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1.
Qiu and Wijbrans [Qiu H.-N. and Wijbrans J. R. (2006) Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing. Geochim. Cosmochim. Acta70, 2354-2370] present three Ar-Ar age spectra for fluid inclusions in garnet from eclogite at Bixiling in the Dabie orogen, east-central China. These Paleozoic ages of 427 ± 20 to 444 ± 10 Ma are interpreted to represent the first formation of Dabie ultrahigh-pressure (UHP) eclogite and thus require subduction of Yangtze crust to have started much earlier than previously accepted. However, no petrographic evidence, such as mineral inclusions in the garnet relating to the particular metamorphic conditions, is presented to substantiate the proposed UHP metamorphic event. Because garnet growth is not uniquely responsible for UHP eclogite-facies metamorphism, a distinction between UHP and high-pressure (HP) metamorphic events must be made in the interpretation of geochronological results. Available data from mineral Sm-Nd and zircon U-Pb dating of eclogites from the same area have firmly established that the UHP eclogite-facies metamorphism took place at Triassic. Neither the age of UHP metamorphism nor the timing of continental collision is reliably constrained by their presented data; the fluid inclusions in garnet must contain inherited 40Ar from UHP eclogite precursor, without considerable resetting of the Ar-Ar isotopic system during Triassic UHP metamorphism. Therefore, their data are either meaningless, or at best viewed as the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic. Furthermore, it is critical for metamorphic geochronology to substantiate the timing of UHP metamorphic event by means of zircon U-Pb in situ dating on coesite-bearing domains of metamorphically grown zircon.  相似文献   

2.
The Songshugou ultramafic massif is located to the north of the Shang‐Dan fault, the Palaeozoic suture between the North and South China blocks. It is the largest Apline‐type ultramafic body in the Qinling orogenic belt of central China, consisting mainly of dunite with a small amount of harzburgite and minor pyroxenite. We present new LA‐ICP‐MS U?Pb dating and trace element results for zircon from two garnet amphibolite samples in the contact metamorphic zone surrounding the massif. One was sampled ～1 m from the massif, the other ～5 m away. The studied zircon grains are small, anhedral, and display typical metamorphic characteristics of low Th/U values (<0.1). The U and Th concentrations of zircon range from several hundred ppm to less than 10 ppm. Cathodoluminescence images show two apparent generations of zircon, with lighter cores and darker rims. Core and rim ages however, are identical within error. These two samples yield identical concordant ages of 506±7 and 510±7 Ma, suggesting that the Songshugou ultramafic massif was emplaced at ～510 Ma. Low HREE concentrations and the absence of Eu anomalies in most analysed zircons suggest that the studied grains most likely formed during garnet amphibolite metamorphism induced by emplacement of the ultramafic massif.

To better understand the cooling history of the massif, 40Ar/39Ar ages of amphibole from three garnet amphibolite specimens in the contact metamorphic zone and one amphobolite sample about 20 m away from the massif were determined. The 40Ar/39Ar ages increase from 372±15 Ma (JSM‐01) near the massif to disturbed, unreliable ‘plateau’ ages of 474±8 Ma (JSM‐03) and 781±146 Ma (JSM‐04) with increasing distance from the ultramafic massif, showing limited heating during exhumation of the massif, followed by slow cooling. Therefore, the Songshugou ultramafic massif does not reflect the Jining orogeny at ～1 Ga. Instead, it was emplaced into the Proterozoic, Qinling Group during the Palaeozoic, probably due to the subduction along the Shang‐Dan fault.  相似文献   

3.
A combination of geological and isotopic-geochronological (K/Ar, 40Ar/39Ar, U-Pb-zircon methods) studies indicates that high-pressure-low-(in part, intermediate) temperature metamorphic rocks of both coherent complexes and inclusions in serpentinite melange were formed during Cretaceous and possibly to a small extent during Paleogene time. The original rocks are mostly of Jurassic and Cretaceous age. Pre-Jurassic metamorphic rocks have not been found within the complexes investigated. The metamorphic histories of many Cuban eclogites and garnet amphibolites were complex, but nevertheless reflect only a single Laramide tectono-metamorphic cycle. The discovery that the high-pressure meta-morphic rocks of Cuba are young confirms that major lateral displacement of crustal masses played a very important role in the structural development of the island.  相似文献   

