Post-collisional Extensional Setting Research Articles

The protoliths of central Himalayan eclogites

Abstract Eclogites represent the highest pressure conditions yet observed from rocks thrust to the surface in the central Himalaya. A detailed investigation of the protolith nature of these eclogites is needed to better understand pre-Himalayan geological history. Retrogressed eclogites were collected from Thongmön (Dingri County) and Riwu (Dinggye County), central Himalaya, China. We investigated the bulk rock major and trace elements, Sr-Nd isotopes, zircon geochronology, and Hf-O isotopes. These retrogressed eclogites experienced five stages of metamorphic evolution from prograde amphibolite-facies to peak eclogite-facies, and high pressure granulites-facies, granulites-facies then final amphibolite-facies overprinting during exhumation. Geochemically, they are subalkaline basalts with high FeO contents and a tholeiitic affinity; trace elements show similarities with enriched mid-ocean ridge basalts. Bulk rocks have a wide range of εNd(t) values from −0.24 to +7.08, and an unusually wide range of initial 87Sr/86Sr ratios of 0.705961–0.821182. Zircon relict magmatic cores from both Thongmön and Riwu eclogites yield a consistent protolith age of ca. 1850 Ma, with enriched heavy rare earth element patterns and significant negative Eu anomalies. These relict cores have oxygen isotopes signatures of δ18O = 5.8–8.1‰, εHf(t) values of –4.85 to +9.59, and two-stage model ages (TDM2) of 1.91–2.81 Ga. Metamorphic overgrowth zircons yield much younger ages of ca. 14 Ma. Integration of all of the above data suggests that the protolith of these central Himalayan retrogressed eclogites might be Proterozoic continental flood basalts of the North Indian Plate, generated under a post-collisional extension setting during the assembly of the Columbia Supercontinent. Occurrence of both Neoproterozoic–early Paleozoic rocks and ca. 1.85 Ga rocks in the regional crystalline rocks may reflect either unrecognized sub-horizontal Main Central Thrust exposure(s) or exhumation of a deeply cut part of the Greater Himalayan Crystalline complex. In combination with previous reports of Late Cretaceous, Neoproterozoic, and similar but younger Paleoproterozoic protolith, it is clear that the central Himalayan eclogites originate from multiple sources of protolith.

The Paleozoic-Mesozoic magmatic evolution of the Eastern Tianshan, NW China: Constraints from geochronology and geochemistry of the Sanchakou intrusive complex

The Sanchakou intrusive complex (containing quartz diorite, porphyritic granodiorite, granodiorite, fine-grained granodiorite, diorite porphyry and hornblende gabbro) is located at the eastern part of the Dananhu-Tousuquan island arc belt, Eastern Tianshan. Zircon U-Pb ages suggest that the above six units were emplaced at 446–238 Ma. The Sanchakou intrusive complex therefore provides a unique opportunity to gain a better understanding of the Paleozoic-Mesozoic magmatism and its evolution in the Eastern Tianshan.The granitoids form a subduction related calc-alkaline to low K (tholeiitic) magmatic series, characterized by enrichment in LILE and depletion in HFSE. They have low initial 87Sr/86Sr, high εNd(t) and εHf(t) values. As well as the above geochemical features, the quartz diorite, porphyritic granodiorite and fine-grained granodiorite have low Nb/U and Ta/U values, relatively high Mg#, Ti/Zr and Ti/Y values, indicating a potential role of the subducted slab in their petrogenesis. The granodiorite units have low Nb/U, Ta/U and Mg# values, and high (La/Yb)N values, suggesting an origin from the melting of juvenile lower crust. The Sanchakou diorite porphyry has positive εNd(t) and εHf(t), low SiO2 and (87Sr/86Sr)i, and high MgO, Mg#, Ti/Y and Ti/Zr values, indicating it was likely sourced from depleted mantle. The Mesozoic hornblende gabbro has low SiO2, relatively high TiO2, MgO, Al2O3 and (La/Yb)N, and minor depletion in Nb-Ta, implying an E-MORB-like mantle source.Integrating available tectonic, stratigraphic and geochemical data, we propose that the Sanchakou quartz diorite, porphyritic granodiorite, granodiorite and fine-grained granodiorite were formed in a double-side subduction system. The younger Permian diorite porphyry was produced in a post-collisional extensional setting and the Triassic hornblende gabbro was likely formed in an intraplate environment.

