New Zircon U-Pb Geochronology of the Post-kinematic Granitic Plutons in the Diancang Shan Metamorphic Massif along the Ailao Shan-Red River Shear Zone and Its Geological Implications

<正>The Ailao Shan-Red River fault zone is the boundary between the Yangtze block to the northeast and the Indochina block to the southwest.It is an important tectonic zone due to its role in the southeastward extrusion of the Indochina block during and subsequent to the Indian-Eurasian collision.Diancang Shan(DCS) high-grade metamorphic complex,located at the northwest extension along the Ailao Shan-Red River(ASRR) shear zone,is a representative metamorphic complex of the ASRR tectonic belt.Structural and microstructural analysis of sheared rocks in the high-grade metamorphic rocks reveals that they are coherent with solid-state high-temperature ductile deformation,which is attributed to left-lateral shearing along the ASRR shear zone.New LA-ICP-MS zircon U-Pb geochronological and microstructural studies of the post-kinematic granitic plutons provide a straightforward time constraint on the termination ductile left-lateral shearing and exhumation of the metamorphic massif in the ASRR shear zone.It is suggested that the left-lateral shearing along the ASRR shear zone ended at ca.21 Ma at relative lower-temperature or decreasing temperature conditions.During or after the emplacement of the young dikes at ca.21 Ma,rapid brittle deformation event occurred,which makes the DCS massif start fast uplift/exhumation and cooling to a shallow crustal level.     

Timing of left-lateral shearing along the Ailao Shan-Red River shear zone: constraints from zircon U–Pb ages from granitic rocks in the shear zone along the Ailao Shan Range, Western Yunnan, China

As the boundary between the Indochina and the South China blocks, the Ailao Shan-Red River (ASRR) shear zone underwent a sinistral strike-slip shearing which is characterized by ductile deformation structures along the Ailao Shan range. The timing issue of left-lateral shearing along the ASRR shear zone is of first-order importance in constraining the nature and regional significance of the shear zone. It has been, therefore, focused on by many previous studies, but debates still exist on the age of initiation and termination of shearing along the shear zone. In this paper, we dated 5 samples of granitic plutons (dykes) along the Ailao Shan shear zone. Zircon U–Pb ages of four sheared or partly sheared granitic rocks give ages of 30.9 ± 0.7, 36.6 ± 0.1, 25.9 ± 1.0 and 27.2 ± 0.2 Ma, respectively. An undeformed granitic dyke intruding mylonitic foliation gives crystallization age of 21.8 ± 1 Ma. The Th/U ratios of zircon grains from these rocks fall into two populations (0.17–1.01 and 0.07–0.08), reflecting magmatic and metamorphic origins of the zircons. Detailed structural and microstructural analysis reveals that the granitic intrusions are ascribed to pre-, syn- and post-shearing magmatisms. The zircon U–Pb ages of these granites provide constraints on timing of the initiation (later than 31 Ma from pre-shearing granitic plutons, but earlier than 27 Ma from syn-shearing granitic dykes) and termination (ca. 21 Ma from the post-shearing granitic dykes) of strong ductile left-lateral shearing, which is consistent with previous results on the Diancang Shan and Day Nui Con Voi massifs in the literature. We also conclude that the left-lateral shearing along the ASRR shear zone is the result of southeastward extrusion of the Indochina block during the Indian–Eurasian plate collision. Furthermore, the left-lateral shearing was accompanied by the ridge jump, postdating the opening, of the South China Sea.     

Onset timing of left-lateral movement along the Ailao Shan–Red River Shear Zone: 40Ar/39Ar dating constraint from the Nam Dinh Area,northeastern Vietnam

Left-lateral motion along the Ailao Shan–Red River (ASRR) Shear Zone has been widely advocated to be the result of the collision between the Indian and Eurasian plates and to account for sea-floor spreading in the South China Sea. Our new ⁴⁰Ar/³⁹Ar data on the south-easternmost outcrop of the Day Nui Con Voi metamorphic massif, northern Vietnam, suggest that the exhumation of metamorphic massif by shearing along the ASRR zone began ∼27 Ma and lasted until ∼22 Ma. A perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of ∼6 cm y⁻¹. Such a good correlation also indicates that the onset of the left-lateral movement of the shear zone may have occurred later than ∼27.5 Ma. This conclusion is consistent with our previous interpretation that collision-induced southeastward extrusion of Indochina along the ASRR Shear Zone postdates the opening of the South China Sea, and that extrusion tectonics in SE China may not be responsible for the opening of the South China Sea.     

