Characteristics and genesis of diachronous Carboniferous volcano-sedimentary sequences: insights from geochemistry,petrology and U–Pb dating in the North Junggar basin,China

ABSTRACT

The subduction of oceanic lithosphere during the Carboniferous Period contributed to the formation of widely distributed subduction-related volcanic rocks within the Junggar basin. These volcanic rock associations contain significant clues for understanding the subduction of the Keramaili oceanic lithosphere and the filling of the remnant oceanic basin. Here, we report regional gravity and magnetic data, petrology, geochemistry, and U–Pb dating for Carboniferous volcanic rocks from the North Junggar basin (NJB). Using samples from well Y-1, we distinguish upper and lower volcanic sequences on the basis of selected geochemical data. An isochronous stratigraphic framework of Carboniferous volcano-sedimentary sequences is then constructed and the petrogenesis of these volcanic rocks is discussed. Finally, we propose an explanation for the genesis of these diachronous Carboniferous volcano-sedimentary sequences. The results show that various volcanic rocks are distributed in different areas of the NJB, and mainly consist of calc-alkaline basalt–andesite–dacite assemblages and alkaline basalt–basaltic andesite–andesite assemblages. The geochemical data also demonstrate a binary nature of the Carboniferous volcanic rocks. In the eastern NJB, the lower and upper volcanic sequences are formed during the early and late Carboniferous, respectively. However, all of these volcano-related sequences in the western of the NJB are formed during the late Carboniferous. These volcano-sedimentary sequences exhibit a ‘ladder-style’ of temporospatial evolution from east to west. The geochemical results also indicate that the upper volcanic rocks include island arc components formed in an extensional setting, whereas the lower volcanic rocks were derived from deep crustal cycling metasomatism by various mantle components in a continental arc environment. Earlier closure of the Keramaili oceanic basin and slab roll-back of the Junggar oceanic lithosphere in eastern versus western Junggar basin led to the formation of these diachronous volcano-sedimentary sequences.     

Petrology, isotope geochemistry and chemical budgets of oceanic gabbros-seawater interactions in the Equatorial Atlantic

 Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between 300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite). Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of gabbroic δ¹⁸O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (⁸⁷Sr/⁸⁶Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry. Received: 8 March 1999 / Accepted: 12 July 1999     

哀牢山缝合带中两类火山岩地球化学特征及其构造意义

在哀牢山构造带的哀牢山断裂与花山－雅邑断裂之间出路大量不同构造环境的岩浆岩。除双沟蛇绿岩外,尚发现有景东火山岩和墨江火山岩。地球化学研究表明,景东火山岩具有类似于富集型洋中脊玄武岩（Ｐ－ＭＯＲＢ）地球化学特征,形成于以双沟蛇绿岩为代表的哀牢山洋盆先期的陆内裂谷构造环境;墨江火山岩具有岛弧火山岩地球化学特征,形成于哀牢山洋盆向西俯冲消减作用下的岛弧构造环境。景东裂谷型火山岩和墨江岛弧火山岩分别代表了     

Metabasalts from the Troodos Massif,Cyprus: Genetic implication deduced from petrography and trace element geochemistry

A wide variety of data support the contention that the Troodos massif, Cyprus is a fragment of oceanic lithosphere formed at a constructive margin some 85 m.y. ago. However, Troodos rocks differ significantly in major and trace element content and mineral parageneses from those formed at present day active constructive margins. It is here argued that the geochemical and petrographic characteristics of the massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.     

Evidence for the Jurassic arc volcanism of the Lolotoi complex,Timor: Tectonic implications

We report the first sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages with geochemical data from metavolcanic rocks in the Lolotoi complex, Timor. The zircon U–Pb ages of two andesitic metavolcanic rocks yield a permissible range of the Middle Jurassic extrusion from 177 Ma to 174 Ma. The geochemical data indicate that the origins of the basaltic and andesitic metavolcanic rocks are products of prolonged oceanic crust and arc magmatism, respectively. They are originated from partial melting of lherzolites, providing an insight into the tectonic evolution of the forearc basements of the Banda volcanic arc. Thus, parts of the Banda forearc basement are pieces of allochthonous oceanic basalts and Jurassic arc-related andesites accreted to the Sundaland during the closure of Mesotethys, and are incorporated later into the Great Indonesian Volcanic Arc system along the southeastern margin of the Sundaland.     

