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1.
The structure and tectonic position of the Neoproterozoic Central Taimyr accretionary belt of northwestern Siberia is dominated by the Faddey and Mamont-Shrenk granite-gneiss terranes, ophiolites, and back-arc volcanic rocks. Granites in the granite-gneiss terranes are S-type and formed between 900 and 850 Ma from 1.9 to 1.8 Ga continental crust. U–Pb and Sm–Nd isotopic studies show that the plagiogranites of the Chelyuskin ophiolite belt formed between 850 and 740 Ma. The ophiolite complex was metamorphosed to garnet amphibolite grade around 600 Ma, which is considered to be when the accretionary belt was obducted onto the Siberian continent. Comparison of principal structures of the Central Taimyr accretionary belt with similar structures in Arctic countries permits definition of the principal stages of the Neoproterozoic destruction of the supercontinent Rodinia, in the Arctic region.  相似文献   

2.
新疆西准噶尔地区是古生代经过俯冲-增生形成的复合造山带,该地区分布有多条蛇绿岩带,其中之一的西准噶尔达拉布特蛇绿岩被认为是最大的一条蛇绿岩带,可能代表了古亚洲洋壳的残余。本文的资料显示蛇绿岩带内的镁铁质岩呈现出N-MORB、E-MORB和似OIB的地球化学特征,通过对阿克巴斯套岩体中的浅色辉长岩LA-ICP-MS锆石年龄测定,获得达拉布特蛇绿岩E-MORB型镁铁质岩的年龄为302±1.7Ma。鉴于达拉布特蛇绿岩中E-MORB和似OIB型镁铁质岩成因的复杂性,结合前人研究成果,对辉长岩锆石U-Pb年龄所代表的意义存在两种可能性：（1）E-MORB型和似OIB型镁铁质岩可能是弧后盆地扩张后期的产物,代表蛇绿岩的年龄,其表明西准噶尔地区可能晚石炭纪还有洋盆存在;（2）E-MORB型镁铁质岩是蛇绿岩消亡阶段由于扩张脊和俯冲带碰撞作用而形成的弧前海山,形成时代晚于达拉布特主体蛇绿岩,但其成因与蛇绿岩的演化密切相关。本文侵向于第二种可能性,认为新疆北部晚石炭-早二叠可能仍存在活动陆缘,俯冲作用仍然存在,扩张脊俯冲形成的板片窗效应导致地幔楔、俯冲板片和沉积物等熔融促使基性岩浆向长英质酸性岩浆转变,从而引发了二叠纪大规模玄武质岩浆底侵,导致了该时期的构造-岩浆-成矿-造山作用的发生。  相似文献   

3.
The Makran mountain range in Southern Iran has been entirely covered by regional mapping on a scale of 1:250,000, but, in contrast to the mapping of Oman on the other side of the Gulf, the recent mapping is little known. The Cenozoic accretionary prism in the south fronts onto a Mesozoic melange zone representing earlier subduction and to the north of this is an eastwards tapering microcontinental sliver, which is followed by a zone of largely intact Mesozoic ophiolite complexes. To the north of this lies the Jaz Murian depression, a desert expanse with no bedrock outcrop. The Sanandaj-Sirjan/Bajgan–Dur-Kan microcontinental sliver, which extends from Turkey through the Zagros to the Makran, is one of a number of microcontinents which detached from Gondwana no later than the early Triassic (NW Iran, Yazd, Tabas, Lut, Birk, Helmand, Farah). During the Mesozoic, these were enclosed by tracts of ocean now represented by ophiolites and the sectors closed at different times, ranging from early Cretaceous to end-Palaeogene. An analogy is drawn with the present-day Mediterranean. The “Sistan Ocean”, in the east, was infilled with an immense thickness of abyssal turbidites during the Eocene before it closed. The field evidence indicates that there was a second Cenozoic accretionary prism here, but sedimentation ceased after the Eocene, on its collision with the back of the main Makran prism. This eastern Sistan trough, now completely occluded, is compared with the still open tract separating the Makran from Oman at the present time and its northern extension which has been similarly occluded by collision beneath the Zagros. Some residual, very shallow inland seas persisted inland from the Makran through the Neogene. The occurrence of blueschists in the inner ophiolite belt of the Makran is discussed. Abyssal turbidite sedimentation continued in the main southern zone of the Makran into the early Miocene, to be succeeded by thick shallow water sequences including reefal limestones. Intense tectonic dislocation at the end of the Miocene was accompanied by a shift of the subduction front to its present position out in the Indian Ocean and uplift, leading to the deposition of spectacular Pliocene–Pleistocene fanglomerates.  相似文献   

