Precambrian Blocks Research Articles

Tarim rotation mechanism and the differential deformation responses along the Tian Shan

SUMMARY Rotation of rigid blocks within continental interiors far from the plate convergence boundary is an unusual process, the dynamics of which is not clear. The Tarim block, as a rigid Precambrian block in central Asia, is surrounded by the Tibetan–Pamir plateau to the south and Tian Shan mountains to the north. Numerous geophysical data suggested that the Tarim block experienced significant clockwise rotation in the Cenozoic. Meanwhile, contrasting deformation patterns and associated topographic responses were observed between the western–central and eastern Tian Shan. The relationship among the India–Asia collision, Tarim rotation, and Tian Shan responses are poorly constrained. Here, a series of large-scale, high-resolution 3-D numerical models were constructed. The model results reveal that the collision of the indenting Indian lithosphere with the southwestern rim of the Tarim block triggers clockwise rotation of the Tarim block. Further on, the Tarim rotation produces differential deformation responses along the strike of Tian Shan, that is convergence-induced higher compression and strong uplifting in central Tian Shan but divergence-induced less compression and moderate uplifting in eastern Tian Shan. Thus, the Tarim rotation serves as an indispensable linkage between the Tibetan plateau evolution and the far-field Tian Shan activation.

FINAL CONSOLIDATION AGE OF THE SOUTHERN PART OF THE SIBERIAN CRATON

Petrographic, geochemical and U-Pb geochronological research studies are done into cataclastic granitoids of the southeastern part of the Irkutsk block of the Sharyzhalgay basement uplift of the Siberian craton – the uplift which is considered southern part of the Tungus superterrane according to most of tectonic schemes. The representative sample of these granitoids corresponds to granodiorite in its geochemical properties and is characterized by high contents of Al2O3, Th, Sr, Ba, low concentrations of K2O, Nb, Y, Yb, a highly fractionated rare earth elements pattern (Lan/Ybn=284), and the absence of europium anomaly. U-Pb geochronological studies of zircon from cataclastic granodiorite were carried out independently by two methods: SIMS and LA-ICP-MS, which showed good consistency of the results. The U-Pb age of the cores of zircon grains with magmatic zoning corresponds to 2893±19 Ma (SIMS method) and 2889±16 Ma (LA-ICP-MS method). These results can be interpreted as the age of the Archean granodiorite protolith. The rims of zircon crystals with the Archean cores, as well as the individual zircon crystals with parallel zoning, yielded ages of 1855±6 Ma (SIMS method) and 1864±5 Ma (LA-ICP-MS method), which record the time of granodiorite transformations. The age of about 1.86 Ga corresponds to the main Early Proterozoic stage of metamorphism, migmatization and magmatism, which show their widespread occurrence in the Sharyzhalgay basement uplift. This age estimate together with the previously published ages for metamorphic and their synchronous magmatic events in the Sharyzhalgay uplift allowed concluding that the Tungus superterrane joined the earlier formed core of the Siberian craton in the time interval 1.85–1.88 Ga. The final stage in the Siberian craton formation is the development of the South Siberian post-collisional magmatic belt, intersecting all large Early Precambrian blocks of the southern part of the Siberian craton, already united into a single structure.

Geochemistry, zircon U-Pb age and Hf isotope for the Huatugou granitoid in western Qaidam: Petrogenesis and tectonic implications

The Qaidam Precambrian block is located in the northeastern Tibetan Plateau and was intruded by numerous Ordovician-Devonian granitoids during and after the closure of the Proto-Tethys Ocean. In the past 20 years, the granitoids within early Paleozoic subduction-collision belts have been investigated in detail. However, the granitoids intruding the inner part of the Qaidam block, bearing the tectonic significance for the whole block, still need to be understood. This study presents new whole-rock geochemical and zircon U-Pb-Hf isotopic data for the Huatugou granitoids in the Qaidam Precambrian block. The investigated granitoids include granodiorite, monzogranite, biotite granite, and muscovite granite, which intruded the Precambrian basements during 451–400 Ma. The granodiorites (451 ± 6 Ma) display adakitic geochemical features and syn-tectonic textures, and their magmas were generated by the partial melting of the lower mafic crust within a thickened continent. The muscovite granites (410 ± 6 Ma), with negative zircon εHf(t) values of −14.5 to −10.4, were crystallized from fractionated S-type magmas, which were derived from the partial melting of ancient crustal materials. The biotite granite (410 ± 3 Ma) and monzogranites (400 ± 4 Ma) are high-temperature A2-type granites. The biotite granite displays positive zircon εHf(t) values of +1.7 to +5.6. Its magma was generated by the high-temperature partial melting of juvenile crustal rocks in a thinned lower crust. The monzogranites exhibit higher SiO2 contents and lower εHf(t) values, and their magmas were derived from the same source but underwent assimilation and fractional crystallization during emplacement. From the thickened to the thinned continent during 451–410 Ma, the western Qaidam block experienced a tectonic transition from compression to extension. Combined with regional geological data, this study suggests that the Qaidam block consisted of the thickened continental crust during subduction processes until the detachment of the subducted slab during the continental collision. The regional extension of the Qaidam block commenced at ∼420 Ma, soon after the exhumation of ultrahigh-pressure metamorphic rocks within the northern Qaidam subduction-collision complex belt.