Lithic-volcanic sandstones derived from oceanic crust in the Franciscan Complex of California: 'sedimental memories' of source rock geochemistry

The Permanente terrane, part of the Franciscan accretionary complex of California, contains an unusual fault-bounded packet bearing lithic-volcanic sandstones and conglomerates. Based on petrographical, sedimentological, stratigraphical and geochemical studies, it is concluded that these rocks were deposited on the flank of an oceanic rise or plateau by sediment gravity flows probably below the carbonate compensation depth. The bulk geochemistry of the sandstones is similar to the chemistry of greenstones in the Permanente terrane, suggesting that the greenstones or rocks similar to them represent the source of the lithic-volcanic sandstones, and that the sandstones geochemically ‘remember’ the chemistry of their source rocks. This ‘sedimental memory’ is impressive: geochemical variation in the sandstones is co-linear with presumable magmatic variation trends in the greenstones. Sandstone geochemistry, plotted on Zr/Y vs. Y and V vs. Ti diagrams, shows that compositions occupy similar fields as greenstones in the Permanente terrane and oceanic crustal compositions in general. Because oceanic crustal sequences can become accreted to continental margins, oceanic crust should be considered a potential source of sediment, especially in continental margin settings. However, differentiating sandstones derived from oceanic arcs and from oceanic crust is extremely difficult unless discriminating geochemical criteria are used.     

Geochemistry of rocks in the Anuy metamorphic dome,Sikhote-Alin: Composition of the protoliths and the possible nature of metamorphism

The paper presents geological, geochemical, and isotopic data on metamorphic rocks in the Anuy block (dome) in the Northern Sikhote-Alin and the surrounding sedimentary rocks of the Samarka accretionary prism. The geochemistry and isotopic composition of the amphibolite-facies metamorphic rocks (variably migmatized gneisses and crystalline schists) in the Anuy block and unmetamorphosed Jurassic-Cretaceous sediments surrounding the block are proved to be similar. All of them corresponded to the erosion products of the transitional-type crust (mature island arcs and active continental margins), have similar major- and trace-element compositions, and Nd model ages of 1.25–1.4 Ga. The geochemistry and isotopic parameters of metapelites in the Anuy block are principally different from those of analogous rocks in the Khanka Massif (the latter rocks are erosion products of the mature crust and have a Nd model age of 1.7–1.9 Ga). The metabasites, which are found as beds and lenses in gneisses and crystalline schists in the Anuy block and among sedimentary rocks surrounding the block, have a composition corresponding to oceanic basalts of the N- and E-MORB types. Based on the synthesis of geological, geochemical and isotopic data it was suggested that the Anuy block could be not a fragment of the basement of an ancient continent (as was believed previously) but rather a complex of the Early Cretaceous granite-metamorphic core of the Cordilleran type.     

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.     

The Itmurundy Pacific-type orogenic belt in northern Balkhash,central Kazakhstan: Revisited plus first U–Pb age,geochemical and Nd isotope data from igneous rocks