Initiation of left-lateral deformation along the Ailao Shan–Red River shear zone: new microstructural,textural, and geochronological constraints from the Diancang Shan metamorphic massif,SW Yunnan,China

The Diancang Shan metamorphic massif, the northwestern extension of the Ailao Shan Massif, is a typical metamorphic complex situated along the NW–SE-trending Ailao Shan–Red River shear zone. Diancang Shan granitic and amphibolitic mylonites collected from sheared high-grade metamorphic rocks were studied using petrographic and electron-backscatter diffraction techniques. Sensitive high-resolution ion microprobe U–Pb dating of zircon grains from the granitic mylonites constrains the timing of shearing. Macro- and microstructural and textural analysis reveals intense plastic deformation of feldspar, quartz, and amphibole under amphibolite-facies conditions, all consistently document left-lateral shearing. Porphyritic monzogranitic mylonite within the shear zone possesses evidence supporting a sequential, progressive process from crystallization during magma emplacement, through submagmatic flow to solid-state plastic deformation. We suggest that the early-kinematic pluton subsequently underwent strong left-lateral strike–slip shearing. The development of complex textures of quartz, feldspar, and amphibole from the granitic and amphibolitic mylonites apparently records successive variation of conditions attending coherent, solid-state high-temperature ductile deformation during regional left-lateral shearing. All magmatic zircons from the mylonitized porphyritic monzogranite give U–Pb ages of 30.95 ± 0.61 million years for the crystallization of the granite. This age provides the timing of onset of left-lateral shearing along the Ailao Shan–Red River shear zone in the Diancang Shan high-grade metamorphic massif.     

Structural geometry and kinematics of the Ailao Shan shear zone: insights from integrated structural,microstructural, and fabric studies of the Yao Shan complex,Yunnan, Southwest China

ABSTRACT

The Yao Shan complex, a massif near the southern segment of the Ailao Shan–Red River (ASRR) shear zone, bears important information on the structural framework of the massif and the kinematics of ductile shearing along the ASRR shear zone. In this contribution, structural, microstructural, quartz c-axis fabric, magnetic fabric, and geochronologic data are used to determine the structural framework of the Yao Shan massif and its tectonic implications for the ASRR shear zone. The Yao Shan complex is characterized by an overall linear A-type antiform that contains a core of high-grade metamorphic rocks with Palaeoproterozoic to Mesozoic protoliths and a mantle of Permo-Triassic low-grade rocks. Both the high-grade metamorphic core and low-grade Permo-Triassic rocks have experienced progressive ductile shearing. Anisotropy of magnetic susceptibility (AMS) results from 17 samples collected along the Xinjie–Pingbian section across the complex show that magnetic lineation (K_max) and foliation (K_max–K_int) are generally subparallel to the corresponding structural elements in the sheared rocks. The shape parameter E values of the magnetic ellipsoids are indicative of dominantly oblate and plane strain, but vary with protolith type and degree of strain among the various rock types. In agreement with the field and microstructural observations, the corrected degree of anisotropy (Pj) values reflect high shear strain in the core rocks and relatively low shear strain in the low-grade strata. A kinematic analysis based on structural and magnetic fabric data shows that both left- and right-lateral shear occurred during the deformation of the Yao Shan complex. Therefore, instead of being an element of the ASRR shear zone, the Yao Shan complex constitutes a crustal-scale inharmonic A-type fold with a fold axis parallel to the stretching lineation. Geochronologic data reveal that the folding occurred coevally with ductile shearing of the middle to lower crust between ca. 30 and 21 Ma.     

