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1.
The Qilian–Qaidam orogenic belt at the northern edge of the Tibetan Plateau has received increasing attention as it recorded a complete history from continental breakup to opening and closure of ocean basin, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. Determining a geochronological framework of the initiation and termination of the fossil Qilian Ocean subduction in the North Qilian orogenic belt plays an essential role in understanding the whole tectonic process. Dating the high-pressure metamorphic rocks in the North Qilian orogenic belt, such as blueschist and eclogite, is the key in this respect. A blueschist from the southern North Qilian orogenic belt was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, and Sm–Nd multichronometric approaches. Pseudosection modeling indicates that the blueschist was metamorphosed under peak P–T conditions of 1.4–1.6 GPa and 530–550 °C. Zircon U–Pb ages show no constraints on the metamorphism due to the lack of metamorphic growth of zircon. Lu–Hf and Sm–Nd ages of 466.3 ± 2.0 Ma and 462.2 ± 5.6 Ma were obtained for the blueschist, which is generally consistent with the U–Pb zircon ages of 467–489 Ma for adjacent eclogites. Lutetium and Sm zoning profiles in garnet indicate that the Lu–Hf and Sm–Nd ages are biased toward the formation of the garnet inner rim. The ages are thus interpreted to reflect the time of blueschist-facies metamorphism. Previous 40Ar/39Ar ages of phengitic muscovite from blueschist/eclogite in this area likely represent a cooling age due to the higher peak metamorphic temperature than the argon retention temperature. The differences of peak metamorphic conditions and metamorphic ages between the eclogites and adjacent blueschists indicate that this region likely comprises different tectonic slices, which had distinct P–T histories and underwent high-pressure metamorphism at different times. The initial opening of the Qilian Ocean could trace back to the early Paleozoic, and the ultimate closure of the Qilian Ocean was no earlier than c. 466 Ma.  相似文献   

2.
Three metapelite samples from the Aksu blueschist terrane, Xinjiang, China, were dated by the 40Ar/39Ar method on separated phengite grains, obtaining plateau ages in the range of 741−757 Ma. In contrast, the measured Rb and Sr isotope data for the three samples yielded isochron ages ranging from 630 Ma to 900 Ma, suggesting large heterogeneity in the blueschist protolith and suppression of diffusional exchange owing to the low-temperature metamorphic conditions. Because the protolith of Aksu blueschist is composed of oceanic materials that formed 40Ar-free phengite during HP and UHP metamorphism and the apparent 40Ar/39Ar plateaus ages in this study are similar to previous K–Ar and Rb–Sr ages, the existence of excess argon in these rocks is considered to be insignificant. As a result, the 40Ar/39Ar plateau ages in this study (ca. 750 Ma) likely represent the approximate time for peak metamorphism, given the low peak metamorphic temperatures for the Aksu blueschist terrane (300−400 °C). This strongly implies that modern style, cold subduction tectonics operated along the margin of the Aksu terrane no later than 750 Ma, in Neoproterozoic time.  相似文献   

3.
A 40Ar/39Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma.These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new 40Ar/39Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T–t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.  相似文献   

4.
The Nevado–Filabride complex, the lowest complex in the Betic hinterland, forms a stack of Alpine nappes. The tectonic units consist of metasedimentary sequences whose ages are not well constrained. Gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex. New radiometric data from U-Pb zircon dating on gneisses from the western Sierra Nevada confirm the presence of late Carboniferous intrusive rocks in the Betic hinterland. These results, combined with available data from the literature and a detailed structural analysis, suggest that the gneisses represent a single late Variscan magmatic event. Evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations, in conjunction with geochemical data, U-Pb zircon (314 ± 7; 304 ± 23 Ma) and 40Ar/39Ar tourmaline (319.85 ± 5.81; 317.85 ± 3.67 Ma) geochronology. A pre-late Carboniferous age for the basal formation of the Nevado–Filabride sequence can be inferred. The gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites, respectively. Superposed tectonic units rather than a continuous Palaeozoic sequence is supported by the occurrence of Palaeozoic rocks at the top of the Nevado–Filabride complex. The nappe tectonics, as evidenced from the contractional character of the unit boundaries and the superposition of higher-grade on lower-grade metamorphic rocks, is additionally supported by the superposition of older on younger rocks as revealed from radiometric data.  相似文献   