4.
The recent fissural volcanism of Kouh-e-Tchah-e-Shahi is undifferentiated and consists mainly in andesitic basalt, rich in olivine, diopside, pigeonite, and labradorbytownite. The evolution, which leads to residual liquids, shows glasses unmiscible between one another. The volcanic arc of Bazman-Taftan and Kouh-e-Tchah-e-Shahi is interpreted as a subducting volcanism of an oceanic crust of the Indian Ocean, deeply diving from Jaz Murian behind the accretional prism of Makran.  相似文献   

5.
The Papuan Ultramafic Belt (PUB) ophiolite is former oceanic crust and upper mantle emplaced onto continental crust in Papua New Guinea (PNG) in a zone of general convergence between the Pacific and Australian plates. The metamorphic sole beneath the ophiolite is best exposed in the Musa–Kumusi divide and comprises a 40- to 300-m-thick body of granulite and amphibolite facies rocks. Geochronological studies on the metamorphic sole, using amphiboles from the granulites and amphibolites, yield measured K–Ar ages ranging from 65.0±0.7 to 57.2±0.6 Ma and average 40Ar–39Ar direct total fusion ages ranging from 67.0±0.7 to 59.5±0.2 Ma. Five of the six 40Ar–39Ar plateau ages, derived from age spectra, lie between 58.6±0.2 and 57.8±0.2 Ma, with an overall mean age of 58.3±0.4 Ma. The large spread in measured K–Ar and 40Ar–39Ar total fusion ages is thought to be caused by the presence of variable amounts of excess argon. The mean plateau age for five samples of 58.3±0.4 Ma is interpreted to mark the time of cooling of the metamorphic sole following peak metamorphism. We suggest that the development of the metamorphic sole and emplacement of the PUB ophiolite onto the PNG crust occurred in a relatively short time interval in the Paleocene.  相似文献   

6.
The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(LaN/SmN)ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(LaN/SmN)ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent.  相似文献   

7.
Part II of this paper reports geochemical and Nd isotope characteristics of the volcanogenic and siliceous-terrigenous complexes of the Lake zone of the Central Asian Caledonides and associating granitoids of various ages. Geological, geochronological, geochemical, and isotopic data were synthesized with application to the problems of the sources and main mechanisms of continental crust formation and evolution for the Caledonides of the Central Asian orogenic belt. It was found that the juvenile sialic crust of the Lake zone was formed during the Vendian-Cambrian (approximately 570–490 Ma) in an environment of intraoceanic island arcs and oceanic islands from depleted mantle sources with the entrainment of sedimentary crustal materials into subduction zones and owing to the accretion processes of the amalgamation of paleoceanic and island arc complexes and Precambrian microcontinents, which terminated by ∼490 Ma. The source of primary melts for the low-Ti basalts, andesites, and dacites of the Lake zone ophiolites and island arc complexes was mainly the depleted mantle wedge above a subduction zone. In addition, an enriched plume source contributed to the genesis of the high-Ti basalts and gabbroids of oceanic plateaus. The source of terrigenous rocks associating with the volcanics was composed of materials similar in composition to the country rocks at a minor and varying role of ancient crustal materials introduced into the ocean basin owing to the erosion of Precambrian microcontinents. The sedimentary rocks of the accretionary prism were derived by the erosion of mainly juvenile island arc sources with a minor contribution of rocks of the mature continental crust. The island arc and accretion stages of the development of the Lake zone (∼540–590 Ma) were accompanied by the development of high- and low-alumina sodic granitoids through the melting at various depths of depleted mantle reservoirs (metabasites of a subducted oceanic slab and a mantle wedge) and at the base of the island arc at the subordinate role of ancient crustal rocks. The melts of the postaccretion granitoids of the Central Asian Caledonides were derived mainly from the rocks of the juvenile Caledonian crust at an increasing input of an ancient crustal component owing to the tectonic mixing of the rocks of ophiolitic and island arc complexes and microcontinents. The obtained results indicate that the Vendian-Early Paleozoic stage of the evolution of the Central Asian orogenic belt was characterized by the extensive growth of juvenile continental crust and allow us to distinguish a corresponding stage of juvenile crust formation.  相似文献   