点苍山变质杂岩新生代变质-变形演化及其区域构造内涵

点苍山变质杂岩体是哀牢山-红河韧性剪切带四个变质杂岩体之一,遭受了多期多阶段变质-变形作用改造。本文重点针对点苍山杂岩的新生代变质-变形作用,尤其是以富铝质高级变质岩即夕线石榴黑云片麻岩和侵位于其中的糜棱岩化细晶花岗质岩石开展了深入研究。对夕线石榴黑云片麻岩的显微构造分析与矿物共生组合研究,确定了高角闪岩相和低角闪岩相变质矿物共生组合,分别为:石榴石(Grt)+夕线石(Sil)+钾长石(Kfs)+黑云母(Bi)+斜长石(Pl)±石英(Q)和夕线石(Sil)+白云母(Ms)+黑云母(Bi)+石英(Q)。对其中的变质锆石进行SHRIMP U-Pb测试,获得了新生代三个阶段的变质作用年龄,即54.2±1.7Ma、31.5±1.5Ma和27.5±1.2Ma.本文还深入研究了侵位于高级变质岩中的一个花岗岩质糜棱岩的宏观与显微构造特点,其LA-ICP-MS年龄为24.4±0.89Ma,代表着同剪切就位花岗质岩浆侵位和结晶年龄。夕线石榴黑云片麻岩中变质锆石从2150～27Ma多期多阶段表观年龄的发育,表明点苍山变质杂岩体具有复杂的构造演化史。点苍山杂岩的多阶段新生代构造-热演化归咎于印度-欧亚板块会聚与碰撞作用(约54Ma)、造山后伸展作用(大约40～30Ma)和沿着哀牢山-红河剪切带大规模左行走滑变形作用(约27～21Ma)。     

哀牢山-红河构造带古新世以来多期活动的构造和年代学证据

哀牢山—红河构造带哀牢山段可划分为东部高级变质带和西部低级变质带。构造分析表明:该构造带由3个不同变形域组成,可能代表其经历的3期左行走滑。第1期走滑发生在整个高级变质带,为拉张性左行走滑,形成角闪岩相L型构造岩。第2期走滑形成高级变质带中的高应变带,变形体制接近简单剪切,形成绿片岩相L-S型糜棱岩。第3期主要发生在低级变质带,为挤压性走滑,形成左行逆冲构造格局,并形成低绿片岩相千糜岩。地质年代学数据证明,3期左行走滑的形成时代分别是:距今58~56Ma、27~22Ma和13~12Ma±。哀牢山—红河构造带第1期左行走滑可能对应于印度与欧亚大陆距今60Ma左右的初始碰撞;第2期变形与青藏高原最强的挤压隆升期一致;第3期事件可能代表距今16~13Ma开始的青藏高原物质进一步东挤。哀牢山—红河构造带的3期主要左行走滑均发生在新生代印度与欧亚大陆的汇聚过程中。     

Thermochronology of the PoSen complex,northern Vietnam: Implications for tectonic evolution in SE Asia

The PoSen complex, located closely adjacent to the southwestern margin of the Red River shear zone represents the uplifted basement of north Vietnam and may record the motion of the shear zone. However, its thermochronological history has not been fully examined yet. Here we applied U–Pb and ⁴⁰Ar/³⁹Ar dating methods to reveal its thermochronological history. U–Pb analysis of composite zircon grains by TIMS yielded an average age of 760 ± 25 Ma, clustering on the concordia line. Twelve SHRIMP U–Pb analyses also yielded a consistent result of 751 ± 7 Ma. Along with the geochemical features, the U–Pb dating results suggest the PoSen complex was a late Proterozoic magmatic complex, which could correspond to the Chengjiang orogeny, a widespread thermal event in southwest China. Results of ⁴⁰Ar/³⁹Ar dating of micas and K-feldspars were in the range of 36–30 Ma, revealing a rapid cooling and exhumation history of the PoSen complex during the late Paleogene. The time span of cooling and exhumation of the PoSen complex is slightly older than the main cooling phases of the Ailao Shan–Red River (ASRR) metamorphic massifs (28–17 Ma), but is synchronous with the early igneous activity stage in the eastern Indo-Asian collision zone of southeast China and north Vietnam. Owing to the ongoing debate about the initiation and offset of the ASRR shear zone, the tectonic force for the late Paleogene cooling of the PoSen complex is still inconclusive. The rapid exhumation of the PoSen complex could be in response to either the detachment of the Neo-Tethyan slab or a transpressional phase of continental subduction along the ASRR shear system in the eastern Indo-Asian collision zone.     

