Metamorphic Conditions of an Archean Core Complex in the Northern Wind River Range, Wyoming

The Archean granulite-facies rocks of the northern Wind RiverRange consist of extensive granitic orthogneisses and migmatiteshosting banded iron formations, amphibolites, metapelites, metabasites,ultramafites and quartzites. Quantitative pressure and temperatureestimates from inclusions within garnet porphyroblasts are 815?5O%Cand 8?1 kb using equilibria buffered by the assemblages spinel-quartz-garnet-sillimaniteand garnet-rutile-ilmenite-sillimanite-quartz. Pressure-temperatureestimates from the groundmass core assemblages of the bandediron formations and hornblende granulites are 750 ?50 %C and5?5 ? 1 kb using garnet-clinopyroxene, garnet-orthopyroxene,and two-pyroxene thermometry, and geobarometers based on theassemblages garnet-quartz-plagioclase-orthopyroxene and orthopyroxene-olivine-quartz.Rim compositions of the matrix minerals indicate nearly isobariccooling from the conditions of 750 %C and 5–5 kb to <600%C at 5 kb. Taken together, the P-T estimates from both thegarnet inclusions and matrix assemblages are consistent witha clockwise P-T-t path for this terrane. Temperature estimates based on oxygen isotope thermometry inthe banded iron formations vary systematically with the degreeof visible late-stage deformation. There is no correlation betweenthe isotopic temperature estimates and those from cation-basedthermometers. The highest pressures and temperatures for theWind River terrane are preserved by the inclusions in garnetporphyroblasts. The ability of these inclusions to preservechemistries corresponding to higher pressures and temperaturesis attributed to the combined effects of inclusion isolationand fixed inclusion volume within the garnet porphyroblasts.Cation-based thermometers in the groundmass preserve lower temperaturesas a result of diffusional partial resetting. Isotopic thermometrywill yield the lowest temperatures if there is even minor retrogradedeformation. Geothermobarometry for the northern Wind River Archean terraneis consistent with a tectonic regime of doubly thickened crust.Peak metamorphic conditions preserved in the cores of the garnetsare compatible with deep burial during the early stages of tectonism.Rapid to intermediate uplift due to erosion of the upper platecould explain the nearly isothermal decompression from 8?0 to5-5 kb. The later, nearly isobaric, cooling path indicated bythe rim compositions of the matrix minerals is consistent withrelaxation of the elevated geotherm.     

The Liaonan Metamorphic Core Complex： Constitution, Structure and Evolution

The Liaonan metamorphic core complex (mcc) has a three-layer structure and is constituted by five parts, i.e. a detachment fault zone, an allochthonous upper plate and an supradetachment basin above the fault zone, and highly metamorphosed rocks and intrusive rocks in the lower plate. The allochthonous upper plate is mainly of Neoproterozoic and Paleozoic rocks weakly deformed and metamorphosed in pre-Indosinan stage. Above these rocks is a small-scale supradetachment basin of Cretaceous sedimentary and volcanic rocks. The lower plate is dominated by Archean TTG gneisses with minor amount of supracrustal rocks. The Archean rocks are intruded by late Mesozoic synkinematic monzogranitic and granitic plutons. Different types of fault rocks, providing clues to the evolution of the detachment fault zone, are well-preserved in the fault zone, e.g. mylonitic gneiss, mylonites, brecciated mylonites, microbreccias and pseudotachylites. Lineations in lower plate granitic intrusions have consistent orientation that indicate uniform top-to-NW shearing along the main detachment fault zone. This also provides evidence for the synkinematic characteristics of the granitic plutons in the lower plate. Structural analysis of the different parts in the mcc and isotopic dating of plutonic rocks from the lower plate and mylonitic rocks from detachment fault zone suggest that exhumation of the mcc started with regional crustal extension due to crustal block rotation and tangential shearing. The extension triggered magma formation, upwelling and emplacement. This event ended with appearance of pseudotachylite and fault gauges formed at the uppermost crustal level. U-Pb dating of single zircon grains from granitic rocks in the lower plate gives an age of 130±5 Ma, and biotite grains from the mam detachment fault zone have 40Ar-39Ar ages of 108-119 Ma. Several aspects may provide constraints for the exhumation of the Liaonan mcc. These include regional extensional setting, cover/basement contact, temporal and spatial coupling of extension and magmatism, basin development and evolution of fault tectonites along detachment fault zone. We propose that the exhumation of the Liaonan mcc resulted from regional extension and thinning of crust or lithosphere in eastern North China, and accompanied with synkinematic intrusion of granitic plutons, formation of detachment fault zone, uplifting and exhumation of lower-plate rocks, and appearance of supradetachment basin.     

