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1.
Abstract The Sambagawa metamorphic belt exposed in central Shikoku records a high-P–T metamorphic event. It is represented by the Oboke nappe and structurally overlying, internally imbricated, Besshi nappe complex. These major structural units are in ductile thrust contact. A melange is developed along a ductile internal tectonic contact within the Besshi nappe complex. Tectonic emplacement of a high-T enclave (Sebadani eclogite) in the melange zone resulted in the development of a contact metamorphic aureole within the host Sambagawa rocks. 36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende from the Sambagawa basic schists which surround the Sebadani enclave are 83.4 ± 0.3 Ma (within contact aureole) and 83.6 ± 0.5 Ma (outside aureole). 40Ar/39Ar plateau ages recorded by muscovite from the same samples are 87.9 ± 0.3 and 89.3 ± 0.4 Ma. Amphibole from the amphibolite within the Sebadani enclave records isotope correlation ages of 93.7 ± 1.1 and 96.5 ± 0.7 Ma (massive interior) and 84.6 ± 1.2 Ma (marginal shear zone). Amphibole within the massive amphibolite is significantly higher in XMg than that within the host Sambagawa basic schists. The older ages recorded by amphibole within the Sebadani enclave are interpreted to date cooling through somewhat higher closure temperatures than which characterize the more Fe-rich amphibole in surrounding schists. The younger amphibole age recorded within the marginal shear zone probably indicates that crystallization of amphibole continued until cooling through the relatively lower amphibole closure temperatures. These results, together with the previously published 40Ar/39Ar ages of the Sambagawa schists, suggest: (i) metamorphic culmination occurred in the Besshi nappe complex at c. 100–90 Ma; (ii) at c. 95 Ma the Besshi nappe complex was internally imbricated and tectonic enclaves were emplaced; (iii) at c. 85 Ma, the composite Besshi nappe was rapidly exhumed and tectonically emplaced over the Oboke nappe (which attained peak metamorphic conditions at c. 75 Ma); (iv) the Besshi and Oboke nappe complexes were further exhumed as a coherent tectonic unit and unconformably overlain by the Eocene Kuma Group at c. 50 Ma. 相似文献
2.
《International Geology Review》2012,54(7):882-893
The Yangjingou gold deposit in Jilin Province lies 11 km south of the large-scale Xiaoxinancha gold–copper deposit. Yangjingou orebodies are structurally controlled fault- or fracture-related auriferous quartz veins. This type of mineralization is significantly different from that of the Xiaoxinancha porphyry gold–copper deposit, and has mineral assemblages and fluid inclusion compositions typical of orogenic gold deposits. We suggest that the Yangjingou deposit is the first orogenic gold deposit discovered in the Yanbian area, even in all of NE China. Here, we present new isotopic dating and trace element analysis of the ore-hosting monzogranite and auriferous quartz veins within the deposit, in order to determine the age and tectonic setting of metallogenesis, and the geological conditions controlling gold mineralization. LA-ICP-MS U–Pb dating of zircons separated from the monzogranite yielded an age of 262.3 ± 1.3 Ma, indicating intrusion during the late Permian. Hydrothermal muscovite from auriferous quartz veins yielded a 40Ar/39Ar plateau age of 241.57 ± 1.2 Ma, indicating that gold mineralization occurred at 241 Ma. Trace element and REE compositions of the monzogranite and auriferous quartz veins are both indicative of the formation from a region of the upper mantle that previously underwent crustal contamination. Geochronological analysis indicates that the diagenesis and mineralization resulting in the Yangjingou gold deposit occurred during the late Permian–Early Triassic. The tectonic evolution of the region and comparison of this deposit with other mineralizing events indicate that the orebody formed during orogenesis associated with collision between the North China and Siberian cratons. 相似文献
3.
