Geochronology,petrogenesis and tectonic implications of the Jurassic Namco–Renco ophiolites,Tibet

The nature of the Namco–Renco ophiolites in the northern Lhasa subterrane is widely disputed. To investigate their formation age, petrogenesis, and tectonic setting, the harzburgites, basalts, and metagabbros of the Namco ophiolite and the harzburgites, lherzolites, gabbros, and diabasic dikes of the Renco ophiolite were selected for whole-rock geochemical and zircon U-Pb dating and in situ Lu-Hf isotopic analyses. The geochemical and geochronological data indicate that the Namco metagabbros were generated at 178.0 ± 2.9 Ma, along with the Namco–Renco peridotites formed in the initial stage of a continental margin basin; whereas the Renco gabbros were developed at 149.7 ± 1.6 Ma, along with the Renco diabasic dikes and Namco basalts formed later in a mature back-arc basin. The Namco–Renco ophiolites were derived from a depleted mantle source with involvement of minor older crustal materials. Combined with the regional geological background, the Namco–Renco ophiolites were likely formed mainly associated with the southward subduction of the Bangong–Nujiang oceanic lithosphere beneath the Lhasa terrane. This study provides new constraints on the formation ages of the Namco–Renco ophiolites and the tectonic evolution of the Namco–Renco Ocean.     

Pre-Cenozoic tectonic attribute and setting of the Song Da zone,VietnamEI北大核心CSCD

Rock association and regional unconformity are identified based on field survey and mapping for the stratigraphy, tectonics, metamorphic rock and igneous rocks in the Song Da zone, the northwestern Vietnam. Pre-Cenozoic in the Song Da zone is divided into three structural layers: (1) the Presinian crystalline basement, (2) the Cambrian-lower part of Upper Triassic, and (3) the upper part of Upper Triassic-Cretaceous. The Pre-Cenozoic structural successions of the Song Da zone show similar features with those of the Yangtze-South China plate: (1) In the Presinian, both of them have TTG complex and Khondalite series of similar ages and experienced similar tectonic evolution; (2) In the Cambrian-early Upper Triassic, the phosphorite and thick-layered limestone are comparable, and the Permian-Triassic basalt in the Song Da zone has genetic mechanism similar to that of the Emeishan basalt; (3) In the late Upper Triassic-Cretaceous, the volcanic-sedimentary faulted basins and thick continental red beds are comparable to those in East China. It can be concluded that the Song Da zone should have been part of the southwestern Yangtze-South China plate, and the boundary between the Yangtze-South China plate and Indochina plate is the Song Ma suture zone which contains ophiolite remnants. Tectonic settings of different structural layers indicate that the Song Da Zone experienced geological events such as the Columbia and Rodinia Supercontinent, sedimentation of the stable platform cover, closure of the Paleo-Tethys, and NW subduction of the Paleo-Pacific plate to the Eurasian plate. The Pre-Cenozoic basement of the Yinggehai Basin is the seaward extension of the Song Da zone, which shows similar tectonic features.     

Geochemistry and tectonic environment of amphiholites of the Zanhuang metamorphic complex北大核心

Amphibolites, one of the kinds of Precambrian basement rocks, are exposed in the south-middle Zanhuang Metamorphic Complex, central-south segment of the Trans-North China Orogen. These metamorphic rocks were carefully studied through field investigations and petrographical and geochemical analyses. It was found that protoliths of the Zanhuang amphibolites are calc-alkaline and tholeiitic rocks with a large range of ΣREE (41.38 × 10-6 -232.55 × 10-5 ) and weak LREE enrichment. The multi-element variation results showed that K, Rb and Ba were relatively concentrated, Nb and Ta were of evident depletion while Ti was of weak depletion. Geochemical characteristics and various relevant geochemical discrimination diagrams showed that the Zanhuang amphibolites were formed in volcanic arcs of continental margin similar to the current continental margin, which was resulted from the subduction between the Western Block and the Eastern Block.     