4.
The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km2 in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.  相似文献   

5.
Pütürge变质地体位于新特提斯构造带南部的土耳其Anatolia逆冲推覆构造带内,形成于欧亚板决与阿拉伯板块之间晚白垩纪碰撞造山事件.Pütürge变质地体主要由变质泥质片岩及片麻岩、花岗质片麻岩、石英岩、角闪岩和大理岩组成,发育类似巴罗型递增变质带的变质带序列,变质程度达高绿片岩相至低角闪岩相.此前该变质地体一直缺乏精确的年代学约束,为此我们采用了二次离子质谱锆石U-Pb测年方法和黑云母40Ar/39 Ar测年方法,对该变质地体进行了年代学研究.结果表明,区内花岗片麻岩原岩形成于84.2±1.1Ma,变质泥质片麻岩中黑云母40Ar/39 Ar年龄所代表的变质时代为83.21±0.1Ma.这说明早白垩世期间岩浆侵入事件不久,Pütürge变质地体就发生了区域变质作用.  相似文献   

6.
Isotope datings of amphibole-bearing mafics and metamafics in the northern part of the Anadyr-Koryak region allow clarification of the time of magmatic and metamorphic processes, which are synchronous with certain stages of the geodynamic development of the northwest segment of the Pacific mobile belt in the Phanerozoic. To define the 40Ar/39Ar age of amphiboles, eight samples of amphibole gabbroids and metamafics were selected during field work from five massifs representing ophiolites and mafic plutons of the island arc. Rocks from terranes of three foldbelts: 1) Pekulnei (Chukotka region), 2) Ust-Belaya (West Koryak region), and 3) the Tamvatnei and El’gevayam subterranes of the Mainits terrane (Koryak-Kamchatka region), were studied. The isotope investigations enabled us to divide the studied amphiboles into two groups varying in rock petrographic features. The first was represented by gabbroids of the Svetlorechensk massif of the Pekulnei Range and by ophiolites of the Tamvatnei Mts.; their magmatic amphiboles show the distribution of argon isotopes in the form of clearly distinguished plateau with an age ranging within 120–129 Ma. The second group includes metamorphic amphiboles of metagabbroids and apogabbro amphibolites of the Ust-Belaya Mts., Pekulnei and Kenkeren ranges (El’gevayam subterranes). Their age spectra show loss of argon and do not provide well defined plateaus the datings obtained for them are interpreted as minimum ages. Dates of amphiboles from the metagabbro of the upper tectonic plate of the Ust-Belaya allochthon points to metamorphism in the suprasubduction environment in the fragment of Late Neoproterozoic oceanic lithosphere in Middle-Late Devonian time, long before the Uda-Murgal island arc system was formed. The amphibolite metamorphism in the dunite-clinopyroxenite-metagabbro Pekulnei sequence was dated to occur at the Permian-Triassic boundary. The age of amphiboles from gabbrodiorites of the Kenkeren Range was dated to be Early Jurassic that confirmed their assignment to the El’gevayam volcanic-plutonic assemblage. These data are consistent with geological concepts and make more precise the available age dates. Neocomian-Aptian 40Ar/39Ar age of amphibolites from the Pekulnei and Tamvatnei gabbroids make evident that mafics of these terranes (varying in geodynamic formation settings and in petrogenesis) were generated in later stages of the development of the West Pekulnei and Mainits-Algan Middle-Late Jurassic-Early Cretaceous island arc systems, presumably due to breakup of island arcs in the Neocomian.  相似文献   

