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1.
The Mersin ophiolite, which is a relic of the late Cretaceous Neotethyan ocean domain in the eastern Mediterranean, is situated on the southern flank of the central Tauride belt. The ophiolite body is cross-cut at all structural levels by numerous mafic dyke intrusions. The dykes do not intrude the underlying melange of platform carbonates. Therefore, dyke emplacement post-dates the formation of the opholite and metamorphic sole but pre-dates the final obduction onto the Tauride platform. The post-metamorphic dyke swarms suggest the geochemical characteristics of Island Arc Tholeiites (IAT). 40Ar/39Ar geochronology of the post-metamorphic microgabbroic-diabasic dykes cutting both mantle tectonites and metamorphic sole revealed ages ranging from 89.6 ± 0.7–63.8 ± 0.9 Myr old, respectively, indicating widespread magmatic activity during the Late Cretaceous-early Palaeocene in the Neotethyan ocean. These data suggest that island arc development in the Neotethyan ocean in southern Turkey was as early as Late Cretaceous.  相似文献   

2.
Palaeo- and Neo-Tethyan-related magmatic and metamorphic units crop out in Konya region in the south central Anatolia. The Neotethyan assemblage is characterized by mélange and ophiolitic units of Late Cretaceous age. They tectonically overlie the Middle Triassic–Upper Cretaceous neritic to pelagic carbonates of the Tauride platform. The metamorphic sole rocks within the Konya mélange crop out as thin slices beneath the sheared serpentinites and harzburgites. The rock types in the metamorphic sole are amphibolite, epidote-amphibolite, garnet-amphibole schist, plagioclase-amphibole schist, plagioclase-epidote-amphibole schist and quartz-amphibole schist. The geochemistry of the metamorphic sole rocks suggests that they were derived from the alkaline (seamount) and tholeiitic (E-MORB, IAT and boninitic type) magmatic rocks from the upper part of the Neotethyan oceanic crust. Four samples from the amphibolitic rocks yielded 40Ar/39Ar isotopic ages, ranging from 87.04 ± .36 Ma to 84.66 ± .30 Ma. Comparison of geochemistry and geochronology for the amphibolitic rocks suggests that the alkaline amphibolite (seamount-type) cooled below 510 ± 25 °C at 87 Ma whereas the tholeiitic amphibolites at 85 Ma during intraoceanic thrusting/subduction. When all the evidence combined together, the intraoceanic subduction initiated in the vicinity of an off-axis plume or a plume-centered spreading ridge in the Inner Tauride Ocean at 87 Ma. During the later stage of the steady-state subduction, the E-MORB volcanics on the top of the down-going slab and the arc-type basalts (IAT/boninitic) detached from the leading edge of the overriding plate, entered the subduction zone after ~2 my and metamorphosed to amphibolite facies in the Inner Tauride Ocean. Duration of the intraoceanic detachment (~87 Ma) and ophiolite emplacement onto the Tauride-Anatolide Platform (Tav?anl? Zone), followed by subsequent HP/LT metamorphism (~82 Ma) spanned ~5 my in the western part of the Inner Tauride Ocean.  相似文献   

