Paleoproterozoic granitoids of the Chuya and Kutima complexes (southern Siberian craton): age,petrogenesis, and geodynamic setting

Detailed geochemical, isotope, and geochronological studies were carried out for the granitoids of the Chuya and Kutima complexes in the Baikal marginal salient of the Siberian craton basement. The obtained results indicate that the granitoids of both complexes are confined to the same tectonic structure (Akitkan fold belt) and are of similar absolute age. U–Pb zircon dating of the Kutima granites yielded an age of 2019±16 Ma, which nearly coincides with the age of 2020±12 Ma obtained earlier for the granitoids of the Chuya complex. Despite the close ages, the granitoids of these complexes differ considerably in geochemical characteristics. The granitoids of the Chuya complex correspond in composition to calcic and calc-alkalic peraluminous trondhjemites, and the granites of the Kutima complex, to calc-alkalic and alkali-calcic peraluminous granites. The granites of the Chuya complex are similar to rocks of the tonalite–trondhjemite–granodiorite (TTG) series and are close in CaO, Sr, and Ba contents to I-type granites. The granites of the Kutima complex are similar in contents of major oxides to oxidized A-type granites. Study of the Nd isotope composition of the Chuya and Kutima granitoids showed their close positive values of εNd(T) (+ 1.9 to + 3.5), which indicates that both rocks formed from sources with a short crustal history. Based on petrogeochemical data, it has been established that the Chuya granitoids might have been formed through the melting of a metabasitic source, whereas the Kutima granites, through the melting of a crustal source of quartz–feldspathic composition. Estimation of the PT-conditions of granitoid melt crystallization shows that the Chuya granitoids formed at 735–776 °C (zircon saturation temperature) and > 10 kbar and the Kutima granites, at 819–920 °C and > 10 kbar. It is assumed that the granitoids of both complexes formed in thickened continental crust within an accretionary orogen.     

Early Proterozoic granitoids of the Olenek complex (northern Siberian craton): petrogenesis and geodynamic setting

The paper deals with geological and geochemical studies of granitoids of the Olenek complex in the Olenek uplift of the basement of the northern Siberian craton. The age of these granitoids was earlier estimated at 2036 ± 11 Ma. The granitoids of the Olenek complex correspond in composition to high-alumina quartz diorites, granites, and leucogranites of the normal petrochemical series. According to geochemical and mineralogical characteristics, the quartz diorites can be assigned to granites of the transitional I-S type, and the granites and leucogranites, to S-type granites. The 8_Nd(T values in the granites of the Olenek complex vary from -0.2 to + 1.4, and the Nd model age is 2.4-2.5 Ga. The quartz diorite is characterized by 8_Nd(T) = + 3.0 and a Nd model age T(DM) = 2.2 Ga. The geochemical characteristics of the granites and leucogranites indicate their formation through the melting of a source of graywacke composition, whereas the quartz diorites resulted, most likely, from the mixing of granitic and basaltic melts. The fact that the granitoids of the Olenek complex intruded the folded rocks of the Eekit Formation but stay virtually undeformed massive bodies suggests that they formed at the postdeformation stage of the regional evolution after the completion of the Paleoproterozoic orogenic events. The intrusion of granitoids marks the completion of the formation of the Early Proterozoic Eekit fold belt on the western (in the recent coordinates) margin of the Birekta terrane of the Olenek superterraine and the final formation of the superterrane structure. At the next stage of magmatism (1.98-1.96 Ga), best pronounced in the uplifts of the basement of the northern Siberian craton, all terranes forming the Anabar and Olenek superterranes assembled into a single structure.     

