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1.
The present study examines the petrology and geochemistry of the Early Paleozoic Motai serpentinites, the South Kitakami Belt, northeast Japan, to reveal the subduction processes and tectonics in the convergent margin of the Early Paleozoic proto-East Asian continent. Protoliths of the serpentinites are estimated to be harzburgite to dunite based on the observed amounts of bastite (orthopyroxene pseudomorph). Relic chromian spinel Cr# [=Cr/(Cr + Al)] increases with decreasing amount of bastite. The compositional range of chromian spinel is similar to that found in the Mariana forearc serpentinites. This fact suggests that the protoliths of the serpentinites are depleted mantle peridotites developed beneath the forearc regions of a subduction zone. The Motai serpentinites are divided into two types, namely, Types 1 and 2 serpentinites; the former are characterized by fine-grained antigorite and lack of olivine, and the latter have coarse-grained antigorite and inclusion-rich olivine. Ca-amphibole occurs as isolated crystals or vein-like aggregates in the Type 1 serpentinites and as needle-shaped minerals in the Type 2 serpentinites. Ca-amphibole of the Type 1 serpentinites is more enriched in LILEs and LREEs, suggesting the influence of hydrous fluids derived from slabs. By contrast, the mineral assemblage, mineral chemistry, and field distribution of the Type 2 serpentinites reflect the thermal effect of contact metamorphism by Cretaceous granite. The Ca-amphibole of the Type 1 serpentinites is different from that of the Hayachine–Miyamori Ophiolite in terms of origin; the latter was formed by the infiltration of melts produced in an Early Paleozoic arc–backarc system. Chemical characteristics of the Ca-amphibole in the ultramafic rocks in the South Kitakami Belt reflect the tectonics of an Early Paleozoic mantle wedge, and the formation of the Motai metamorphic rocks in the forearc region of the Hayachine–Miyamori subduction zone system, which occurred at the Early Paleozoic proto-East Asian continental margin. 相似文献
2.
Re-Os geochronology of Cu and W-Mo deposits in the Balkhash metallogenic belt,Kazakhstan and its geological significance 总被引：2，自引：0，他引：2
Xuanhua Chen Wenjun Qu Shuqin Han Seitmuratova Eleonora Nong Yang Zhengle Chen Fagang Zeng Andao Du Zhihong Wang 《地学前缘(英文版)》2010,1(1)
<正>The Central Asian metallogenic domain(CAMD) is a multi-core metallogenic system controlled by boundary strike-slip fault systems.The Balkhash metallogenic belt in Kazakhstan,in which occur many large and super-large porphyritic Cu—Mo deposits and some quartz vein- and greisen-type W—Mo deposits,is a well-known porphyritic Cu—Mo metallogenic belt in the CAMD.In this paper 11 molybdenite samples from the western segment of the Balkhash metallogenic belt are selected for Re—Os compositional analyses and Re—Os isotopic dating.Molybdenites from the Borly porphyry Cu deposit and the three quartz vein-greisen W—Mo deposits—East Kounrad.Akshatau and Zhanet—all have relatively high Re contents(2712—2772μg/g for Borly and 2.267—31.50μg/g for the other three W—Mo deposits),and lower common Os contents(0.670—2.696 ng/g for Borly and 0.0051—0.056 ng/g for the other three).The molybdenites from the Borly porphyry Cu—Mo deposit and the East Kounrad,Zhanet,and Akshatau quartz vein- and greisen-type W—Mo deposits give average model Re—Os ages of 315.9 Ma,298.0 Ma,295.0 Ma,and 289.3 Ma respectively.Meanwhile,molybdenites from the East Kounrad,Zhanet,and Akshatau W—Mo deposits give a Re—Os isochron age of 297.9 Ma,with an MSWD value of 0.97.Re-Os dating of the molybdenites indicates that Cu—W—Mo metallogenesis in the western Balkhash metallogenic belt occurred during Late Carboniferous to Early Permian(315.9—289.3 Ma),while the porphyry Cu—Mo deposits formed at—316 Ma,and the quartz vein-greisen W—Mo deposits formed at ~298 Ma.The Re—Os model and isochron ages thus suggest that Late Carboniferous porphyry granitoid and pegmatite magmatism took place during the late Hercynian movement.Compared to the Junggar-East Tianshan porphyry Cu metallogenic belt in northwestern China,the formation of the Cu—Mo metallogenesis in the Balkhash metallogenic belt occurred between that of the Tuwu-Yandong in East Tianshan and the Baogutu porphyry Cu deposits in West Junggar. Collectively,the large-scale Late Carboniferous porphyry Cu—Mo metallogenesis in the Central Asian metallogenic domain is related to Hercynian tectono-magmatic activities. 相似文献
3.
