超高压变质带的全球分布及其大地构造意义

对全球28个超高压变质岩产地的地质对比研究发现,超高压变质带的发育,与弧-陆和陆-陆碰撞造山带关系密切。碰撞造山带多发育于活动的大陆边缘。在那里,冷的陆壳物质可以深俯冲到另一个相对稳定的未俯冲的板块之下,其深度可以超过90~120 km,在高压达>2.5 GPa和温度约600℃或更高条件下,导致超高压特征矿物如,柯石英、金刚石等的生成。在空间分布上,超高压变质带集中于欧亚大陆及其周缘,澳洲和北美目前尚未有报道。从时代上看,超高压变质带多出现于显生宙,前寒武纪只有晚元古代（泛非运动期）少数两例。说明以超高压变质带为特征的碰撞造山作用,是显生宙以来,陆壳增生达到一定的规模后,才出现的构造体制;Rodinia和Pangea古大陆的裂解,使地温梯度降低,从而有利于超高压变质带的生成。     

变质作用、板块构造及超级大陆旋回

麻粒岩相超高温变质作用（GUHTM）主要发育于新太古代至寒武纪岩石中;推测在深部较年轻的,特别是新生代造山带岩石中也会有GUHTM存在。岩石中最初出现GUHTM记录意味着产生瞬时极高热流处的地球动力学发生了改变。许多GUHTM带可能发育于类似现代大陆弧后的构造背景中。在较热的地球上,超大陆及其裂解形成的循环组合,尤其是经岩石圈减薄的洋盆卷入到其外翻过程中可能产生比现代太平洋边缘更热的大陆弧后。中温榴辉岩 高压麻粒岩相变质作用（EHPGM）也是最先发现于新太古代岩石记录中,并发育于从元古宙至古生代岩石中。EHPGM带是对GUHTM带的补充,并经常认为是记录了从俯冲至碰撞造山作用的过程。在元古宙岩石记录中的蓝片岩明显记录了与现代俯冲作用相关的低热流梯度。以发育柯石英(±硬柱石)或金刚石为特征的硬柱石蓝片岩和榴辉岩（高压变质作用,HPM）及超高压变质岩（UHPM）主要是在显生宙形成。HPMUHPM记录了显生宙俯冲碰撞造山带早期碰撞过程中的低热流梯度及陆壳的深俯冲作用。尽管与直觉不同,在超级大陆聚敛期（Wilson旋回洋盆打开和关闭）的大陆地块增生过程,许多HPMUHPM带看来确实是通过小洋盆关闭而发育起来的,反映双重热体制的双重变质带仅发育于新太古代以来的岩石记录中。双重热体制是现代板块构造的特点,而双重变质作用则是板块构造在岩石记录中的特征性标志。尽管构造样式很可能不同,新太古代以来GUHTM和EHPGM带的发育证明“元古宙板块构造体制”的开始。以冷俯冲和大陆地壳深俯冲至地幔,以及其中的部分又从深达300 km处发生折返为标志,“元古宙板块构造体制”在新元古代进化为“现代板块构造体制”,这个转变可由岩石中的HPMUHPM证明。记录这种极端条件的变质带年龄是不一致的,而变质作用发生时间与各大陆岩石圈聚合到超级克拉通（如Superia/Sclavia）或超级大陆（如Nuna (Columbia), Rodinia, Gondwana, 和Pangea）的时间却是一致的。     

Precambrian geodynamics: Concepts and models

In contrast to modern-day plate tectonics, studying Precambrian geodynamics presents a unique challenge as currently there is no agreement upon paradigm concerning the global geodynamics and lithosphere tectonics for the early Earth. This review is focused on discussing results of recent modeling studies in the context of existing concepts and constraints for Precambrian geodynamics with an emphasis placed on three critical aspects: (1) subduction and plate tectonics, (2) collision and orogeny, and (3) craton formation and stability. The three key features of Precambrian Earth evolution are outlined based on combining available observations and numerical and analogue models. These are summarized below:

• •Archean geodynamics was dominated by plume tectonics and the development of hot accretionary orogens with low topography, three-dimensional deformation and pronounced gravitational tectonics. Mantle downwellings and lithospheric delamination (dripping-off) processes are likely to have played a key role in assembling and stabilizing the hot orogens on a timescale up to hundreds of millions of years. Both oceanic-like and continental-like lithospheres were rheologically weak due to the high Moho temperature (> 800 °C) and melt percolation from hot partially molten sublithospheric mantle.
• •Wide spread development of modern-style subduction on Earth started during Mesoarchean–Neoarchean at 3.2–2.5 Ga. This is marked by the appearance of paired metamorphic complexes and oldest eclogite ages in subcontinental lithospheric mantle. Numerical models suggest that the transition occurred at mantle temperatures 175–250 °C higher than present day values, and was triggered by stabilization of rheologically strong plates of both continental and oceanic type. Due to the hot mantle temperature, slab break-off was more frequent in the Precambrian time causing more episodic subduction compared to present day.
• •Wide spread development of modern-style (cold) collision on Earth started during Neoproterozoic at 600–800 Ma and is thus decoupled from the onset of modern-style subduction. Cold collision created favorable conditions for the generation of ultrahigh-pressure (UHP) metamorphic complexes which become widespread in Phanerozoic orogens. Numerical models suggest that the transition occurred at mantle temperatures 80–150 °C higher than present day values and was associated with stabilization of the continental subduction. Frequent shallow slab break-off limited occurrence of UHP rocks in the Precambrian time.

Further progress in understanding Precambrian geodynamics requires cross-disciplinary efforts with a special emphasis placed upon quantitative testing of existing geodynamic concepts and extrapolating back in geological time, using both global and regional scale thermomechanical numerical models, which have been validated for present day Earth conditions.     

碰撞带热结构与碰撞成矿系统

造山带热结构对大陆碰撞带的形态大小、构造式样、岩浆活动和变质作用具有重要控制作用。然而,热结构对碰撞成矿作用的控制还不清楚。本文概述比利牛斯、阿尔卑斯、加里东、扎格罗斯、青藏高原和华力西等全球主要碰撞带的热结构与成矿系统发育特征,对比各个造山带内不同矿床类型成矿温度变化,探讨热结构对碰撞成矿的控制作用。研究表明,碰撞带主要发育盆地流体有关的密西西比河谷型铅锌矿床、变质流体有关的造山型金矿床和岩浆热液有关矿床(斑岩铜矿床、云英岩型钨锡矿床和岩浆热液有关的铌钽锂铍矿床等)。其中,前两者在大多数碰撞带内均有发育,代表了大陆碰撞成矿作用的基本类型。这些矿床的成矿温度在热碰撞带比较高而在冷碰撞带则偏低。岩浆热液有关矿床一般只出现在比较热的碰撞带内,这些热碰撞带的温度压力条件有很大区域在湿固相线以内,热扰动能够造就地壳发生部分熔融形成含矿岩浆。     

Exhumation mechanisms of melt‐bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates

A series of 2D petrological–thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high‐temperature–ultrahigh pressure metamorphic (HT–UHPM) rocks. To this end, the models include fluid‐ and melt‐induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large‐scale crustal stacking, shallow crustal delamination, trans‐lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid‐ and melt‐induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid‐ and melt‐related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large‐scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high‐temperature range of P–T conditions documented from UHPM rocks, including those retrieved from HT–UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100–1200 °C, are still to be achieved in experiments using numerical models.     

Proterozoic orogens in southern Peninsular India: Contiguities and complexities

The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents – Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ∼400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P–T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale “flower structures”, transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics. Our synthesis of the spatial distribution, geometry, kinematics and the transpressional strain of the shear zone systems provides insights into the tectono-metamorphic history of the Proterozoic orogens of southern India and their contiguity and complexities. Recent understanding of subduction, accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in Earth history, although the major and common structural architecture was built during the final assembly of the Gondwana supercontinent.     