Tatsuya Fujimoto Shigeru Otoh Yuji Orihashi Takafumi Hirata Takaomi D. Yokoyama Masanori Shimojo Yoshikazu Kouchi Hokuto Obara Yasuo Ishizaki Kazuhiro Tsukada Toshiyuki Kurihara Manchuk Nuramkhan Sersmaa Gonchigdorj 《Resource Geology》2012,62(4):408-422
A dropstone‐bearing, Middle Permian to Early Triassic peri‐glacial sedimentary unit was first discovered from the Khangai–Khentei Belt in Mongolia, Central Asian Orogenic Belt. The unit, Urmegtei Formation, is assumed to cover the early Carboniferous Khangai–Khentei accretionary complex, and is an upward‐fining sequence, consisting of conglomerates, sandstones, and varved sandstone and mudstone beds with granite dropstones in ascending order. The formation was cut by a felsic dike, and was deformed and metamorphosed together with the felsic dike. An undeformed porphyritic granite batholith finally cut all the deformed and metamorphosed rocks. LA‐ICP‐MS, U–Pb zircon dating has revealed the following 206Pb/238U weighted mean igneous ages: (i) a granite dropstone in the Urmegtei Formation is 273 ± 5 Ma (Kungurian of Early Permian); (ii) the deformed felsic dike is 247 ± 4 Ma (Olenekian of Early Triassic); and (iii) the undeformed granite batholith is 218 ± 9 Ma (Carnian of Late Triassic). From these data, the age of sedimentation of the Urmegtei Formation is constrained between the Kungurian and the Olenekian (273–247 Ma), and the age of deformation and metamorphism is constrained between the Olenekian and the Carnian (247–218 Ma). In Permian and Triassic times, the global climate was in a warming trend from the Serpukhovian (early Late Carboniferous) to the Kungurian long and severe cool mode (328–271 Ma) to the Roadian to Bajocian (Middle Jurassic) warm mode (271–168 Ma), with an interruption with the Capitanian Kamura cooling event (266–260 Ma). The dropstone‐bearing strata of the Urmegtei Formation, together with the glacier‐related deposits in the Verkhoyansk, Kolyma, and Omolon areas of northeastern Siberia (said to be of Middle to Late Permian age), must be products of the Capitanian cooling event. Although further study is needed, the dropstone‐bearing strata we found can be explained in two ways: (i) the Urmegtei Formation is an autochthonous formation indicating a short‐term expansion of land glacier to the central part of Siberia in Capitanian age; or (ii) the Urmegtei Formation was deposited in or around a limited ice‐covered continent in northeast Siberia in the Capitanian and was displaced to the present position by the Carnian. 相似文献
4.
T. Scheiber J. Berndt B. D. Heredia K. Mezger O. A. Pfiffner 《International Journal of Earth Sciences》2013,102(8):2097-2115
The Suretta nappe of eastern Switzerland contains a series of meta-igneous rocks, with the Rofna Porphyry Complex (RPC) being the most prominent member. We present LA-ICP-MS U–Pb zircon data from 12 samples representing a broad spectrum of meta-igneous rocks within the Suretta nappe, in order to unravel the pre-Alpine magmatic history of this basement unit. Fine-grained porphyries and coarse-grained augengneisses from the RPC give crystallization ages between 284 and 271 Ma, which either represent distinct magma pulses or long-lasting magmatic activity in a complex magma chamber. There is also evidence for an earlier Variscan magmatic event at ~320–310 Ma. Mylonites at the base of the Suretta nappe are probably derived from either the RPC augengneisses or another unknown Carboniferous–Permian magmatic protolith with a crystallization age between 320 and 290 Ma. Two polymetamorphic orthogneisses from the southern Suretta nappe yield crystallization ages of ~490 Ma. Inherited zircon cores are mainly of late Neoproterozoic age, with minor Neo- to Paleoproterozoic sources. We interpret the Suretta nappe as mainly representing a Gondwana-derived crustal unit, which was subsequently intruded by minor Cambrian–Ordovician and major Carboniferous–Permian magmatic rocks. Finally, the Suretta nappe was thrust into its present position during the Alpine orogeny, which hardly affected the U–Pb system in zircon. 相似文献
5.
