西天山造山带构造单元划分及古生代洋陆转换过程

西天山造山带位于哈萨克斯坦—准噶尔板块与卡拉库姆—塔里木板块的结合部,是由一系列前寒武纪微陆块、古生代洋壳残片及陆缘弧相互拼贴而成的多聚合带、多成矿带,其独特的造山-成矿过程受到了国内外的广泛关注。本文通过构造单元划分与编图,建立了古生代西天山造山带的构造格架,认为古生代西天山造山带的构造演化依次经历了:罗迪尼亚大陆裂解与北天山早古生代多岛洋盆形成阶段(Z-O_2),北天山早古生代多岛洋盆闭合与南天山洋盆开始形成阶段(O_3-S),南、北天山洋晚古生代洋盆形成与发展阶段(D-C_1),南、北天山晚古生代洋盆全面闭合与天山碰撞造山带形成阶段(C1-C_2)和碰撞后板内演化阶段(C_2-P)。     

Zircon U–Pb geochronological,trace element,and Hf isotopic constraints on the genesis of the Fe and Cu skarn deposits in the Qiman Tagh area,Qinghai Province,Eastern Kunlun Orogen,China

Fe and Cu skarn deposits are very important skarn types worldwide, but it is currently unclear whether the nature of intrusions related to Fe and Cu skarn deposits exerts a key influence on variations in metal associations between Fe and Cu skarn deposits. The Qiman Tagh area of Qinghai Province (QTQP), located in the Eastern Kunlun Orogen (EKO), provides a good opportunity to address this issue. Here, integrating new zircon U–Pb ages, trace elements and Hf isotopes from this study with published data, we constrain the sources of magma associated with Fe and Cu skarn deposits within the QTQP and discuss their role in controlling differences between Fe and Cu skarn deposits.Combined with published data, two discrete suites of the intrusions associated Fe and Cu skarn deposits have been recognized in the QTQP: (1) 245.1 ± 1.5 Ma granodiorite (related to a 245.5 ± 1.6 Ma Cu skarn deposit) has zircon εHf(t) values of −11.9 to −2.1; (2) 235–224 Ma monzonites, quartz monzonites, granodiorite porphyries, monzogranites, and granites associated with 234–225 Ma Fe skarn deposits are characterized by relatively high zircon εHf(t) values (−5.1 to +5.9). The Sr–Nd–Hf isotopic data suggest that the intrusions of Suite 1 and 2 were dominantly derived by partial melting of a Mesoproterozoic juvenile mafic lower crust. Suite 2 intrusions associated with Fe skarn deposits have more mantle components in their magma sources than rocks of Suite 1 that are related to a Cu skarn deposit. Furthermore, zircon εHf(t) values of intrusions associated with Fe and Cu skarn deposits in the QTQP show a negative correlation between mantle components in the magma sources and the contents of Cu and Zn in these deposits. Zircon trace elements indicate that the intrusions associated with Fe skarn deposits are relatively less oxidized than the rock associated with Cu skarn deposit in the QTQP, reflecting a positive correlation between crustal components in the magma sources and oxygen fugacity of the magmas. This indicates that different proportions of mantle and crustal materials in the magma sources may affect oxygen fugacity and Fe contents of the magmas, which possibly leads to the variations in metal associations between Fe and Cu skarn deposits in the QTQP. Zircon U–Pb ages, trace elements and Hf isotopic compositions, combined with geological, geochronological, and geochemical evidence, indicates that having different proportions of mantle components in the magma sources of intrusions associated with Fe and Cu skarn deposits is one of the most critical factors controlling differences in metal association between Fe and Cu skarn deposits.     

华北克拉通中部造山带中条山地区绛县群地层划分与构造背景

中条山地区是华北克拉通中部造山带的重要组成部分,保存了中部造山带演化的重要记录。绛县群划分为横岭关亚群和铜矿峪亚群,横岭关亚群为一套由石英岩和绢云片岩组成的变质碎屑岩;铜矿峪亚群则以酸性、基性火山岩为主,夹有正常沉积碎屑岩的“双峰式”火山岩。本文对绛县群岩石组合及地层特征开展详细调查研究,经过对绛县群火山岩、侵入岩和碎屑锆石同位素年龄的综合分析,认为绛县群的形成年龄大约在2200~ 2120Ma之间,时代为古元古代。结合碎屑岩、火山岩的地球化学特征和华北克拉通中部造山带的演化过程,推测绛县群形成的构造环境很可能为古活动大陆边缘。其中,横岭关亚群沉积在活动大陆边缘盆地,而铜矿峪亚群则形成于活动大陆边缘火山弧环境。     

勉略构造带庄科岩片洋壳型变玄武岩锆石U-Pb年龄和Lu-Hf同位素特征:新元古代勉略洋盆存在的直接证据

庄科岩片位于南秦岭南缘勉略构造带.前人系统的岩石地球化学研究表明,庄科岩片变玄武岩属拉斑玄武岩系列,整体表现出N-MORB特征,为勉略洋盆残余,但一直未获得高精度的测年数据.本文首次获得庄科岩片洋壳型变玄武岩LA-ICP-MS锆石U-Pb年龄为999±4Ma(MSWD=0.08,n=25)、991±6Ma(MSWD=1...     

