西藏雅鲁藏布江缝合带中段沉积混杂岩的变质特征及构造意义

通过岩相学与矿物化学分析和视剖面图模拟计算对雅鲁藏布江缝合带中段地区沉积混杂岩带中多硅白云母石英片岩和黑硬绿泥石片岩的变质特征进行研究。多硅白云母石英片岩峰期矿物组合为多硅白云母+绿泥石+钠长石+石英+榍石,峰期条件为366℃/6.4 kbar;黑硬绿泥石片岩峰期矿物组合为阳起石+绿泥石+绿帘石+钠长石+石英+榍石+钛铁矿,峰期条件为363℃/6.1 kbar,黑硬绿泥石为峰后降温减压阶段的产物。雅鲁藏布江缝合带中段地区的沉积混杂岩变质温压条件达到了高压绿片岩相,指示俯冲深度21～23 km,地温梯度15～18℃/km,其变质特征与俯冲洋壳被上部地幔楔刮削进入增生楔系统经历变质作用形成的低级变质岩十分相似。结合前人年代学数据,研究认为雅鲁藏布江缝合带中段地区的沉积混杂岩在增生楔深部经历高压绿片岩相变质作用,形成于单一的特提斯洋洋壳向欧亚大陆南缘俯冲的构造体制之下印度-亚洲大陆初始碰撞时期。     

2013-24 科技期刊亮点

花岗岩浆指示北秦岭早古生代大陆碰撞过程西北大学地质学系张成立等研究发现,北秦岭构造带存在与500Ma高压-超高压变质、450Ma中压麻粒岩相和420Ma角闪岩相退变质作用分别对应的3期花岗岩浆作用,指示北秦岭在早古生代期间经历了由大陆碰撞到碰撞后再造的演化历史。     

索伦-林西缝合带北侧西段中二叠统哲斯组砂岩碎屑分析及其构造意义

由于索伦-林西缝合带的地表露头较差,关于其存在与否和延伸状况仍然存在着争论.为了进一步研究其俯冲碰撞过程,对缝合带以北的满都拉地区发育的中二叠统哲斯组磨拉石沉积地层中砂岩进行碎屑成分分析.砂岩碎屑成分及物源分析结果表明,在盆地南部发育有与索伦—林西缝合带俯冲碰撞作用有关的晚古生代岩浆弧.     

柴达木盆地及相邻地区地质构造演化

用板块构造理论对柴达木盆地及相邻地区的地质构造演化进行了论述,并划分出华北板块、塔里木板块、秦祁昆板块与巴颜喀拉-扬子板块四部分,各部分先后于早古生代时期、晚古生代后期及中生代早期由其间发育的深断裂-缝合带相拼接。中新生代时期印度板块向北碰撞、俯冲于西藏板块之下,第三纪中晚期俯冲作用加剧,柴达木地区随着青藏高原整体迅速隆升而陷落,形成了柴达木构造断陷盆地。     

塔里木-中朝板块北缘的志留纪岛弧公婆泉群火山岩地质地球化学及构造意义

公婆泉群位于北山中部，可分为：北、南两带。南带为弧后盆地玄武岩-安山岩组合，北带为岛弧火山岩组合，西段窑洞努如为火山角砾岩和熔火山角砾岩组合，中段公婆泉一带为玄武岩-安山岩-流纹岩-粗面岩组合，东段东七一山地区为玄武岩-安山岩-流纹岩组合。岩石组合，地球化学综合分析认为，公婆泉群是中-晚志留世哈萨克斯坦板块俯冲与塔里木-中朝板块的产物。岛弧西段窑洞努如位于近大陆或者在水下陆壳。公婆泉位于近海沟陆壳，而东七一山则远离海沟，是在洋壳的基础上发育的。说明塔里木-中朝板块是一个统一的板块，同时在早古生代存在哈萨克斯坦板块向塔里木-中朝板块的俯冲。     

门源县松树南沟金矿床地质构造特征

松树南沟金矿位于红沟-庄浪加里东期陆缘裂谷带的中西段,南与中祁连前武纪隆起带前加里东期大坂-兴隆山陆块毗邻。区域构造背景反映了一个在前寒武纪基底上于晚奥陶世拉张的被动陆缘裂谷环。通过对矿区地质构造特征的系统研究,确定了松树南沟金矿床位于古亚洲构造域中朝板块北祁连板段北祁连山环太平洋型俯冲带(缝合带),并靠近大坂山深大断裂带。构造(接触带构造、片理化带和断裂构造)为矿液运移提供通道和储矿空间。     