The Itmurundy zone/belt is located in the northern Balkhash area of central Kazakhstan. Geologically it belongs to the Kazakh orocline located in the western Central Asian Orogenic Belt (CAOB), north of the Tarim craton and west of the Junggar block. The Itmurundy belt, which surprisingly has remained unstudied in terms of up-to-date geochronological, geochemical and isotope methods compared to other regions of the CAOB, was revisited and reinvestigated. The belt possesses a very complicated geological structure and hosts rocks of mantle, orogenic and post-orogenic associations. This paper focuses on the orogenic association and presents original geological data, first U–Pb age and first up-to-date geochemical and Nd isotope data from igneous rocks. The orogenic association of the Itmurundy belt includes volcanic and sedimentary rocks of three formations, Itmurundy (O_1-2), Kazyk (O_2-3) and Tyuretai (O₃–S₁), and represents an accretionary complex. The most lithologically diverse Itmurundy Fm. (O_1-2) consists of oceanic basalt, pelagic chert, hemipelagic siliceous mudstone and siltstone, and greywacke sandstones. Both sedimentary and igneous rocks were strongly deformed by syn- and post-accretion processes, which, in places, formed duplex structures. The igneous rocks are basalt/dolerite/gabbro, andesibasalt, trachybasalt and diorite. The diorite yielded a U–Pb age of ca. 500 Ma. The subalkaline volcanic and subvolcanic rocks belong to the tholeiitic series. Based on major oxides three groups of rocks can be distinguished: high-Ti, mid-Ti and low-Ti. The rocks of these three groups are variably enriched in LREE (La_N = 122, 23 and 2 in average, respectively) showing LREE enriched (high-Ti), LREE depleted (mid-Ti) and flat (low-Ti) REE patterns. The high-Ti group shows enrichment in Nb, Th, and Zr compared to the mid-Ti and low-Ti groups. The low-Ti group is special for the Nb troughs in primitive mantle normalized multi-element diagrams, which are typical of supra-subduction settings. The values of εNd are mostly positive for the mid-Ti and low-Ti groups, but negative for the high-Ti group. The geochemical features of the igneous rocks suggest their formation in oceanic (oceanic floor and oceanic island/seamount) and supra-subduction (intra-oceanic arc) settings. In general, the structural position, lithology and deformation styles of Itmurundy sedimentary and igneous rocks and the geochemical features of the igneous rocks all accord well with the models of Ocean Plate Stratigraphy (OPS) and Pacific-type orogeny. Thus, the Itmurundy belt at northern Balkhash represents an Ordovician-Silurian Pacific-type orogenic belt formed at a convergent active margin of the Paleo-Asian Ocean.     

北祁连榴辉岩相变沉积岩的特征及其构造意义

在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少.本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果.变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t= 450～520℃,p=1.9～2.3 GPa,与相邻榴辉岩的温压条件一致.地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境.变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800 Ma左右和540～600 Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代-早古生代新生洋壳或增生物质.同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表.     

The nature of the Palaeozoic oceanic basin at the southwestern margin of Gondwana and implications for the origin of the Chilenia terrane (Pichilemu region,central Chile)

The Coastal Accretionary Complex of central Chile constitutes the product of early Carboniferous to Late Triassic subduction at the rear of Chilenia, a continental terrane likely derived from Laurentia and accreted to southwestern margin of Gondwana during the Mid to Late Devonian. The complex contains basaltic metavolcanic sequences of the subducted oceanic lithosphere accreted to the active margin. In this paper, we address the tectonic setting of these rocks by means of a geochemical study in the coastal area of Pichilemu region, central Chile. The accreted fragments of oceanic crust occupy different structural levels, exhibit variable metamorphic grade, and have geochemical fingerprints that reveal a compositional heterogeneity of the subducted oceanic crust. The amphibolites have N to E-MORB compositions. Greenschist units include N-MORB and E-MORB transitional to OIB, and blueschists and greenschists interleaved within a single metavolcanosedimentary sequence have OIB signatures. Neodymium isotopic systematics indicate depleted and enriched mantle sources, whereas strontium isotopic systematics indicate seawater/rock interaction. The variety of rocks suggests formation in an oceanic setting characterized by shallow and deep mantle sources, such as plume-influenced ridge. Based on the geological, petrological, geochemical, and isotopic characteristics, we propose that the metavolcanic protoliths of the Pichilemu region formed relatively close to the western margin of the Chilenia terrane during the initial stage (late Cambrian–Early Devonian) of seafloor development and drifting of this continental block. Geochemical similarities with oceanic units accreted to the active margin south of the Pichilemu region indicate a regional pattern of the oceanic crust subducted under the Palaeozoic Chilean margin between, at least, 34°S and 39°S latitude, strongly supporting the activity of a mantle plume. This, in turn, can be correlated with the location of the Pacific plume generation zone in early Palaeozoic era, corroborating a Laurentian origin for the Chilenia terrane.     