Petrology of the tin-bearing granite-leucogranites of the Piaoak Massif, Northern Vietnam

The Piaoak tin-bearing granite-leucogranites located in the Caobang Province of Northern Vietnam compose a stock-like hypabyssal body. Host rocks are represented by Early Devonian carbonate sequences and Early Triassic ??black?? shales. The geochronological age of the Piaoak granite-leucogranites corresponds to the Late Cretaceous: T = 83.5 ± 6.2 Ma, ⁸⁷Rb/⁸⁶Sr method; T = 89.7 ± 1.0 Ma, ³⁹Ar/⁴⁰Ar method. The massif has a simple basic to acid order: two-mica and muscovite granite-leucogranite ?? raremetal aplites, pegmatites ?? tin-bearing greisens and hydrothermal veins. The petrographic and microstructural studies revealed a sharp change in crystallization conditions of the granite-leucogranite magma at the late magmatic stage and formation of muscovite via incongruent melting of protolithionite. The study of melt and coexisting fluid inclusions showed that solidus crystallization occurred under fluid-saturated conditions at 635?C600°C. In composition, the granite-leucogranites of the Piaoak Massif correspond to the raremetal-plumasite geochemical type (according to L.V. Tauson), and reach Li-F facies in terms of their rare-element composition. The composition of aplites and pegmatites demonstrates that granite-leucogranite magma did not accumulate lithophile and volatile components in the residual melt during differentiation, but was initially enriched in rare-metals. It is most probable that the melt was generated from Proterozoic lithotectonic complexes and overlaying Lower Triassic ??black?? shales.     

变质核杂岩的旋扭成因：滇东南老君山变质核杂岩的构造解析

通过滇东南老君山变质核杂岩特征及周围构造环境的力学分析及历史分析认为,该变质核杂岩实质上是旋扭构造的砥柱,而旋扭构造又源于文山－麻栗坡断裂的走滑运动,拆离伸展构造晚于旋扭构造。因此提出了变质核杂岩旋扭成因的新看法,并认为越北古陆的形成是由于印度板块与扬子板块的碰撞导致红河断裂和文山－麻栗坡断裂的走滑运动进而引起的旋扭隆升。后期叠加伸展拆离作用。     

西藏拉轨岗日核杂岩盖层变质分带特征及其地质意义

藏南拉轨岗日由一系列链状的热穹隆构成, 总体呈东西向延伸, 每一个热穹隆是一个变质核杂岩, 核部发育大量变质岩, 基底与盖层之间发育拆离断层.通过对拉轨岗日变质带及其特征变质矿物进行化学成分分析和温度压力估算, 得出拉轨岗日变质带的分带规律及矿物成分、变质温度、压力、深度的变化规律, 为拉轨岗日变质核杂岩的热活动提供了佐证.      

Formation of Polycrystalline Graphite Aggregates in High-Pressure Metamorphic Rocks from the Kokchetav Massif,Northern Kazkhstan

Doklady Earth Sciences - An inclusion of graphite in zircon from the diamond- and tourmaline-bearing rocks of the Kokchetav massif (Northern Kazakhstan) has been studied. The inclusion, associated...     