Cooling rates based on the retrograde diffusion of Fe2+ and Mg between garnet and biotite inclusions commonly show two contrasting scenarios: a) narrow closure temperature range with apparent absence of retrograde diffusion; or b) high result dispersion due to compositional variations in garnet and biotite. Cooling rates from migmatites, felsic and mafic granulites from Ribeira Fold Belt (SE Brazil) also show these two scenarios. Although the former can be explained by very fast cooling, the latter is often the result of open-system behaviour caused by deformation. Retrogressive cooling during the exhumation of granulite-facies rocks is often processed by thrusting and shearing which may cause plastic deformation, fractures and cracks in the garnet megablasts, allowing chemical diffusion outside the garnet megablast – biotite inclusion system.However, a careful use of garnets and biotites with large Fe/Mg variation and software that reduces result dispersion provides a good correlation between closure temperatures and the size of biotite inclusions which are mostly due to diffusion and compositional readjustment to thermal evolution during retrogression.Results show that felsic and mafic granulites have low cooling rates (1–2 °C/Ma) at higher temperatures and high cooling rates (∼100 °C/Ma) at lower temperatures, suggesting a two-step cooling/exhumation process, whereas migmatites show a small decrease in cooling rates during cooling (from 2.0 to 0.5 °C/Ma). These results agree with previously obtained thermochronological data, which indicates that this method is a valid tool to obtain meaningful petrological cooling rates in complex high-grade orogenic belts, such as the Ribeira Fold Belt. 相似文献
4.
《Geological Journal》2018,53(3):1005-1021
The Heihe–Nenjiang–Hegenshan suture zone has long been accepted as the major tectonic boundary between the Xing'an and Songliao blocks and extends through the Great Xing'an Range in NE China, but its location of the northern segment between the Moguqi and Nenjiang areas and its timing remain unclear. We address these issues by presenting zircon LA‐ICP‐MS U–Pb ages, Lu‐Hf isotopes, bulk‐rock major, and trace elemental data for mylonitized rhyolites collected from the Moergenhe Formation in the Nenjiang area and for gabbros of the Moguqi area, respectively. The mylonitized rhyolites, which display an arc‐related geochemical affinity with enrichment in Th and U, and depletion of Nb, Ta, and Ti, and gently right‐tilted rare earth element (REE) patterns (light REE [LREE]/heavy REE [HREE] =4.53–7.60), as well as the εHf (t) values (+6.4 to +11.8) of analyzed zircons, indicate an origin by partial melting of potentially young lower continental crust of a subducting slab. The zircon LA‐ICP‐MS U–Pb data show the formation age of the mylonitized rhyolites is 352.4 Ma. The analyzed gabbros with an emplacement age of 352.6 Ma have high concentrations of Th and U, slightly enriched LREE patterns and relative low LREE/HREE ratios (4.3 to 4.6). These features, together with their high positive εHf (t) values (+7.7 to +15.2), suggest that they were likely derived from the partial melting of a depleted mantle source that was metasomatized by subduction‐related fluids. Combined with the geochemical features of the coeval igneous rocks from the northern Great Xing'an Range, these results reveal that the existence of an early Carboniferous NE‐trending magmatic arc (ca. 350–330 Ma), extending along the west of the Heihe–Nenjiang–Hegenshan suture zone, gives more constraints on the amalgamation of the Xing'an and Songliao blocks along the Nenjiang–Moguqi areas and indicates that the amalgamation should have terminated by at least the end of the early Carboniferous. 相似文献
5.