喀喇昆仑地区早二叠世弧后盆地

喀喇昆仑山北坡明铁盖地区早二叠世的基性火山岩岩石化学和地球化学特征表明,它是一种既具有在硅铝质基底上扩张的洋脊玄武岩性质,又具有钙碱性岛弧拉斑玄武岩特点的弧后盆地火山岩。这一弧后盆地的形成与沿西金乌兰湖—空喀山口至乔戈里峰一线的古特提斯洋盆的扩张及向北俯冲、消减相联系,构成了塔里木板块南部晚古生代活动大陆边缘。     

全球洋岛和洋底高原海山中的中酸性岩——数据挖掘及其意义

洋岛类型的海山和洋底高原类型的海山主要由玄武岩组成, 中酸性岩出露很少, 因此很少引起学术界的关注。 我们认为即便中酸性岩很少, 研究他们也是很有意义的, 因为这些中酸性岩不仅可能与洋岛和洋底高原玄武岩的构造背景有关, 也可能与洋岛和洋底高原在形成中酸性岩时的温度和压力变化有关, 这对正确认识洋岛和洋底高原形成的地球动力学背景是很有意义的。 本次研究收集了 GEOROC 数据库中的数据总量为 44 404 个, 经过清洗留下有效数据 3 908 个, 分为洋岛中酸性岩（OIG）、洋底高原中酸性岩（OPG）和洋底高原中酸性岩中的岛弧部分（OPAG）3 类。 学术界通常认为, 洋岛代表热点, 洋底高原温度相对较低, 属于大火成岩省。 然而, 本文对全球洋岛和洋底高原的中酸性岩浆岩的研究表明, 分布在洋岛和洋底高原的中酸性岩主要是板内环境的富碱性的粗面岩和碱性流纹岩, 洋底高原边缘可能受俯冲带的影响, 部分中酸性岩具有岛弧的地球化学特征。 按照中酸性岩的 Sr-Yb 分类, 可知洋岛中酸性岩大多属于南岭型和广西型, 洋底高原海山中酸性岩主要属于南岭型, 广西型很少, 指示洋底高原中酸性岩形成的深度比洋岛浅, 温度也比洋岛低, 同时表明中酸性岩大多形成的压力较低而温度很高, 指示伴有高热中酸性岩的洋岛和洋底高原是地球上的热点。 本文认为, 洋岛和洋底高原中酸性岩并非学术界普遍认为的是玄武岩和碧玄岩分离结晶形成的, 因为洋岛和洋底高原岩浆岩如果是双峰式分布的话, 则不大可能是分离结晶的, 其中的酸性岩部分可能是玄武岩部分熔融形成的。 同时有些岩浆演化的关系表明, 中酸性岩在哈克图解中与玄武岩受不同因素制约, 说明并非演化的关系。     

Le magmatisme basique calcoalcalin d'âge dévono-dinantien du nord du Massif Central,témoin d'une marge active hercynienne: arguments géochimiques et isotopiques Sr/Nd

Abstract

Samples of volcanic rocks from the main outcrops of Devono-Dinantian series in northern Massif-Central have been studied for Sr and Nd isotopes and immobile incompatible trace elements. In addition, two intrusive bodies of dioritic composition have been dated at 365 ± 3 Ma (Aydat) and 360 ± 1 Ma (Beaumont-Huriel), using the U/Pb zircon method. Together with geochemical data, these ages show that all the studied rocks belong to the same igneous episode. Based on trace element and radiogenic isotopes (⁸⁷Sr/⁸⁶Sr_i from 0,7041 to 0,7057; εNd_i from +1.5 to +5.0), the Late Devonian and Early Carboniferous igneous rocks bear distinct similarities with magmas produced in modern active margin settings. Combined with independent evidence for the occurrence of Devonian oceanic lithosphere in the northern branch of the Variscides, these geochemical affinities suggest that southward subduction of oceanic crust prevailed during the Late Devonian in northern Massif-Central. As a corollary, it appears that true continent-continent collision did not occur before Early Carboniferous times. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.     