7.
《International Geology Review》2012,54(10):1270-1293
ABSTRACT

The Chinese southwestern Tianshan HP–UHP/LT metamorphic complex possesses well-preserved mafic layers, tectonic slices/blocks, boudins/lens of different sizes, and lithology embedded within dominant metavolcanoclastics. A recent study on the ultra-high pressure (UHP) eclogite revealed a short timescale of exhumation (≤10 Ma, ~315 ± 5 Ma). However, controversies still exist on some key questions: (1) the reasonable interpretation of spatially close-outcropped high pressure (HP) and UHP slices with respect to regional geodynamics, and (2) if the previous regional scatter Ar–Ar ages proved the existence of internally coherent sub-belts or troubled by dating on samples with notable 40Ar retention. This study focusses on detailed P–T–time (phengite Ar closure) recovery of samples from a HP eclogite lens and its host rock, the UHP thick-layered eclogite. Based on data from bulk–rock, microprobe analysis, and muscovite Ar–Ar chronological dating, we link phengite growth to potential garnet growth stages via thermodynamic modelling. Facilitated by the P–T–Ar retention% graph, we collect all the regional muscovite Ar–Ar data together with results in this study for evaluating the significance of regional muscovite Ar–Ar ages and set back to geodynamics. According to modelling results, the HP lens eclogite reached peak metamorphism at ~550°C, 2.50 GPa with an Ar–Ar muscovite plateau age of 316.9 ± 1.0 Ma that could date the mass phengite growth event during prograde metamorphism. In contrast, the UHP layered eclogite experienced UHP peak burial at ~510°C, 2.95 GPa, and then to HP peak metamorphism at ~560°C, 2.60 GPa with ~311.6 ± 0.7 Ma plateau age that may constrain the cooling age during early exhumation. Noteworthy, both of them share a quite similar early exhumation path despite bearing contrasting prograde metamorphic experiences. With considering updated regional exhumation pattern, this might imply the existence of a potential deep juxtaposing (capture) process between HP slices and exhumating UHP complex, at about 45–60 km depth along subduction plate interface.  相似文献   

8.
The Xiongdian eclogite occurring in the Sujiahe tectonic melange zone at Luoshan County, Henan Province, in the western Dabie Mountains, is typical high-pressure (HP)-ultrahigh-pressure (UHP) and medium-temperature eclogite. The occurrence, internal texture and surface characteristics of zircons in eclogite were studied rather systematically petrographically combined with the cathodoluminescence (CL) and scanning electron microscope (SEM) methods. Zircons are mainly hosted in garnet and other metamorphic minerals with sharp boundaries, have a multifaceted morphology and are homogeneous or exhibit a metamorphic growth texture in the interior, thus indicating that they are the product of metamorphism. SHRIMP analyses give zircon 206Pb/238U ages of 335 to 424 Ma and show a certain degree of radiogenic Pb loss; therefore it may be inferred that the age of 424? Ma represents the minimum age of a HP-UHP metamorphic age. From the above analyses coupled with previous Sm-Nd, 40Ar-39Ar, U-Pb and 207Pb/206Pb age d  相似文献   

9.
《China Geology》2021,4(1):67-76
The Pamir Plateau comprises a series of crustal fragments that successively accreted to the Eurasian margin preceded the India-Asia collision, is an ideal place to study the Mesozoic tectonics. The authors investigate the southern Tashkorgan area, northeastern Pamir Plateau, where Mesozoic metamorphic and igneous rocks are exposed. New structural and biotite 40Ar-39Ar age data are presented. Two stages of intense deformation in the metamorphic rocks are identified, which are unconformably covered by the Early Cretaceous sediment. Two high-grade metamorphic rocks yielding 128.4 ± 0.8 Ma and 144.5 ± 0.9 Ma 40Ar-39Ar ages indicate that the samples experienced an Early Cretaceous cooling event. Combined with previous studies, it is proposed that the Early Cretaceous tectonic records in the southern Tashkorgan region are associated with Andean-style orogenesis. They are the results of the flat/low-angle subduction of the Neotethyan oceanic lithosphere.©2021 China Geology Editorial Office.  相似文献   