Geological characteristics, tectonic setting and preliminary interpretations of the Jilau gold–quartz vein deposit, Tajikistan

The southern Tien Shan metallogenic province of Central Asia hosts a number of important gold resources including the Jilau gold–quartz vein system in western Tajikistan. These deposits were formed at the late stages of continent–continent collision in association with subduction-related magmatism, metamorphism and continental margin deformation attributed to the Central Asian Hercynian Orogeny. Jilau is hosted by a Hercynian syntectonic granitoid intrusive that was emplaced into bituminous dolomite country rocks. Economic mineralisation is associated with a dilational jog within a high-angle, oblique dextral-reverse slip shear zone that was undergoing brittle–ductile deformation. The orebody takes the form of shear-zone subparallel quartz veins and lenses that emanate from a steeply plunging ore shoot of veins and stringers within a silicified and sulphidised granodiorite core. It is thought to have formed by a dynamic process in which fluid flow was governed by a fault-valve mechanism. Numerous cycles of fluid pressure build-up, fault failure, jog dilation, fluid flow, phase separation of low salinity H₂O–CO₂–CH₄(–N₂) fluids, and sealing took place. Gold appears together with scheelite and bismuth minerals predominantly as inclusions in arsenopyrite in quartz veins and altered wall-rock, and is mainly associated with quartz containing fluid inclusions enriched in CH₄. The correlation between high gold grades and high CH₄ concentrations suggests that components of the mineralising fluids were derived from, or passed through, the reducing, carbonaceous rocks in the contact aureole of the intrusive. The occurrence of Au and W in an adjacent Hercynian skarn deposit and in the Jilau orebody, infers that the ore metals in both these systems were ultimately derived from a magmatic source. Received: 15 April 1999 / Accepted: 30 December 1999     

Structure,age, and tectonic development of the Huoshishan–Niujuanzi ophiolitic mélange,Beishan, southernmost Altaids

The Huoshishan–Niujuanzi ophiolitic mélange (HNO) is located near the central part of the Beishan Orogenic Belt in the southernmost Altaids. The HNO consists of ultramafic rocks, cumulate gabbros, gabbros, plagiogranites, diorites, diabases, basalts, andesites, rhyolitic volcaniclastic rocks and siliceous sedimentary rocks, many of which are in a schist matrix (Gongpoquan Group). Geochemical data of the mafic rocks indicate a calc-alkaline or a mixture of calc-alkaline and tholeiitic rocks with negative Nb, Ta and positive Pb, Ba and La anomalies, suggesting formation in an island arc or supra-subduction zone setting. A gabbro from a block in the mélange in the Niujuanzi area has a zircon age of 435.0 ± 1.9 Ma and a plagiogranite with an age of 444.3 ± 1.9 Ma, and another gabbro from the Huoshishan area has an age of 410.5 ± 3.7 Ma. The schist matrix has a zircon age of 512 ± 5.3 Ma and contains Silurian, Devonian and Carboniferous fossils, thus the mélange formed in the late Carboniferous or later. Our structural analysis of fault planes in the HNO, the crenulation cleavages (S₂) of the schist, and fold axial planes of early Permian sandy limestone/quartz veins and late Permian sandstones indicates that the mélange underwent a north-to-south compression, and the orientation of stretching lineations, slickensides and fold hinge lines implies that the HNO experienced top-to-the north (or -northwest) movement. The entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late Paleozoic. The emplacement age of the HNO was probably near the end-Permian based on the age of the youngest rocks in the ophiolitic mélange, and by the presence of a late Permian unconformity. From our work, integrated with published regional data, we outline a comprehensive geodynamic model for the central BOC.     

Geospatial technology and structural analysis for geological mapping and tectonic evolution of Feiran–Solaf metamorphic complex,South Sinai,Egypt