5.
The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, 40Ar/39Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The 40Ar/39Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite 40Ar/39Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.  相似文献   

6.
翟庆国  李才  王军  陈文  张彦 《岩石学报》2009,25(9):2281-2288
藏北羌塘中部沿龙木错-双湖-线出露一条低温高压变质带,目前已有多处蓝片岩的报道.然而,除冈玛错地区产有典型的蓝闪石外,多数地区并没有典型蓝闪石的报道.绒玛蓝片岩位于羌塘中部高压变质带的中段,是该带中规模最大、保存最好的蓝片岩,对蓝片岩进行了详细的岩石学和矿物学研究,钠质角闪石主要为蓝闪石、青铝闪石、钠闪石和镁钠闪石.对蓝片岩中蓝闪石和多硅白云母进行了40Ar/39Ar定年,获得了227.3±3.8Ma和215±1.5Ma的坪年龄,分别代表蓝片岩快速俯冲消减和俯冲作用结束开始折返抬升的时代.绒玛蓝片岩岩石学、矿物学和40Ar/39Ar年代学研究为羌塘中部高压变质带的研究提供了新的资料.  相似文献   

7.
《Gondwana Research》2014,25(3-4):1038-1050
The New England Orogen of easternmost Australia is dominated by suites of Palaeozoic to earliest Mesozoic rocks that formed in supra-subduction zone settings at Gondwana's eastern margin. On the northern New South Wales coast at Rocky Beach, Port Macquarie, a serpentinite mélange carries rare tectonic blocks of low-grade, high-pressure, metamorphic rocks derived from sedimentary and igneous protoliths. Dominant assemblages are glaucophane + phengite ± garnet ± lawsonite ± calcite ± albite blueschists and lawsonite-bearing retrogressed garnet + omphacite eclogites. In some blocks with sedimentary protoliths, eclogite forms folded layers within the blueschists, which is interpreted as Mn/(Mn + Fe) compositional control on the development of blueschist versus eclogite assemblages. Review of previous studies indicates pressure–temperature conditions of 0.7–0.5 GPa and ≤ 450 °C. Three samples of high-pressure metasedimentary rocks contain Archaean to 251 ± 6 Ma (Permo-Triassic) zircons, with the majority of the grains being Middle Devonian to Middle Carboniferous in age (380–340 Ma). Regardless of age, all grains show pitting and variable rounding of their exteriors. This morphology is attributed to abrasion in sedimentary systems, suggesting that they are all detrital grains. New in situ metamorphic zircon growth did not develop because of the low temperature (≤ 450 °C) of metamorphism. The Permo-Triassic, Devonian and Carboniferous zircons show strong heavy rare earth element enrichment and negative europium anomalies, indicating that they grew in low pressure igneous systems, not in a garnet-rich plagioclase-absent high pressure metamorphic environment. Therefore the youngest of these detrital zircons provides the maximum age of the metamorphism. A titanite + rutile porphyroblast within an eclogite has a U–Pb age of 332 ± 140 Ma (poor precision due to very low U abundances of mostly < 1 p.p.m.) and provides an imprecise direct age for metamorphism. In the south of the Port Macquarie area, the Lorne Basin ≥ 220 Ma Triassic sedimentary and volcanic rocks unconformably overlie serpentinite mélange, and provide the minimum age of the high-pressure metamorphism. Our preferred interpretation is that the 251 Ma zircons are detrital and thus the Port Macquarie high-pressure metamorphism is constrained to the end of the Permian–Early Triassic. Emplacement of the serpentinite mélange carrying the Rocky Beach high-pressure rocks might have been due to docking of a Permian oceanic island arc (represented by the Gympie terrane in southern Queensland?) and an Andean-style arc at the eastern Australian margin (expressed in the New England Orogen by 260–230 Ma north-south orientated magmatic belts). Alternatively, if the 251 Ma grains are regarded as having grown in thin pegmatites, then the dominant Devonian–Carboniferous detrital population still indicates a maximum age for the high pressure metamorphism of ca. 340 Ma. A ≤ 340 Ma age of metamorphism would still be much younger than the previously suggested ca. 470 Ma (Ordovician) age, which was based on Ar–Ar dating of phengites.  相似文献   