An orocline in the eastern Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogen in the world with considerable Phanerozoic juvenile crustal growth. The eastern segment of the CAOB is occupied by NE China and its adjacent areas, which locates in a triangle area surrounded by Siberian Craton to the northwest, North China Craton (NCC) to the south and Pacific oceanic plate to the east. Therefore, the NE China is a key area to study the geological evolution of multiple tectonic systems and overprinting, which has become a hot research topic. In the past five decades, especially the last two decades, there have been many detailed geological investigations carried out and a lot of new data reported in NE China. A progress has been achieved in tectonic evolution, however, there are still many open questions and arguments dealing with the tectonic models, correlation of tectonic units, amalgamation of different blocks and their tectonic affinity. In this study, we did a detailed review of the tectonic evolution of NE China and regional comparison and correlation of the different tectonic units in the eastern segment of CAOB. We re-subdivided NE China into two old blocks of Erguna block (EB) and Jiamusi block (JB) with Precambrian basement and three accretionary terranes of Xing'an accretionary terrane (XAT), Songliao accretionary terrane (SAT) and Zhangguangcai accretionary terrane (ZGCAT), which are separated from each other by the Xinlin-Xiguitu suture (XXS), Hegenshan-Heihe suture (HHS), Longfengshan suture (LFS) and Mudanjiang-Yilan suture (MYS), respectively. The ZGCAT is dominantly composed of early Paleozoic magmatic arc materials and minor late Paleozoic igneous rocks with an old Yichun mini-block, while the SAT consists dominantly of the late Paleozoic magmatic rocks with two small old blocks of the Xilinhot and Longjiang mini-blocks. According to the tectonic correlation and comparation of different tectonic units, we established a new orocline tectonic model for the eastern CAOB: 1) the XAT and ZGCAT accreted to the southern (present position) margin of Ereendavaa-Erguna-Mamyn block (EEMB) and Bureya-Jiamusi-Khanka block (BJKB) with the closure of Xinlin-Xiguitu-Heilongjiang ocean during the early Paleozoic; 2) Subsequently, the SAT accreted to the southern (present position) margin of integrated XAT-ZGCAT terrane with the closure of Hegenshan-Nenjiang-Longfengshan ocean during the late Paleozoic. These initially W-E linear shaped accretionary orogenic belts were ultimately bent southward through Paleozoic time and constituted a huge Paleozoic orocline, NE China Orocline, which collided with NCC by a scissor-like style closure of Paleo-Asian Ocean (PAO) from west to east along the Solonker-Xar Moron-Changchun-Yanji suture (SXCYS) during the late Permian-middle Triassic. The NE China Orocline, together with Tuva-Mongol Orocline and Kazakhstan Orocline in the western CAOB, constituted a huge multiple orocline tectonic system in the CAOB during the Paleozoic era. Our study will contribute to the understanding on tectonic evolutions of CAOB and the NE Asian, and suggests that the orocline should be a common tectonic model for accretionary orogeny. • NE China consists of two Precambrian blocks and three Paleozoic accreted terranes. • A new orocline tectonic model is established for first time in NE China. • Blocks in NE China had been integrated in later Early Carboniferous.

