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1.
The Khangai batholith is one of the largest groups of granitoid plutons produced in Central Asia in the Late Permian–Early Triassic, at 270–240 Ma. The batholith occurs in the Khangai collage of Precambrian terranes, which include Early Precambrian crustal blocks (Dzabkhan and Tarbagatai) and Early to Late Neoproterozoic structures of the Songino block in their surroundings. The axial zone of this collage is overprinted by a basin filled with Devonian volcanic–siliceous rocks and Early to Middle Carboniferous terrigenous rocks. The isotopic parameters (Nd and Pb) of granitoids in the Khangai batholith indicate that the melts were derived from compositionally contrasting crustal sources and a single mantle one. The massifs hosted in the Precambrian blocks were produced with the involvement of lower crustal material, with various ages of the origin of the crust and its differentiation into upper and lower ones. The crust of the Tarbagatai and Dzabkhan blocks was produced in the Early Archean and was differentiated at the Archean–Proterozoic boundary. The crust of the Songino block was formed in the Paleoproterozoic and differentiated in the Early Neoproterozoic. According to the Pb and Nd isotopic parameters of granitoids in the Khangai Basin, the regional continental crust was close to the juvenile one, i.e., the continental crust of the Khangai Basin had still not been differentiated by the time when the Khangai batholith was produced. A single mantle source was involved in the origin of the melts of granitoids of the Khangai batholith in various tectonic blocks. The evolution of the Pb isotopic composition of this sources is consistent with the Stacey–Kramers model at µ = 9.5. This source can be identified with the enriched mantle, which has a higher U/Pb ratio than the depleted mantle and lower εNd(T) of 0 to +2.  相似文献   

2.
A comparative analysis of within-plate (intracontinental) and orogenic magmatic series formed during various evolution stages of the East European Craton (EEC) was performed using geological-petrological, geochemical, and isotopic data. The example of Baltic shield indicates that the compositions and tectonic settings of mantle melts in the Early Precambrian (Archean and Early Paleoproterozoic) significantly differed from those in the Phanerozoic. The Early Precambrian magmas were dominated by high-Mg low-Ti melts of the komatiite-basaltic and boninite-like series; this tectonomagmatic activity was determined by the ascent of mantle superplumes of the first generation, which originated in the depleted mantle. In the interval of 2.3–2.0 Ga, high-Mg mantle melts gradually gave place to the Fe-Ti picrites and basalts that are typical of within-plate Phanerozoic magmatism; at ～2 Ga, plume tectonics of the Early Precambrian gave way to plate tectonics. This is considered to be linked to the activity of mantle superplumes of the second generation (thermochemical), which originated from the liquid metallic core/mantle interface. Owing to the presence of fluid components, these superplumes reached much higher levels, where spreading of their head portions led to the active interaction with overlaying thinned rigid lithosphere. Sm-Nd isotopic studies showed that orogenic Neoarchean and Middle Paleoproterozoic magmatism of the Baltic shield was connected to the melting of the lithospheric mantle and crust; the melting of crustal sources gave rise to felsic members of the considered complexes. The systematic geochemical variations observed in these rocks with time presumably reflect a general trend toward an increase of the thickness of the continental crust serving as the basement for orogens. Beginning at ～2 Ga, the Meso, Neoproterozoic, and Phanerozoic including, no systematic variations were observed in the isotopic-geochemical characteristics of within-plate magmatism. All considered age sections demonstrate that isotopic-geochemical characteristics of parental mantle melts were strongly modified by crustal contamination. Mesoproterozoic magmatism of EEC was unique in the development of giant anorthosite-rapakivi granite complexes. Kimberlites and lamproites were repeatedly formed within EEC in the time interval from 1.8 to 0.36 Ga; their maximal development was noted in the Late Devonian. It was shown that only kimberlites derived from weakly enriched mantle are diamondiferous in the Arkhangelsk province; in the classic diamond provinces (Africa and Yakutia), diamondiferous kimberlites were derived from both depleted and enriched mantle.  相似文献   

