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1.
《International Geology Review》2012,54(8):1020-1038
ABSTRACTThe La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende 40Ar/39Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous. 相似文献
2.
苏鲁超高压变质带胡家林超镁铁质岩成因及构造意义 总被引：1，自引：1，他引：0
胡家林超镁铁质杂岩体产于苏鲁超高压变质带中部,纯橄岩和(石榴)单斜辉石岩呈不连续透镜体产于蛇纹石化橄榄岩中。纯橄岩遭受了部分蛇纹石化(烧失量=6.6%～13.2%),全岩富集强相容元素(Ni、Cr、Co)和Ir族PGE(IPGE;Ir、Os、Ru)及高IPGE/PPGE值,亏损Al、Ti、V,具高Mg~#橄榄石(Fo=91.7～92.4)和高Cr~#(0.68～0.76)尖晶石。纯橄岩高耐熔地球化学及矿物化学特征和古老的大陆岩石圈地幔相一致,表明其原岩来源于弧前地幔,代表了华北克拉通古老的大陆岩石圈地幔残留。(石榴)单斜辉石岩全岩呈相对低含量的强相容元素(Cr、Ni、Co)和IPGE,高含量的Al、Ti、V和流体迁移元素(Sr、Pb和Ba),球粒陨石标准化REE配分图呈明显"上凸"型,具低Mg~#橄榄石(Fo=76.6～76.8)和低Al_2O_3(2.76%)和高SiO_2(54.56%～56.87%)的单斜辉石。全岩组成和矿物化学表明其原岩为俯冲带内超镁铁质火成堆晶岩,最初岩浆由地幔岩高程度部分熔融的熔体和俯冲带中富H_2O流体和/或熔体构成。(石榴)单斜辉石岩原岩曾被地幔流带入扬子大陆俯冲板片和上覆地幔楔之间的俯冲通道,经历了超高压变质作用和生成大量石榴石。(石榴)单斜辉石岩在折返过程中,与大陆岩石圈地幔楔剥离的蛇纹石化橄榄岩及纯橄岩相结合,形成超镁铁质杂岩体,整体被低密度的俯冲板片(主要由花岗质片麻岩和变质沉积岩组成)裹挟,折返至地壳浅部。 相似文献
3.
氦同位素研究确定,大别山榴辉岩等超高压岩石矿物并非来自地幔,而是生成于岩石圈地幔顶部。结合深部地球物理,提出一个岩石圈地幔顶部形成超高压矿物的模型。即表壳岩石俯冲到岩石圈地幔顶部,形成超高压变质岩,然后由于地壳隆升、剥蚀,出露到地表。冲入大别山地区岩石圈地幔顶部的表壳岩石之所以会形成超高压变质岩是因为它同时受到板块会聚的强大压力和蘑菇云地幔产生的高温。沿六安—黄石综合地球物理、地球化学剖面实测的热流剖面显示,南大别构造带的莫霍面温度达到1307℃,超高压变质作用所需要的高温条件至今依然存在。已有文献表明黏塑性的大陆板块在碰撞俯冲时,岩石圈地幔的变形远比通常认定的那种刚性板块俯冲要复杂。俯冲呈对冲形式,方向大都向下,在岩石圈地幔中,俯冲板块和制动板块像麻花一样相互楔入,在深部甚至改变俯冲方向,制动板块反而向俯冲板块俯冲。当岩石圈地幔顶部局部熔融时,无疑俯冲物质将向局部熔融层扩散,在高温高压下发生超高压变质作用。大别山变形晚期,在核部形成“背形穹隆”。将形成于岩石圈地幔顶部的超高压变质岩带到地表,接受剥蚀而出露地表。已有资料表明,全球主要超高压变质岩的分布带与古特提斯洋分布有关,古特提斯洋碰撞带是全球最长的一条陆内碰撞俯冲带。它们是否均为黏塑性板块之间的软碰撞、在邻近碰撞带的岩石圈地幔顶部是否都有高温的区域则尚待验证。 相似文献
4.