Preliminary discussion on tectonic setting of the magmatic-metamorphic complex in the northern Dunhuang region: Insight into southward extension style of the southern Central Asian Orogenic Belt

中亚造山带南缘如何向南扩展，对深入理解增生型造山作用和大陆地壳生长机制以及中亚构造域与特提斯构造域的衔接具有重要科学意义。作为中亚造山带南缘的关键构造单元，敦煌构造带大地构造属性长期备受关注且颇有争议。传统观点认为敦煌构造带是古亚洲洋南侧的前寒武纪稳定大陆地块，以刚性块体的形式参与了中亚造山带南缘的最终拼贴过程。然而，近年来研究认为敦煌构造带卷入了古亚洲洋南部的俯冲增生造山过程，属于中亚造山带南缘的增生系统。显然，这一争议限制了对中亚造山带南缘向南扩展方式及增生造山过程的理解。敦煌北部三危山地区出露一套古生代岩浆-变质杂岩，是解开这一争论的关键。本文综合前人研究基础及新的资料，归纳了这套岩浆-变质杂岩的野外岩石-构造组合、地球化学和年代学等方面特征：该岩浆-变质杂岩整体显示二元结构特征，即较老的增生杂岩为基底，弧岩浆岩侵入或不整合覆盖其上；其中岩浆岩属于中钾-高钾钙碱性系列中酸性岩浆岩，富集大离子亲石元素（LILE）和轻稀土元素（LREE），亏损高场强元素（HFSE），与典型的弧岩浆岩类似，并且微量元素组成特征反映中酸性岩浆的源区与俯冲沉积物部分熔融有关；岩浆作用大致归为510Ma、460~410Ma和370~360Ma三期。岩浆岩中结晶锆石不一致的ε_Hf（t）值（既有正值，又有负值）以及继承锆石的存在表明，岩浆源区既有古老地壳物质的加入，也有新生地壳物质的形成。以上这些特征与发育在增生杂岩之上的增生弧十分类似，因此本文提出敦煌北部岩浆-变质杂岩的属性为古生代增生弧，并且该增生弧与其南部的红柳峡俯冲增生杂岩共同勾勒出敦煌构造带自北向南增生弧-增生杂岩的基本构造格架，即敦煌构造带的大地构造属性实为造山带而非稳定地块。结合区域地质背景及敦煌地区与北山地区古生代至早中生代构造-热事件的对应关系，认为敦煌造山带属于中亚造山带中段南缘的增生系统，中亚造山带中段以增生弧-增生杂岩的形式向南扩展至敦煌地区。;The southward extension style of the southern Central Asian Orogenic Belt (CAOB) is significant for understanding the process of accretionary orogeny and growth mechanism of continental crust, as well as the connection between the Central Asian tectonic domain and Tethys tectonic domain. As a key tectonic unit located in the southern CAOB, the tectonic attribution of Dunhuang tectonic belt has long been concerned and controversial. Traditionally, the Dunhuang tectonic belt is considered to be a Precambrian stable continental block on the southern side of the Paleo-Asian Ocean (PAO), which participated in the final collage process of the southern margin of the CAOB in the form of rigid plates. However, in recent years, studies have suggested that the Dunhuang tectonic belt is involved in the subduction-orogenic process in the southern PAO, and belongs to the accretionary system of the southern CAOB. This controversy has limited the understanding of the southward expansion mode and accretive orogenic process of the southern margin of the CAOB. The focus of the debate is whether the Dunhuang tectonic belt is a Precambrian block or a Phanerozoic orogenic belt. The Paleozoic magmatic-metamorphic complex in the Sanweishan area is the key to reveal the tectonic attribution of Dunhuang tectonic belt. Integrating the published and new data, this paper sums up the field rock-structure association, geochemistry, and geochronology of the magmatic-metamorphic complex. In the field, the magmatic-metamorphic complex show dual structure, namely the older accretionary complex as the basement, and the arc magmatic rocks intrude or unconformity overlie on it. The magmatic rocks belong to medium potassium-high potassium calc-alkaline series, enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field strength elements (HFSE), which resembles the arc magmatic rocks. And the trace elements compositions indicate that the magma sources are perhaps related to partial melting of subducted sediment. The magmatism can be roughly classified into three periods: 510Ma, 460~410Ma, and 370~360Ma. The discordant ε_Hf(t) values (both positive and negative) of crystalline zircons in magmatic rocks and the existence of inherited zircons indicate that both the reworking of ancient crust and juvenile material were added to the magmatic sources. These characteristics are very similar to the accretionary arcs, so we propose that the magmatic-metamorphic complex in the northern Dunhuang region represent a Paleozoic accretionary arc. This accretionary arc and the southside Hongliuxia subduction-accretion complex collectively outline the basic tectonic framework of the Dunhuang orogenic belt. In combination with the relations of tectono-thermal events between Dunhuang and Beishan regions during Paleozoic to Early Mesozoic eras, it is considered that the Dunhuang orogenic belt belongs to the accretionary system of the southern margin of the middle section of CAOB. The middle section of CAOB extends southward to Dunhuang region in the form of accretionary arc-accretionary complex.

