Diverse tectonic settings of formation of the metaigneous rocks in the Jurassic metamorphic accretionary complexes (Refahiye,NE Turkey) and their geodynamic implications

Two isolated metamorphic accretionary complexes of Jurassic age, the Refahiye and Kurtlutepe metamorphic rocks, crop out as tectonic slices within the coeval suprasubduction-zone ophiolite at the southern margin of the Eastern Pontides (NE Turkey), close to the ?zmir-Ankara-Erzincan suture. The Refahiye metamorphic rocks are made up of greenschist, marble, serpentinite, phyllite and minor garnet amphibolite, garnet micaschist and metachert. The whole unit was metamorphosed under garnet-amphibolite-facies conditions and strongly retrogressed during exhumation. The Kurtlutepe metamorphic rocks consist of subgreenschist-facies metavolcanics, metavolcaniclastics, marble, calc-phyllite, and minor serpentinite and metachert. Metabasites in the Refahiye metamorphic rocks are represented by four distinct geochemical affinities: (i) cumulate “flavor,” (ii) alkaline oceanic island basalt (OIB), (iii) enriched mid-ocean ridge basalt (E-MORB) and (iv) tholeiitic island arc basalt (IAB). On the other hand, the Kurtlutepe metavolcanic rocks display only tholeiitic to calc-alkaline island arc geochemical affinities. The metabasic rocks with OIB affinities were interpreted as parts of the accreted oceanic islands, and those with E-MORB affinities as parts of accreted ridge segments close to oceanic islands and/or plume-distal mid-ocean ridges with a mantle previously metasomatized by plume components. The metabasic rocks with IAB affinities might have been derived from the overlying suprasubduction ophiolite and/or arc domain by a number of tectonic or sedimentary processes including tectonic slicing of accretionary complex and overlying fore-arc ophiolite, juxtaposition of the magmatic arc with subduction zone by strike slip faults, submarine gravity sliding and debris flows or subduction erosion. However, totally recrystallized nature of the metabasic rocks together with field relations does not allow any inference on the processes involved. The Kurtlutepe metavolcanic rocks might represent collided and accreted oceanic island arc with the subduction zone. Attempted subduction of an intraoceanic island arc may also explain the magmatic lull during Late Jurassic–Early Cretaceous in the Eastern Pontides.     

A sheeted dyke complex within the Coolac Ophiolite,southeastern New South Wales

A unit composed of sheeted dykes and an associated unit of sodic felsic rocks have been found within the Coolac Ophiolite, about 5 km east of Coolac township, southeastern N.S.W. The dyke complex consists of small multiple dykes intruding gabbro screens. About half the dykes are basaltic in composition and half are ande‐sitic. Felsic differentiates occur as minor intrusions within the dyke complex, and also constitute the sodic felsic unit. It is suggested that the dyke complex and associated felsic rocks be included in the Honeysuckle Beds, and that the absence of a mappable dyke unit further south within the Honeysuckle Beds is caused by tectonic dismembering of the ophiolite. The dyke complex bears considerable resemblance to the sheeted dyke members of other ophiolite sequences. However, the scale of development is roughly 10 times smaller than at Troodos or Newfoundland, and one‐way chilling is not well developed. The mechanism of intrusion in the Coolac dyke complex therefore is probably not symmetrical spreading about a well defined axis, but a diffuse injection of small multiple dykes within a relatively broad extensional zone. The environment in which such a mechanism might develop, and which is suggested by geochemical evidence, including the high proportion of intermediate and felsic rocks, is that of a small basin adjacent to or within an island‐arc system.     

藏北中生代火山岩的岩石化学与地球化学初步研究

在西藏冈底斯山以北、唐古拉山以南地区,分布着侏罗纪到白垩纪的基性和中性火山岩。它们与本区的超基性岩空间上接近,或者呈断层接触。对这些岩石组合的成因有不同的解释。有人认为是大陆碰撞缝合带的蛇绿岩(常承法与郑锡澜,1974),有人认为是岛弧与边缘海的产物。近来,张旗(1983)在丁青发现了比较连续的蛇绿岩组合。要正确地对这些岩石的构造环境进行解释,必须进行详细的岩石学、岩石化学和微量元素地球化学研究。本文对采自拉弄沟、罗布中(图1)两地的玄武岩和采自切里湖的安山岩的岩石化学和微量元素地球化学做了初步研究,这里将初步结果及讨论作一报道。     