The Sinai Peninsula constitutes an important district of the Egyptian lands where it forms a triangular portion in northeastern Egypt. The southern Sinai metamorphic complexes are the northern uppermost part of the Arabian–Nubian Shield revealing the upper and middle crust from the East African Orogeny, in which they tectonically evolved. The Feiran–Solaf metamorphic complex (FSMC) of Sinai, Egypt is one of the highest grades metamorphic complexes of a series of basement domes that trends NW and crops out throughout the Arabian–Nubian Shield. The main aim of the present study is to apply the geospatial technology and to represent the capability of the geospatial technology to estimate the combined influence of lithology and structure studies, and to construct the lithological and structural maps of FSMC. Furthermore, detailed structural analysis is carried out to reveal the different ductile and brittle deformational events and proposed the tectonic evolutionary model for the study area. Mainly geospatial technology and structural analysis software have been used to go well with the aim of the present study. Developing specific image processing of satellite images and structural analysis were succeeded to discriminate the various lithological rock units, and the geological structural features of the FSMC, using geographic information system tools to construct the different thematic maps, were extracted. The present detailed investigations of the enhanced satellite images, structural analysis, and field verification reveal that the FSMC reached its present tectonic setting through more than four deformational phases concluding that the Pan-African Najd Fault System continued in Sinai and was reactivated during Red Sea tectonics as indicated by the dextral shear zone (Rihba) bordering the northern side of the FSMC.     

Active tectonic and palaeoseismological features of the western section of Mustafakemalpaşa Fault; Bursa,NW Anatolia

The Mustafakemalpa?a Fault (MF), located among Manyas, Ulubat and Orhaneli faults, is a right lateral strike-slip and 47 km in length. The MF begins with a pressure ridge at the west and exhibits complex jog terminations at east ends in restraining left stepovers. The western section of the fault bounds Miocene and Quaternary units and continues towards ?lyasç?lar. The central segment of the fault, starts with approximately 750-m leftward stepover, exhibits a sinusoidal geometry between Kapakl?oluk and Kabulbaba. In this section, MF traverses mountainous terrain and cuts Ophiolite, Jurassic limestones and Miocene detritals, forming dextral faulting features and gaining reverse component. The eastern section exhibits left stepping en-echelon pattern and consists 2.5-km offset on the Orhaneli River. In this study, palaeoseismological findings related to the Holocene activity and active tectonic properties of the MF are presented. The trenches exposed mismatched stratigraphy, demonstrating evidence of events across the fault. We identified three events (before BC 2190, later AD 1425 and 1850) that have occurred during the past 4000 years. We suggest a long non-characteristic recurrence interval and ~0.7 mm/y slip-rate for MF, based on trench data and offset of the Late Pliocene drainage of Orhaneli River.     

Detrital zircon geochronology of Devonian quartzite from tectonic mélange in the Mianlue Suture Zone,Central China: provenance and tectonic implications

ABSTRACT

Devonian quartzite occurs as blocks within a phyllite matrix in Puziba area of the Mianlue Suture Zone (MLSZ) in central China. The depositional time of the quartzite is younger than 425 Ma (mainly Early Devonian), constrained by the zircon U–Pb geochronology data from the quartzite, cross-cutting relationships with granite, and palaeontology evidence. The detrital zircons in the quartzite show typical magmatic features with four main age peaks at: 2676–2420 Ma (11.6% of the population), 1791–1606 Ma (4.8%), 997–817 Ma (26.5%), and 597–425 Ma (17.5%). In combination with the zircon εHf(t) values, we propose that the quartzite in the MLSZ was sourced from Neoproterozoic and Palaeozoic magmatic and sedimentary rocks in the South Qinling Block and the South China Block (particularly from the Bikou Terrane), with minor contributions from Archaean and Palaeoproterozoic magmatic units from both of the South and North China blocks. The blocks of quartzite, slate, marble, metasandstone, and chert blocks in the phyllite matrix in the Puziba area show a typical block-in-matrix texture in a tectonic mélange, and provide significant evidence from sedimentary rock blocks rather than ophiolite or volcanic rock for the existence of the MLSZ.     

Diagnostic features and field-criteria in recognition of tectonic,sedimentary and diapiric mélanges in orogenic belts and exhumed subduction-accretion complexes

