首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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   

2.
Investigation of an eclogite xenolith, discovered in a Cretaceous granite from the Central Domain of the Dabieshan massif in eastern China, yields new petrological insights into the high to ultrahigh-pressure metamorphism, experienced by the Qinling-Dabie orogen. Prior to inclusion as a xenolith in the granite during the Early Cretaceous, this eclogite xenolith had recorded a complex metamorphic evolution that complies with subduction and exhumation processes experienced by the continental crust of the South China Block. Well-preserved mineral parageneses substantiate the prograde and retrograde stages revealed by inclusions in porphyroblastic garnet and zoned minerals such as garnet, omphacite and amphibole in the matrix. The relatively low P/T re-equilibration during a late metamorphic stage was textually inferred by the presence of aluminous and calcic-subcalcic amphiboles such as katophorite, edenite, taramite and pargasite as main matrix phases. According to our U/Pb, Rb/Sr and new 40Ar/39Ar geochronological results, namely109 ± 1 and 112 ± 2 Ma plateau ages for muscovite and amphiboles, respectively, two successive but distinct cooling stages account for the thermal history of the granite–migmatite gneiss dome that forms the Central Dabieshan Domain. We argue that prior to the Cretaceous doming, the Central Dabieshan Domain experienced a tectono-metamorphic evolution similar to that observed in the high-pressure to ultra high-pressure units developed in the Southern Dabieshan Domain and Hong’an massif.  相似文献   

3.
The paper presents new data on age, geochemistry, and Sr and Nd isotope composition of rocks from the Akatui massif and comagmatic rocks from the lower unit of the Kailas Formation (Akatui volcano-plutonic association), localized within the Aleksandrovskii Zavod depression. The amphibole 40Ar/39Ar age date the monzogabbro of the early phase of the Akatui massif at 154.8 ± 4.4 Ma; the monzonite of the main phase yields a 40Ar/39Ar age of 160.7 ± 3.9 Ma, and the shoshonite basalt of the lower unit of the Kailas Formation yields a 40Ar/39Ar age of 161.5 ± 1.7 Ma. The leading petrogenetic mechanism for the Akatui volcano-plutonic association is crystal fractional differentiation of melts with minor crustal contamination, which can be suggested from the mineralogical and petrographic features and geochemical and isotope characteristics of rocks. The geochemical data for the Akatui volcano-plutonic association show LILE, LREE, U, Th, and Pb enrichment with a characteristic depletion in high-field strength elements (HFSE), such as Nb and Ti. They are also depleted in P. Sr-Nd isotope data (87Sr/86Sr(160 Ma) = 0.70642-0.70688 and £Nd(160 Ma) = − 0.6 to − 2.2) suggest an EMII-type mantle source and could also indicate a negligible degree of crustal contamination in the evolved melts.  相似文献   

4.
The plutonic complex of the Meissen massif (northern margin of the Bohemian massif) comprises dioritic to mainly monzonitic and granitic rocks. The diorite to monzonite intrusions show major and trace element patterns typical for shoshonitic series. The chemical signatures of less crustally contaminated diorites are similar to arc-related shoshonitic rocks derived from continental lithospheric mantle (CLM) sources previously enriched by subduction of altered oceanic crust. Laser step heating 40Ar/39Ar analyses on actinolitic to edenitic amphiboles from geographically different occurrences of the monzonitic intrusion yielded concordant plateau ages as well as total gas ages ranging from 329.1±1.4 to 330.4±1.4?Ma and from 330.4±2.1 to 330.6±1.8?Ma, respectively. These cooling ages are indistinguishable from sensitive highresolution ion microprobe (SHRIMP) 238U/206Pb intrusion ages measured on magmatic zircon rims from the monzonite (Nasdala et al., submitted). This shows that the monzonite intrusion is probably not related temporally to active subduction because it postdates eclogites of the adjacent Saxonian Erzgebirge by approximately 20?Ma. The shoshonitic magmas intruded during strike-slip tectonism along the Elbe valley zone. The enrichment of their mantle sources may be of Upper Devonian/Lower Carboniferous age or older. Intrusions of shoshonitic to ultra-potassic (K-rich) rocks during the Upper Visean/Namurian are widespread in the Moldanubian zone. Based on similar ages and structural relationships a similar post-collisional setting to the Meissen shoshonitic rocks can be demonstrated. Most of these occurrences cut high-grade nappe units which were subducted during the Upper Devonian/Lower Carboniferous. In contrast to the Meissen massif, at least the ultra-potassic members of the Central and the South Bohemian batholiths were derived from CLM sources enriched by fluids or melts released from subducted oceanic crust and by greater portions of crustal material. Despite the similar post-collisional geodynamic setting of the K-rich intrusions, different enrichment processes generated mid-European Hercynian CLM sources with heterogeneous major and trace element and isotopic signatures.  相似文献   