The Uluguru Mountains of eastern Tanzania represent a key area to unravel the tectonic history of Gondwana assembly along the Neoproterozoic East African Orogen. On the basis of combined structural and 40Ar/39Ar geochronological investigations, we have re‐interpreted the structural architecture of the region. Texturally‐late, ductile to cataclastic low‐angle tectonic contacts are recognized, which delimit the contacts between the main lithological units that form a post‐metamorphic, NW‐verging nappe stack. 40Ar/39Ar geochronology places this nappe‐forming event at c. 550 Ma, post‐dating tonalitic dyke emplacement at c. 580 Ma. Results from this study (i) demonstrate that a distinct and later episode of convergence occurred in the region at c. 550 Ma, when previous structural elements of the East African Orogen were finally juxtaposed; and (ii) extends models involving the polyphase assembly of Gondwana to East Africa. 相似文献
6.
华北地台北缘乌兰哈雅地区发现二叠纪未-三叠纪初蒙古寺-盘羊山-乌兰哈雅推覆构造,将晚太古代色尔腾山岩群绿片岩系推覆到震旦系什那干组灰岩和古生代碎屑岩系之上.推覆面走向近东西向,延伸长度大于50 kn,推覆方向180～230o,推覆距离大于4.5 km.推覆界面被中三叠世(U-Pb同位素年龄231 Ma)二长花岗岩侵入,又被中侏罗统大青山组不整合覆盖.本推覆构造的发现改变了前人认为本区不存在古生代末大型推覆构造的看法,对了解华北地台(板块)构造发展历史与地壳演化有重要意义.说明华北地台(板块)晚古生代末存在大型陆内造山事件,其动力来源推测为华北板块与华南板块的碰撞所产生的巨大挤压应力. 相似文献
7.
黑龙江省东部古生代—早中生代的构造演化:火成岩组合与碎屑锆石U-Pb年代学证据 总被引：17，自引：0，他引：17
黑龙江省东部松嫩—张广才岭地块与佳木斯地块之间的演化历史以及古亚洲洋构造体系与环太平洋构造体系的叠加与转化一直是地学领域研究的热点问题之一。依据该区古生代—早中生代火成岩的年代学与岩石组合研究,结合碎屑锆石的年代学研究成果,讨论了松嫩—张广才岭地块与佳木斯地块之间的演化历史以及两大构造体系叠加与转化的时间。锆石U-Pb定年结果表明:黑龙江省东部古生代—早中生代岩浆作用可划分成8期:早奥陶世(485Ma)、晚奥陶世(450Ma)、中志留世(425Ma)、中泥盆世(386Ma)、早二叠世(291Ma)、中二叠世(268 Ma)、晚三叠世(201～228 Ma)以及早侏罗世(184 Ma)。早奥陶世—中志留世,岩浆作用主要分布在松嫩—张广才岭地块的东缘,并呈南北向带状展布,主要由闪长岩-英云闪长岩-二长花岗岩组成,显示活动陆缘—碰撞的构造演化历史,揭示松嫩—张广才岭地块与佳木斯地块于中志留世(425Ma)已经拼合在一起,这也得到了早泥盆世地层碎屑锆石年代学的支持。中泥盆世,火山作用分布在佳木斯地块东缘和松嫩—张广才岭地块上,前者为双峰式火山岩组合,后者为A型流纹岩,它们共同揭示该区处于一种碰撞后的伸展环境。早二叠世,佳木斯地块东缘发育一套钙碱性火山岩组合,揭示古亚洲洋俯冲作用的存在,而同期的张广才岭地区则发育一套典型的双峰式火成岩组合,揭示了陆内伸展环境的存在。中二叠世,同碰撞型火山岩分布于佳木斯地块东缘及东南缘,其形成可能与佳木斯地块和兴凯地块的碰撞拼合有关。晚三叠世,张广才岭地区存在的双峰式火山岩和敦—密断裂东南区发育的A型流纹岩均显示陆内的伸展环境,其形成应与古亚洲洋最终闭合后的伸展环境相联系。此外,结合牡丹江断裂两侧均发育中—晚二叠世花岗岩以及佳木斯地块上晚三叠世—早侏罗世岩浆作用的缺失,暗示松嫩—张广才岭地块与佳木斯地块在三叠纪早期沿牡丹江断裂可能存在一次裂解事件。而早—中侏罗世陆缘(东宁—汪清—珲春)钙碱性火山岩和陆内(小兴安岭—张广才岭)双峰式火成岩组合的出现,结合牡丹江断裂两侧"张广才岭群"和"黑龙江群"构造混杂岩的就位,暗示松嫩—张广才岭地块与佳木斯地块在早—中侏罗世再次拼合,这也标志着环太平洋构造体系的开始。 相似文献
8.