Late Mesoproterozoic–early Neoproterozoic quartzite–schist sequences of the Aktau–Mointy terrane (Central Kazakhstan): Provenance, crustal evolution, and implications for paleotectonic reconstruction

Metasedimentary complexes represented by quartzite–schist Kiik Group quartzite–schists play a significant role in the Aktau–Mointy terrane structure, which is one of the largest Precambrian blocks in the western Central Asian Orogenic Belt (CAOB). U-Pb LA-ICP-MS dating of detrital zircons from quartzites in different Kiik Group areas and related magmatic age data establish that accumulation occurred in the late Mesoproterozoic to early Neoproterozoic (from 1150 to 920 Ma). Concordant age estimates of detrital zircons from Kiik Group quartzites predominately fell within intervals of 1149–2105 and 2272–2850 Ma with the prominent peaks at 1217, 1332, 1466, 1582, 1621, 1748, 1849, 2517 and 2689 Ma, and minor peaks at 1992, 2282, 2791 and 2823 Ma. Hf-in-zircon and Nd whole rock data suggest that the primary stages of continental crustal growth in the Aktau–Mointy terrane were associated with the tectono-magmatic processes of the Neoarchean, Paleoproterozoic, and Mesoproterozoic. Juvenile continental crust production occurred during the Neoarchean and Paleoproterozoic, with older complex tectono-magmatic reworking, and juvenile crust production finally prevailed in the mid-Mesoproterozoic. Based on the age patterns and Hf isotopic compositions of detrital zircons from the Aktau–Mointy terrane and other terranes in the western CAOB, the crustal basement of the Aktau–Mointy terrane exhibits certain similarities with those of the Northern Kazakhstan and Issyk–Kul terranes. They all exhibit a close tectonic affinity with the Chinese Central Tian Shan terranes, Yili Block, North and China Craton, while differing from the Tarim Craton. In the Rodinia supercontinent structure, the Aktau–Mointy terrane was probably located near the Sveconorwegian Orogen in the western Baltica with other Kazakhstan and Northern Tian Shan Precambrian blocks.

Exploration of a magnetite deposit buried under thick and conductive sediments with multiple-source CSEM

The North China Craton (NCC) hosts abundant mineral resources, making it economically an important Precambrian block in China. The thick conductive loess strata in NCC presents a great challenge for successful application of controlled-source electromagnetic method (CSEM) by masking the deeper targets due to thick loess strata of conductive nature. In this work, we proposed a multiple-source CSEM method that addresses these weaknesses by using multiple-sources with inline and broadside configurations. The short offset was adopted to increase the S/N. Joint inversion of inline and broadside sources datasets enhance the detection accuracy. Based on an indirect detection strategy and the known ore-forming information, a deeply seated ore body at 1157 m was delineated.

Formation and stabilization of the Dunhuang Block, NW China: Constraints from the Late Paleoproterozoic A-type granites of the Dunhuang Complex