西藏白朗地区蛇绿岩火山岩中单斜辉石的化学特征

近年来,应用岩石的地球化学和矿物学特征区分不同构造位置的现代火山岩,已被人们所注意。同时,根据这种观测资料所确定的一般特征,亦已广泛地应用于古火山岩,并获得了一些古地质环境的概念。     

Petrochemical Investigation of the Antique Ophiolite (Philippines): Implications on Volcanogenic Massive Sulfide and Podiform Chromitite Deposits

Abstract: The Antique ophiolite, located in Panay island (west‐central Philippines), corresponds to several tectonic slices within the suture zone between the Philippine Mobile Belt (PMB) and the North Palawan Block (NPB). It includes dismembered fragments of a basaltic sequence, dominantly pillow‐lavas with minor sheet flows, rare exposures of sheeted dikes, isotropic gabbros, subordinate layered mafic and ultramafic rock sequences and serpentinites. Most of the ophiolite units commonly occur as clasts and blocks within the serpentinites, which intrude the whole ophiolitic body, as well as, the basal conglomerate of the overlying Middle Miocene sedimentary formation. The volcanic rock sequence is characterized by chemical compositions ranging from transitional (T)‐MORB, normal (N)‐MORB and to chemistry intermediate between those of MORB and island arc basalt (IAB). The residual upper mantle sequence is harzburgitic and generally more depleted than the upper mantle underlying modern mid‐oceanic ridges. Calculations using whole‐rock and mineral compositions show that they can represent the residue of a fertile mantle source, which have undergone degrees of partial melting ranging from 9‐22.5 %. Some of the mantle samples display chondrite‐nor‐malized REE and extended multi‐element patterns suggesting enrichments in LREE, Rb, Sr and Zr, which are comparable to those found in fore‐arc peridotites from the Izu‐Bonin‐Mariana (IBM) arc system. The Antique ultramafic rocks also record relatively oxidizing mantle conditions (Δlog f_O2 (FMQ)=0.9‐3.5). As a whole, the ophiolite probably represents an agglomeration of oceanic ridge and fore‐arc crust fragments, which were juxtaposed during the Miocene collision of the PMB and the NPB. The intrusion of the serpentinites might be either coeval or subsequent to the accretion of the oceanic crust onto the fore‐arc. Volcanogenic massive sulfide (VMS) deposits occur either in or near the contact between the pillow basalts and the overlying sediments or interbedded with the sediments. The morphology of the deposits, type of metals, ore texture and the nature of the host rocks suggest that the formation of the VMS bodies was similar to the accumulation of metals around and in the subsurface of hydrothermal vents observed in modern mid‐oceanic ridge and back‐arc basin rift settings. The podiform chromitites occur as pods and subordinate layers within totally serpentinized dunite in the residual upper mantle sequence. No large coherent chromitite deposit was found since the host dunitic rocks often occur as blocks within the serpentinites. It is difficult to evaluate the original geodynamic setting of the mineralized bodies since the chemistry of the host rocks were considerably modified by alteration during their tectonic emplacement. A preliminary conclusion for Antique is that the VMS is apparently associated with a primitive tholeiitic intermediate MORB‐IAB volcanic suite, the chemistry of which is close to the calculated composition of the liquid that coexisted with the podiform chromitites.     

Composition and geodynamic setting of the volcanic rocks from ophiolites of the Ust’-Belaya Mountains,Chukchi Peninsula

In addition to ophiolites in the structure of the Otrozhnaya sheet, the igneous rocks were established within the Middle Devonian-Lower Carboniferous tuffaceous-terrigenous complex earlier considered to be the cover of the ophiolite association. In order to establish their geodynamic formation setting, the geochemical study of igneous rocks was conducted. The volcanic rocks from the ophiolite complex are similar to MORB; subvolcanic rocks of tuffaceous-terrigenous complex have a suprasubduction origin. An abundance of pyroclastic rocks and the type of sediments allow us to conclude about their formation in an island arc setting. The existence of the Middle Devonian-Lower Carboniferous island arc complex within the Ust’-Belaya Mountains gives rise to continue the Koni-Taigonos arc inside the region and testifies to its subsistence in the Devonian.     