西昆仑地区元古宙岩浆侵入作用及构造-岩浆演化过程

通过对西昆仑地区元古代侵入岩的岩石类型、形成时代和岩石地球化学资料的综合分析,探讨各个构造单元侵入岩形成期次、岩石成因及构造-岩浆演化过程。铁克里克断隆带元古宙中酸性侵入岩以A型花岗岩为主,是塔里木板块古老基底在高温低压条件下发生部分熔融的产物。西昆仑造山带古元古代和中元古代早期中酸性侵入岩为钙碱性I型花岗岩,是变玄武岩在低温条件下部分熔融条件下形成的,而古元古代晚期和新元古代中酸性侵入岩则是高温条件下老基底岩系部分熔融而形成的A型花岗岩。甜水海地块仅发育新元古代侵入岩,为S型花岗岩,是高温高压环境下甜水海地块古老基底部分熔融而形成。根据侵入岩岩浆演化规律,将西昆仑地区元古宙划为4个演化阶段:12 426~1 567Ma:以铁克里克断隆带A型花岗岩为代表的塔里木板块陆内演化,以西昆仑造山带钙碱性-拉斑质I型花岗岩为代表的陆缘弧。21 301~1 000Ma:铁克里克断隆带和西昆仑造山带均以陆内演化性质的A型花岗岩为主。31 000~851 Ma:甜水海地块S型花岗岩可能是陆-陆碰撞导致地壳加厚的产物,指示甜水海地块可能作为Rodinia超大陆的一员发生聚合拼接作用。4815~644 Ma:铁克里克断隆带和西昆仑造山带均存在碱性基性岩浆岩和A型花岗岩的双峰式侵入岩组合,指示塔里木地块和西昆仑地块可能作为Rodinia超大陆组成部分,在该阶段发生了裂解作用。通过对元古宙侵入岩的系统分析,西昆仑地区不同构造单元地壳演化有一定差异,经历了不同演化过程。     

The Kalar Compex, Aldan-Stanovoi shield, an ancient anorthosite-mangerite-charnockite-granite association: Geochronologic, geochemical, and isotopic-geochemical characteristics

The autonomous (massif-type) anorthosite massifs of the Kalar Complex (2623 ± 23 Ma) intrude high-grade metamorphic rocks of the Kurulta tectonic block at the junction of the Aldan and Dzhugdzhur-Stanovoi fold area. These rocks belong to the most ancient anorthosite-mangerite-charnockite-granite (AMCG) magmatic association, whose origin was constrained to the Mesoproterozoic (1.8–1.1 Ga). The charnockites are typical high-potassium reduced granites like rapakivi, which affiliate with the A type. The Nd and Pb isotopic composition of these rocks suggests their predominantly crustal genesis, whereas the anorthosites were most probably produced by a mantle magma that was significantly contaminated by crustal material at various depth levels. The intrusions of the Kalar Complex were emplaced in a postcollision environment, with the time gap between the collisional event and the emplacement of these massifs no longer than 30 m.y. The southern Siberian Platform includes two definitely distinguished and spatially separated AMCG associations, which have different ages and tectonic settings: (i) the Late Archean (2.62 Ga) postcollision Kalar plutonic complex and (ii) the Early Proterozoic (1.74–1.70 Ga) anorogenic Ulkan-Dzhugdzhur volcano-plutonic complex.     

Age and sources of the Paleoproterozoic premetamorphic granitoids of the Goloustnaya block of the Siberian craton: Geodynamic applications

This paper reports the results of geological, geochronological, and isotope geochemical investigations of two premetamorphic granite massifs of the Goloustnaya block of the Baikal salient of the basement of the Siberian craton and granite gneisses from the migmatite–gneiss sequence of this block. The U–Pb zircon age of the granites of the Khomut massif is 2153 ± 11 Ma. The age of the Elovka massif was previously determined by us as 2018 ± 28 Ma. The Khomut and Elovka granites underwent structural and metamorphic transformations accompanied by migmatization. An age of 1.98–1.97 Ga was obtained for the structural and metamorphic processes in the Goloustnaya block from the analysis of margins of zircon grains from the Khomut granites and zircon from the granite gneisses. The biotite granites of the Khomut massif show transitional I–S-type geochemical characteristics, which allowed us to suggest that they were derived by melting of a crustal source of intermediate–acid composition. The Khomut granites show positive ε_Nd(T) values from +2.0 to +2.2 and a Nd model age of 2.4 Ga, which may indicate their formation owing to the reworking of the Paleoproterozoic juvenile continental crust. The combined isotope geochemical data are consistent with collision of island arcs as a possible environment for the formation of the Khomut granites. The formation of these granites was not related to the development of the structure of the Siberian craton, similar to a few other anorogenic magmatic complexes of the margin of the Chara–Olekma terrane of the Aldan shield with ages of ~2.2–2.1 Ga, including the granites of the Katugin complex. The biotite–amphibole granites of the Elovka massif with an age of ~2.02 Ga are geochemically similar to I-type granites. The geochemical characteristics of these granites, including elevated Sr and Ba and low Nb and Ta contents, were inherited from a subduction-related source. Negative ε_Nd(T) values from–0.9 to–1.8 and rather high contents of K₂O and Th allow us to suppose a metamagmatic crustal source for the granites of the Elovka massif. The combined isotope geochemical characteristics of the Elovka granites suggest that a mature island arc or an active continental margin is the most probable environment of their formation. The estimates of the age of structural and metamorphic processes affecting the Goloustnaya block (1.98–1.97 Ga) coinciding with the time of similar transformations in the central part of the Aldan shield and eastern Anabar shield (1.99–1.96 Ga) indicate wide occurrence of collisional events of similar age in the Siberian craton and allow us to consider this age interval as an early large-scale stage of the formaiton of the structure of the Siberian craton.     