3.
In this study, LA-ICP-MS U–Pb zircon dating were used to determine the age of the newly discovered plagiogranite suite intruding gabbro and volcanic units of Mersin ophiolites from the Inner Tauride Belt. Obtained U–Pb zircon ages from the plagiogranite yielded crystallization ages of 93.0?±?1.5 to 94.2?±?2.4 Ma (Turonian–Cenomanian) supporting the idea of Late Cretaceous active subduction factory in the Tauride Suture Zone. The plagiogranites are mainly granodioritic, and tonalitic in composition, and contain mafic microgranular enclaves (MME) ranging from 10 to 45 cm in size. The plagiogranites are geochemically defined by low K2O (0.02–1.03 wt%) and TiO2 (0.17–1.88 wt%) and comparatively high Na2O (2.3–10.2 wt%) and SiO2 (70–78 wt%) compositions together with depletion in Ti, Ta, and Nb. The tectonomagmatic discrimination diagrams, trace, and REE-normalized multi-element patterns indicate that the plagiogranites are distinctive calc-alkaline, I-type volcanic arc granites. Plagiogranites are furthermore characterized by the diffuse presence of isotropic pseudomorphic growth of secondary calcic-amphibole (edenite and actinolite) over a pristine not preserved Ca-inosilicate. Inverse geothermobarometry models indicate a secondary amphibole genesis at ca. 600 °C and 1.5–1.7 kbar, suggesting HT-metasomatism affecting the already intruded plagiogranites. While it is already accepted that Mersin ophiolite complex is generated in a supra-subduction zone, this study represents a new contribution on the evolution of the Mersin ophiolite during the Late Cretaceous Neotethys subduction and could shed light on the genesis of plagiogranites in arc-environments.  相似文献   

4.
Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the 40Ar/39Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the 40Ar/39Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the 40Ar/39Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.  相似文献   

5.
40Ar/39Ar dating was conducted on the Da Lien granite related to greisen‐skarn type polymetallic (W‐CaF2‐Cu‐Bi‐Au) mineralization in Nui Phao, northern part of Vietnam in the South China Plate. Biotite and muscovite separates from the biotite‐muscovite granite and greisenized granite indicate four plateau ages: 82.2 ± 0.4 Ma, 82.8 ± 0.3 Ma, 81.5 ± 0.3 Ma and 82.5 ± 0.4 Ma. The plateau ages were not significantly influenced by excess 40Ar in dated minerals or by loss of radiogenic 40Ar due to hydrothermal activities. The results indicate that solidification of granite related to the polymetallic mineralization occurred in the Late Cretaceous between 82.8 Ma and 81.5 Ma.  相似文献   

6.
The Tengchong-Lianghe tin district in northwestern Yunnan, China, is an important tin mineralization area in the Sanjiang Tethyan Metallogenic Domain. There are three N–S trending granite belts in the Tengchong-Lianghe area, with emplacement ages ranging from Early Cretaceous to Late Cretaceous and Early Cenozoic. Tin mineralization is spatially associated with these granitic rocks. However, the petrogenetic link between the tin deposits and the host granites is not clear because of the lack of age data for the tin mineralization. We investigate the possibility of direct dating of cassiterite from three typical tin deposits in the Tengchong-Lianghe tin district, using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). In situ LA-MC-ICP-MS dating of seven cassiterite samples from the Lailishan (LLS-1 and LLS-2), Xiaolonghe (XLH, WDS, DSP, and HJS), and Tieyaoshan (TYS) tin deposits yielded well-defined 206Pb/207Pb–238U/207Pb isochron ages. To assess the accuracy of the in situ U/Pb dating of cassiterite, 40Ar/39Ar dating of coexisting muscovite (in samples LLS-1, DSP, and TYS) was also performed. The cassiterite in situ U/Pb ages (47.4?±?2.0, 71.9?±?2.3, and 119.3?±?1.7 Ma, respectively) are in excellent agreement with the coexisting muscovite 40Ar/39Ar ages (48.4?±?0.3, 71.9?±?1.4, and 122.4?±?0.7 Ma, respectively). The U/Pb ages of cassiterite combined with the 40Ar/39Ar ages of muscovite indicate that there are three tin mineralization events in this district: the Lailishan tin deposit at 47.4?±?2.0 to 52?±?2.7 Ma, the Xiaolonghe tin deposit at 71.6?±?2.4 to 3.9?±?2.0 Ma, and the Tieyaoshan tin deposit at 119.3?±?1.7 to 122.5?±?0.7 Ma. These ages are highly consistent with the zircon U/Pb ages of the host granites. It is su.ggested that the Cretaceous tin mineralization might have taken place in a subduction environment, while the Early Tertiary tin metallogenesis was in a postcollisional geodynamic setting.  相似文献   