3.
Data on the petro- and geochemical characteristics of mantle xenoliths in kimberlites, which sampled the mantle beneath Early Precambrian tectonic structures (Archean cratons: the basement of the Eastern Siberian Platform, Karelian, Kaapvaal, Wyoming, Western Dharvar; Early and Middle Proterozoic foldbelts: Western Olenek, Natal, and Halls Creek), and xenoliths in alkaline basalts, which sampled the mantle benath Late Proterozoic-Phanerozoic structures (foldbelts: Central Asian, Mozambique, southern tip of South America, and Central German) indicate the following: (1) The major and trace element and REE composition of the mantle is different beneath Early Precambrian structures and Late Proterozoic-Phanerozoic foldbelts and reflects the degree of partial melting of the primitive mantle and its depletion in magmaphile components beneath ancient structures compared to young ones. (2) The original composition of the mantle was different beneath the Early Precambrian and Late Proterozoic structures in terms of both major oxides and incompatible trace elements and REE and their ratios; the composition of the mantle beneath the Eastern Siberian Platform, Wyoming, and Karelian cratons is different in terms of Zr/Y, La/Sm, Ce/Sm, Gd/Yb, and Lu/Hf. (3) The degree of melting of the primitive mantle decreases with depth, as follows from the negative correlation between the MgO/SiO2 ratio and pressure (i.e., depth) and the positive correlation between the Al2O3/MgO ratio and pressure in the xenoliths. (4) The Y, Zr, Ti, Sm, Gd, and Yb conncentrations and the sum of HREE in the mantle decrease with increasing degree of melting; correspondingly, the material most strongly depleted in these incompatible trace elements and REE composes the upper levels of the lithospheric continental mantle.  相似文献   

4.
The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules. The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat flow.  相似文献   

5.
Abundant gold deposits are distributed along the margins of the North China Craton (NCC). Occurring throughout the Precambrian basement and located in or proximal to Mesozoic granitoids, these deposits show a consistent spatial–temporal association with Late Jurassic–Early Cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock. Their ages are mainly Early Cretaceous (130–110 Ma) and constrain a very short period of metallogenesis. Sr–Nd–Pb isotopic tracers of ores, minerals and associated rocks indicate that gold and associated metals mainly were derived from multi-sources, i.e., the wall rocks (Precambrian basement and Mesozoic granites) and associated mafic rocks.Previous studies, including high surface heat flow, uplift and later basin development, slow seismic wave speeds in the upper mantle, and a change in the character of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, have been used to suggest that ancient, cratonic mantle lithosphere was removed from the base of the NCC some time after the Ordovician, and replaced by younger, less refractory lithospheric mantle. The geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization (130–110 Ma) indicate that they were derived from an ancient enriched lithospheric mantle source; whereas, the mafic dikes and volcanic rocks younger than 110 Ma were derived from a relatively depleted mantle source, i.e., asthenospheric mantle. According to their age and sources, relation to magmatism and geodynamic framework, the gold deposits were formed during lithospheric thinning. The removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature. The fluids derived from the lower crust were mixed with magmatic and meteoric waters, and resulted in the deposition of gold and associated metals.  相似文献   

6.
Analysis of the morphology of the recent uplifts on Precambrian cratons and geological–geophysical data on the structure of the crust and mantle indicate that these structures were formed due to expansion of the crustal rocks as a result of retrograde metamorphism. This occurred due to the contribution of large volumes of deep fluids to the complexes of the Early Proterozoic rocks, which underwent high-grade metamorphism in the lower crust. Later, these complexes were moved to the shallower depths after denudation of thick overlap sequences from the craton’s surfaces. The calculation of the volumetric expansion effects using P–T diagrams for the main types of metamorphic rocks shows that this mechanism could have prompted the uplifts of the Precambrian crust in recent time with amplitudes from 100–200 to 1000–1500 m.  相似文献   