10.
Continuous laser probe 40Ar-39Ar technique has been taken to carry out in situ analysis onto the metamorphic garnet and plagioclase from high-pressure basic granulites in Sanggan area of the North China craton. Garnet porphyroblasts was formed in the high-pressure granulite facies episode. In the symplectite assemblage arround garnet, plagioclase is one of the garnet breakdown products. Ar analysis of garnet porphyroblasts defines an 40Ar-39Ar isochron which gives out an age of 2510Ma, that indicates the high-pressure granulite facies metamorphic age. So, the Archaean high-pressure granulite metamorphism has been confirmed by this age dating. Another 40Ar-39Ar isochron age of 1968Ma has been obtained from Ar data of plagioclase. That should represent the age of garnet breakdown reaction. The >500Ma gap between the age of high-pressure metamorphism and garnet breakdown does not support the isothermal decompression P-T path given by petrological view. The symplectite assemblages are more likely to be formed during another medium-pressure metamorphism overprint. This conclusion will give a strong constraint on the crustal evolution of Sanggan area in the North China craton.  相似文献   

11.
The Rhodope Domain in NE Greece consists of different tectonometamorphic complexes involved in the Alpine collisional history between the Eurasian and African plates. In the Kechros Complex, which is the lowermost tectonic unit in the East Rhodope, a lense of kyanite eclogite occurs within orthogneiss and common eclogites are found between serpentinized peridotite and underlying pelitic gneisses. In kyanite eclogite, the high-pressure (HP) mineral assemblage is Grt?+?Omp (Jd35–55)?+?Ky?+?Ph?+?Qz?+?Rt?+?(indirectly inferred Tlc?+?Law); a Na-rich tremolite and zoisite formed at or near peak metamorphic conditions. In common eclogites, the HP mineral assemblage is Grt?+?Omp (Jd29–41)?+?Rt and, with less certainty, Amp (Gln-rich?+?Brs?+?Wnc?+?Hbl)?±?Czo. The inclusions in garnet are glaucophane, actinolite, barroisite, hornblende, omphacite, clinozoisite, titanite, rutile and rarely paragonite and albite. In kyanite eclogite, peak P–T conditions are constrained at 2.2?GPa and 615°C using garnet–omphacite–phengite geothermobarometry and very similar values of 585?±?32°C and 2.17?±?0.11?GPa with the average P–T method, by which conditions of formation could also be narrowed down for the common eclogite (619?±?53°C and 1.69?±?0.17?GPa) and for a retrogressed eclogite (534?±?36°C and 0.77?±?0.11?GPa). Ages for the HP metamorphism in the Kechros Complex are not yet available. A Rb–Sr white mica age of 37?Ma from orthogneiss records a stage of the exhumation. The HP event may be coeval with the Eocene HP metamorphism (49–55?Ma) recorded in the Nestos Shear Zone in Central Rhodope and in the Attic-Cycladic crystalline belt, where it is interpreted as the result of subduction and final closure of the Axios/Vardar ocean and subsequent subduction of the Apulian continental crust (a promontory of the Africa continent) under the southern margin of the European continent in the late Cretaceous and early Tertiary.  相似文献   

12.
Results of integrated 40Ar/39Ar, Rb-Sr, and Sm-Nd geochronological studies of the Lukinda dunite-troctolite-gabbro-anorthosite massif in the northeast of the Selenga-Stanovoi superterrane, Central Asian Fold Belt, are presented. It is shown that this massif is much younger than formerly thought: 249 ± 14 to 251 ± 15 Ma vs. Paleoproterozoic. This date of magmatism corresponds to one of the stages of the formation of the Selenga-Vitim belt, which ranks among the largest Phanerozoic volcanoplutonic belts in Central Asia.  相似文献   