ABSTRACTThe results of SHRIMP U-Pb ages and in situ Hf isotope of zircons from three granites in the southern Manzhouli region of northeast China, provide new data to understand the subduction process of Mongol-Okhotsk Plate beneath the Erguna massif. SHRIMP U-Pb zircon geochronology results yield an age of 265.5 Ma (middle Permian) for fine-grained monzogranite. Rocks from the Early–Middle Triassic are mainly granodiorite (247.4 ± 4.6 and 249.3 ± 4.9 Ma), the granites are with SiO2 = 60.0–77.4 wt.%, Al2O3 = 12.3–16.8wt.% and Na2O/K2O = 0.7–1.9. Chemically, they are metaluminous to peraluminous and belong to the high-K calc-alkaline series. Enrichments in the large ion lithophile elements (e.g., Rb, Ba, and K) and depletions in the high field strength elements (e.g., Nb, Ta, and Ti) are typical for these rock types. The monzogranite (~265 Ma) and granodiorite (~247 Ma) contain zircons with εHf(t) values of 6.3–8.5 and 5.1–7.9, yielding TDM2 model ages of 888–752 and 958–774 Ma, respectively. These geochemical and zircon Hf isotopic data indicate that primary magmas for Middle Permian–Early Triassic granites crystallized from primary magmas generated by Neoproterozoic crustal materials, formed in an active continental margin setting. The andesite of the Gegenaobao formation is similar with the Izu–Bonin–Mariana arc, relating to subduction initiation. Based on the characteristics of exposed rocks and zircon U-Pb ages of andesite and granitoid rocks in the study area, we conclude the onset subduction of Mongol-Okhotsk Plate beneath the Erguna massif may occur at early-middle Permian. 相似文献
9.
U–Pb zircon analyses from three meta-igneous and two metasedimentary rocks from the Siviez-Mischabel nappe in the western Swiss Alps are presented, and are used to derive an evolutionary history spanning from Paleoarchean crustal growth to Permian magmatism. The oldest components are preserved in zircons from metasedimentary albitic schists. The oldest zircon core in these schists is 3.4 Ga old. Detrital zircons reveal episodes of crustal growth in the Neoarchean (2.7–2.5 Ga), Paleoproterozoic (2.2–1.9 Ma) and Neoproterozoic (800–550 Ma, Pan-African event). The maximum age of deposition for the metasedimentary rocks is given by the youngest detrital zircons within both metasedimentary samples dated at ~490 Ma (Cambrian-Ordovician boundary). This is in the age range of two granitoid samples dated at 505 ± 4 and 482 ± 7 Ma, and indicates sedimentation and magmatism in an extensional setting preceding an Ordovician orogeny. The third felsic meta-igneous rock gives a Permian age of intrusion, and is part of a long-lasting Variscan to post-Variscan magmatic activity. The zircons record only minor disturbance of the U–Pb system during the Alpine orogeny. 相似文献
10.