Precambrian terranes in the southwestern framing of the Siberian craton: isotopic provinces,stages of crustal evolution and accretion-collision events

We studied geology and main rock assemblages of the Precambrian Kan, Arzybei, and Derba terranes of the Central Asian Fold Belt which border the Siberian craton in the southwest. The Precambrian terranes include three isotopic provinces (Paleoproterozoic, Mesoproterozoic, and Neoproterozoic) distinguished from the Sm-Nd isotope compositions of granitoids, felsic metavolcanics, and metasediments. The terranes formed in three stages of crustal evolution: 2.3–2.5, 0.9–1.1, and 0.8–0.9 Ga. Proterozoic juvenile crust was produced by subduction-related magmatism; it was originally of transitional composition and transformed into continental crust by potassic plutonism as late as the Late Vendian-Cambrian. Terrigenous sediments in the Arzybei and Derba terranes vary in T(DM) Nd model ages from 1.0 to 2.0 Ga. The Nd ages of the underlying metavolcanics and lowest T(DM) of metasediments indicate that terrigenous sedimentation started in the Neoproterozoic. It was maintained by erosion of Mesoproterozoic-Neoproterozoic crust and, to a lesser extent, of Early Precambrian rocks on the craton margin or in Paleoproterozoic terranes. Ar-Ar dating of amphiboles and biotites from metamorphic rocks and U-Pb dating of zircons from granitoids yielded 600–555 and 500–440 Ma, respectively, corresponding to the Vendian and Early Paleozoic stages of nearly synchronous metamorphism and plutonism. Accretion and collision events caused amalgamation of the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic terranes in the Vendian and their collision with the Siberian craton. The lateral growth of the paleocontinent completed in the Late Ordovician.     

Neoproterozoic nappes and superposed folding of the Itremo Group, west-central Madagascar

The Precambrian geology of west-central Madagascar is reviewed and re-interpreted in light of new field observations, Landsat Thematic Mapper image analysis, and U–Pb geochronology. The bedrock of the area consists of: (1) late Archean (to Paleoproterozoic) migmatite gneiss and schist; (2) Mesoproterozoic stratified rocks (Itremo, Amborompotsy, and Malakialina Groups) perhaps deposited unconformably on the older metamorphic rocks (1, above); (3) Proterozoic ( 1000 Ma–720 Ma) plutonic rocks emplaced into both units above (1 and 2), and; (4) latest Neoproterozoic to middle Cambrian ( 570–520 Ma) granitoids emplaced as regionally discordant and weakly foliated plutons throughout the regions.

The effects of Neoproterozoic orogenic processes are widespread throughout the region and our observations and isotopic measurements provide important constraints on the tectonic history of the region: (i) Archean gneisses and Mesoproterozoic stratified rocks are the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). (ii) This continental fragment (both basement and cover) was extensively invaded by subduction-related plutons in the period from  1000 Ma to  720 Ma that were emplaced prior to the onset of regional metamorphism and deformation. (iii) Continental collision related to Gondwana's amalgamation began after  720 Ma and before  570 Ma. Collision related deformation and metamorphism continued throughout the rest of the Neoproterozoic with thermal effects that lasted until  520 Ma. The oldest structures produced during continental collision were km-scale fold- and thrust-nappes with east or southeast-directed vergence (present-day direction). They resulted in the inversion and repetition of Archean and Proterozoic rocks throughout the region. During this early phase of convergence warm rocks were thrust over cool rocks thereby producing the present distribution of regional metamorphic isograds. The vergence of the nappes and the distribution of metamorphic rocks are consistent with their formation within a zone of west or northwest-dipping continental convergence (present-day direction). (iv) Later upright folding of the nappes (and related folds and thrusts) produced km-scale interference fold patterns. The geometry and orientation of these younger upright folds is consistent with E–W horizontal shortening (present-day direction) within a sinistral transpressive regime. We relate this final phase of deformation to motion along the Ranotsara and related shear zones of south Madagascar, and to the initial phases of lower crustal exhumation and extensional tectonics within greater Gondwana.     

