Paleozoic Tectonothermal Event Research Articles

Microtexture, geochemistry and geochronology of monazite and zircon from the Jialu deposit in the Lesser Qinling: Implications for multi-stage magmatic and metamorphic events in the southern margin of the North China Craton

The Lesser Qinling area is the largest outcrop of the Precambrian basement in the southern margin of the North China Craton (S-NCC). Notably, many carbonatite-related polymetallic deposits are distributed in this area, including the Jialu deposit with light and heavy REE mineralization. Carbonatite and granite from the Jialu deposit contain diverse monazite and zircon grains, and deciphering the age distributions and chemical compositions of these monazites and zircons is crucial for comprehending the multi-stage magmatic and metamorphic events in the S-NCC. Type I monazite from banded REE ore veins yields a U-Pb age of 217.0 ± 5.0 Ma, representing the formation age of carbonatite. Type II monazite from granite yields consistent U-Pb and Sm-Nd ages of ∼1.95 Ga, representing the emplacement age of granite. Type III monazite is hydrothermal in origin, and yields two dominant age peaks at ∼550 Ma and ∼525 Ma. Types I, II and III zircons are xenocrysts from the Taihua Group and yield two main age peaks at ∼2.35 and ∼2.17 Ga. Of note, type III zircon also records an early Paleozoic tectonothermal event by a lower intercept age of 532 ± 59 Ma, which overlaps the ages recorded by hydrothermal monazite. The early Paleozoic tectonothermal events discovered may be a manifestation of the global Pan-African Orogeny in the S-NCC. This new finding further implies that the S-NCC was significantly affected by the assembly of the Gondwana supercontinent during the early Paleozoic.

Paleozoic remelting of carbonatite in Bayan Obo (China): Further insights into the formation of a giant REE deposit

Currently, it has been confirmed that the ore-hosting dolomite of the giant Bayan Obo deposit is the Mesoproterozoic igneous carbonatite, but the roles, that the Paleozoic tectonothermal events played, are still poorly understood. In this study, we present a comprehensive investigation on the geochemistry of various carbonates to understand the generation of the Bayan Obo carbonatite. Hydrothermal monazite Th-Pb dating (peaking at ∼ 420 Ma) identified an atypical coarse-grained dolomite, which occurring as veins, and cutting through typical fine dolomitic facies. Typical coarse dolomitic facies and atypical coarse-grained dolomite are significantly different in terms of MgO, FeO, MnO, Y, Al, Si, V, Ti, Ni, Sc, and ∑REE contents and the C-O-Sr isotopic compositions, and atypical coarse-grained dolomite has lower Ba and Sr contents, lower Co/Ni ratio, with strong Eu positive anomaly and more scattered element concentration, suggesting that it has a different origin from typical coarse dolomitic facies, crystallizing from a relatively low-temperature and oxidized environment. The pervasiveness of magma flow structure and magnetite mass in the contact zone between the granite plutons and the fine dolomitic facies indicates that, at ∼ 270 Ma, the heat flux from granitic magma triggered melting of the fine dolomitic facies, forming remelted calcitic carbonatite. In addition, Th-Pb ages of monazite ranging from 1350 Ma to 240 Ma and the REE-poor properties of both remelted carbonatites suggest that both Paleozoic thermal events only caused REE activation and redistribution, without introducing additionally exogenous ore-forming metals. Accordingly, we propose that the dolomite mass in Bayan Obo are Mesoproterozoic mantle-derived carbonatites (including ore-hosting dolomite and carbonatite dike), which underwent remelting at ∼ 440 Ma and ∼ 270 Ma, respectively.

Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 – 689 °C/10.0 – 11.6 kbar to 780 – 807 °C/9.9 – 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 – 689 °C/4.1 – 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 – 1.85 and 1.95 Ga. The ca. 1.82 – 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low-pressures. These zircon results provide evidence for 1.8 – 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 ± 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 ± 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 – 720℃) and titanite U-Pb (710 – 780℃) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of Paleozoic orogeny. In addition, apatite U-Pb dating yields 236.6 ± 4.3 and 300 ± 10 Ma for samples from the southern and central Dunhuang area, respectively, indicating different cooling histories. Combining with previous results, we propose that the Paleozoic metamorphism occurred in a long-lived hot continental arc with slow cooling rates (0.3 – 5℃/Myr), rather than an subduction–accretionary complex. Our study highlights the decoupling of metamorphic zircon U-Pb ages and P-T paths, which has significant implications for P-T-t path studies in polyphase metamorphic terranes.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Reworked early Precambrian basement of the Dunhuang Block (NW China) as revealed by titanite U–Pb dating

The Dunhuang Block in the Central Asian Orogenic Belt (CAOB) consists of Archean to Paleoproterozoic crystalline basement, along with Paleozoic complexes that are related to the evolution of the CAOB. A current main controversy concerns whether the whole Dunhuang Block has experienced the Paleozoic tectonothermal events. Titanite, a common U–bearing accessory mineral in igneous and metamorphic rocks, is susceptible to magmatic and metamorphic processes, and thus tends to record relatively late tectonothermal events for metamorphic rocks. In this study, we conducted LA–ICPMS U−Pb dating on titanites from the Archean to Paleoproterozoic rocks in the Dunhuang Block. The titanites are likely metamorphic in origin and yielded U–Pb ages of 409−309 Ma. These ages can be divided into two major groups at 409−390 Ma and 354−309 Ma, corresponding to an early-stage retrograde of high–grade metamorphism and a later-stage magmatism, respectively. Overall data and regional geology indicate that the whole Dunhuang Block has been intensively reworked by the Paleozoic tectonothermal events, leading to metamorphic and magmatic overprints of the early Precambrian basement rocks during the evolution of the CAOB in the Paleozoic. Therefore, the Dunhuang Block should not be considered as an independent Paleozoic orogenic belt. More importantly, our study demonstrates that titanite U–Pb dating can be used as a potential geochronologic tool to reveal late tectonothermal events of the reworked Precambrian metamorphic terranes.

Possible imprints of late Paleoproterozoic orogeny in the Dunhuang terrane, NW China: Constraints from igneous and metapelitic rocks

The Dunhuang terrane is a NEE trending tectonic belt confined by the Tarim Craton, Beishan Orogen, Alxa Block and Altyn Tagh Fault. To constrain the late Paleoproterozoic geotectonic background of this terrane, a synthetic research on petrology, geochemistry, zircon and monazite geochronology, and zircon Lu-Hf and whole-rock Nd isotopes of late Paleoproterozoic (ca. 1.9–1.8 Ga) igneous rocks and metapelites was conducted. The Sanweishan-Dongshuigou Na-rich rhyolite and granite formed at ca. 1.84–1.82 Ga exhibit enriched light rare earth elements (LREEs), depleted heavy rare earth elements (HREEs) and insignificant Eu anomalies, with low Y (3.44–6.39 ppm) and Yb (0.29–0.53 ppm) but relatively high Mg# (54.5–58.5) and Sr (368–643 ppm), and variable Cr (5.58–29.7 ppm) and Ni (2.82–15.6 ppm) contents, as well as negative to positive εHf(t) (−6.1 to +6.4) values, indicating subduction-related arc magmatism accompanied by crustal-mantle interaction. The Shuwozi granitic gneiss crystallized at ca. 1.81 Ga displays S-type granite affinity (A/CNK > 1.1) and has low Sr/Y (1.0–1.3) and (Gd/Yb)N (1.50–1.98) ratios, and negative εHf(t) (−20.3 to −5.1) and εNd(t) (−4.3) values, suggesting partial melting of crustal rocks at shallow crustal depth. The Hanxia amphibolite with protolith age of ca. 1.86–1.84 Ga is featured by depleted LREE and flat HREE patterns, insignificant Eu anomalies, low Nb/Ta (11.0–12.0), Ta/Yb (0.02–0.04), Th/Yb (0.01–0.04) and Th/Ta (0.7–1.3) but high Ti/V (22.9–26.7) ratios, and positive εNd(t) (2.7–4.0) values, resembling depleted mantle-derived N-MORB- or BABB (back-arc-basin basalt) geochemical affinities. The metapelite, characterized by mineral assemblage of garnet, biotite, sillimanite, plagioclase, K-feldspar, quartz and rutile, documented ca. 1.81 Ga upper amphibolite-facies metamorphism with peak metamorphic P-T conditions being restricted to 690–810 °C and 5.3–6.4 kbar based on conventional geothermobarometry. These magmatic and metamorphic rocks were variously altered and deformed during Paleozoic (ca. 430–360 Ma) tectonothermal event. Integrated with published data, it can be inferred that the Dunhuang terrane underwent ca. 1.89–1.81 Ga subduction-collisional orogeny related to late Paleoproterozoic (ca. 2.0–1.8 Ga) global assembly of the Columbia supercontinent, similar as those recorded in the northern Tarim Craton, Alxa Block and Quanji massif, and then overprinted Paleozoic (ca. 440–360 Ma) orogenic process, corresponding to the previously proposed composite orogeny in the Dunhuang terrane.

