The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

The geological characteristics of ultrahigh-pressure （UHP） metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithlc assemblage of typical oceanic crust, the peak metamorphic temperature 〈600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, reepectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler eubducted into the mantle depth end then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior end cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alpa and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.     

哈山构造带火山岩储层发育特征及控制因素

利用薄片、岩石元素全分析、扫描电镜等多种分析测试手段,结合测井资料分析,开展准西北缘哈拉阿拉特山(哈山)造山带火山岩发育背景、岩性组合、储层成因及控制因素研究。结果表明:研究区上石炭统火山岩形成于陆缘岛弧及大洋岛弧背景,发育火山角砾岩、凝灰岩、玄武岩、安山岩及流纹岩组合;主要储集空间类型为构造裂缝及溶蚀孔缝;岩性岩相是成储基础,成岩作用控制了储层演化,构造演化是火山岩成储的主控因素和关键;在推覆体前翼、构造转折端和多期次断层交汇处的凝灰岩、火山角砾岩发育区以及受长期风化淋滤作用改造的火山角砾岩及火山熔岩发育带,为火山岩储层有利发育区。     

新疆阿希矿区火山岩岩石化学特征及其形成环境

通过对新疆阿希矿区下石炭统火山岩岩石化学和微量元素地球化学的研究，探讨了该区火山岩形成的大地构造背景与喷发环境，认为这套火山岩属钙碱性系列，形成于与板块俯冲作用有关的活动大陆边缘或成熟岛弧带中近大陆一侧，是陆相火山喷发的产物。     

Lawsonite-bearing eclogites in the north Qilian and north Altyn Tagh： Evidence for cold subduction of oceanic crust

Laboratory experiments and thermal models predic that lawsonite-bearing eclogite should be the dominan rock types for typical oceanic subduction zone[1,2] However, eclogite containing unaltered lawsonite is rare in nature and has been described only from …     

Petrology, geochemistry and geodynamics of basic granulite from the Altay area, North Xinjiang, China

The basic granulite of the Altay orogenic belt occurs as tectonic lens in the Devonian medium- to lower-grade metamorphic beds through fault contact. The Altay granulite (AG) is an amphibole plagioclase two-pyroxene granulite and is mainly composed of two pyroxenes, plagioclase, amphibole and biotite. Its melano-minerals are rich in Mg/(Mg Fe^2 ),and its amphibole and biotite are rich in TiO2. The AG is rich in Mg/(Mg Fe^2 ), Al2O3 and depletion of U, Th and Rbcontents. The AG has moderate ∑REE and LREE-enriched with weak positive Eu anomaly. The AG shows island-arc pattern with negative Nb, P and Ti anomalies, reflecting that formation of the AG may be associated with subduction. Geochemical and mineral composition data reflect that the protolith of the AG is calc-alkaline basalt and formed by granulite facies metamorphism having peak P-T conditions of 750℃-780℃ and 0.6-0.7 Gpa. The AG formation underwent two stages was suggested. In the early stage of oceanic crustal subduction, calc-alkaline basalt with island-arc environment underwent granulite facies metamorphism to form the AG in deep crust, and in the late stage, the AG was thrust into the upper crust.     

冀北赤城红旗营子杂岩的地质特征及意义

运用野外地质填图、岩石地球化学分析以及同位素年龄等方法探讨冀北赤城红旗营子杂岩的岩石组合、构造属性、变质演化以及时代归属等特征,为重新划分冀北赤城的红旗营子群提供依据。结果显示红旗营子杂岩由云州表壳岩及其内部的退变榴辉岩、蛇纹岩(蛇纹石化方辉橄榄岩)和蛇纹石化(橄榄)大理岩等透镜状构造岩块组成,它们一起经历了晚古生代(321~326 Ma B.P.)区域角闪岩相变质事件。其中,云州表壳岩的岩性主要为石榴黑云斜长片麻岩,原岩是形成于大陆岛弧环境的泥质(含泥质)砂岩,沉积作用发生在晚古生代(340~369Ma B.P.),属于晚古生代沉积-变质地层;石榴黑云斜长片麻岩之中的退变榴辉岩、蛇纹岩(蛇纹石化方辉橄榄岩)和蛇纹石化(橄榄)大理岩等构造岩块应属"外来"的洋壳残片,成因上可能与古亚洲洋的洋壳向南俯冲消减有关。赤城红旗营子杂岩的岩石组合、构造属性、变质演化以及同位素地质年代学等特征明显不同于冀北其他地区的红旗营子群,冀北红旗营子群需要重新划分。     

Discovery of Late Paleozoic retrograded eclogites from the middle part of the northern margin of North China Craton

The northern margin of the North China Craton (NCC), located between the Paleo-Asian Ocean tectonic region on the north and the NCC on the south, is a key region for studying the tectonic evolution of NCC. A Pre-cambrian retrograded eclogite (2500 Ma or 1800 Ma) was reported in Baimashi near Hengshan Mountain in the NCC, which is characterized by the vermicular symplec-tite of diopside and plagioclase with absence of ompha-cite[1,2]. In Hongqiyingzi Group from the middle part of the …     

两广交界处岑溪二叠纪岛弧型玄武岩及其古特提斯性质的讨论

两广交界处的岑溪二叠纪岛弧型玄武岩是华南地区(狭义)首次报道的、有较为可靠同位素年龄的晚古生代活动大陆边缘型玄武岩。对该地区玄武岩和细碧角斑岩的主量元素、微量元素(包括稀土元素)和同位素年代学进行了综合研究，论证了岑溪细碧角斑岩建造是与古特提斯域大陆边缘岛弧或弧间盆地有关的二叠纪火山喷发产物。     