新疆中天山古生代侵入岩浆序列及构造演化 总被引：1，自引：0，他引：1
新疆中天山构造岩浆带是中亚造山带的重要组成部分,广泛分布着古生代花岗质侵入体。本研究重点对中天山南缘巴音布鲁克及巴伦台地区的花岗质侵入体进行了LA-ICP-MS锆石U-Pb测年,并获得了岩体侵位年龄由老到新分别为463±3Ma(石英闪长岩)、437±5Ma(石英闪长岩)、424±3Ma(二长花岗岩)、383±4Ma(二长花岗岩)、356±3Ma(二长花岗岩)和303±5Ma(正长花岗岩)。综合区域地质分析认为,中天山古生代侵入岩浆活动可分为四个构造岩浆演化阶段:(1)晚寒武世—晚奥陶世阶段,Terskey洋盆和南天山洋盆自新元古代打开形成广阔洋盆,Terskey洋盆在晚寒武世开始初次俯冲,于晚奥陶世洋盆闭合,南天山洋盆于早奥陶世初次俯冲,具有自西向东、由早到晚的俯冲特点;(2)早志留世—中泥盆世阶段,南天山洋盆持续向北俯冲,该阶段北天山洋开始向南侧俯冲,在伊犁地块北缘形成了弧岩浆;(3)晚泥盆世—早石炭世阶段,南天山洋盆闭合于晚泥盆世末期,在早石炭世中晚期进入残余洋盆演化阶段;(4)晚石炭世—早二叠世阶段,该阶段为后碰撞伸展环境,区域上为陆内演化阶段。 相似文献
11.
西藏双湖纳若地区蓝片岩中青铝闪石40Ar/39Ar定年为222.5Ma,其原岩时代争论颇大,以往多数研究者认为早于泥盆纪,甚至为中新元古代,称阿木岗群。在1∶10万填图和实测剖面的基础上,系统采集了相关样品,获得了蓝片岩原岩Sm-Nd法等时线年龄分别为272±7Ma,268±5.6Ma和252±4.6Ma,结合区域地质条件和双湖地区蓝片岩的产状,认为双湖地区的蓝片岩原岩时代为二叠纪,将双湖地区原定为前泥盆纪或中新元古代阿木岗群时代重新划定为二叠纪,恰格勒拉蛇绿杂岩时代确定为早二叠世晚期,为龙木错—双湖古特提斯洋的演化提供了重要年代依据。 相似文献
12.
Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7?Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3?Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6?±?1.1?Ma, 310.5±1.2?Ma, and 303.3±0.2?Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300?Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360?Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences. 相似文献
13.
This study describes a previously unidentified Neoproterozoic mafic dyke emplaced in the northern flank of the Langshan Tectonic Belt. This dyke intruded into the micaquartz schist of the Zhaertaishan Group, and yielded an age of 908 ± 8 Ma. The youngest U-Pb ages of micaquartz schist from the Zhaertaishan Group in the Langshan area were 1118 ± 33 Ma,1187 ± 3 Ma and 1189 ± 39 Ma,suggesting that the depositional age of the protolith of the schist was between 908 ± 8 Ma and 1118 ± 33 Ma. In addition, 436 U-Pb age data and 155 Lu-Hf isotopic data from six samples in the Langshan Tectonic Belt and one Permian greywacke from the Wuhai area show distinct differences between the northern and southern flanks of the Main Langshan area. The U-Pb ages of the northern flank are primarily Meso-Neoproterozoic; similar ages have not been identified in the southern flank to date.Moreover, two-stage Hf model ages of the northern flank feature three age peaks at ～900 Ma,～1700 Ma and ～2600 Ma; this differs from Hf model ages of the southern flank, which feature one strong age peak at ～2700 Ma. These results suggest that the northern and southern flanks of the Main Langshan area have different geochronologic characteristics and should be divided further. Based on the U-Pb ages and Hf model ages, the northern and southern flanks of the Main Langshan area are named the North and South Langshan Tectonic Belts. Comparison of the U-Pb age and two-stage Hf model age distributions from the North Langshan Tectonic Belt, South Langshan Tectonic Belt, Alxa Block and the North China Craton(NCC) reveal that the North Langshan Tectonic Belt is similar to the Alxa Block and that the South Langshan Tectonic Belt is similar to the NCC. In addition, the zircon U-Pb age of 860 ±7 Ma commonly observed in the Alxa Block was detected in the Permian greywacke from the Wuhai area of the NCC, which suggests that the amalgamation of the North and South Langshan Tectonic belts(i.e.,the amalgamation of the Alxa Block and the NCC), occurred between Devonian and late Permian. 相似文献
14.
Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion. 相似文献
15.
16.
In the western fold-and-thrust belt of the southern Urals, the Kübler and Árkai indices determined on shales, slates and phyllites record an increase from lower late diagenetic to epizonal grade from west to east. The metamorphic grade varies strongly within the different tectonic segments, which are separated by major thrusts. The increase of diagenetic and incipient metamorphic grade from the footwall to the hanging wall of all major Upper Palaeozoic thrusts indicates a pre-Permo/Triassic origin. West of the Avzyan thrust zone, the diagenetic to incipient metamorphic grade is related to the Palaeozoic basin development and reached the final grades in Late Carboniferous to Early Permian times. East of the first Avzyan thrust in the Yamantau anticlinorium, the diagenetic to lower greenschist metamorphic grade is possibly of Neoproterozoic origin and might be related to the development of the Neoproterozoic basin at the eastern margin of the East European Craton. The eastern part of the Yamantau anticlinorium was exhumed below 200 °C in the Late Carboniferous or Early Permian. The diagenetic grade of the autochthonous Palaeozoic sedimentary units increases toward the stack of Palaeozoic nappes and might partly be caused by the deformational process due to the emplacement of the Palaeozoic nappes. Within the Timirovo thrust sheet, the decrease of metamorphic grade with stratigraphic age developed prior to the emplacement of the nappes. The upper anchizonal metamorphic grade of the Upper Devonian slates of the Zilair nappe results from the deformation process related to the Lower Carboniferous nappe emplacement. 相似文献
17.
A combined geochemical (whole-rock elements and Sr-Nd-Pb isotopes, zircon trace elements and Hf isotopes) and geochronological (zircon U–Pb ages) study was carried out on the relatively low-grade meta-basites and meta-granitoids from Longjingguan within the central Dabie ultrahigh-pressure (UHP) metamorphic zone, east-central China. Zircon investigations indicate that the meta-basites were formed at ∼772 Ma and subsequently experienced granulite-facies metamorphism at ∼768 Ma and a later thermal overprint at ∼746 Ma, while the meta-granitoids recorded three groups of zircon ages at ca. 819 Ma, 784 Ma and 746 Ma. The meta-granitoids can be subdivided into low-Si and high-Si types, and they were derived from mid-Neoproterozoic partial melting of the Neoarchean and Paleoproterozoic metamorphic basement rocks of the South China Block, respectively. These Neoproterozoic zircon ages are consistent with the protolith ages of the Dabie Triassic UHP meta-igneous rocks. In addition, the low-grade rocks have bulk-rock Pb isotope compositions overlapping with the UHP meta-igneous rocks. Therefore, the low-grade meta-basites and meta-granitoids could be interpreted as counterparts of the UHP meta-igneous rocks in this area, suggesting the same petrogenesis for their protoliths in the Neoproterozoic.Trace element patterns indicate that the low-grade rocks have better preserved their protolith compositions than their equivalent UHP rocks, and thus they are more suitable for elucidating the Neoproterozoic evolution of the northern margin of the South China Block. Zircon ages combined with geochemical features strongly suggest that the protoliths of the meta-granitoids and meta-basites were formed in a magmatic arc and a continental rifting setting, respectively. More specifically, the granitoids derived from partial melting of Neoarchean and Paleoproterozoic basement materials at ∼819 Ma in a magmatic arc setting, whereas the precursors of the meta-basites are products of a continental rifting event at about 784 to 772 Ma. The obtained results provide new geochronological and geochemical constraints for the Neoproterozoic evolution of the northern margin of the South China Block, which can further contribute to the understanding of the breakup of the supercontinent Rodinia. 相似文献
18.