8.
The southern North China craton hosts numerous world-class porphyry Mo and Pb-Zn-Ag vein deposits. Whether or not the Pb-Zn-Ag veins are genetically associated with the porphyry Mo system remains contentious. Here we focus on the genetic relationships between the Sanyuangou Pb-Zn-Ag vein deposit and the world-class Donggou porphyry Mo deposit, and discuss the potential implications from the spatial and temporal relationships between porphyry and vein systems in the southern North China craton.At Sanyuangou, vein-hosted sulfide mineralization mainly comprises pyrite, sphalerite, and galena, with minor chalcopyrite, pyrrhotite, bornite, tetrahedrite, covellite, polybasite and argentite. The mineralization is hosted by a quartz diorite stock, which has a zircon U-Pb age of 1756 ± 9 Ma. However, sericite from alteration selvages of Pb-Zn-Ag sulfide mineralization yields a well-defined 40Ar/39Ar plateau age of 115.9 ± 0.9 Ma. Although nominally younger, the sericite 40Ar/39Ar age is similar to the age of the nearby Donggou porphyry Mo deposit (zircon U-Pb age of 117.8 ± 0.9; molybdenite Re-Os ages of 117.5 ± 0.8 Ma and 116.4 ± 0.6 Ma). Pyrite from Donggou has elevated contents of Mo and Bi, whereas pyrite from Sanyuangou is enriched in Cu, Zn, Pb, Ag, Au, and As. This trace element pattern is consistent with metal zonation typically observed in porphyry related metallogenic systems. Pyrite grains from Sanyuangou have lead isotopes overlapping those from Donggou (17.273–17.495 vs. 17.328–17.517 for 206Pb/204Pb, 15.431–15.566 vs. 15.408–15.551 for 207Pb/204Pb, and 37.991–38.337 vs. 38.080–38.436 for 208Pb/204Pb). Collectively, the geological, geochronological, and geochemical data support a magmatic-hydrothermal origin for the Sanyuangou Pb-Zn-Ag deposit and confirm that the Pb-Zn-Ag veins and the Donggou Mo deposit form a porphyry-related magmatic-hydrothermal system.Given the widespread Pb-Zn-Ag veins and Mo mineralized porphyries in many districts of the southern North China craton, the model derived from this study has broad implications for further exploration of Mo and Pb-Zn-Ag resources in the area.  相似文献   