硼的地球化学性质及其在俯冲带的循环与成矿初探 总被引：2，自引：1，他引：1
硼是广泛应用于化工、农业、材料科学及核工业领域的重要元素。硼与氢的核聚变反应是未来具备运用潜力的清洁能源。硼作为典型的亲石元素,是高度不相容元素。硼元素容易富集于蚀变洋壳及蛇纹石化地幔橄榄岩中。而在板块俯冲过程中,由于硼具有强的流体活动性,会优先赋存于流体中。因此,当蛇纹石化的大洋岩石圈及覆于其上的沉积物在俯冲过程中发生脱水,这使得弧前地幔楔发生大规模的蛇纹石化。此时大量硼元素很可能随俯冲流体释放并封存于弧前地幔楔中。目前已发现的超大型硼矿床主要位于聚合型板块边缘,尤其土耳其拥有世界上最大的硼酸盐储量。我们推测这些矿床的形成基础条件之一可能与弧前高度蛇纹石化的地幔楔有关。尤其是在洋 陆俯冲环境,弧前蛇纹岩或蛇绿混杂岩首先通过俯冲侵蚀再循环到火山弧岩浆中,使得岩浆更富集硼。随后弧火山喷发大量富硼的火山岩、岩浆热液及水气。在岩浆冷却过程中,硼元素析出、沉淀于火山表面,并伴随风化、侵蚀过程汇聚至碰撞造山带的封闭湖盆之中。此外,干冷的气候条件下也进一步促进了硼的成矿。我国具有形成大型、超大型硼矿的地质条件,应加大研究及探勘力度,并适当购买硼作为战略储备。 相似文献
5.
Along the Rio Muni transform margin, the transition from continental to oceanic crust occurs across a region of approximately 75-km width. The crust in this transition region, termed proto-oceanic crust (POC), is neither purely oceanic nor continental in composition and structure. Improved seismic reflection images from the PROBE deep-imaging dataset, combined with gravity modelling, have shed new light on the structural architecture of the margin and the composition of the POC. On these newly migrated seismic reflection sections, four fracture zones associated with large steps in the Moho are identified, splitting the POC into three segments. Models in which these segments are composed of oceanic or stretched continental crust do not provide satisfactory predictions of the gravity anomaly. A model of serpentinized peridotite for two segments of POC, with the third segment composed of oceanic crust in between, does satisfy the observed gravity anomaly. Three alternative geological scenarios are proposed to explain the segmentation and composition of the POC: (a) serpentinized upper mantle becoming unroofed and emplaced at basement surface level along detachment surfaces confined to discrete segments by the fracture zones, (b) oblique-slip on transform faults that allow the circulation of water into the mantle and emplacement of serpentinized upper mantle material; or (c) intense faulting of anomalous oceanic crust as a result of magma depletion allowing hydrothermal circulation and the emplacement of serpentinized peridotites. 相似文献
6.
In order to contribute to a better understanding of exhumation related retrogression processes ultrahigh-pressure (UHP) mafic and ultramafic rocks from the Chinese Continental Scientific Drilling (CCSD) in the Maobei eclogite body of the Sulu ultrahigh-pressure metamorphic belt in eastern China were studied for their magnetofabrics. Variably retrogressed eclogites and serpentinized ultramafic rocks were retrieved from the depth interval of 100 to 1000 m of the borehole. A vein network of irregular shaped veins with a retrograde metamorphic assemblage cuts across the eclogite foliation at both low and high angles. SEM imaging of the eclogites documents that magnetite associated with retrograde pargasitic amphiboles developed around shape-preferred garnet. SEM imaging of the serpentinized ultramafic rocks documents that magnetite rims grew around shape-preferred garnet and that magnetite formed within a mesh texture related to serpentinization. Syn-serpentinization magnetite growth increased bulk susceptibilities, but reduced anisotropy. Maximum susceptibility axes from both eclogites and serpentinized garnet-peridotites trend N-S, i.e. parallel with the stretching lineation as defined by olivine and omphacite grains. This implies that the magnetic fabric mimics the UHP fabric and survived retrogression and that the fabric is inherited from the UHP stage. As a consequence, retrogression was not associated with substantial ductile deformation and the mafic-ultramafic Maobei body behaved as a rigid body within a ductile deforming quartzo-feldspathic matrix during exhumation. Internal strain is restricted to brittle fracturing, associated fluid circulation and vein formation facilitating retrograde reactions in the mafic-ultramafic rocks. 相似文献
7.
John Wakabayashi 《International Geology Review》2017,59(5-6):599-620
ABSTRACTSedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks. 相似文献
8.