5.
The Dzheltula alkaline massif is located in the Tyrkanda ore region of the Chara–Aldan metallogenic zone of the Aldan–Stanovy Shield (South Yakutia). The region contains separate placer gold objects, which are being explored at the present time, and ore-bearing Mesozoic alkaline intrusions, which are weakly studied due to their poor accessibility. The Dzheltula massif (DM) is the largest exposed multiple-ring intrusion within the Tyrkanda ore region; therefore, it is considered as a typical object for geological, petrological, geochronological, and metallogenic studies. The DM consists of five magmatic phases of syenite composition. 40Ar–39Ar dating has established that the crystallization age of the oldest phase, the leucocratic syenite porphyry (pulaskite), is 121.1 ± 1.3 Ma. The crystallization age of the cross-cutting phases represented by syenite–porphyry dikes (laurvikites and pulaskites) ranges from 120.1 ± 2 to 118.3 ± 2.1 Ma. The youngest phase of the massif, trachyte, crystallized at 115.5 ± 1.6 Ma. According to the mineralogical and geochemical studies, two types of ore mineralization, namely gold and uranium–thorium–rare-earth (U–Th–REE), are established within the DM. The gold mineralization was found in the quartz–chlorite–pyritized metasomatites. It is confined to the NNE- and NNW-trending fault zones and coincides with the strike of the syenite porphyry dike belt. Uranium–thorium–rare-earth mineralization has been established in the quartz–feldspathic metasomatites localized in the outer contact of the massif. The juxtaposition of mineralization of different types in some zones of the Dzheltula syenite massif significantly increases the ore potential of the studied object within the Tyrkanda ore region.  相似文献   

6.
The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar–Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have 143Nd/144Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have 143Nd/144Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas.  相似文献   

7.
The Lukinda dunite–troctolite–gabbro massif in the Selenga–Stanovoy superterrane on the southeastern framing of the Siberian Platform was earlier considered Precambrian. The performed 40Ar/39Ar dating of the massif plagioclase yielded an Early Permian age (285 ± 7.5 Ma). The main specific petrochemical features of the intrusion rocks during their crystallization differentiation are an increase in SiO2 and CaO contents and a decrease in FeOtot content, with TiO2 content remaining low and showing minor variations. A specific geochemical feature of the Lukinda massif ultrabasite–basites is a slight domination of LREE over HREE, with (La/Yb)N= 1.0–8.2. The depletion of the massif rocks in LILE (except for Sr and Ba), REE, and HFSE suggests that the massif formed on an active continental margin.  相似文献   

