Precambrian Metamorphic Complex Research Articles

The Age, Composition, and Provenances of Terrigenous Rocks in the South of the Sangilen Block of the Central Asian Orogenic Belt

Abstract —We have first determined the petrogeochemical, Sm–Nd isotope, and U–Th–Pb geochronological (detrital-zircon LA-ICP-MS) characteristics of terrigenous rocks from a thick sequence in the south of the Sangilen block, which is part of the Central Asian Orogenic Belt (CAOB). The rocks are heterogeneous in facies composition: From west to east, there is a transition from graywacke sandstones with an admixture of pyroclastics to much more silicic lithoid arenites. Geochronological data on detrital zircons from the predominant graywacke sandstones mark the following intervals of concordant ages (Ma): 787–907, 1870–2236, 2613–2725, and 2900–2980. With regard to the oldest determined age (early Cambrian, 520 Ma) of the igneous rocks intruding these sandstones, the possible period of accumulation of terrigenous sequences in the south of the Sangilen block is estimated at 790–520 Ma. The obtained geochemical, Sm–Nd isotope, and U–Th–Pb geochronological data point to Neoproterozoic and early Precambrian island arc and continent-marginal complexes as probable provenances for the terrigenous deposits of the “southern band”. The relative proportions of rocks of these complexes in the deposits changed with distance from the sedimentary basin (from west to east, in modern coordinates): The portion of early Neoproterozoic juvenile rocks decreased, and the portion of early Precambrian metamorphic complexes increased. The Paleoproterozoic and Mezoarchean detrital zircons found in the terrigenous rocks of the Sangilen block could not originate from any known Precambrian complexes of the Tuva–Mongolian microcontinent. This gives grounds to regard other craton blocks of the CAOB as their sources.

Precambrian obducted serpentinites in the Rhodope Massif

The Precambrian metamorphic complex in the Rhodope Massif is built of two lithostratigraphic units: the lower is an ancient granite-gneiss continental crust – Prarhodopian Group (PRG), and the upper one – a Neoproterozoic metamorphosed volcano-sedimentary rock complex – Rhodopian Group (RG). The lower stratigraphic levels of the RG are occupied by an ophiolitic association consisting of serpentinites, amphibolites, and metagabbros. The serpentinites constantly occupy the same level between the continental gneisses surface of the PRG and the base of the RG. The high degree of serpentinization (85–95%) indicates low temperature hydration metamorphism on the surface of an ultrabasic ocean plate. The formation of the Rhodope ophiolitic association has taken place in a Neoproterozoic supra-subduction zone in three stages: a. serpentinization at the ocean floor; b. obduction of serpentinite fragments, scraped from soft and plastic hydrated coat of the sliding ultrabasic plate; c. SSZ-type autochthonous Neoproterozoic (610–566 Ma) basic volcanism, including and covering serpentinite bodies. This determines a heterogeneous nature of the ophiolitic association. The lower granite-gneiss complex – PRG may have been a part of some microcontinent after the breaking of the supercontinent Rodinia. The formation of a supra-subduction zone – SSZ and the obduction of serpentinite fragments started during ocean closure preceding the amalgamation of supercontinent Gondwana.

Фосфаты Стрельцовского рудного поля Юго-Восточного Забайкалья (Часть 3)

The conclusions’ reliability of geologists who created the largest uranium ore base in the country on the territory of the Streltsovsky ore field (SOF) in the South-Eastern Transbaikalia, about the belonging of spatially isolated phosphate mineralization and powerful industrial mineralization to a final stage of Late Mesozoic tectonomagmatitic activation, is clarified. The relevance of the research lies in the need to study the geological materials at the SOF and the surrounding area to assess the prospects for involving in the exploitation of phosphate mineralization developed on the flanks of uranium deposits developed under the state program of the TOP. The object of study is the “blind” Argunskoye and Zherlovoye deposits and their western flanks on the surface, and the subject is the phosphate mineralization developed in the block of terrigenous-carbonate rocks. The purpose of the research is to clarify the time and features of the formation of phosphates, and the task is to assess the industrial significance of phosphate mineralization for decision-making. The methodology of scientific research is based on the interpretation of geological facts given in the literature and stock materials, use of concepts, terms, creation of hypotheses and their consequences, consistency of the comparison of which with the established facts determines the relatively high reliability of geological results. The author has used a descriptive method that takes into account a system of interrelated consistent positions, and a method of theoretical modeling, which raises the strategic goals of nature cognition to a higher scientific level. The results of the research are presented in three articles (parts). In the second part, phosphates of the region and SOF are characterized. It is established that the Talan manifestation of apatites belongs to the phosphate sorption-biogenic metalliferous formation of the Precambrian metamorphic rocks complex and combines the promising second and third most important industrial types of deposits. It is recommended to perform geological exploration at the evaluation stage and to test the electrothermal method for producing fused phosphate-magnesium fertilizers. In the third part, the known types of phosphogenesis and the genetic features of their manifestation in PSA are considered. Attention is drawn to the advantage of the recommended electrothermal method for processing apatite-carbonate and apatite-silicate rocks of the Talan manifestation