7.
The Mersin ophiolite, represented by approximately 6-km-thick oceanic lithospheric section on the southern flank of the Taurus calcareous axis, formed in the Mesozoic Neo-Tethyan ocean some time during Late Cretaceous in southern Turkey. The ultramafic and mafic cumulates having over 3 km thickness consist of dunite ± chromite, wehrlite, clinopyroxenite at the bottom and pass into gabbroic cumulates in which leucogabbro, olivine-gabbro and anorthosite are seen. Crystallization order is olivine (Fo91−80) ± chromian spinel (Cr# 60-80), clinopyroxene (Mg#95−77), plagioclase (An95.6−91.6) and orthopyroxene (Mg#68−77). Mineral chemistry of ultramafic and mafic cumulates suggest that highly magnesian olivines, clinopyroxenes and absence of plagioclase in the basal ultramafic cumulates are in good agreement with products of high-pressure crystal fractionation of primary basaltic melts beneath an island-arc environment. Major, trace element geochemistry of the cumulative rocks also indicate that Mersin ophiolite was formed in an arc environment. Coexisting Ca-rich plagioclase and Forich olivine in the gabbroic cumulates show arc cumulate gabbro characteristics. Field relations as well as the geochemical data support that Mersin ophiolite formed in a supra-subduction zone tectonic setting in the southern branch of the Neo-Tethys in southern Turkey.  相似文献   

8.
The Güira de Jauco metamorphic sole, below the Moa-Baracoa ophiolite (eastern Cuba), contains strongly deformed amphibolites formed at peak metamorphic conditions of 650–660°C, approximately 8.6 kbar (~30 km depth). The geochemistry, based on immobile elements of the amphibolites, suggests oceanic lithosphere protholiths with a variable subduction component in a supra-subduction zone environment. The geochemical similarity and tectonic relations among the amphibolites and the basic rocks from the overlying ophiolite suggest a similar origin and protholith. New hornblende 40Ar/39Ar cooling ages of 77–81 Ma obtained for the amphibolites agree with this hypothesis, and indicate formation and cooling/exhumation of the sole in Late Cretaceous times. The cooling ages, geochemical evidence for a back-arc setting of formation of the mafic protoliths, and regional geology of the region allow proposal of the inception of a new SW-dipping subduction zone in the back-arc region of the northern Caribbean arc during the Late Cretaceous (ca. 90–85 Ma). Subduction inception was almost synchronous with the main plume pulse of the Caribbean–Colombian Oceanic Plateau (92–88 Ma) and occurred around 15 million years before arc-continent collision (75 Ma–Eocene) at the northern leading edge of the Caribbean plate. This chronological framework suggests a plate reorganization process in the region triggered by the Caribbean–Colombian mantle plume.  相似文献   

9.
The Borgulikan ore field is localized in the west of the Umlekan-Ogodzha volcanoplutonic belt made up of various igneous (upper-Amur granite-granodiorite (140–134 Ma), Burunda monzodiorite-granodiorite (130–127 Ma), and Taldan andesite (127–123 Ma)) and superposed (Early Cretaceous Gal’ka trachybasalt-rhyolite (119–115 Ma) and Late Cretaceous trachybasalt-trachyandesite (97–94 Ma)) complexes. 40Ar/39Ar dating of porphyry intrusions breaking through the Taldan volcanic complex and associated with Cu-Mo-(Au) mineralization yielded the following ages: early (dark) “pre-ore” quartz monzodiorite porphyrites — 125.8±0.7 Ma (groundmass) and 125.2±1.8 Ma (biotite phenocrysts); late (cream) “syn-ore” quartz monzodiorite porphyrites — 122.6±0.7 Ma (biotite phenocrysts). In age and many geochemical features the quartz monzodiorite porphyrites are close to the Taldan complex volcanics. Both of these rocks seem to belong to the same volcanoplutonic association.  相似文献   

