Field report:Research along the Yarlung Suture Zone in Southern Tibet,a persistent geological frontier

The Yarlung Suture Zone in Southern Tibet marks the boundary between India and Asia-formerly separated by an ocean basin-and is a critical record of the tectonic processes that created the Tibetan Plateau. The Yarlung Suture Zone is also a frontier research area, as difficulty of access has limited research activity, providing ample opportunities for new discoveries. This paper documents field research conducted by the authors along the Yarlung suture zone in eastern Xigaze(Shigatse, Rikaze)County, ~250 km west of the city of Lhasa, in July 2017. The goal of this research was to map the Suture Zone structure in detail, and more specifically to understand the branching relationships between two major fault systems-the Great Counter Thrust and Gangdese Thrust. A summary of early geological exploration is included to provide context for this research.     

山东省莱州-安丘铁成矿带成矿规律探析

成矿带内主要赋矿层位为古元古代粉子山群和荆山群变质地层,铁矿成因可归纳3种成因类型,即层控沉积变质型、受断裂构造控制的高中温热液型和岩浆熔离型。沉积变质铁矿层控特征明显,其赋存层位为古元古代粉子山群小宋组;热液型铁矿严格受断裂构造、层间构造所控制,不论在垂向或水平方向上与蛇纹岩或磁铁蛇纹岩均呈过渡关系,矿体多呈形态复杂的透镜状产于古元古代粉子山群小宋组和荆山群野头组祥山段;岩浆熔离型铁矿位于昌邑大店断裂与平度断裂相接部位,成矿母岩为古元古代莱州岩套西水夼细粒变辉长岩,矿体赋存于古元古代荆山群野头组中。     

Relationship between Thermal Metamorphism of Coal and the Periodic Variation Law of Geochemical Behavior of Elements: A Study on the C2 Coal Seam in the Fengfeng Mining Area of the Handan Coalfield in Hebei,China

This study highlights the response of the periodic variation of the geochemical behavior of elements to the thermal metamorphism of coal by considering the differentiation mode and differentiation degree of elements of the C2 coal seam in the Fengfeng mining area of the Handan Coalfield in Hebei, China. The periodic variation of the geochemical behavior of elements was observed to change towards a certain direction as the degree of metamorphism of a geological body increased. Based on the coincidence degree (or similarity degree) between the geochemical behavior of elements and periodic variation of elements, the 57 elements in this study were divided into two levels. The periodic variation of the geochemical behavior of first-level elements was largely synchronous with that of their first ionization energy, suggesting that only one differentiation mode exists and the elements are mainly inorganically associated. The differentiation mode of the second-level elements deviated to a certain extent from their first ionization energy; the larger the deviation, the more complicated and diverse was the differentiation mode. Among the second-level elements, the grade of coal metamorphism has evident and intuitive effects on the proportion of elements with particular structural features, such as the 4q+3 type of elements and the odd-odd elements. In addition, the distribution of elements in organic and inorganic matter within coal are subject to the structural features of the elements. The differentiation mode and differentiation intensity of elements could be characterized by the hierarchical parameter and differentiation intensity. The hierarchical parameter and differentiation intensity of certain elements showed a good positive or negative correlation with Rmax in coal. The 57 elements in this study were quantitatively ordered according to the degree of magmatic hydrothermal fluid influence and thermal metamorphism of coal through graphs depicting the goodness of fit, correlation coefficient with Rmax, and differentiation intensity. The results of this study are consistent with the results of previous field research, illustrating the scientific significance and application value of this study on the periodic variation of the geochemical behavior of elements.     

Cambrian metamorphism of the Qaidam block:Constraints from phase equilibrium modeling and monazite U-Pb datingSCIEI北大核心CSCD

柴达木地块位于青藏高原东北缘的祁连-阿尔金-昆仑早古生代造山系之中,它的西段出露一套(超)高温变质岩组合:变泥质岩、长英质片麻岩、基性麻粒岩、钙硅酸盐岩、含橄榄石大理岩及少量Mg-Al麻粒岩。本文以相平衡模拟和独居石U-Pb年代学为主要手段,限定柴西缘变泥质岩的变质作用P-T-t轨迹。变泥质岩记录了顺时针P-T轨迹,其中,压力峰期条件约为0.89GPa和800℃,温度峰期条件约为0.64GPa和825℃,退变质条件为0.58GPa和800℃至0.37±0.05GPa和702±50℃。变泥质岩的独居石U-Pb年龄集中在517~496Ma之间,三个样品的加权平均年龄分别为508±2Ma(MSWD=2.0)、506±1Ma(MSWD=1.7)、506±1Ma(MSWD=1.3)。本文将其解释为独居石的形成年龄,并认为独居石主要形成于退变质过程中的残留熔体结晶阶段(0.58GPa和800℃左右)。结合已有的工作可以得出,柴达木地块西段麻粒岩相变质作用具有顺时针P-T轨迹和1000~1300℃/GPa的峰期T/P值,并且在高温变质条件(>800℃)持续了超过30Myr。该类高T/P型变质作用最有可能发生在大型碰撞造山带内,可以与冈瓦纳大陆内部的晚泛非期的高T/P型变质作用对比,很可能是与冈瓦纳大陆最终拼合有关的晚泛非期造山事件的体现。     