8.
张能  李剑波  杨云松  那福超 《岩石学报》2012,28(4):1291-1304
弯岛湖蛇绿混杂岩带是金沙江缝合带西段的重要组成部分。蛇绿岩混杂于上三叠统变质碎屑岩夹变质火山岩中,成份主要为镁铁质-超镁铁质杂岩,岩石类型有变质橄榄岩、变质堆晶质辉长岩及其伴生的岛弧型花岗岩系。岩石化学及地球化学特征表明:蛇绿岩主要为低Ti(岛弧-弧后)型、富集型洋中脊(E-MORB)拉斑玄武岩;与之共(伴)生的基性喷出岩、辉绿岩脉属板内洋岛型裂谷型碱性玄武岩及其过渡类型系列。在变质辉长岩获得全岩Sm-Nd等时线年龄值为232±11Ma,代表了镁铁质-超镁铁质杂岩的形成年龄,可能为洋壳初始俯冲变质的时间。在蛇绿岩上覆的硅质岩中发现有中三叠世拉丁晚期至晚三叠世卡宁早期的放射虫化石,表明弯岛湖镁铁质-超镁铁质杂岩可能形成于中三叠世多岛洋盆或弧后盆地构造环境。  相似文献   

9.
The Hegenshan ophiolite in the Inner Mongolian-Daxinganling Orogenic Belt (IMDOB), northern China, consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, mafic lavas and dikes, intruded by younger granodiorite dikes. The ultramafic rocks are highly depleted, serpentinized harzburgites with minor dunite, characterized by relative enrichment in large ion lithophile elements (LILE, e.g., Ba and Rb) and light rare earth elements (LREE). They are interpreted to be oceanic mantle that has undergone extensive melt extraction and variable degrees of metasomatism. The cumulate rocks consist mainly of gabbro and troctolite with LREE-depleted chondrite-normalized REE patterns showing significant positive Eu anomalies. They are enriched in LILE, depleted in Nb, and have high positive εNd(t) (+8 to +11), suggesting derivation from a subduction-modified N-MORB-like source. The gabbros and mafic dikes have essentially the same age (295 ± 15 and 298 ± 9 Ma, respectively). The mafic dikes have flat to LREE-depleted, chondrite-normalized REE patterns, are depleted in Nb, enriched in LILE and have N-MORB-type Nd isotopic signatures (εNd(t) = +8.1 to +10). The mafic lavas, erupted at 293 ± 1 Ma, can be divided into two groups; one composed of strongly deformed metabasalts similar in chemical and Nd–Sr isotopic compositions to the mafic dikes, and the other composed of undeformed and unmetamorphosed basalts with oceanic island basalt (OIB)-like trace element signatures and Nd isotopic compositions. The granodiorite dikes, which were intruded at 244 ± 4 Ma, have LREE-enriched, chondrite-normalized REE patterns with no Eu anomalies. Their abnormally high εNd(t) values (+6.3 to +6.8) and low ISr (0.70412 to 0.70450) suggest formation from melts derived from thickened oceanic crust during or shortly after closure of the Paleo-Asian Ocean. The structure, lithology and geochemistry of the Hegenshan ophiolite suggest that it is a Cordilleran-type body formed in a supra-subduction zone (SSZ) environment and amalgamated by collision of several fragments of Paleo-Asian lithosphere. Final emplacement and amalgamation occurred in the latest Permian or earliest Triassic.  相似文献   