8.
Zircon UPb dating by SIMS of the Mont-Louis granite yields an age of 305±5 Ma, intrepreted to reflect the igneous emplacement age of the massif. It is in agreement with the Hercynian syntectonic character of Pyrenees granite. 40Ar/39Ar on hornblende, biotite and K-feldspar permit, to estimate the massif cooling. A rapid temperature decrease (≈30 °C/Ma) is revealed from Westphalian to Late Stephanian, coeval with the emplacement of a laccolithe in the upper crust. Then, the cooling rate decreases to ≈1 °C/Ma. This would be consistent with a long time residence for the pluton from the Late Palaeozoic to the Early Cainozoic at 6–8 km depth. To cite this article: O. Maurel et al., C. R. Geoscience 336 (2004).  相似文献   

9.
Detailed 40Ar/39Ar geochronology on single grains of muscovite was performed in the Variscan Tanneron Massif (SE France) to determine the precise timing of the post-collisional exhumation processes. Thirty-two plateau ages, obtained on metamorphic and magmatic rocks sampled along an east–west transect through the massif, vary from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation of the lower crust that lasted about 20 Ma during late Carboniferous. In the eastern part of the massif, the closure of the K–Ar isotopic system is at 311–315 Ma, whereas in the middle part of the massif it closes earlier at 317–321 Ma. These cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a major ductile fault, the La Moure fault that separates both domains. In the western part of the massif, the ages decrease from 318 to 303 Ma approaching the Rouet granite, which provides the youngest age at 303.6 ± 1.2 Ma. This age distribution can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex. These ages argue in favour of a cooling of the magmatic body at around 15 Ma after the country rocks in the western Tanneron. The emplacement of the Rouet granite in the core of an antiform is responsible for recrystallization and post-isotopic closure disturbances of the K–Ar chronometer in the muscovite from the host rocks. These new 40Ar/39Ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later stage of collision. During the first stage between 320 and 310 Ma, the differential motion of tectonic blocks limited by ductile shear zones controls the post-collisional exhumation. This event could be related to orogen parallel shearing associated with crustal-scale strike-slip faults and regional folding. The final exhumation stages at around 300 Ma take place within the tectonic doming associated to magmatic intrusions in the core of antiformal structures. Local ductile to brittle normal faulting is coeval to Upper Carboniferous intracontinental basins opening.  相似文献   

10.
Charoite is a unique mineral and a rock of the same name. It is known from the only deposit in the world at the contact with Early Cretaceous syenite and other alkaline rocks of the Malyi Murun massif. The data on 40Ar/39Ar dating of tinaksite, tokkoite, and frankamenite associated with charoite are reported in this paper. All these minerals, except for frankamenite, have shown clear plateau ages overlapping with each other within the analytical errors. The weighted mean of the plateau ages of tinaksite, tokkoite, and microcline is 135.86 ± 0.26 Ma. Considering that these minerals are syngenetic to charoite, this age is the time of charoite crystallization. Frankamenite with an older age (137.55 ± 0.46 Ma) may reflect the polychronous crystallization of the charoite association.  相似文献   

11.
Résumén

Des analyses 40Ar-39Ar ont été entreprises sur diverses formations effusives et intrusives permirnnes de Provence orientale. Celles-ci ont précédemment fourni des âges K/Ar conventionnels compris entre 220 et 2 76 Ma, discordants entre différents auteurs.

Deux âges-plateau obtenus sur un plagioclase d’une coulée basique (δ 1) (278 ± 0.4 Ma) et sur un adulairr d’un filon de fluorite-baryte intrusif dans une formation volcanique acide du massif de l’Estérel (All) (263,8 ± 0,7 Ma), ainsi que d’autres âges issus de spectres d’âge plus ou moins perturbés (discutés dans le texte), montrent que l’essentiel du volcanisme effusif du massif de l’Estérel s’est probablement déroulé pendant une période de l’ordre de 14 Ma, sensiblement plus faible que celle proposée antérieurement à partir des données K/Ar. Le filon minéralisé de fluorite-baryte de la caldeira de Maurevieille (Nord-Est du massif de l’Estérel) est contemporain du volcanisme permien. A l’Ouest du massif, des dykes basiques intrusifs dans le massif des Maures n’ont pu être datés à cause de leur altération importante, niais ils s’intègrent bien dans la tectonique distensive permienne E-W de la région.  相似文献   