Sericite 40Ar/39Ar and zircon U-Pb dating of the Liziyuan gold deposit, West Qinling orogen, central China: Implications for ore genesis and tectonic setting

The West Qinling orogen in central China contains several dozens of gold deposits. However, absolute ages of gold mineralization and its relations to regional tectonism, magmatism, and metamorphism remain not tightly constrained for most of these gold deposits. Here we integrate 40Ar/39Ar dating of sericite from various hydrothermal alteration assemblages and U-Pb dating of zircon grains from magmatic rocks within and surrounding the Liziyuan gold deposit in the northern belt of the West Qinling orogen to provide new insights into the age and tectonic setting of gold mineralization. The Liziyuan gold deposit consists of six ore camps (Yuzigou, Liushagou, Jiancaowan, Kuangou, Suishizi, and Yingfang) that are hosted in Ordovician greenschist facies metamorphosed sedimentary-volcanic rocks and, less significantly, the Middle Triassic Tianzishan monzogranite pluton intruding the metamorphic rocks. Gold mineralization consists of auriferous quartz-pyrite veins and subordinate sulfide disseminations in hydrothermally altered wall rocks, with minor calcite-polymetallic sulfide veins in the monzogranite-hosted orebodies. Ore-related alteration assemblages are dominated by quartz, sericite, pyrite, chlorite, and calcite, with gold occurring mostly as native gold grains enclosed in or filling microfractures of pyrite and quartz. Barren alteration assemblages, dominated by K-feldspar, quartz, sericite, and pyrite, are locally developed along eastern margin of the Tianzishan monzogranite. Two sericite aggregates extracted from gold ores in the Liushagou camp yield well-defined 40Ar/39Ar plateau ages of 200.1 ± 1.2 and 198.8 ± 1.1 Ma (2σ), whereas those from barren alteration assemblages in the Tianzishan monzogranite, which hosts the Suishizi ore camp, give rise to older 40Ar/39Ar plateau ages of 211.9 ± 1.2 and 211.3 ± 1.3 Ma (2σ). The Tianzishan monzogranite pluton and several dioritic to granitic stocks and dykes in and surrounding the gold mines have zircon U-Pb ages ranging from 241.3 ± 1.2 to 212.4 ± 0.9 Ma (2σ). These new isotopic ages suggest that the Liziyuan gold deposit formed at the Triassic-Jurassic boundary, arguably representing the youngest orogenic-type gold mineralization event over the northern belt of the West Qinling orogen. Gold mineralization postdates magmatism in the Liziyuan gold deposit at least by about 10 million years, precluding a possible genetic relation between the two. Rather, the ore genesis was likely related to metamorphic fluids released from Paleozoic sedimentary and volcanic rocks in relation to the Qinling orogenesis. In contrast, the barren hydrothermal alteration assemblages were likely caused by fluids exsolved from a magma presumably represented by the ca. 212 Ma diorite dyke. Results presented here, when combined with independent geological and geochemical data, suggest that the Liziyuan gold deposit formed most likely under a post-collisional extensional setting related to the West Qinling orogeny.

Mineralization age, tectonic setting and ore genesis of the Wuxing Pt–Pd-rich magmatic Cu-Ni sulfide deposit, Northeast China

• Wuxing deposit with age of 516.7 Ma is reported. • Wuxing intrusive rocks experienced independent diagenetic processes. • The Wuxing deposit was formed in a post–collision extensional setting. The Wuxing deposit in Northeast China, tectonically located in the Jiamusi–Khanka Block, is a Pt–Pd-rich Cu-Ni sulfide deposit related to mafic–ultramafic rocks. The zircon U–Pb ages of pyroxenites , gabbros , porphyritic granites , and leucogranits in the mining area are 516.7 ± 2.6, 513.8 ± 2.0, 505.4 ± 2.3 and 499.5 ± 2.9 Ma, respectively, indicating that the diagenesis and metallogenesis of the Wuxing deposit occurred during the Cambrian. The ore-forming complex is cut by quartz diorite porphyritic and plagiogranite porphyry dikes. Zircon U–Pb ages of the quartz diorite porphyrite and plagiogranite porphyry are 206.6 ± 2.1 and 201.6 ± 1.3 Ma, respectively. The U–Pb ages indicate the metallogenic age constraint of 516.7 ± 2.6 Ma for the Wuxing deposit, rather than Cambrian and Triassic magmatic activities. The Hf isotopic composition ε Hf (t) of zircons in the pyroxenites and gabbros are −11.5 to −7.7 and −5.8 to −2.5, with the two–stage model ages (t DM2 ) of 2198–1958 and 1841–1633 Ma, respectively. Geochemically, the mafic–ultramafic complexes are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and depleted in high-field strength elements (HFSE), indicating these rocks originated from the enriched mantle. The porphyritic granites are high-K calc-alkaline rocks (Rittman index δ = 1.92–2.00), and the leucogranites are low-K alkaline rocks ( δ = 4.76–4.86) with ε Hf (t) and t DM2 of −9.2 to +0.8 and 2037–1411 Ma, respectively. The porphyritic granites and leucogranites are characterized by low A/CNK values (0.91–1.05), 10,000 Ga/Al values (1.76–2.36), and Zr saturation temperature (731–823 ℃) similar to those of I-type granites. Along with contemporaneous intrusions, the Wuxing deposit is inferred to have formed in a post-collisional extension environment. Consequently, the origin of the Wuxing deposit is likely related to the evolution of enriched mantle magma during the Cambrian post-collision extensional setting of the Jiamusi–Khanka Block.