Bassam A. Abuamarah 《International Geology Review》2020,62(5):630-648
ABSTRACTThe Bir Umq ophiolite is one of the most important ophiolitic successions in the Arabian Shield, and represents an excellent case for the study of the tectonomagmatic evolution of the earliest Precambrian events in the juvenile part of the Arabian-Nubian Shield (ANS). It is a dismembered ophiolite, which includes a serpentinized peridotite with small amounts of gabbro and mélange, and is overlain by the Sumayir formation. The mantle section of the Bir Umq ophiolite has been pervasively sheared and folded during its emplacement and is extensively serpentinized, carbonated and silicified, resulting in the common development of magnesite and listwaenite along the shear zones. Listwaenite occurs in the form of upstanding ridges due to its resistance to erosion. Antigorite is the main serpentine mineral, which, however, has low amounts of lizardite and chrysotile, indicating that the present serpentinites formed by prograde metamorphism. The ophiolitic rocks of Bir Umq have undergone regional metamorphism up to the greenschist to amphibolite facies. The presence of mesh and bastite textures indicates harzburgite and dunite protoliths. The serpentinized peridotite preserves rare relicts of primary minerals such as olivine, pyroxene and Cr-spinel. The serpentinized ultramafics of Bir Umq have high Mg# [molar Mg/(Mg+Fe2+); 0.90–0.93), low CaO, and Al2O3 contents similar to that of the environment of the suprasubduction zone. Additionally, they are characterized by the depletion of some compatible trace elements (e.g., Nb, Sr, Ta, Zr, Hf and REE), but show a wide variation in the Rb and Ba. Moreover, they are enriched in some elements that have affinities for Mg-rich minerals such as Ni, Cr, V, and Co. Fresh relics of olivine have high Fo (av. 0.91) and NiO (av. 0.42) contents, similar to those in the mantle olivine. The fresh Cr-spinel has high Cr# (0.68) and low TiO2 content (av. 0.11), similar to those in modern fore-arc peridotites. The composition of both orth- and clinopyroxenes confirms the fore-arc affinity of the studied ultramafics. The present study indicates that the protoliths of the serpentinized ultramafics of Bir Umq have high partial melt degrees, which is consistent with the characteristics of ultramafic rocks formed in a subarc environment (fore-arc) within a suprasubduction zone system. 相似文献
9.
内蒙古索伦山地区出露蛇绿岩,其研究对探讨古亚洲洋演化具有重要意义。对内蒙古索伦山地区蛇绿岩进行了系统的调查和研究,探讨了其就位机制与时限。 结合索伦山蛇绿岩地质特征和区域地质背景综合分析,认为研究区蛇绿岩组合包括地幔与洋壳组分。索伦山地区蛇绿岩存在较为完整的蛇绿岩组合模式,出露地幔岩石组合为蛇纹石化纯橄榄岩、蛇纹石化二辉-方辉橄榄岩、橄榄辉石岩和硅化碳酸盐化蚀变超基性岩(风化壳)等。蛇绿岩组合中洋壳组分为辉长岩、辉绿岩、玄武岩和硅质岩。蛇绿岩就位机制划分为4种,即碰撞仰冲型、增生底垫型、俯冲剥离型和角流型。其中,俯冲剥离型就位机制表现为岩石组合齐全完整的特征,产出形态为岩块、岩片,其中岩块、岩片与基质为构造断层接触;在俯冲带近大陆一侧常形成岛弧岩浆岩等特征。索伦山蛇绿岩地质特征与俯冲剥离型就位机制特征完全相符,故索伦山蛇绿岩就位机制大致为洋中脊俯冲剥离型。根据大洋岩石圈形成之后在10 Ma之内就位这一原则,结合索伦山地区辉长岩SHRIMP锆石U Pb年龄为(2807±53) Ma,认为索伦山蛇绿岩就位时限在270 Ma左右。 相似文献
10.