10.
内蒙古贺根山蛇绿岩形成时代及构造启示   总被引：16，自引：5，他引：11  
贺根山蛇绿岩位于兴蒙造山带北缘,发育完整的地幔橄榄岩、堆晶岩和基性熔岩组合,伴生有放射虫硅质岩,但贺根山蛇绿岩的形成时代一直存在争议,给兴蒙造山带北部构造演化阶段划分造成了很大障碍。锆石U-Pb年代学研究表明,贺根山蛇绿岩中辉长闪长岩(341±3Ma)和玄武岩(359±5Ma)结晶年龄为早石炭世早期,同时玄武岩继承锆石峰值年龄为晚泥盆世早期(375±2Ma),这些继承锆石呈短柱状、棱角状,生长环带宽缓,多为补丁状、平坦状,为典型的基性岩浆锆石,表明最迟在晚泥盆世早期洋壳物质已经开始形成。上石炭统格根敖包组火山岩与蛇绿岩局部呈喷发不整合接触,该组的晶屑凝灰岩夹层时代为晚石炭世(323±3Ma),提供了蛇绿岩构造侵位年龄的上限。因此,将贺根山蛇绿岩形成时代定为晚泥盆世-早石炭世,侵位时代为晚石炭世。侵入地幔橄榄岩中的部分基性岩脉时代为早白垩世(132±1Ma、139±3Ma和120±1Ma),它们含有大量继承锆石(144±1Ma~2698±25Ma),继承锆石峰值年龄密切响应了兴蒙造山带北部早白垩世之前复杂的岩浆及构造事件,这些基性岩脉是燕山期伸展环境下的岩浆产物,并非早白垩世蛇绿岩。结合前人的工作成果和区域岩浆岩、地层时空分布特征,建立了兴蒙造山带北部晚古生代构造演化历程:二连贺根山一线早泥盆世处于剥蚀阶段,中泥盆世陆壳拉张出现新生洋盆,晚泥盆世早期洋盆持续扩张形成新生洋壳,早石炭世晚期洋壳开始向北俯冲消减,并持续增生至西伯利亚活动陆缘,晚石炭世洋盆陆续闭合,部分已经构造侵位的蛇绿岩被晚石炭世火山岩不整合覆盖,贺根山蛇绿岩正是该洋盆的残余产物。  相似文献   

11.
Five samples of muscovite from mylonites of the earlier Tanlu ductile shear zone on the eastern margin of the Dabie Mountains yield 40Ar/39Ar ages ranging from 178 Ma to 196 Ma. Three of them have reliable plateau ages of 188.7±0.7 Ma, 189.7±0.6 Ma and 192.5±0.7 Ma respectively, which indicates a syn-orogenic, sinistral strike-slip thermal event. This displacement movement derived from the continent-continent collision of the North and South China blocks took place in the Early Jurassic and after uplifting of high-pressure to ultrahigh-pressure slabs to the mid-crust. It is suggested that during the collision the Tanlu fault zone was an intracontinental transform fault caused by differential subduction speeds. The 40Ar/39Ar ages of mylonite whole-rock and muscovite from the later Tanlu ductile shear zone suggest another sinistral strike-slip cooling event at 128 Ma. During this strike-slip faulting, large-scale intrusion and doming uplift occurred in the eastern part of the Dabie orogenic belt. Data o  相似文献   

12.
The Kiziltepe ophiolitic thrust sheet in the Bolkar Mountains of Turkey occurs between two subparallel ophiolite belts bounding the Tauride carbonate platform and represents a remnant of the Cretaceous Neo-Tethyan oceanic lithosphere. It is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra-oceanic subduction zone in the Neo-Tethys 92-90 Ma. Blueschist-facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing P/T with cooling. Regional tectonic constraints suggest a Maastrichtian age for the timing of this blueschist-facies metamorphism. Sodic amphibole-rich veins and crossite/Mg-riebeckite rims on hornblende suggest that growth of blueschist-facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries. We infer a counterclockwise P-T-t trajectory during which metamorphism was accompanied/succeeded by rapid uplift along the northern edge of the Tauride belt in Late Cretaceous-early Tertiary time.  相似文献   