The first data on the Early Proterozoic sialic basement in the eastern West Siberian Platform (studies of the Tyn'yar rhyolite–granite pluton)

Despite the long history of research, the presence of Precambrian complexes in the West Siberian basement has not been proven. The Tyn'yarskaya 100 and Tyn'yarskaya 101 wells were drilled in the Vakh–Elogui interfluve, in the eastern West Siberian Plate (eastern Khanty-Mansi Autonomous District). At a depth of 1790 m, they stripped a rhyolite extrusion, which graded into A-type alkali granitoids with rare-metal and REE mineralization (thorite, thorogummite, pitchblende, REE-carbonates, chevkinite, and others) downsection. This volcanoplutonic complex is Early Permian (K–Ar age, ～ 270 Ma; Rb–Sr age, 275.7 Ma; Sm–Nd age, 276 Ma; U–Pb age, 277 Ma). Some zircon grains from granites are much older (2049 ± 23 Ma, SHRIMP II), suggesting a relationship between the Early Permian granitic magma and the ancient matter. This might have been a granite-metamorphic basement, the partial melting of which produced the Tyn'yar rhyolite–granite body. The Sm–Nd model ages also suggest the participation of a Precambrian substratum in the formation of the rocks under study. Thus, it is quite possible that the Tyn'yar area is underlain by a Proterozoic (～ 2 Ga) sialic basement, which is an edge of the Siberian Platform thinned by Late Proterozoic–Early Paleozoic rifting and extension.     

Age of the Vishnyakovskoe deposit of rare-metal pegmatites (East Sayan): U-Pb geochronological study of manganotantalite

The U-Pb age of the manganotantalite from rare-metal pegmatites of the Vishnyakovskoe deposit (East Sayan Belt) has been assessed at 1838 ± 3 Ma. The acquired data indicate the pegmatites of this deposit and associated granites of the Sayan complex belong to the postcollision South Siberian igneous belt (1.88–1.84 Ga), which stretches along the southwestern frame of the Siberian Craton by more than 2500 km, from the Yenisei Ridge to the Aldan Shield. Formation of this igneous belt is related to joining (starting from about 1.9 Ga BP) of the series of continental microplates and island arcs to the Siberian Craton; this led to final stabilization of the craton at about 1.8 Ga BP.     

Composition,age, and tectonic position of granitoids of the Shmakovka complex

The geological, geochemical, and geochronological data on the granitiods of the Shmakovka massif, which represents a petrotype of the synonymous complex (southern Russian Primorye), show that the granitoid intrusions of the Shmakovka Complex play a “coupling” role, occurring in different blocks of the Khanka composite terrane. The geochemical and isotopic features of the granitoids indicate that their formation resulted from melting of a “mixed,” substantially metapelite, source similar to the most intensely metamorphosed rocks of the Khanka massif. According to U–Pb measurements, the granitoids are 490 ± 1 Ma old. The analysis of the distribution of Early Paleozoic I-, S-, and A-type granitoids in southern Primorye reveals that Late Cambrian–Early Ordovician endogenic events marked the amalgamation of Precambrian–Early Paleozoic blocks and the eventual formation of the Bureya–Jiamusi superterrane (Bureya–Khanka orogenic belt).     

