Late Paleozoic Fluid Systems and Their Ore-forming Effects in the Yuebei Basin,Northern Guangdong,China

Based on detailed and systematic researches of the geology of ore deposits, fluid inclusions and isotope geochemistry etc., and regarding the Late Paleozoic fluid system of the Yuebei Basin as an integrated object in this paper, we have revealed the temporo-spatial evolution law of the basin＇s fluid system and discussed its ore-forming effects by simulating and analyzing the distribution of ore-forming elements, the fluid thermodynamics and dynamics of evolution processes of this basin. The results show that Late Paleozoic ore-forming fluid systems of the Yuebei Basin include four basic types as follows. （1） The sea floor volcanic-exhalation system developed during the rapid basin slip-extension stage in the Mid-Late Devonian, which affected the Dabaoshan region. It thus formed the Dabaoshan-type Cu-Pb-Zn-Fe sea floor volcanic-exhalation sedimentary deposits. （2） The compaction fluid system developed during the stable spreading and thermal subsidence-compression stage of the basin in the Mid-Late Devonian. The range of its effects extended all over the whole basin. It resulted in filling-metasomatic deposits, such as the Hongyan-type pyrite deposits and pyrite sheet within the Fankou-type Cu-Pb-Zn-S deposits. （3） The hot water circulation system of sea floor developed during the stage of basin uplifting and micro-aulacogen from the late Late Carboniferous to Middle Carboniferous. The range of its effects covered the Fankou region. It thus formed MVT deposits, such as the main orebody of the Fankou-type Pb-Zn-S deposits. （4） The gravity fluid system developed during the stage of fold uplifting and the basin closed from Middle Triassic to Jurassic, forming groundwater hydrothermal deposits, e.g. the veinlet Pb-Zn-calcite orebodies of the Fankou-type Pb-Zn- S deposits. Migration and concentration of the ore-forming fluids were constrained by the state of temporo-spatial distribution of its fluid potential. Growth faults not only converged the fluids and drove them to move upwards, but also the fluids often crossed the faults to the edges of the basin at the bottom of these faults and the lithologic interfaces, and even migrated to the basin＇s edges from top to bottom along the faults, which may be one of the basic reasons for the stratabound deposits to cluster mainly along the contemporaneous faults on the inner border of the basin. The superposed mineralization resulting from the multi-stage activity of contemporaneous faults and ore-forming fluid systems in the basin may be one of the key factors for forming superlarge ore deposits.

Tectonic Framework of Late Paleozoic Intrusions in Xingxingxia： Implications for Final Closure of South Tianshan Ocean in East Tianshan

This work carried out systematic geological field investigation, petrography observation, zircon geochronology and whole rock geochemistry on Late Paleozoic intrusions in the Xingxingxia region near the Xinjiang-Gansu provincial boundary, western China, aiming to constrain the Late Paleozoic tectonic framework of the Xingxingxia region and the final closure time of South Tianshan Ocean in the East Tianshan. The Xingxingxia area is located in the east part of the Tianshan orogen, and adjacent to the north of the Tarim Basin. The Late Paleozoic magma activities in the Xingxingxia region can be mainly divided into three stages. The first stage includes intrusive magma activities under a collision setting between Late Ordovician to the Late Devonian. The second stage is intrusive magma activities under a subduction setting during（304±3）–（278±3） Ma, and the third stage involves intrusive magma activities under a collision and post-collision setting during（268±5）–（259.9±2.6） Ma. The final suture zone of South Tianshan Ocean should be between the Central Tianshan Block and South Tianshan accretionary complex. Based on previous work, both the first stage magma activities（i.e., intrusive magmatic activities between the Late Ordovician to Late Devonian） and the Hongliuhe ophiolitic complex indicate a close event between Central Tianshan Block and South Tianshan Accretionary Complex. The 304±3 Ma dioritic metamorphic gneiss of the XingX ingxia complex and the 278±3 Ma diorite are all island arc calc-alkaline rocks, the 289±3 Ma gabbro is island arc tholeiitic gabbro formed by magma from metasomatic enrichment mantle. All these results indicate that the second stage of magmatic activities is under a subduction setting. The third stage magma activities i.e. the granitic magma activities of（268±5）–（259.9±2.6） Ma occurred at a transitional setting from compressional to post-collision extensional tectonic setting. Thus, around（268±5）–（260±3） Ma, the final closure of the South Tianshan Ocean occurred and the Tianshan orogen shifted into the intracontinental evolution stage. During and after the closure process, a wide range of metamorphism and large dextral strike-slip faults developed.