The geochemical composition and temporal evolution of A-type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust. Moreover, these aspects may provide an indication of when orogenic processes ended and craton formation/stabilization began in Early Precambrian blocks. In this study, we report the results of zircon U–Pb ages, Hf isotope data and whole-rock geochemistry for the Proterozoic Huoyanshan potassic granites in the Dunhuang Block, northwest China. Zircon U–Pb dating reveals that the Huoyanshan granites were emplaced at ca. 1791–1786 Ma, representing a Late Paleoproterozoic magmatism event. The granites have high SiO2 (70.71–72.20 wt%), total alkali contents (K2O + Na2O = 8.81–8.96 wt%; K2O/Na2O = 1.57–1.66), TiO2/MgO ratios (2.19–2.57), low CaO (0.77–1.27 wt%), MgO contents (0.12–0.16 wt%), and Mg# values (7.72–8.67), with A/CNK ratios of 0.96–1.04, pointing to alkaline and metaluminous to weakly peraluminous affinities. These granites are characterized by high total rare-earth element concentrations (∑REE = 419–755 ppm) and variable (La/Yb)N values (8–24) with significantly negative Eu anomalies (Eu/Eu* = 0.31–0.42). The primitive mantle-normalized trace element patterns suggest that the Huoyanshan plutons are depleted in Ba, Nb, Ta, Sr, P, and Ti, but enriched in Rb, Th, and Pb. Additionally, the granites have a high zirconium saturation temperature of 855–897 °C, with an average of 878 °C. These geochemical characteristics indicate that Huoyanshan potassic granites are an aluminous A-type granite, and further classified as A2-type. Together with the zircon Hf isotope data (εHf(t) = −11.21 to −2.76), indicates that these granites were generated from a crustal source. The heterogeneous Hf isotope compositions in zircons are principally caused by the disequilibrium melting of Zr-bearing sources. In addition, two-stage Hf model ages of zircons from the granites are ca. 2.6–3.1 Ga and plot on the average crustal evolution trend of Archean tonalite–trondhjemite–granodiorite (TTG) gneiss (~2.5 to 3.0 Ga) in the Dunhuang Block. Thus, we propose that A-type granites in the study area were generated from the disequilibrium partial melting of Archean tonalitic to granodioritic rocks under high-temperature and low-pressure conditions according to their low Al2O3 + CaO + Na2O + K2O (molar) values and high 10000 × Ga/Al values (>2.9). Subsequently, the magma experienced prominent fractional crystallization of K-feldspar, plagioclase, orthopyroxene, and/or magnetite. Our results, withavailabledata, suggest that the Huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting, and serve as a precursor for the formation and stabilization of the Dunhuang Block during the Late Paleoproterozoic. Furthermore, it is probable that the post-collisional magmatism event at ca. 1.79 Ga in the Dunhuang Block was closely related to the assembly of the Columbia supercontinent.

Olonkhuduk Anorthosite Pluton of the Baidaric Terrane of the Central Asian Orogenic Belt: Geological Position and Age

Abstract???Anorthosites of the Olonkhuduk pluton of the Central Asian Orogenic Belt are dated at 1772 ?? 1 Ma (ID TIMS U-Pb zircon method). Similar age estimate (1784 ?? 10 Ma) was previously obtained for anorthosite of the Khungilingol pluton from the Ider block of the Tarbagatai terrane. These data indicate a significant time gap (70???60 Ma) between the collision that formed the Early Precambrian blocks of the Baidaric and Tarbagatai terranes in the range 1860???1850 Ma and the emplacement of the anorthosites. The termination of accretion-collision processes and consolidation of the Early Precambrian block of the Baidaric terrain are marked by the postkinematic subalkaline granites with an age of 1825 ?? 5 Ma. Geochemically, the anorthosites of the Olonkhuduk and Khungilingol plutons are similar to the typical anorthosites of ancient cratons. Sm-Nd isotopic data indicate a mixed source of these anorthosites: juvenile mantle component of Paleoproterozoic age and crustal component of Neoarchean age. It can be assumed that the primary mafic magma of the anorthosites experienced a significant crustal contamination. Based on the similar geologic setting, age, and composition, the anorthosites of the Olonkhuduk and Khungilingol plutons can be ascribed to a single within-plate complex. The age values obtained for the anorthosites coincide with the estimated age of rift magmatism in the North China craton at 1.8???1.75 Ga and the emplacement time of mafic dikes swarms at 1778 ?? 3 Ma (SIMS U-Pb method). Thus, the considered Early Precambrian blocks by the end of the Paleoproterozoic (about 1900???1850 Ma) could be a part of the Columbia supercontinent (Rogers and Santosh, 2002).

Age of rocks of the Shurmak Formation according to U-Pb dating of zircon by LA-ICP-MS method (south-eastern Tuva)

At the south-eastern part of Tuva, early Proterozoic volcanogenic-sedimentary deposits have been studied including conglomerates from Shurmak formation. As the result of U-Pb zircon dating from tuff with andesitic composition, conglomerate matrix and volcanicomictic sandstones from this formation, it has been established that it was accumulated at around 500 Ma. The dating results for zircons from tuff are a geochronological date and indicate the time of manifestation of andesitic volcanism synchronous with accumulation of Shurmak Formation deposits. Given the close age of zircons of the main populations in both tuff and clastic rocks, one can assume that the formation of the latter is related directly to the manifestations of volcanism on this territory at this time. The presence in the tuff of xenogenic zircons and in volcanicomictic sandstones and matrix of Shurmak Formation of detrital zircons with Neoproterozoic and Paleoproterozoic age, as well as Sm-Nd isotopic data indicate that these volcanic and sedimentary processes in the early Cambrian time took place within the range if Precambrian block of earth’s crust. The main supplier of clastic material during the formation of the Early Cambrian Shurmak Formation were volcanic events synchronous with sedimentation. Other sources of clastic material entering the sedimentation basin were Neoproterozoic granitoids and, less commonly, Paleoproterozoic and Neoarchean rocks. Combination with the underlying rocks of the Mugur Formation of the Erza metamorphic complex occurred as a result of tectonic processes in the post-Cambrian time, since the sedimentation of the Shurmak Formation deposits and the processes of granulite metamorphism in the rocks of the metamorphic complex occurred almost simultaneously.