9.
《Chemical Geology》2007,236(1-2):134-166
The ∼ 5000 km3 Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the 40Ar/39Ar and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution 238U–206Pb zircon dating of Tertiary bentonites and Fish Canyon Tuff; a test for age “concordance” by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a ≥ 0.4 Ma age difference between the U–Pb zircon ages and 40Ar/39Ar sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U–Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 200 40Ar/39Ar measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system.Disequilibrium-corrected 206Pb/238U ages obtained for 29 single-crystal and three multi-grain analyses span an interval of ∼ 28.67–28.03 Ma and yield a weighted mean age of 28.37 ± 0.05 Ma (95% confidence level), with MSWD = 8.4. The individual dates resolve a range of ages in excess of analytical precision, covering ∼ 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the “chemical abrasion” (CA) technique [Mattinson, J.M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1–2): 47–66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 ± 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the ∼ 28.0 Ma 40Ar/39Ar eruption age of FCT – even when considering the possibility that the latter may be low by as much as ∼ 1% due to a miscalibration of the 40K decay constants – and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the 40Ar/39Ar method.Even though data statistics preclude unambiguous conclusions, 40Ar/39Ar dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 ± 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology, 145: 117–152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2–0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a “staircase” argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon magma indicate that the body remained above its solidus (∼ 700 °C) for an extended period of time (> 105 years). At these temperatures, argon volume diffusion is thought to be fast enough to prevent accumulation of radiogenic Ar. If this statement were true, an existing isotopic record should have been completely reset within a few hundred years, regardless of the phase and initial age of the phenocryst. As these minerals are unlikely to be xenocrysts that were incorporated within such a short time span prior to eruption, we suggest that a fraction of radiogenic Ar can be retained > 105 years, even at T ∼ 700 °C.  相似文献   

10.
Blueschists and eclogites located in the Tasmanides of eastern Australia preserve evidence of contrasting modes of exhumation. A review of structural, metamorphic, geochronological and geochemical data indicates that these HP metamorphic rocks can be sub-divided into three main groups: (i) eclogite–blueschists with calc-alkaline and tholeiitic affinities contained within thick sedimentary sequences (called continental HP rocks); (ii) moderate-pressure (< 9 kbar) blueschist of arc to MORB-type composition within sedimentary or serpentinite mélange zones (called accretionary HP rocks) and (iii) eclogites of MORB-type composition with or without a pervasive blueschist overprint contained within serpentinite (called exotic HP rocks). Three different modes of exhumation can be ascribed to the different rock types, namely: (i) exhumation influenced by the buoyancy of continental slabs; (ii) exhumation of accretionary HP rocks by corner flow and/or extensional collapse in the accretionary wedge or (iii) discontinuous exhumation of eclogites triggered by slab rollback and trench retreat. We suggest that a dominant west-dipping, eastward migrating subduction zone can explain the distribution and formation of HP metamorphic rocks in the Tasmanides.Thermobarometric and geochronological data from eclogites and blueschists in the Peel–Manning Fault System (New England Orogen) also provide evidence for discontinuous exhumation of subducted oceanic rocks. These data indicate that eclogites were exhumed from depths of ~ 70 km to ~ 30 km during the Ordovician (490–470 Ma), with terminal exhumation and exposure along the Peel–Manning Fault system probably occurring during the Permian. Based on these timing constraints, we suggest a model where HP rocks reside between depth-dependant exhumation circuits for considerable lengths of time.  相似文献   

11.
The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect (inferred resources of 4–5 Mt fine-Cu with a grade of 0.72% Cu; 30–50 t fine-gold with a grade of 0.23 g/t Au) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U–Pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma (Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the second about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic andesites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U–Pb zircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast, the 40Ar/39Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close temporal relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite from quartz-sericite vein (hosted in DMGP) yields a 40Ar/39Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-forming magmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U–Pb ages (106–121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subducting northward during the Early Cretaceous.  相似文献   

12.
The genesis of polymetallic deposits in southern Altay, NW China has been disputed between a syngenetic seafloor hydrothermal process and an epigenetic orogenic-type mineralization. The Dadonggou Pb–Zn deposit occurs as NW-trending veins in the Devonian Kangbutiebao Formation volcanic-sedimentary sequence in the Kelan basin, southern Altay. A set of integrated zircon U–Pb and biotite 40Ar/39Ar geochronological data were applied to constrain the forming ages of the ores and their country rocks. Three samples of host volcanic rocks yielded weighted mean 206Pb/238U ages of 397.1 ± 4.5 Ma, 391.7 ± 3.6 Ma and 391.1 ± 4.2 Ma, respectively, indicating that the Kangbutiebao Formation was deposited in a Devonian back-arc basin. Two biotite samples separated from the Pb–Zn-containing quartz veins yielded 40Ar/39Ar plateau ages of 205.9 ± 2.1 Ma and 204.3 ± 2.2 Ma, respectively, which represent the age of the Pb–Zn mineralization that is attributed to the closure of the Kelan back-arc basin and the Late Triassic orogeny. Combining the available geological and geochronological data, this contribution outlines the successive evolution from the development of a Devonian back-arc basin to the Late Triassic post-subduction orogeny, and proposes that the Dadonggou Pb–Zn deposit is an epigenetic orogenic-type deposit placed in the Late Triassic orogeny.  相似文献   