End Late Paleozoic tectonic stress field in the southern edge of Junggar Basin

This paper presents the end Late Paleozoic tectonic stress field in the southern edge of Junggar Basin by interpreting stress-response structures （dykes, folds, faults with slickenside and conjugate joints）. The direction of the maximum principal stress axes is interpreted to be NW-SE （about 325°）, and the accommodated motion among plates is assigned as the driving force of this tectonic stress field. The average value of the stress index Rt is about 2.09, which indicates a variation from strike-slip to compressive tectonic stress regime in the study area during the end Late Paleozoic period. The reconstruction of the tectonic field in the southern edge of Junggar Basin provides insights into the tectonic deformation processes around the southern Junggar Basin and contributes to the further understanding of basin evolution and tectonic settings during the culmination of the Paleo- zoic.

Late Paleozoic Sequence-Stratigraphic Frameworks and Sea Level Changes in Dianqiangui Basin and Its Adjacent Areas with Systematic Revision of Regional Unconformities

The formation process of the Dianqiangui basin, a special basin, occurred after the Caledonian orogeny, in the south of Guizhou, the west of Guangxi and the southeast of Yunnan, experienced three periods: it began in the Devonian, persisted in the Carboniferous, and became fiercer in the Permian. Controlled by syndepositional fault-zones, varieties of isolated carbonate platforms, large and small, were developed in the background of a deep-water basin, namely, an inter-platform ditch. And a special paleogeographical Late Paleozoic pattern marked by “platform-basin-hill-trough” was produced in both the Dianqiangui basin and its adjacent areas. Affected by regional tectonic activities and the global changes in the sea level, the platform carbonates and coal measures superimposed each other cyclically on the attached platform. The reef-building on the isolated platform and the margin of the attached platform corresponds to the development of the shale succession in the deep-water basin. All of these elementary characteristics reflect a regular and sophisticated filling succession of the Dianqiangui basin, a result of the dual controls of the regionally tectonic activities and the eustacy. Based on the two elementary features of the third-order sequences, i.e. the regularity of sedimentary-facies succession in space and the simultaneity of environmental changes in time, 25 third-order sequences could be discerned in the Upper Paleozoic strata in the Dianqiangui basin and its adjacent areas. On the basis of the two kinds of facies-changing surfaces and the two kinds of diachronisms in stratigraphic records, the regional Late Paleozoic sequence-stratigraphic framework in the Dianqiangui basin and its adjacent areas can be established. There are two types of facies-changing surfaces and two types of diachronisms in stratigraphic records: the static type, a result of the change in sedimentary facies in space, and the dynamic type, a result of the change in time. These two types of facies-changing surfaces led to the generation of the two types of diachronisms: the diachronism of facies-changing surfaces that was formed by the static facies-changing surfaces, and the diachronism of punctuated surfaces that was formed by the dynamic facies-changing surfaces. The two types of facies-changing surfaces and the two types of diachronisms in stratigraphic records are the key to the establishment of the sequence-stratigraphic framework. The sequence boundaries could be divided geologically into four types: tectonic unconformity, sedimentary unconformity, drowned unconformity and their correlative surfaces. All of these four types can be further grouped into exposed punctuated surfaces and deepened punctuated surfaces. The tectonic unconformity is similar to Type Ⅰ sequence boundary, and the sedimentary unconformity is similar to Type Ⅱ sequence boundary defined by Vail et al.. In terms of sequence stratigraphy, the tectonic unconformities of the Ziyun movement, the Qiangui epeirogeny and the Dongwu revolution as well as the drowned unconformity in the transitional period from the Permian to the Triassic can be systematically defined and their geological characteristics are briefly presented.