10.
张天羽  李才  苏犁  解超明  吴彦旺  王明 《地质通报》2014,33(11):1662-1672
在藏北羌塘中部日湾茶卡地区发现蛇绿混杂岩,岩石单元包括堆晶辉长岩、辉长岩岩墙、玄武岩等。通过对该蛇绿岩中堆晶辉长岩的地球化学研究发现,它们属于E-MORB型,与桃形湖和果干加年山地区的蛇绿岩比较相似。堆晶辉长岩(R12T11)中用于LA-ICP-MS定年的锆石具有明显的Ce正异常和Eu负异常,属于典型的岩浆成因锆石;12个测试点的年龄加权平均值为442.7Ma±3.4Ma,表明日湾茶卡蛇绿岩形成于早志留世。日湾茶卡蛇绿岩是龙木错—双湖—澜沧江缝合带内一处新的早古生代洋壳残片,将为进一步揭示龙木错—双湖—澜沧江洋的构造演化提供新依据。  相似文献   

11.
准噶尔、天山和北山52个蛇绿岩的地质特征、地球化学性质和同位素年代学资料系统集成研究表明它们可以分为14条蛇绿(混杂)岩带。绝大多数蛇绿岩呈"岩块+基质"的混杂岩型式沿重要断裂带(构造线)线状分布,少数蛇绿岩以构造岩片叠置方式面状产出。混杂岩的基质有蛇纹岩(碳酸盐化蛇纹岩)和糜棱岩化细碎屑岩两类,岩块既有地幔橄榄岩、基性杂岩和基性火山岩等蛇绿岩组分,也有其它非蛇绿岩组分岩石。堆晶岩出露局限,典型席状岩墙群没有发育。这些蛇绿岩可归类为SSZ(Supra-Subduction Zone)和MORB(Mid-Ocean Ridge)两种类型,前者玄武岩具大离子亲石元素(LILE)富集和高场强元素(HFS)亏损特征,后者不显示该特点;洋岛玄武岩(OIB)既可出现在SSZ型蛇绿混杂岩中,也可为MORB型的组成部分;SSZ型蛇绿混杂岩辉长岩和玄武岩比MORB型具有相对更富集的Sr-Nd同位素组成,但部分形成于弧后(间)盆地的SSZ型蛇绿岩与MORB型一致,具有近亏损地幔的Sr-Nd同位素组成。已确认的最老蛇绿岩为西准噶尔572 Ma玛依勒,次之为北山542~527 Ma月牙山—洗肠井和西准噶尔531 Ma唐巴勒,最年轻蛇绿岩为325 Ma北天山巴音沟和321 Ma北山芨芨台子。根据蛇绿岩证据,结合近年来中亚造山带古地磁、岩浆岩、高压—超高压变质岩和构造地质方面的进展,可以推断埃迪卡拉纪末期—早寒武世,古亚洲洋已达到一定规模宽度,发育洋岛和洋内弧;早古生代时期,多岛洋格局发育至鼎盛期,一系列弧地体分别归属哈萨克斯坦微陆块周缘的科克切塔夫—天山—北山线性弧、成吉思弧、巴尔喀什—西准噶尔弧体系和西伯利亚南部大陆边缘弧体系;晚古生代时期,古亚洲洋于石炭纪末期闭合,增生杂岩和弧地体组成哈萨克斯坦拼贴体系和蒙古拼贴体系两个巨型山弯构造。  相似文献   

12.
The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.  相似文献   

13.
New 40Ar/39Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at 42 Ma to 10 kbar and 400 °C at 39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at 35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex.  相似文献   