Paleozoic Sanweishan arc in the northern Dunhuang region, NW China: The Dunhuang block is a Phanerozoic orogen, not a Precambrian block

The Dunhuang region is located to the south of the Central Asian Orogenic Belt and connecting the Tarim Craton westerly and North China Craton easterly, respectively. It is an important position for understanding the geodynamics and tectonic framework of Central Asia. However, the issue about whether it is a Phanerozoic orogen or a Precambrian block is still controversial. Magmatic arcs are the most prominent and laterally persistent structural elements of orogenic belts and used to reconstruct now-disrupted orogenic belts. This paper concerns the petrogenesis and tectonic setting of andesitic-dacitic-rhyolitic volcaniclastic rocks and dacitic porphyries in the Sanweishan area, northern Dunhuang region. Volcaniclastic rocks overlie the metasedimentary rocks by fault and dacitic porphyries intrude into the metasedimentary rocks. The calc-alkaline signature and relative enrichment of large-ion lithophile elements of volcaniclastic rocks and dacitic porphyries seemingly favor a subduction setting. Zircon U-Pb dating results reveal the deposition at 424–414 Ma for volcaniclastic rocks and emplacement at ca. 364 Ma for dacitic porphyries. The coeval granitoid plutons in the Sanweishan area show arc geochemical signature and are originated from partial melting of the subducted sediments based on the available data. Paleozoic volcanic-subvolcanic rocks and plutons in this area probably represent a magmatic arc developed on the accretionary complex, resembling the arc magmatic rocks formed in the Nankai complex, SW Japanese Island arc. Our results provide constraints on the existence of the Paleozoic magmatic arc in the northern Dunhuang region and suggest the so-called “Dunhuang block” is virtually a Phanerozoic orogen rather than a Precambrian block.

Age and composition of plagiogneisses from the Yangiyugan, Artic area: the first evidence of Precambrian blocks existing in the basement of the West Siberian Platform

So far, Precambrian formations were not found in oil gas fields of the Western Siberian Platform (i.e., in central, western, and northern parts), where they were assumed to be. This article analyzes data on the mineral composition, geochemistry, and age of plagiogneisses from the Yangiyugan oil exploration area (Arctic part of Western Siberia, 80 km to the southeast of Salekhard). Rocks underlying this basement were naturally formed aggregates and are composed almost entirely of quartz and oligoclase, with the presence of muscovite and biotite, amphiboles (edenite, ferroedenite, and ferropargasite), clinozoisite–epidote, almandine garnet, clinochlore, microcline, magnetite, titanomagnetite, rutile, and fluorapatite. The mineral association tends to change with depth and plagiogneisses formed from trondhjemites (or leucocratic plagiogranites) under conditions of increasing temperature and amphibolites–facies metamorphism. First, trondhjemites occurred in the Ediacaran period about 566 ± 3 Ma (the dating of zircons by SHRIMP-II). The Sm-Nd isochron age of 691 ± 58 Ma also indicates Neoproterozoic origin for the gneiss protolith. It turned out that metamorphism of trondhjemite and its transformation into plagiogneiss took place in the early Ordovician (486 ± 4 Ma). The most recent tectonic–thermal events (U-Pb of 329 ± 9 and Rb-Sr of 292 ± 23 Ma) in the area are apparently associated with collisional processes and diaphthoresis. The metamorphic complexes of the studied area are the deep segments of a middle Paleozoic island arc system or, more likely, several island arcs. These segments were brought to the upper crust by rifting and graben formation about 250 million years ago. The isotope readings and geochronologic data in this article are the first direct evidence on the presence of Precambrian blocks in the pre-Jurassic oil-gas-bearing basement of the Western Siberian Platform.