Plate tectonics in the late Paleozoic

As the chronicle of plate motions through time, paleogeography is fundamental to our understanding of plate tectonics and its role in shaping the geology of the present-day. To properly appreciate the history of tectonics--and its influence on the deep Earth and climate-it is imperative to seek an accurate and global model of paleogeography. However, owing to the incessant loss of oceanic lithosphere through subduction, the paleogeographic reconstruction of ＇full-plates＇ （including oceanic lithosphere） becomes increasingly challenging with age. Prior to 150 Ma ~60% of the lithosphere is missing and re- constructions are developed without explicit regard for oceanic lithosphere or plate tectonic principles; in effect, reflecting the earlier mobilistic paradigm of continental drift. Although these ＇continental＇ re- constructions have been immensely useful, the next-generation of mantle models requires global plate kinematic descriptions with full-plate reconstructions. Moreover, in disregarding （or only loosely applying） plate tectonic rules, continental reconstructions fail to take advantage of a wealth of additional information in the form of practical constraints. Following a series of new developments, both in geo- dynamic theory and analytical tools, it is now feasible to construct full-plate models that lend themselves to testing by the wider Earth-science community. Such a model is presented here for the late Paleozoic （410-250 Ma） together with a review of the underlying data. Although we expect this model to be particularly useful for numerical mantle modeling, we hope that it will also serve as a general framework for understanding late Paleozoic tectonics, one on which future improvements can be built and further tested.

Chronological Constraints on Late Paleozoic Collision in the Southwest Tianshan Orogenic Belt, China: Evidence from the Baleigong Granites

The Baleigong granites, located in the western part of the southwestern Tianshan Orogen(Kokshanyan region, China), records late Paleozoic magmatism during the late stages of convergence between the Tarim Block and the Central Tianshan Arc Terrane. We performed a detailed geochronological and geochemical study of the Baleigong granites to better constrain the nature of collisional processes in the Southwest Tianshan Orogen. The LA-ICP-MS U-Pb zircon isotopic analyses indicate that magmatism commenced in the early Permian(~282 Ma). The granite samples, which are characterized by high contents of SiO2(67.68-69.77 wt%) and Al2O3(13.93-14.76 wt%), are alkali-rich and Mg-poor, corresponding to the high-K calc-alkaline series. The aluminum saturation index(A/CNK) ranges from 0.93 to 1.02, indicating a metaluminous to slightly peraluminous composition. Trace element geochemistry shows depletions in Nb, Ta, and Ti, a moderately negative Eu anomaly(δEu=0.40-0.56), enrichment in LREE, and depletion in HREE((La/Yb)N=7.46-11.78). These geochemical signatures are characteristic of an I-type granite generated from partial melting of a magmatic arc. The I-type nature of the Baleigong granites is also supported by the main mafic minerals being Fe-rich calcic hornblende and biotite. We suggest that the high-K, calc-alkaline I-type granitic magmatism was generated by partial melting of the continental crust, possibly triggered by underplating by basaltic magma. These conditions were likely achieved in a collisional tectonic setting, thus supporting the suggestion that closure of the South Tianshan Ocean was completed prior to the Permian and was followed(in the late Paleozoic) by collision between the Tarim Block and the Central Tianshan Arc Terrane.

On Geodynamic Evolution of Simao Region (Southwestern Yunnan, China) during Late Paleozoic and Triassic

Over the last years, the Simao region, southwestern Yunnan, China, turned out to be a very promising target to elucidate plate tectonic processes around the Permian Triassic boundary within the Tethyan domain of Asia. New data from this area reveal that Upper Paleozoic compressional deformations occurred along the Lancangjiang and in areas to the east. Along Lancangjiang, an angular unconformity is exposed, which separates quartz phyllites—formed during a Carboniferous tectono metamorphic event—from Triassic red beds to roofing rhyolites. The acidic volcanics were often said to be remnants of a volcanic arc that was active during the Triassic subduction and closure of an oceanic realm along the Lancangjiang zone. According to our new data, however, these volcanics indicate most probably an Upper Triassic stage of rifting. In the Yunxian anticline (NW of Simao), an angular unconformity of intra Permian age is exposed where Carboniferous to lower Middle Permian strata, which were deposited in a rather deep basin, are unconformably overlain by a shallow marine sequence of upper Middle to Upper Permian sediments. The angular unconformity is of the same age as the syn orogenous sediments first described and dated from the Phetchabun region in Thailand and hence a convincing argument for a Late Variscan orogeny forming an extensive zone of mountain building that can be traced through the central parts of mainland Southeast Asia.