Multiple episodes of deformation during the tectonic evolution of orogenic belts and ancient subduction-accretion complexes cause obfuscation of primary block-in-matrix fabric of mélanges, and thereby making the recognition of their tectonic, sedimentary or diapiric origin difficult. Here we present a comprehensive overview and synthesis of a diverse set of field-based stratigraphic and structural criteria, which are at the base of geological mapping rules, to differentiate between various mélange types, developed by disparate geological processes and mechanisms. We first define the current concepts of mélange and mélange nomenclature, and describe the most diagnostic features of tectonic, sedimentary and diapiric mélanges at different scales. We discuss some of the main issues complicating the application of these diagnostic criteria, such as: (i) similarities between the block-in-matrix fabric of different mélange types formed in partially lithified sediments at shallow structural levels, (ii) transformation of fabric elements with increased depth due to tectonic reworking and recrystallization processes, (iii) significance of “exotic” versus “native” blocks in mélange matrix, and (iv) age relationships between blocks and matrix in a mélange. We introduce two additional criteria in approaching these complexities and in recognizing different processes of polygenetic mélanges formation in the field when primary diagnostic fabrics were reworked by multiple deformational events. These new criteria are based on (i) the coherence between lithological compositions of mélange components (blocks and matrix) and characteristics and tectonic evolution of the geodynamic setting of their formation (“tectonic environment criterion”), and (ii) specificity and kinematic coherence in the distribution of deformation between blocks and the matrix (“deformation criterion”). The discussed diagnostic criteria can be applied to all field-based investigations of mélanges and broken formations in orogenic belts and exhumed subduction-accretion complexes around the world, regardless of their location, age, and tectonic history.     

Structure,emplacement, and tectonic setting of Late Devonian granitoid plutons in the Teplá–Barrandian unit,Bohemian Massif

The Štěnovice and Čistá granodiorite–tonalite plutons are small (~27 and ~38 km², respectively) intrusions that are largely discordant to regional ductile structures in the center of the upper-crustal Teplá–Barrandian unit, Bohemian Massif. Their whole-rock and trace-element compositions are consistent with medium-K calc-alkaline magma, generated above a subducted slab in a continental margin arc setting. The U–Pb zircon age of the Štěnovice pluton, newly determined at 375 ± 2 Ma using the laser ablation ICP-MS technique, is within the error of the previously published Pb–Pb age of 373 ± 1 Ma for the Čistá pluton. The two plutons also share other characteristics that are typical of concentrically expanded plutons (CEPs), such as elliptical cross-section in plan view, steep contacts, inferred downward-narrowing conical shape, faint normal zoning, and margin-parallel magmatic foliation decoupled from the regional host-rock structures. We interpret the Štěnovice and Čistá plutons as representing the initial Late Devonian stage of much more voluminous early Carboniferous arc-related plutonism (represented most typically by the Central Bohemian Plutonic Complex) in the upper crust of the central Bohemian Massif. These two plutons are important tectonic elements in that they indicate an overall shift of the arc-related plutonic activity from the ~NW to the ~SE, accompanied with a general compositional trend of the magmas from medium-K calc-alkaline to shoshonitic/ultrapotassic. Such a pattern is compatible with SE-directed subduction of the Saxothuringian Ocean beneath the Teplá–Barrandian overriding plate as a cause of arc-related magmatism in this part of the Bohemian Massif.     

Geochemistry,petrogenesis and tectonic setting of the peraluminous granites in the nacheng gold deposit,Yongning, Guangdong北大核心

The Nacheng granite locates in the Yong ning area, Guangdong Province. This paper studied petrographical and chronological characteristics of this granite as well as its petrogenesis and geological significance. The Nacheng granite mainly consists of quartz (40%-50%), orthoclase (25%-30%), palgioxlase (15%-20%) with minor accessory minerals of quartz apatite and magnetite. Geochemical analyses show that the manziying granite has high content of SiO2 (75. 56%) , K2O (4.72%), Al2O3 (13. 34%) and total alkali (Na2O+K2O=7. 49%), but relatively low MgO (0. 04%-1. 29%) and CaO (0. 39%-2. 21%). The ratios of K2O/Na 2O are generally higher than 1. The Rittmann indexes are between 1-2. 43, belonging to the high-K calc-alkaline series. The values of A/CNK vary from 1. 12 to 1. 32 with an average of 1. 10, showing the characteristics of peraluminous S type granites. Trace element analyses show that the Nacheng granite has relatively high REE content of 82. 6 × 10-6 225. 68 × 10-1, and displays right-inclined V-shape REE patterns with relatively high fractionations of LREE and HREE[(La/Yb)N = 1. 51-9. 66] and pronounced negative Eu anomalies (δEu = 0. 31-0. 74) in the chondrite-normalized REE diagrams, suggesting a typical crust-derived grandite. The magmas may be divided into two types: "low-temperature" type from mudstones and "high-temperature" type from sandstones. The Nacheng granites were formed from the post-orogenic tectonic setting.     