Age and tectonic setting of the Kengurak-Sergachi gabbro-anorthosite massif (the Selenga-Stanovoi superterrane,southern frame of the Siberian craton)

The results of geochemical and geochronological study of the Kengurak-Sergachi gabbroanorthosite massif in the Selenga-Stanovoi superterrane, southern frame of the Siberian craton, are presented. According to geochemical peculiarities, the massif rocks are close to the autonomous “massif-type anorthosite.” The massif age corresponds to 1866 ± 6 Ma based on the results of U-Pb zircon dating. The Kengurak-Sergachi massif was intruded most likely in post-collision epoch concurrently to formation of the South Siberian giant post-collision magmatic belt (1.87–1.84 Ga) extending along the southwestern flank of the Siberian craton.     

Late Paleozoic granitoids of western Transbaikalia: magma sources and stages of formation

Geochemical and geochronological studies of the main types of granitoids of the Angara-Vitim batholith (AVB) and granites of the Zaza complex in western Transbaikalia were carried out. U-Pb (SHRIMP-II) and Rb-Sr dating yielded the age of autochthonous gneiss-granites of the Zelenaya Griva massif (325.3±2.8 Ma), quartz syenites of the Khangintui pluton (302.3±3.7 Ma) and intruding leucogranites of the Zaza complex (294.4±1 Ma), monzonites of the Khasurta massif (283.7±5.3 Ma), and quartz monzonites of the Romanovka massif (278.5±2.4 Ma). The U-Pb and Rb-Sr dates show that the Late Paleozoic magmatism in western Transbaikalia proceeded in two stages: (1) 340–320 Ma, when predominantly mesocratic granites of the Barguzin complex, including autochthonous ones, formed, and (2) 310–270 Ma, when most AVB granitoids formed. We suggest that at the early stage, crustal peraluminous granites formed in collision geodynamic setting. At the late (main) stage, magmatism occurred in postorogenic-extension setting and was accompanied by the formation of several geochemical types of granitoids: (1) typical intrusive mesocratic granites of the Barguzin complex, similar to those produced at the first stage; (2) melanocratic granitoids (monzonitoids, quartz syenites), which were earlier dated to the early stage of the AVB evolution; (3) leucocratic medium-alkali (peraluminous) granites of the Zaza intrusive complex; and (4) some alkali-granite and syenite intrusions accompanied by alkaline mafic rocks. The diversity of granitoids that formed at the late stage of magmatism was due to the heterogeneous composition of crust protoliths and different degrees of mantle-magma participation in their formation.     

The Vendian age of granodiorites and plagiogranites of the Tallainskii complex (Baikal–Muya Belt): U–Pb isotope data

This work presents the results of U–Pb isotope dating of zircons from granodiorites and plagiogranites of the Tallainskii gabbro–granodiorite–plagiogranite complex of the Karalon–Mamakan zone of the Baikal–Muya belt, ascribed to the Tallainskii pluton. The age datings obtained for granodiorite of the Eleninskii massif (605 ± 6 Ma) and plagiogranite of the Ust-Berezovo massif (609 ± 6 Ma) are in close agreement within the limits of error. Taking into account previously published data, the emplacement of the Tallainskii complex occurred within the age interval of 615–603 Ma in connection with postcollision extension. The “island arc” geochemical characteristics of granodiorites and plagiogranites can be explained by magmatic differentiation and (or) participation in the formation of a melt source enriched in the suprasubduction component involved in petrogenesis during the preceding Neoproterozoic period.     

Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes,Baikal Region)

Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500-465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss-migmatite-granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K₂O/Na₂O values, and rare-element composition the migmatite-plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li-F and Rb-Be-Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region.     