Late Paleozoic to Mesozoic extension in southwestern Fujian Province, South China： Geochemical, geochronological and Hf isotopic constraints from basic-intermediate dykes

The tectonic evolution of SE China block since late Paleozoic remains debated. Here we present a new set of zircon U-Pb geochronological, Lu-Hf isotopic data and whole-rock geochemistry for two stages of basicintermediate dykes from the southwestern Fujian. The samples were collected from the NE-trending （mainly diabases） and NW-trending （mainly diabasic diorites） dykes and yielded zircon U-Pb ages of 315 and 141 Ma, with eHf（t） values of -8.90 to 7.49 and -23.39 to -7.15 （corresponding to TDM2 values of 850 to 1890 Ma and 737 to 2670 Ma）, respectively. Geochemically these rocks are characterized by low TiO2 （0.91-1.73 wt.%） and MgO （3.04-7.96 wt.%）, and high A1203 （12.5-16.60 wt.%） and K20 （0.60-3.63 wt.%）. Further they are enriched in LREEs and LILEs （Rb, Ba, Th and K）, but depleted in HFSEs （Nb, Ta and Zr）. The tectonic discrimination analysis revealed that the dykes were formed in an intraplate extensional envi- ronment. However, the NW trending dykes show crust-mantle mixed composition, which indicate an extensional tectonic setting with evidence for crustal contamination. The SE China block experienced two main stages of extensional tectonics from late Carboniferous to early Cretaceous. The tectonic evolution of the SE China block from late Devonian to Cretaceous is also evaluated.

Geochemistry of Sulfur and Hazardous Elements in Late Paleozoic Coals and Partings from Taozao Coalfield, Shandong Province, China

The analyses of different sulfur forms, the trace elements in pyrites using electron microprobe and the trace elements in coal using INAA (instrumental neutron activation analysis) of the Late Paleozoic coals from the Taozao coalfield in Shandong Province, China, conclude that most sulfur (>75 %) in high-sulfur coal of Taiyuan Formation occurred as pyrite, in which many hazardous elements co-existed and their concentrations varied with their geological origin. The concentrations of hazardous elements in high-sulfur coals from Taiyuan Formation, composing mainly of Cu, As, U, Pb, Mo and Co, are much higher than those in the low-sulfur coals from Shanxi Formation and Shihezi Formation, because the influence of seawater during and after coal accumulation in Taiyuan Formation is stronger than those in Shanxi and Shihezi formations. Moreover, the element As is related to Fe, and both elements exist mainly in the form of pyrite. The element U is richer in the coal influenced by seawater. In addition, the coal affected by the magmatism contains more U, too. When high-sulfur coals are processed with heavy media washing to remove sulfur and minerals, the majority of hazardous elements will also be removed from the coals.

Detrital zircon constraints on late Paleozoic tectonism of the Bogda region(NW China)in the southern Central Asian Orogenic Belt

The Chinese North Tianshan(CNTS)in the southern part of the Central Asian Orogenic Belt(CAOB)has undergone multistage accretion-collision processes during Paleozoic time,which remain controversial.This study addresses this issue by tracing the provenance of Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS through U-Pb dating and Lu-Hf isotopic analyses of detrital zircons.New detrital zircon U-Pb ages(N=519)from seven samples range from 261±4 Ma to 2827±32 Ma.The most prominent age peak is at 313 Ma and subordinate ages vary from 441 Ma to 601 Ma,with some Precambrian detrital zircon ages(~7%)lasting from 694 Ma to 1024 Ma.The youngest age components in each sample yielded weighted mean ages ranging from 272±9 Ma to 288±5 Ma,representing the maximum depositional ages.These and literature data indicate that some previously-assumed"Carboniferous"strata in the Bogda area were deposited in the Early Permian,including the Qijiaojing,Julideneng,Shaleisaierke,Yangbulake,Shamaershayi,Liushugou,Qijiagou,and Aoertu formations.The low maturity of the sandstones,zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East Junggar Arc and the Harlik-Dananhu Arc in the CNTS.The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc.Zircon EHf(t)values have increased since^408 Ma,probably reflecting a tectonic transition from regional compression to extension.This event might correspond to the opening of the Bogda intraarc/back arc rift basin,possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean.A decrease of zirconεHf(t)values at^300 Ma was likely caused by the cessation of oceanic subduction and subsequent collision,which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous.