The Yaean Metamorphic Core Complex and Extensional Detachment Fault in Inner Mangolia, China

The article describes the characteristics of the Yagan metamorphic core complex, especially the associated detachment fault and various extensional structures in its footwall. The age of the complex is discussed in some detail as well. The basic features of the Yagan metamorphic complex (Jurassic in age) are similar to those of the metamorphic core complex (Tertiary in age) in the Cordilleran area; they are as follows: (a) mylonitic gneisses in the footwall, (b) chloritized sheared mylonitic rocks, (c) pseudotachylites and flinty cataclasites or microbreccias, (d) unmetamorphosed or epimetamorphic rocks in the hanging wall with a layer of fault gouges or incohesive fault breccia next to the detachment fault. In contrast to its Cordilleran counterpart, however, there are many extensional faults with different styles (from dactile low-angle normal faults through brittle-ductile to brittle high-angle normal faults)in the footwall.     

Multistage Extension and Age Dating of the Xiaoqinling Metamorphic Core Complex,Central China

There are two extensional systems in the Xiaoqinling metamorphic core complex (XMCC). One is the detachment fault system developed along the peripheries of the XMCC, which extended in an ESE-WNW direction and whose upper plate moved towards the WNW. The other extensional system includes the retrograde shear zones and normal faults developed within the XMCC, which represent the collapse of the XMCC. Ar-Ar and K-Ar dating shows that the extension of the detachment fault system continued from 135 to 123 Ma, i.e. in the late stage of its evolution at about 127 Ma. The collapse represented by the extensional system within the XMCC was operative during 120(106 Ma, and its main activity occurred about 116 Ma ago. These suggest that the XMCC experienced two extensional stages in its evolution, i.e., the syn-orogenic regional extension and post-orogenic collapse extension.     

The Yagan Metamorphic Core Complex and Extensional Detachment Fault in Inner Mangolia,China1

Abstract The article describes the characteristics of the Yagan metamorphic core complex, especially the associated detachment fault and various extensional structures in its footwall. The age of the complex is discussed in some detail as well. The basic features of the Yagan metamorphic complex (Jurassic in age) are similar to those of the metamorphic core complex (Tertiary in age) in the Cordilleran area; they are as follows: (a) mylonitic gneisses in the footwall, (b) chloritized sheared mylonitic rocks, (c) pseudotachylites and flinty cataclasites or microbreccias, (d) unmetamorphosed or epimetamorphic rocks in the hanging wall with a layer of fault gouges or incohesive fault breccia next to the detachment fault. In contrast to its Cordilleran counterpart, however, there are many extensional faults with different styles (from dactile low-angle normal faults through brittle — ductile to brittle high — angle normal faults) in the footwall.     

福建东山坑北复式岩体的地质特征及其变质变形作用

在平潭—东山构造带南段深变质的东亚带和浅变质的西亚带的过渡带．出露一套变质变形特征极其复杂的动力变质岩系。其主体是一个由早侏罗世、晚侏罗世及晚侏罗世—早白垩世石英闪长岩、花岗闪长岩、二长花岗岩、正长花岗岩等组成的坑北复式岩体。该岩体可划分6个基本单元，并与区域上同时代侵入岩的谱系单位划分进行对比，归并为东山序列、古美山、漳浦超单元。该区发育三期韧-(脆)性变形及动力变质作用。据R1-R2图解及巴尔巴林岩浆构造分类．坑北复式岩体中各超单元(单元)侵入岩分别形成于不同的构造环境。     