7.
拉萨地体南部早侏罗世岩浆岩的成因和构造意义   总被引：9，自引：7，他引：2  
董昕  张泽明 《岩石学报》2013,29(6):1933-1948
本文从拉萨地体南部原来被认为是前寒武纪变质基底的冈底斯岩群中厘定出了一套早侏罗世的岩浆岩.锆石U-Pb年代学研究表明,这些岩浆岩侵位于202～ 180Ma.岩石类型包括辉长闪长岩、二长岩和花岗闪长岩,是一套中酸性、偏铝质钙碱性、Ⅰ型花岗岩类.微量元素表现出消减带富集大离子亲石元素、亏损高场强元素的特征,并具有岛弧花岗岩的亲缘性.锆石Hf同位素研究表明,加查地区中酸性岩石来自新生地壳物质的熔融,偏基性岩石来自于亏损地幔.而桑日地区的酸性岩石来自于古老地壳物质的重熔.本文认为包括研究区在内的南拉萨地体中的晚三叠世-早侏罗世岩浆岩为俯冲到南拉萨地体之下的松多洋壳断离或回卷,软流圈地幔上涌,地幔楔熔融并加热上覆地壳的产物.  相似文献   

8.
The work is dedicated to most important abiotic processes of the Early Precambrian, effect of which is recorded in continental crust, and to complementary processes in subcontinental mantle. We intend to figure out when a certain process was triggered first in the past and what indications suggest its further activity, evolution and possible cessation in subsequent geological history. Considerations are based on described natural objects characterizing particular geological events and enabling the cause-and-effect interpretation in order to understand different viewpoints known from publications. Considered in the work are the early Precambrian greenstone belts and ophiolites, island-arc systems and ecologites, magmatism unconnected with subduction zones (rifting-related, plateau basalts, dykes, kimberlites) and anorthosites representing a group of heterochronous intrusions of complicated genesis. Main considerations are premised with a brief review of the earliest geodynamic phenomena associated with meteorite impacts by termination of the planetary accretion.  相似文献   

9.
The oldest crystalline complexes of the Early Caledonian superterrane of Central Asia were formed in the Early Precambrian. They are exposed in the basement of microcontinents, which represent old cratonic fragments. Among the latters are the crystalline complexes of the Tarbagatai block previously ascribed to the Dzabkhan microcontinent. It was shown that the crystalline complexes of the Tarbagatai block have a heterogeneous structure, consisting of the Early Precambrian and later Riphean lithotectonic complexes. Structurally, the Early Precambrian complexes are made up of tectonic sheets of gneisses, migmatites, and gneiss granites of the Ider Complex that are cut by gabbroanorthosite massif. The Riphean Jargalant Complex comprises alternating hornblende crystalline schists and biotite (sometimes sillimanite-bearing) gneisses with marble horizons. The upper age boundary of the Riphean Complex is determined by the subautochthonous granitoids with age about 810 Ma. The presence of the Riphean high-grade rocks indicates that structures with newly formed crust were formed in the paleooceanic framing of the Early Precambrian blocks of the Rodinia supercontinent by the Mid-Late Riphean. Divergence that began at that time within old Rodinian cratons and caused rifting and subsequent break-up of the supercontinent was presumably changed by convergence in the paleooceanic area.  相似文献   

10.
吴宗絮 《现代地质》1994,8(2):133-138
本文根据变质岩矿物对的平衡热力学研究结果，提出冀东地区早前寒武纪陆壳平均地温分布曲线，并讨论现代陆亮地温曲线的两种可能形态．早前寒武纪在相当于中、下地壳的部分，地温的平均梯度约为７．３℃／ｋｍ，它预示了对流热传输占有重要地位（约占６５％）．基于传导热传输的假设，现代地温分布是一条平滑的曲线，而基于中地壳是一个含水的低速层以及对流热传输的假设，现代地温分布则可能是一条折线．退变质作用引起的放热与扩容可能是诱发地震的能量来源之一．  相似文献   

11.
The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian “cratonal terrane”. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1–1.4 Ga at ?Nd(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2?1.3 Ga at ?Nd(T) from ?7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880?780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval.  相似文献   