13.
The Sistan Suture Zone (SSZ) of eastern Iran is part of the Neo‐Tethyan orogenic system and formed by convergence of the Central Iranian and Afghan microcontinents. Ar Ar ages of ca. 125 Ma have been obtained from white micas and amphibole from variably overprinted high‐pressure metabasites within the Ratuk Complex of the SSZ. The metabasites, which occur as fault‐bounded lenses within a subduction mélange, document peak‐metamorphic conditions in eclogite or blueschist facies followed by near‐isothermal decompression resulting in an epidote–amphibolite‐facies overprint. 40Ar/39Ar step heating experiments were performed on a phengite + paragonite mixture from an eclogite, phengites from two amphibolites, and paragonite from a blueschist; ‘best‐fit’ ages from these micas are, respectively, 122.8 ± 2.2, 124 ± 13, 116 ± 19 and 139 ± 19 Ma (2σ error). Barroisite from an amphibolite yielded an age of 124 ± 10 Ma. The ages are interpreted as cooling ages that record the post‐epidote–amphibolite stage in the exhumation of the rocks. Our results imply that both the high‐pressure metamorphism and the epidote–amphibolite‐facies overprint occurred prior to 125 Ma. Subduction of oceanic lithosphere along the eastern margin of the Sistan Ocean had therefore begun by Barremian (Early Cretaceous) times. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

14.
The Malkhan granite-pegmatite system located in Central Transbaikalia, in the southwestern portion of the Malkhan-Yablonovy structure-formational zone of the Caledonian folding comprises two granite massifs (Bolsherechensk and Oreshny) and a miarolitic pegmatite field of the same name, which adjoins the Chikoi deep-seated fault and Lower Cretaceous Chikoi rift depression in the north. The first 40Ar/39Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on K-feldspar, muscovite, and lepidolite from the Oktyabrskaya pegmatite vein. According to these data, the age of the granitepegmatite system is 123.8–127.6 Ma, which is consistent with the age of Lower Cretaceous rocks from the Chikoi depression. The intimate spatial relationship and isochronism between the Chikoi depression and the Malkhan granite-pegmatite system are strongly suggestive of a rift regime that affected its evolution, thus highlighting the need to regard the evolution of this system as being intimately related to depression development. Such a model can easily be realized within the framework of the concept of a metamorphic core complex, which was used to explain the nature of Transbaikal-type rift depressions and conjugate granite-gneiss swells.  相似文献   

15.
通过对郯庐断裂带南段桐城地区高压-超高压变质带详细的岩石学和构造学研究,将研究区从空间结构上划分为三个构造单元:上部低温-高压单元、中部中温-高压单元和下部超高压单元。根据研究区多期构造变形分析,共识别出了五期有区域构造地质学含义的事件(D_1-D_5):D_1代表高压-超高压变质岩中-晚三叠世同碰撞早期折返过程;D_2表征了高压-超高压变质岩晚三叠世同碰撞晚期折返过程;D_3记录了早白垩世中大别变质核杂岩的形成,也即整个中国东部晚中生代大规模伸展构造在研究区的表现;D_4可能标志着郯庐断裂走滑构造对高压-超高压造山带的叠加;D_5表现为脆性正断作用,控制了晚白垩世-古近纪潜山半地堑盆地的形成。这些结果表明了研究区所经历构造演化的复杂性,其构造几何形态很难用郯庐断裂左行平移南大别超高压变质岩来解释,也不支持桐城地区存在巨大走滑作用的证据。  相似文献   

16.
In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics.

The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons.

North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the 40Ar/39Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands.

The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the 40Ar/39Ar dates.

