Post-collisional Magmatism And Geodynamic Process From The North Qilian Orogen To Southern Margin Of The Alxa Block,NW China

Orogen can involve a complete Wilson cycle from continental rift,ocean spreading to ocean/continent subduction and collision,which are accompanied with various magmatism in different stages.Post-collisional magmatism develops in the last phase of orogenic process in response to lithospheric extension and upwelling of asthenospheric mantle and it can record the composition and heat mechanism of lithosphere after orogenic collision,and also plays a significant role in crust-mantle interaction and crust growth.Qilian orogenic belt is a typical early Paleozoic orogenic belt in China and provide a nature laboratory for evaluating magmatic process in a multiphase orogen.However,little attention has been paid to post-collisional magmatism in the North Qilian orogen and it is not clear about their spatial and temporal distribution and deep dynamic process.Besides,there are also some Paleozoic magmatism in the joint area of the North Qilian and the Alxa block.Whether these magmatism is controlled by the North Qilian orogeny or has an affinity with the tectonic development of Alxa block is still not well understood.In our study,we present U-Pb dating,geochemical and Sr-Nd-Hf isotopic composition for post-collisional intrusive rocks of eight plutons from the North Qilian to the southern margin of the Alxa block.Combined with previous works,we have discussed petrogenesis of different intrusive rocks from the North Qilian to the southern margin of the Alxa block and established the spatial and temporal frame of these post-collisional magmatism.Our results are summarized as follows:1.The zircon U-Pb dating results show that the magma crystallization ages of eight plutons are 450-250 Ma from the North Qilian to the southern margin of the Alxa block.They are divided into five periods:?1?450-440 Ma Jiling and Xijing hornblende gabbro,Xijing high Ba-Sr granitoids and Lianhuashan and Mengjiadawan high Sr/Y granodiorites;?2?430-425 Ma low Sr/Y Mengjiadawan and Yangqiandashan I-type granitoids;?3?425-410 Ma Xinkaigou and Shengrongsi A-type and alkali-feldspar granites;?4?410-400 Ma Xijing high Ba-Sr granites and?5?late Paleozoic?320-250 Ma?Xiaokouzi gabbro and northern Xijing pyroxenite.2.Jiling and Xijing hornblende gabbros?450-440 Ma?have similar geochemical characteristics.They have similar enriched Sr-Nd isotopic composition and high Th/Yb and Th/La ratios.We note that Jiling and Xijing hornblende gabbros may be caused by partial melting of enriched mantle modified by addition of sediment.Xijing high Ba-Sr granitoids?450-440 Ma?range from monzodiorite to quartz monzonite.They have similar enriched Sr-Nd-Hf isotopic composition.Major and trace element calculations suggest that the⁴40 Ma Xijing high Ba-Sr granitoids experienced up to 60%fractionation at 4-6 kbar with a crystallizing assemblage dominated by hornblend,clinopyroxene and plagioclase.Trace element and Sr-Nd isotopic modelling suggest incorporation of⁵%slab-derived sediment into mantle magma source of these high Ba-Sr granitoids.Mengjiadawan and Yangqiandashan high Sr/Y granodiorites?450-440 Ma?have high Sr/Y ratios?52-91?,low Y and HREE contents,which implied abundant garnet or hornblend but minor or no plagioclase in their source.Melting experiments have shown that such high Sr/Y geochemical signatures can be achieved by thickened crust.Mengjiadawan quartz diorites and Yangqiandashan granodiorites?430-425 Ma?are both high high-K calc-alkaline and contain biotite and hornblende as major mafic minerals,belonging to I-type granitoids.Nd and Hf model ages?1.3-1.5 Ga?suggest that its magma was predominantly derived from Meso-proterozoic crustal materials.Xinkaigou and Shengrongsi granites?425-410 Ma?are A type or alkali-feldspar granites.They are possibly derived from partial melting of felsic crustal material under lower pressure condition.Xijing granites?410-400 Ma?have similar Sr-Nd-Hf isotopic composition with Late Ordovician Xijing high Ba-Sr rock series.Combined with major and trace element calculation,it is indicated that early Devonian Xijing high Ba-Sr granites could have been formed by about 80%fractional crystallization at lower pressures?1-2 kbar?with a crystallizing assemblage dominated by amphibole,plagioclase and K-feldspar.Xiaokouzi gabbros?317 Ma?have depleted Sr-Nd isotopic composition,indicating that they are from partial melting of mantle material.Xijing pyroxenite have low SiO₂,high MgO,high Cr contents and depleted Sr-Nd isotopic characteristic,suggested to be cumulates from depleted mantle.3.Our study shows that the type of post collisional magmatism are complex.Early Palaeozoic post-collisional magmatism from the North Qilian to the southern margin of the Alxa show roughly decreasing from northern and southern towards the interior of the North Qilian.And different rock types are related to their longitude:the plutons in the north and south display hornblende gabbro,high Ba-Sr granitoids and high Sr/Y granodiorites,and then low Sr/Y I-type granitoids appear inwards.In the middlemost longitude,the A-type and alkali feldspar granites occur in the SRS and XKG region.In the late Paleozoic,Xinkaigou gabbro and Xijing pyroxenite occur along the Longshoushan fault,which is more closed with the Alxa block.4.We note that the spatial and temporal variations of the post-collisional early Palaeozoic magma suites were caused by lithospheric delamination after the collision between the Central Qilian and the Alxa block.The delamination firstly occurred in the edge of the thickened lithosphere and partial melting of enriched mantle which was influenced by sediment addition in early subduction stage produced Xijing and Jiling hornblende gabbro,high Ba-Sr rock series and partial melting of thickened lower crust generate high Sr/Y granodirotes.With delamination propagated from margin part of the thickened lithosphere to inward beneath the North Qilian and southern margin of the Alxa block,some low Sr/Y I type granitoids,A type or alkali feldspar granites were produced.Xijing high Ba-Sr granites also occurred at low pressure in the late stage of lithosphere delamination.In the late Paleozoic stage only a few magmatism occurred along Longshoushan fault which might be more closely related with the Chaganchulu back-arc basin development.