13.
The Jiehe gold deposit, containing a confirmed gold reserve of 34 tonnes (t), is a Jiaojia-type (disseminated/stockwork-style) gold deposit in Jiaodong Peninsula. Orebodies are hosted in the contact zone between the Jurassic Moshan biotite granite and the Cretaceous Shangzhuang porphyritic granodiorite, and are structurally controlled by the NNE- to NE-striking Wangershan-Hedong Fault. Sulphide minerals are composed predominantly of pyrite with lesser amounts of chalcopyrite, galena, and sphalerite. Hydrothermal alteration is strictly controlled by fracture zones, in which disseminated sulfides and native gold are spatially associated with pervasive sericitic alteration. Mineralogical, textural, and field relationships indicate four stages of alteration and mineralization, including pyrite-bearing milky and massive quartz (stage 1), light-gray granular quartz–pyrite (stage 2), quartz–polysulfide (stage 3) and quartz–carbonate (stage 4) stages. Economic gold is precipitated in stages 2 and 3.The Jiehe deposit was previously considered to form during the Eocene (46.5 ± 2.3 Ma), based on Rb-Sr dating of sericite. However, 40Ar/39Ar dating of sericite in this study yields well-defined, reproducible plateau ages between 118.8 ± 0.7 Ma and 120.7 ± 0.8 Ma. These 40Ar/39Ar ages are consistent with geochronological data from other gold deposits in the region, indicating that all gold deposits in Jiaodong formed in a short-term period around 120 Ma. The giant gold mineralization event has a tight relationship with the extensional tectonic regime, and is a shallow crustal metallogenic response of paleo-Pacific slab subduction and lithospheric destruction in the eastern NCC.  相似文献   

14.
Eclogites are high-pressure/low-temperature metamorphic rocks and are regularly considered as an indicator of ancient subduction zones. Eclogites have recently been found in the North Shahrekord metamorphic complex (NSMC) of the Sanandaj–Sirjan zone and represent the only ones within the Zagros orogen. Their occurrence and timing are important for the reconstruction of convergence history and geodynamic evolution of the Neo-Tethys Ocean and Zagros orogen. White mica from the eclogites and an associated paragneiss give 40Ar/39Ar ages ranging from 184.3 ± 0.9 to 172.5 ± 0.8 Ma and represent the age of cooling through the closure temperature for phengitic white mica. The NSMC also comprises the ductile NW–SE trending North Shahrekord Shear Zone (NSSZ), which is located in the northeast of the Main Zagros Reverse Fault. The NSMC consists mainly of various metasedimentary rocks, orthogneiss and small-sized bodies of metabasic rocks containing also the eclogites. Furthermore, pre-metamorphic granitoids represent part of the NSMC. The North Shahrekord eclogites are composed of garnet, omphacite, zoisite, Ca–Na amphibole, phengite and rutile. The highly deformed and metamorphosed granitoids yield hornblende and biotite 40Ar/39Ar ages 170.1 ± 0.9 Ma and 110.7 ± 0.3 Ma, respectively. According to the new age dating results of eclogites, the rocks are the oldest high-pressure metamorphic rocks in the Zagros orogenic belt testifying the Neo-Tethys Ocean subduction. Our new data indicate that the eclogites formed during Early Jurassic subduction of a Panafrican microcontinental piece from the northern margin of the Neo-Tethyan Ocean under the Central Iranian microplate. We suggest that initiation of subduction in Neo-Tethyan Ocean occurred a few million years prior to 184 Ma (Pliensbachian stage).  相似文献   