A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented.  相似文献   


17.
We describe, date and constrain the P–T conditions of a syntectonic inverted metamorphic sequence associated with continental collision and crustal‐scale thrusting in one of the key regions of the late Palaeozoic Variscan belt of Western Europe – the Champtoceaux Complex (Armorican Massif, France), interpreted as a trace of the Variscan suture zone between Laurussia and Gondwana. The Complex consists of several stacked units, some of them eclogite‐bearing, that are sandwiched between two main pieces of continental crust – the Parautochthon and the Upper Allochthon. Moderately to steeply dipping foliation parallels the main lithological boundaries. From the bottom to the top of the metamorphic rock pile, the following sequence testifies to the syntectonic temperature increase: chlorite–biotite‐bearing metagreywackes (Parautochthon); orthogneisses with eclogite lenses; micaschists with chloritoid–chlorite–garnet; orthogneisses; micaschists with staurolite–biotite–garnet with chloritoid inclusions (Lower Allochthon); and migmatites with boudins of eclogite and kyanite–biotite–garnet‐bearing metapelitic lenses (Upper Allochthon). Mylonitic amphibolites with lenses of serpentinized peridotite mark the boundary between the Lower Allochthon and the overlying Upper Allochthon, suggesting the presence of a major thrust. It is inferred that the latter is responsible for the development of the inverted metamorphic zoning. Multiequilibrium thermobarometry and pseudosections calculated with thermocalc indicate that equilibration temperatures of the syntectonic peak metamorphic assemblages increase upwards in the rock pile from <500 °C in the Parautochthon to >650 °C in the Upper Allochthon. All units equilibrated at similar pressures between 7 and 10 kbar. In the Upper Allochthon, chronological results on muscovite suggest initial cooling from c. 343 Ma (muscovite Rb–Sr) to c. 337 Ma (muscovite 40Ar–39Ar). A subsequent very rapid temperature decrease is suggested by the synchronous closure of the muscovite and biotite K–Ar and biotite Rb–Sr isotopic systems (c. 337–335 Ma). This cooling is also recorded in the Upper Micaschists of the Lower Allochthon and in the Parautochthon with muscovite 40Ar–39Ar ages of c. 336–334 and 332 Ma, respectively. Ages of c. 343 Ma inferred from disturbed muscovite spectra from the Parautochthon are possibly linked to a previous higher pressure metamorphic event in this unit. It is suggested that the development of the inverted metamorphic zoning in the Champtoceaux Complex is due to the emplacement of a hot nappe over colder units and is contemporaneous with major crustal thrusting and associated pervasive ductile deformation. The preservation of this inverted field gradient was possible because of fast cooling, tentatively associated with the syn‐compressional denudation of the tectonic pile, expressed by the detachment at the top of the nappe pile. The efficiency of cooling is best shown by the near‐coincidence of Rb–Sr and 40Ar–39Ar ages, obtained on both sides of the major thrust. Finally, we highlight similarities with other regions of the West‐European Variscan belt (Iberian massif, French Massif Central) and suggest that inverted metamorphic zoning is systematically associated with the contact between the Lower and Upper Allochthons.  相似文献   