Lithospheric and sublithospheric deformation under the Borborema Province of northeastern Brazil from receiver function harmonic stripping

Abstract. The depth-dependent anisotropic structure of the lithosphere under the Borborema Province in northeast Brazil has been investigated via harmonic stripping of receiver functions developed at 39 stations in the region. This method retrieves the first (k=1) and second (k=2) degree harmonics of a receiver function dataset, which characterize seismic anisotropy beneath a seismic station. Anisotropic fabrics are in turn directly related to the deformation of the lithosphere from past and current tectonic processes. Our results reveal the presence of anisotropy within the crust and the lithospheric mantle throughout the entire province. Most stations in the continental interior report consistent anisotropic orientations in the crust and lithospheric mantle, suggesting a dominant northeast–southwest pervasive deformation along lithospheric-scale shear zones developed during the Brasiliano–Pan-African orogeny. Several stations aligned along a northeast–southwest trend located above the (now aborted) Mesozoic Cariri–Potiguar rift display large uncertainties for the fast-axis direction. This non-azimuthal anisotropy may be related to a complex anisotropic fabric resulting from a combination of deformation along the ancient collision between Precambrian blocks, Mesozoic extension and thermomechanical erosion dragging by sublithospheric flow. Finally, several stations along the Atlantic coast reveal depth-dependent anisotropic orientations roughly (sub)perpendicular to the margin. These results suggest a more recent overprint, probably related to the presence of frozen anisotropy in the lithosphere due to stretching and rifting during the opening of the South Atlantic.

Indentation tectonics of the Fanshi Block in the Trans-North China Orogen

The Trans-North China Orogen (TNCO) is a tectonic zone that records significant magmatic, metamorphic, and tectonic events during the Paleoproterozoic. The central part of the TNCO comprises several Precambrian blocks (the Hengshan Block, the Wutai Block, the Fuping Block, and the Fanshi Block) that are separated by three Paleoproterozoic ductile shear zones as tectonic boundaries. The Hengshan Block and the Fuping Block are represented by a group of large-scale gneissic domes, whereas the Wutai Block characterized by the ENE–WSW-trending synclinorium, and the framework of the Fanshi Block is dominated by strike-slip ductile shear zones superimposed on tight folds. We present detailed structural analysis of the Zhujiafang ductile shear zone in the central Hengshan, the Wangjiahui ductile shear zone at the northern margin of the Wutaishan, and the metamorphic complex between these two shear zones. The macro- and microstructures indicate that the ductile shear zones show different kinematics. The Zhujiafang ductile shear zone is E–W-trending and record sinistral strike-slip shearing, and the Wangjiahui ductile shear zone is WSW–ENE-trending and record dextral strike-slip shearing. Both of shear zones have similar deformation ages according to the 40Ar/39Ar dating results, and they cut through all the gneissosity and the fold developed in these metamorphic rocks. The kinematic difference between the Zhujiafang and the Wangjiahui ductile shear zones clearly define the Fanshi Block sandwiched between them, which is characterized by movement from west to east. The Fanshi block differs from the Hengshan Block and the Wutai Block in terms of deformation history, metamorphism, and magmatism. In addition, a series of NE-trending ductile–brittle thrust nappes inside the Wutai Block under the influence of the indention of the Fanshi Block inside. Therefore, we propose that the Fanshi Block was indented to the TNCO from west to east, accompanied by strong strike-slip ductile shearing and thrusting during the late Paleoproterozoic.

Comparison of main characteristics of different Precambrian blocks in the Trans-North China Orogen

Comparison of main characteristics of different Precambrian blocks in the Trans-North China Orogen

Tectonic framework for metallogeny of precambrian structures on the example Kolyma and Omolon terranes (North-East of the Asia)

On example Omolon and Kolyma terranes (North-East of the Asia) showed a influence of the regional tectonic position and geodynamic evolution of Precambrian blocks on the structure of the ore deposits. The first of that is stable consolidated structure, and formation of basement was full completed to early of Neoproterozoic. The second example is composite terrane, similar to fold-trusted belt. Terranes with consolidated Early Precambrian crust was a stable structure for the all of geological history. Further ages of the compression and tension not to make any significance changes of that. Formed by this processes ore-controlled and ore-hosted structures correspond a deep and steep faults of the basement. By this pathways, subsequent tectonic and magmatic events permeated far into the terranes. That was favourable for forming large and hi-grade deposits but restrict the range of deposit types. Contrariwise, tectonic structure of the trust-faulted terranes was mobile for the whole timing scale. There Precambrian rocks compose of allokhtones, and trust-fault structure was multiple renewed. That was promote of forming wide range types of mineralization, but caused of dissipation of metals on numerous small-scale structures. Consequently, age and level of consolidation of the basement is a one of the most important factors of metallogeny of the Precambrian terranes. That defined the style of faulting structure (domination of steep or shallow faults), depth of the faults, morphology and geometry of the ore bodies.