Early Paleozoic magmatism in northern Kontum Massif, Central Vietnam: Insights into tectonic evolution of the eastern Indochina Block

The Chu Lai and Dai Loc granitic complexes are distributed on two sides of the early Paleozoic Tam Ky – Phuoc Son suture zone (TPSZ) in the northern Kontum Massif (KTM), central Vietnam. Studies of their petrogeneses and tectonic setting provide new insights into the tectonic evolution of the eastern Indochina Block and its affinity with the South China Block (SCB). Zircon U-Pb dating indicates that the Chu Lai and Dai Loc granites formed at 445–452 Ma and 415–420 Ma, respectively. The Chu Lai granites have high ASI, K2O/Na2O and Rb/Sr as well as strongly negative zircon εHf(t) (−14.6 to −2.9) and complex inherited zircon components, characteristics of typical S–type granites. The Chu Lai granites may be subdivided into two groups based on their geochemistry. However, both groups have similar zircon Hf isotopes and inherited–zircon age spectra, suggesting that they probably were derived from different degrees of partial melting of the same late Neoproterozoic meta–greywacke source at different crustal levels. The Dai Loc granites have variable major– and trace–element contents, ASI, K2O/Na2O, Rb/Sr, and zircon Hf–isotope compositions, indicating that most of them are S–type granites and some are I–type granites, suggesting derivation from heterogeneous sources. The petrology, geochemistry, geochronology and formation temperatures suggest that the Chu Lai granites formed in a syn–collisional setting between the KTM and Truong Son Belt (TSB) and the Dai Loc granites in a post–collision extensional setting triggered by the upwelling asthenosphere mantle. Similarities of age spectra and Hf isotope compositions of inherited zircons from the Chu Lai granites with detrital zircons from the Neoproterozoic sedimentary rocks in Western Cathaysia Block and Tethyan Himalaya suggest they are adjacent each other during the late Neoproterozoic. The ages and zircon Hf–isotope compositions of early Paleozoic granites between the KTM and TSB are consistent with those in the SCB, but different from many other microcontinents from the northern margin of East Gondwana, suggesting that the SCB was close to the KTM and TSB and experienced similar early Paleozoic tectonothermal events. It can be inferred that there was a concealed early Paleozoic subduction zone south of the SCB, connecting with the TPSZ in central Vietnam, and that the far–field effects of the subduction of an unknown oceanic plate triggered the early Paleozoic intracontinental orogeny in the SCB.

Paleozoic tectonothermal event in Mt. Dongbatu, Dunhuang terrane, southernmost Central Asian Orogenic Belt (CAOB): Implications for petrogenesis and geological evolution