Refining the age of magmatism in the Altos Cuchumatanes,western Guatemala,by LA–ICPMS,and tectonic implications

The Altos Cuchumatanes Range is made up of a core of igneous and metamorphic rocks, surrounded by lower Palaeozoic and Mesozoic sedimentary strata. These units constitute the westernmost exposure of basement rocks in Guatemala and represent some of the most important crustal units in the Maya Block. New laser ablation–inductively coupled plasma mass spectrometry U-Pb zircon geochronology allows better definition of their igneous ages, inheritance and petrologic evolution. The Altos Cuchumatanes magmatism occurred during the Middle Ordovician (461 Ma) and lower Pennsylvanian (312–317 Ma), replicating similar age trends present in southern Mexico (Acatlán Complex) and the Maya Block, from Chiapas to central Guatemala (Rabinal-Salamá area) and Belize (Maya Mountains). The U-Pb inheritance from cores of the studied zircons makes it possible to decipher the pre-magmatic history of the area. During the Late Ordovician to Permo-Carboniferous, the Altos Cuchumatanes and Maya Block were located adjacent to northeastern Mexico, near the Mixteco terrane, where Ordovician megacrystic granites intruded a passive-margin sedimentary sequence. The Ordovician granites present at the southern limit of the Maya Block, in the Altos Cuchumatanes, in central Guatemala and in Belize, are the result of partial crustal melting during the initial opening of the Rheic Ocean, when both Maya and Mixteco terranes would have lain close to NW Gondwana until the closure of that ocean. The crystallization of the early Pennsylvanian granites seems to be the result of an E-dipping subduction zone that accommodated convergence between Laurentia and Gondwana.     

Morphotectonics and tectonic processes of the Southwest Indian OceanEI北大核心CSCD

The Southwest Indian mid-ocean ridge (SWIR) is an ultraslow spreading ridge. Based on the submarine bathymetric data, we develop a new division principle on submarine morphotectonics and subdivide the SWIR into the seven-order tectonic geomorphologic units. Taking its submarine morphotectonics in the middle segment and adjacent seafloors of the mid-ocean ridge between Discovery II and Gallieni transform faults as a sample, this paper systematically analyzes its tectonic evolution, segmentation, segmentation and propagation mechanism, the formation of the central rift valley, the ridge-plume interactions, and the ocean ridge jumping. The results showed that the mid-ocean ridges can be divided into four three-order morphotectonics units (i.e., one-order segments of mid-ocean ridge), from west to east, which are separated by the Andrew Bain, the Prince Edwards, the Discovery II, and the Gallieni transform faults, respectively, corresponding to ridge landforms associated with a strongly hotspot-affected ridge, a weakly hotspot-affected ridge, and a normal ultraslow spreading ridge. Each segment can be further subdivided into three or four secondary segments. This paper focuses only on the segmentation and division from fourth-order to seventh-order morphotectonics units between the Discovery II and the Gallieni transform faults (i.e., the fourth-order morphotectonics unit of mid-ocean ridges can be subdivided into other three secondary units). Here the seventh-order morphotectonics unit consists of segments of laterally-aligned rifts (shear zone), en echelon rifts, and other transverse-faulting structures. The mid-ocean ridge segment experienced three oceanic ridge jumping at about 80 Ma, 60 Ma and 40 Ma, respectively, which were affected by the Marion and Crozet hotspots, or the Madagascar Plateau, etc. The oceanic processes of the SWIR are related to the Gondwana breakup, and its tectonic processes has been analyzed in detail as the periodic pull-apart extension, domino-style half-graben, graben subsidence, oceanic core complex, etc. in axial mid-oceanic ridge since 20 Ma. ©, 2015, Science Press. All right reserved.     

