Structural constraints on the evolution of the Meatiq Gneiss Dome (Egypt), East-African Orogen

Amphibolite-grade quartzofeldspathic gneiss domes surrounded by greenschist-grade island arc and ophiolitic assemblages is a characteristic feature of the Arabian–Nubian Shield in the Eastern Desert of Egypt. The mode of formation of these domes, including the Meatiq Gneiss Dome, is controversial, as is the protolith age of these gneisses. Reinvestigation of selected segments of the Eastern Desert Shear Zone (EDSZ), a high-strain zone separating the eugeoclinal units from the underlying quartzofeldspathic gneisses show it to be a top-to-the NW shear zone which was later folded about a NW–SE trending fold axis (long axis of the gneiss dome). Kinematic indicators (shear bands, duplex structures, etc.) along the north-eastern and south-western flanks of the dome therefore show apparent left-lateral and right-lateral strike-slip displacement across the EDSZ. These observations are in conflict with most previous tectonic models which link formation of the dome to extension in a NW–SE oriented corridor bordered by two sub-parallel left-lateral NW–SE oriented strike-slip faults. Emplacement of upper crustal, low-grade, eugeoclinal rocks tectonically on top of middle crustal amphibolite-grade quartzofeldspathic gneisses indicates that the EDSZ may represents an extensional fault with a possible break-away zone in the southern part of the Eastern Desert. Alternatively it can be explained as the result of two (or more) tectonometamorphic events with an intervening episode of erosion and exhumation of high grade rocks prior to emplacement of the eugeoclinal thrust complex. Recent U–Pb TIMS ages on syntectonic orthogneisses and post-tectonic granites in the area show that shearing and subsequent doming must be younger than 630 Ma, possibly as young as 600 Ma.     

The najd fault system revisited; a two-way strike-slip orogen in the saudi arabian shield

The Najd strike-slip fault system extends over the northeastern Arabian Shield in a zone >1200 km in length and >300 km wide. Faults trend NW-SE with strike lengths >500 km but small sinistral displacements of <25 km. Cumulative displacement across the zone is >240 km. Najd faults were active in the late Proterozoic and post-date cratonization of the Shield. Associated secondary structures include grabens, thrust faults, folds and dike swarms. In the southwest of the Najd system, near Zalm, initial faulting was dextral and began earlier than formerly thought. Emplacement of a plutonic complex was controlled by Najd fractures of dextral geometry and displacements. The same fractures were active before and after deposition of a group of volcanosedimentary rocks in grabens orientated consistently with development in a dextral strike-slip regime. Graben deformation was controlled by sinistral motion along the same fractures responsible for graben development and also by younger fractures of sinistral geometry and displacement. Dike swarms in the area are also consistent with early dextral and later sinistral shear of Najd trend. Structures in the Zalm area occur throughout the Najd system and the consistent chronology of older dextral structures dislocated and deformed by younger sinistral faults suggests a reversal in the sense of motion of the Najd system as a whole.     

Distribution of oceanic and continental leads in the Arabian-Nubian Shield

New common lead data for feldspar, whole-rock, and galena samples from the Arabian-Nubian Shield, together with data from previous work, can be divided into two main groups. Group I leads have oceanic (mantle) characteristics, whereas group II leads have incorporated a continental-crustal component of at least early Proterozoic age. The group I leads are found in rocks from the Red Sea Hills of Egypt and the western and southern parts of the Arabian Shield. Group II leads are found in rocks from the northeastern and eastern parts of the Arabian Shield, as well as from the southeastern Shield near Najran. They are also found in rocks to the south in Yemen, to the east in Oman, and to the west at Aswan, Egypt. This distribution of data suggests that the Arabian-Nubian Shield has an oceanic core flanked by rocks that have developed, at least in part, from older continental material. Two mechanisms are suggested by which this older lead component could have been incorporated into the late Proterozoic rocks, and each may have operated in different parts of the Shield. The older lead component either was derived directly from an underlying early Proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent.New U-Pb zircon data indicate the presence of an early Proterozoic basement southeast of Jabal Dahul in the eastern Arabian Shield. These data, together with 2,000-Ma-old zircons from the Al Amar fault zone, verify the implication of the common lead data that at least a part of the eastern Arabian Shield has an older continental basement.Because continental margins are particularly favorable locations for development of ore deposits, these findings may have important economic implications, particularly for tin, tungsten, and molybdenum exploration.     