华北克拉通北缘喀喇沁变质核杂岩早白垩世构造演化过程与形成模式

对于喀喇沁变质核杂岩早白垩世构造过程与形成模式长期以来存在着不同的认识。通过详细的野外构造观察及擦痕应力场反演,并结合前人年代学数据,有效地制约了喀喇沁变质核杂岩早白垩世的构造演化,并对其形成模式进行了分析。结果表明,起源于晚侏罗世的喀喇沁变质核杂岩,在早白垩世(141～100 Ma)再次经历了强烈的伸展与岩浆活动。在此伸展活动中,沿着核部杂岩两侧分别发育了NE走向、倾向相反的楼子店-八里罕和上店-东风大型正断层,进而控制两侧半地堑式的小牛与平庄盆地的发育。在这两条边界断层的伸展运动及随后的均衡隆升中,核部杂岩不断抬升与剥露,先后发育了NE-NNE向的伸展韧性变形带与脆性正断层。这些早白垩世韧性和脆性伸展构造一致指示拉张方向为NW-SE向。综合分析表明,区内早白垩世伸展活动经历了141～134 Ma的初始伸展与同构造岩体侵位阶段、133～126 Ma的边界断层强烈活动与核部快速抬升阶段以及125～100Ma的均衡隆升阶段。喀喇沁变质核杂岩在早白垩世的伸展活动中转变为地垒式伸展穹窿,其强烈伸展活动出现在华北克拉通破坏峰期,动力学背景是古太平洋板块俯冲导致的远场弧后拉张。     

扎日加变质核杂岩的确定及地质意义

产于北巴颜喀拉山扎日加地区的变质核杂岩具有典型的二层结构。由构造片岩、构造片麻岩、千枚岩等组成穹状背斜核,其中心为燕山期二长花岗岩体,盖层为巴颜喀拉山群浅变质砂板岩。核部与盖层之间为剥离断层。它的厘定表明巴颜喀拉沉积盆地下面存在中元古界的古老基底,并对大场金矿的成因类型提出了新的认识。     

医巫闾山变质核杂岩中金矿的稳定同位素特征

医巫闾山变质核杂岩中金矿及相关岩石的氢、氧、铅、硅、碳、硫稳定同位素测试结果为:成矿流体δD为-97‰~-72‰,δ18O为0.3‰~3.8‰,在δ18O-δD分布图上,位于变质水、岩浆水与当地雨水之间;其他稳定同位素数据及综合资料均说明,以排山楼金矿为代表的该区金矿成矿具有长期性、复杂性和多源性;医巫闾山变质核杂岩构造系统对区内金矿有一定控制作用.     

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.     

庐山变质核杂岩的变质变形及构造意义

庐山出露的变质核杂岩的构造层次具有典型的三层结构,即最上面的未变质沉积盖层、中间韧性流变层、核部的变质基底,其间分别以上剥离断层和拆离断层为分界线。上部沉积盖层以脆性变形为主,中间韧性流变层变形程度不一,但远离拆离带变质变形逐渐减弱,而核部的变质基底以强塑性变形为特征。对变质核杂岩南北两条地质剖面的分析,得到三个构造层次的岩石变质变形及岩性变化特征。区域动力学分析反映变质核杂岩东部原拆离带在五里正断层附近。拆离带中多硅白云母压力计得出的变质压力为0.6~1.1 GPa;锆石U-Pb年龄测试得到变质核杂岩的隆升时代为早白垩世。     

福建蒲洋变质核杂岩构造及其控矿作用初探

蒲洋变质核杂岩属古大陆边缘型,由变质杂岩核和基底滑脱剥离带组成,因后期构造破坏,褶叠层和未变质盖层出露不齐全,变质杂岩核为加里东期－印支期的中酸性侵入岩,具片麻状构造;基底滑脱剥离带发育于中元古界麻源群变质岩中,具片麻状构造,并在此基础眼育了中浅层次低角度正断层系,肖坂地区金矿床采变质核杂岩生成演化的控制,近核部陡倾角断裂控制石英脉型金矿床,西北侧低角度正断层控制构造蚀变岩金矿床,从构造控矿角度分     