The origin and emplacement of the Lajishankou ophiolite complex in the South Qilian belt: evidences from geological mapping

Many ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block.     

Geochemistry and petrogenesis of the Yishak Volcanic Sequence,Kudi ophiolite,West Kunlun (NW China): implications for the magmatic evolution in a subduction zone environment

The Yishak Volcanic Series (YVS) is part of the Late Cambrian Kudi ophiolite in the western Kunlun Mountains, NW China, which marks the oldest suture zone in the Tibetan Plateau. The YVS is a well-preserved sequence comprising five conformable units (A to E) of basalts to andesites, with mafic dykes in Units A to D. These rocks are sequentially characterized upward by geochemical features of N-MORB (Unit A), E-MORB (Unit B), island-arc basalt (Unit C), BABB (Unit D and mafic dykes) and boninite series rocks (Unit E), reflecting complex interactions among various source components. The evolution of magma compositions from MORB-dominated to boninite series rocks clearly indicates temporally increasing subduction signatures, which, in combination with the extensional tectonic background, suggests that the Kudi ophiolite most likely formed in a spontaneous nucleation of subduction zone.     

Devonian supra-subduction zone setting for the Princhester and Northumberland Serpentinites: implications for the tectonic evolution of the northern New England Orogen

The Princhester Serpentinite of the Marlborough terrane of the northern New England Orogen is a remnant of upper mantle peridotite that was partially melted at an oceanic spreading centre at 562 Ma, and subsequently interacted with Late Devonian island arc basalts in an intra-oceanic supra-subduction zone (SSZ) setting. The full range of rare-earth element (REE) contents, including U-shaped patterns, can be explained by a single process of reaction of partially melted, depleted peridotite with Late Devonian calc-alkaline and island arc tholeiite magmas by equilibrium porous flow, fractionating the REE by a chromatographic column effect. The Northumberland Serpentinite on South Island of the Percy Group has similar REE and high field strength element (HFSE) contents to the most depleted samples of the Princhester Serpentinite, supporting a common origin. However, spinel compositions suggest that the Northumberland Serpentinite interacted with boninitic magmas. The REE and mineral geochemistry indicates that the Princhester and Northumberland Serpentinites both represent part of the mantle component of a disrupted SSZ ophiolite. The ophiolite is considered to have formed above an east-dipping subduction zone, based on the geochemistry of Devonian island arc basalts between Mt Morgan and Monto, which include compositions identical to dykes and gabbroic blocks within the Princhester Serpentinite. Blockage of the subduction zone by collision with the Australian continent during the Late Devonian led to slab breakoff and the reversal of subduction direction, trapping the Late Devonian ophiolite in a forearc position. Its location, in a forearc setting above a growing accretionary wedge, conforms to the definition of a Cordilleran-type ophiolite. This interpretation is consistent with current views that most ophiolites are formed from young, hot and thin oceanic lithosphere at forearc, intra-arc and backarc spreading centres in a SSZ setting, and that emplacement follows genesis by 10 million years or less. Late Devonian crustal growth may have been widespread in the New England Orogen, because the disrupted ophiolite assemblage of the Yarras complex in the southern New England Orogen is probably of this age. Extensional tectonism at the end of the Carboniferous dismembered the Princhester – Northumberland ophiolite, removed the crustal section, and produced windows of accretionary wedge rocks within the fragmented ophiolite. The Princhester Serpentinite, together with fault slices of metasedimentary rocks, was thrust westward as a flat sheet over folded strata of the Yarrol Forearc Basin by a Late Permian out-of-sequence thrust during the Hunter – Bowen Orogeny, completing the emplacement of the Marlborough terrane. The Princhester and Northumberland Serpentinites could have been displaced by strike-slip movement along the Stanage Fault Zone or an equivalent structure. There is no record in the northern New England Orogen of SSZ ophiolites and volcanic arc deposits of Cambrian age, as exposed along the Peel Fault. Partial melting of the Princhester Serpentinite at an oceanic spreading centre at 562 Ma, recorded by mafic intrusives displaying N-MORB chemistry, was an earlier event that was outboard of any Early Paleozoic subduction zone along the margin of the Australian continent, and cannot be regarded as representing the early history of the New England Orogen. It is possible that the formation of intra-oceanic arcs in latest Silurian and Devonian time was the first tectonic event common to both the southern and northern New England Orogen.     