Late Precambrian Metamorphic Complexes of the Ulutau Massif (Central Kazakhstan): Age, Composition, and Formation Settings of Protoliths

This work presents the results of a study of metamorphic rocks of the Bekturgan and Balazhezdy groups of the Precambrian Ulutau massif (Central Kazakhstan), which have been traditionally identified with the most ancient Early Precambrian complexes, comprising the massif’s basement. The protoliths of the metamorphic rocks, represented by lavas and tuffs of basalt and rhyolite–trachyrhyolite composition, sandstones, and tuffaceous sandstones, comprise a contrasted volcanic-sedimentary sequence. The obtained U‒Th–Pb zircon ages (LA–ICP MS, SHRIMP II) demonstrate that the formation of this sequence occurred during a narrow (762–788 Ma) interval during the second half of Tonian (Neoproterozoic). The metamorphic transformations of the rocks were completed before an intrusion of the alkaline syenites of the Karsakpai complex (673 ± 2 Ma), which were not subject to metamorphism. The whole-rock Nd isotopic compositions of the metamorphic rocks indicate the origin of their protoliths due to erosion and partial melting of the more ancient complexes of the Precambrian continental crust of the Ulutau massif. The Hf isotopic compositions of zircons indicate that the main stages of formation and transformation of the Ulutau continental crust occurred during the Neoarchean–Neoproterozoic and coincided with the main stages of the Precambrian magmatism in the western part of the Central Asian Orogenic Belt.

Hydrochemical characterization of shallow groundwater from the Mesta River basin

Groundwater is a valuable natural resource and an important source of drinking and domestic water supply. There are two types of groundwater in the Mesta River Basin: HCO3- > Ca2++Mg2+ (1st type waters) and HCO3- < Ca2++Mg2+ < HCO3-+SO42- (2nd type waters). The waters of the first type formed in the silicate rocks, are widespread in the high mountain, mountain and low mountain area. These waters are attached to the region of regional fracturing of the Precambrian metamorphic complex represented by gneisses, granitogneisses, amphibolites and to the fractured environment of the southern Bulgarian granites and granitoids, quartzites, conglomerates and marls of the Paleozoic. The waters of the second type are characteristic of the valleys covered by Pliocene and Quaternary sediments with minimal precipitation. Groundwater from the Mesta River catchment area is mainly hydrocarbonate-calcium-magnesium with high sodium content in the southern part of the catchment area. The content of anions in groundwater is in the sequence: HCO3- > SO42-> Cl-. The cations are arranged in the order: Ca2++Mg2+ > (Na++K+) or Ca2+ > (Na++K+) > Mg2+. Groundwater is very soft to moderately hard, fresh, with TDS content up to 0.5 g/l and chloride content below 10 mg/l.

Stages and conditions of metamorphism of mafic granulites in the Early Precambrian complex of the Angara–Kan terrane (southwestern Siberian Craton)

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~1.77 Ga) and two-pyroxene granulites (~1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf values for the core and rims. The positive εHf values (up to +6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).

New data on the Mesozoic magmatism of the Bechasyn zone, in the Greater Caucasus, and estimation of the age of the regional metamorphism

The results of the local U–Th–Pb isotope dating of accessory zircons from gabbroic rocks of the Khasaut subvolcanic complex are considered. Dikes of gabbroic rocks, which were previously mapped as subvolcanic intrusions in the structure of the Precambrian metamorphic complex, were studied in the Bechasyn zone of the crystallinicum of the Greater Caucasus, in the riverbed of the Khasaut River (the left tributary of the Malka River). The U–Pb age of igneous zircons from these rocks of ~220 Ma does not support the Proterozoic age and is evidence of possible manifestation of Triassic magmatism in the Greater Caucasus. The recent data on the zircon age of the protolith of the Bechasyn complex indicate that some sedimentary rocks are younger than 440 Ma and the peak of regional metamorphism of greenschist facies in the Bechasyn zone, taking the thermochronological corrections into account, probably corresponded to 400–420 Ma.

