碰撞后岩浆活动对于了解造山带垮塌和去根过程及陆壳生长具有重要意义.总结了柴北缘超高压变质带中形成于400～360 Ma的碰撞后花岗岩-辉长岩侵入体和镁铁质岩脉的年代学和地球化学特征.其中,花岗岩侵入体具有典型的I-型花岗岩特征,形成于壳幔相互作用的岩浆混合.来自地幔的镁铁质岩脉可以划分为两组:(1)392～375 Ma中基性岩脉;(2)约360 Ma超基性岩脉.其地球化学特征表明,镁铁质岩脉的微量元素和同位素随形成时间的变新而逐渐亏损,地幔源区从岩石圈地幔变为软流圈地幔.这种源自地幔的镁铁质岩浆活动是碰撞后岩浆活动开始和造山带垮塌的关键指标.结合碰撞后岩浆作用的特征,提出了一个地球动力学模型来解释柴北缘约35百万年(Ma)的造山带垮塌去根过程,在395～375 Ma发生缓慢的岩石圈地幔侵蚀,360 Ma前岩石圈发生拆沉作用,岩石圈地幔垮塌,同时软流圈地幔上升.地幔岩浆的加入表明碰撞后阶段是大陆生长的重要时期.

Potassic to ultrapotassic and bimodal volcanic rocks related to the collisional evolution of a continental orogen can provide valuable insights into the properties of the mantle and crust-mantle interactions. Here we report two distinct suites of volcanic sequences, subalkaline and alkaline, from the Oulongbuluke block, north of the North Qaidam ultrahigh-pressure metamorphic belt. The subalkaline suite is characterized by bimodal compositions. Both mafic and felsic rocks show enrichment of LREEs and LILEs but depletion of HFSEs and EM-I type Sr-Nd isotopic compositions (ISr = 0.70725 to 0.70811 and epsilon(Nd)(t) =-11.9 to-8.7). The alkaline mafic rocks are potassic to ultrapotassic in composition and characterized by enrichment in LREEs, depletion in Nb-Ta and enriched Sr-Nd isotopic compositions (ISr = 0.70793 to 0.71394 and epsilon(Nd)(t) =-7.4 to-3.5), similar to the EM-II mantle source. The subalkaline/alkaline felsic rocks were most likely fractionated from parental mantle-derived mafic rocks, according to their geochemical characteristics. U-Pb zircon geochronology indicates the sub -alkaline suite formed at 382-378 Ma, while the alkaline suite formed over a long period from 441 Ma to 372 Ma. The subalkaline and alkaline mafic magmas are derived from two distinctly enriched mantle sources: subducted fluid metasomatized mantle and the potassium-rich melt metasomatized mantle, respectively. We suggest that the mantle beneath the continental collision belt was heterogeneous and metasomatized by oceanic subduction -zone fluids and potassic-rich melts during subduction of the Proto-Tethys Ocean at similar to 530-445 Ma. Melting of the metasomatized lithospheric mantle, triggered by the break-off of an oceanic slab during the continental collision at 445-430 Ma, and by delamination of the lithospheric mantle during orogenic collapse at 380-360 Ma, caused the formation of the potassic to ultrapotassic alkaline and bimodal subalkaline volcanic rocks.

Post-collisional magmatism contains important clues for understanding processes of orogenic belts, potentially including unrooting and collapse. Here we report new geochronological and geochemical data for a suite of postcollisional mafic dykes in the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt. Two groups of magmatic rocks can be distinguished: (1) Middle Devonian (392-375 Ma) basic-intermediate dykes; (2) Upper Devonian (-360 Ma) ultrabasic dykes. Whole-rock geochemistry and zircon Hf isotopes reveal that the intermediate-basic dykes are derived from a lithospheric mantle source, whereas the ultrabasic dykes are melting of asthenosphere mantle. We suggest that such mantle-derived mafic magmatism is the critical indicator for the start of post-collisional magmatism and orogen unrooting and collapse. We propose a geodynamic model explaining the activities of the lithospheric and asthenospheric mantle during the post-collisional stage, which reveals a - 35 million year unrooting process, from slow lithospheric mantle erosion between -395-375 Ma, to final collapse by lithosphere delamination and asthenosphere upwelling at -360 Ma. Addition of juvenile mantle materials to the crust by the post-collisional mafic magmatism suggests that the post-collisional stage is an important period for continental growth in Earth's history.

The Qilian Orogen, which lies between the Alashan Block in the north and the Qaidam Block in the south, records the entire tectonic evolution of Proto-Tethys Ocean from its spreading, subduction, closing, marginal accretion to continental collision. There are three different types of parallel ophiolite belts extending in the Qilian Orogen, from south to north, including: (1) the South Qilian ophiolitic complex belt with oceanic plateau-type, MOR-type and backarc-type ophiolites in the south; (2) the Tuoleshan MOR-type ophiolite belt in the middle; and (3) the Zoulangnanshan back-arc-basin (BAB)-type ophiolite belt in the north. The southern oceanic plateau-type ophiolite belt is represented by Lajishan-Yongjing ophiolite, which is a typical ophiolite fragment of oceanic plateau originated from the Cambrian mantle plume activity during ca. 525 similar to 500Ma. The middle ophiolite belt extends along Aoyougou, Yushigou, Binggou and Yongdeng, representing the ophiolite complex of the MOR-type oceanic crust with the ages of 550 similar to 495Ma. The northern ophiolite belt contains forarc-type and BAB-type ophiolites. The forarc-type ophiolite, represented by the Dachadaban ophiloite, formed in response to the processes from subduction initiation, forearc extension to back-arc basin development during 517 similar to 487Ma; the BAB-type ophiolite, represented by the Jiugequan and Laohushan ophiolites, are the SSZ-type ophiolite and the products of back arc basin extension during Ordovician (490 similar to 445Ma). The three ophiolite belts are the products formed in different tectonic settings during the history of Qilian Ocean from Neoproterozoic to Early Paleozoic, and thus is of great significance in understanding the tectonic evolution of Proto-Tethys Ocean in Qin (ling)-Qi (lian)-Kun (lun) Orogenic Belt. The timing and distribution of the three ophiolite belts and arc volcanics constrain a northward subduction polarity.