老挝班康姆铜金矿床火山-侵入杂岩地球化学特征及地质意义

班康姆铜金矿床是近年来新发现的大型铜金矿床。前人研究初步表明研究区成矿作用与古特提斯洋壳俯冲消减所产生的岩浆作用密切有关,但是缺乏岩石地球化学证据。笔者于研究区地表、钻孔采集了13件火山岩-侵入岩新鲜岩石,开展了主量元素、微量元素、稀土元素分析测试。研究表明该区安山质-侵入杂岩具有岛弧钙碱性火山岩的地球化学特征,构造环境判别图显示该套岩石属于大洋弧环境,形成于大洋消减带,与洋壳板片俯冲有关。安山质-侵入杂岩来源于地幔楔,并经历了结晶分异作用,而煌斑岩形成于板片消减而富集的地幔部分熔融。通过与黎府构造带矿床稀土、微量元素特征对比,证实班康姆矿区与Chatree矿区火山岩及金矿床形成于同一期构造岩浆成矿作用,进一步表明了黎府构造岩浆带在晚二叠世—早三叠世发生了一次重要的区域成矿作用。     

滇西腾冲地块高黎贡群变质沉积岩时代与原特提斯洋俯冲/增生：来自碎屑锆石U- Pb定年和岩石地球化学证据

滇西腾冲地块高黎贡群变质沉积岩时代和构造背景的厘定对正确认识原特提斯构造域演化过程及腾冲地块与冈瓦纳大陆之间的关系十分关键。岩石学、岩石地球化学结果表明,高黎贡群变质岩由变质沉积岩和变质岩浆岩组成,前者以片岩和副片麻岩为主,夹少量大理岩和石英岩,其原岩由一套杂砂岩、泥岩夹少量灰岩、硅质岩岩石组合,为深海-半深海相沉积物,形成于活动大陆边缘环境。碎屑锆石LA-ICP-MS U-Pb定年结果表明高黎贡群变质沉积岩中的锆石主要来源于与罗迪尼亚、冈瓦纳超大陆拼合及原特提斯洋俯冲有关的岩浆岩(900~1000Ma和500~600Ma),少量来源于中元古代地层(1500~1600Ma和2300~2400Ma)。4件样品中最年轻碎屑锆石群的加权平均年龄(507~510Ma)及没有出现有意义的小于470Ma碎屑锆石,表明高黎贡群变质沉积岩原岩形成于510~470Ma,是晚寒武世-早奥陶世早期原特提斯洋壳向冈瓦纳大陆下俯冲过程中,在俯冲带上盘沉积的含有大量该期火成岩碎屑的斜坡相沉积物。     

Petrogenetic character and provenance of metabasalts in the aspiring and Torlesse Terranes, South Island, New Zealand: Implications for the gold endowment of the Otago Schist?

We report quantitative X-ray diffraction, whole-rock geochemical and Sm–Nd isotope data for metabasalts from the Aspiring and Torlesse Terranes in the South Island of New Zealand. These rocks underlie the Mesozoic metasedimentary Otago Schist which is anomalously enriched in gold and host to at least one world-class orogenic gold deposit at Macraes (> 125 t Au). Geochemical and Nd isotopic similarities between the samples point to a common history of the two terranes rather than the amalgamation of one or two allochthonous plate fragments. Furthermore, geochemical and Nd data suggest the metabasalts within both terranes formed in an oceanic but essentially non-subduction-related setting. The origin of the Aspiring and Torlesse basalts can be linked to the formation of an oceanic plateau that had resulted from a (?Permian) mantle plume initiation event proximal to a mid-oceanic rise or triple junction. Given the intrinsically gold-enriched nature of certain oceanic-character mafic rocks, the anomalous gold endowment of the Otago Schist may have been enhanced via the accretion and subduction of a gold-enriched oceanic plateau fragment. The metabasalts are generally enriched in gold (up to 13 ppb) compared to their enclosing metasedimentary rocks (typically ca. 1 ppb), with sulphide-rich metabasaltic rocks having up to 550 ppb Au. However, the relatively small volume of metabasalts in the Otago Schist precludes these rocks as the principal source for Otago Schist orogenic gold, with a primarily metasedimentary source of the gold potentially having a limiting effect on the overall endowment of the Otago Schist. This approach, that employs petrogenetic fingerprinting of potentially fertile source rocks for the assessment of gold endowment, might prove useful in the conceptual exploration targeting of relatively immature and poorly exposed terrains.     