12.
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   

13.
Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by 40Ar/39Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation 40Ar/39Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of published K–Ar ages (112 ± 5 Ma) for one K-feldspar sample of the Malyy Murun syenite. Tinaksite is characterized by a similar spread of ages (from 133.0 ± 3 Ma to 115.7 ± 4.3 Ma) within a single grain. We suggest that charoitites originated due to the interaction of metasomatic agents derived from the Malyy Murun magma and country rocks. Timing of magma emplacement and charoitite crystallization is reflected by the older cluster of ages, whereas the younger ages are due to a secondary process.  相似文献   

14.
Vertical displacements on the SW–NE Têt fault (Eastern Pyrenees Axial Zone, France), which separates the Variscan Canigou-Carança and Mont-Louis massifs, were constrained using a thermochronologic multi-method approach. 40Ar/39Ar data from the granitic Mont-Louis massif record its Variscan cooling history and reveal no ages younger than Early Cretaceous, while the Canigou-Carança gneiss massif records systematically younger 40Ar/39Ar ages. These younger 40Ar/39Ar ages in the Canigou-Carança gneiss massif are the result of partial to total rejuvenation of argon isotopic systems related to a thermal flow coeval with the Cretaceous HT-BP metamorphism in the North Pyrenean Zone. Only the deepest rocks from the Canigou-Carança suffered this extensive Mid-Cretaceous thermal overprint probably due to differential burial around 4 km at that time. The post Mid-Cretaceous vertical displacements along the Têt fault are recorded by “low” temperature thermochronology using K-feldspar 40Ar/39Ar, zircon and apatite fission track and (U–Th)/He datings. The Mont-Louis granite samples experienced a long period of protracted cooling reflecting a lack of thermo-tectonic activity in this area from Late Palaeozoic to Early Cenozoic, followed by cooling from 55–60 Ma to Late Eocene at a mean rate of 15–20°C/Ma in the final stage. This cooling stage corresponds to Têt fault reactivation with a reversed component, promoting exhumation of the Mont-Louis roof zone contemporaneously with the south-vergent Pyrenean thrusting. In the Canigou-Carança massif, the main cooling event occurred from 32 to 18 Ma at a maximum rate of 30°C/Ma during Early Oligocene followed by a more moderate rate of 3°C/Ma from Late Oligocene to Early Burdigalian, coeval with the normal reactivation of the Têt fault in brittle conditions that accommodated the final exhumation of the massif during the opening of the Gulf of Lion.  相似文献   

15.
黑龙江杂岩构造折返的岩石学与年代学证据   总被引：12，自引：9，他引：3  
赵亮亮  张兴洲 《岩石学报》2011,27(4):1227-1234
黑龙江杂岩主要出露在佳木斯地块西缘,沿牡丹江断裂分布,为佳木斯地块与松嫩地块拼合过程中形成的构造混杂岩。杂岩以强烈变形的糜棱岩为主体,其中含有大量规模不等、变形程度不同的变橄榄岩、变辉长岩、蓝闪石片岩(变玄武岩)及变硅质岩和大理岩等岩块或岩片。蓝闪石片岩岩块多经强烈的构造置换呈宽度和延伸有限的条带或岩片与长英质糜棱岩相间分布,发育以青铝闪石、冻蓝闪石、黑硬绿泥石、多硅白云母和钠长石等为代表的新生变质矿物组合,变形叶理呈北东向展布。但在局部地区仍保存有规模较大,且未受变形作用改造的蓝闪石片岩岩块,以不定向生长的蓝闪石、钠长石和绿帘石变质矿物组合为特征。块状蓝闪石片岩边部变形特征明显,变形叶理与周围糜棱岩叶理产状一致。岩块与变形岩石的组构关系表明,黑龙江杂岩至少经历了两期重要的变质事件,即以蓝闪石片岩岩块为代表的早期高压变质事件和以糜棱岩为代表的晚期变质-变形事件。在桦南地区长英质糜棱岩中获得的白云母40Ar/39Ar年龄分别为176.5±1.9Ma和184.5±2.1Ma,该年龄在黑龙江杂岩的变形岩石中普遍存在。鉴于蓝闪石片岩岩块被这一变质-变形事件所改造,以及变质矿物组合由高压向低压的转变关系,表明黑龙江杂岩在早-中侏罗世经历了一次快速的构造抬升事件。这一事件不但使佳木斯地块西缘以蓝片岩为代表的俯冲杂岩发生构造折返,而且对该区晚中生代盆地的形成与演化起着重要的控制作用。  相似文献   