西昆仑早古生代构造演化及其对塔西南盆地的影响

在板块构造理论基础上，通过区内蛇绿岩、与俯冲有关的岩浆岩及相关沉积等综合研究，对西昆仑早古生代构造演化进行了重建：震旦纪－寒武纪西昆仑从塔里木大陆分裂出去，并在它们之间形成“昆仑洋”；寒武纪－奥陶纪“昆仑洋”洋壳向南俯冲于西仑之下，形成库地俯冲带、库地弧前盆地、西昆仑早古生代火山弧及麻扎－黑卡弧后盆地；志留纪－早泥盆世，“昆仑洋”消失，西昆仑早古生代火山弧与塔里木大陆碰撞，并形成塔里木前陆冲断带和泥盆纪前陆盆地磨拉石及塔里木大陆内泥盆纪的“红层相”     

内蒙古达茂旗乌德缝合带特征及大地构造意义

乌德缝合带位于华北地块和北缘增生带之间，显示早古生代属于典型的沟－弧－盆体系。与碰撞造山作用同时，形成区域性构造混杂带，地层学和同位素地质年代资料表明，造山作用发生于加里东期，造山作用的动力学过程主要表现为由北向南的洋壳俯冲和向北的陆壳仰冲，并伴随右旋剪切滑移运动。     

西藏康马热伸展变质核杂岩构造研究

西藏喜马拉雅特提斯北亚带内沿定日—康马一线展布近东西向变质核杂岩带，在这个带上康马变质核杂岩是一典型代表。该变质核杂岩核部由花岗岩及花岗岩边部片麻岩组成，岩体之上分别为早古生代构造片岩带、石炭—二叠纪褶叠层带及三叠—白垩纪板岩带组成。研究表明，构造片岩带构成了该变质核杂岩的拆离断层系，拆离断层剪切指向为上层系相对下层系自南而北运动，推测其形成于三叠纪—白垩纪。其后由于雅鲁藏布江缝合带的俯冲、碰撞，使得早期拆离系叠加岩浆热动力形成穹隆体。     

论北秦岭加里东期造山作用

提出北秦岭加里东造山带由华北地块南缘，以未变质—浅变质沉积岩系为主体的前陆褶断带和北秦岭以中—深变质岩系及强烈岩浆活动为特征的山根带两个主要部分组成。认为它自成体系，相应的变质、变形、岩浆作用发育。该造山带是早古生代未秦岭古洋盆沿商丹—桐柏一线俯冲、消减，促使秦岭古岛弧系碰撞华北地块，造成弧后边缘海闭合所致，是显生宙秦岭造山带形成演化进程中的重大构造事件之一。     

Nd isotopic disequilibrium between minerals and Rb-Sr age of the secondary phengite in eclogite from the Yangkou area, Qingdao, eastern China

There are two generations of white micas in retrograded coesite-bearing eclogite from the Yangkou area near Qingdao, eastern China. The secondary phengite developed along the folliations in eclogite is the majority of the white micas. Nd and Sr isotopic disequilibriums between garnet and retrograded omphacite as well as secondary phengite have been observed. Consequently, the Rb-Sr ages ((193 ± 4) Ma―(195 ± 4) Ma) given by the tie lines of the secondary phengite + garnet or whole rock may predate the formation time of the phengite. The Rb-Sr age of (183 ± 4) Ma given by the secondary phengite + retrograde omphacite is much closer to the formation time of the phengite indicating the retrograde age of eclogite instead of a cooling age of eclogite at 500℃.     

Lower time limit on the UHPM rock exhumation: Discovery of eclogite pebbles in the Late Jurassic conglomerates from the northern foot of the Dabie Mountains, eastern China

Eclogite pebbles are first discovered in late Jurassic conglomerates of the Fenghuangtai Formation from the northern foot of the Dabie Mountains, eastern China. The eclogite shows relatively strong retrograde metamor- phism. The major remnant minerals are garnet, phengite, quartz, rutile, clinozoisite, fingerprint(or worm)-form sym- plectite and pseudomorphic enclosure of coesite in garnet. End-member components of the garnet mainly made up of grossular (27.7%-37.8%), almandinc (45.5%-49.0%) and pyrope (12.3%-25.7%). End-member components of the pyrope is less than 30%, i.e. equivalent to C type eclogite. Si-cations of the phengite are 3.44-3.54 p.f.u. (taking O=11 as standard). It is estimated that the pressures are 2.5-3.2 GPa, the temperatures are 600-900℃. Discovery of the eclogite pebbles in the late Jurassic conglomerates from the northern foot of the Dabie Mountains suggests that the HP-UHP metamorphic rocks of the Dabie Mountains, which were formed in Triassic, had been exposed to the surface in the late Jurassic or before it by exhumation.     