SOVETSKAYA GEOLOGIYA (SOVIET GEOLOGY) IN COVER-TO-COVER TRANSLATION, 1960, NOS. 11 AND 12

The Río Negro-Juruena Province (RNJP) occupies a large portion of the western part of the Amazonian Craton and is a zone of complex granitization and migmatization. Regional metamorphism, in general, occurred in the upper amphibolite facies. The granites and gneisses of the RNJP yield Rb-Sr and Pb-Pb whole-rock isochron dates ranging from 1.8 Ga to 1.55 Ga, with initial ⁸⁷Sr/⁸⁶Sr ratios of ～ 0.703 and a single-stage model μ₁ value of ～ 8.1. In order to improve the geochronological control, SHRIMP U-Pb zircon ages, conventional U-Pb zircon ages, and additional Pb-Pb whole-rock isochron ages were determined for samples of granitoids and gneisses from the Papuri-Uaupés and Guaviare-Orinoco rivers areas (northern part of the province) and Jamari-Machado rivers and Pontes de Lacerda areas (southern part). The granitoids from the northern part of the province yield conventional U-Pb zircon ages of 1709 ± 17 Ma and 1521 ± 31 Ma, and SHRIMP U-Pb concordant zircon results of 1800 ± 18 Ma. Samples of gneissic rocks from the southern part of the RNJP yielded SHRIMP U-Pb concordant ages of 1750 ± 24 Ma and 1570 ± 17 Ma and a Pb-Pb whole-rock isochron age of 1717 ± 120 Ma. These new U-Pb and Pb-Pb results confirm the previous Rb-Sr and Pb-Pb geochronological evidence that the main magmatic episodes within the RNJP occurred between 1.8 and 1.55 Ga, and suggest that this crustal province constitutes a segment of continental crust newly added to the Amazonian Craton at the end of the Early Proterozoic. In the area of the RNJP, there are several anorogenic rapakivi-type granite plutons. Because of the absence of recognized Archean material within the basement rocks, it is reasonable to consider the Early to Middle Proterozoic continental crust as the magmatic source for the rapakivi granite intrusions.     

全吉地块金泉山—化石沟一带古生代 花岗质岩体地球化学及其构造意义

柴达木盆地北缘之全吉地块花岗质岩体大量发育,具多期次多阶段特征。通过对全吉地块金泉山—化石沟一带古生代花岗质岩体岩石学、岩石化学特征及单颗粒锆石U-Pb同位素定年,发现该区花岗岩有4次侵入,侵入时代分别为早奥陶世(471~476Ma)、中奥陶世(459±5Ma)、早志留世(423±4Ma)和中泥盆世(366±2Ma)。岩石地球化学研究显示,该4期花岗岩均具典型的钙碱性特征,轻稀土富集、重稀土轻度亏损、Eu轻微负异常—正异常,大离子亲石元素K2O、Rb、Ba、Th等相对强烈富集,高场强元素Nb、Ta、Hf、Zr及Yb明显亏损,除第4期岩体即具I型,又具S花岗岩特征外,其它各期次均属I型花岗岩,总体显示岩体具壳源特征,为板块碰撞前消减地区花岗岩,研究推测,金泉山—化石沟一带古生代花岗质岩体第1、2组年龄为全吉地块与柴达木陆块碰撞的时代,第3组年龄反映了深俯冲地下的板块由于拆沉而折返的时代,第4组年龄为碰撞隆起后造山带伸展、滑塌的时代。     

Main stages and geodynamic regimes of the Earth’s crust formation in East Antarctica in the Proterozoic and Early Paleozoic

Geological, geochemical, and isotopic data (U-Pb for zircon and Sm-Nd for whole-rock samples) are summarized for Proterozoic and Early Paleozoic geological complexes known from various regions of East Antarctica. The main events of tectonothermal and magmatic activity are outlined and correlated in space and time. The Paleoproterozoic is characterized as a period of rifting in Archean blocks, their partial mobilization, and formation of a new crustal material over a vast area occupied by present-day East Antarctica. In most areas, this material was repeatedly reworked at the subsequent stages of evolution (1800–1700, 1100–1000, 550–500 Ma). Complexes of Mesoproterozoic juvenile rocks (1500, 1400–1200, 1150–1100 Ma) arising in convergent suprasubduction geodynamic settings are established in some areas (basalt-andesite and tonalite-granodiorite associations with characteristic geochemical signatures). The evolution of the Proterozoic regions in East Antarctica may be interpreted as a Wilson cycle with the destruction of the Archean megacontinent 2250 Ma ago and the ultimate closure of the secondary oceanic basins by 1000 Ma ago. The Mesoproterozoic regions make up a marginal volcanic-plutonic belt that combines three provinces of different ages corresponding to consecutive accretion of terranes 1500–1150, 1400–950, and 1150–1050 Ma ago. The Neoproterozoic and Early Paleozoic tectonomagmatic activity developed nonuniformly. In some regions, it is expressed in ductile deformation, granulite-facies metamorphism, and postcollision magmatism; in other regions, a weak thermal effect and anorogenic magmatism are noted. The evolution of metamorphic complexes in the regime of isothermal decompression and the intraplate character of granitoids testify to the collision nature of the Early Paleozoic tectonomagmatic activity.     