Unravelling the protracted U-Pb zircon geochronological record of high to ultrahigh temperature metamorphic rocks: Implications for provenance investigations

The assessment of detrital zircon age records is a key method in basin analysis, but it is prone to several biases that may compromise accurate sedimentary provenance investigations. High to ultrahigh temperature (HT-UHT) metamorphism (especially if T > 850 °C) is herein presented as a natural cause of bias in provenance studies based on U-Pb detrital zircon ages, since zircon from rocks submitted to these extreme and often prolonged conditions frequently yield protracted, apparently concordant, geochronological records. Such age spreading can result from disturbance of the primary U-Pb zircon system, likewise from (re)crystallization processes during multiple and/or prolonged metamorphic events. In this contribution, available geochronological data on Archean, Neoproterozoic and Palaeozoic HT-UHT metamorphic rocks, acquired by different techniques (SIMS and LA-ICP-MS) and showing distinct compositions, are reassessed to demonstrate HT-UHT metamorphism may result in modes and age distributions of unclear geological meaning. As a consequence, it may induce misinterpretations on U-Pb detrital zircon provenance analyses, particularly in sedimentary rocks metamorphosed under such extreme temperature conditions. To evaluate the presence of HT-UHT metamorphism-related bias in the detrital zircon record, we suggest a workflow for data acquisition and interpretation, combining a multi-proxy approach with: (i) in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources, and (ii) the integration of a petrochronological investigation to typify fingerprints of the HT-UHT metamorphic event. The proposed workflow is validated in the investigation of one theoretical and one natural example allowing a better characterization of the sedimentary sources, maximum depositional ages, and the tectonic setting of the basin. Our workflow allows to the appraisal of biases imposed by HT-UHT metamorphism and resulting disturbances in the U-Pb detrital zircon record, particularly for sedimentary rocks that underwent HT-UHT metamorphism and, finally, suggests ways to overcome these issues.     

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block,southern India: From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane(SGT)of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly.This block is composed of three sub-blocks:the Neoarchean Northern Madurai block,Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block.The margins of these blocks are well-known for the occurrence of ultrahigh-temperature(UHT)granulite facies rocks mostly represented by Mg-Al metasediments.Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block.The major rock types of the layered intrusion include spinel orthopyroxenite,garnet-bearing gabbro,gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture.The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel,cordierite and orthopyroxene.The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions.Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980℃.We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases,which yield temperature above 1000℃.FMAS petrogenetic grid,Al-in-orthopyroxene isopleth,conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature(P–T)path and near isothermal decompression.The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca.2.0 Ga and the metamorphic overgrowths yield weighted ²⁰⁶Pb/²³⁸U mean ages ca.520 Ma.Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded ²⁰⁶Pb/²³⁸U weighted mean ages of ca.532 Ma and 523 Ma marking the timing of metamorphism.We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting,defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks.The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.     

Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution

The Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.     

北山青白口纪红山铁矿床成因

北山青白口纪晚期含铁层系的沉积海盆长度大于200km,宽大于50km。已经发现多处大型沉积变质磁铁矿床;其中,红山铁矿床,已提交铁矿石储量1.6亿吨;天湖铁矿床,铁矿石储量1.3亿吨;杨岭铁矿床,铁矿石远景资源量1亿吨左右。北山青白口系上部皆有中基性到中性火山岩,并都有与火山活动成因相关的沉积变质铁矿存在,这反映了青白口纪是既有稳定的以碳酸盐沉积为主的滨浅环境又有半活动到活动的碎屑岩夹火山岩的半深海环境。（1）红山铁矿床铁矿石中含有大量灰紫色碧玉团块和硅质条带,通过硅质岩的地球化学特征研究发现,该矿区硅质岩为海底热水沉积物,代表了呈凝胶体沉积在海底上的热液喷发的二氧化硅相和铁。（2）通过研究围岩蚀变类型发现其是在含铁高温热卤水在喷流通道涌出时,把成矿前的矿层底板沉积岩系进行岩水反应。本矿区的围岩蚀变作用是一种海底变质作用。（3）红山铁矿床沉积环境由北东到南西依次为滨浅海沉积环境、浅海深水环境以及半深海海底喷发环境;（4）研究认为红山铁矿为与火山活动有关的沉积变质型铁矿床。     

High-temperature–low-pressure metamorphism and the production of S-type granites of the Hillgrove Supersuite,southern New England Orogen,NSW, Australia

