Lithospheric dripping in a soft collision zone:Insights from late Paleozoic magmatism suites of the eastern Central Asian Orogenic Belt

The closure of Paleo-Asian Ocean is considered to have occurred along the Solonker Suture in the southernmost segment of the Central Asian Orogenic Belt(CAOB),the largest Phanerozoic accretionary orogen on the globe.The suture branches to the east to form the northern Hegenshan-Heihe Suture and the southern Solonker-Changchun Suture.The Hegenshan-Heihe Suture is an ideal natural laboratory for studying the post-collisional geodynamic processes operating in a soft collision zone driven by divergent double-sided subduction.Here we report results from an integrated study of the petrology,geochronology,geochemistry,and Sr-Nd-Hf isotopic compositions of the Early Carboniferous-Early Permian mag-matic suite in the Hailar Basin of the Xing'an-Erguna Block.The Early Carboniferous igneous rocks are represented by 356-349 Ma andesitic tuffs,exhibiting typical subduction-related features,such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements.These features,together with the relatively depleted Sr-Nd-Hf isotopic compositions,constant Nb/Y values,but highly variable Rb/Y and Ba values indicate that these rocks were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids.The Late Carboniferous-Early Permian magmatic suite(317-295 Ma)is characterized by high Sr contents(313-1080 ppm)and low Y contents(5-13 ppm),and these can be subdivided into calc-alkaline adakitic rocks and high-K calc-alkaline adakitic rocks.The calc-alkaline adakitic rocks have higher values of Sr/Y,(Sm/Yb)_(source normalized),and Mg#,and lower values of Y,_(Ybsource normalized),and K_(2)O/Na_(2)O than the high-K calc-alkaline adakitic rocks,which suggests that the former was generated by partial melting of foundered lower continental crust and the latter by partial melting of normal lower continental crust.Based on our new data,in conjunction with those in previous studies,we conclude that the tectonic evolution of the Hegenshan-Heihe Suture involved Early Carboniferous double-sided subduction of the Nenjiang Ocean,latest Early Carboniferous soft collision between the Xing'an-Erguna and Songliao blocks,and Late Carboniferous-Early Permian post-collisional extension.We also propose a new geodynamic scenario in which removal of the lithospheric root might have occurred in a soft collision zone during the post-collision period via repeated and localized lithospheric dripping,which results from combined effects of hydration weakening of the lithosphere caused by pre-collision subduction and asthenospheric stirring triggered by slab break-off.

电子碰撞激发产生软X光激光的理论研究

用一维非平衡辐射流体力学程序JB19对激光照射Au、Se、Ge、Cu等靶进行了模拟计算,并对形成的等离子体状态做了详细描述。计算表明只要选取适当的激光介质(包括材料、形状和结构)和相匹配的激光条件就可以得到产生粒子数反转和激光增益的最佳等离子体状态。计算结果与LLNL(劳伦斯利弗莫尔实验室),NRL(海军实验室),CEL(里梅尔实验室)实验观察值符合得很好。

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt(CAOB)in NE China is a key area for investigating continental growth.However,the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood.NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts.The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities,respectively.In contrast,the Xing’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes.These blocks and terranes were separated by the Xinlin-Xiguitu,Heilongjiang,Nenjiang,and Solonker oceans from north to south,and these oceans closed during the Cambrian(ca.500 Ma),Late Silurian(ca.420 Ma),early Late Carboniferous(ca.320 Ma),and Late Permian to Middle Triassic(260-240 Ma),respectively,forming the Xinlin-Xiguitu,Mudanjiang-Yilan,Hegenshan-Heihe,Solonker-Linxi,and Changchun-Yanji suture zones.Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean(PAO),namely,the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans.The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west.The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south.The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner.A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO,which led to“soft collision”of tectonic units on each side,forming huge accretionary orogenic belts in central Asia.

新疆博格达地区早二叠世软沉积物变形构造:弧后碰撞前陆盆地地震记录

早二叠世下岌岌槽子群发育于博格达弧后碰撞前陆盆地缓坡带,为一套长石含量较高的陆源碎屑岩。古流向、沉积相带分布研究表明,当时的古坡向向南。岌岌槽子群中首次发现大量软沉积物变形构造,包括链环层理、水塑性褶皱、液化沙脉、沙侵蘑菇、碟状泄水构造、球枕构造、流化砾岩、底辟流化砾岩坨、滑混层、同沉积断层等,均是与地震驱动相适配的变形构造。这些变形构造常沿限定的岩层发育,上、下均为未变形的岩层,显示事件变形特征。大量滑混层褶皱枢纽和球枕构造轴面实测结果显示,它们既没有优势方位,其位态也与古坡向无关,进一步证实最有可能的驱动机制是地震。由于古地震活动直接与东天山造山活动密切相关,因此,研究软沉积物变形构造的时、空发育特征能够成为认识天山造山过程和相关盆地发育的一个新视角。

Permian-Triassic magmatic evolution of granitoids from the southeastern Central Asian Orogenic Belt:Implications for accretion leading to collision

Many orogenic belts in the world exhibit accretionary and collisional orogenic phases to varying extents.How accretion evolves into collision of the Central Asian Orogenic Belt(CAOB),the largest Phanerozoic accretionary orogenic belt,is an intriguing question.In this paper,we present new U-Pb age,geochemical and isotopic data for Permian-Triassic granitoids from middle Inner Mongolia,Northern China in the southeastern CAOB,and delineate the magmatic transition from subduction to(soft)collision.The magmatic record of soft collision is identified and characterized by thickened lower crust-derived high Sr/Y granitoids with a sub-linear distribution along the Solonker suture zone.Granitoids from Early Permian to Late Permian became more enriched in whole-rock Nd and zircon Hf isotopic compositions(εNd(t)values from 2.4 to-19.5,εHf(t)values from 11.6 to-33.7),indicating increasing incorporation of old crust.The change in peak timing of magmatism from west(ca.264 Ma)to east(ca.251 Ma)along the Solonker suture zone implies"scissor-like"closure of the Paleo-Asian Ocean.Integrated with previous studies,a three-stage tectonic model from the Permian to Triassic by accretion leading to collision on the south-eastern margin of CAOB is proposed.(1)Early Permian(>ca.285 Ma):Juvenile magmatism on an active continental margin with double-sided subduction of the Paleo-Asian Ocean;(2)Middle Permian to Middle Triassic(ca.285–235 Ma):Magma source transition from juvenile to old crust induced by a tectonic switch from arc to"scissor-like"closure and subsequent intracontinental orogenic contraction;(3)Late Triassic(<ca.235 Ma):A-type and alkaline magmatism in response to post-collisional extension.