Abdar-Khoshutula intrusive-dike series: Evolution and origin of granitoids in Early Mesozoic magmatic area (Central Mongolia)

The dike belt and separate intrusive bodies of the Abdar–Khoshutula series were formed in the NE-trending linear zone, southwest of the Daurian–Khentei batholith, in the peripheral part of the Early Mesozoic magmatic area, on the western termination of the Mongol–Okhotsk belt. The granitoids of this series are subdivided into following geochemical types: anatectic granitoids of the calc-alkaline and subalkaline series, alkaline rocks, and plumasite rare-metal leucogranites (Li–F granites). The entire series was formed within approximately 12–15 Ma. Its geochemical evolution follows two trends, which correspond to two stages of the granitoid magmatism. The early stage was responsible for the formation of granitoids of two phases of the Khoshutulinsky Pluton and alkaline syenites with similar trace element distribution patterns. However, syenites, as agpaitic rocks, are significantly enriched in Ba, Zr, and Hf. The late stage of the intrusive- dike series resulted in the formation of the dike belt and Abdar Massif of rare-metal granites. These rocks show enrichment in Li, Rb, Cs, Nb, Ta, Sn, and Y, and deep negative anomalies of Ba, Sr, La, and Ce, which are best expressed in the late amazonite–albite granites of the Abdar intrusion and ongonites of the dike belt. The intrusive-dike series in the magmatic areas of different age of Mongolia and Baikal region are characterized by the wide compositional variations, serve as important indicators of mantle-crustal interaction and differentiation of granitoid magmas, and could highlight the nature of zonal areas within the Central Asian Fold Belt. Obtained geochemical data indicate a potential opportunity to concentrate trace and ore components during long-term evolution of the intrusive-subvolcanic complexes, which could be indicators of the evolution of the ore-magmatic systems bearing rare-metal mineralization.     

赞比亚谦比西铜矿花岗岩年龄及其指示意义

赞比亚谦比西铜矿位于新元古代卢菲利安弧构造带内。矿体呈似层状分布于新元古代罗安组的砂页岩中。罗安组地层不整合于古元古代穆瓦系砂砾岩之上,穆瓦系砂砾岩不整合于下部谦比西花岗岩基底之上。锆石U-Pb测年结果表明谦比西花岗岩年龄为(1984±6)Ma~(1986±6)Ma;穆瓦系年龄上限为(1932±8)Ma。谦比西花岗岩岩石地球化学、稀土元素与球粒陨石配分特征均表现为S型花岗岩的特征。区域地质资料表明,可能由于古元古代班韦卢地块与坦桑尼亚太古宙克拉通碰撞作用诱发了卢菲利安古元古代花岗岩基底的形成;太古宙刚果克拉通于早元古代(2100~1800 Ma)期间活化,并形成一稳定块体。     

Signature of Precambrian extension events in the southern Siberian craton

We investigate extension events in the southern Siberian craton between 1.8 and 0.7 Ga. Signature of Late Paleoproterozoic within-plate extension in the Northern Baikal region is found in 167 4± 29 Ma dike swarms. A Mesoproterozoic extension event was associated with intrusion of the 1535 ± 14 Ma Chernaya Zima granitoids into the Urik-Iya graben deposits. Neoproterozoic extension recorded in the Sayan-Baikal dike belt (740-780 Ma dike complexes) was concurrent with the breakup of the Rodinia supercontinent and the initiation of the Paleoasian passive margin along the southern edge of the Siberian craton. The scale of rifting-related magmatism and the features of the coeval sedimentary complexes in the southern Siberian craton indicate that Late Paleoproterozoic and Early Mesoproterozoic extension did not cause ocean opening, and the Paleoasian Ocean opened as a result of Neoproterozoic rifting.     