12.
A brief geological and petrographic characterization of the Early Precambrian dike complexes of the Kola region is given along with data on new estimates of dike age and analysis of their distribution over the entire Fennoscandian Shield. The emplacement of dikes in the Archean core of the shield continued after consolidation of the sialic crust 2.74?C1.76 Ga ago. After the Svecofennian Orogeny, dikes continued to form in the west in the area of newly formed crust, while the amagmatic period began in the Archean domain. The intense formation of dikes in the Svecofennian domain lasted approximately for 1 Ga (1.8?C0.84 Ga). The younger igneous rocks in the crustal domains of different age are less abundant and localized at their margins. A similar distribution of dikes is characteristic of other shields in different continents. This implies that the formation of the sialic crust in the shields is not completed by its consolidation and formation of the craton. For 1 Ga after completion of this process, the crust is underplated by mantle-derived magmas. This process is reflected at the Earth??s surface in the development of mantle-derived mafic and anorogenic granitoid magmatism. The process of crust formation is ended as the subcratonic lithosphere cools and the amagmatic period of the craton history is started. Beginning from this moment, the manifestations of cratonic magmatism were related either to the superposed tectonomagmatic reactivation of the cold craton under the effect of crust formation in the adjacent mobile belts or to the ascent of mantle plumes.  相似文献   

13.
敦煌复合造山带前寒武纪地质体的组成和演化   总被引：3，自引：3，他引：0  
赵燕  孙勇 《岩石学报》2018,34(4):963-980
敦煌复合造山带位于塔里木克拉通东端,是连接塔里木克拉通和华北克拉通的重要纽带。近年来,敦煌基础地质研究取得了重大进展。本文简要回顾了敦煌基础地质研究历史和现状,系统归纳了区内前寒武纪地质单元时空分布特征及前寒武纪构造-热事件序列,初步讨论了前寒武纪大陆地壳形成和演化规律、前寒武纪结晶基底亲缘性及构造演化过程,提出:(1)敦煌造山带前寒武纪结晶基底形成于ca.3.1~1.6Ga,构造-热事件主要划分为新太古代(ca.2.7~2.6Ga和2.6~2.5Ga)、古元古代晚期(ca.2.0~1.8Ga)和中元古代早期(1.8~1.6Ga)三个阶段;(2)新太古代早期(ca.2.7~2.6Ga)和新太古代晚期(2.6~2.5Ga)是敦煌造山带大陆地壳形成的主要阶段;古元古代晚期(ca.2.0~1.8Ga)和中元古代早期(1.8~1.6Ga)主要是古老大陆地壳物质再循环阶段,也有少量新生陆壳物质的形成;(3)敦煌造山带前寒武纪结晶基底最初拼合事件可能发生在新太古代末期(~2.5Ga),之后经历了古元古代晚期(ca.2.0~1.8Ga)汇聚、碰撞造山过程,直到中元古代早期(1.8~1.6Ga),造山活动结束,前寒武纪结晶基底最终固结,进入稳定发展阶段;(4)前寒武纪结晶基底最终稳定固结之后,即～1.6Ga之后,敦煌前寒武纪结晶基底可能进入长达12亿年的静寂期,一直处于稳定状态,目前没有发现相关的岩浆-变质-沉积记录(类似于地盾状态),直至古生代志留纪开始活化(～440Ma),卷入古亚洲洋南缘俯冲、碰撞造山过程并被强烈改造。  相似文献   

14.
早前寒武纪地质及深成构造作用研究进展   总被引：4，自引：0，他引：4  
早前寒武纪地质的研究进展主要表现在准大陆克拉通早期构造演化,克拉通及古老造山带深层结构,元古代超大陆恢复对比、早期地壳性质及生长等主要问题上开展多学科研究计划的实施。其中,同位素年代学,特别是锆石Ｕ－Ｐｂ方法,地震反射、Ｐ－Ｔ计算及古地磁研究对前寒武纪地质学的进展具有重要的推动作用。和个古陆克拉通区域地质学的持质研究积累,不断产生新的认识,这种新的科学思想涉及到早期陆壳组成及区划,太古代克拉通化历史,太古代－元古代界限及性质,元古代造山带网络与克拉进陆块拼合,大陆下地壳剖面及其组成等同题。华北早前寒武纪地质演化研究中的重要问题包括：华北麻粒岩相带与克拉通基底构造的关系,克拉通基底构造区域,早期陆壳性质及其记录的重大构造一热事件幕,华北克拉通与世界典型陆块构造演化对比等。  相似文献   