15.
The PoSen complex, located closely adjacent to the southwestern margin of the Red River shear zone represents the uplifted basement of north Vietnam and may record the motion of the shear zone. However, its thermochronological history has not been fully examined yet. Here we applied U–Pb and 40Ar/39Ar dating methods to reveal its thermochronological history. U–Pb analysis of composite zircon grains by TIMS yielded an average age of 760 ± 25 Ma, clustering on the concordia line. Twelve SHRIMP U–Pb analyses also yielded a consistent result of 751 ± 7 Ma. Along with the geochemical features, the U–Pb dating results suggest the PoSen complex was a late Proterozoic magmatic complex, which could correspond to the Chengjiang orogeny, a widespread thermal event in southwest China. Results of 40Ar/39Ar dating of micas and K-feldspars were in the range of 36–30 Ma, revealing a rapid cooling and exhumation history of the PoSen complex during the late Paleogene. The time span of cooling and exhumation of the PoSen complex is slightly older than the main cooling phases of the Ailao Shan–Red River (ASRR) metamorphic massifs (28–17 Ma), but is synchronous with the early igneous activity stage in the eastern Indo-Asian collision zone of southeast China and north Vietnam. Owing to the ongoing debate about the initiation and offset of the ASRR shear zone, the tectonic force for the late Paleogene cooling of the PoSen complex is still inconclusive. The rapid exhumation of the PoSen complex could be in response to either the detachment of the Neo-Tethyan slab or a transpressional phase of continental subduction along the ASRR shear system in the eastern Indo-Asian collision zone.  相似文献   

16.
The north-west Turkish blueschists represent a subducted passive continental margin sequence dominated by metaclastic rocks and marble. The depositional age of the blueschist protoliths are probably Palaeozoic to Mesozoic, while the age of the high-pressure/low-temperature metamorphism is Late Cretaceous. Blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal material that locally shows incipient blueschist metamorphism and by spinel peridotite slices. The metaclastic rocks with regional jadeite and glaucophane, which comprise the lower part of the blueschist unit, make up an over 1000-m-thick coherent sequence in the Kocasu region of north-west Turkey. Rare metabasic horizons in the upper parts of the metaclastic sequence with sodic amphibole + Iawsonite but no garnet indicate lawsonite blueschist facies metamorphism. The blueschist metaclastics in the Kocasu region are practically free of calcium and ferric iron and closely approximate the NFMASH system in bulk composition. Two low-variance mineral assemblages (with quartz and phengite) are jadeite + glaucophane + chlorite + paragonite and chloritoid + glaucophane + paragonite. The metaclastics comprise up to several-metres-thick layers of jadeite schist with quartz, phengite and nearly 100 mol% jadeite. Phase relations in the metaclastics show that the chloritoid + glaucophane assemblage, even in Fe2+-rich compositions, is stable in the jadeite stability field. In the NFASH system the above assemblage without the accompanying garnet has a narrow thermal stability field. Mineral equilibria in the metaclastics involving chloritoid, glaucophane, jadeite, paragonite and chlorite indicate metamorphic P-T conditions of 20 ± 2 kbar and 430 ± 30 d? C, yielding geothermal gradients close to 5d? C km-1, one of the lowest geotherms recorded. Blueschists in the Kocasu region, which have been buried to 70 km depth, are tectonically overlain by the volcanosedimentary sequence and by peridotite buried not deeper than 30 km. Phengites from two jadeite schists were dated by Ar/Ar laser probe; they give an age of 88.5 ± 0.5 Ma, interpreted as the age of metamorphism. Blueschists and the overlying peridotite bodies are intruded by 48-53-Ma-old granodiorite bodies that were emplaced at 10 km depth. This suggests that the exhumation of blueschists by underplating of cold continental crust, and normal faulting at the blueschist-peridotite, interface occurred during the Late Cretaceous to Palaeocene (88-53 Ma).  相似文献   