40Ar/39Ar geochronology constraints on the Middle Tertiary basement extensional exhumation,and its relation to ore-forming and magmatic processes in the Eastern Rhodope (Bulgaria)

The interaction of distinct geologic processes involved during late orogenic extensional exhumation history of the metamorphic units in the Eastern Rhodope is refined by new and reviewing ⁴⁰Ar/³⁹Ar geochronological and structural data. Minerals with different closure temperatures from metamorphic rocks investigated in this study are combined with those from magmatic and ore-forming hydrothermal rocks in two late stage metamorphic domes – the Kesebir-Kardamos and the Biala reka-Kehros domes. The 38-37 Ma muscovite and biotite cooling ages below 350°-300°C characterize basement metamorphic rocks that typified core of the Kesebir-Kardamos dome, constraining their exhumation at shallow crustal levels in the footwall of detachment. These ages are interpreted as reflecting last stage of ductile activity on shear zone below detachment, which continued to operate under low-temperature conditions within the semi-ductile to brittle field. They are close to and overlap with existing cooling ages in southern Bulgaria and northern Greece, indicating supportively that the basement rocks regionally cooled between 42-36 Ma below temperatures 350°-300°C. The spatial distribution of ages shows a southward gradual increase up structural section, suggesting an asymmetrical mode of extension, cooling and exhumation from south to the north at latitude of the Kesebir-Kardamos dome. The slightly younger 36.5-35 Ma crystallization ages of adularia in altered rocks from the ore deposits in the immediate hanging-wall of detachments are attributed to brittle deformation on high-angle normal faults, which further contributed to upper crustal extension, and thus constraining the time when alteration took place and deformation continued at brittle crustal levels. Silicic dykes yielded ages between 32-33 Ma, typically coinciding with the main phase of Palaeogene magmatic activity, which started in Eastern Rhodope region in Late Eocene (Priabonian) times. The ⁴⁰Ar/³⁹Ar plateau ages from the above distinct rock types span time interval lasting approximately ca. 6 Ma. Consequently, our geochronologic results consistently indicate that extensional tectonics and related exhumation and doming, epithermal mineralizations and volcanic activity are closely spaced in time. These new ⁴⁰Ar/³⁹Ar age results further contribute to temporal constraints on the timing of tectonic, relative to ore-forming and magmatic events, suggesting in addition that all above mentioned processes interfered during the late orogenic extensional collapse in the Eastern Rhodope region.     

A new understanding of Demala Group complex in Chayu Area,southeastern Qinghai-Tibet Plateau: Evidence from zircon U-Pb and mica 40Ar/39Ar dating

The Chayu area is located at the southeastern margin of the Qinghai-Tibet Plateau. This region was considered to be in the southeastward extension of the Lhasa Block, bounded by Nujiang suture zone in the north and Yarlung Zangbo suture zone in the south. The Demala Group complex, a set of high-grade metamorphic gneisses widely distributed in the Chayu area, is known as the Precambrian metamorphic basement of the Lhasa Block in the area. According to field-based investigations and microstructure analysis, the Demala Group complex is considered to mainly consist of banded biotite plagiogneisses, biotite quartzofeldspathic gneiss, granitic gneiss, amphibolite, mica schist, and quartz schist, with many leucogranite veins. The zircon U-Pb ages of two granitic gneiss samples are 205 ± 1 Ma and 218 ± 1 Ma, respectively, representing the ages of their protoliths. The zircons from two biotite plagiogneisses samples show core-rim structures. The U-Pb ages of the cores are mainly 644 –446 Ma, 1213 –865 Ma, and 1780 –1400 Ma, reflecting the age characteristics of clastic zircons during sedimentation of the original rocks. The U-Pb ages of the rims are from 203 ± 2 Ma to 190 ± 1 Ma, which represent the age of metamorphism. The zircon U-Pb ages of one sample taken from the leucogranite veins that cut through granitic gneiss foliation range from 24 Ma to 22 Ma, interpreted as the age of the anatexis in the Demala Group complex. Biotite and muscovite separates were selected from the granitic gneiss, banded gneiss, and leucogranite veins for ⁴⁰Ar/³⁹Ar dating. The plateau ages of three muscovite samples are 16.56 ± 0.21 Ma, 16.90 ± 0.21 Ma, and 23.40 ± 0.31 Ma, and the plateau ages of four biotite samples are 16.70 ± 0.24 Ma, 16.14 ± 0.19 Ma, 15.88 ± 0.20 Ma, and 14.39 ± 0.20 Ma. The mica Ar-Ar ages can reveal the exhumation and cooling history of the Demala Group complex. Combined with the previous research results of the Demala Group complex, the authors refer that the Demala Group complex should be a set of metamorphic complex. The complex includes not only Precambrian basement metamorphic rock series, but also Paleozoic sedimentary rock and Mesozoic granitic rock. Based on the deformation characteristics, the authors concluded that two stages of the metamorphism and deformation can be revealed in the Demala Group complex since the Mesozoic, namely Late Triassic-Early Jurassic (203 –190 Ma) and Oligocene –Miocene (24 –14 Ma). The early stage of metamorphism (ranging from 203 –190 Ma) was related to the Late Triassic tectono-magmatism in the area. The anatexis and uplifting-exhumation of the later stage (24 –14 Ma) were related to the shearing of the Jiali strike-slip fault zone. The Miocene structures are response to the large-scale southeastward escape of crustal materials and block rotation in Southeast Tibet after India-Eurasia collision.©2021 China Geology Editorial Office.     