中国显生宙造山带麻粒岩相高级变质岩石的地质特征、变质时代、P-T轨迹及其形成的大地构造背景

本文重点介绍我国显生宙造山带中麻粒岩的地质特征、岩石类型、P-T轨迹、变质时代及其形成的大地构造背景。我国显生宙造山带主要包括阿尔泰造山带、南天山-西南天山造山带、西昆仑造山带、东昆仑造山带、阿尔金-柴北缘造山带、北秦岭造山带、南秦岭勉略造山带、东秦岭-桐柏-大别造山带、班公湖-怒江造山带和喜马拉雅中东段造山带。这些造山带中麻粒岩的围岩有许多为蛇绿岩套或蛇绿混杂岩带,部分为副片麻岩和花岗质片麻岩,并一起经历了麻粒岩相变质改造,造山带中大多出现一种高压麻粒岩,有的与榴辉岩并存,但少数造山带中(例如阿尔泰造山带)多种压力类型麻粒岩并存,既有低-高压泥质麻粒岩、中低压基性麻粒岩、高压基性和长英质麻粒岩,又有高温-超高温泥质麻粒岩。变质时代除个别为新元古代晚期外,变质时间多为加里东、海西、印支、燕山、喜山期。麻粒岩的P-T轨迹除西天山木札尔特河低压麻粒岩具逆时针轨迹,反映大陆弧构造环境外,其它都是具有等温降压(ITC)特点的顺时针轨迹,形成的大地构造环境大部分为洋陆俯冲碰撞环境,少部分为陆-陆碰撞环境。目前显生宙造山带中麻粒岩的研究大多数尚在起步阶段,少数研究较详细,不少造山带中麻粒岩的类型和变质时代以及形成的构造背景还不清楚,有待深入研究,新的麻粒岩产地有待发现。     

Geochronological and geochemical study of gneiss–schist complexes and associated granitoids,Beishan Orogen,southern Altaids

The tectonic nature of metamorphic terranes and their role in orogenesis are problematic. Here we present new U–Pb ages and geochemical data for widespread metamorphic rocks and associated granitoids from Northwest China. Orthogneisses from the metamorphic complexes have crystallization ages of ～457, ～452, and ～526 Ma. One paragneiss (schist) has a maximum depositional age of 312 ± 7 Ma. Three foliated granites were emplaced at ～450, ～349, and ～410 Ma, and all lack inherited Precambrian ages. The metamorphic terranes may have undergone multiple petrotectonic events as revealed by the metamorphic ages. Both the orthogneisses and granitoids show enrichment in large ion lithophile elements (LILEs) and light rare Earth elements (LREEs), and depletion in high field strength elements (HFSEs), which indicate that they formed in a subduction-generated accretionary arc setting. Our study demonstrates that the metamorphic terranes in the Beishan area, originally considered as Precambrian basement with suspected Neoarchaean to Palaeoproterozoic ages, are actually parts of early Palaeozoic arcs. The protoliths were probably metamorphosed arc plutonic and sedimentary rocks. Combined with other studies, we speculate that the Beishan Orogen formed by progressive arc accretion during the latest Neoproterozoic to early Palaeozoic time. This new interpretation has implications for other high-grade metamorphic terranes within orogens that have been assumed to represent ancient or pre-existing micro-continental blocks. If so, the importance of collision as a mechanism of mountain building has been overestimated, and the accretionary process as a mechanism of continental growth has been underestimated.     