17.
The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541 ± 13 Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461 ± 15 Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715 ± 180 Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related ~ 2530 Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial176Hf/177Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.  相似文献   

18.
The precise constraints on the timing of metamorphism of the Changhai metamorphic complex is of great importance considering the prolonged controversial issue of the north margin and the extension of the Sulu–Dabie HP–UHP Belt. While the monazite U–Th–Pb and muscovite 40Ar/39Ar techniques are widely accepted as two of the most powerful dating tools for revealing the thermal histories of medium–low grade metamorphic rocks and precisely constraining the timing of metamorphism. The Changhai metamorphic complex at the SE Jiao–Liao–Ji Belt, North China Craton consists of a variety of pelitic schist and Grt–Ky-bearing paragneiss, and minor quartzite and marble. Analyses of mineral inclusions and back-scattered electric (BSE) images of monazites, combined with LA–ICP–MS U–Th–Pb ages for monazites and 40Ar/39Ar ages for muscovites, provide evidence of the origin and metamorphic age of the Changhai metamorphic complex. Monazites separates from various Grt–Mus schists and Grt–Ky–St–Mus paragneisses exhibit homogeneous BSE images from cores to rims, and contain inclusion assemblages of Grt + Mus + Qtz ± Ctd ± Ky in schist, and Grt + Ky + St + Mus + Pl + Kfs + Qtz inclusions in paragneiss. These inclusion assemblages are very similar to matrix minerals of host rocks, indicating they are metamorphic rather than inherited or detrital in origin. LA–ICP–MS U–Th–Pb dating reveals that monazites of schist and paragneiss have consistent 206Pb/238U ages ranging from 228.1 ± 3.8 to 218.2 ± 3.7 Ma. In contrast, muscovites from various schists show slightly older 40Ar/39Ar plateau ages of 236.1 ± 1.5 to 230.2 ± 1.2 Ma. These geochronological and petrological data conclude that the pelitic sediments have experienced a metamorphic event at the Mid–Late Triassic (236.1–218.2 Ma) rather than the Paleoproterozoic (1950–1850 Ma), commonly regarded as the Precambrian basement for the Jiao–Liao–Ji Belt. Hence, the Changhai metamorphic complex should be considered as a discrete lithotectonic group.This newly recognized Mid–Late Triassic metamorphic event (236.1–218.2 Ma) for the Changhai metamorphic complex is coeval with the HP–UHP metamorphic event (235–220 Ma) for Sulu–Dabie rocks. This leads us to speculate that the metamorphism of the Changhai complex belt along the SE margin of the North China Craton was genetically related to the Mid–Late Triassic collision of the North China and South China cratons. By the same token, the Sulu–Dabie HP–UHP Belt may have extended through Yantai, and the southern Yellow Sea, and to the southern side of the Changhai metamorphic complex.  相似文献   

19.
The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.  相似文献   

20.
The mineral separate GA1550 biotite has become an international standard for K/Ar and 40Ar/39Ar dating studies, although it was prepared as an intralaboratory standard at ANU to monitor tracer depletion from a gas pipette. It is one of a small number of samples that has been calibrated against 38Ar tracers, some of which had been mixed with known amounts of atmospheric argon, so that a so-called primary calibration has been performed. By measuring GA1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1.342 (± 0.007) × 10? 9 mol/g, and together with the measured K content of 7.645 (± 0.050) weight percent, we derive a best estimate for the K/Ar age as 98.5 ± 0.5 Ma, where the error is derived from averaging the ages determined relative to the 38Ar tracer.  相似文献   

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