Phase transition in the subducted oceanic lithosphere and generation of the subduction zone magma

Two metamorphic processes, i.e. subsolidus dehydration and partial melting occurring in MORB, metasediments and peridotite of subducted oceanic lithosphere are discussed on the basis of available experimental work and phase equilibrium modeling. Phase diagrams of hydrous MORB show that in most cold subduction P-T （pressure-temperature） regimes a large portion of water in the basic layer has released below the onset of blueschist facies （〈 20 km）, and at a depth （60--70 km） of transition from lawsonite blueschist to lawsonite eclogite facies through glaucophane dehydration; only a smaller portion of water will escape from the slab through dehydration of lawsonite and chloritoid in the depth range suitable for arc magma formation; and a very small portion of water stored in lawsonite and phengite will fade into the deeper mantle. The role of amphibole for arc magma formation is still arguable. In cold subduction P-Tregimes, the dehydration of chlorite and talc in AI-poor metasediments, and chloritoid and carpholite in AI-rich metapelites at a depth around 80--100 km will make some con- tributions to the formation of arc magma. Comparatively, dehydration of serpentine in hydrated peri- dotite occurs at depths of 120--180 km, playing an important role in the arc magmatism. Subduction of oceanic crust along warm P-T regimes will cross the solidi at a depth over 80 km, resulting in partial melting under fluid-saturated and fluid-absent conditions in the metasediments involving biotite and phengite, and in the basic rocks involving epidote and amphibole. The melt compositions of the basic crust are adakitic at pressures 〈 3.0 GPa, but become peraluminous granitic at higher pressures.     

Mineral inclusions of zircon and UHP metamorphic evidence from paragneiss and orthogneiss of pre-pilot drillhole CCSD-PP2 in north Jiangsu Province, China

Coesite inclusions, together with omphacite, jadeite, garnet and phengite inclusions, were identified in zircons separates from almost all gneissic core samples of pre-pilot drillhole CCSD-PP2 by the Laser Raman spectroscopy and the cathodoluminescence method. These data indicate that gneissic rocks consisting of paragneisses and orthogneisses ubiquitously experienced UHP metamorphism. This research may be of great significance for an in-depth study of the subduction-exhumation mechanism of the Sulu UHP metamorphic belt and selecting the drilling site for the Chinese Continental Scientific Drilling Project.     

Characteristics and geological significance of the eclogites from the Tongbai Mountains, Henan Province

Some eclogites and the related high-P metamorphic belt have been found for the first time from the Tongbai Mountains, Henan Province, where the eclogites and their probable retrogressive products ——garnet amphibolites——occur as lenses in mica-schists. The fresh eclogites comprise garnet, omphacite, quartz, rutile, phengite, barroisite, etc. The compositional features of garnet and available geothermobarometers indicated that the eclogites belong to the low-T type. The mica-schists contain phengite, quartz, garnet and rutile that are also attributed to high-P assemblage. The eclogites and related high-P belt probably belong to the Early Paleozoic in age, which is distinct from the Indosinian Dabie high- and ultrahigh-P metamorphic belt.     

Implications of micro-compositions of garnet and biotite from high-grade meta-pelites

Based on detailed studies on the compositional zoning of garnet and biotite in pelitic rocks from the Jingshan group of granulite facies in north Jiaodong, P-T pseudosections with isopleths of Fe/(Fe + Mg) in garnet and biotite were calculated in the KF-MASH system for two representative rocks of sillimanite-garnet-cordierite-biotite gneiss ( Vbi/ Vg>1 ) and sillimanite-garnet gneiss (Vbi/ Vg<0.2) using the software THERMOCALC and the internally consistent thermodynamic dataset. With a comparison of the calculated Fe/(Fe+ Mg) values in garnet and biotite in the peak P-T fields constrained by peak mineral assemblages with the measured ones, it is concluded that the coarse garnet crystals with diffusion zoning from high grade meta-pelites can preserve their peak compositions even when they have experienced a cooling event, and that biotite crystals surrounded by fetsic minerals in biotite-rich rocks with Vbi/Vg> 1 can nearly preserve their peak compositions, and biotites in garnet-rich rocks with Vbi/Vg<0. 2 cannot preserve their peak compositions due to the influence of grain-boundary fluid.     