Geology, geochronology and tectonic setting of the Chaihulanzi gold deposit in Inner Mongolia, China

The Chaihulanzi is a Precambrian metamorphic rock-hosted gold deposit in the northern margin of the North China Craton (NCC). The local outcropping lithologies include mainly Archean metamorphic rock, Paleoproterozoic carbonatite, and Phanerozoic diorite, granite, aplite and mafic to felsic dikes. Due to the lack of data, the deposit was variably proposed to be orogenic, intrusion-related, or mesothermal type, and the mineralization was loosely constrained to be Paleozoic or Mesozoic. Based on field geology, petrological/mineralogical observations, whole-rock geochemistry, and zircon U-Pb-Hf isotopes of ore-related aplite, and sericite Ar-Ar dating of gold-ore veins, we discussed the ore-causative magmatism, and the age and tectonic setting of gold mineralization. Our results show the presence of an early (native gold-dominated) and a late (gold-silver telluride-dominated mineralization) stage: The early-stage gold mineralization was genetically linked to the Middle-Late Permian aplite (LA-ICP-MS zircon U-Pb age: 264.5 ± 1.3 Ma), whose ages are similar to the gold ore veins (sericite Ar-Ar age: 263.67 ± 0.99 Ma). The distinct negative zircon εHf (-18.31 to -15.45) and model age (TDMC = 2.1–2.4 Ga) indicate a crustal magma source, probably from partial melting of the NCC basement. The whole-rock trace element compositions and zircon U-Pb ages, together with the tectonic evolution of northern NCC margin imply a post-collisional extension setting (after the Paleo-Asian ocean closure) for the gold mineralization. The newly-discovered late-stage quartz-carbonate ore veins occur as fracture infill, and crosscut early-stage quartz-sericite veins and Mesozoic volcanic wallrocks, implying that the late-stage mineralization occurred in the Mesozoic or after. The two-stage gold mineralization overprinting makes Chaihilanzi and its periphery highly prospective for gold exploration.

Reworking of juvenile crust beneath the Bangong–Nujiang suture zone: Evidence from Late Cretaceous granite porphyries in Southern Qiangtang, Central Tibet

Crustal reworking in the collisional zones is commonly considered as an important process to result in continental crust differentiation and maturation. However, the reworking mechanism remains unclear. Here, a combined study of zircon SIMS U–Pb age, whole-rock major and trace element, Sr–Nd–Hf and zircon Hf–O isotopic geochemistry was carried out for the Duoma–Namuqie (DM–NMQ) granite porphyry intrusions, including DM, NMQ I, II and III intrusions, in southern Qiangtang, central Tibet. Zircon SIMS U–Pb dating yielded ages of 79–76 Ma, suggesting that they were formed in the later stage of Late Cretaceous. The DM granite porphyries are metaluminous to peraluminous with slightly low SiO2 (69.2–71.4 wt%) and high MgO (0.67–0.79 wt%) contents. They are depleted in high-field-strength elements (HFSEs) and enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), and show weakly negative Eu anomalies. The NMQ III granite porphyries show similar elemental features to the DM granite porphyries, but have relatively high SiO2 (69.8–72.2 wt%) and low MgO (0.25–0.28 wt%) contents. Both of them belong to I-type granites. The NMQ I and II granite porphyries are relatively differentiated with high SiO2 (76.4–77.9 wt%), low MgO (0.11–0.16 wt%) contents, and significantly negative Eu anomalies. Extremely low LREE contents and notable REE tetrad effect can be observed in the NMQ II granite porphyries. The NMQ I and II intrusions are fractionated I-type granites. The whole-rock Sr–Nd–Hf and zircon Hf–O isotope data exhibit an enriched trend from the DM granite porphyries ((87Sr/86Sr)i = 0.7049–0.7050, εNd(t) = 1.2–1.6, εHf(t)whole-rock = 9.9–12.0, εHf(t)zircon = 7.4–11.5, δ18O = 6.2–7.1‰) to the NMQ III granite porphyry ((87Sr/86Sr)i = 0.7058–0.7067, εNd(t) = −0.9 to −1.1, εHf(t)whole-rock = 7.0–7.4, εHf(t)zircon = 2.7–9.5, δ18O = 7.2–7.6‰) to the NMQ I and II granite porphyries (εNd(t) = −3.0 to −4.3, εHf(t)whole-rock = 5.3–6.6, εHf(t)zircon = 1.5–7.4, δ18O = 6.6–9.2‰). We suggest that the DM–NMQ granite porphyries were mainly derived by partial melting of the juvenile mafic lower crust with variable volumes of ancient crustal components involved in their magma sources. The way in which these granite porphyries were formed provides evidence for the crustal differentiation and maturation in a post-collisional extensional setting triggered by the far-field rollback of the subducted Neo-Tethys oceanic slab

The Pilot Knob iron ore deposits in southeast Missouri, USA: A high-to-low temperature magmatic-hydrothermal continuum