Structure and tectonic evolution of the Late Jurassic–Early Cretaceous Wandashan accretionary complex,NE China

The Late Jurassic–Early Cretaceous Wandashan accretionary complex (AC) in NE China is a key region for constraining the subduction and accretion of the Palaeo-Pacific Ocean; however, the protoliths and structure of the region remain poorly understood, resulting in debates regarding crustal growth mechanisms and subduction-related accretionary processes in Northeast China. In this contribution, we integrate detailed field observations, ocean plate stratigraphy (OPS) reconstruction, and associated geological data to determine the structure and tectonic evolution of the Wandashan AC. The Wandashan AC formed through the progressive incorporation of OPS units along an oceanic trench. The observed OPS comprises, in ascending order, Permian basalt and limestone, Middle Triassic–Early Jurassic chert, Middle Jurassic siliceous shale and mudstone, and Late Jurassic–Early Cretaceous turbidite. Numerous NNE–SSW-striking thrust faults have segmented the OPS into a series of bedding-parallel tectonic slices that were successively thrust over the Jiamusi massif along a basal thrust (the Yuejinshan Fault), producing a large-scale imbricate thrust system. The Wandashan AC underwent oceanward accretion via multiple deformational processes. The OPS units were detached and rearranged along or within a decollement through offscraping, underplating, thrusting, and duplexing. The units were then emplaced over the Jiamusi massif along the basal thrust. The timing of accretion and thrusting is constrained to the latest Middle Jurassic to earliest Early Cretaceous (ca. 167–131 Ma). Reconstructed accretion-related structural lines within the Wandashan AC trend dominantly NE–SW, close to the direction of Jurassic extension at the eastern Asian continental margin. Large-scale left-lateral strike-slip movement on the Dunmi Fault during the late Early Cretaceous resulted in the folding of structural lines within the Wandashan AC, producing their present-day westward-convex orientation.     

Geology,geochronology and geochemistry of large Duobaoshan Cu-Mo-Au orefield in NE China: Magma genesis and regional tectonic implications