Structure,Timing, and Mechanism of the Pliocene and Late Miocene Uplift Process of the Ailao Shan-Diancang Shan,SE Tibet,China

The uplift of the Ailao Shan-Diancang Shan(ASDS) along the Ailao Shan-Red River(ASRR) shear zone is an important geological event in the southeastern margin of Qinghai-Tibet Plateau tectonic domain in the Late Cenozoic,and it preserves important information on the structures,exhumational history and tectonic evolution of the ASRR shear zone.The uplift structural mode and uplift timing of the ASDS is currently an important scientific topic for understanding the ASDS formation and late stage movements and evolution of the ASRR shear zone.The formation of the ASDS has been widely considered to be the consequence of the strike-slip movements of the ASRR shear zone.However,the shaping of geomorphic units is generally direct results of the latest tectonic activities.In this study,we investigated the timing and uplift structural mechanism of the ASDS and provided the following lines of supportive evidence.Firstly,the primary tectonic foliation of the ASDS shows significant characteristic variations,with steeply dipping tectonic foliation developed on the east side of the ASDS and the relatively horizontal foliation on the west side.Secondly,from northeast to southwest direction,the deformation and metamorphism gradually weakened and this zone can be further divided into three different metamorphic degree belts.Thirdly,the contact relationship between the ASDS and the Chuxiong basin-Erhai lake is a normal fault contact which can be found on the east side of the ASDS.40Ar/39 Argeochronology suggests that the Diancang Shan had experienced a fast cooling event during 3–4 Ma.The apatite fission track testing method gives the age of 6.6–10.7 Ma in the Diancang Shan and 4.6–8.4 Ma in the Ailao Shan,respectively.Therefore the uplift of the ASDS can be explained by tilted block mode in which the east side was uplifted much higher than the west side,and it is not main reason of the shearing movements of the ASRR shear zone.The most recent uplift stages of the ASDS happened in the Pliocene(3–4 Ma) and Late Miocene(6–10 Ma).     

哀牢山构造带晚渐新世地壳深熔与熔/流体迁移:锆石U-Pb年龄与微量元素证据

哀牢山构造带是青藏高原东南缘重要的边界构造带,其内出露的深变质岩系一直被认为是古老的变质基底岩石。利用LA-ICP-MS原位微区分析技术对哀牢山深变质岩系锆石进行U-Pb年龄、微量元素分析。结果表明深变质岩系的原岩有728±8Ma、727±3Ma、231±4Ma的花岗质岩石和其它年龄的碎屑岩,变质时代为27.8~23.7Ma。综合野外地质特征和分析结果,我们认为哀牢山深构造带在晚渐新世27.8~23.7Ma发生了大规模的地壳深熔作用,现今所见深变质岩系是由不同时代、不同岩性的原岩在晚渐新世(27.8~23.7Ma)变质形成,不全是古老的变质基底岩石。深熔过程中熔/流体发生了明显的迁移。哀牢山变质带具有混合岩化特征的岩石很可能是峰期变质作用后减压熔融的产物。晚渐新世地壳深熔作用与左行走滑剪切是哀牢山深变质带折返过程中近似同时发生的两种不同变质表现形式,两者相互影响、相互制约。     

Cenozoic high-K alkaline magmatism and associated Cu–Mo–Au mineralization in the Jinping–Fan Si Pan region,southeastern Ailao Shan–Red River shear zone,southwestern China–northwestern Vietnam

The Jinping–Fan Si Pan (JFP) Cenozoic magmatic and Cu–Mo–Au metallogenic belt in the southeastern part of the Ailao Shan shear zone host the Tongchang, Chang′an, Habo, and Chinh Sang Cu–Mo–Au deposits. These deposits form an integrated epithermal-porphyry regional mineralization system associated with 40–32 Ma high-K alkaline magmatism. The magmatic rocks in the belt have relatively low TiO₂ (<0.73 wt%), P₂O₅ (<0.29 wt%), and FeO^* (<4.99 wt%), and high Na₂O (2.86–4.75 wt%) and K₂O (4.01–7.98 wt%). They also have high contents of incompatible trace elements, and are enriched in LILE (Rb, Ba, K, Sr) and LREE. They have marked Nb, Ta, Ti and P depletion in primitive mantle-normalized spidergrams, and plot close to the EMII mantle field in the Sr–Nd isotopic diagram. These characteristics are similar to those of the Eocene high-K alkaline rocks along the northern Ailao Shan belt, eastern Tibet plateau. The sulfur and lead isotope analyses of sulfide minerals from both the ores and related magmatic rocks confirm the involvement of a magmatic ore fluid. The Cenozoic alkaline intrusions and Cu–Mo–Au mineralization in the JFP were formed prior to the initiation of left-lateral shearing along the Ailao Shan shear zone. The magmas appear to have been derived from enriched mantle, possibly with mixing of materials from the buried Tethyan oceanic lithosphere, and/or crust.     