The Mesoproterozoic St. Francois Mountains igneous terrane in southeast Missouri, USA, contains eight major and several minor IOA/IOCG-type deposits. This study focuses on the Pilot Knob deposits, i.e., the largely massive Pilot Knob Magnetite (PKM) deposit and the Pilot Knob Hematite (PKH) deposit, which is located 240 m stratigraphically above the PKM and consists of variably mineralized bedded hematite and ore hosted in brecciated volcanic agglomerates. The PKM deposit was previously shown to be of magmatic and magmatic-hydrothermal origin, although its formation has not been precisely dated. The origin of the PKH deposit (i.e., sedimentary vs. hydrothermal) and its genetic relationship to the PKM, remain controversial.We present new U-Pb data on apatite intergrown with massive magnetite in the PKM deposit and provide the first precise age for the formation of the PKM ore at 1437.7 ± 5.8 Ma. Petrographic observations of PKH ore, bulk rock compositions, and the mineral chemistry of hematite, which contains up to 2.7% Ti, suggest that the hematite in the PKH deposit crystallized from acidic and hypersaline hydrothermal fluids at a temperature between 200 and 250 °C. The Fe isotopic composition of 9 bedded (δ56Fe = 0.05–0.30‰, average 0.13‰) and 3 brecciated hematite samples (δ56Fe = −0.19 to 0.01‰, average −0.06‰) from the PKH deposit are slightly lighter than the published δ56Fe results of magnetite from the PKM deposit (δ56Fe = 0.06–0.27‰, average 0.17‰). However, all isotopic signatures fall within the magmatic range, indicating that iron in both deposits was originally sourced from a magma. Because of the hydrothermal origin of the PKH deposit, the iron isotopic compositions of the PKM and PKH ores that imply a shared/similar iron source, and the spatial proximity of both deposits, we argue that the PKM and PKH deposits are genetically related and represent two endmembers of a high-to-low temperature magmatic-hydrothermal continuum. In this scenario, ore fluids exsolved from the magma that facilitated the formation of the PKM deposit migrated upwards, infiltrated existing sedimentary structures near the surface, and precipitated hydrothermal hematite ore while preserving the original bedded and brecciated structures.Geochemical signatures of the rhyolites/rhyodacites that host the PKM deposit imply that these rocks are A2-type felsic rocks that were emplaced in a post-collisional extensional setting. Bulk silicate Earth normalized patterns of the PKM deposit and wall rocks display a negative slope from Cs to Lu with negative Nb and Ta anomalies, indicating a hydrous source for the rhyolites and rhyodacites, possibly a subduction-modified subcontinental lithospheric mantle (SCLM). These geochemical signatures support a proposed tectonic setting of the St. Francois Mountains, wherein the igneous terrane developed on a growing continental margin. Episodic mafic-to-intermediate magmatism, and subsequently exsolved hydrothermal fluids, may have formed the cluster of IOA/IOCG-type deposits in the igneous terrane between ~1500 and ~1440 Ma. Within such a context, the PKM and PKH deposits may represent a shallow, small-scale snapshot of processes similar to the ones that form the IOA-IOCG continuum: a deeper magmatic event that exsolved a hydrothermal fluid that forms an overlying ore body.

Duobagou Permian–Triassic granites from the Dunhuang orogenic belt, NW China: implications for the tectonic evolution of the southernmost Central Asian Orogenic Belt

AbstractThe Duobagou Permian–Triassic granites of the Dunhuang orogenic belt are of great importance in understanding the tectonic evolution of the southernmost Central Asian Orogenic Belt. LA-ICP-MS U–Pb zircon ages indicate that Permian–Triassic granitic intrusions from the Duobagou area formed at 276–274 Ma and 246 ± 1 Ma. These granites have high SiO2, Na2O and K2O, but low Al2O3, CaO and MgO contents and belong mainly to the high-K calc-alkaline I-type granite series. Based on whole-rock geochemistry and Sr–Nd and zircon Hf isotopes, the Duobagou Permian–Triassic granites were dominantly derived from the partial melting of lower continental crust formed during late Palaeoproterozoic to Mesoproterozoic times in a post-collisional extensional setting. Permian granites with zircon ϵHf(t) values of −5.4 to +3.1 and Hf model ages of TDM2 = 1.14–1.70 Ga indicate the involvement of a mantle component in their petrogenesis. Triassic granites with higher zircon ϵHf(t) values (+0.5 to +3.8) and TDM2 = 1.08–1.31 Ga suggest more juvenile sources caused by a greater contribution of mantle-derived melts, indicating a significant crustal growth. Regional extension from lithospheric delamination and heating from asthenospheric upwelling were proposed to have triggered the partial melting of lower crust, resulting in the generation of the Permian–Triassic magmatism. This may have been the mechanism for the significant crustal growth during Permian and Triassic times in the southernmost Central Asian Orogenic Belt.