《Journal of Structural Geology》2001,23(6-7):1031-1042
The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende 40Ar/39Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The 40Ar/39Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The 40Ar/39Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones. 相似文献
19.
The NE-trending Bayanwula Shan–Lang Shan is an important tectonic belt lying between the North China Plate (NCP) to the east and the Alxa block to the west. An understanding of its nature and the timing of deformation are essential to understand the relationship between the NCP and the Alxa block. Two phases of ductile deformation have been observed in this belt. Large-scale top-to-the-west ductile thrusting characterized the early deformation in the Bayanwula Shan–Lang Shan. Nearly east–west trending quartz stretching lineations and lineations formed by amphibole and biotite are well developed. Different types of sheath and oblique folds with east–west trending fold hinges are also developed in the region. The shear strain of this ductile thrust is up to 17. The ductile deformation may have resulted from the top-to-the-west thrusting of the northern part of the NCP over the Alxa block, and may have occurred ca. 351 Ma (biotite 40Ar/39Ar age). Later ductile deformation was expressed as NE-trending sinistral shear along the entire Bayanwula Shan–Lang Shan and likely occurred ca. 250 Ma (biotite and muscovite 40Ar/39Ar ages); this shear may have resulted from the collision between the Yangtze and North China plates to the south during the Triassic. Combined with recently obtained detrital zircon U–Pb ages for the area, the ductile deformation events in the eastern Alxa block indicate that the block may not have been part of the NCP, at least before the end of the Devonian. Both blocks were located in the Paleo-Asian Ocean during the Paleozoic and collided or amalgamated with each other at the end of the Devonian. 相似文献
20.
黑龙江嘉荫连珠山金矿床成岩成矿年代学及其地质意义 总被引：2，自引：0，他引：2
连珠山矿床位于小兴安岭北麓,是一个产于花岗岩内部断裂体系的蚀变岩型金矿床。为了限定其成矿时代与成矿地质背景,本文对其赋矿围岩黑云母二长花岗岩和石英闪长岩进行了岩石地球化学、锆石U-Pb和绢云母40Ar/39Ar年代学的系统研究。定年结果表明黑云母二长花岗岩形成时代为中三叠世(243.7±1.3Ma,MSWD=0.77,n=12),岩浆上侵过程中受到早古生代(474~438Ma)和中二叠世(267~261Ma)的岩浆混染,而石英闪长岩形成于晚三叠世(215.3±1.3Ma,MSWD=0.35,n=17);黄铁绢英岩化矿石中的绢云母40Ar/39Ar坪年龄为194.2±2.0Ma,指示为早侏罗世成矿;元素地球化学特征显示连珠山侵入岩为准铝质-弱过铝质、高钾钙碱性岩石系列,具有I型花岗岩的地球化学属性;轻稀土元素富集,相对亏损重稀土元素,且具有弱的负Eu异常;富集大离子亲石元素(LILE),亏损高场强元素(HFSE)。结合小兴安岭-张广才岭地区已有年代学资料和区域构造演化特征,认为其成矿地质背景为兴蒙造山晚期与古太平洋板块俯冲转换期,或成矿发生在兴蒙造山期后的伸展阶段。 相似文献