Geochemistry of lamprophyres at the Daping gold deposit,Yunnan Province,China: Constraints on the timing of gold mineralization and evidence for mantle convection in the eastern Tibetan Plateau

Cenozoic lamprophyre dykes occur widely along the Ailao-Shan-Red-River (ASRR) shear zone related to the Indian–Eurasian collision. Two generations of lamprophyres have been found at the Daping gold deposit in the southern part of the ASRR shear zone and have been investigated by using phlogopite ⁴⁰Ar/³⁹Ar dating and whole-rock major and trace element as well as Sr and Nd isotope geochemical analyses. The ⁴⁰Ar/³⁹Ar plateau ages of phlogopite from the two generations of lamprophyres bracket the emplacement of auriferous quartz veins in the Daping deposit between 36.8 ± 0.2 Ma and 29.6 ± 0.2 Ma, consistent with the timing of gold mineralization in other parts of the ASRR shear zone. Geochemical data suggest that these lamprophyres most likely originated from a subduction-modified mantle source consisting of phlogopite-bearing spinel lherzolite, which underwent partial melting with contributions from crust materials. In particular, the second generation lamprophyres are characterized by more primitive geochemical features than the first, suggesting that secular source evolution probably resulted from post-collisional slab break-off mantle convection and remelting from ascending asthenosphere after subducted lithosphere break-off. Widespread and episodic occurrences of lamprophyres and other potassic volcanism in the eastern Tibetan Plateau were probably related to the onset of transtensional tectonics along the ASRR shear zone during Oligocene. A genetic model involving transtensional tectonics has been proposed for lamprophyres and gold mineralization in the ASRR shear zone.     

哀牢山-红河剪切带左行走滑作用起始时间约束

位于哀牢山-红河剪切带NW延伸方向上的点苍山变质杂岩体遭受强烈的左行走滑剪切变形、变质作用改造,岩石中保存了典型的高温矿物组合以及由它们构成的宏观和微观高温变形构造特征,其中糜棱岩中具有极其发育的长石矿物拉伸线理而形成典型的L与LS型构造岩是其一个明显的特征。本文对点苍山地区高温糜棱岩主要矿物开展了显微构造与矿物变形、变形机制及组构分析,并对于遭受高温糜棱岩化改造的一个花岗质岩体开展了SHRIMP锆石U-Pb定年分析。结果表明岩石中长石、角闪石、石英等主要矿物具有典型的达角闪岩相条件下的高温晶质塑性变形和动态生长特征,它们也为走滑剪切变形活动提供了充分的微观构造证据。对于点苍山高温糜棱岩化改造的眼球状或似斑状二长花岗岩的显微构造分析结果表明,这套花岗质岩石从走滑剪切前期岩浆的侵位之后经历了早期强烈的岩浆期后交代作用—亚岩浆流动—高温固态塑性剪切变形的递进演化过程。由此可见,岩浆的上升与就位受左行走滑剪切作用的制约,岩体又遭受了强烈剪切变形改造。同时对这套构造前期就位花岗质岩石中的锆石进行定年分析,获得33.88±0.32Ma的岩浆结晶年龄,为此,我们有充分的理由认为,在点苍山地区哀牢山-红河剪切带左行走滑剪切作用的起始时间至少应该为早渐新世30.88±0.32Ma。     

Continental origin of the Bibong eclogite,southwestern Gyeonggi massif,South Korea

Eclogite is a high-pressure (HP) metamorphic rock that provides important information about the subduction of both continental and oceanic crusts. In this study we present SHRIMP zircon U–Pb isotopic data for a suite of the basement gneisses to investigate the origin of the Proterozoic Bibong eclogite in the Hongseong area, South Korea. Zircon grains from the basement felsic gneisses yielded Paleoproterozoic protolith ages ranging from ca. 2197 to 1880 Ma, and were intruded by syenite at ca. 750 Ma. A HP regional metamorphic event of Triassic age (ca. 255–227 Ma) is recorded in the zircon rims of the country rocks, which is also observed in the zircons from the eclogite. The contacts between the Bibong eclogite and its host rocks support an origin for the Proterozoic protoliths, indicating continental intrusions. The Hongseong area thus preserves evidence for the Triassic collision, indicating a tectonic linkage among the northeast Asian continents.     