Sedimentology and sequence stratigraphy of the mixed clastic-carbonate deposits in the Late Paleozoic icehouse period:A case study from the northern Qaidam Basin

The widely-developed,mixed clastic-carbonate succession in the northern Qaidam Basin records the paleo-environment changes under the glacial activity during the Late Paleozoic icehouse period in the context of regional tectonic stability,however,the depositional environment and sequence stratigraphy characteristics of the mixed deposits is rarely reported and still not clear.Combined the latest drilling wells data,we analyzed the sedimentary and stratigraphic characterization of the mixed strata via detailed field outcrops and core observations and thin section microscopic observations and recognized three depositional systems,including progradational coastal system,incised valley system,and carbonate-dominated marine shelf system,and identified four third-order sequences,SQ1,SQ2,SQ3 and SQ4,consisting of LST,TST,and HST.The depositional environment is overall belonged to marine-continental transition context and shifted from marine to continental environment frequently,showing an evolutionary pattern from marine towards terrestrial-marine transition and then back into the marine environment again in the long-term,which was controlled by the regional tectonic subsidence and the high-frequency and large-amplitude sea-level changes due to the Late Paleozoic glacial activity.The result is of significance in understanding the evolution of the Qinghai-Tibet Plateau and the sedimentation-climate response.

Geothermal Field and Tectono-Thermal Evolution since the Late Paleozoic of the Qaidam Basin,Western China

The Qaidam basin is the largest intermountain basin inside Tibet, and is one of the three major petroliferous basins in western China. This study discussed the geothermal field and tectono-thermal evolution of the basin, in an effort to provide evidence for intracontinental or intraplate continental dynamics and basin dynamics, petroleum resources assessment, and to serve petroleum production.

Stratigraphic framework of late Paleozoic in northeastern China

Late Paleozoic strata in northeastern China are distributed in a zonal pattern around the old-land on the Jiamusi-Mongolia Block. They are composed of active deposits in the regular distributed tectonic lithofacies zones. This indicates that the late Paleozoic strata belong to continental margin deposits. According to the strong conformability of the sedimentary strata in the same continental margin and distinct differences among the three continental margins, three stratigraphical regions of the Jiamusi-Mongolia Stratigraphical Province are recognized along the northern, southern and eastern margins of the Jiamusi-Mongolia Block, named respectively as Xing＇an Stratigraphical Region, Inner Mongolia grass-Songhua River Stratigraphical Region and Baoqing-Hunchun Stra- tigraphical Region. Due to the characteristics of continental margin deposits and active sediments, the strata can be correlated on the level of formation by the methods of analysing the rock association in the same stratigraphic region. Therefore, some revisions of the lithologieal formations of the late Paleozoic strata in northeastern China have been made, and a new chart of lithostratigraphic correlation has been proposed. Furthermore, the present stratigraphic framework is setting on the International Stratigraphic Chart on the level of stage, after comprehen- sive researches to lithostratigraphy, biostratigraphy and chronostratigraphy, especially the conodont biostratigra- phy and isotopic ages of volcanic rocks obtained in recent years.

Post-Late Paleozoic Collisional Framework of Southern Great Altai

We outline the post-Late Paleozoic （latest Permian to Cenozoic） collisional framework of the southern Great Altai （Central Asia） produced by the convergence between the Tuva-Mongolia and Junggar continental terranes （microplates）. The collisional structures in the region classified on the basis of their geometry and deformation style, dynamic metamorphism, and compositions of tectonites are of three main types： （1） mosaic terranes made up of large weakly deformed Paleozoic blocks separated by younger shear zones; （2） contractional deformation systems involving structures formed in post-Late Paleozoic time, parallel faults oriented along collisional deformation systems, and relict lenses of Paleozoic orogenic complexes; and （3） isolated zones of dynamic metamorphism composed mostly of collisional tectonites different in composition and alteration grade.

MAGMATIC ACTIVITY RELATED TO LATE PALEOZOIC RIFTING IN CENTRAL QIANGTANG PLATEAU

Carboniferous—Lower Permian volcanic rocks and small\|scale basic and ultrabasic intrusions occur in Chabu\|Chasang region of central Qiangtang plateau in northern Tibet Detailed studies of petrology and geochemistry of magmatic rocks further indicate that there were really a Late Paleozoic rift valley in Chabu\|Chasang area, and no so\|called Paleo\|Tethys suture zone existed there. The rift initially split in early Carboniferous, access the peak in Lower Permian, is closed and folded during Late Permian. The volcanic rocks composed of mainly basalts, a small amount of basaltic andesites and andesites, are zonally distributed, and occur alternately with flysch or flyschoid sandstones, slates, pebbled slates, radiolarian cherts and carbonate rocks. The sedimentary facies change rapidly toward both sides and show rapid deposits of proximal gravity flow.