Late Silurian to Early Devonian volcanics in the East Kunlun orogen, northern Tibetan Plateau: Record of postcollisional magmatism related to the evolution of the Proto-Tethys Ocean

The late Early Paleozoic volcanic rocks in the East Kunlun orogen are crucial to determining collisional processes related to the evolution of the Proto-Tethys Ocean. We report new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd isotopes from these volcanic rocks and constrain their tectonic setting and magmatic processes. This volcanic suite consists mainly of foliated metarhyolites and minor metabasalts. Zircon U-Pb ages indicate that the bimodal volcanic rocks formed from Late Silurian to Early Devonian (ca. 420−409 Ma). The basalts show tholeiitic geochemical signatures characterized by low contents of SiO2 (47.19–54.83 %), MgO (2.21–7.52 %), and K2O/Na2O ratios (0.32−0.77%) and high contents of TiO2 (1.80–2.91 %). Their chondrite-normalized REE patterns are characterized by enrichments in LREEs (LREE/HREE = 3.68–6.09) with slight Eu anomalies (δEu = 0.67–1.00); their patterns are similar to the reference line of oceanic island basalt (OIB). The primitive mantle-normalized trace element diagram also shows similarities with OIBs and features overall enrichments in large ion lithophile and high field strength elements, except for Nb and Ta. The εNd(t = 422 Ma) values range narrowly from −1.79 to +1.32. These features suggest that the basalts were most likely derived from an asthenospheric mantle that was contaminated by small volumes of subcontinental lithospheric mantle (SCLM)/crust. The variations in major and trace elements show that the basalts experienced fractional crystallization of olivine, pyroxene and plagioclase. In contrast, the metarhyolites have high SiO2 (65.85–70.83 %), Na2O + K2O (6.71–10.09 %), K2O/Na2O ratios (1.69–2.80 %) and low MgO, Ni, and Cr contents. Their trace element signatures show enrichments in LILEs and LREEs (e.g. Cs, Rb, Ba), depletion of HFSEs (e.g. Nb, Ti, Ta) and high negative Eu anomalies. These features suggest a crustal origin lacking interactions with mantle melts. Considering all the geochemical and tectonic events of this area, it is concluded that the Maoniushan Formation volcanic rocks in EKOB formed in a postcollisional extensional setting related to asthenospheric mantle upwelling and continental delamination processes during the Late Silurian to Early Devonian. These tectonic-magmatic events, together with previous data, suggest that the Proto-Tethys Ocean had closed and evolved to postcollisional collapse stage since the Late Silurian.

The Kamusite A2-type granites in the Karamaili tectonic belt, Xinjiang (NW China): tracing staged postcollisional delamination in the eastern Junggar

AbstractThe Kamusite pluton is located in the eastern Junggar area, the westernmost segment of the Karamaili structural belt, and is predominantly composed of medium granite and microgranite with an exposure area of 30 km2. The U–Pb zircon geochronology of the Kamusite granites indicates that they crystallized in the late Carboniferous period (328–321 Ma). These granites exhibit high contents of SiO2 (76.09–77.85 wt %) and K2O + Na2O (8.01–9.06 wt %), but low MgO (0.01–0.14 wt %), CaO (0.07–0.32 wt %) and TiO2 (0.01–0.13 wt %) contents, showing alkalic–calcic, weakly peraluminous and ferroan features. They are depleted in Ba, Sr, Ti and P and enriched in Rb and some high-field-strength elements (Hf, Nd, Ta and Y); their rare earth element patterns are slightly right-leaning with strongly negative Eu anomalies, high 10 000 * Ga/Al (3.4–5.36, >2.6) and especially high Y/Nb ratios (1.61–10.33, >1.2), showing the geochemical characteristics of A2-type granite. These granites were produced by partial melting in a high-temperature, low-pressure, reduced and anhydrous environment and experienced extensive fractional crystallization, which concomitantly resulted in tin mineralization. Combining the high positive zircon ϵHf(t) values of +10.9 to +15.76 with young Hf (TDM2) model ages (638–330 Ma), it can be suggested that underplating-related mantle-derived materials were the original source of the Kamusite A2-type granites; namely, these granites formed by the partial melting of juvenile crust. The record of large-scale magmatism indicates that the whole tectonic belt was in a postcollisional extensional setting induced by staged delamination from west to east during the late Carboniferous to early Permian periods.

Multiphase Late Devonian to Carboniferous volcanic events in the west of Oyu Tolgoi, southeastern Mongolia: New geochronological, geochemical, and isotopic constraints on tectonic history

The Late Paleozoic volcanic rocks distributed in southeastern Mongolia are key to reconstructing the tectonic processes along the south margin of the Central Asian Orogenic Belt during this period. In this study, representative volcanic samples were collected in the west of Oyu Tolgoi near the eastern border of the South Gobi and Trans-Altai Zones. The geochronological results reveal intermittent Late Devonian to Early Carboniferous volcanic belts with zircon UPb ages of 360, 320, and 340 Ma for the south, center, and north belts, respectively. Geochemical and isotopic results imply comparable petrogenesis for the samples from different belts. The basic samples are primarily derived from the different degrees of partial melting of a mantle source, and their compositions are slightly affected by the assimilation of crustal materials with different maturities. The intermediate to acidic samples were more likely produced by partial melting of a crust-derived source triggered by underplating of the mantle magma. The low Mg# and apparent Eu anomalies of the extremely high-Si rhyolites sampled from a bimodal-like volcanic sequence in the south belt indicate strong fractional crystallization after derivation from the partial melting of the juvenile crust. In conjunction with the spatiotemporal distribution pattern of magma and the observations recorded in previous studies, we propose that the Late Devonian to Early Carboniferous volcanic rocks from the south and north belts are a continental arc formed from the northward subduction of the Tsagaan Uul terrane under the Mongolian microcontinent and that the Late Carboniferous volcanism in the center belt was produced under a post-collision extensional setting. The subsequent increasingly intense extension was also responsible for the local Late Carboniferous to Early Permian calc-alkaline to alkaline granitic plutons.