Magmatic rocks in the Charlie-Gibbs Fracture Zone, North Atlantic Ocean

Two basalt groups corresponding to different geodynamic environments were distinguished based on results of additional geochemical studies of materials obtained during Cruise 15 of the R/V Akademik Kurchatov and literature data on the Charlie-Gibbs Fracture Zone: (1) predominantly low-Ti tholeiites that are most closely similar to spreading basalts generated during the initial opening of the North Atlantic (southeastern margin of Greenland and the Vöring and Rockall plateaus) and (2) high-Ti differentiated basalts of variable alkalinity, which are deep, low-volume partial melts likely derived with the participation of a mantle plume. Compositional variations in the basalts were studied in the active and passive parts of the fracture zone. Our results bridge the gap in detailed geochemical data on oceanic crustal rocks and can be used in reconstructions of ancient geodynamic environments.     

Geotectonic evolution of the Western Cordillera of Colombia: New aspects from geochemical data on volcanic rocks

The Western Cordillera of Colombia (WCC) is part of the Basic Igneous Complex (BIC), which is one of the world's largest ophiolitic complexes, extending from Costa Rica through Panama and Colombia to Ecuador. Major and trace element data on 32 volcanic rocks from the central and northern parts of the Western Cordillera are presented; no data have been available to date for volcanic rocks from the northern parts of the Western Cordillera. Petrographical and geochemical investigations show that the rocks are altered and have undergone low-grade metamorphism. The subalkaline rocks are represented by tholeiitic basalts, calc-alkaline basic andesites, andesites, and one dacite. It is concluded that a mature oceanic island arc existed in the Cretaceous, in what is now the northern part of the Western Cordillera. The tectonics of the region, particularly the intensive imbrication of the chain, indicates the presence of a paleo-subduction zone with an oceanic island arc that accreted on the old continental margin. These new data, combined with new and previous data from the central part of the BIC of Colombia, suggest that volcanic rocks of the Western Cordillera can be interpreted as allochthonous slabs. These slabs were imbricated with back-arc and fore-arc sediments and tonalitic bodies during the closing of a back-arc basin in northwestern South America and accretion of an oceanic island arc. Oblique subduction accreted these different areas to the continental margin during Late Cretaceous and early Tertiary times. Two plate-tectonic models are proposed: a) development of the calc-alkaline volcanic rocks in the northern parts of the Western Cordillera, separated by tholeiitic rocks, formed along a transform fault represented by the tholeiitic basalts of the central and southern parts of the Western Cordillera; or b) development of an oceanic island arc along the Cretaceous continental margin of northwestern South America. In the central and southern parts of this island arc, accretion took place early and therefore only an island-arc tholeiitic suite was formed.     

大别山榴辉岩岩石学及地球化学特征

本文分析了大别山地区榴辉岩的产出特点,矿物化学、岩石化学及元素地球化学特征。所获资料表明大别山榴辉岩与围岩一起经历了超高压变质作用,但其化学成分特征与围岩存在一定的差异,兼有大陆玄武岩和大洋玄武岩的特征。这些岩石大部分可能为板块俯冲过程中带入的大洋岩石残片。在深部变质形成榴辉岩后又随大别杂岩一起被抬升到地表,在抬升过程中经历了不同程度的退变质作用。