榴辉岩的地震波性质：对苏鲁超高压变质带地壳成分和折返机制的探讨 总被引：5，自引：3，他引：5
本文总结了榴辉岩的高温高压弹性波速测量结果,并将其应用于苏鲁超高压变质带地震资料的解释。由于榴辉岩具有高密度和高波速,它们和长英质片麻岩、大理岩、石英岩、角闪岩、麻粒岩、蛇纹石化橄榄岩的界面可以产生强反射。如果俯冲的陆壳物质以榴辉岩与围岩互层的形式在上地幔保留下来,就可能在造山带的上地幔产生地震反射。根据CCSD孔区地震剖面所建立的地壳成分模型表明:苏鲁超高压带地壳浅部的高速层可归因于夹在花岗质片麻岩、副片麻岩、角闪岩等岩石中的榴辉岩和超基性岩;中地壳主要由中酸性片麻岩、斜长角闪岩和副片麻岩组成;下地壳以中基性麻粒岩为主。在该超高压变质带现今的深部地壳,榴辉岩含量很少或几乎没有。因此,折返的超高压变质岩是以构造岩片的形式沿一系列剪切带逆冲并覆盖在正常的中下地壳之上,深部榴辉岩的缺乏可能与下地壳拆沉作用无关。 相似文献
11.
The main hole (MH), and pre-pilot holes PP1, and PP3 of the Chinese Continental Scientific Drilling Project (CCSD) penetrated three different garnet peridotite bodies in the Sulu ultrahigh pressure (UHP) metamorphic belt, which are 80 m, 120 m, and 430 m thick, respectively. The bodies occur as tectonic blocks hosted in eclogite (MH peridotite) and gneisses (PP1 and PP3 peridotites). The peridotites in the MH are garnet wehrlites, whose protoliths were ultramafic cumulates based on olivine compositions (Fo79-89) and other geochemical features. Zoned garnet and omphacite (with 4-5 wt.% Na2O) are typical metamorphic minerals in these rocks, and, along with P-T estimates based on mineral pairs, suggest that the rocks have undergone UHP metamorphism. SHRIMP U-Pb isotope dating of zircon from the garnet wehrlite yielded a Paleozoic protolith age (ca. 346-461 Ma), and a Mesozoic UHP metamorphic age (ca. 220-240 Ma). The peridotites in PP1 consist of interlayered garnet (Grt)-bearing and garnet-free (GF) peridotite. Both types of peridotite have depleted mantle compositions (Mg# = 90-92) and they display transitional geochemical features. The intercalated layers probably reflect variations in partial melting rather than pressure variations during metamorphism, and the garnets may have been formed by exsolution from orthopyroxene during exhumation. These peridotites were probably part of the mantle wedge above the subduction zone that produced the UHP metamorphism and thus belonged to the North China Block before its tectonic emplacement. The exhumation of the subducted Yangtze Block brought these mantle fragments to shallow crustal levels. The ultramafic rocks in PP3 are dominantly dunite with minor garnet dunite. Their high Mg# (92-93) and relatively uniform chemical compositions indicate that they are part of a depleted mantle sequence. The presence of garnet replacing spinel and enclosing pre-metamorphic minerals such as olivine, clinopyroxene and spinel suggests that these rocks have undergone progressive metamorphism. SHRIMP U-Pb isotope dating of zircon from these rocks yielded two age groups: 726 ± 56 Ma for relic magmatic zircon grains and 240 ± 2.7 Ma for the newly formed metamorphic zircon. The older group is similar in age to granitic intrusions within the Dabie-Sulu belt, suggesting that the PP3 garnet peridotite may record the early emplacement of the peridotite into the crust. The younger dates coincide with the age of UHP metamorphism during continent-continent collision between the Yangtze and North China Blocks, suggesting that these peridotites were subducted to depths equivalent to the coesite facies and later exhumed. Thus, the garnet peridotites in the CCSD cores include both ultramafic rocks that existed originally in the subducted plate and rocks from the mantle wedge above the subducted plate, i.e., part of the North China Block. 相似文献
12.