13.
abstract

Although numerous ages have been obtained for the Chinese southwestern Tianshan high pressure/ultrahigh pressure-low temperature (HP/UHP-LT) metamorphic belt in the past two decades, its exhumation history is still controversial. The poor age constraint was related to the appealing low metamorphic temperatures and excess Ar commonly present under HP/UHP conditions. This study aims to provide new age constraints on the orogen’s exhumation by obtaining 40Ar/39Ar mica ages using the conventional step-heating technique, with emphasis on the avoidance of excess Ar contamination. From a cross section along the Kekesu Valley, four samples, three from the HP-LT metamorphic belt (TK050, TK051, and TK081) and one from the southern margin of the low pressure metamorphic belt (TK097), were selected for 40Ar/39Ar dating. Phengites from garnet glaucophane schist TK050 and the surrounding rock garnet phengite schist TK051 yield comparable plateau ages of 321.4 ± 1.6 and 318.6 ± 1.6 Ma, respectively, while epidote mica schist TK081 gives a younger plateau age of 293.3 ± 1.5 Ma. Considering the chemical compositions of phengites, mineral assemblages, and microstructures in the thin slices, we suppose that the former represents the time the HP rocks retrograded from the peak stage (eclogite facies) to the (epidote)-blueschist facies, whereas the latter reflects greenschist facies overprinting. Biotite and muscovite from two-mica quartzite TK097 give similar plateau ages of 253.0 ± 1.3 and 247.1 ± 1.2 Ma, interpreted to date movement on the post collisional transcrustal South Nalati ductile shear zone. By combining our new ages with published data, a two-stage exhumation model is suggested for the Chinese southwestern Tianshan HP/UHP-LT metamorphic belt: initial fast exhumation to a depth of about 30–35 km by ~320 Ma was followed by relatively slow (~1 mm year–1) uplift to ~10 km by ~293 Ma.  相似文献   

14.
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   

15.
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16.
Variscan to Alpine magmatic activity on the North Tethys active Eurasian margin in the Caucasus region is revealed by 40Ar/39Ar ages from rocks sampled in the Georgian Crystalline basement and exotic blocs in the Armenian foreland basin. These ages provide insights into the long duration of magmatic activity and related metamorphic history of the margin, with: (1) a phase of transpression with little crustal thickening during the Variscan cycle, evidenced by HT-LP metamorphism at 329–337 Ma; (2) a phase of intense bimodal magmatism at the end of the Variscan cycle, between 303 and 269 Ma, which is interpreted as an ongoing active margin during this period; (3) further evolution of the active margin evidenced by migmatites formed at ca. 183 Ma in a transpressive setting; (4) paroxysmal arc plutonic activity during the Jurassic (although the active magmatic arc was located farther south than the studied crystalline basements) with metamorphic rocks of the Eurasian basement sampled in the Armenian foreland basin dated at 166 Ma; (5) rapid cooling suggested by similar within-error ages of amphibole and muscovite sampled from the same exotic block in the Armenian fore-arc basin, ascribed to rapid exhumation related to extensional tectonics in the arc; and finally (6) cessation of ‘Andean’-type magmatic arc history in the Upper Cretaceous. Remnants of magmatic activity in the Early Cretaceous are found in the Georgian crystalline basement at c. 114 Ma, which is ascribed to flat slab subduction of relatively hot oceanic crust. This event corresponds to the emplacement of an oceanic seamount above the N Armenian ophiolite at 117 Ma. The activity of a hot spot between the active Eurasian margin and the South Armenian Block is thought to have heated and thickened the Neo-Tethys oceanic crust. Finally, the South Eurasian margin was uplifted and transported over this hot oceanic crust, resulting in the cessation of subduction and the erosion of the southern edge of the margin in Upper Cretaceous times. Emplacement of Eocene volcanics stitches all main collisional structures.  相似文献   