Geophysical evidence for crustal and mantle weak zones controlling intra-plate seismicity – the 2017 Botswana earthquake sequence

Large earthquakes away from plate boundaries pose a significant threat to human lives and infrastructure, but such events typically occur on previously unknown faults. Most cases of intra-plate seismicity result from compression related to far-field plate boundary stresses. The April 2017 MW 6.5 earthquake in central Botswana, and subsequent events, occurred in a region with no previously known large earthquakes, occurred away from major present day tectonic activity, and accommodate extension rather than compression. Here, we present results from an integrated geophysical study that suggests the recent earthquakes may be a sign of future activity, controlled by the collocation of a weak upper mantle and weak crustal structure, between otherwise strong Precambrian blocks. Magnetotelluric data highlights Proterozoic continent accretion structure within the region, and shows that recent extension and seismicity occurred along ancient thrust faults within the crust. Our seismic velocity and resistivity models suggest a weak zone in the uppermost mantle, that does not persist to greater depths, and is therefore unlikely to represent mantle upwelling. The Botswana events may therefore be indicative of top-down extension as a response to large scale extensional forces.

Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group, Central Qilian Block, NW China

The Central Qilian Block is a Precambrian block in the Qilian Orogen, which has long drawn international attention for the study of orogeny and continental dynamics. The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north. Based on detailed fieldwork, this study presents a systematic study of petrography, mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones. The garnet metamorphic zone is composed of micaschist, garnet-bearing micaschist and felsic leptynite, with interlayered plagioclase amphibolite. The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist, sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the outer rim, the contents of almandine and pyrope slightly decrease and grossular slightly increase. Biotite in both metamorphic zones is ferro-biotite. Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone. Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage (4.5–5.0 kbar, 520–530 °C), a peak P stage (9.8–10.2 kbar, 560–570 °C), a stage of thermal relaxation (8.0–8.5 kbar, 580–590 °C) and finally a retrograde stage (6.8–7.0 kbar, 560–580 °C). Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage (5.5–6.0 kbar, 540–550 °C) and a peak stage (7.8–8.5 kbar, 660–690 °C). The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism, which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.

Tectonic affinity and evolution of the Precambrian Qilian block: Insights from petrology, geochemistry and geochronology of the Hualong Group in the Qilian Orogen, NW China

The Qilian block is a Precambrian block in the Qilian Orogen, which marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth for the Qilian block remains unclear and several models have been proposed. We present a systematic study of petrology, geochemistry and geochronology of paragneisses and intermediate to basic intrusions from the Hualong Group, which represents the lower part of Precambrian basement of the block. The protoliths of paragneisses are wackes with a maximum deposition age of ca. 900 Ma formed at an active continental margin during assembly of the Rodinia supercontinent. Their clastic input was derived from a single source granitic material of ca. 922 Ma and the magmas of their source rocks mainly came from reworked Paleoproterozoic crustal sources. The sediments were deposited close to source with a minor degree of weathering and limited sorting. Cooling after amphibolite facies metamorphism affected the paragneisses at 439.6 ± 4.9 Ma as suggested by a biotite 40Ar/39Ar age. Lenticular quartz diorites with a LA-ICPMS zircon U-Pb age of 900 ± 22 Ma are adakitic rocks formed by partial melting of lower crustal material. The quasi-lamellar hornblendites have a zircon U-Pb age of 469 ± 2 Ma and a biotite 40Ar/39Ar age of 465.4 ± 3.1 Ma. They were derived from a subduction-related metasomatic mantle and underwent fast cooling during their ascent. It is inferred that the Qilian block is a continental fragment that rifted from the South China craton during break-up of the Rodinia supercontinent, which resulted in formation of the North Qilian ocean and the South Qilian ocean as two branches of the global early Paleozoic ocean. The Hualong block, represented by the Hualong Group, is a fragment broken away from the Qilian block during this stage and situated in the South Qilian ocean. Combining our results with existing data, we identify a sequence of seafloor spreading related to a mantle plume activity (ca. 600–500 Ma), northward intra-oceanic subduction (ca. 540–469 Ma), oceanic crust-continent subduction (ca. 470–446 Ma) and continent-continent collision (ca. 446–440 Ma) between the Qilian block and the Qaidam block.

Nd and Pb Isotopic Composition of Granitoids in the Khangai Batholith as an Indicator of Crust-Forming Processes in the Terranes of the Central Asian Orogenic Belt

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 eNd(T) of 0 to +2.