18.
U–Pb sensitive high resolution ion microprobe (SHRIMP) zircon geochronology, combined with REE geochemistry, has been applied in order to gain insight into the complex polymetamorphic history of the (ultra) high pressure [(U)HP] zone of Rhodope. Dating included a paragneiss of Central Rhodope, for which (U)HP conditions have been suggested, an amphibolitized eclogite, as well as a leucosome from a migmatized orthogneiss at the immediate contact to the amphibolitized eclogite, West Rhodope. The youngest detrital zircon cores of the paragneiss yielded ca. 560 Ma. This date indicates a maximum age for sedimentation in this part of Central Rhodope. The concentration of detrital core ages of the paragneiss between 670–560 Ma and around 2 Ga is consistent with a Gondwana provenance of the eroded rocks in this area of Central Rhodope. Metamorphic zircon rims of the same paragneiss yielded a lower intercept 206Pb/238U age of 148.8±2.2 Ma. Variable post-148.8 Ma Pb-loss in the outermost zircon rims of the paragneiss, in combination with previous K–Ar and SHRIMP-data, suggest that this rock of Central Rhodope underwent an additional Upper Eocene (ca. 40 Ma) metamorphic/fluid event. In West Rhodope, the co-magmatic zircon cores of the amphibolitized eclogite yielded a lower intercept 206Pb/238U age of 245.6±3.9 Ma, which is interpreted as the time of crystallization of the gabbroic protolith. The metamorphic zircon rims of the same rock gave a lower intercept 206Pb/238U age of 51.0±1.0 Ma. REE data on the metamorphic rims of the zircons from both the paragneiss of Central Rhodope and the amphibolitized eclogite of West Rhodope show no Eu anomaly in the chondrite-normalized patterns, indicating that they formed at least under HP conditions. Flat or nearly flat HREE profiles of the same zircons are consistent with the growth of garnet at the time of zircon formation. Low Nb and Ta contents of the zircon rims in the amphibolitized eclogite indicate concurrent growth of rutile. Based on the REE characteristics, the 148.8±2.2 Ma age of the garnet–kyanite paragneiss, Central Rhodope and the 51.0±1.0 Ma age of the amphibolitized eclogite, West Rhodope are interpreted to reflect the time close to the (U)HP and HP metamorphic peaks, respectively, with a good approximation. The magmatic zircon cores of the leucosome in the migmatized orthogneiss, West Rhodope, gave a lower intercept 206Pb/238U age of 294.3±2.4 Ma for the crystallization of the granitoid protolith of the orthogneiss. Two oscillatory zircon rims around the Hercynian cores, yielded ages of 39.7±1.2 and 38.1±0.8 Ma (2σ errors), which are interpreted as the time of leucosome formation during migmatization. The zircons in the leucosome do not show the 51 Ma old HP metamorphism identified in the neighboring amphibolitized eclogite, possibly because the two rock types were brought together tectonically after 51 Ma. If one takes into account the two previously determined ages of ca. 73 Ma for (U)HP metamorphism in East Rhodope, as well as the ca. 42 Ma for HP metamorphism in Thermes area, Central Rhodope, four distinct events of (U)HP metamorphism throughout Alpine times can be distinguished: 149, 73, 51 and 42 Ma. Thus, it is envisaged that the Rhodope consists of different terranes, which resulted from multiple Alpine subductions and collisions of micro-continents, rather similar to the presently accepted picture in the Central and Western Alps. It is likely that these microcontinents were rifted off from thinned continental margins of Gondwana, between the African and the European plates before the onset of Alpine convergence.  相似文献   

19.
北秦岭松树沟榴辉岩的确定及其地质意义   总被引：9，自引：8，他引：1  
陈丹玲  任云飞  宫相宽  刘良  高胜 《岩石学报》2015,31(7):1841-1854
松树沟石榴石角闪岩(榴闪岩)呈透镜状产于松树沟超镁铁岩旁侧的斜长角闪岩中,一直以来被认为是形成于接触交代变质或麻粒岩相变质过程。详细岩相学及矿物元素分析,在榴闪岩的基质矿物、石榴石幔部及锆石包体中发现残留的绿辉石,而且石榴石也保存了明显的进变质主、微量元素成分环带,表明松树沟榴闪岩为榴辉岩退变质的产物,至少经历了从角闪岩相到榴辉岩相再到角闪岩相的三阶段顺时针PT演化过程。锆石定年结果得到榴辉岩的变质年龄为500±8Ma,原岩结晶时代为796±16Ma,与秦岭岩群北侧官坡超高压榴辉岩的变质年龄和原岩年龄完全一致,也与北秦岭区域高压-超高压变质时代和原岩的结晶时代一致。表明松树沟榴辉岩与北秦岭造山带已发现的高压-超高压变质岩石一起都应是古生代大陆深俯冲作用的结果,而松树沟超镁铁岩可能是俯冲的大陆板片在折返过程中携带的俯冲隧道中的交代地幔岩。  相似文献   

20.
Summary Retrograde eclogites and serpentinites from the Hochgr?ssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. 40Ar/39Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date. Received June 11, 2000; revised version accepted January 2, 2001  相似文献   

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