The Appinite-Migmatite Complex of Sanabria, NW Iberian Massif, Spain

The Sanabria appinitic rocks and host migmatites form an unusual,non-peri-batholithic complex in which all the typical membersof the appinite suite are present. It differs from most appiniticcomplexes in the deeper level of emplacement and the close temporaland spatial association with migmatites. Consequently, manyin situ relationships that resulted from the invasion of maficmagma into a crustal anatectic zone are extremely well preserved.The complex shows unequivocal relations between members of theappinitic suite and between these and migmatites derived byanatexis of a gneissic formation (Ollo de Sapo gneiss). Theserelations point to derivation of monzodiorites and biotite dioritesby hydrous basalt fractionation combined with fluid-assistedmelting of the crustal rocks surrounding the appinitic intrusions.This hydrous basic magma may be derived from an enriched regionof the mantle associated with subduction. Petrogenetic modelshave been tested using a combination of field relations andgeochemical data. Despite the complexity of the processes involved,it is concluded that water played an important role in the petrogenesisof the intermediate and mafic magmas. Reaction between monzodioritemelts and the host migmatites was responsible for the generationof a range of intermediate rocks within the complex. The needfor water to facilitate magma generation in both the mantleand the crust suggests that melting is linked with subduction.This interpretation has important implications because appiniticmagmatism may be considered as indicative of subduction processesinvolved not only in the generation of the mafic end-membersof the suite, but also in the generation of batholiths withwhich the appinitic rocks are spatially and temporally associated. KEY WORDS: appinite; monzodiorite; migmatite; Variscan orogen; Iberian massif     

Metamorphic P-T Paths from Cordillera Darwin, a Core Complex in Tierra del Fuego, Chile

P–T conditions and prograde P–T paths have beencalculated for amphibolite-grade pelites and amphibolites fromCordillera Darwin, Tierra del Fuego, Chile. Peak P–T conditionsare nearly all within the kyanite stability field; temperaturesgenerally show an increase with increasing grade, but pressureshave a less consistent trend, possibly increasing slightly fromgarnet to kyanite grade. P–T paths from pelites show heatingby 80–100C during loading of 0•2–3 kbar. Texturalanalysis and previous structural work indicate that this segmentof the path correlates with back-folding deformation. P–Tpaths from two Mg-rich garnet amphibolites suggest a decreasein pressure of as much as 3 kbar with 25–50C of heatingfrom the kyanite stability field to the sillimanite, and areconsistent with pervasive, minor development of fibrolitic sillimanitealong plagioclase grain boundaries. Together, the P–Tpath segments from pelites and amphibolites constitute a clockwiseP–T trajectory. The proposed clockwise P–T paths are consistent with theinterpretation that Cordillera Darwin represents an extensionallyexhumed metamorphic core complex, in which loading during garnetgrowth in the pelitic rocks was succeeded by differential upliftduring garnet growth in magnesian amphibolites. ^* Present address: Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706     

早元古陡岭群变质杂岩的岩石地球化学特征

河南省淅川县大陡岭至西峡县田关一带分布的陡岭群变质杂岩,由角闪岩相的变质岩石组成,主要包括片麻岩、钙硅酸粒岩、斜长角闪岩,夹少量的大理岩、石墨片岩和极少量的石英岩。岩石地球化学特征表明：片麻岩的原岩为一套泥砂质的沉积碎屑岩,具有后太古沉积岩的组成特点,形成于活动大陆边缘环境;透辉石变粒岩的原岩为钙砂质的沉积碎屑岩,但可能与片麻岩有着不同的物源区;斜长角闪岩有明显不同的两种产出状态和不同的两类稀土模式,一类呈夹层状与片麻岩互层产出,总稀土含量较低和不强的轻重稀土分离,原岩为非大洋型的拉斑玄武质火山岩;另一类则呈透境状产出,并具有很高的总稀土含量和强的轻重稀土分离,及明显的Ｅｕ负异常,原岩为拉斑玄武质的深成侵入岩。陡岭群变质杂岩有与秦岭群类似的岩石组合、原岩建造和时代等特征,形成于非大洋的活动大陆边缘环境。