17.
Long-lived intra-oceanic arcs of Izu-Bonin-Marianas (IBM)-type are built on thick, granodioritic crust formed in the absence of pre-existing continental crust. International Ocean Discovery Program Expedition 350, Site U1437, explored the IBM rear arc to better understand continental crust formation in arcs. Detailed petrochronological (U–Pb geochronology combined with trace elements, oxygen and hafnium isotopes) characterizations of zircon from Site U1437 were carried out, taking care to exclude potential contaminants by (1) comparison of zircon ages with ship-board palaeomagnetic and biostratigraphic ages and 40Ar/39Ar geochronology, (2) analysing zircon from drill muds for comparison, (3) selectively carrying out in situ analysis in petrographic thin sections, and (4) minimizing potential laboratory contamination through using pristine equipment during mineral separation. The youngest zircon ages in Site U1437 are consistent with 40Ar/39Ar and shipboard ages to a depth of ~1390 m below sea floor (mbsf) where Igneous Unit Ig 1 yielded an 40Ar/39Ar age of 12.9 ± 0.3 Ma (all errors 2σ). One single zircon (age 15.4 ± 1.0 Ma) was recovered from the deepest lithostratigraphic unit drilled, Unit VII (1459.80–1806.5 mbsf). Site U1437 zircon trace element compositions are distinct from those of oceanic and continental arc environments and differ from those generated in thick oceanic crust (Iceland-type) where low-δ18O evolved melts are produced via re-melting of hydrothermally altered mafic rocks. Ti-in-zircon model temperatures are lower than for mid-ocean ridge rocks, in agreement with low zircon saturation temperatures, suggestive of low-temperature, hydrous melt sources. Zircon oxygen (δ18O = 3.3–6.0‰) and hafnium (εHf = + 10–+16) isotopic compositions indicate asthenospheric mantle sources. Trace element and isotopic differences between zircon from Site U1437 rear-arc rocks and the Hadean detrital zircon population suggest that preserved Hadean zircon crystals were probably generated in an environment different from modern oceanic convergent margins underlain by depleted mantle.  相似文献   

18.
Noblesse multi-collector noble gas mass spectrometer is specially designed for multi-collection of Ar isotopes with different beam sizes, especially for small ion beams, precisely, and hence is perfectly suitable for 40Ar/39Ar geochronology. We have analyzed widely used sanidine, muscovite, and biotite standards with recommended ages of ~ 1.2–133 Ma, with the aim to assess the reliability of Noblesse for 40Ar/39Ar dating. An ESI MIR10 30W CO2 laser was used for total fusion or incremental heating samples. Extracted gases were routinely purified by four SAES NP10 getters (one at ~ 400 °C and others at room temperature). A GP50 getter and a metal cold finger cooled by liquid N (? 196 °C) were also attached for additional purification if necessary. The Ar isotopes were then measured by Noblesse using Faraday or multiplier according to the signal intensities. Over a period of 1.5 months 337 air calibrations produced a weighted mean 40Ar/36Ar of 296.50 ± 0.08 (2σ, MSWD = 4.77). Fish Canyon sanidine is used to calculate J-values, which show good linear relationship with position in irradiation. The age of four mineral standards (Alder Creek sanidine, Brione muscovite, Yabachi sanidine, and Fangshan biotite) are within error of the accepted ages. Five Alder Creek sanidine aliquots yielded an age range of 1.174–1.181 ± 0.013 Ma (2σ) which broadly overlaps the established age of the standard and the uncertainty approaches those of the foremost Ar/Ar laboratories in the world. The weighted mean ages of four Brione muscovite aliquots (18.75 ± 0.16 Ma, 2σ), five Yabachi sanidine aliquots (29.50 ± 0.19 Ma, 2σ), and three Fangshan biotite aliquots (133.0 ± 0.76 Ma, 2σ) are consistent with the recommended values of these standards, and the uncertainties are typical of modern Ar/Ar laboratories world-wide.  相似文献   