영주화강암과 안동화강암의 SHRIMP U-Pb 저어콘 연대와 그 의미

영남육괴 북동부 지역에서 선캠브리아 변성암 복합체와 고생대 퇴적암층을 관입한 영주화강암과 안동화강암의 저반에 대한 SHRIMP U-Pb 저어콘 연대측정을 실시하였다. 영주화강암 저반의 대부분을 차지하는 각섬석-흑운모 토날라이트와 등립질 흑운모 화강섬록암은 각각 약 187Ma와 186Ma이다. 안동화강암 저반에서는 주 구성암체인 안동심성암체의 흑운모 화강섬록암과 극조립질 흑운모 화강암이 각각 182Ma와 186Ma를 나타낸다. 따라서 영주화강암과 안동화강암 저반의 연대는 오차범위 내에서 거의 일치한다. 한편 안동화강암 저반의 각섬석 반려암은 시료 중 가장 고기인 약 194 Ma이고, 영주화강암 저반 중 세립질 복운모 화강암으로 주로 구성된 춘양화강암은 가장 젊은 약 175 Ma이다. 이러한 연대측정 자료는 영남육괴 북동부인 영주-안동 지역에서의 쥬라기 대보 화성활동은 쥬라기 초에 고철질 마그마의 관입으로 시작되었고, 전기 쥬라기 중엽에 정점에 이르러 다량의 화강암질 마그마가 관입하였으며, 최종적으로 전기 쥬라기의 끝 무렵에 소량의 보다 분화된 화강암질 마그마가 관입하였음을 지시한다. SHRIMP zircon U-Pb age dating is carried out for the Yeongju and Andong granite batholiths intruding the Precambrian metamorphic complex and Paleozoic sedimentary formations within the NE Yeongnam Massif, Korea. Dating of zircons from a hornblende-biotite tonalite and an equigranular biotite granodiorite in the Yeongju granite has yielded ages of ca. 187 Ma and ca. 186 Ma, respectively. Also, dating of zircons from a biotite granodiorite and a very coarse-grained biotite granite in the Andong granite has yielded ages of ca. 182Ma and ca. 186Ma, respectively. These data indicate that the main intrusions of the Yeongju and Andong granite batholiths occur almost at the same age. The oldest age of ca. 194 Ma has been determined on zircons from a hornblende gabbro in the Andong granite, and the youngest age of 175 Ma is obtained from the Chunyang granite pluton, mainly consisting of fine-grained two-mica granite, of the Yeongju batholith. These results indicate that Jurassic Daebo magmatism in the Yeongju-Andong area, NE Yeongnam massif, started early at the Early Jurassic with an intrusion of mafic magma, and followed by an emplacement voluminous granite magma during the middle of the Early Jurassic, and was finalized with the emplacement of relatively small amount of much evolved granite magma at the end of Early Jurassic.

The main old lands in China and assembly of Chinese unified continent

The main old lands in China include the North China Block (NCB), South China Block (SCB) and Tarim Block (TRB), all of which have individual tectonic evolving histories. The NCB experienced complex geological evolution since the early Precambrian onwards, and carries important records from the old continental nuclei, giant crustal growth episode and cratonization (stabilitization), then to the Paleoproterozoic rifting-subduction-accretion-collision with imprints of the Great Oxygen Event (GOE), and to the Late Paleoproterozoic-Neoproterozoic multi-stage rifting representing North China platform tectonic features. The TRB has two-layer basement of the Early Precambrian metamorphic complexes and Neoproterozoic sedimentary sequences. Three till sheets have been reported. The SCB consists of the Yangtze Block (YZB) and Cathaysia Block (CTB) that were cohered in the Neoproterozoic. The YZB recorded tectonic processes of the Early Precambrian crustal growth, 1.0–0.9 Ga and 0.8–0.6 Ga metamorphic-magmatic events, and two Neoproterozoic glaciations. The CTB consists of ca. 1.8 Ga, 1.0 to 0.9 Ga and ca. 0.8 Ga granitic gneisses and metamorphic rocks, indicating there was a vast Precambrian basement. The Neoproterozoic sedimentary rocks overlie partly on the basement. That the YZB and CTB have a Neoproterozoic uniform cover layer illustrates the SCB should form, at least, during 1.0–0.9 Ga, corresponding to the Rodinia Supercontinent. The Central Chinese Orogenic System with high-ultra-high-pressure metamorphic rocks supports a suggestion that the above-mentioned three old lands were collided to assemble a unified Chinese Continent during the Pangea orogenic period.