M. Umeda 《Island Arc》1998,7(4):637-646
Five radiolarian zones, from the Upper Silurian to Middle Devonian, are discriminated from the tuffaceous successions of the Joryu and Nakahata Formations of the Yokokurayama Group of the Yokokurayama area and the Konomori area in the Kurosegawa Belt, Southwest Japan. The definition of the zones is based on the first appearance biohorizon of the characteristic species. The zones are the Pseudospongoprunum sagittatum, Futobari solidus, Trilonche (?) sp. A, Glanta fragilis and Protoholoeciscus hindea zones, in ascending order. The preliminary age assignments for the zones are discussed on the basis of the comparison with other previous documented faunas. The age determination of the formations suggests the presence of unconformities and the episodic sedimentation of the tuffaceous strata in the Yokokurayama Group. 相似文献
6.
《Geodinamica Acta》2013,26(3-4):141-155
Abstract Magmatic and metamorphic events, imprinted in the crystalline rocks of the so-called core mountains inside the Alpine structure of the Inner Carpathians, allow the re-construction of the history of the Rheic Ocean opening, its development and its final closure. Intra-Carpathian core-mountains are the remnants of the continents that drifted away from Gondwana and docked, initially, with Baltica as part of Avalonia and later on as parts of the Gondwana-derived Armorica Terrane Group or as a separate micro-continent. All magmatic suites, mafic and felsic, present in the Carpathians core mountains, show similarities to those found in the European Variscan Belt. All described- and dated metamorphic and magmatic events also have equivalents in the evolution of the Caledonian-Variscan Belts of Europe. The most pronounced feature of all Carpathian core mountains is the syn-collisional, multistage I/S granitoid magmatism (370-340 Ma) related to subduction, mafic-magma influx, extensional decompression and slab melting. That episode marked the Laurussia - Gondwana collision and closure of the Rheic Ocean, as in the whole of Central and Western Europe. The Carpathian core-mountains, currently dispersed inside the Alpine mountain chain, can be considered the broken fragments of the eastern prolongation of the Variscan orogenic belts – possibly part of the Moldanubian Unit. 相似文献
7.
The Palaeoproterozoic tectonic evolution of the Jiao–Liao–Ji Belt (JLJB) in the North China Craton remains controversial, particularly in the absence of adequate evidence for oceanic plate subduction. Here, we report SHRIMP U–Pb zircon ages, whole-rock geochemistry, and Sm–Nd isotope data on felsic tuffs from the Anshan–Helan–Lianshanguang area in the Liaodong Peninsula which provides important insights into the early tectonic evolution of the JLJB. The zircon SHRIMP U–Pb ages suggest formation of the tuffs crystallized at ca. 2.17 Ga, which coincide with the crystallization of the voluminous Liaoji granitoids in this belt. The felsic tuffs show calc-alkaline features and are characterized by a wide range of SiO2 (61.89–72.55 wt.%), MgO (1.35–5.80 wt.%), Cr (36.0–67.8 ppm) and Ni (5.88–28.8 ppm) contents, and low TiO2 (0.42–0.64 wt.%) and total Fe2O3 (2.14–5.64 wt.%) concentrations. They also display enrichment in LREE and depletion in Nb, Ta, and Ti similar to the features of continental arc andesites. Their wide variation in whole-rock εNd(t) (−2.6 to +3.4) combined with the synchronous magmatic activity in this region suggests that they were derived from mixing of mafic (e.g., those in the Liaohe suite) and felsic (e.g., the Liaoji granitoids) magmas, followed by fractional crystallization. In view of the widespread distribution of magmatism, including the Helan felsic tuffs and Liaoji granitoids, and the synchronous sedimentary rocks in the Liaodong Peninsula, we propose that the evolution of the JLJB involved an initial back-arc extension stage, followed by arc-continent collision. 相似文献
8.