16.
The Elna Cu(Au)–porphyry deposit is one of the typical ore objects in the northeastern margin of the Argun superterrane facing the Mongolia–Okhotsk foldbelt. Mineralization includes zones of argillization with fine quartz veins in granodiorite of the Elna massif. The geochronological 40Ar/39Ar studies of hydrothermal near-ore metasomatites and magmatic rocks of the deposit show that the age of host granitoids is 126 ± 2 Ma, which corresponds to the upper age boundary of granitoids from the Burinda Complex, whereas the age of overprinted hydrothermal processes is 122–117 Ma. The age of mineralization correlates well with the age of the thermal event in East Asia. An intense stage of magmatism including both volcanic and intrusive forms occurred in this period.  相似文献   

17.
The Late Triassic Central Patagonian Batholith is a key element in paleogeographic models of West Gondwana just before to the break-up of the supercontinent. The preexisting classification of units of this batholith was mainly based on isotopic and geochemical data. Here we report the results of field mapping and petrography, backed up by three new 40Ar/39Ar biotite ages, which reveal previously unnoticed relationships of the rocks in the batholith. Based on the new information we present a reorganization of units where the batholith is primarily formed by the Gastre and the Lipetrén superunits. The Gastre Superunit is the oldest magmatic suite and is composed of I-type granites which display evidence of felsic and mafic magma interaction. It is formed by 4 second-order units: 1) equigranular hornblende–biotite granodiorites, 2) porphyritic biotite–hornblende monzogranites, 3) equigranular biotitic monzogranites and 4) hornblende quartz-diorites. Emplacement depth of the Gastre Superunit is bracketed between 6 and 11 km (1.8–3 kbar), and the maximum recorded temperatures of emplacement are comprised between 660 and 800 °C. The recalculated Rb/Sr age is 222 ± 3 Ma and the porphyritic biotite–hornblende monzogranites yielded a 40Ar/39Ar age in biotite of 213 ± 5 Ma. On the other hand, the Lipetrén Superunit is made up by fine-grained biotitic monzo- and syenogranites that postdate magma hybridization processes and intrude all the other units. The recalculated Rb/Sr age for this suite is identical to a 40Ar/39Ar age in biotite extracted from one of its monzogranites (206.4 ± 5.3 and 206 ± 4 Ma, respectively). This and the observed textural features suggest very fast cooling related to a subvolcanic emplacement. An independent unit, the “Horqueta Granodiorite”, which has previously been considered as the record of a Jurassic intrusive stage in the Central Patagonian Batholith, gave a 40Ar/39Ar age in biotite of 214 ± 2 Ma. This and the reexamination of available isotopic data allow propose that this granodiorite unit is part of the Late Paleozoic intrusives in the region. The Late Triassic Central Patagonian Batholith is overlain by 190–185 Ma volcano-sedimentary rocks, suggesting that it was exposed sometime between the latest Triassic and earliest Jurassic times, roughly coeval with a major accretionary episode in the southwestern margin of Gondwana.  相似文献   