Kyanite–sillimanite metamorphism of the Precambrian complexes,Transangarian region of the Yenisei Ridge

Four Precambrian metamorphic complexes in the vicinity of regional faults in the Transangarian region of the Yenisei Ridge were examined. Based on geothermobarometry and P-T path calculations, our geological and petrological studies showed that the Neoproterozoic medium-pressure metamorphism of the kyanite–sillimanite type overprinted regionally metamorphosed low-pressure andalusite-bearing rocks at about 850 Ma. A positive correlation between rock ages and P-T estimates for the kyanite-sillimanite metamorphism provide evidence of the regional structural and tectonic heterogeneity. The medium-pressure metamorphism was characterized by (1) the development of deformational structures and textures, and kyanite-bearing blastocataclasites (blastomylonites) with sillimanite, garnet, and staurolite after andalusite-bearing regional metamorphic rocks; (2) insignificant apparent thickness of the zone of medium-pressure zonal metamorphism (from 2.5 to 7 km), which was localized in the vicinity of the overthrusts; (3) a low metamorphic field gradient during metamorphism (from 1–7 to 12 °C/km); and (4) a gradual increase in lithostatic pressure towards the thrust faults. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence for near-isothermal loading. This event was justified within the framework of the crustal tectonic thickening model via rapid overthrusting and subsequent rapid uplifting and erosion. The results obtained allowed us to consider medium-pressure kyanite-bearing metapelites as a product of collision metamorphism, formed either by unidirectional thrusting of rock blocks from Siberian craton onto the Yenisei Ridge in the zones of regional faults (Angara, Mayakon, and Chapa areas) or by opposite movements in the zone of splay faults of higher orders (Garevka area).     

Multi-Layered Low-Angle Ductile Detachment System in Dabie-Sulu UHPM and HPM Belts,China

INTRODUCTIoNTheDabie-Suluregion,Chinahasbeenrecognizedasthelargestultrahigh-pressuremetamorphic(UHPM)(peakp>2.7GPa)andHPMbeltsintheworld,andattractedexten-siveinterestfrominternationalgeoscientists.AwealthofpublicationsexistsabouttheoccurrenceofUHPMindexmin-erals,thegeochronology,metamorphicPTpaths,andthelarge-scaleconceptualtectonicevolutionofUHPMandhigh-pressuremetamorphic(HPM)rocksintheDabie-Suluregion.Morerecentoverviewsaregiven'byErnstetal.(1995),Liouetal.(l996),andHackere…     

Geochemistry of phlogopite, diopside, calcite, anhydrite and apatite pegmatites and syenites of southern Madagascar: evidence for crustal silicocarbonatitic (CSC) melt formation in a Panafrican collisional tectonic setting

The phlogopite, diopside, calcite, anhydrite and apatite pegmatites of Ampandrandava and Beraketa are examples for the many other pegmatites of similar silicocarbonatitic composition found in the Bekily and Betroka-Beraketa Precambrian belts of southern Madagascar. The two studied pegmatites and associated syenites crystallised from immiscible silicocarbonatitic and peralkaline syenitic melts in a time span between 515 and 504 Ma in the final extensional phase of the Panafrican continental collision and connected metamorphic/metasomatic event. Model T _Nd ages suggest that the melts were produced by partial melting of 3.5 Ga partially evaporitic continental crust. The studied pegmatites and genetically associated syenitic rocks are very rare examples for crustal silicocarbonatitic melts generated in a Panafrican collisional setting. The overwhelming majority of carbonatites and associated peralkaline rocks are mantle derived, much poorer in phosphate and sulfate and found in a cratonic environment. In light of the present results, genetic models for other sulfate- and phosphate-rich magmatic rocks (e.g., phlogopite–apatite–calcite mineralisations in the Grenville-Hasting formation in Canada and in the Sludyanka group in Eastern Siberia) should be reevaluated.     