西秦岭中段日多玛岩体锆石U-Pb年龄、地球化学特征及其构造意义

日多玛花岗闪长岩体位于西秦岭中段美武岩体附近,对该岩体进行了岩相学、锆石U-Pb年代学和岩石地球化学方面的研究。日多玛花岗闪长岩体含有较丰富的暗色微粒包体,花岗闪长岩和暗色微粒包体LA-ICP-MS锆石U-Pb年龄分别为(236.8±3.6) Ma(MSWD=4.1)和(242.7±1.6) Ma(MSWD=1.4),属于印支早期。日多玛岩体具有富钾(2.58%~2.75%)、富碱(Na₂O+K₂O=5.57%~5.76%)、弱过铝质(A/CNK=1.48~1.51)特征,属于高钾钙碱性I型花岗岩类。日多玛岩体稀土元素表现为轻重稀土元素分馏较明显(LREE/HREE=10.4~11.9)、呈右倾特征,具有中等Eu负异常(δEu=0.68~0.81),岩石富集K、Rb、Ba、Th和U等大离子亲石元素,明显亏损Nb、Ta、Ti和P等高场强元素。岩石地球化学特征表明,日多玛花岗闪长岩来源于下地壳高钾玄武质岩石的部分熔融。此外,花岗闪长岩体具有较高的Mg^#(57~61)、Cr(149×10^-6~185×10^-6)和Ni(36×10^-6~47×10^-6),显示有少量幔源物质加入,暗色微粒包体可能代表了这种幔源岩浆。结合区域地质背景分析,认为日多玛花岗闪长岩体形成于后碰撞构造环境,可能与俯冲洋壳的断离作用有关。     

Three-dimensional P-wave Velocity Structure Modelling of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Crustal Architecture and Metallogenic Implications

In this study, we compiled and analyzed 69310 P-wave travel-time data from 6639 earthquake events. These events (M ≥ 2.0) occurred from 1980s to June 2019 and were recorded at 319 seismic stations (Chinese Earthquake Networks Center) in the study area. We adopted the double-difference seismic tomographic method (tomoDD) to invert the 3-D P-wave velocity structure and constrain the crust-upper mantle architecture of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt (MLYB). A 1-D initial model extracted from wide-angle seismic profiles was used in the seismic tomography, which greatly reduced the inversion residual. Our results indicate that reliable velocity structure of the uppermost mantle can be obtained when Pn is involved in the tomography. Our results show that: (1) the pattern of the uppermost mantle velocity structure corresponds well with the geological partitioning: a nearly E–W-trending low-velocity zone is present beneath the Dabie Orogen, in contrast to the mainly NE-trending low-velocity anomalies beneath the Jiangnan Orogen. They suggest the presence of thickened lower crust beneath the orogens in the study area. In contrast, the Yangtze and Cathaysia blocks are characterized by relatively high-velocity anomalies; (2) both the ultra-high-pressure (UHP) metamorphic rocks in the Dabie Orogen and the low-pressure metamorphic rocks in the Zhangbaling dome are characterized by high-velocity anomalies. The upper crust in the Dabie Orogen is characterized by a low-velocity belt, sandwiched between two high velocity zones in a horizontal?direction, with discontinuous low-velocity layers in the middle crust. The keel of the Dabie Orogen is mainly preserved beneath its northern section. We infer that the lower crustal delamination may have mainly occurred in the southern Dabie Orogen, which caused the mantle upwelling responsible for the formation of the granitic magmas emplaced in the middle crust as the low-velocity layers observed there. Continuous deep-level compression likely squeezed the granitic magma upward to intrude the upper crustal UHP metamorphic rocks, forming the ‘sandwich’ velocity structure there; (3) high-velocity updoming is widespread in the crust-mantle transition zone beneath the MLYB. From the Anqing-Guichi ore field northeastward to the Luzong, Tongling, Ningwu and Ningzhen orefields, high-velocity anomalies in the crust-mantle transition zone increase rapidly in size and are widely distributed. The updoming also exists in the crust-mantle transition zone beneath the Jiurui and Edongnan orefields, but the high-velocity anomalies are mainly stellate distributed. The updoming high-velocity zone beneath the MLYB generally extends from the crust-mantle transition zone to the middle crust, different from the velocity structure in the upper crust. The upper crust beneath the Early Cretaceous extension-related Luzong and Ningwu volcanic basins is characterized by high velocity zones, in contrast to the low velocity anomalies beneath the Late Jurassic to Early Cretaceous compression-related Tongling ore field. The MLYB may have undergone a compressive-to-extensional transition during the Yanshanian (Jurassic–Cretaceous) period, during which extensive magmatism occurred. The near mantle–crustal boundary updoming was likely caused by asthenospheric underplating at the base of the lower crust. The magmas may have ascended through major crustal faults, undergoing AFC (assimilation and fractional crystallization) processes, became emplaced in the fault-bounded basins or Paleozoic sequences, eventually forming the many Cu-Fe polymetallic deposits there.     