东构造结那木拉断裂带上新世以来强烈活动的年代学证据

为揭示东喜马拉雅构造结那木拉断裂带上新世以来强烈活动特征,对采集自那木拉断裂带的三件基岩样品进行黑云母40Ar/39Ar、 磷灰石裂变径迹两种热年代学方法测年;并利用"Pecube"软件对测得年龄数据及断裂带两侧已发表年龄数据进行定量模拟计算.测试结果显示黑云母40 Ar/39 Ar年龄范围为4.44±0.71 Ma～...     

新疆阿克塔斯金矿床黑云母花岗岩锆石U-Pb和绢云母39Ar/40Ar测年及地质意义

阿克塔斯金矿床位于新疆阿尔泰造山带南缘,矿体赋存于黑云母花岗岩与中泥盆统北塔山组中性火山岩接触带,矿石类型主要为石英脉型和蚀变岩型。矿区中酸性侵入岩体的岩石类型为二长花岗岩、黑云母花岗岩。近矿的黑云母花岗岩LA-ICP-MS锆石U-Pb年龄为(329.5±1.8)Ma,成岩时代为早石炭世;主成矿期的金-黄铁矿-石英细脉中的绢云母~(40)Ar/~(39)Ar坪年龄为(247.7±1.3) Ma、等时线年龄为(245.1±1.4)Ma,成矿时代为早三叠世。研究表明,阿克塔斯金矿成岩与成矿时代相距80 Ma,为碰撞造山和后碰撞造山不同地质作用的产物。该矿床具有造山型金矿的特点,额尔齐斯断裂带的右形剪切走滑作用控制金矿成矿过程。     

New <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age constraints on cooling and unroofing history of the metamorphic host rocks (and igneous intrusion associates) from the Bulfat Complex (Bulfat area), NE-Iraq

The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~?40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~?80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. ⁴⁰Ar/³⁹Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78?±?0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~?400 °C/Ma and ~?3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78?±?0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).     

GEOCHRONOLOGY OF ~(Ar)/~(39)Ar DATING IN THE BASEMENT ROCKS IN EASTERN KUNLUN MOUNTAINS AND ITS TECTONIC IMPLICATIONS

GEOCHRONOLOGY OF ~(Ar)/~(39)Ar DATING IN THE BASEMENT ROCKS IN EASTERN KUNLUN MOUNTAINS AND ITS TECTONIC IMPLICATIONS     

武夷山北缘断裂带动力学研究

华南武夷山北缘边界被绍兴－兴山－东乡断裂带所限。该断裂带到少保留了三期构造事件的形迹，第一期发生在８００Ｍａ～９００Ｍａ的晚元古代，呈ＮＷ向ＳＥ的区域推覆韧剪变形运动，以构造混杂岩和区域绿片岩相－角闪岩相变质，强烈的褶皱和韧剪变形为特征，对应于古洋盆关闭，华南复合地体与江南岛弧撞焊接过程，第二期发生在４５８Ｍａ～４２１Ｍａ的志留纪，表现为从北向南的韧剪变形运动，伴有左旋走滑韧性剪切，以糜棱岩化和进变质作用为特征．黒云母多变为硅线石。该期变形使第一期构造形迹被强烈选加置换。其动力学背景与闽东南地体朝武夷山的拼贴增生事件有关。第三期属中生代陆内变形，是一种高构造位的左旋走滑脆性剪切，以岩石的破裂和岩块的水平位移为特征．并具转换拉伸性质，导致中生代火山沉积盆地的形成。     