北山洋晚志留世—早泥盆世构造演化:内蒙古白云山蛇绿混杂岩带南部侵入岩年代学、地球化学的制约

内蒙古北山白云山蛇绿混杂岩带南部中酸性侵入岩主要以脉岩形式产出。采用LA-ICP-MS锆石U-Pb定年和岩石地球化学分析,对脉岩的岩石类型、年代学、构造背景等进行研究,为北山洋的拼合碰撞时限提供年代学约束。研究结果显示:石英闪长岩的年龄为(420.0±1.0) Ma,花岗闪长岩的年龄为(404.6±2.6) Ma,表明脉岩形成时期为晚志留世—早泥盆世。中酸性侵入岩脉SiO₂含量为58.38%~69.46%,K₂O含量为0.74%~1.38%,Al₂O₃含量为15.54%~16.95%,Na₂O含量为4.15%~5.36%,岩石类型为钙碱性石英闪长岩和过铝质低钾拉斑系列的花岗闪长岩。石英闪长岩与花岗闪长岩源于陆缘火山弧区和同碰撞区,二者轻稀土元素相对重稀土元素明显富集,δEu负异常,高场强元素Ta、Nb亏损,显示出岛弧环境的典型特征,表明由于洋壳俯冲作用,研究区在晚志留世—早泥盆世发育了岛弧岩浆岩侵入体。随着俯冲作用深入,北山洋在晚志留世—早泥盆世早期完成闭合碰撞,并在研究区形成同碰撞花岗岩,碰撞时间应早于(404.6±2.6) Ma。     

Geochemistry of subalkaline and alkaline extrusives from the Kermanshah ophiolite,Zagros Suture Zone,Western Iran: implications for Tethyan plate tectonics

The Kermanshah ophiolite is a highly dismembered ophiolite complex that is located in western Iran and belongs to the Zagros orogenic system. The igneous rocks of this complex consist of both mantle and crustal suites and include peridotites (dunite and harzburgite), cumulate gabbros, diorites, and a volcanic sequence that exhibits a wide range in composition from subalkaline basalts to alkaline basalts to trachytes. The associated sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-water sedimentary rocks (e.g., dolomite, limestone, and pelagic sediments, including umber). Also present are extensive units of radiolarian chert. The geochemical data clearly identifies some of the volcanic rocks to have formed from two distinct types of basaltic melts: (i) those of the subalkaline suite, which formed from an initial melt with a light rare earth elements (LREE) enriched signature and incompatible trace element patterns that suggest an island arc affinity; and (ii) those of the alkaline suite with LREE-enriched signature and incompatible trace element patterns that are virtually identical to typical oceanic island basalt (OIB) pattern. The data also suggests that the trachytes were derived from the alkaline source, with fractionation controlled by extensive removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse volcanics together with the occurrence of a variety of Triassic–Cretaceous sedimentary rocks and radiolarian chert indicate that the studied volcanic rocks from the Kermanshah ophiolite represent off-axis volcanic units that were formed in intraplate oceanic island and island arc environments in an oceanic basin. They were located on the eastern and northern flanks of one of the spreading centers of a ridge-transform fault system that connected Troodos to Oman prior to its subduction under the Eurasian plate.     

The Late Proterozoic ophiolite of Sol Hamed,NE Sudan

The Sol Hamed complex, a sequence from ultramafics, through gabbros, thin sheeted dykes, to pillow lavas, is an ophiolite. It was obducted, tilted nearly to vertical, eroded and covered unconformably by the Nafirdeib Series. This is a volcaniclastic series, dated at 712 ± 58 Ma, the lower part of which includes conglomerates with ophiolitic clasts, olistostromes, one with large oolitic limestone slabs, felsitic and dacitic tuffs, turbidites and black shales, with andesites above. Some of the structures in the ophiolite are attributed to sub-oceanic deformation. Deformation after the deposition of the Nafirdeib Series produced folds and cleavage trending about NE-SW and, more locally, a cleavage trending NW in a shear zone where the ophiolite was thrust northeastwards over the Nafirdeib Series. The ophiolite and the Nafirdeib Series were intruded by the batholithic granite dated elsewhere at ca. 669 Ma. The ophiolite is thought to occur on a suture with the Nafirdeib Series representing part of an island arc sequence.     