In this contribution, systematic petrographical, whole-rock geochemical, geochronological and zircon Lu-Hf isotopic investigations were conducted on Paleozoic (ca. 410–400 Ma) metamorphic-magmatic rocks from Mt. Dongbatu in the central Dunhuang terrane. The metamorphic rocks in the Shangkouzi area are of meta-calcsilicate rocks, and granitoid rocks occurring within these rocks are geochemically of trondhjemite. Whereas, metamorphic rocks in the Gangou area are metabasaltic rocks. LA-ICP MS zircon U-Pb dating results reveal that these metamorphic-magmatic rocks recorded ca. 1.6 Ga magmatic event and metamorphic events of ca. 410–400 Ma and ca. 370 Ma. Field occurrence, whole-rock geochemistry, mineral composition and zircon Hf isotopes suggest that the 1.6 Ga mantle-derived basaltic magma, together with iron-rich and Ca-rich components and mica-quartz schist of Paleoproterozoic supracrustal rocks probably contributed to protolith of meta-calcsilicate rocks in the Shangkouzi area, and granitoid rocks within meta-calcsilicate rocks were possibly generated by partial melting of calcsilicate rocks.Combined with previously published data, it is suggested that the protoliths of Paleozoic high-grade metamafic rocks in the Dunhuang terrane are predominantly composed at least of basaltic rocks produced by magmatic events of 2.5–1.8 Ga, ca. 1.8 Ga and ca. 1.6 Ga. The Paleozoic granitoid rocks in the Dunhuang terrane might have been derived from different magma sources as partial melting of subducted plate, or thickened lower continental crust, or pre-existing basaltic rocks produced by 1.6 Ga and 1.8 Ga magmatic activities, and calcsilicate rocks genetically associated basaltic magmatism at ca. 1.6 Ga. Considering regional tectonic evolution, it can be concluded that the Dunhuang terrane was probably in a post-collisional extension setting at ca. 1.6 Ga, producing certain amount of basaltic and calcsilicate rocks. From the beginning of Silurian (ca. 440 Ma), the Dunhuang terrane was involved in the orogenic processes related to the closure of the southern Paleo-Asian Ocean, becoming the active continental margin of the southernmost CAOB.

Two kinds of authigenic xenotime overgrowths in response to an Early Paleozoic tectonothermal event in South China

This study has documented two kinds of xenotime overgrowths from the Cryogenian-Ediacaran sedimentary rocks in South China. The identical tetragonal dipyramidal structures prompted one type of xenotime to overgrow detrital zircon grains, whereas the other type coexists with rutile crystals. SIMS Pb-Pb, NanoSIMS, EPMA, and TEM imaging analyses suggest these xenotime overgrowths, together with their rutile substrate could be of hydrothermal origin; formed by means of percolating fluids leaching ilmenite and REE(Y) bearing silicate minerals. Iron, Ti, and REE(Y) ions leached in this way were combined with O, S, and P anions to precipitate rutile, xenotime, iron oxides, and pyrite in other pore spaces. Such fluid induced activities were probably induced by the ca. 460–390 Ma Wuyi-Yunkai Orogeny in South China. The most important finding is that the rutile-xenotime assemblage was precipitated almost simultaneously, rendering a dissolution-reprecipitation process impossible. Absence of dissolution margins on detrital zircons suggest detrital Fe-Ti oxides are more prone to be leached by fluids than zircon grains, probably because the latter has more stable chemical properties.

Tectono-thermal evolution of the southwestern Alxa Tectonic Belt, NW China: Constrained by apatite U-Pb and fission track thermochronology

The Central Asian Orogenic Belt (CAOB) is regarded to have undergone multiple phases of intracontinental deformation during the Meso-Cenozoic. Located in a key position along the southern CAOB, the Alxa Tectonic Belt (ATB) connects the northernmost Tibetan Plateau with the Mongolian Plateau. In this paper we apply apatite U-Pb and fission track thermochronological studies on varieties of samples from the southwestern ATB, in order to constrain its thermal evolution. Precambrian bedrock samples yield late Ordovician-early Silurian (~430–450Ma) and late Permian (~257Ma) apatite U-Pb ages; the late Paleozoic magmatic-sedimentary samples yield relatively consistent early Permian ages from ~276 to 290Ma. These data reveal that the ATB experienced multiple Paleozoic tectono-thermal events, as the samples passed through the apatite U-Pb closure temperature (~350–550°C). We interpret these tectonic events to record the long-lived subduction-accretion processes of the Paleo-Asian Ocean during the formation of the southern CAOB, with possible thermal influence of the Permian Tarim mantle plume. Apatite fission track (AFT) data and thermal history modelling reveal discrete low-temperature thermal events for the ATB, inducing cooling/reheating through the AFT partial annealing zone (~120–60°C). During the Permian, the samples underwent rapid cooling via exhumation or denudation from deep crustal levels to temperatures <200°C. Subsequent thermal events in the Triassic were thought to be associated with the final amalgamation of the CAOB or the closure of the Paleotethys. During the Jurassic-Cretaceous the study area experienced heating by burial, followed by renewed cooling, which may be related with the construction and subsequent collapse of the Mongol-Okhotsk Orogeny, or the Lhasa-Eurasia collision and subsequent slab break-off. These results indicate that the ATB may have been stable after late Cretaceous in contrast to the Qilian Shan and Tianshan. Finally, our results indicate differential exhumation scenario occurred across the southwestern ATB during the Cretaceous.

Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860±6Ma and 435±5Ma, along with variably disconcordant ages with lower intercept ages of 442±41Ma. The zircon core ages range from 3015Ma to 851Ma, with three major age populations at 930–865Ma, 1850–1200Ma and 2650–2400Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736Ma, 1835–1775Ma and 2720–2500Ma. Metamorphic ages of ca. 860Ma and ca. 435Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction-collision process within the Cathaysia Block at around 860Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440Ma.

Gold mineralization in the Jiangnan Orogenic Belt of South China: Geological, geochemical and geochronological characteristics, ore deposit-type and geodynamic setting

Located in the southeastern margin of the Yangtze Block and generally interpreted as the Neoproterozoic collisional product of the Yangtze with the Cathaysia Blocks of South China, the Jiangnan Orogenic Belt (JOB) contains a number of gold (Au) (-polymetallic) ore deposits and mineral showings, mostly hosted by Neoproterozoic low-grade metamorphic volcaniclastic and sedimentary rocks. The mineralization styles mainly include auriferous quartz veins and disseminated mineralization in altered mylonite and cataclasite that are developed along shear zones, fracture zones and inter- or intra-formational fault zones closely related to regional folding and shearing deformation. Three gold mineralizing epochs are recognized in the JOB. The ca. 423–397Ma mineralization was associated with the early Paleozoic tectonothermal event(s), which induced widespread emplacement of Silurian S-type granites, low-grade metamorphism and enrichment of gold in the Neoproterozoic rocks (i.e., forming Au source beds). The second Au mineralization epoch, occurring at ca. 176–170Ma (Jurassic), was related to the subduction of the Paleo-Pacific plate beneath the South China continental margin. The third and most important Au mineralization epoch took place at ca. 144–130Ma (early Cretaceous), when a Basin-and-Range tectonic pattern was developed, characterized by NE–NNE-trending strike-slip faults, granitic domes and metamorphic core complexes (MCC), and basins filled with red bed lithologies. C, H, O, He-Ar, S and Pb isotopic and fluid-inclusion data suggest that the ore fluids were predominantly metamorphic and/or magmatic, with variable input of mantle-derived fluids and the progressive involvement of meteoric waters in the later stages of mineralization. Ore materials were mostly contributed by the Neoproterozoic source beds, plus a possible contribution from mantle- or magma-derived components. The Au (-polymetallic) deposits in the JOB, particularly those formed in the early Cretaceous, share many geological and geochemical features with the orogenic-type and Carlin-type deposits. In the context of tectonic evolution of South China, the gold mineralization in the JOB may be considered an “intracontinental reactivation type”, characterized by synchronous development of Au–polymetallic mineralization, reactivation of stuctures developed in Neoproterozoic metamorphic rocks, and widespread granite emplacement in the late Mesozoic.