Songbai Peng T.M. Kusky X.F. Jiang L. Wang J.P. Wang H. Deng 《Gondwana Research》2012,21(2-3):577-594
We report the presence of a Grenvillian ophiolite on the northern margin of the Yangtze craton, drastically changing current ideas about South China's role in plate reconstructions of the Rodinia supercontinent. Strongly deformed amphibolites that locally show relict pillow lavas, isotropic and layered metagabbro, diabase dikes, serpentinized dunite and harzburgite with podiform chromite are dated at circa 1100–985 Ma (U–Pb zircon). The ophiolite is structurally dismembered and thrust over the Proterozoic shelf sequence that covers the north margin of the Yangtze craton, and overrode a flysch to conglomerate-wildflysch unit shed from the ophiolite and a magmatic arc terrane and deposited on the older Yangtze carbonate platform. The youngest clasts in the conglomerate are circa 861–813 Ma (U–Pb zircon), giving a maximum age for ophiolite emplacement. Fine-grained layered amphibolites exhibit slightly depleted-flat type REE curves with no obvious Eu anomalies, and are N-MORB type tholeiites. Metagabbro has typical cumulate textures, flat REE distributions and obvious positive Eu anomalies. The REE characteristics of serpentinized dunites show a U-shape of slight loss of middle REE, representing cumulates metasomatized by LREE slightly enriched mantle. All these features indicate that the metamafic–ultramafic rocks from the Proterozoic Miaowan Formation form a structurally dismembered ophiolite resting above an ophiolitic wildflysch, sitting on top of the Proterozoic shelf sequence on the Yangtze craton. The ophiolite is contemporaneous with an arc sequence preserved to the north on the edge of the Yangtze craton, suggesting that the entire ophiolitic forearc–arc was accreted to the Yangtze craton between 1000 and 850 Ma. Xenocrystic zircons in granite clasts in the basal wildflysch unit have ages consistent with Australian affinity, and detrital zircons in the arc sequence also show derivation from Australia, suggesting that the arc formed on the Australian segment of Rodinia before collision with the Yangtze craton. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of a Neoproterozoic oceanic basin on the north margin of the Yangtze craton, and demonstrates that the Yangtze craton first collided with Rodinia on its northern margin, with subsequent accretion of the Cathaysian block on the southern margin of the craton. 相似文献
13.
扬子陆核崆岭高级变质地体内出露一套强变形的基性-超基性岩岩石组合, 主要呈似层状、透镜状分布于崆岭群中, 该套变基性-超基性岩组合对扬子陆块早期构造演化过程具有重要意义.通过同位素稀释法(isotope dilution thermal ionization mass spectrometry, 简称ID-TIMS)获得该套岩石组合中蛇纹石化方辉橄榄岩全岩Sm-Nd等时线年龄为1063±12Ma, 说明Sm-Nd同位素体系可用于对超低含量、发生强蚀变作用的超基性岩样品进行定年.样品Nd同位素组成相对均一(εNd(t)值为6.90~7.32), 表明形成于封闭体系中, 其对应Nd同位素两阶段模式年龄为1.13~1.09Ga, 与形成年龄接近, 说明来自亏损软流圈地幔部分熔融.结合区域上已有的中元古代末期到新元古代早期构造岩浆事件研究, 认为在该时期扬子陆块可能由多个微陆块组成, 就扬子陆核而言, 其与扬子陆块西侧之间很可能存在分隔的大洋. 相似文献
14.
中国大陆科学钻探工程的科学目标及初步成果 总被引：34，自引：20，他引：34
中国大陆科学钻探工程的主孔位于苏鲁超高压变质带南部的东海县，其穿过的岩石曾位于板块会聚边界的地幔深处，是研究大陆深俯冲及地幔动力学的最佳地区。中国大陆科学钻探的科学目标旨在通过对钻孔中获取的全部连续岩心、液态和气态样品及原位测井数据进行的全方位测量与综合研究，建立5000m孔深的各类多学科精细剖面，再造北中国板块与扬子板块会聚边界深部三维物质的组成和分布及三维结构构造；阐明板块会聚边缘的深部流体作用、壳一幔相互作用及地幔中物质循环和流变学；寻找超深地幔条件下形成的特征矿物，揭示超高压变质成矿机理；建立结晶岩地区地球物理理论模型和解释标尺；揭示超高压变质岩石的形成与折返模型及板块会聚边界的深部动力学机制。通过5km深孔营造的特殊地下空问，研究现代地壳的物理、化学及生物作用，并将建立亚洲第一个大陆科学钻探深孔长期观察实验站。中国大陆科学钻探工程已取得初步成果。主孔2000m岩心的深度和方位准确归位；建立了岩性、地球化学、构造、矿化、岩石物性、地下流体及各类测井等精细剖面；揭示了发现除超镁铁质岩外的各类岩石中的柯石英；通过SHRIMP测年准确确定超高压及退变质事件的年龄；初步揭示了超高压及退变质过程中流体的变化及水一岩作用；划分了构造岩片单元，确定了其边界的韧性剪切带性质，并发现早期构造信息；建立的随岩性变化的弹性波速度和热导率连续剖面对地震反射和热结构提供了岩石物性的制约；VSP地震剖面揭示了孔区深部的构造岩片结构，以及强地震反射层和大型韧性剪切带有关；发现地下特殊气体一甲烷、二氧化碳及氦气的异常，以及气体异常越往深部越频繁出现的规律。经DNA分析，在超高压岩心中发现在极端条件下形成的微生物。 相似文献
15.