U-Pb and Ar/Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, ⁴⁰Ar/³⁹Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The ⁴⁰Ar/³⁹Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite ⁴⁰Ar/³⁹Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.     

哀牢山-红河剪切带中段多阶段新生代花岗岩脉:同位素年代学及对于剪切应变型式转变时间的约束

哀牢山-红河剪切带是新生代印度板块与欧亚板块碰撞过程中发育的大规模走滑型剪切带,其发育对于碰撞过程中印支地块的南东向逃逸以及藏东南地区构造格局的形成具有重要的贡献。与剪切带演化相关,伴随发育多阶段花岗岩脉就位,它们为限定剪切变形时限、阐明剪切作用属性提供了重要证据。本文在野外观察基础上,应用显微构造和EBSD石英c-轴组构分析查明花岗岩脉的构造特点与应变型式,同时采用锆石LA-ICP-MS测年方法获得岩脉侵位与结晶年龄。年龄分析结果表明,岩脉年龄分别为27.09±0.48Ma、25.17±0.23Ma和25.16±0.50Ma,其中年龄为27.09±0.48Ma的花岗岩脉具有糜棱岩化现象,其变形特征体现为中温变形后叠加低温变形,且剪切变形形式由一般剪切转换为简单剪切;年龄为25.17±0.23Ma的花岗岩脉表现出同剪切晚期构造特征,且具有较低温度简单剪切变形特点;25.16±0.50Ma的切穿糜棱叶理,矿物未见变形,可能代表剪切期后岩脉。结合区域构造,推测剪切方式由纯剪为主的剪切向由单剪为主的剪切转换发生在27Ma和25Ma之间,哀牢山-红河剪切带中段在约25Ma走滑运动结束。     

Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and Ar/Ar geochronological constraints

A kinematic and geochronological study has been carried out on the Triassic high-strain shear zones in Hainan Island, the southern South China Block. There are WNW- and NE-trending high-strain shear zones with greenschist- to amphibolite-facies metamorphism in this island. Kinematic indicators suggest a dextral top-to-the-NNE thrust shearing for the WNW-trending high-strain shear zones and a sinistral top-to-the-SE thrust shearing for the NE-trending shear zones. The quartz c-axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system. The timing of shearing for these shear zones has been constrained by the ⁴⁰Ar/³⁹Ar dating analyses of synkinematic minerals. Middle Triassic (242–250 Ma) and late Triassic–early Jurassic (190–230 Ma) have been identified for the WNW- and NE-trending shear zones, respectively. A synthesis of these kinematic and thermogeochronological data points to a two-stage tectonic model for Hainan Island, that is, top-to-the-NNE oblique thrusting at 240–250 Ma followed by top-to-the-SE oblique thrusting at 190–230 Ma. In combination with the available data from the southern South China and Indochina Blocks, it is inferred that South Hainan and North Hainan have affinity to the Indochina and South China Blocks, respectively. The tectonic boundary between South Hainan and North Hainan lies roughly along the WNW-trending Changjiang–Qionghai tectonic zone probably linking to the Song Ma and Ailaoshan zones. The middle Triassic structural pattern of Hainan Island is spatially and temporally compatible with those of the South China and Indochina Blocks, and thus might be a derivation from the amalgamation of the Indochina with South China Blocks in response to the closure of the Paleotethys Ocean and subsequent subduction/collision.     