Age of mafic granulites from the early precambrian metamorphic complex of the Angara-Kan terrain (Southwestern Siberian Craton): U-Pb and Lu-Hf isotope and REE composition of zircon

Age of mafic granulites from the early precambrian metamorphic complex of the Angara-Kan terrain (Southwestern Siberian Craton): U-Pb and Lu-Hf isotope and REE composition of zircon

Upper-Crust Shear-Wave Velocity of South Korea Constrained by Explosion and Earthquake Data

Abstract This study presents the 3D variations of shear-wave velocity of the upper crust in the southern Korean Peninsula, estimated using earthquake data as well as refraction-experiment data using large explosions. By combining two different data sets in different period ranges, we obtained group velocities over an extended bandwidth. The inversion of the group velocities with the extended bandwidth secured better resolution and produced much better agreement between the group velocity data and the prediction from the inversion model. Our 3D images of the shear-wave velocities correlate very well with the local geology and tectonics, showing low velocities in the Cretaceous sedimentary basins in the Gyeongsang basin and the Honam shear zone and high velocities in the Gyeonggi and Yeongnam massifs. The velocities of the Okcheon fold belt are lower than those of the two massifs and higher than those of the Gyeongsang basin. The velocity profiles of three tectonic zones, the two massifs and the fold belt, are different from the velocity profile of the Gyeongsang basin. While the three tectonic zones show a rapid increase from 2.6 km/s at the surface to 3.2 km/s at 1-km depth, the Gyeongsang basin shows the steepest velocity gradient between 2–3 km depth. The depth of the Gyeongsang basin inferred from the steepest velocity gradient in the vertical velocity sections is estimated at ∼3 km, while for the small basins along the Honam shear zone, it is estimated to be less than 2 km. High shear-wave velocities are found in the northeast area of the Gyeonggi massif and at the southwest tip of the Yeongnam massif, where a Precambrian metamorphic complex is widespread.

P–T evolution of the Precambrian Metamorphic Complex, NW Iran: a study of metapelitic rocks

AbstractThe Mahneshan Metamorphic Complex (MMC) is one of the Precambrian terrains exposed in the northwest of Iran. The MMC underwent two main phases of deformation (D1 and D2) and at least two metamorphic events (M1 and M2). Critical metamorphic mineral assemblages in the metapelitic rocks testify to regional metamorphism under amphibolite‐facies conditions. The dominant metamorphic mineral assemblage in metapelitic rocks (M1) is muscovite, biotite I, Garnet I, staurolite, Andalusite I and sillimanite. Peak metamorphism took place at 600–620°C and ∼7 kbar, corresponding to a depth of ca. 24 km. This was followed by decompression during exhumation of the crustal rocks up to the surface. The decrease of temperature and pressure during exhumation produced retrograde metamorphic assemblages (M2). Secondary phases such as garnet II biotite II, Andalusite II constrain the temperature and pressure of M2 retrograde metamorphism to 520–560°C and 2.5–3.5 kbar, respectively. The geothermal gradient obtained for the peak of metamorphism is 33°C km−1, which indicates that peak metamorphism was of Barrovian type and occurred under medium‐pressure conditions. The MMC followed a ‘clockwise’ P–T path during metamorphism, consistent with thermal relaxation following tectonic thickening. The bulk chemistry of the MMC metapelites shows that their protoliths were deposited at an active continental margin. Together with the presence of palaeo‐suture zones and ophiolitic rocks around the high‐grade metamorphic rocks of the MMC, these features suggest that the Iranian Precambrian basement formed by an island‐arc type cratonization. Copyright © 2010 John Wiley & Sons, Ltd.