Radioactive elements in collisional and within-plate Sodic-Potassic Granitoids: Accumulation levels and metallogenic significance

New data are reported on the content of radioactive elements in the Precambrian Na-K granitoids from the southwestern margin of the Siberian Craton, Aldan and Ukrainian shields, and Kursk-Voronezh Massif. Analytical data on other regions were generalized for comparison. Two global epochs of Na-K granitoid magmatism bearing elevated contents of radioactive elements (U, Th, K) were distinguished in the Early Precambrian (in Ga): Neoarchean (2.8-2.6) and Late Paleoproterozoic (1.9-1.75). Mesoarchean (3.1-2.8 Ga) epoch of Na-K granite formation has been additionally distinguished at the Australian, South African, and Canadian shields. These epochs of granitization provided high maturity of the crust: geochemical differentiation of the oldest continental blocks and their geochemical and metallogenic specialization for trace elements and RAE. In the southern margin of the Siberian Craton, the most intense granite formation occurred in the Late Paleoproterozoic. The extended South Siberian belt of collisional and within-plate Na-K granitoids is characterized by intense influx of RAE and other trace elements in the upper crustal shell. The southwestern margin of the craton (Yenisei Range) was spanned by repeated Late Neoproterozoic Na-K granite formation, with wide development of collisional and within-plate Na-K granites having elevated Th content and [Th]/[U] ratio. The higher RAE concentrations are typical of within-plate Paleo and Neoproterozoic granitoids. The highest uranium content was found in the postcollisional and within-plate Na-K granites and subalkaline leucogranites. Uranium ore concentrations were formed at the riftogenic stages of evolution of these crustal blocks, when within-plate subalkaline acid magmatism and accompanying hydrothermal metamorphism overprinted granitized crystalline massifs, including high-U sedimentary and volcanic complexes. Areas with the most favorable geological-geochemical environments for the formation of uranium mineralization were distinguished in the southern margin of the Siberian Craton and its nearest folded framing.     

Structure and evolution of the continental crust in the Baikal Fold Region

A summary of original Nd isotopic data on granitoids, silicic volcanics, and metasediments of the Baikal Fold Region is presented. The available Nd isotopic data, in combination with new geological and geochronological evidence, allowed recognition of the Early Baikalian (1000 ± 100 to 720 ± 20 Ma) and Late Baikalian (700 ± 10 to 590 ± 5 Ma) tectonic cycles in the geological evolution. The tectonic stacking, deformation, metamorphism, and granite formation are related to orogenic events that occurred 0.80–0.78 Ga and 0.61–0.59 Ga ago. The crust-forming events dated at 1.0–0.8 Ga and 0.70–0.62 Ga pertain to each cycle. The Early Baikalian crust formation developed largely in the relatively narrow and spatially separated Kichera and Param-Shamansky zones of troughs in the Baikal-Muya Belt. The formation and reworking of the Late Baikalian continental crust played the leading role in the Karalon-Mamakan, Yana, and Kater-Uakit zones and in the Svetlinsky Subzone of the Anamakit-Muya Zone in the Baikal-Muya Belt. In general, three large historical periods are recognized in the evolution of the Baikal Fold Region. The Early Baikalian period was characterized by prevalence of reworking of the older continental crust. The Late Baikalian-Early Caledonian period is distinguished by more extensive formation and transformation of the juvenile crust. The third, Late Paleozoic period was marked by reworking of the continental crust with juxtaposition of all older crustal protoliths. Two models of paleogeodynamic evolution of the Baikalian fold complexes are considered: (1) the autochthonous model that corresponds to the formation of suboceanic crust in rift-related basins of the Red Sea type and its subsequent reworking in the course of collision-related squeezing of paleorifts and intertrough basins and (2) the allochthonous model that implies the formation of fragments of the Baikal-Muya Belt at the shelf of the Rodinia supercontinent, their subsequent participation in the evolution of the Paleoasian ocean, and their eventual juxtaposition during Late Baikalian and Early Caledonian events in the structure of the Caledonian Siberian Superterrane of the Central Asian Foldbelt.