用陆块旋转解释藏东南渐新世—中新世伸展作用——来自点苍山及邻区变质核杂岩的证据

沿红河断裂带(RRFZ)分布的点苍山变质核杂岩是一个不完整的变质核杂岩,它由两个特征迥异的单元组成,包括被同构造二长花岗岩侵入角闪岩相构造岩组成的下盘和绿片岩相的拆离断层带。下盘岩石包括具有高温构造组合,具有指示左行走滑剪切运动方向的L型糜棱岩或LS型糜棱岩。拆离断层带是一个上盘向E到SE伸展剪切的低温剪切带,由具有剪应变和压应变的典型S-L糜棱岩构成。低温构造岩也包括发育于下盘的几个糜棱岩化似斑状二长花岗岩侵入体。变质核杂岩与西侧覆盖未变质的中生代沉积岩并置,东部受第四纪断层作用影响为沿洱海分布的更新世—全新世沉积盆地。通过对点苍山变质核杂岩的构造研究,结合邻区变质核杂岩的地质年代学及古地磁学分析,我们认为:位于东南亚红河断裂和实皆断裂带之间的扇形区域内出现的变质核杂岩与渐新世—中新世时期区域性伸展作用有关,而伸展作用是由印支地块的差异性旋转产生的,其原因是由于约33Ma开始斜向俯冲的印度板块的顺时针旋转和回退所致。     

Time constraints on deformation of the Ajjaj branch of one of the largest Proterozoic shear zones on Earth: The Najd Fault System

In active tectonic regions, shear zones play an important role in re-configuring the structure of the lithosphere. One of the largest shear zones on Earth is the Najd Fault System of the Arabian–Nubian Shield. The main active phase of this shear zone was during the last stages of the Pan-African Orogeny (ca. 630–540 Ma). Six samples of intrusive rocks that were emplaced into the shear zone at different stages during its active phase are used to illustrate the progressive evolution of the Ajjaj shear zone. A sample of coarse-grained diorite, with an intercept U–Pb zircon age of 696 ± 6 Ma, shows very weak deformation. Two samples from deformed granodiorite–tonalite intrusions at the border of the Ajjaj shear zone show conspicuous degrees of deformation, and define two U–Pb clusters of concordia ages at 747 ± 12 Ma–668 ± 8 Ma and 742 ± 5 Ma–702 ± 12 Ma. Two samples of granites show mylonitic foliation with flattened quartz and biotite parallel to the trend of the shear zone. These samples yield U–Pb ages of 601 ± 3 Ma–584 ± 3 Ma. Another granite sample is undeformed and shows cross-cutting relations with the shear foliation of the Ajjaj shear zone. It yields a concordia age of 581 ± 4 Ma. The metamorphic rocks of the Hamadat complex host the Ajjaj shear zone, and have been useful in determining the metamorphic P-T conditions attending the activity of the shear zone. The peak metamorphism of the Hamadat Complex is 505–700 °C at two ranges of pressure 8–11 and 14.5 ± 2 kbar. New data confine the activation of the Ajjaj shear zone in a limited period of time between 604 Ma and 581 Ma and the operation at different crustal levels with a maximum depth of 58 km.     

Integrated potential-field and seismic constraints on the structure of the Archean metasedimentary English River belt,Western Superior craton,Canada

The English River Subprovince is a prominent belt of metasedimentary rocks in the Archean Western Superior Province. The structure of its western half was investigated by using techniques of enhancement and automatic interpretation of magnetic data, and integration of magnetic-derived information with seismic and gravity data. The results indicate that a suite of exposed felsic plutons that intruded the belt at ca. 2698 Ma extends under most of the metasedimentary rocks that are exposed at the surface. The thickness of the metasedimentary rocks is interpreted to be less than 1 km in areas where it is underlain by the members of this intrusive suite. In other areas, the metasedimentary rocks attain thicknesses of 3–4 km and appear to be underlain by rocks similar to the gneissic rocks that are exposed in the adjacent metaplutonic Winnipeg River Subprovince. The integration of enhanced magnetic data with gravity data indicates that the large gravity anomaly that extends along the English River belt correlates well spatially and morphologically with the extensive suite of felsic intrusions that underlies the belt, suggesting that the crustal component of the gravity anomaly is related to this suite of intrusions. We interpret the source of the gravity anomaly as a dense unit comprising anhydrous mineral assemblages that formed within these felsic intrusions in response to low-pressure, high-temperature metamorphism that affected the belt at ca. 2691 Ma. On the basis of geochronological, geological and geophysical constraints, we propose that this metamorphic episode is linked to the continuation of magmatism at depth after the emplacement of the ca. 2698 Ma felsic plutons, being ultimately related to the advection of mantle heat into the crust during a period of regional extension.