14.
The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks.  相似文献   

15.
Oman has two ophiolites – the better known late Cretaceous northern Oman (or Semail) ophiolite and the lesser known and smaller, Jurassic Masirah ophiolite located on the eastern coast of the country adjacent to the Indian Ocean. A number of geological, geochronological and geochemical lines of evidence strongly suggest that the northern Oman ophiolite did not form at a mid-ocean ridge but rather in a supra-subduction zone setting by fast spreading during subduction initiation. In contrast the Masirah ophiolite is structurally part of a series of ophiolite nappes which are rooted in the Indian Ocean floor. There are significant geochemical differences between the Masirah and northern Oman ophiolites and none of the supra-subduction features typical of the northern Oman ophiolite are found at Masirah. Geochemically Masirah is MORB, although in detail it contains both enriched and depleted MORB reflecting a complex source for the lavas and dykes. The enrichment of this source predates the formation of the ophiolite. The condensed crustal section on Masirah (ca. 2 km) contains a very thin gabbro sequence and is thought to reflect its genesis from a cool mantle source associated with the early stages of sea-floor spreading during the early separation of eastern and western Gondwana. These data suggest that the Masirah ophiolite is a suitable analogue for an ophiolite created at a mid-ocean ridge, whereas the northern Oman ophiolite is not. The stratigraphic history of the Masirah ophiolite shows that it remained a part of the oceanic crust for ca. 80 Ma. The chemical variability and enrichment of the Masirah lavas is similar to that found elsewhere in Indian Ocean basalts and may simply reflect a similar provenance rather than a feature fundamental to the formation of the ophiolite.  相似文献   

16.
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(2807±53) Ma,认为索伦山蛇绿岩就位时限在270 Ma左右。  相似文献   

17.
Inliers of 1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012±12 Ma anorthosite–mangerite–charnockite–granite (AMCG) suite; (2) El Catrı́n unit: ≥1350 Ma orthogneiss migmatized at 1106±6 Ma; and (3) El Marquez unit: ≥1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106±6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978±3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 °C at 977±12 Ma is documented by 40Ar/39Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of 25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia.  相似文献   

18.
We present new regional petrologic, geochemical, Sr–Nd isotopic, and U–Pb geochronological data on the Turonian–Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian–Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90–88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian–Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and Jarabacoa blocks, the mainly Campanian group IV basalts of the Peña Blanca, Siete Cabezas and Pelona–Pico Duarte Formation, represent the subsequent stage of back-arc spreading and off-axis non-arc-like magmatism, caused by migration of the arc toward the northeast. These basalts have geochemical affinities with the mantle domain influenced by the Caribbean plume, suggesting that mantle was flowing toward the NE, beneath the extended Caribbean island-arc, in response to rollback of the subducting proto-Caribbean slab.  相似文献   

19.
Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced  600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean 206Pb/238U age of 733 ± 7 Ma, indicating ophiolite formation at  730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt ( 810–780 Ma and  730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.  相似文献   

20.
蛇绿岩中的花岗质岩石成因类型与构造意义   总被引：27，自引：0，他引：27  
花岗质岩石在蛇绿岩的岩石组合中只占很小部分（通常＜10％），但是对于蛇绿岩的成因研究，特别是对蛇绿岩形成和演化过程的精确年龄测定，具有重要的意义。简要回顾了蛇绿岩中花岗质岩石的研究历史。近年来的研究表明，蛇绿岩中的花岗质岩石不仅包括直接由大洋玄武质岩浆结晶分异形成的花岗质岩石，即传统意义上的大洋斜长花岗岩，还包括蛇绿岩在形成、演化过程中本身形成的和侵入的花岗质岩石。根据成因，可以将这些花岗质岩石分为 4类：结晶分异型花岗岩、剪切型花岗岩、俯冲型花岗岩和仰冲型花岗岩。系统地综述了它们的地球化学特征以及形成的构造背景。  相似文献   

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