Early Devonian mafic igneous rocks in the East Kunlun Orogen, NW China: Implications for the transition from the Proto- to Paleo-Tethys oceans

The tectonic evolution of the Proto- and Paleo-Tethys oceans had a significant influence on ocean-continent distributions in East Asia in the Phanerozoic, and major implications for continental growth in the region. However, it remains ambiguous when and how the Proto-Tethys Ocean transformed into the Paleo-Tethys Ocean. Here we present petrologic, mineralogical, chronological and geochemical data for Early Devonian mafic igneous rocks, located in the East Kunlun Orogen. The mafic igneous rocks include basaltic lavas and diabase dykes, and are tholeiitic in composition. Geochemical and Sr–Nd isotopic data indicate that the basaltic lavas were derived from melting of a spinel-bearing asthenospheric mantle (E-MORB) at normal mantle potential temperatures (1384–1400 °C) with negligible (1–4%) crustal contamination. The diabase dykes probably originated from melting of a spinel-bearing lithospheric mantle metasomatized by subduction-related fluids, with 5–20% crustal contamination, and crystallized at 1100–1135 °C. Both basaltic lavas and diabase dykes have the geochemical characteristics of within-plate basalts. Magmatic zircons from the mafic rocks yield Early Devonian ages (407–403 Ma), postdating the East Kunlun ultrahigh-pressure metamorphism by 19–25 Myr. Comparing our results with the location and timing of the high- and ultrahigh-pressure metamorphic belt, we conclude that the mafic igneous rocks formed in a post-collisional extensional setting. Their generation was associated with both the terminal stages of the Proto-Tethys orogenic belt, (with orogenic collapse promoted by repeated and localized delamination of lithospheric mantle), and early continental rifting related to the evolution of the Paleo-Tethys Ocean to the south. The period of ~426–390 Ma is important for the transition from Proto- to Paleo-Tethys oceans in the East Kunlun Orogen.

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.

Early-Late Devonian Post-Collision Extension Due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40Ar-39Ar Dating

Ordovician-Silurian subduction, Early Devonian arc-contient collision and followed post-collision extension are recorded in the north of the North China Craton. Most previous research has focused on the first two processes. Discussion on the post-collisional extension and its tetonic regime is still limited. In this study, 40Ar-39Ar muscovite ages obtained from the central part of the northern North China Craton were analyzed to shed light on the timing of post-collision extension. Garnet muscovite schist and muscovite quartz schist in the Jianshan Formation yielded 40Ar-39Ar muscovite plateau ages of 412 ± 3 Ma and 391 ± 3 Ma, respectively. Two other 40Ar-39Ar muscovite plateau ages (389 ± 2 Ma and 397 ± 2Ma) were obtained for two Mesoproterozoic monzogranites intruding into the Jianshan Formation. Based on previous research, the northern North China Craton underwent a collision event with Bainaimiao arc at c. 415 Ma, followed by post-collision extension in early Late Paleozoic. Therefore, combined with the newly acquired muscovite 40Ar-39Ar dating results, the Jianshan Formation might go through regional metamorphism at c. 412 Ma during the collision process. Subsequently, the Jianshan Formation and monzogranites intruding into it went through rapid exhumation along with metamorphism at c. 397–389 Ma in a post-collision extensional setting. The muscovite 40Ar-39Ar ages provide new markers for the exhumation history and the post-collisional extension setting during Early-Late Devonian in the study area. Furthermore, slab breakoff as the cause for this extensional setting is argued by the emplacement of the Early-Late Devonian alkaline rocks.

Late Paleoproterozoic mafic-intermediate dykes from the southern margin of the North China Craton: Implication for magma source and Columbia reconstruction