Magmatic and metamorphic evolution of the Shotur Kuh metamorphic complex (Central Iran)

Metamorphic basement rocks, that are exposed beneath the very low-grade to unmetamorphosed Upper Jurassic-Eocene formations north of the Torud fault zone within the Great Kavir Block, were investigated to elucidate the origin of their protoliths and the pressure and temperature conditions of metamorphism. The basement, previously assumed as a pre-Cambrian metamorphic complex, is mostly formed by amphibolite-facies orthogneisses (tonalite, granodiorite, and granite) with amphibolites and small amounts of metasediment-micaschists. Major- and trace-element geochemistry in combination with U–Pb age dating of zircon showed that the protoliths formed during Late Neoproterozoic continental arc magmatism that has also been identified in other tectonic blocks of Central Iran. In addition to quartz, feldspar(s), micas in orthogneisses, and amphibole + plagioclase in amphibolite, all rocks may contain garnet that shows prograde zoning. Kyanite was found only in some Al-rich amphibolite together with gedrite. The PT conditions of the rocks, based on conventional geothermobarometry and the pseudosection method, show a medium-pressure amphibolite-facies metamorphism. Ar–Ar age dating of muscovite reveals that this metamorphism occurred in the Middle Jurassic (166 Ma) and related to the closure of the Neotethyan basin.

Kyanite–sillimanite metamorphism of the Precambrian complexes, Transangarian region of the Yenisei Ridge

Four Precambrian metamorphic complexes in the vicinity of regional faults in the Transangarian region of the Yenisei Ridge were examined. Based on geothermobarometry and P-T path calculations, our geological and petrological studies showed that the Neoproterozoic medium-pressure metamorphism of the kyanite–sillimanite type overprinted regionally metamorphosed low-pressure andalusite-bearing rocks at about 850 Ma. A positive correlation between rock ages and P-T estimates for the kyanite-sillimanite metamorphism provide evidence of the regional structural and tectonic heterogeneity. The medium-pressure metamorphism was characterized by (1) the development of deformational structures and textures, and kyanite-bearing blastocataclasites (blastomylonites) with sillimanite, garnet, and staurolite after andalusite-bearing regional metamorphic rocks; (2) insignificant apparent thickness of the zone of medium-pressure zonal metamorphism (from 2.5 to 7 km), which was localized in the vicinity of the overthrusts; (3) a low metamorphic field gradient during metamorphism (from 1–7 to 12 °C/km); and (4) a gradual increase in lithostatic pressure towards the thrust faults. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence for near-isothermal loading. This event was justified within the framework of the crustal tectonic thickening model via rapid overthrusting and subsequent rapid uplifting and erosion. The results obtained allowed us to consider medium-pressure kyanite-bearing metapelites as a product of collision metamorphism, formed either by unidirectional thrusting of rock blocks from Siberian craton onto the Yenisei Ridge in the zones of regional faults (Angara, Mayakon, and Chapa areas) or by opposite movements in the zone of splay faults of higher orders (Garevka area).

Regularities of volcanosedimentary iron accumulation in the Early Precambrian

Issue of the correlation between ferruginous-siliceous accumulation and volcanism at early stages of the continental lithosphere evolution is considered. Magnetite quartzites of the Early Precambrian volcanosedimentary greenstone and intensely metamorphosed complexes are subdivided into volcanosedimentary and sedimentary genetic types. Major features of the evolution of ferruginous-siliceous accumulation in the Early Precambrian are elucidated and a model of the evolution of volcanogenic iron accumulation is proposed. Principles of the prognosis of magnetite mineralization in the Early Precambrian metamorphic, primary volcanosedimentary, and greenstone complexes are elaborated.

Reconstruction of the primary stratigraphy and correlation of the Precambrian metamorphic complexes in the Rhodope massif

The lithological analysis of the metamorphic rocks in the Rhodope massif, taking into consideration the lateral variability of their primary composition, polymetamorphic effects and tectonic deformations, demonstrates that prior to the metamorphism the upper Rhodopian Supergroup is composed of two complexes: lower volcano-sedimentary and upper calcareous ones. The lower complex after the metamorphism is composed of metaophiolites (serpentinites, amphibolites, metagabbroids) in the lower levels), mica schists, marbles, quartzites, jaspilites, called Variegated Formation. The upper complex is composed of marbles interlayered with schists. There are two types of Variegated Formation: à. Western Rhodopian (Satovcha) type with ophiolites prevailing in lower layers and b. Central Rhodopian (Lukovitsa) type with sedimentary rocks prevailing. A new correlation scheme of the lithostratigraphic units from Precambrian blocks in South Bulgaria is presented.