The Guleman ophiolite,one of the most important ophiolitic massifs of the Southeast Anatolian Ophiolitic Belt,consists of a core of serpentinized mantle rocks overlain by an ultramafic sequence,layered and isotropic gabbro,and sheeted dykes.The ophiolite structurally overlies the Lower Miocene Lice Formation and is overlain by young sandstones and shales of the Upper Maashtrichtian-Lower Eocene Hazar Complex and Middle Eocene Maden Complex.The Guleman ophiolite tectonically overlain by Precambrian to Upper Triassic Bitlis metamorphic massif.The mantle peridotites compose mainly of fresh and in place serpentinized harzburgite tectonite with local bands and lenses of dunites with large-sized chromitite pods.The Guleman peridotites commonly show porphyroclastic texture,high-temperature fabrics such as kink-bands in olivines.According to microprobe analyses,the harzburgite and dunite have low Ca O and Al2O3 abundance similar to Mariana forearc,and their average Cr-(=Cr/(Cr+Al)atomic)ratio of Cr-spinelsis surprisingly high(0.63)besides Fo content of olivine is between 90.9 to 92.3 in peridotites.According to Mg#(Mg/(Mg+Fe2+))versus Cr#in spinel diagram,the degree of partial melting is higher than 35%and spinel values plot in the forearc peridotites field.The Gulemanharzburgites have low Ca O,Al2O3 and Ti O2 contents in orthopyroxene and clinopyroxene lammelles,resembling those of depleted harzburgites from modern forearcs and different from moderately depleted abyssal peridotites.Consequently,we propose that the Guleman peridotites form in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between Arabian and Anatolian plates. 相似文献
16.
Tomographic evidence for the subducting oceanic crust and forearc mantle serpentinization under Kyushu, Japan 总被引：1，自引：0，他引：1
We determined high-resolution three-dimensional P- and S-wave velocity (Vp, Vs) structures beneath Kyushu in Southwest Japan using 177,500 P and 174,025 S wave arrival times from 8515 local earthquakes. A Poisson's ratio structure was derived from the obtained Vp and Vs values. Our results show that significant low-Vp, low-Vs and high Poisson's ratio zones are extensively distributed along the volcanic front in the uppermost mantle, which extend and dip toward the back-arc side in the mantle wedge. In the crust, low-Vp, low-Vs and high Poisson's ratio anomalies exist beneath the active volcanoes. The subducting Philippine Sea slab is clearly imaged as a high-Vp, high-Vs and low Poisson's ratio zone from the Nankai Trough to the back-arc. A thin low-velocity zone is detected above the subducting Philippine Sea slab in the mantle wedge, and earthquakes in the upper mantle are distributed along the transition zone between this thin low-velocity zone and the high-velocity Philippine Sea slab, which may imply that oceanic crust exists on the top of the slab and the forearc mantle wedge is serpentinized due to the slab dehydration. The seismic velocity of the subducting oceanic crust with basaltic or gabbroic composition is lower than that of the mantle according to the previous studies. The serpentinization process could also dramatically reduce the seismic velocity in the forearc mantle wedge. 相似文献
17.