青藏高原中的古特提斯体制与增生造山作用

青藏高原古特提斯体系的特征表现为古特提斯洋盆中多条状地体的存在,多俯冲、多岛弧增生体系的形成和多地体汇聚、碰撞造山的动力学环境,其构架包括4条代表古特提斯洋壳残片的蛇绿岩或蛇绿混杂岩(昆南-阿尼玛卿蛇绿岩带、金沙江-哀牢山-松马蛇绿岩带、羌中-澜沧江-昌宁-孟连蛇绿岩带和松多蛇绿岩带)、5条火山岩浆岛弧带(布尔汗布达岛弧岩浆带、义敦火山岩浆岛弧带、江达-绿春火山岛弧带、东达山-云县火山岛弧带和左贡-临沧岛弧-碰撞岩浆带)、4个陆块或地体(松潘-甘孜地体、羌北-昌都-思茅地体、羌南-保山地体)、3条洋壳深俯冲形成的高压-超高压变质带(金沙江得荣高压变质带、龙木错-双湖高压变质带、松多高(超)压变质带),以及5条弧前增生楔或增生杂岩(西秦岭增生楔、巴颜喀拉-松潘-甘孜增生楔、金沙江增生楔、双湖-聂荣-吉塘-临沧增生楔、松多增生杂岩)。古特提斯洋盆的俯冲增生造山作用普遍存在于青藏高原古特提斯复合造山体中,构成与多条古特提斯蛇绿岩带(缝合带)相伴随的俯冲增生杂岩带(链)。古特提斯俯冲增生杂岩带包括由弧前强烈变形的沉积增生楔、以及高压变质岩、岛弧岩浆岩、蛇绿岩和外来岩块组成的混杂体,代表在洋盆俯冲过程中的活动陆缘的地壳增生。     

Geology, Geochemistry and Tectonic Significance of Mafic-ultramafic Rocks of Mesoproterozoic Phulad Ophiolite Suite of South Delhi Fold Belt, NW Indian Shield

A thick sequence of mafic-ultramafic rocks, occurs along a major shear zone (Phulad lineament), running across the length of Aravalli Mountain Range for about 300 kms. It has been suggested, that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics. The geological and tectonic significance of the complex is assessed using field relationships, petrography and geochemistry. Structurally, the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite (mantle component) followed by layered cumulus gabbroic rocks (crustal component). A complex of non-cumulus rocks comprising hornblende schists, gabbros, sheeted dykes and pillowed basalts structurally overlies layered gabbros. Huge bodies of diorite intrude volcanics.

Geochemical classification suggests that all non-cumulus mafic rocks are sub-alkaline basalts except one variety of dykes which shows mildly alkaline character. The sub-alkaline rocks are tholeiite to calc-alkaline with boninite affinity. Tectono-magmatic variation diagrams and MORB normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks.

The mafic rocks of Phulad Ophiolite Suite are zoned across the strike in terms of their distribution from west to east. The hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes. The alkaline dyke occurs at the easternmost part. The rocks of Phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction. The geochemical interpretation presented in this study, together with discussion of lithological association is used to decipher the tectonic evolution of the Mesoproterozoics of NW Indian shield.     

Geochemical characteristics of ophiolitic rocks from the southern margin of the Sivas basin and their implications for the Inner Tauride Ocean,Central-Eastern Turkey

