Rb, Ba and Pb), and depletion in high-field-strength elements (e.g., Nb, Ta and Ti). Late Carboniferous volcanic rocks can be dicided into two types:Ⅰ and Ⅱ. The Ⅰ type rock show tholeiite and the Ⅱ type rock show calc-alkaline. The zircon U-Pb age of Late Carboniferous volcanic rocks yield a Late Carboniferous age (321 ± 2 Ma and 319 ± 2 Ma). The Ⅰ type volcanic rocks with low Th/Yb, Ba/Yb, Ba/Nb ratio also show flat MORB-like REE and slightly enrichment in LILE. The Ⅱ type with moderate Th/Yb, Ba/Yb, Ba/Nb ratio and show slightly enrichment in LREE and LILE. The relatively low initial 87Sr/86Sr and positive eNd(t) of the these rocks, suggest that the Early Carboniferous magmas and Late Carboniferous magmas have similar characteristic with Okinawa Trough and Mariana Trough BABB. Therefore, the Early Carboniferous volcanic rocks come from incipient rifting in the early stage of back-arc basin extension and the Late Carboniferous volcanic rocks come from a mature back-arc basin spreading in the later stage of extension.(4) Based on the above three points, we propose a model which show a tectonic evolution in Northeast Xinjiang area during Late Devonian to Permian. There are two Oceans existed in Northeast Xinjiang area before Late Devonian. The Junggar Ocean closed during the end of the Devonian. Post-collisional compression and extension began during the Early Carboniferous and was accompanied by ca 340 Ma arc-like magmatism and ca 320 Ma bimodal magmatism, suggesting a tectonic transition. The slab subduction, slab breakoff and slab rollback provide a plausible explanation for the complex spatial and temporal geochemical evolution of magmatism in Junggar since the Devonian. The Tianshan Ocean closed during the end of the Carboniferous. The Jueluotage back-arc basin opened in two tectonic stages: incipient rifting in the early stage of back-arc basin extension during the Early Carboniferous and a mature back-arc basin spreading in the later stage of extension during the Late Carboniferous. It is suggested that the Tianshan Ocean plate subducted Northward. In the Bogda-Haerlike area, we have identified two episodes of Late Paleozoic bimodal suites and propose that the essentially bimodal character of this complex reflects the features of back ofback-arc rifting magmatism.(5) The closure of the Junggar Ocean and Tianshan Ocean suggest that The Soutnwest of Central Asian Orogenic Belt have been finished the Ocean subduction. As time goes by, the Northeast Xinjiang area began to relax.