古太平洋板块中生代俯冲活动与南海中生界

中国大陆东部边缘发育的中生代混杂岩增生带、中侏罗世末期以来形成的NE—NNE向构造格局以及大规模岩浆活动形成的NNE向火山岩带等地质构造特征揭示,中生代发生了古太平洋板块向中国大陆东部边缘NNW向的俯冲活动。位于南海东部的厚层残余中生界揭示了与此同时南海广泛的沉积作用。此次研究结果证实,南海东部的中生界并非是中特提斯海的东向延伸,而是西太平洋构造域的组成部分。

秦岭造山带斜向俯冲—碰撞与南秦岭金—铅锌矿床成矿作用的响应

南秦岭金-多金属矿床聚集区是秦岭成矿带乃至我国中央造山成矿带的重要组成部分,该卡林型-类卡林型金矿床、热水沉积改造铅锌矿床和沉积改造汞锑矿床及聚集区的形成与成矿演化,成矿地质背景,容矿与含矿建造,矿床类型、规模及时空分布规律,均与现代板块构造活动机制有密切的耦合关系。具体分析了造山带俯冲碰撞活动过程中,成矿构造地质背景、成矿盆地环境、沉积建造、成矿作用与矿床分布规律对造山作用的积极响应。

Material transportation and fluid-melt activity in the subduction channel: Numerical modeling

The subduction channel is defined as a planar to wedge-like area of variable size,internal structure and composition,which forms between the upper and lower plates during slab subduction into the mantle.The materials in the channel may experience complex pressure,temperature,stress and strain evolution,as well as strong fluid and melt activity.A certain amount of these materials may subduct to and later exhume from>100 km depth,forming high to ultra-high pressure rocks on the surface as widely discovered in nature.Rock deformation in the channel is strongly assisted by metamorphic fluids activities,which change composition and mechanical properties of rocks and thus affect their subduction and exhumation histories.In this study,we investigate the detailed structure and dynamics of both oceanic and continental subduction channels,by conducting highresolution petrological-thermomechanical numerical simulations taking into account fluid and melt activities.The numerical results demonstrate that subduction channels are composed of a tectonic rock melange formed by crustal rocks detached from the subducting slab and the hydrated mantle rocks scratched from the overriding plate.These rocks may either extrude sub-vertically upward through the mantle wedge to the crust of the upper plate,or exhume along the subduction channel to the surface near the suture zone.Based on our numerical results,we first analyze similarities and differences between oceanic and continental subduction channels.We further compare numerical models with and without fluid and melt activity and demonstrate that this activity results in strong weakening and deformation of overriding lithosphere.Finally,we show that fast convergence of orogens subjected to fluid and melt activity leads to strong deformation of the overriding lithosphere and the topography builds up mainly on the overriding plate.In contrast,slow convergence of such orogens leads to very limited deformation of the overriding lithosphere and the mountain building mainly occurs on the subducting plate.

Deep carbon recycling and isotope tracing:Review and prospect

Deep carbon recycling is an essential part of the global carbon cycle.The carbonates at the bottom of the ocean are brought to the mantle by subduction.Subsequently, deep carbon is released to the atmosphere in the form of CO2 through volcanism.At present, research on deep carbon recycling is still at its early stage.The proportion of subduction-related carbon and primary mantle-derived carbon in CO2 released by volcano is an important issue.Carbon isotopes can easily distinguish organic carbon from inorganic carbon.However, ～95% of subduction-related and primary mantle-derived carbon released by volcano is inorganic, which carbon isotopes find difficult to distinguish.Recently, Ca and Mg isotope geochemistry has provided important tools for tracing crust-derived material recycling.Here we focus on this topic by introducing the principles of C, Ca, and Mg isotopes in tracing deep carbon recycling and previous research results.We also summarize the research progress on the total storage and phases of deep carbon, CO2 fluxes which depend on the release via volcanism, the partial melting of the carbon-bearing mantle, and carbon behaviour during oceanic subduction.

The transport of water in subduction zones

The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low(?5?C/km), lawsonite may carry water into great depths of ?300 km. In the hot subduction zone where the geothermal gradient is high(>25?C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ?60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.

Timing of the initial collision between the Indian and Asian continents

There exist three mainstream opinions regarding the timing of the initial collision between the Indian and Eurasian continents,namely,65±5,45±5,and 30±5 Ma.Five criteria are proposed for determining which tectonic event was related to the initial collision between India and Asia:the rapid decrease in the rate of plate motion,the cessation of magmatic activity originating from the subduction of oceanic crust,the end of sedimentation of oceanic facies,the occurrence of intracontinental deformation,and the exchange of sediments sourced from two continents.These criteria are used to constrain the nature of these tectonic events.It is proposed that the 65±5 Ma tectonic event is consistent with some of the criteria,but the upshot of this model is that the magmatic activity originating from the Tethyan subduction since the Mesozoic restarted along the southern margin of the Asian continent in this time after a brief calm,implying that the subduction of the Neotethys slab was still taking place.The magmatic activity that occurred along the southern margin of the Asian continent had a 7-Myr break during 72-65 Ma,which in this study is interpreted as having resulted from tectonic transformation from subduction to transform faulting,indicating that the convergence between the Indian and Asian continents was once dominated by strike-slip motion.The 30±5 Ma tectonic event resulted in the uplift of the Tibetan Plateau,which was related to the late stage of the convergence between these two continents,namely,a hard collision.The 45±5 Ma tectonic event is in accordance with most of the criteria,corresponding to the initial collision between these two continents.

Magmatism during continental collision, subduction, exhumation and mountain collapse in collisional orogenic belts and continental net growth: A perspective

Continental orogens on Earth can be classified into accretionary orogen and collisional orogen.Magmatism in orogens occurs in every periods of an orogenic cycle,from oceanic subduction,continental collision to orogenic collapse.Continental collision requires the existence of prior oceanic subduction zone.It is generally assumed that the prerequisite of continental deep subduction is oceanic subduction and its drag force to the connecting passive-margin continental lithosphere during continental collision.Continental subduction and collision lead to the thickening and uplift of crust,but the formation time of the related magmatism in orogens depends on the heating mechanism of lithosphere.The accretionary orogens,on the other hand,have no strong continental collision,deep subduction,no large scale of crustal thrusting,thickening and uplift,and no UHP eclogite-facies metamorphic rocks related to continental deep subduction.Even though arc crust could be significantly thickened during oceanic subduction,it is still doubtful that syn-or post-collisional magmatism would be generated.In collisional orogens,due to continental deep subduction and significant crustal thickening,the UHP metamorphosed oceanic and continental crusts will experience decompression melting during exhumation,generating syn-collisional magmatism.During the orogen unrooting and collapse,post-collisional magmatism develops in response to lithosphere extension and upwelling of asthenospheric mantle,marking the end of an orogenic cycle.Therefore,magmatism in orogens can occur during the continental deep subduction,exhumation and uplift after detachment of subducted oceanic crust from continental crust,and extensional collapse.The time span from continental collision to collapse and erosion of orogens(the end of orogenic cycle)is 50–85 Myr.Collisional orogens are the key sites for understanding continental deep subduction,exhumation,uplift and orogenic collapse.Magmatism in collisional orogens plays important roles in continental reworking and net growth.