The Kamieniec Metamorphic Belt comprises a volcano-sedimentary succession exposed within a collision zone between the Saxothuringian and Brunovistulian crustal domains of the European Variscides. The studied rocks recorded two metamorphic episodes. The first episode, M1, occurred at conditions of c. 485 ± 25 °C and 18 ± 1.8 kbar related to burial within a subduction zone. The subsequent episode, M2, was linked to the final phases of exhumation to mid-crustal level, associated with pressure and temperature (P–T) conditions ranging from c. 520 ± 26 °C and 6 ± 0.6 kbar through 555 ± 28 °C and 7 kbar ± 0.7 to ~590 ± 30 °C and 3–4 ± 0.4 kbar. The documented deformation record is ascribed to three events, D1 to D3, interpreted as related to the burial and subsequent exhumation of the Kamieniec Metamorphic Belt. The D1 event must have witnessed the subduction of the Kamieniec Metamorphic Belt rock succession whereas the D2 event was associated with the exhumation and folding of the Kamieniec Metamorphic Belt in an E-W-directed shortening regime. A subsequent folding related to the D2 event was initiated at HP conditions, however, the planar fabric produced during a late stage of the D2 event, defined by a low-pressure mineral assemblage M2, indicates that the D2 final stage was synchronous with the onset of the M2 episode. Consequently, the entire D2 event seems to have been associated with the exhumation of the Kamieniec Metamorphic Belt to mid crustal level. The third deformation event D3, synchronous with the M2 episode, marked the last stage of the exhumation, and was linked to emplacement of granitoid veins and lenses. The latter resulted in heating and rheological weakening of the entire rock succession and in the formation of non-coaxial shear zones. 相似文献
9.
10.
Mengyan Shi Quanlin Hou Chunming Wu Quanren Yan Nannan Cheng Qian W. L. Zhang Hao Y. C. Wang 《Geological Journal》2021,56(8):3959-3973
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic collages in the world with prominent juvenile crust addition, where the southernmost margin of the CAOB located is one hot topic in the international studies. The Dunhuang Block located to the south of the Beishan Orogen is a key position for outlining the tectonic framework of the south margin of the CAOB. However, at present, the tectonic attribution of the Dunhuang Block is still controversial. In this paper, we focus on the origins and protoliths of the meta-mafic rocks from the Hongliuxia complex in the southern Dunhuang Block for determining the tectonic attribution of the Dunhuang Block. In the field, the meta-mafic rocks are mainly exposed as tectonic blocks within the metasedimentary rocks displaying the “block-in-matrix” fabrics or as tectonic slices juxtaposed by faults. Geochemically, the meta-mafic rocks could be divided into three groups based on the rare earth elements (REE) and trace elements: the group 1 samples are depleted in the LREE relative to HREE and show flat HREE patterns, similar to the mid-oceanic ridge basalt; the group 2 samples show flat REE patterns without obvious differentiation between the LREE and HREE, like the oceanic plateau basalt; the group 3 samples are enriched in LREE relative to LREE, and have negative Nb, Ta, and Ti anomalies, similar to the island arc basalt. Conclusively, the protoliths of the meta-mafic rocks dominantly belong to the oceanic plate components. They were metamorphosed during Early Silurian to Middle Devonian in different depths of the subduction zone documented in previous studies. So, we suggest the Hongliuxia complex is an exhumed Palaeozoic subduction–accretion complex (SAC) formed during the oceanic plate subduction. This result confirms the Palaeozoic subduction event occurred in the Dunhuang region and indicates that the Dunhuang Block is a Palaeozoic Orogen rather than a part of Precambrian continent block. Considering the regional geology context, we speculate the Dunhuang Orogen belongs to the southernmost margin of the CAOB. 相似文献