Number of dismembered ophiolite bodies crop out between Sivas and Malatya on the top of the Eastern Tauride platform in the central-eastern Turkey. One of which at the southern margin of the Sivas basin in the Tecer Mountain area comprises melange and the lower part of an oceanic lithospheric section on top of the Tauride platform. The mantle tectonites are characterized by variably serpentinized harzburgites and dunites, and are intruded by numerous isolated dykes. The gabbroic cumulates consist of olivine gabbro, gabbro and gabbronorite. The major and trace element geochemistry of the mafic cumulate rocks suggests that the primary magma was compositionally similar to those observed in modern island-arc tholeiitic sequences. The isolated dykes are exclusively basaltic in composition and display geochemically two distinct subgroups: Group I is represented by high TiO₂ (.87–1.47 wt.%) and other incompatible elements, whereas Group II is characterized by low TiO₂ (.36–.66 wt.%) and other incompatible elements. The Group I isolated diabase dykes have flat to slightly LREE-depleted profiles (La/Yb_N = .32–.79), whereas the Group II isolated diabase dykes are more depleted in general and have a LREE-depleted character (La/Yb_N = .19–.49). This suggests that the isolated dykes were derived from an island arc tholeiitic magma (Nb/Y = .02–.05) with different degrees of partial melting (Group II > Group I) and relatively high oxygen fugacity in intra-oceanic subduction zone. The ophiolitic rocks in the study area may well be compared with the Divri?i ophiolite to the southeast. All the evidence suggests that the isolated dykes in the Tecer Mountain area differ from the alkaline isolated dykes cutting the Divri?i ophiolite. Since the late stage dykes (~76 Ma) in the Divri?i area are alkaline, the tholeiitic isolated dykes in the present study should have been emplaced prior to the alkaline dykes during Late Cretaceous SSZ-spreading (~90 Ma) within the Inner Tauride Ocean.     

Geochemistry of arc volcanic rocks of the Zagros Crush Zone,Neyriz, Iran

The northeastern margin of the Tethyan Neyriz ophiolite complex in southwestern Iran is tectonically juxtaposed under cataclastically-deformed island arc volcanic–volcaniclastic rocks. We document this arc component of the Zagros Crush Zone in the Neyriz area, and describe its petrographic and geochemical characteristics. The arc unit which we call the Hassanabad Unit, is tectonically intercalated with Cretaceous limestone in the cataclastic shear zone around the Hassanabad pass north of Neyriz.Analyses of the distributions of the major, rare earth and other trace elements in the volcanic rocks of the Hassanabad Unit reveal a dominantly calc-alkaline island arc composition. Volcanogenic sandstone and sedimentary breccia, with clasts of basalt, andesite and diorite, are cataclastically intercalated with pillowed calc-alkaline island arc volcanic rocks, pelagic limestone and radiolarian chert. Trace element geochemistry corroborates the petrographic evidence that the poorly-sorted and angular volcanogenic sediments were derived locally from the island arc volcanic and intrusive rocks. The emplacement of the volcanic arc rocks adjacent to the thrust sheets of the crustal and mantle sequences of the Neyriz ophiolite was probably a result of subduction-related processes during closure of the Tethys ocean during the Late Cretaceous.     

川西盐边群的优地槽岩石组合

<正> 古优地槽岩石组合往往难以得到完整保存的记录。蛇绿岩的上冲作用使得蛇绿岩的岩石层序受到破坏;消减作用又常使消减带的复理石岩系变形,成为关系复杂、断裂交错的冲断席;最后的弧带造山作用或碰撞造山作用(Coleman,1975;Dickinson,1977)又进一步使这些岩石褶曲、断裂,于是,原来的层序和面貌便难以辨认。     

Petrology of the Tekirova (Antalya) ophiolite (Southern Turkey): evidence for diverse magma generations and their tectonic implications during Neotethyan-subduction