Assembly of central Gondwana along the Zambezi Belt: Metamorphic response and basement reactivation during the Kuunga Orogeny

Central Gondwana was assembled by three continental collisions in relatively quick succession: late Cryogenian East Africa Orogen, early Ediacaran West Antarctica Orogen and late Ediacaran Kuunga Orogen. The Kuunga Orogen involved diachronous closure of the South Adamastor–Khomas–Mozambique Oceans and accretion of Kalahari Craton and cratonic elements in Antarctica, with a previously assembled North Gondwana. The two older orogens were still hot and deforming at the time of final assembly by the Kuunga Orogen, and were therefore reworked and re-metamorphosed. The Central Kuunga Orogen is comprised of the Lufilian Arc, Zambezi Belt, Malawi–Unango Complex and the Lurio Belt. This region was the site of earliest collision in the Kuunga Orogen at ~575 Ma, and involved collision of two buoyant, previously metamorphosed rigid basement promontories. Pivoting on the Zambezi Belt indenters led to clockwise rotation of the Kalahari Craton and oblique collision within the Damara Belt ~20–30 m.y. later. The Central Kuunga Orogen is a relatively cold collisional belt dominated by eclogite, whiteschist and Barrovian series metamorphic parageneses, and contrasts with the paired metamorphic response in the Damara Belt to the west, and low-P/high-T metamorphism in the East Kuunga Orogen. Metamorphic parageneses are preserved from each stage of the full Wilson Cycle: from initiation of continental lithosphere thinning at ~940 Ma, widespread rifting between 725 and 805 Ma, and passive margin sedimentation until ~580 Ma. Eclogite-facies subduction parageneses indicate consumption of ocean lithosphere was underway by ~630–660 Ma. Collision at ~575 Ma involved deep burial of continental crust and formation of very high-P, low T/depth metamorphic parageneses, followed by Barrovian series thermal peaks at ~545 and ~525 Ma. Isostatic compensation and stress switches associated with plate reconfigurations once Gondwana was assembled, resulted in exhumation and local extension in an intra-continental setting from ~518 Ma.     

Geochemical and Sr–Pb–Nd isotopic characteristics of the Shakhtama porphyry Mo–Cu system (Eastern Transbaikalia,Russia)

The Shakhtama Mo–Cu porphyry deposit is located within the eastern segment of the Central Asian Orogenic Belt, bordering the southern margin of the Mongol–Okhotsk suture zone. The deposit includes rocks of two magmatic complexes: the precursor plutonic (J₂) and ore-bearing porphyry (J₃) complexes. The plutonic complex was emplaced at the final stages of the collisional regime in the region; the formation of the porphyry complex may have overlapped with a transition to extension. The Shakhtama rocks are predominantly metaluminous, I-type high K calc-alkaline to shoshonitic in composition, with relatively high Mg#, Ni, Cr and V. They are characterized by crustal-like I_Sr (0.70741–0.70782), relatively radiogenic Pb isotopic compositions, ε_Nd(T) values close to CHUR (−2.7 to +2.1) and Nd model ages from 0.8 to 1.2 Ga. Both complexes are composed of rocks with K-adakitic features and rocks without adakite trace element signatures. The regional geological setting together with geochemical and isotopic data indicate that both juvenile and old continental crust contributed to their origin. High-Mg# K-adakitic Shakhtama magmas were most likely generated by partial melting of thickened lower crust during delamination and interaction with mantle material, while magmas lacking adakite-like signatures were probably generated at shallower levels of lower crust. The derivation of melts, related to the formation of plutonic and porphyry complexes involved variable amounts of old Precambrian lower crust and juvenile Phanerozoic crust. Isotopic data imply stronger contribution of juvenile mantle-derived material to the fertile magmas of the porphyry complex. Juvenile crust is proposed as an important source of fluids and metals for the Shakhtama ore-magmatic system.     

