北祁连干沙鄂博稀土元素矿床地质和矿床地球化学特征

干沙鄂博稀土元素矿床位于北祁连造山带中段,是该地区唯一大型稀土元素矿床。矿体产于早泥盆世碱性岩体及其接触带中,围岩蚀变表现为霓长岩化、碳酸岩化和萤石化,矿石类型主要为霓辉正长岩型和霓辉正长斑岩型,矿物组合为氟碳钙铈矿+黄铜矿+霓辉石+钠长石+钠闪石+萤石+方解石。通过详细的岩相学和矿相学观察,划分出4个成矿期,包括岩浆成矿期(Ⅰ)、早期热液成矿期(Ⅱ)、晚期热液成矿期(Ⅲ)和表生成矿期(Ⅳ)。其中,早期热液成矿期为主成矿期,包括3个成矿阶段:霓石-钠长石-氟碳钙铈矿阶段(Ⅱ_1)、钠闪石-氟碳钙铈矿阶段(Ⅱ_2)、霓石-方解石-氟碳钙铈矿阶段(Ⅱ_3)。脉石矿物石英的δ~(18)O值为7.3‰～11.3‰,δD值为-89.4‰～-68.8‰,δ~(13)C_(V-PDB)值为-10.5‰～-7.9‰,表明成矿流体以岩浆热液为主,有大气降水的参与。矿石中硫化物δ~(34)S_(V-CDT)值介于-7.1‰～-1.3‰之间,平均-4.4‰,总硫同位素δ~(34)S_(ΣS)为-3.9‰;矿石硫化物铅同位素组成基本一致,~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Pb/~(204)Pb比值分别为18.417～18.524、15.637～15.769和38.425～38.863,与矿区早泥盆世含矿碱性岩体岩石铅同位素组成十分相似,推测成矿物质主要来源于该碱性岩体。     

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.     

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.     

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。     

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.     

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.     

东天山马庄山金矿区赋矿石英斑岩的岩石成因和构造背景:元素地球化学、U-Pb年代学和Sr-Nd-Hf同位素约束

马庄山金矿位于东天山东段,矿区出露有大量的石英斑岩,总体上呈北东走向,沿矿区中部断裂侵入。石英斑岩呈灰白色,具斑状结构,斑晶主要为石英,少量正长石和斜长石,基质为长英质和少量暗色矿物。石英斑岩的Na_2O+K_2O含量为3. 55%～9. 67%,Al_2O_3含量(10. 37%～14. 28%),K_2O/Na_2O比值为2. 57～66. 5,显示富碱高钾、过铝质特征;稀土含量中等(65×10~(-6)～161×10~(-6)),轻重稀土分异中等([La/Yb]N=6. 15～12. 5),相对富集轻稀土元素、具弱的负Eu异常。微量元素总体富集Rb、K、Th、U和Pb,亏损Sr、Ba、P、Nb和Ti等元素,与活动大陆边缘弧火成岩的微量元素特征一致。马庄山石英斑岩的SiO_2与P_2O_5呈现负相关关系,Y、Th与Rb均呈现正相关的关系,说明研究区的次火山岩属于I型花岗岩类。对马庄山石英斑岩内岩浆锆石进行LA-ICP-MS U-Pb定年,获得一致曲线年龄为315. 4±0. 6Ma(MSWD=0. 67),加权平均年龄为316. 0±2. 0Ma(MSWD=0. 23),将矿区内以石英斑岩为主的次火山岩侵位时间限定在314～318Ma。石英斑岩具有较高的(87Sr/86Sr)i值(0. 7077～0. 7102),较低的εNd(t)值(-1. 62～1. 82),Nd的二阶段模式年龄(tDM2)为0. 9～1. 2Ga,εHf(t)值为-3. 2～0. 4,tDM2在1. 28～1. 51Ga之间,暗示岩石来自于下地壳物质重熔,可能有部分地幔物质加入。综合石英斑岩的地质、地球化学和年代学特征,认为东天山东段,在晚石炭世,马庄山地区处于与俯冲相关的活动陆缘弧环境。     

内蒙古太平沟钼矿床流体包裹体特征及成矿动力学背景

太平沟斑岩型钼矿床位于大兴安岭北段,处于东西向古生代古亚洲构造成矿域和北北东向中新生代滨西太平洋构造成矿域的转换、叠合部位.该矿床形成于131.5～127.5Ma,是大陆内部构造-岩浆活化的产物,成矿系统发育于早白垩世初期华北及中国东部构造体制由挤压向伸展转变的背景下.太平沟钼矿的流体成矿过程包括早、中、晚3个阶段,分别以石英-黄铜矿-黄铁矿、石英-黄铁矿-辉钼矿和石英-碳酸盐组合为特征.石英中发育水溶液型、含/富CO_2型和含子矿物型流体包裹体,子矿物包括石盐、黄铜矿等,但晚阶段仅发育水溶液型包裹体.成矿早、中、晚各阶段流体包裹体的均一温度分别集中在320～390℃、240～320℃和140～200℃,盐度变化为从＞66.8 wt%NaCl eqv.、2.4 wt%～33.8 wt% NaCl eqv.到＜10 wt%NaCl eqv..流体系统由早阶段的高温、高盐度、富CO_2的岩浆热液,经流体沸腾、CO_2逸失、温度降低等过程导致大量金属硫化物沉淀,演化为晚阶段低温、低盐度、贫CO_2的大气降水热液.     

Ore fluid geochemistry of the Jinlongshan Carlintype gold ore belt in Shaanxi Province, China

THEJINLONGSHANGOLDOREBELTINZHEN’ANCOUN TY,SOUTHERNSHAANXIPROVINCE,ISLOCATEDINTHEWEST ERNQINLINGGOLDPROVINCE(NO.16INFIG.1;CHEN YANJINGETAL.,2004).ITWASDISCOVEREDINTHEDEVO NIANSTRATAINTHELATE1980S).ITSGEOLOGICALSETTING ANDMETALLOGENICEVOLUTIONARESIMILARTOT…