The SW Antalya Complex is an assemblage of Mesozoic carbonate platform, margin and ophiolitic rocks which record the formation and tectonic emplacement of a small Mesozoic ocean basin. The late Cretaceous ophiolitic rocks are located at two localities, namely the relatively intact Tekirova ophiolite to the east of Kemer zone and the dismembered Gödene ophiolite to the west of Kemer zone. The Tekirova (Antalya) ophiolite comprises harzburgitic tectonites, ultramafic to mafic cumulates, isotropic gabbros and sheeted dikes. Numerous isolated dikes, ranging in thickness from 5 cm to 10 m, intruded the crustal rocks at different structural levels. The isotropic gabbros are represented by gabbro, diorite and quartz diorite rocks with granular to ophitic–subophitic textures. The isolated dikes are characterized by dolerite, diabase and microdiorite with ophitic, intersertal and microgranular textures. These rocks exhibit tholeiitic to alkaline compositions. New geochemical data presented in this paper from the isolated dikes and isotropic gabbros suggest that there are three main types of parental basic magmas that form the oceanic crustal rocks of the Tekirova (Antalya) ophiolite. These are (1) IAT series which can be referred to the Group I isolated dikes and isotropic gabbros; (2) low-Ti boninitic series characterized by the Group II isolated dike and isotropic gabbros; and (3) OIB-type including the Group III isotropic gabbros. The geochemical evidence suggests that the crustal rocks of the Tekirova (Antalya) ophiolite were generated from a progressive source depletion from island arc tholeiites (IAT) to boninites. Therefore, a fore-arc tectonic setting seems likely for the generation of the crustal rocks from the Tekirova (Antalya) ophiolite in the southern branch of Neotethys during the Late Cretaceous. The OIB-type alkaline isotropic gabbros are thought to have resulted from either (1) a late-stage magmatic activity fed by melts that originated within an asthenospheric window due to slab break-off or (2) subduction of a ridge system which generated OIB source across the asthenospheric window that has been no influence of fluids from the subducted slab into the overlying mantle wedge, shortly before the emplacement of the Tekirova (Antalya) ophiolite onto the Tauride platform.     

西准噶尔柳树沟西南玛里鸦蛇绿岩地质特征

玛里鸦蛇绿岩套,位于西准噶尔推覆构造达尔布特铲状断裂南侧,岩石类型齐全,层序基本完整,具有自身的岩石学、岩石化学、稀土元素分布、锶同位素比值等特征.它代表岛弧陆源侧活动边缘海或弧间盆地构造环境.系俯冲洋壳局部熔融,地幔物质底辟升流产物.侵位于早—中石炭世.     

The Zhaheba ophiolite complex in Eastern Junggar (NW China): Long lived supra-subduction zone ocean crust formation and its implications for the tectonic evolution in southern Altaids

The composite Zhaheba ophiolite complex, exposed in Eastern Junggar in the Southern Altaids, records an unusually long record of oceanic crust and magmatic arc evolution. The Zhaheba ophiolite complex consists of ultramafic rocks, gabbro, diorite, basalt and chert intruded by diabase dikes and diorite porphyry. These rocks are overlain by a several-km-thick section of tuffaceous rocks, volcaniclastic sedimentary rocks, and intermediate volcanic rocks. The igneous rocks of the ophiolite complex show negative Nb and Ta anomalies and LREE enrichment relative to HREE, suggesting the influence of fluids derived from a subducting oceanic slab. The LA-ICPMS U–Pb age of zircons from gabbro is 495.1 ± 3.5 Ma. Zircon ages from diorite and basalt are 458.3 ± 7.2 Ma and 446.6 ± 6.0 Ma, respectively. The basalt is locally overlain by bedded chert. Diabase dikes and diorite porphyry yield the U–Pb ages of 421.5 ± 4.1 Ma and 423.7 ± 6.5 Ma, respectively. The age of stratigraphically lower part of the overlying volcanic–volcaniclastic section is constrained to be about 410 Ma, the maximum depositional age of the tuffaceous sandstone from U–Pb detrital zircon ages. Late rhyolite at the top of the stratigraphic section yielded a U–Pb zircon age of 280.3 ± 3.7 Ma. The age and stratigraphic relationships for the Zhaheba ophiolite complex and related rocks suggest that the period of ~ 70 Ma of initial supra-subduction magmatism was followed by construction of a mature island arc that spanned an additional 140 Ma. Many other ophiolites in the southern Altaids appear to record similar relationships, and are represented as substrates of oceanic island arcs covered by island arc volcanism in supra-subduction zone. The occurrence of the Zhaheba ophiolite complex with tuffaceous and intermediate to felsic volcanic rocks is different from the rock association of classic Tethyan SSZ ophiolites but similar to some ophiolites in North America. Although the Zhaheba ophiolite belt is flanked by the Dulate arc in the north and Yemaquan arc in the south, it cannot stand a suture between two arcs. It is suggested that Devonian–Carboniferous Dulate arc was built on the late Cambrian–middle Ordovician Zhaheba supra-subduction oceanic crust. The late Carboniferous rocks and early Permian rocks in Dulate arc are interpreted to form in the extensional process within Zhaheba–Dulate arc composite system.