Tectono-thermal events in East Kunlun, Northern Tibetan Plateau: Evidence from zircon U–Pb geochronology

The widely distributed high-grade gneisses in the East Kunlun Orogenic Belt (EKOB) are keys to understand the Precambrian tectonic evolution of the Northern Tibetan Plateau. In this study, new LA-ICP-MS zircon U–Pb ages from paragneiss and schist of the Proterozoic Jinshuikou Group and quartzite of the Proterozoic Binggou Group are reported in an attempt to evaluate the Neoproterozoic and Paleozoic tectono-thermal events of the EKOB. These geochronologic data can be classified into 4 groups: Group 1 ages ranging from 2243Ma to 3701Ma are represented by inherited zircons from protolith and confirm the existence of Eoarchean to Paleoproterozoic continental nucleus in the source region of the Jinshuikou Group. Group 2 ranging from 928Ma to 1849Ma yields lower intercept ages of 0.9–1.0Ga which represent the Neoproterozoic tectono-thermal event. This event, similar to that of the northern margin of Qaidam, might be a response to the assembly of Rodinia. Group 3 ranges from Neoproterozoic to early Paleozoic with lower intercept ages which are identical to the weighted mean ages of Group 4. These two age groups confirm the tectono-thermal event related to Paleozoic oceanic subduction. Moreover, based on the youngest age of 2.2Ga in Group 1 and the upper intercept age of 1.8Ga in Group 2, the depositional timing of the Jinshuikou and Binggou groups can be defined as Paleoproterozoic and Mesoproterozoic, respectively.

Late Paleozoic anorogenic magmatism of the Gobi Altai (SW Mongolia): Tectonic position, geochronology and correlation with igneous activity of the Central Asian Orogenic Belt

Geological and geochronological U–Pb (zircon ID-TIMS) and 40Ar/39Ar (amphibole) data were obtained for six anorogenic magmatic associations in the Gobi Altai, southwestern Mongolia, in order to reveal its relationships with Late Paleozoic tectonothermal events in the southern part of the Central Asian Orogenic Belt (CAOB). In the Gobi Altai massifs of alkaline granites and grabens filled with bimodal volcanic suites occur as a chain 450km long east–southeast direction. The massifs and volcanic fields are controlled by contacts of pre-Ediacaran continental crustal blocks with Late Neoproterozoic–Cambrian juvenile ophiolite and arc-type complexes of the Lake zone. Gobi Altai alkaline granites bear sodic-rich mafic silicate minerals and possess typical properties of ferroan A-type granites. Our geochronological data constrain emplacement time of the Gobi Altai alkaline granites and bimodal volcanic rocks at 293±1–279±1Ma. This age interval coincides with timing of the emplacement of I-type granites (with a few S-type granites) and gabbroic intrusions in the Mongolian and Chinese Altai as well as with numerous massifs of A-type granites, layered mafic–ultramafic intrusions, and eruption of bimodal volcanic rocks between the southern edge of the Siberia craton and the northern margins of the Tarim and North China cratons. Hence, the Gobi Altai anorogenic magmatic complexes were produced by a large-scale Late Paleozoic magmatic event (or events) that encompassed a wide spectrum of terranes in CAOB. The most realistic models explaining these magmatic processes are: (i) the effect of the Tarim mantle plume, (ii) slab (or slabs) break-off after subduction blocking due to accretion of terranes of various types.

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.

Zircon U–Th–Pb–Hf isotopes of the basement rocks in northeastern Cathaysia block, South China: Implications for Phanerozoic multiple metamorphic reworking of a Paleoproterozoic terrane