Late Paleozoic alkaline magmatism in Western Transbaikalia,Russia:Implications for magma sources and tectonic settings

This paper presents the results of geochronological(40Ar-39Ar,U-Pb SHRIMP Ⅱ),petrological and geochemical studies of the Late Paleozoic complexes of alkaline rocks(Zimovechinsky,Tuchinsky and Koma) located within the Vitim Plateau(the western part of the Mongol-Okhotsk Orogenic Belt).The rocks were formed at 310-280 Ma.It is coeval with Late Paleozoic magmatism within the Central Asian Orogenic Belt.The εNd(T) values show large variations from-2.1 to +3.3 as well as the initial Sr(I) isotopic ratios from 0.7042 to 0.7138,that demonstrate strong isotopic heterogeneity of the magmatic source.The geochemical characteristics of the rocks show pronounced positive Pb and negative Ti,Zr-Hf anomalies that can be explained by involvement of the subducted component in primary melts.The rocks intruded in a setting of extension at the active continental margin of the Siberian Craton during subduction of Mongol-Okhotsk oceanic crust under the Siberian Craton.

华夏陆块晚新元古代-早古生代沉积物源的组成与演化及其对冈瓦纳大陆重建的意义

华南板块是冈瓦纳大陆的重要组成部分,其在冈瓦纳大陆中的古地理位置是恢复古大陆格局的关键要素,也是地质学界争论的焦点。本文汇总并对比了华南东南部华夏陆块多个盆地的成冰纪-奥陶纪碎屑沉积岩的岩石地层学、全岩地球化学和碎屑锆石数据,厘清了华夏陆块晚新元古代-早古生代三次重要的沉积-构造事件:晚成冰世砾岩指示的近源沉积事件,晚埃迪卡拉世硅质岩指示的海进事件,以及中寒武统底部平行不整合界面、砂岩中砾石的广泛出现指示的构造抬升事件。成冰纪-奥陶纪沉积岩中含有大量格林威尔早期(1300~1000 Ma)和晚期(1000~900 Ma)的碎屑锆石,分别主要来自澳大利亚和印度板块。印度东部为华夏盆地长期提供物质补给,而澳大利亚西部在晚埃迪卡拉世从主要源区变为次要源区。中寒武世华夏陆块抬升使内部850~700 Ma岩浆岩出露成为主要物源之一,是对外部构造应力的远程响应。因此,华夏陆块在晚新元古代-早古生代位于东冈瓦纳北缘,与印度、澳大利亚具有长期稳定的联系,毗邻东南极洲。在冈瓦纳大陆聚合过程中,印度和澳大利亚泛非期造山带的形成、抬升和剥蚀影响了华夏陆块沉积盆地的物源变化。

柴北缘尕海南山晚志留世—晚泥盆世火山岩浆组合对早古生代造山后伸展时限的约束

柴达木盆地北缘(柴北缘)构造带经历了早古生代的大洋俯冲到大陆俯冲,形成了广为人知的柴北缘超高压变质带。早古生代造山带何时开始垮塌一直存在争论,火山岩和侵入岩作为深地岩石探针能为约束地壳活动提供关键制约。应用锆石LA–ICP–MS U–Pb年代学和Lu–Hf同位素方法对柴北缘东段尕海南山地区出露的牦牛山组火山碎屑岩和侵入其中的花岗岩开展研究。锆石U–Pb年代学结果显示,牦牛山组火山碎屑岩的形成时代约为423 Ma,侵入其中的花岗岩的形成时代为370 Ma,表明火山岩喷发的年龄在晚志留世,后期侵入的花岗岩结晶年龄为晚泥盆世;锆石Lu–Hf同位素结果显示,晚志留世熔结凝灰岩ε_(Hf)(t)值集中在–11.5~–8.3,其两阶段Hf模式年龄集中在1945~2133 Ma,显示火山岩主要源于古老地壳物质熔融;而晚泥盆世侵入的花岗岩的ε_(Hf)(t)值分布在3.9~9.1,其两阶段的Hf模式年龄集中在792~1118 Ma,显示花岗岩主要源于中—新元古代地壳物质的部分熔融。结合对区域地质、岩石学等资料的综合分析认为,晚志留世—早泥盆世时期,大陆深俯冲导致的强烈造山作用造成柴北缘地壳发生明显加厚,加厚的欧龙布鲁克地壳基底发生部分熔融,形成了该时期的火山岩;晚泥盆世时期,加厚地壳的拆沉作用导致软流圈地幔上涌,引发区域地壳伸展,上涌的软流圈物质与地壳相互作用并发生部分熔融作用。因此区域牦牛山组形成时代跨度较大,不能笼统地用牦牛山组代表造山结束的时限,晚泥盆世岩浆岩的出现才预示着柴北缘地区进入显著的地壳伸展状态。