Duobaoshan is the largest porphyry-related Cu-Mo-Au orefield in northeastern(NE)Asia,and hosts a number of large-medium porphyry Cu(PCDs),epithermal Au and Fe-Cu skarn deposits.Formation ages of these deposits,from the oldest(Ordovician)to youngest(Jurassic),have spanned across over 300 Ma.No similar orefields of such size and geological complexity are found in NE Asia,which reflects its metallogenic uniqueness in forming and preserving porphyry-related deposits.In this study,we explore the actual number and timing of magmatic/mineralization phases,their respective magma genesis,fertility,and regional tectonic connection,together with the preservation of PCDs.We present new data on the magmatic/mineralization ages(LA-ICP-MS zircon U-Pb,pyrite and molybdenite Re-Os dating),whole-rock geochemistry,and zircon trace element compositions on four representative deposits in the Duobaoshan orefield,i.e.,Duobaoshan PCD,Tongshan PCD,Sankuanggou Fe-Cu skarn,and Zhengguang epithermal Au deposits,and compiled published ones from these and other mineral occurrences in the orefield.In terms of geochronology,we have newly summarized seven magmatic phases in the orefield:(1)Middle-Late Cambrian(506-491 Ma),(2)Early and Middle Ordovician(485-471 Ma and~462 Ma),(3)Late Ordovician(450-447 Ma),(4)Early Carboniferous and Late-Carboniferous to Early Permian(351-345 and 323-291 Ma),(5)Middle-Late Triassic(244-223 Ma),(6)Early-Middle and Late Jurassic(178-168 Ma and~150 Ma),and(7)Early Cretaceous(~112 Ma).Three of these seven major magmatic phases were coeval with ore formation,including(1)Early Ordovician(485-473 Ma)porphyry-type Cu-Mo-(Au),(2)Early-Middle Triassic(246-229 Ma)porphyry-related epithermal Au-(Cu-Mo),and(3)Early Jurassic(177-173 Ma)Fe-Cu skarn mineralization.Some deposits in the orefield,notably Tongshan and Zhengguang,were likely formed by more than one mineralization events.In terms of geochemistry,ore-causative granitoids in the orefield exhibit adakite-like or adakite-normal arc transitional signatures,but those forming the porphyry-/epithermal-type Cu-Mo-Au mineralization are largely confined to the former.The varying but high Sr/Y,Sm/Yb and La/Yb ratios suggest that the ore-forming magmas were mainly crustal sourced and formed at different depths(clinopyroxene-/amphibole-/garnet-stability fields).The adakite-like suites may have formed by partial melting of the thickened lower crust at 35-40 km(for the Early Ordovician arc)and>40 km(for the Middle-Late Triassic arc)depths.The Early Jurassic Fe-Cu skarn orecausative granitoids show an adakitic-normal arc transitional geochemical affinity.These granitoids were likely formed by partial melting of the juvenile lower crust(35-40 km depth),and subsequently modified by assimilation and fractional crystallization(AFC)processes.In light of the geological,geochronological and geochemical information,we proposed the following tectonometallogenic model for the Duobaoshan orefield.The Ordovician Duobaoshan may have been in a continental arc setting during the subduction of the Paleo-Asian Ocean,and formed the porphyry-related deposits at Duobaoshan,Tongshan and Zhengguang.Subduction may have ceased in the latest Ordovician,and the regional tectonics passed into long subsidence and extension till the latest Carboniferous.This extensional tectonic regime and the Silurian terrestrial-shallow marine sedimentation had likely buried and preserved the Ordovician Duobaoshan magmatic-hydrothermal system.The south-dipping Mongol-Okhotsk Ocean subduction from north of the orefield had generated the Middle-Late Triassic continental arc magmatism and the associated Tongshan PCD and Zhengguang epithermal Au mineralization(which superimposed on the Ordovician PCD system).The Middle Jurassic closure of Mongol-Okhotsk Ocean in the northwestern Amuria block(Erguna terrane),and the accompanying Siberia-Amuria collision,may have placed the Paleo-Pacific subduction system in NE China(including the orefield)under compression,and formed the granodiorite-tonalite and Fe-Cu skarn deposits at Sankuanggou and Xiaoduobaoshan.From the Middle Jurassic,the consecutive accretion of Paleo-Pacific arc terranes(e.g.,Sikhote-Alin and Nadanhada)onto the NE Asian continental margin may have gradually distant the Duobaoshan orefield from the subduction front,and consequently arc-type magmatism and the related mineralization faded.The minor Late Jurassic and Cretaceous unmineralized magmatism in the orefield may have triggered mainly by the far-field extension led by the post-collisional(Siberia-Amuria)gravitational collapse and/or Paleo-Pacific backarc-basin opening.     

Analysis of tectonic stress field and potential activities in Lingshan Fault Zone,GuangxiEI北大核心CSCD

Tectonic stress is one of the most active factors in the earth crust, therefore figuring out its mode of action and form of conversion is of great significance to guide the research on fault activity. Through a survey on many structural features in the Lingshan area, which include vertical folds, conjugated joints and scratch on faults surface, etc., we investigated the revolution characteristics of the tectonic stress field, mode of action and sequence of tectonic stress in the Lingshan area by structural analysis method. The results show that the region experienced four periods of function of tectonic stress field which came from four different directions: NW, NE, nearly EW and nearly SN. The force of NW appeared early than that of NE, and then the compression of nearly EW. We concluded that the structural framework of the study area has been formed by the effect of SN compressed stress which is still active today. Meanwhile, based on the geologic analysis of the mechanism of tectonic stress, we analyzed the structural stress quantitatively in the Lingshan area using finite element numerical simulation. The simulation demonstrated that the overlapping parts of strain intensive zone in higher value of maximum principal stress and lower value of maximum principal stress, such as, fault-bend parts, the fault intersections and the faults disappears site, have higher energy and prone to reactive in the later period. We think that the north of Mountain Luoyang, Fengtong, Tieling, Luwu are high stress concentration area, and the energy release is minor. And thus, we proposed a high-risk district and a sub-high-risk district in the Lingshan area is according to the size and scope of the stress. ©, 2015, Science Press. All right reserved.     