15.
秦岭区前寒武纪构造格局与演化问题探讨   总被引：28，自引：5，他引：23  
秦岭造山带中夹持大量前寒武纪岩层、地块,筛除后期改造与变位,昨原,它们复杂而多样,不仅是秦岭造山带研究的基础,也是研究早期中国大陆壳形成演化的关键地带。现有的地质、地球化学研究,揭示秦岭区早前寒武纪具非统一多块分离拼合特征,晚期已成为早期化南块的边缘带。中新元古代相应于Ｇｒｅｎｖｉｌｌｉａｎ造山事件和Ｒｏｄｉｎｉａ超大陆全球性构造时期,秦岭区从扩张裂谷系与小洋盆兼杂并存的垂向加积增生构造体制向侧向  相似文献   

16.
This contribution emphasizes first-order structural and metamorphic characters of Precambrian accretionary orogens to understand the kinematics and thermomechanical state of the continental lithosphere in convergent settings involving massive juvenile magmatism. We define a new class of orogens, called ultra-hot orogens (UHO), in which the weakest type of lithosphere on Earth is deformed. UHO are characterized by (1) distributed shortening and orogen-scale flow combining vertical and horizontal longitudinal advection, under long-lasting convergence, (2) homogeneous thickening by combined downward movements of supracrustal units and three-dimensional mass redistribution in the viscous lower crust, and (3) steady-state, negligible topography and relief leveled by syn-shortening erosion and near-field sedimentation. The flow analysis of UHO provides clues to understanding crustal kinematics beneath high plateaus and suggests that the seismic reflectivity pattern of hot orogens is an image of the layering produced by lateral flow of the lower crust and associated syn-kinematic plutonism.In between the UHO and the modern cold orogens (CO), developed by shortening of lithosphere bearing a stiff upper mantle, two classes of orogens are defined. Hot orogens (HO, representative of Cordilleran and wide mature collisional belts) share flow pattern characteristics with UHO, but involve a less intense magmatic activity and develop high topographies driving their collapse. Mixed-hot orogens (MHO, representative of magmatic arcs and Proterozoic collisional belts) are orogens made of UHO-type juvenile crust and display CO-like structure and kinematics. This classification points to the fundamental link between the presence of a stiff lithospheric mantle and strain localization along major thrusts in convergent settings. A high Moho temperature (> 900 °C), implying thinning of the lithospheric mantle, enhances three-dimensional flow of the lithosphere in response to convergence. Overall, this classification of orogens emphasizes the space and time variability of uppermost mantle temperature in controlling plate interactions and continental growth.  相似文献   

17.
Systematized geological, geochronologic, geochemical, and Sm-Nd isotopic geochemical data obtained over the past decade on A-granitoids in the Aldan Shield and in adjacent mobile belts surrounding it in the south make it possible to identify the sources from which the rocks were derived and the geodynamic environments in which they were generated. The territory in question provides evidence of five episodes of Early Precambrian within-plate magmatism, including the derivation of A-granites: at 2.62, 2.40?C2.52, 2.07, 1.87?C1.88, and 1.70?C1.74 Ga. Although all of the granitoids were derived within plates, the environments of their derivation were different: (i) postcollisional lithospheric extension at 2.64 and 1.87?C1.88 Ga in an anorogenic environment and (ii) in relation to the activity of mantle plumes at 2.40?C2.52, 2.07, 1.74?C1.70 Ga. The postcollisional magmatism generated only potassic granitoids of the subalkaline type, whereas the anorogenic magmatic rocks comprise both subalkaline granitoids (of K series) and alkaline granites (of Na series), which are intensely fractionated and strongly enriched in incompatible elements. A-granitoids in the Aldan Shield and its surrounding folded structures were derived from mixed mantle-crustal sources. The sources of the subalkaline granitoids were dominated by the material of the continental lower crust, while the alkaline granitoids were derived from mantle sources. Thereby the mantle source material of the anorogenic granitoids consisted of an OIB-type component, and the postcollisional granitoids were derived from MORB and OIB sources.  相似文献   