敦密断裂带白垩纪两期重要的变形事件

本文报道了敦密断裂带糜棱岩中黑云母~(40)Ar/~(39)Ar定年结果和大规模走滑-逆冲断裂的几何学、运动学特征及其形成时代,以便揭示断裂带两期变形事件的构造属性。黑龙江省密山市花岗质糜棱岩中黑云母~(40)Ar/~(39)Ar加权平均年龄为132.2±1.2Ma,它是敦密断裂带经历伸展事件的冷却年龄,也是东北亚大陆边缘在早白垩世Hauterivian期-Albian期发生强烈区域伸展作用的产物。密山市至辽宁省清原县系列大型走滑-逆冲断层和断层相关褶皱揭示出在晚白垩世晚期-末期发生右旋走滑-逆冲事件,该事件规模大,影响范围广,导致整个断裂带遭受到强烈改造,形成对冲式断裂系统。将研究区走滑-逆冲断裂与山东省郯庐断裂带中段挤压构造对比,认为郯庐断裂带北段和中段在晚白垩世末期都发生了强烈的走滑-逆冲事件,它们具有相同的构造特征和构造属性。     

A middle–late Ediacaran volcano‐sedimentary record from the eastern Arabian‐Nubian shield

The Ediacaran Jibalah Group comprises volcano‐sedimentary successions that filled small fault‐bound basins along the NW–SE‐trending Najd fault system in the eastern Arabian‐Nubian Shield. Like several other Jibalah basins, the Antaq basin contains exquisitely preserved sedimentary structures and felsic tuffs, and hence is an excellent candidate for calibrating late Ediacaran Earth history. Shallow‐marine strata from the upper Jibalah Group (Muraykhah Formation) contain a diversity of load structures and intimately related textured organic (microbial) surfaces, along with a fragment of a structure closely resembling an Ediacaran frond fossil and a possible specimen of Aspidella. Interspersed carbonate beds through the Muraykhah Formation record a positive δ¹³C shift from ?6 to 0‰. U‐Pb zircon geochronology indicates a maximum depositional age of ~570 Ma for the upper Jibalah Group, consistent with previous age estimates. Although this age overlaps with that of the upper Huqf Supergroup in nearby Oman, these sequences were deposited in contrasting tectonic settings on opposite sides of the final suture of the East African Orogen.     

中国阿尔泰造山带南缘额尔齐斯断裂带的构造变形及意义

额尔齐斯断裂是中亚造山带中的一条重要深大断裂,对于额尔齐斯断裂运动性质一直有着走滑断层、逆冲断层和压扭性断层等不同看法.本文在中国阿尔泰造山带南缘开展了详细的构造地质学工作,研究结果表明,额尔齐斯断裂及其次级断裂组成额尔齐斯断裂带.额尔齐斯断裂带在中国境内是一条宽约20 ～ 40km,长约400km,经受不同程度构造作用的强应变带,剪切作用影响范围遍布整个中国阿尔泰造山带南缘.额尔齐斯断裂带经历了左行走滑和右行走滑两个阶段.结合前人有关韧性剪切带成因型金矿、同构造岩体侵位与变形关系及对变质岩石40Ar/39Ar年代学研究,本文认为额尔齐斯断裂带的左行走滑构造形成于早二叠世(283～275Ma).早二叠世之后,额尔齐斯断裂带叠加了右行走滑事件,其活动时限可能为晚二叠世(260 ～ 245 Ma),其规模远远小于前期的左行走滑构造.额尔齐斯断裂带走滑活动性质的确定,为二叠纪北疆及整个中亚造山带造山后调整过程中不同的构造方式提供了佐证.     

大别山东端郯庐断裂带由走滑向伸展运动转换的40Ar-39Ar年代学记录

通过对大别山东端郯庐左旋韧性剪切带中一系列含白云母、黑云母、斜长石和钾长石的糜棱岩样品⁴⁰Ar-³⁹Ar同位素年代学研究,发现大别山东端的郯庐断裂带在距今139Ma之前发生过一次左行平移运动,随后转变为伸展活动,由走滑向伸展活动转换的时间介于距今139Ma至128Ma之间。在伸展活动中,大别山东端的郯庐走滑韧性剪切带发生了缓慢的抬升和冷却,从而导致封闭温度较低的矿物记录了较小的年龄。黑云母110Ma±以及斜长石97~92Ma的⁴⁰Ar-³⁹Ar年龄值指示郯庐断裂带的伸展活动一直持续到距今90Ma±。     

Sub-surface geometry of Ar Rika and Ruwah faults from gravity and magnetic surveys

Ten global positioning system (GPS)–gravity profiles were conducted to provide sub-surface geometry of two sections of the Najd Fault System (NFS) Ruwah and Ar Rika faults, six in the Afif and four in the Al Muwayh area about 500 and 650 km west of Riyadh, respectively. GPS surveys were collected in differential GPS (DGPS) mode, allowing a large area to be covered in limited time. DGPS is utilized for the advantages of accuracy, economy, and speed. Output DGPS location coordinates were used in free-air and Bouguer reductions; terrain corrections were applied using a 3-arcsecond digital elevation model; finally, isostatic and decompensative corrections were applied. Integration of the resulting decompensative isostatic residual anomalies and aeromagnetic map has mapped the NFS very accurately. Modeling the gravity field crossing the Ruwah fault zone revealed that it is associated with low gravity anomalies probably due to a complex of lower density crushed rocks and modeled the geometry of the subsurface structure of Ar Rika fault as an inclined fault with reverse movement that would imply a compression component (post-dated the shearing) parallel to the plane of the cross-section.     