Petrogenesis of Late Carboniferous granitoids in the Chihu area of Eastern Tianshan,Northwest China,and tectonic implications: geochronological,geochemical, and zircon Hf–O isotopic constraints

ABSTRACT

This contribution presents new SIMS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf–O isotope systematic on an example of Late Carboniferous granodiorite and porphyritic granodiorite intrusions from the Chihu area of Eastern Tianshan, Xinjiang. SIMS zircon U–Pb dating indicates that the Chihu granodiorite and porphyritic granodiorite formed at 320.2 ± 2.4 Ma and 314.5 ± 2.5 Ma, respectively. These rocks are metaluminous to weakly peraluminous with an A/CNK value of 0.92–1.58, as well as low 10000 Ga/Al, Zr + Nb + Y + Ce, and Fe₂O₃^T/MgO values, which suggest an I-type normal island arc magmatic suite. The porphyritic granodiorite has a slightly higher Sr/Y ratio (28–37) and lower Y (6.9–11.7 ppm) and Yb (0.98–1.49 ppm) contents, suggesting mild adakite affinities. In situ Hf–O isotopic analyses using LA-ICP-MS-MC and SIMS indicate that the ε_Hf(t) and δ¹⁸O values of granodiorite zircons vary from +11.5 to +14.9 and 4.80 to 5.85 ‰, respectively, similar to values for porphyritic granodiorite zircons, which vary from +11.9 to +17.2 and 3.78 to 4.71 ‰, respectively. The geochemical and isotopic data imply that the Chihu granodiorite and porphyritic granodiorite share a common origin, most likely derived from partial melts of the subduction-modified mantle. Based on the regional geological history, geochemistry of the Chihu intrusions, and new isotopic studies, we suggest that the Late Carboniferous magma was generated during the period of the northward subduction of the Palaeo-Tianshan ocean plate beneath the Dananhu–Tousuquan island arc.     

Geochemistry and tectonic significance of peridotites from the South Sandwich arc–basin system, South Atlantic

Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc–basin system and for the nature of interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling shows that they represent the residues from 15–20% melting at oxygen fugacities close to the QFM buffer. The dunites have U-shaped REE patterns, low incompatible element abundances and high Cr# (0.66–0.77). Petrography and geochemistry indicate that the latter are the product of intense interaction between peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of lesser interaction between peridotite and melt saturated with olivine ± clinopyroxene. The data demonstrate that the harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centre), and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich arc magmas. The second dredge locality, near the South Sandwich Trench–Fracture Zone intersection, yielded rocks ranging from lherzolite to harzburgite that could similarly have resulted from a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and island arc tholeiite. The South Sandwich peridotites have a similar petrogenetic history to those from Conical Seamount in the Mariana forearc in the sense that both involved interaction between arc magma and pre-existing mantle lithosphere of different provenance. However, the precise compositions of the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, data from the South Sandwich and Izu–Bonin–Mariana systems emphasise the potential significance of peridotite geochemistry in unravelling the complex tectonic histories of forearcs past and present. Received: 31 August 1999 / Accepted: 3 December 1999     

Discovery of eclogite in the Bangong Co–Nujiang ophiolitic mélange,central Tibet,and tectonic implications

An eclogite has been recently identified within ophiolitic mélange in the western segment of the Bangong Co–Nujiang suture zone, at Shemalagou in the Gaize area of central Tibet. The eclogite consists of garnet, omphacite, phengite, rutile, quartz, diopside, and amphibole. The omphacite, which has not been recognized in the suture zone until this study, occurs as rare relics within diopside grains in the eclogite. Phase equilibria modeling shows that the eclogite formed under P–T conditions of 22–28 kbar and 600–650 °C with a low geothermal gradient of ca. 8 °C/km, suggesting that it formed during the subduction of oceanic crust. The protoliths of the eclogite and coexisting garnet amphibolites have geochemical characteristics similar to those of normal mid-ocean ridge basalt (N-MORB), confirming that the eclogites formed from oceanic crust. The presence of high-pressure (HP) eclogite indicates that the ophiolitic mélange in the Bangong Co–Nujiang suture zone underwent oceanic subduction and was subsequently exhumed. We conclude that this ophiolitic belt represents a newly identified HP metamorphic belt in the Tibetan Plateau, adding to the previously recognized Songduo and Longmucuo–Shuanghu eclogite belts. This discovery will result in an improved understanding of the tectonic evolution of the Bangong Co–Nujiang suture zone and the Tibetan Plateau as a whole.