西藏罗布莎不同类型铬铁矿的特征及成因模式讨论 总被引：6，自引：2，他引：4
蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿是铬的主要来源。已有的研究表明,豆荚状铬铁矿形成于洋中脊或俯冲带的浅部地幔环境。但随着近些年在豆荚状铬铁矿及围岩地幔橄榄岩中不断发现金刚石等深部矿物,人们也开始质疑豆荚状铬铁矿的浅部成因理论。本文系统研究了西藏雅鲁藏布江蛇绿岩带东段的罗布莎豆荚状铬铁矿床,识别出两类铬铁矿,一类以方辉橄榄岩为围岩的致密块状铬铁矿(Cr1#),另一类是以纯橄岩壳为围岩的浸染状铬铁矿(Cr2#)。两类铬铁矿在铬尖晶石的矿物化学成分、PGE和Re-Os同位素特征上存在较大差别,属不同演化过程的结果。地幔橄榄岩的地球化学特征指示罗布莎橄榄岩中存在由低铬且轻稀土亏损和高铬且轻稀土富集的两类方辉橄榄岩。在此基础上,提出豆荚状铬铁矿为多阶段形成的新认识,经历了早期俯冲至地幔过渡带(410~660km)的陆壳和洋壳物质被脱水和肢解,过渡带产生的热和流体促成了地幔的熔融和Cr的释放和汇聚;铬铁矿浆在地幔柱/地幔对流驱动下,运移至过渡带顶部冷凝固结,并有强还原性的流体进入,后者携带了深部形成的金刚石、斯石英等高压矿物,并进入"塑性-半塑性地幔橄榄岩"中;随着物质向上移动,深度降低,早期超高压相矿物发生相变,如斯石英转变成柯石英,高压相的铬铁矿中出溶成柯石英和单斜辉石;在侵位过程和俯冲带环境,含水熔体与方辉橄榄岩反应形成了不含超高压矿物的规模相对较小的浸染状铬铁矿(Cr2#)及纯橄岩壳。 相似文献
18.
The microstructures, major- and trace-element compositions of minerals and electron backscattered diffraction (EBSD) maps of high- and low-Cr# [spinel Cr# = Cr3+/(Cr3+ + Al3+)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo-Tethyan Ocean. The high-Cr# (0.77–0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction-produced magmas in a shallow, mature mantle wedge. Some high-Cr# chromitites show crystal-plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high-Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low-Cr# (0.49–0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB-like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high-Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low-Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo-Tethyan subduction at ~130–120 Ma. This two-episode subduction model can provide a new explanation for the coexistence of high- and low-Cr# chromitites in the same volume of ophiolitic mantle. 相似文献
19.
《International Geology Review》2012,54(12):1506-1522
Garnet orthopyroxenites from Maowu (Dabieshan orogen, eastern China) were formed from a refractory harzburgite/dunite protolith. They preserve mineralogical and geochemical evidence of hydration/metasomatism and dehydration at the lower edge of a cold mantle wedge. Abundant polyphase inclusions in the cores of garnet porphyroblasts record the earliest metamorphism and metasomatism in garnet orthopyroxenites. They are mainly composed of pargasitic amphibole, gedrite, chlorite, talc, phlogopite, and Cl-apatite, with minor anhydrous minerals such as orthopyroxene, sapphirine, spinel, and rutile. Most of these phases have high XMg, NiO, and Ni/Mg values, implying that they probably inherited the chemistry of pre-existing olivine. Trace element analyses indicate that polyphase inclusions are enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and high field strength elements (HFSE), with spikes of Ba, Pb, U, and high U/Th. Based on the P–T conditions of formation for the polyphase inclusions (?1.4 GPa, 720–850°C), we suggest that the protolith likely underwent significant hydration/metasomatism by slab-derived fluid under shallow–wet–cold mantle wedge corner conditions beneath the forearc. When the hydrated rocks were subducted into a deep–cold mantle wedge zone and underwent high-pressure–ultrahigh-pressure (HP–UHP) metamorphism, amphibole, talc, and chlorite dehydrated and garnet, orthopyroxene, Ti-chondrodite, and Ti-clinohumite formed during prograde metamorphism. The majority of LILE (e.g. Ba, U, Pb, Sr, and Th) and LREE were released into the fluid formed by dehydration reactions, whereas HFSE (e.g. Ti, Nb, and Ta) remained in the cold mantle wedge lower margin. Such fluid resembling the trace element characteristics of arc magmas evidently migrates into the overlying, internal, hotter part of the mantle wedge, thus resulting in a high degree of partial melting and the formation of arc magmas. 相似文献
20.
The microstructures, major‐ and trace‐element compositions of minerals and electron backscattered diffraction (EBSD) maps of high‐ and low‐Cr# [spinel Cr# = Cr3+/(Cr3++Al3+)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo‐Tethyan Ocean. The high‐Cr# (0.77‐0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction‐produced magmas in a shallow, mature mantle wedge. Some high‐Cr# chromitites show crystal‐plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high‐Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low‐Cr# (0.49‐0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB‐like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high‐Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low‐Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo‐Tethyan subduction at ～130‐120 Ma. This two‐episode subduction model can provide a new explanation for the coexistence of high‐ and low‐Cr# chromitites in the same volume of ophiolitic mantle. 相似文献