Dyke swarms formed in extensional settings and are important proxies to track mantle source characteristics as well as continent and supercontinent break-up. The late Paleoproterozoic mafic-intermediate dykes in the Xiaoqinling region, southern margin of the North China Craton (NCC), provide a window to investigate the late Paleoproterozoic magma sources and tectonic evolution. Although many studies have been conducted on this region, the timing of late Paleoproterozoic extension and tectonic setting remain debated. Here we present results from LA-ICP-MS zircon U-Pb geochronology, Lu-Hf isotopic analyses and geochemistry of these dykes, together with an evaluation of the published geochronological and geochemical data for the regional dyke activity. The zircon U-Pb data show emplacement ages of 1751–1841 Ma for the dykes and compiled age data indicate widespread late Paleoproterozoic dyke emplacement during 1.85–1.75 Ga, with the majority of formed at 1.84–1.80 Ga. The geochemical characteristics, along with zircon εHf(t) values (showing two distinct ranges: 0.7–7.4 and −5.1–0), indicate multiple magma sources for the 1.84–1.80 Ga dykes from asthenosphere and depleted subcontinental lithosphere sources followed by interaction and mixing with continental crust. Geochemically, the dykes are enriched in Ba, K and P and depleted in Nb, Ta and Sr. The geological and geochemical features of these dykes are consistent with their formation within a post-collisional extension setting. We also evaluate the significance of these dykes in terms of paleogeographic correlations within the Columbia supercontinent through correlations with the North China Craton, Indian Shield and West Australia Craton.

Early–Middle Permian post-collisional granitoids in the northern Beishan orogen, northwestern China: evidence from U–Pb ages and Sr–Nd–Hf isotopes

The geochemistry and Sr–Nd isotope, zircon U–Pb, and zircon Hf isotope compositions are reported for monzogranites and granodiorites from the Hazhu area in the northern Beishan orogen, northwestern China. Zircon U–Pb dating yielded ages of 270.1 ± 1.1 and 277.4 ± 1.2 Ma for the monzogranites and 263.6 ± 1.2 and 262.2 ± 1.1 Ma for the granodiorites. These monzogranites and granodiorites are metaluminous to weakly peraluminous I-type and belong to mid-K calc-alkaline and high-K calc-alkaline series. They exhibit high Mg# values and moderate degrees of differentiation (D.I. = 70.7–88.1). They are enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements. They show high (87Sr/86Sr)i ratios of 0.6995–0.7070 and high εNd(t) values of 4.37–5.70 with Nd model ages (TDM) of 522–789 Ma, suggesting a juvenile crustal origin. Furthermore, their εHf(t) values are all positive, and Hf isotopic crustal model ages ([Formula: see text] = 394–1097 Ma) also indicate a juvenile crustal origin. According to the data obtained in this study and other regional geological data acquired recently, the Hazhu granitoids were derived from common sources of melting from the Neoproterozoic to late Paleozoic juvenile crusts. The younger intrusions (granodiorites) are more basic, likely as a result of more juvenile lower crust being melted along with asthenospheric upwelling, which led to the addition of more basic components. These granitoids formed in a post-collisional setting. The tectonic regime transformed from an arc-related compressional setting to post-collisional extension, likely as a result of lithospheric extension and thinning in response to oceanic lithospheric delamination. These granitoids in the northern Beishan orogen were probably emplaced in a post-collisional extensional setting and suggest vertical continental crustal growth in the southern Central Asian Orogenic Belt.

Petrogenesis of Siluro-Devonian rhyolites of the Tobique Group in the northwestern Appalachians (northern New Brunswick, Canada): tectonic implications for the accretion history of peri-Gondwanan terranes along the Laurentian margin

Abstract Uppermost Silurian–Lower Devonian felsic rocks in the bimodal volcanic suite of the Tobique Group in the northwestern mainland Appalachians (northern New Brunswick, Canada) form part of an overstep sequence deposited across the accreted vestiges of Iapetus Ocean on composite Laurentia. Whereas the mafic rocks of the bimodal volcanic suite are continental tholeiites, the felsic rocks are peraluminous and possess geochemical characteristics of A2-type granites emplaced in post-collisional extensional settings. The major and trace element compositions of the felsic rocks indicate that they were generated by dehydration melting of late Precambrian granitoid rocks triggered by heat derived from the rising mafic magma. Unlike the basalts, which have positive ε Nd ( t ) values, the felsic rocks have values close to chondrites (−1.6 to +1.1), which is consistent with derivation from a crustal source. The rapid transition from compressional to extensional magmatism in latest Silurian–Early Devonian times in this part of Ganderia is probably due to Late Silurian Ganderia–Laurentia collision followed by slab breakoff. Based on Sm–Nd isotopic characteristics in their respective igneous rocks, both Ganderia and Avalonia are underlain by similar Neoproterozoic lower–middle crust and subcontinental lithospheric mantle.

Petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites of the Songnen Block, NE China: Implications for the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans

The petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites in NE China offers new insights into the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans. This study reports new LA–ICP–MS zircon UPb ages, geochemical data, and zircon Hf isotopic compositions of syenogranites and quartz diorite porphyries exposed on the northern margin of the Songnen Block, NE China. Zircon UPb age data indicate that the syenogranites and quartz diorite porphyries were emplaced at ca. 319 and 120 Ma, respectively. The syenogranites have high K2O contents of 4.15–4.79 wt% and (K2O + Na2O)/CaO ratios of 33–102, and low MgO (0.06–0.20 wt%) and P2O5 (0.01–0.02 wt%) contents. Their remarkably high HREE contents and Y/Nb ratios (2.07–2.40), significantly negative Eu anomalies (Eu/Eu⁎ = 0.15–0.28), and low Sr contents (29.7–85.8 ppm) indicate that the syenogranites are A-type granites. They have positive zircon εHf(t) values of 8.4–12.1 and Neoproterozoic Hf TDM2 ages of 795–561 Ma, suggesting a juvenile lower crustal source. The quartz diorite porphyries have relatively high Na2O and MgO, and moderate K2O contents. Their A/CNK values (0.95–1.18) indicate a transition from being metaluminous to weakly peraluminous. They are enriched in LILEs (e.g., Rb, K, and Ba) and depleted in HFSEs (e.g., Nb, Ta, Ti, and P). Their low Y (11.1–14.4 ppm) and Yb (0.98–1.34 ppm) contents and high Sr (589–1131 ppm), Cr, and Ni contents and Sr/Y ratios indicate that they were generated by partial melting of oceanic crust. Based on these data and regional geological investigations, we propose that the syenogranites on the northern margin of the Songnen Block were formed in a post-collision extensional setting, likely due to closure of the Paleo-Asian Ocean between the Xing'an and Songnen blocks. Formation of the Early Cretaceous quartz diorite porphyries may have been triggered by partial melting of the Paleo-Pacific flat slab. Apart from rollback of the Paleo-Pacific oceanic slab, the main reason for the eastward migration of magmatism during the Cretaceous was the trench-ward dehydration, eclogitization, and sinking of the residual Paleo-Pacific flat slab.

Geochronology, geochemistry, and Hf isotopic compositions of early Permian syenogranite and diabase from the northern Great Xing’an Range, northeastern China: petrogenesis and tectonic implications

Geodynamic evolution in the late Paleozoic is significant for understanding the final amalgamation of the Central Asian Orogenic Belt (CAOB). No consensus has yet been reached regarding the late Paleozoic geodynamic evolution of the northern Great Xing’an Range (GXR) in northeastern China, the eastern CAOB. Furthermore, late Paleozoic syenogranite–diabase dyke association is present in the Xiaokele area in northern GXR. It provides an important opportunity to understand the nature of magmatism and the geodynamic evolution during this period. This paper presents new zircon U–Pb ages, zircon Hf isotopic compositions, and geochemical data of whole rocks for Xiaokele syenogranite and diabase. Zircon U–Pb dating suggests that the Xiaokele syenogranite (292.5 ± 0.9 Ma) and diabase (298.3 ± 1.5 Ma) were emplaced during the early Permian. The Xiaokele syenogranites have high SiO2 contents, low MgO contents, and enriched zircon εHf(t) values, suggesting that their primary magma was generated by the partial melting of the juvenile crustal material. The Xiaokele diabases have low SiO2 contents, high MgO contents, are enriched in large-ion lithophile elements, depleted in high-field-strength elements, and exhibit enriched zircon εHf(t) values. They derived from a lithospheric mantle source that had previously been metasomatized by slab-derived fluids. Combined with previous research results, we believe that the continent–continent collision between the Xing’an and Songliao blocks occurred during the late early Carboniferous – early late Carboniferous (330–310 Ma), and the two blocks were transformed into a post-collisional extensional setting during the latest Carboniferous – early Permian.

Evolution of the Mesozoic Yuljeon Basin in South Korea and its tectonic implication

The Mesozoic Yuljeon Basin (YB) is a small scale non–marine sedimentary basin in the northeastern part of the Gyeonggi Massif in the Korean Peninsula. The YB consists of two lithostratigraphic units, the Munamdong and Yuljeon formations. Based on an integrated study of stratigraphy, structural geology, geochronology, and geochemistry, major episodes of the YB have been investigated. In the late Triassic (after 227 Ma), the YB was formed in a tectonic setting, wherein crust was extended and uplifted, which resulted in deposition of the Munamdong Formation. In the latest early Jurassic (circa 180 Ma), the Yuljeon Formation was deposited in association with volcanic activity in a continental magmatic arc setting. On entering the middle Jurassic (circa 174 Ma), extension in the YB stopped and then basin inversion occurred due to contractional deformation. The middle Jurassic thermal pulse in the area generated various scale igneous rocks and induced hydrothermal circulations, which created suitable conditions for fault activity with help of fluid pressure. This study demonstrates that the YB formed with sedimentation in a post–collisional extensional setting related to the Songrim Orogeny, and then it was enlarged with volcanic–related sedimentation in a continental arc setting related to the Daebo Orogeny, and lastly it was followed by basin inversion with arc magmatism in the same continental arc setting, reflecting the temporal and spatial changes in regional tectonics. Moreover, the general provenance characteristics for the early Mesozoic strata in the eastern North China Block, Gyeonggi Massif of South Korea, and southwest Japan, can be geochronologically correlated. The sedimentary rocks record the distinctive tectonothermal histories of East Asia such as the middle Paleoproterozoic orogens in the eastern North China Block, the Permo–Triassic continental collision between North and South China Blocks, and the subduction of Paleo–Pacific plate during the Mesozoic.