阿拉善地块晚古生代构造演化新模式:来自岩浆岩和沉积岩的记录

本文根据阿拉善地块岩浆岩和沉积岩的记录分析,结合造山带岩石圈演化,提出阿拉善地块随着古亚洲洋在晚志留世末闭合,发生弧-陆或陆-陆碰撞、以及与相邻地块的碰撞,使陆壳得到广泛加厚;晚古生代时期以伸展为特征,早石炭世造山带岩石圈初始垮塌,致使阿拉善地块北部珠斯楞—杭乌拉带由泥盆纪相对稳定的被动陆缘,转为活动陆缘,并形成一套前弧碎屑岩-火山岩沉积;晚石炭世乌力吉—宗乃山—沙拉扎山带岩石圈初始垮塌、诺尔公—红古玉林带造山带岩石圈初始垮塌,由此构成阿拉善地块岩石圈部分垮塌,使中南部广泛区域沉降,形成一套后弧环境的海相碳酸盐-碎屑岩沉积岩石组合,并伴随同时期的辉长岩等基性岩体侵入;二叠纪造山带岩石圈完全垮塌,大规模软流圈上涌物质和热,使俯冲的洋壳物质、加厚下地壳混合物质、交代岩石圈地幔衍生新生物质等被加热熔融,形成高(超)钾岩浆储库。早二叠世,因侧向强迫重力翻转或受塔里木早二叠世地幔柱活动的触发,导致高(超)钾质岩浆沿珠斯楞、乌力吉、诺尔公陆壳中的岩石圈不连续挤出,以气球膨胀式短时、快速侵位,形成以珠斯楞、乌力吉、诺尔公为代表的二叠纪巨量花岗岩带,该时期也应该是阿拉善地块最重要的大规模成矿时期。阿拉善地块是华北克拉通的组成部分,至今在其南部依然保留着大于200 km厚的岩石圈。阿拉善地块在早古生代时期汇聚,伴随古亚洲洋、祁连洋的形成、演化与消亡及与华北地块相互作用,使阿拉善地块被“破坏”;晚古生代时期伸展,伴随造山带岩石圈垮塌被“肢解”,总体上属于西部“华北克拉通破坏”范畴,显生宙以来的构造-岩浆事件,是西部“华北克拉通破坏”的响应。

塔里木盆地西北缘乌什西次凹的地层系统和构造特征

乌什西次凹位于塔里木盆地西北缘,隶属于库车坳陷的乌什凹陷。它位于南天山主山脉(哈尔克山)和塔里木盆地柯坪—温宿凸起之间。区域构造格局上,这里是塔里木克拉通向西北自然延伸的部分。次凹北缘的阿合奇断裂是南天山造山带和塔里木克拉通的分界。乌什西次凹内发育与柯坪—温宿凸起相似的晚前寒武纪—古生代地层系统,包括寒武系、奥陶系和石炭系的烃源岩。新近系—第四系碎屑岩建造直接不整合于变形的古生代被动大陆边缘碎屑岩—碳酸盐岩沉积建造之上;剖面上,向南天山方向加厚,向塔里木克拉通方向减薄,呈现典型的前陆盆地剖面结构特征。这是一个晚新生代陆内前陆盆地,叠加在晚海西期—燕山期古隆起之上。这里的构造变形主要有3期,分别是中海西期、晚海西期—印支期和晚喜山期。变形以厚皮冲断构造及其相关褶皱为主,薄皮构造不发育。平面上,主构造线走向NE-SW。剖面上,以南天山向塔里木冲断为主。