Phase equilibria of HP-UHP mica-schists in Jiangling,Eastern Dabieshan orogen

The HP-UHP metamorphic terrane of Jiangling, eastern Dabieshan comprises extensively distributed granitic gneisses, mica-schists and numerous eclogite blocks. The mica-schists generally contain garnet, phengitic muscovite, biotite, plagioclase, quartz, rutile and a small amount of epidote and hornblende. Study on petrography and phases equilibria in the NCKMnFMASH system indicates that the present mineral assemblages in mica-schists are not in equilibrium. The earlier stage of mineral assemblage represented by garnet and phengite reflects a HP-UHP condition. The garnet compositions and the phengite Si contents give a PT condition of 580-00 ℃ at 2.6-2.8 GPa. The garnet zonation records an earlier progressive metamorphic process which may be associated with the appearance of glaucophane, jadeite and lawsonite. The later stage of mineral assemblage characterized by the presence of biotite and plagioclase reflects a PT condition of 620-635℃ at 0.9-1.1 GPa, belonging to the HP amphibolite facies. The main mineral assemblage in mica-schists from the Jiangling region has recorded a complete HP-UHP metamorphic process.     

Phase equilibria of HP–UHP mica-schists in Jiangling, Eastern Dabieshan orogen

The HP–UHP metamorphic terrane of Jiangling, eastern Dabieshan comprises extensively distributed granitic gneisses, mica-schists and numerous eclogite blocks. The mica-schists generally contain garnet, phengitic muscovite, biotite, plagioclase, quartz, rutile and a small amount of epidote and hornblende. Study on petrography and phases equilibria in the NCKMnFMASH system indicates that the present mineral assemblages in mica-schists are not in equilibrium. The earlier stage of mineral assemblage represented by garnet and phengite reflects a HP–UHP condition. The garnet compositions and the phengite Si contents give a PT condition of 580–600℃ at 2.6–2.8 GPa. The garnet zonation records an earlier progressive metamorphic process which may be associated with the appearance of glaucophane, jadeite and lawsonite. The later stage of mineral assemblage characterized by the presence of biotite and plagioclase reflects a PT condition of 620–635℃ at 0.9–1.1 GPa, belonging to the HP amphibolite facies. The main mineral assemblage in micaschists from the Jiangling region has recorded a complete HP–UHP metamorphic process.     

Coupled isomorphic substitution and exsolution of pyroxene,rutile, apatite and quartz in supersilicic garnet

Dissolution of pyroxene in garnet at ultrahigh pressures produces supersilicie garnet with the coupled substitutions of Si^Ⅵ M^Ⅵ= A^Ⅵ A^Ⅵ and Si^Ⅵ Na^Ⅷ=A^Ⅵ M^Ⅷ,which are enhanced by rising pressure. The supersilicic garnet and exsolution of pyroxene, rutile, apatite and quartz in garnet during decompression were found in natural rocks,pointing to the importance in studying mantle-derived rocks and ultrahigh pressure metamorphism related to plate deep subduction. Ti, P, K and H2O enters garnet via the substitutions of Ti = Si, P^Ⅵ Na^Ⅷ = Si^Ⅵ CaⅧ, Si^Ⅵ K^Ⅷ = AI^Ⅵ M^Ⅷ, and [(OH)4]^4- = [SiO4]^4- or [4H]^4 = Si^4 respectively. The possible entering of Eskola pyroxene component M0.5AlSi2O6 in clinopyroxene, together with the common pyroxene component M2Si2O6, into garnet can lead to the presence of the substitution of Si^Ⅵ 0.5□^Ⅷ= A^Ⅵ 0.5M^Ⅷ in garnet structure, which plays a key role in the exsolution of rntile, apatite and quartz in garnet. Two new breakdown reactions are thus proposed on the basis of the new coupled substitution, which can be regarded as a theoretical model for the exsolntion of the 3 minerals in garnet. The real exsolution may be a combination of several breakdown reactions.     

Discovery of eclogite and its geological significance in Qiangtang area, central Tibet

Eclogites have been found in the margin of the Qinghai-Tibet Plateau respectively since the 1990s. First Eocene eclogite from Himalayan belt was discov- ered in Kaghan valley, northern Pakistan in 1991, in which coesite was identified[1,2]. Then two eclog…