The Cathaysia block records a complex tectonic history, the understanding of which is central to the debate on the evolution of the South China Block and its position in various supercontinent assemblies. Here we investigate two key formations from this block, the Mayuan Group in the northern Fujian Province, northeastern Cathaysia block and its equivalent the Badu Group in southwestern Zhejiang Province. Previous studies traced the Paleoproterozoic records from the Badu Group whereas a Neoproterozoic age was proposed for the Mayuan Group. The rocks sampled in this study from both groups show similar mineral assemblages of garnet+sillimanite+biotite+plagioclase+quartz±K-feldspar±muscovite±graphite as well as high contents of SiO2 and Al2O3, typical of amphibolite- to granulite-facies metapelitic rocks. Zircon U–Pb data yield two discordia intercept ages of ~1990Ma and ~2450Ma from one sample and discordia intercept ages of ~3.5Ga, 2.5Ga, 1.86Ga and 233Ma from another in the Badu Group. Zircons in two samples from the Mayuan group yield intercept ages of 1859Ma and 249Ma in one sample and ~2.6Ga, 1.87Ga, 257Ma and a weighted mean 206Pb/238U age of 248Ma in the other. The ca. 1.86–1.87Ga and 230–250Ma ages are interpreted to represent the time of metamorphic reworking because zircon grains of these ages tend to have low Th/U ratios, flat HREE patterns and unzoned internal texture as revealed by cathodoluminescene (CL) images. These results confirm that the Badu Group is a Paleoproterozoic lithostratigraphic unit and also suggest that at least part, if not all, of the Mayuan Group is Paleoproterozoic.Evidence for Paleozoic metamorphic reworking that is considered to have affected the whole of Cathaysia block is not revealed in this study; in contrast our data clearly show obvious Mesozoic metamorphic reworking at ca. 230–250Ma. Zircon ɛHf(t) values range from −19 to +11 with a peak at −5.5 and show TDMC(Hf) ranging from 1.9 to 4.1Ga with a peak at ca. 2.7–3.0Ga suggesting that a major crustal growth took place during this time. This interpretation is consistent with the previously suggested crustal growth peaks of ~2.7Ga and ~2.9Ga. A synthesis of the reliable geochronological data gathered so far on Phanerozoic metamorphic reworking of the northeastern Cathaysia block reveals that the imprints of these tectonothermal events is differently distributed in the different zones. Rocks metamorphosed during the Paleozoic tectonothermal event dominantly occur in the western zone whereas those reworked by high-grade metamorphism during the Mesozoic tectonothermal event mainly outcrop in the eastern zone. Our study alerts the previous notion of a uniform distribution of the reworked rocks by high-grade metamorphism all across the northeastern Cathaysia block and provides new insights on the evolution of the South China Block.

Middle Paleozoic basaltic and kimberlitic magmatism in the northwestern shoulder of the Vilyui Rift, Siberia: relations in space and time

A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian–Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui–Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume–lithosphere interaction.

Pan-African and early Paleozoic tectonothermal events in the Nyainrong microcontinent: Constraints from geochronology and geochemistry

To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 206Pb/238U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P2O5 and Al2O3 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.

Pb Ages of Detrital Zircons In Relation To Geodynamic Evolution: Paleozoic of the Northern Gemericum (Western Carpathians, Slovakia)

Abstract New U-Pb sensitive high-resolution ion microprobe (SHRIMP) detrital-zircon ages from early Paleozoic to Permian siliciclastic metasediments are used to constrain provenance and paleogeographic affinity of the Northern Gemericum Unit (NGU) in the Western Carpathians (Central Europe). The samples span the time period from ∼ 480 Ma to ∼ 260 Ma and permit the recognition of provenance changes that reflect the realm's geodynamic evolution. The detrital-zircon age spectrum of one early Paleozoic sample resembles the Neoproterozoic and Archean ages of the Cadomian belt (550–650 Ma) and the West African craton provenances (1.8–2.7 Ga). The origin of the early Paleozoic NGU (Rakovec) basin is inferred to be the rifting as followed by back-arc spreading which occurred at the Cambrian–Ordovician boundary (ca. 480 Ma), as continental pieces started to split off from the Gondwanan margin. The deformation and closure of the early Paleozoic basin is coincident with the 355 Ma peak of the post-Tournaisian detrital-zircon age assemblage. The detrital-zircon age spectra of two post-Mississippian and two post-Artinskian samples indicate high exhumation and erosion rates in the Variscan collision belt. Pennsylvanian–Permian sedimentary troughs, formed between 318 and 275 Ma, record contemporaneous unroofing of the Variscan collision belt and magmatic activity during post-orogenic transtensional tectonics. Zircon age spectra reflect these three Paleozoic (480–419 Ma, 340–380 Ma, 320–275 Ma) tectono-thermal events and permit the recognition of provenances, both of which supporting the interpreted changes in geotectonic regime from Ordovician rifting to Variscan compression and the subsequent extension. The zircon age data constrain the provenance linkage of the low-grade NGU basement to the Cadomian fragments within the West-European Variscides.

An early Paleozoic tectonothermal event in western Mongolia: Implications for regional extension of the Cambrian orogenic belt

An early Paleozoic tectonothermal event in western Mongolia: Implications for regional extension of the Cambrian orogenic belt