内蒙古呼和浩特变质核杂岩韧性拆离带40Ar-39Ar定年及其构造含义

晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。     

Ar-Ar Geochronology in a gneiss dome within the Zagros Orogénic Belt

Exhumation of the Tutak mantled gneiss dome without significant cooling has taken place in a doubly plunging anticline within the Sanandaj-Sirjan HP-LT metamorphic belt in the Zagros Thrust System of Iran. Reconstruction of structural evolution of the Tutak gneiss dome at the contact between Arabian and Iranian plates by ⁴⁰Ar/³⁹Ar geochronology exhibits history of the closure of Neo-Tethyan Ocean. There are two granites of different ages in the core of dome; the oldest corresponds to the central Iranian continental crust and was deformed at about 180 Ma. The younger granite was emplaced in the NE–SW transpression system. The timing of strain-related fabrics and exhumation history of the region illustrates the closure of Neo-Tethys and beginning of continent-continent collision at about 77 Ma, as constrained by a well defined plateau ⁴⁰Ar/³⁹Ar age obtained on biotite. Then, the biotite age corresponds to the second stage of emplacement of the Bendenow granite-gneiss which illustrating repeated orogenic events. Continuing deformation without interruption that by now has been created at about 77 Ma, was largely restricted to the transpression and high proportion of simple shear components relative to the pure shear components along the NE–SW.     

黑龙江黄松群碎屑锆石LA-ICP-MSU-Pb定年

对黑龙江省东部敦—密断裂以南的太平岭边缘隆起带上的黄松群杨木组石榴黑云片岩和阎王殿组黑云石英片岩进行了锆石LA-ICP-MS U-Pb测年,结果表明:杨木组石榴黑云片岩中锆石~(206)Pb/~(238)U加权平均峰值年龄分别为281.8 Ma、317.2 Ma、424.7 Ma、529.3 Ma;阎王殿组黑云石英片岩中锆石~(206)Pb/~(238)U加权平均峰值年龄分别为310.4 Ma、384.4 Ma、484.7 Ma;两个组所获得的最年轻的锆石~(206)Pb/~(238)U年龄分别为280±4 Ma和285±8 Ma。此外,前人对黄松群研究结果表明其变质作用时代为晚二叠世—早三叠世。因此,黄松群时代为二叠纪,海西期和加里东期岩浆岩构成了黄松群沉积物源的主体。     

The Bani Ghayy group; sedimentation and volcanism in pull-apart grabens of the Najd strike-slip orogen,Saudi Arabian shield

The cratonisation of the Arabian Shield involved the accretion of ensimatic arc terranes in the west and south followed by continental collisions along the Nabitah and Al Amar sutures and the onset of the Najd strike-slip orogen. Volcanosedimentary successions which post-date these collisions have been proposed as the products of magmatism in transitional volcanic arcs. One such succession, the Bani Ghayy group, lies unconformably above a series of volcanosedimentary rocks associated with an older active continental margin and was deposited in three separate, elongate, fault-bounded grabens. Each graben contains coarse fanglomerate and fan delta deposits along the margin and proximal greywackes toward the centre. Reef limestones occur locally whereas volcanogenic rocks are widespread. The latter are geochemically bimodal and with the sediments are typical of continental rifting environments although deformed ultramafic rocks within internal reverse faults suggests that the grabens were floored, in part, by primitive mantle derived crust. Deposition of the Bani Ghayy group post-dates incipient motions along fractures of the Najd fault system. The orientation of the Bani Ghayy grabens and the geometry of the boundary faults are consistent with development in a regime of northwest-trending dextral shear. However, the Bani Ghayy grabens are displaced and deformed by later movements of the same faults but in a sinistral sense. Other successions correlated with the Bani Ghayy also preserve features consistent with deposition in continental grabens and the possibility that they represent arc volcanism is discounted.     

Thermal Evolution of the Tanlu Fault Zone on the Eastern Margin of the Dabie Mountains and Its Tectonic Implications

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