滇西碧罗雪山地区忙怀组火山岩地球化学特征

滇西碧罗雪山一带,澜沧江岛弧带上发育了中三叠纪忙怀组火山岩,岩石类型主要是流纹岩.常量元素SiO2、K2O、Na2O、Al2O3明显偏高,MgO较低,TiO2中等的特征.里特曼指数σ1.39~3.01,为钙碱性系列,在log t-logσ图解中全部落入造山带区.碧罗雪山一带流纹岩的大离子亲石元素含量变化比较小,微量元素表现为大离子亲石元素Rb、Sr、Ba明显富集,高场强元素Th也相对富集.Eu负异常明显,轻稀土元素富集,稀土分配模式为轻稀土富集右斜型.微量元素在Rb-(Yb+Nb)图解及Rb-(Yb+Ta)图解中大多数落入碰撞带花岗岩区和火山弧花岗岩区.综合岩石地球化学特征及构造环境判别图解指示,碧罗雪山地区忙怀组火山岩为澜沧江洋由西向东俯冲消减,由被动大陆边缘转化为活动大陆边缘过程中形成的岛弧火山岩.     

Discovery of Mg-righ volcanic rock series in western Altay area, Xinjiang and its geologic significance

A series of Mg-rich volcanic rocks, including rare Mg-rich dacites in the Asherle copper mine, western Altay Area, has been found. This suite of Mg-rich volcanic rocks is characterized by high SiO2, MgO and low Ti contents. Compared with the island arc tholeiite and MORB, these rocks are depleted in HFSE and are therefore similar to boninite. The dacites show V-shaped distribution REE patterns whereas the basalts display flat or LREE-depletion patterns. Based on their geochemical features and occurrence, it is suggested that they were formed in oceanic island arc setting. Its petrogenesis is related to the slab movement. At the early stage, the peridotite of the ocean upper mantle diapirically uprised to the shallow level and started to melt owing to subduction of the ocean slab, forming the island arc tholeiite with pillow structure. At the late stage, the residual refractory peridotites melted again, giving rise to the formation of the parent magma of Mg-rich basalt.     

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.     

Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate). Current models predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5?km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9?km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.     

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.     

Rapid change in drift of the Australian plate records collision with Ontong Java plateau

The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world's oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present (40)Ar-(39)Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale.     

西藏班戈县蓬错地区富铌玄武岩地球化学特征及地质意义

系统的岩石化学与地球化学研究表明西藏蓬错地区存在富铌玄武岩。SiO_2含量为45.84%~55.88%,TiO_2含量为1.05%~3.25%,与加拿大Superior省太古代Wawa绿岩带的富铌玄武岩相当。Na_2O含量为0.92%~5.10%,P_2O_5含量为0.23%~1.19%,与新生代富铌玄武岩相同。稀土分布模式为无明显铕异常或弱负铕异常的右倾曲线,轻重稀土分馏较强烈。虽然铌含量很高,但Th与轻稀土元素的强烈富集,在微量元素蛛网图中铌依然显示亏损,高场强元素Ta、Ti也呈弱负异常。微量元素地球化学特征显示,蓬错富铌玄武岩形成于班公湖—怒江结合带中段的洋内弧,是adakite熔体交代的地幔楔橄榄岩部分熔融的产物,其源区处于石榴石—尖晶石稳定域,作用深度60~100 km。蓬错富铌玄武岩的发现和厘定,表明班公湖—怒江带中段在中—晚侏罗世经历了洋内俯冲消减过程,揭示了自北向南的洋内俯冲极性,也暗示了班公湖—怒江结合带多次俯冲改造的复杂性。     

论海沟—岛弧—弧后盆地复合体系的形成机制

海沟一岛孤一弧后盆地复合体系的成因与软流圈及岩石圈的流变性有关,以前者为主,并遵循流体运动的最小阻力原理。俯冲有两种类型:一是大洋岩石圈向大陆岩石圈的俯冲,二是大洋岩石圈之间的俯冲。弧后扩张及岛弧升高在时间上的滞后现象,也可以用此原理来说明。     

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。     

新疆富蕴扎河坝地区蛇绿岩带基本特征及大地构造意义

扎河坝地区蛇绿岩带属扎河坝—阿尔曼太蛇绿岩带之组成部分，主要由无序的蛇纹岩、变辉长岩、变辉绿岩及少量变质基性火山岩、硅质岩构成其主体。常见外来混杂岩块。据其组合特征可分为蛇绿岩、混杂岩两亚带。经初步研究认为：蛇绿岩为残留洋壳的一部分，其形成时代为奥陶纪—志留纪。加里东晚期洋壳发生转化，至泥盆纪发育一套中基性海相沟弧型火山岩组合。因此，该带及其北部至额尔齐斯带南缘地区，应为西伯利亚板块华力西增生带。     

Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge

The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.     

An overview of adakite petrogenesis

ADAKITE IS A PETROLOGIC TERM THAT DEFANT AND DRUM-MOND[1] FIRST INTRODUCED ~ 15 YEARS AGO TO REFER TO “VOL- CANIC OR INTRUSIVE ROCKS IN CENOZOIC ARCS ASSOCIATED WITH SUBDUCTION OF YOUNG (≤25 MA) OCEANIC LITHO- SPHERE”; THESE ROCKS ARE “CHARACTERIZED B…