The high-temperature–low-pressure Wongwibinda Metamorphic Complex of the southern New England Orogen is bound by S-type granite plutons of the Hillgrove Supersuite to the north, east and south. New U–Pb geochronology of five samples of the Hillgrove Supersuite demonstrates that plutonism in the complex involved two pulses: ca 300 Ma and ca 292 Ma. This indicates that plutonism partially overlaps the age of high-T–low-P metamorphism (296.8 ± 1.5 Ma), but also postdates it. Zircon grains identified as xenocrysts based on age (≥310 Ma) have U–Pb–Hf isotopic character that largely overlaps detrital grains in the host Girrakool Beds, indicating that accretionary complex crust is the likely source of these xenocrysts. The ¹⁷⁶Hf/¹⁷⁷Hf initial character for zircon for the ca 300 Ma plutons (three samples) is less radiogenic than those in the ca 292 Ma plutons (two samples). The progression in ¹⁷⁶Hf/¹⁷⁷Hf initial character for zircon infers an increasing mantle component in the Hillgrove Supersuite with time. These data are evidence of a rift tectonic setting, where mantle-derived magmas are predicted to more readily migrate to shallower crustal levels as the crust thins and becomes hotter. Additionally, early episodes of partial melting in the system melt-depleted the metasedimentary sources, thus reducing the S-type component as anatexis progressed. The evolution of the Hillgrove Supersuite coincides with a period of early Permian slab roll back and extension accompanied by crustal rifting and thinning, leading to high-T–low-P metamorphism, anatexis and S-type granite production and the development of rift basins such as the Sydney–Gunnedah–Bowen system.     

冀东地区新太古代麻粒岩相变质作用及其大地构造意义

冀东地区的早前寒武纪基底保留有太古宙克拉通普遍发育的"穹窿-龙骨构造",如卢龙-双山子表壳岩系呈近南北向带状分布于以TTG片麻岩为主构成的太平寨卵形构造域、迁安片麻岩穹窿和安子岭片麻岩穹窿之间,洒河桥线性构造带呈北东向切割太古宙构造线分布。在太平寨卵形域和洒河桥线性带中常见有零散分布的麻粒岩相表壳岩块体和古元古代变质基性岩墙。表壳岩块体包括基性和泥砂质麻粒岩和BIF型铁矿等,变质基性岩墙也发育高压麻粒岩相组合,变质锆石年龄为～1.81Ga。本文总结了近年来对该区麻粒岩的研究进展。太平寨卵形域中的基性麻粒岩以中粒二辉麻粒岩为主,有些样品的角闪石周围出现叠加变质形成的微粒矿物组合,个别样品见辉石周围发育石榴石冠状体,形成‘红眼圈’结构;利用稀土元素温度计确定二辉麻粒岩的峰期达到了～1000℃的超高温条件,麻粒岩中的锆石仅记录新太古代末期变质年龄(～2.50Ga),与周围TTG质岩石的结晶时间近于相同。洒河桥线性带中的基性麻粒岩以细粒高压麻粒岩组合为主,偶见中粒二辉麻粒岩残留,其峰期P-T条件分别为800～860℃/1.0～1.2GPa和950～1070℃/1.0GPa;麻粒岩中锆石主体记录新太古代末期变质年龄,但出现少量古元古代变质锆石(1.97～1.83Ga),石榴石-全岩Lu-Hf等时线年龄为1.77～1.78Ga。由此推测太平寨和洒河桥地区都经历了新太古代末期超高温麻粒岩相变质作用,又在古元古代晚期遭受了高压麻粒岩相差异性叠加,太平寨地区受叠加较弱,仍然保留太古宙的卵形构造,而洒河桥一带受叠加较强,形成了线性变形带。太平寨卵形域的泥砂质麻粒岩可识别出4阶段的变质矿物组合:包括包体组合、峰期组合、固相线(或最后)组合以及叠加组合。相平衡模拟表明固相线组合的P-T条件为870～890℃/～0.7GPa,峰期组合可达到1000℃/1.1GPa,石榴石中富钙斜长石包体指示麻粒岩早期经历了低压高温变质阶段。由此构建麻粒岩P-T轨迹为逆时针型,包括3个阶段:低压加热至超高温(AG-I),近等温升压至压力峰期(～1.1GPa)(AG-II),和峰后降压降温至固相线(AG-IIIa)以及在亚固相线下的降温降压过程(AG-IIIb)。锆石定年表明泥砂质麻粒岩原岩沉积年龄稍早于2.50Ga,变质年龄为2.48～2.50Ga。泥砂质麻粒岩的峰期变质条件和时代均与二辉麻粒岩一致,叠加组合的P-T条件与高压麻粒岩相似,反映泥砂质麻粒岩也受到了古元古代晚期变质作用影响。依据太平寨麻粒岩的逆时针型P-T轨迹,推测麻粒岩相变质作用包括如下构造过程:(i)AG-I指示表壳岩层受到后续地幔极高温岩浆喷发被埋深加热,或者受到下部TTG质岩浆海的加热过程;(ii)AG-II指示被加热的岩石(总伴有BIF铁矿层)被破碎并在在密度驱动下沉入岩浆海深部,达到下地壳深度;(iii)AG-IIIa/b对应沉入岩浆海深部的岩石伴随穹窿上升发生减压冷却的过程。很多地质观测和数值模拟研究表明太古宙克拉通的形成受太古宙特有的垂直构造体制控制,与太古代之后线性造山带的构造体制完全不同。