3D model of Svecofennian Accretionary Orogen and Karelia Craton based on geology,reflection seismics,magnetotellurics and density modelling: Geodynamic speculations

A 3D model of deep crustal structure of the Archaean Karelia Craton and late Palaeoproterozoic Svecofennian Accretionary Orogen including the boundary zone is presented. The model is based on the combination of data from geological mapping and reflection seismic studies, along profiles 1-EU, 4B, FIRE-1-2a-2 and FIRE-3-3a, and uses results of magnetotelluric soundings in southern Finland and northern Karelia. A seismogeological model of the crust and crust–mantle boundary is compared with a model of subhorizontal velocity-density layering of the crust. The TTG-type crust of the Palaeoarchaean and Mesoarchaean microcontinents within the Karelia Craton and the Belomorian Province are separated by gently dipping greenstone belts, at least some of which are palaeosutures. The structure of the crust was determined mainly by Palaeoproterozoic tectonism in the intra-continental settings modified by a strong collisional compression at the end of the Palaeoproterozoic. New insights into structure, origin and evolution of the Svecofennian Orogen are provided. The accretionary complex is characterized by inclined tectonic layering: the tectonic sheets, ~15 ​km thick, are composed of volcanic-sedimentary rocks, including electro-conductive graphite-bearing sedimentary rocks, and electro-resistive granitoids, which plunge monotonously and consecutively eastward. Upon reaching the level of the lower crust, the tectonic sheets of the accretionary complex lose their distinct outlines. In the seismic reflection pattern they are replaced by a uniform acoustically translucent medium, where separate sheets can only be traced fragmentarily. The crust–mantle boundary bears a diffuse character: the transition from crust to mantle is recorded by the disappearance of the vaguely drawn boundaries of the tectonic sheets and in the gradual transition of acoustically homogeneous and translucent lower crust into transparent mantle. Under the effect of endogenic heat flow, the accretionary complex underwent high-temperature metamorphism and partial melting. Blurring of the rock contacts, which in the initial state created contrasts of acoustic impedance, was caused by partial melting and mixing of melts. The 3D model is used as a starting point for the evolutionary model of the Svecofennian Accretionary Orogen and for determination of its place in the history of the Palaeoproterozoic Lauro-Russian intracontinental orogeny, which encompassed a predominant part of the territory of Lauroscandia, a palaeocontinent combining North American and East European cratons. The model includes three stages in the evolution of the Lauro-Russian Orogen (~2.5, 2.2–2.1 and 1.95–1.87 ​Ga). The main feature of the Palaeoproterozoic evolution of the accretionary Svecofennian Orogen and Lauroscandia as a whole lay in the causal link with evolution of a superplume, which initiated plate-tectonic events. The Svecofennian–Pre-Labradorian palaeo-ocean originated in the superplume axial zone; the accretionary orogens were formed along both continental margins due to closure of the palaeo-ocean.     

Late Mesozoic magmatism in the East Qinling Orogen,China and its tectonic implications