柴北缘前寒武纪岩体（地层）变质年龄 及其对早古生代造山作用的指示

柴北缘前寒武纪岩体（地层）分布广泛。为确定柴北缘地区前寒武纪岩体（地层）受早古生代碰撞造山作用的影响,采用LA-ICP-MS技术,对大柴旦地区前寒武纪黑云斜长片麻岩、斜长角闪岩及石榴子石斜长角闪岩中的锆石进行了U-Pb同位素定年。黑云斜长片麻岩获得479~472Ma的变质年龄,斜长角闪岩获得440Ma和470Ma 2个变质年龄,石榴子石斜长角闪岩获得418.8Ma±3.0Ma的变质年龄。初步确定,柴北缘早古生代造山作用对前寒武纪岩体构成了3次强度不等的变质作用叠加,分别为大洋俯冲末期阶段(495~467Ma)岛弧花岗岩弱热烘烤变质作用、大陆碰撞造山阶段(467~423Ma)区域变质作用、S型花岗岩热动力变质作用和大陆后碰撞造山阶段(423~371Ma) I型花岗岩强烈接触热变质作用。     

2D thermomechanical modelling of continent–arc–continent collision

Arc–continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin. We present 2D petrological–thermo-mechanical models of arc–continent collision and investigate geodynamic regimes of this process. The model includes spontaneous slab bending, dehydration of subducted crust, aqueous fluid transport, partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes. Results point to two end-member types of subsequent arc–continent collisional orogens: (I) orogens with remnants of accretion prism, detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments; and (II) orogens mainly consisting of the closed back-arc basin suture, detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment-derived magmatic rocks. Transitional orogens between these two endmembers include both the suture of the collapsed back-arc basin and variable amounts of magmatic production. The orogenic variability mainly reflects the age of the subducting oceanic plate. Older, therefore colder and denser oceanic plates trigger subduction retreat, which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere. In this case, subducted sediments are not heated enough to melt and generate magmatic plumes. On the other hand, young and less dense slabs do not retreat, which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans-lithospheric plumes. We have also investigated the influences of convergence rate and volcanic/plutonic rocks' ratio in newly forming lithosphere. The predicted gross-scale orogenic structures find similarities with some natural orogens, in particular with deeply eroded orogens such as the Variscides in the Bohemian Massif.     

柴北缘前寒武纪岩体（地层）变质年龄及其对早古生代造山作用的指示

柴北缘前寒武纪岩体（地层）分布广泛。为确定柴北缘地区前寒武纪岩体（地层）受早古生代碰撞造山作用的影响,采用LA—ICP—MS技术．对大柴旦地区前寒武纪黑云斜长片麻岩、斜长角闪岩及石榴子石斜长角闪岩中的锆石进行了u—Pb同位素定年。黑云斜长片麻岩获得479-472Ma的变质年龄,斜长角闪岩获得440Ma和470Ma2个变质年龄,石榴子石斜长角闪岩获得418．8Ma±3．0Ma的变质年龄。初步确定,柴北缘早古生代造山作用对前寒武纪岩体构成了3次强度不等的变质作用叠加,分别为大洋俯7中末期阶段（495-467Ma）岛弧花岗岩弱热烘烤变质作用、大陆碰撞造山阶段（467-423Ma）区域变质作用、S型花岗岩热动力变质作用和大陆后碰撞造山阶段（423-371Ma）I型花岗岩强烈接触热变质作用。     

On Continent-Continent Point-Collision and Ultrahigh-Pressure Metamorphism

Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite- and diamond-bearing eclogites and eclogite facies metamorphic rocks formed at 650-800℃ and 2.6-3.5 GPa, and most of the protoliths of UHP rocks are volcanic-sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducte     

http://www.sciencedirect.com/science/article/pii/S1674987114000309

In the early 1980s, evidence that crustal rocks had reached temperatures 〉1000 ℃ at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphic-metasomatic reworking of the lithospheric mantle was realized. In parallel, new developments in instrumentation, the expansion of in situ analysis of geological ma- terials and increases in computing power opened up new fields of investigation. The robust quantifi- cation of pressure （P）, temperature （T） and time （t） that followed these advances has provided reliable data to benchmark geodynamic models and to investigate secular change in the thermal state of the lithosphere as registered by metamorphism through time. As a result, the last 30 years have seen sig- nificant progress in our understanding of lithospheric evolution, particularly as it relates to Precambrian geodynamics.