EPMA Th-U-Pb Monazite Dating of Zhongtiao and Lüliang Precambrian Metamorphic Complexes

Zhongtiao and Lüliang Precambrian metamorphic complexes are essential parts of the southern segment of Trans-North China Orogen (TNCO), the North China Craton (NCC). Their metamorphism and related granitic magmatism enable to better understand the nature and the evolution of the TNCO, the NCC, with great scientific significance. EPMA Th-U-Pb monazite dating of two monzogranitic rock samples from Yongji and Henglingguan intrusions in the Zhongtiao Precambrian complex yields two peak ages of 1884.7 Ma-1849.9 Ma, and 1743.5–1738.8 Ma, respectively. The former is interpreted as the main metamorphic ages and the latter as the ages of fluid activation. Five samples from the Lüliang Precambrian complex were dated by EPMA Th-U-Pb monazite dating method. The garnet two-mica schist and the granitoid gneiss recorded three age peaks or range of >1902 Ma, 1883.3 Ma-1865.6 Ma, and 1731.3 Ma, corresponding to early magmatic and anatectic events, peak metamorphism, and late fluid activation, respectively. The two samples of garnet granite yield the ages of 1882.8 Ma-1850.9 Ma, corresponding to the anatectic granitic magmatism, which simultaneously occurred with the peak metamorphism. The unmetamorphosed granite vein yields the age of 1742.6 Ma, which is interpreted as the forming age of the granite vein and together with the age of 1731.3 Ma shown by the garnet two-mica schist may imply important fluid-magmatic activation events. The above dating results reveal that the peak metamorphism occurred during 1885 Ma-1849 Ma in the Zhongtiao and the Lüliang Precambrian metamorphic complexes, accompanied by the S-type granitic magmatism. The age range of the peak metamorphism and cogenetic S-type granitic magmatism in the Zhongtiao and Lüliang Precambrian complexes are consistent with the metamorphic and the deformation events in the Hengshan–Wutai–Fuping complexes, further proving that the amalgamation between the Western and the Eastern continental blocks in the NCC occurred during late Paleoproterozoic.

P-T paths and problems of high-temperature polymetamorphism

Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the Tmin of the older P-T trajectory corresponded to Tmax of the younger trajectory, and often Tmax–Tmin > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.

Discovery of radiolarian fossils in the Xixiang Group, the Southern Qinling, central China, and its implications

The Xixiang Group in the southern Qinling has been believed to be a Precambrian metamorphic complex. Radiolarian fossils were found in the muddy chert interlayers of volcanic rocks in the Sunjiahe Formation of the group. The ages of the fossils are Late Devonian and Early Carboniferous in the lower member and Early Carboniferous in the middle and upper members of the formation. The complex in the Xixiang area can be determined as a Paleozoic-Triassic melange which connects with the Mianxian- Lueyang ophiolitic melange.

Caledonian ductile thrusting in a Precambrian metamorphic complex: The Moine of northwestern Scotland

Structural models drawn from external thrust-belts have been applied to a part of the metamorphic Caledonides in northwestern Scotland. Internal deformation of the amphibolite-facies Moine rocks was accomplished primarily by northwest-directed ductile thrusting (“sliding”), which was succeeded within a significant portion of the Moine by upright folding above a basal detachment. The Moine can be regarded as a stack of three major nappes, each of which has distinctive structural, stratigraphic, and metamorphic characteristics. Caledonian shortening within the Moine is of the order of 100 km, yet geothermometry and geobarometry confirm that all tectonic units had equilibrated at similar mid-crustal depths during a Precambrian, “Ardgourian” metamorphic event (∼1000 Ma). Kyanite-grade Caledonian metamorphism implies that ductile thrusting and steep refolding took place beneath a considerable overburden, although an intervening stage of uplift is attested to by the Caledonian reworking of low-pressure thermal aureoles. Final uplift of the complex took place during or after emplacement onto the foreland by the Moine thrust and related structures. Prior to Caledonian tectonics, the Moine was only a few kilometres thick but hundreds of kilometres in lateral extent, perhaps as a result of post-Ardgourian, pre-Caledonian crustal stretching. The pre-existing geometry of the interface between Moine cover and Lewisian basement exerted a major control on Caledonian structural development.