18.
40Ar/39Ar isotopic ages of magnesioarfvedsonite (725.9 ± 6.3 myr) and phlogopite (637.6 ± 5.7 myr) from carbonatites of the Penchenga rock complex located in the South Trans Angara segment of the Yenisei Ridge have been determined. Based on this, the formation of the rock complex is associated with the start of the period of Neoproterozoic (725–700 Ma) rifting and alkaline magmatism in the region, the latter being associated with the former. The appearance of zones penetrable for magma of the “disseminated” rifting type is associated with the development of late collision shear dislocations in a consolidated marginal continental crust. The primary melts had a plume nature, but matter from different mantle sources intermixed in the environment of the changing geodynamic regime. The younger age of the mica was apparently caused by partial remelting of carbonatites under the influence of heat and fluids generated during the formation of the adjacent large granite massif of a younger age.  相似文献   

19.
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   

20.
This study firstly presents chemical and initial Os-isotopic compositions of Os-Ir-Ru minerals of two ultramafic formations of Polar Siberia, which are exemplified by Guli clinopyroxene-dunite massif of the Maimecha-Kotui Province and the Kunar dunite-harzburgite massif from the Chelyuskin ultramafic belt of the Taimyr Peninsula. The study employed a range of methods, including electron microprobe analysis, negative thermal ionization mass spectrometry (N-TIMS) and laser ablation attached to an inductively coupled plasma mass spectrometry (LA MC-ICP-MS). The majority of platinum-group minerals (PGM) from the Guli massif are Os-(Ir-Ru) solid solutions or Os-rich minerals. At Kunar, minerals of Ru-Os-Ir system (i.e., osmium, ruthenium, iridium and rutheniridosmine) dominate the PGM assemblage. The ruthenium trend in the mineral compositions is due to the formation of these minerals under high pressures and temperatures at considerable depths. The 187Os/188Os values of Os-rich minerals from the Guli massif range from 0.12309 ± 0.00002 to 0.12606 ± 0.00003 (n = 168). The initial Os-isotopic composition of PGM from the central block of the Guli massif is characterized by the 187Os/188Os values, varying in the range 0.12404–0.12606. Osmiumrich minerals from the southwestern block of the Guli massif are characterized by the least “radiogenic” 187Os/188Os values (i.e., 0.12309–0.12341). Low relative to the chondritic universal reservoir (CHUR) 187Os/188Os values are indicative of a near-to-chondritic source of platinum-group elements (PGE). The most “productive” stage of PGM formation at Guli (n = 121) is recorded in the time interval of 545–615 Ma. The older model 187Os/188Os ages of osmium minerals are characteristic of the southwestern block of the Guli massif (e.g., 745–760 Ma). The results of the initial Os-isotopic composition for Os-rich alloys are consistent with a model, in which PGM were formed during multi-stage melt depletion events in the mantle. This agrees well with the suggestion that the Guli massif consists of heterogeneous blocks of ultramafic rocks. The 187Os/188Os ratio in the investigated PGM from the Kunar massif varies in a wider range (0.1094–0.1241, n = 28). For the dominant set of PGM samples (n = 25), regardless of their chemical composition, four groups of the initial osmium isotopic compositions can be estimated, with average 187Os/188Os values of 0.1217 ± 0.0002 (n = 7), 0.1223 ± 0.0002 (n = 7), 0.1230 ± 0.0002 (n = 6) and 0.1238 ± 0.0003 (n = 6), respectively. The average model Re-Os ages for the defined groups of the Kunar massif are consistent with Late Riphean age interval (e.g., 975 ± 42 Ma, 892 ± 42 Ma, 791 ± 28 Ma and 681 ± 42 Ma, respectively). Significant variations in the 187Os/188Os values and model ages for Ru-Os-Ir alloys at Kunar are close to those from other duniteharzburgite massifs of the Earth, pointing out for their prolonged multi-stage evolution within the upper mantle.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号