19.
Number of dismembered ophiolite bodies crop out between Sivas and Malatya on the top of the Eastern Tauride platform in the central-eastern Turkey. One of which at the southern margin of the Sivas basin in the Tecer Mountain area comprises melange and the lower part of an oceanic lithospheric section on top of the Tauride platform. The mantle tectonites are characterized by variably serpentinized harzburgites and dunites, and are intruded by numerous isolated dykes. The gabbroic cumulates consist of olivine gabbro, gabbro and gabbronorite. The major and trace element geochemistry of the mafic cumulate rocks suggests that the primary magma was compositionally similar to those observed in modern island-arc tholeiitic sequences. The isolated dykes are exclusively basaltic in composition and display geochemically two distinct subgroups: Group I is represented by high TiO2 (.87–1.47 wt.%) and other incompatible elements, whereas Group II is characterized by low TiO2 (.36–.66 wt.%) and other incompatible elements. The Group I isolated diabase dykes have flat to slightly LREE-depleted profiles (La/YbN = .32–.79), whereas the Group II isolated diabase dykes are more depleted in general and have a LREE-depleted character (La/YbN = .19–.49). This suggests that the isolated dykes were derived from an island arc tholeiitic magma (Nb/Y = .02–.05) with different degrees of partial melting (Group II > Group I) and relatively high oxygen fugacity in intra-oceanic subduction zone. The ophiolitic rocks in the study area may well be compared with the Divri?i ophiolite to the southeast. All the evidence suggests that the isolated dykes in the Tecer Mountain area differ from the alkaline isolated dykes cutting the Divri?i ophiolite. Since the late stage dykes (~76 Ma) in the Divri?i area are alkaline, the tholeiitic isolated dykes in the present study should have been emplaced prior to the alkaline dykes during Late Cretaceous SSZ-spreading (~90 Ma) within the Inner Tauride Ocean.  相似文献   

20.
West of the Main Uralian fault, the main suture in the southern Urals, 40Ar/39Ar apparent ages of amphibole, muscovite and potassium feldspar are interpreted as cooling ages. A fast exhumation of the metamorphic complex of Kurtinsky during Upper Carboniferous time is indicated by the small age difference (15 Ma) between cogenetic amphibole and muscovite. Differentiated movement in the footwall of the Main Uralian fault along strike is indicated by the age difference of 70 Ma between the metamorphic complexes of Kurtinsky (north) and Maksyutov (south). No Upper Paleozoic (Uralian) medium- to high-temperature event is recorded in 40Ar/39Ar data from the metamorphic complex of Beloretzk (MCB). An amphibole age of 718±5 Ma and the occurrence of mafic intrusions might signal the break-up of Rodinia and therefore indicate the rifting period followed by the separate movement of the "Beloretzk terrane". Muscovite ages of approximately 550±5 Ma, the unique pre-Ordovician tectonometamorphic evolution of the MCB and the Late Vendian sedimentary history of the western Bashkirian Megaanticlinorium (BMA) imply the existence of a Neoproterozoic orogeny at the eastern margin of Baltica. This orogeny might have been initiated by the accretion of the "Beloretzk terrane". The metamorphic grade of the overlain Silurian shales and the K/Ar microcline ages from the "Beloretzk terrane" give evidence for a new thermal event at approximately 370 Ma. A microcline age of 530–550 Ma obtained for the Vendian conglomerate in the western BMA suggests that a maximum temperature of approximately 200°C was reached in Cambrian or Vendian times. An orthoclase age (590–630 Ma) of the Vendian Zigan flysch deposits might be inherited from the eastern source area, the Cadomian orogen. An orthoclase age (910–950 Ma) from the Riphean Zilmerdak conglomerate coincides with a documented decrease in the subsidence rate of the Upper Riphean basin.  相似文献   

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