18.
The paper reports data on the chemical composition of mantle peridotite xenoliths from kimberlites and alkaline basalts that represent the continental lithospheric mantle (CLM) beneath Early Precambrian and Late Proterozoic-Cenozoic structures, respectively. In order to identify compositional trends during the melting of primitive material and propose the most reliable criteria for constraining the conditions of this process and its degree, we analyzed literature data on the melting of spinel and garnet peridotites within broad temperature and pressure ranges. It was determined that the degree of melting (F%) of pristine peridotite of composition close to that of the primitive mantle (PM) can be deduced from the Mg/Si and Al/Si ratios in the residue; an equation was proposed for evaluating F from the Mg/Si ratio. The Ca/Al ratio of residues at low (1–1.5 GPa) pressures and degrees of melting from 2–3 to 20–25% increases several times but decreases with increasing F at pressures higher than 3 GPa. The Na partition coefficient between melt and residue decreases at increasing pressure and approaches one at a pressure close to 20 GPa. Residues after low-degree melting are strongly depleted in Ti, Zr, Y, and Nb but are enriched in Cr. The application of these criteria to the composition of xenoliths brought to the surface from the mantle occurring beneath tectonic structures of various age led us to conclude that compositional heterogeneities of CLM (particularly the variations in the concentrations of major and certain siderophile elements) are controlled, first of all, by the melting of the mantle source material. These processes occurred under various thermodynamic conditions (T, P, and $ f_{O_2 } $ f_{O_2 } ) and differed in their intensity, and this predetermined the compositional diversity of the residual mantle material (its concentrations of Mg, Al, Si, Ca, Na, K, Ni, Co, V, and Cr). Our results are principally consistent with the hypothesis of the global magmatic ocean. It is thought that the early phases of its consolidation were variably controlled by the fractionation of minerals, for example, majorite. Moreover, heterogeneities in the distribution of siderophile elements could be partly predetermined by changes in the properties of these elements at ultrahigh temperatures and pressures. The processes of partial melting were the most intense during the early evolution of the mantle (perhaps, in the Early Precambrian), and hence, the mantle has different chemical composition beneath Archean cratons and Phanerozoic foldbelts.  相似文献   

19.
An important role of the early Neoproterozoic juvenile crustal growth in the formation of the Khangai group of Precambrian terranes in the Central Asian Orogenic Belt was demonstrated by the example of the Holbo Nur Zone of the Songin Block. Magmatic complexes of this zone correspond to different settings of the Early Neoproterozoic ocean: oceanic islands, mid-ocean ridges, intraoceanic island arcs, and turbidite basins. Obtained data on volcanic rocks and associated granitoids constrain a timing of the island-arc magmatic complexes, at least within the interval of 888–859 Ma. The comparison of structures of the Songino and Tarbagatai blocks of the Khangai group of terranes showed that they share many common features in their geology and evolution and may be united into the single Songino–Tarbagatai terrane. This terrane was formed owing to the Early Neoproterozoic (~800 Ma) accretion of the ocean island, spreading, island-arc, and turbidite complexes of the oceanic plate to a stable continental massif represented by the Early Neoproterozoic Ider Complex of the Tarbagatai Block. The involvement of the Dzabkhan terrane into a Khangai collage of terranes is constrained between the formation of the volcanic rocks of the Dzabkhan Formation (~770–755 Ma), which are unknown in the Songino–Tarbagatai terrane, and the Tsagaan-Olom carbonate cover (~630 Ma), overlying both the Dzabkhan and Songino–Tarbagatai terranes. It was proposed that the formation of the Precambrian terranes of the Central Asian Orogenic Belt began from the Early Neoproterozoic accretion to the Rodinia supercontinent. The fragmentation of the latter above a mantle superplume at the end of the Early Neoproterozoic spanned also the newly formed fold area. This led to the formation of terranes, which included both fragments of the Paleoproterozoic craton and Early Neoproterozoic structures. Subsequent amalgamation of these Precambrian crustal fragments into composite terranes possibly occurred at the end of the early Baikalian tectonic phase.  相似文献   

20.
《地学前缘(英文版)》2020,11(5):1727-1742
Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT) is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma) diabases display low SiO_2(46.52-49.24 wt.%) with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm) and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zircon ε_(Hf)(t)(-12.0 to-24.7) and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma) arc intrusions in western BAT can be divided into Periods Ⅰ and Ⅱ at approximately 450 Ma.The Period Ⅰ(450 Ma) intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zircon ε_(Hf)(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the Period Ⅱ(≤450 Ma) intrusions are characterized by variable zircon e_(Hf)(t)(-15.0 to+11.5) with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(45) of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from Period Ⅰ to Ⅱ can be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.  相似文献   

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