The Qinling Orogen in Central China records the history of a complex geological evolution and tectonic transition from compression to extension during the Late Mesozoic,with concomitant voluminous granitoids formation.In this study,we present results from petrological,geochemical,zircon U-Pb-Lu-Hf isotopic studies on the Lengshui felsic dykes from Luanchuan region in the East Qinling Orogen.We also compile published geochronological,geochemical,and Hf isotopic data from Luanchuan region and present zircon Hf isotopic contour maps.The newly obtained age data yield two group of ages at～145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes,with the ～145 Ma interpreted as response to the peak of magmatism in the region,and the ～140 Ma as the timing of formation of the felsic dykes.The corresponding Hf isotopic data of the granite porphyries display negativeeHit)values of-16.67 to-4.61,and Hf crustal model ages(T_(DM~C_)of 2255-1490 Ma,indicating magma sourced from the melting of Paleo-to Mesoproterozoic crustal materials.The compiled age data display two major magmatic pulses at 160-130 Ma and 111-108 Ma with magmatic quiescence in between,and the zircon Hf isotopic data display/ε_(Hf)(t)values ranging from-41.9 to 2.1 and T_(DM)~c values of3387-1033 Ma,suggesting mixed crustal and mantle-derived components in the magma source,and correspond to multiple tectonic events during the Late Mesozoic.The Luanchuan granitoids are identified as 1-type granites and most of these are highly fractionated granites,involving magma mixing and mingling and crystal fractionation.The tectonic setting in the region transformed from the Late Jurassic syn-collision setting to Early Cretaceous within-plate setting,with E-W extension in the Early Cretaceous.This extension is correlated with the N-S trending post-collisional extension between the North China Craton and Yangtze Craton as well as the E-W trending back-arc extension triggered by the westward Paleo-Pacific Plate subduction,eventually leading to lithospheric thinning,asthenospheric upwelling,mafic magma underplating,and crustal melting in the East Qinling Orogen.     

华北中部造山带左权变质杂岩ca. 2.5Ga和ca. 1.9Ga变质年龄记录及其地质意义

华北克拉通早前寒武纪基底由多个微陆块组成,其主期拼合时代是困扰地质学家的一个重大课题。华北中部造山带作为新太古代-古元古代一条重要的碰撞型造山带已得到广泛共识。大量高精度年代学资料显示,华北中部造山带至少记录了大约1. 85Ga、1. 95Ga和2. 5Ga的三组变质年龄信息。但目前,2. 5Ga左右的变质年龄仅在华北中部造山带中部的阜平和赞皇等少数杂岩区有零星报道。左权变质杂岩位于华北中部造山带中南段东侧、阜平杂岩以南,向东紧邻赞皇杂岩,是洞悉早前寒武纪时期华北克拉通基底形成及演化过程的一个重要窗口。杂岩区出露多种早寒武纪变质岩石,其中长英质黑云斜长片麻岩分布范围最广,局部暗色矿物富集;斜长角闪岩或角闪片麻岩多以透镜状或似层状方式产出于长英质片麻岩中;杂岩区南部发育多个小型磁铁矿矿床。本文对研究区多种类型岩石样品进行了细致的岩相学、锆石U-Pb年代学和锆石稀土元素研究,发现多数样品中发育变质成因锆石,记录至少两组变质年龄信息。第一组年龄(1903Ma)仅被个别角闪片麻岩样品保存,反映了杂岩区所经历的变质峰期或近峰期阶段的时代;第二组年龄(2483～2507Ma)分布广泛,所代表的变质事件发生在区域片麻理之前,与华北克拉通约25亿年时发生的大规模构造热事件有关。     

From intrabasinal volcanism to far-field tectonics: causes of abrupt shifts in sediment provenance in the Devonian–Carboniferous Drummond Basin,Queensland

The Drummond Basin of central Queensland preserves a large-volume succession of little studied, predominantly fluviatile, coarse-grained sedimentary rocks of mid-Mississippian age. The stratigraphy of the basin has been subdivided into three sedimentary cycles. The Cycle 1/Cycle 2 boundary records a distinct, but poorly understood change in provenance from a volcanic-dominated succession related to initial basin rifting (Cycle 1) to a quartz-rich, craton-derived succession (Cycle 2). Cycle 3 has been thought to mark a resumption of intrabasinal volcanism and related sedimentation. The purpose of this study was to enhance the understanding of the basin-wide siliciclastic sedimentation of Cycles 2 and 3, and causes for the changes in sediment provenance. This objective was achieved by constraining large-scale spatial and temporal depositional trends and investigating sediment transport pathways into and through the basin. Petrographic, QFL, paleocurrent and conglomerate clast analyses were undertaken. The observations presented here have several implications relevant to understanding the stratigraphy of the Drummond Basin and regional tectonic events at this time. Cycle 3 is revised here primarily to be a continuation of Cycle 2-style basement-derived sedimentation, rather than recording a resumption of volcanism in the area, as per prevailing models. Quartz-rich sedimentation in the Drummond Basin was, therefore, more long-lived than previously envisaged, and once established, was not significantly disrupted by volcanism. Cycle 2 formation thicknesses appear highly variable across the basin. This is unlikely to be a result of pre-existing rift-related topography as suggested in previous models. The thickness variations are more likely related to sediment bypassing and post-depositional deformation in the area. The distinctive coarse-grained, relatively quartz-rich sedimentation of Cycles 2 and 3 is unusual in its volume and extent. The sediment was transported into the basin from its southern/southwestern margin, implying long-distance transport and extrabasinal sediment supply. While the specific source terrain(s) remain unknown, one plausible tectonic driver was far-field influence of the intraplate Alice Springs Orogeny.