Back-arc Extensional Setting Research Articles

LA-ICP-MS zircon U–Pb ages, geochemistry, and genesis of Paleozoic granites from Biezhentao Mountain, Western Tianshan, Xinjiang

The geologic history of the Western Tianshan region is important for understanding the evolution of the Central Asian Orogenic Belt and accretionary orogenesis. Paleozoic and Mesozoic igneous rocks are present along the northern margin of the Yili Block; however, studies of Paleozoic subduction and collisional events have been relatively limited. Published geochronologies of Middle Devonian magmatic rocks in this region are also lacking. Therefore, this study analyzed the zircon U–Pb ages and major and trace elemental compositions of three granite types collected from Biezhentao Mountain (Wenquan County, Western Tianshan). The medium-grained diorite (384.1 ± 3.6 Ma) and diorite–porphyrite (382 ± 3.2 Ma) are silica-rich, weakly peraluminous, alkali-rich, enriched in large-ion lithophile elements, depleted in high field strength elements, and belong to the calc–alkaline series, with A-type granite characteristics. The monzogranite (423.3 ± 9.4 Ma) exhibits A-type granite characteristics and belongs to the peraluminous calc–alkaline series. The findings suggest that the granitoids recorded two stages of tectonomagmatism during the Middle Devonian–Late Silurian. The Middle Devonian medium-grained diorite and diorite–porphyrite formed in a back-arc extensional setting, whereas the Late Silurian monzogranite formed in an active continental margin setting during the subduction of the North Tianshan Ocean. These results provide insights into the tectonic and magmatic processes that occurred during the evolution of the Western Tianshan region and the formation and evolution of the Central Asian Orogenic Belt.

Late Neoarchean-early Paleoproterozoic K-rich granitoid rocks and mafic dykes from the Liaodong Bay Depression, Bohai Sea Basin: Implications for cratonization of the North China Craton

Late Neoarchean to early Paleoproterozoic potassic granites and mafic dykes are widely distributed across the Eastern Block (EB) of the North China Craton (NCC), which provide important information about the geodynamic origin and tectonic evolution of Archean terranes. Here, we are first to report the zircon U–Pb–Hf isotopic and whole-rock compositions of potassic granites and mafic dykes from the Liaoxi Rise of the Liaodong Bay Depression (LBD) in the offshore Bohai Sea Basin (OBSB) of the northeastern NCC. The potassic granites have crystallization ages of 2499–2460 Ma and are characterized by variable SiO2 content (63.2 to 72.3 wt%) and high K2O (3.27–6.88 wt%). In addition, they show geochemical features akin to post-collisional related granites and possess positive zircon εHf(t) values of 0.9 to 4.95, suggesting that they mainly originated from the partial melting of high-K mafic rocks and formed under the post-collisional setting. The mafic dykes have a crystallization age of 2485 Ma. They show high Nb (8.64–9.39 ppm) and high Nb/U ratio (10.3–14.8), as well as radiogenic initial zircon Hf isotopes (﹢1.26–﹢4.39). These values indicate affinities with Nb-enriched basalts and that they originated from mantle-derived magma under a subduction-related back-arc extensional setting. Combined with previous research results, we believed that magmatic rocks of the LBD area may have formed under remote effects of oceanic slabs being subducted from the northwest to southeast of the eastern Hebei–western Liaoning Province back-arc basin system during the late Neoarchean to early Paleoproterozoic. Meanwhile, the K-rich granitoid rocks and coeval mafic dyke sequences imply the increased maturity of the continental crust and late Neoarchean to early Paleoproterozoic basement initiation of cratonization in the NCC.

Micro-Inclusions in Corundum from Puesto La Peña Complex Ouachitites, Mendoza, Argentina: A Window to the Mantle

Abstract Corundum xenocrysts from ouachititic dikes related to the ultrapotassic Puesto La Peña Complex, Mendoza, Argentina, contain inclusions of oxides, silicides, and silicates. Oxides identified are hibonite, carmeltazite, and as yet unnamed minerals consisting of ZrTiO3, (Al,Ti,Zr) oxide, and (Mn,REE) oxide. An unnamed Ti2(Mn,Fe)4Si5 phase was also recognized. Silicates include wodegongjieite and grossmanite. All minerals are enriched in rare earth elements. The formation of these mineral inclusions under extreme P-T conditions would have occurred in the primeval Earth, preserved in the lithospheric mantle and protected from oxidation by crystallization of corundum through the introduction of mantle-derived magma-transported fluids. The corundum crystals would have been transported by a rising plume in a back-arc extensional setting during the Miocene and incorporated as xenocrysts in an alkaline ouachititic magma, conferring its peraluminous signature.

Volcanism straddling the Miocene–Pliocene boundary on Patmos and Chiliomodi islands (southeastern Aegean Sea): insights from new 40Ar ∕ 39Ar ages

Abstract. The island of Patmos, in the eastern Aegean Sea, consists almost entirely of late Miocene to Pliocene volcanic rocks. The magmatism in the Aegean is governed by subduction of the African plate below the Eurasian plate, back-arc extension, slab rollback, slab edge processes and westward extrusion of central Anatolia to the west along the Northern Anatolian Fault into the Aegean domain. The evolution of the Aegean basin is that of a back-arc setting, with a southerly trend in the locus of both convergent tectonics and back-arc stretching, allowing intermittent upwelling of arc, lithospheric and asthenospheric magmas. Here, we present new 40Ar/39Ar age data for Patmos and the nearby small island of Chiliomodi to place this volcanism in a new high-resolution geochronological framework. High-resolution geochronology provides a key to understanding the mechanisms of both the tectonic and magmatic processes that cause the extrusion of magma locally and sheds light on the tectonic evolution of the larger region of the back-arc basin as a whole. The volcanic series on Patmos is alkalic, consistent with a back-arc extensional setting, and ranges from trachybasalt to phonolites, trachytes and rhyolites, with SiO2 ranging from 51.6 wt % to 80.5 wt %, K2O ranging from 2 wt % to 11.8 wt % and extrusion ages ranging from 6.59 ± 0.04 (0.14) Ma to 5.17 ± 0.02 (0.11) Ma. Volcanism on Patmos and adjacent Chiliomodi can be understood as a combination of mantle and crustal tectonic processes including the influence of transform faults and rotational crustal forces that also caused the widening of the southern Aegean basin due to two opposite rotational poles in the east and west and rollback of the subducting slab south of Crete.

Middle Permian basic and acidic volcanism in the Istanbul zone (NW Turkey): evidence for post-variscan extensional magmatism

ABSTRACT The Istanbul Zone (NW Turkey) forms the eastward extension of Avalonia and was subjected to deformation, uplift and erosion for a time period of 40–50 Ma following the collision with the Sakarya Zone during Early to Late Carboniferous. This paper deals with the petrology and age of the volumetrically minor basic and acidic volcanism at the lowermost horizons of Middle Permian continental red beds, which are overlain by Lower Triassic marine sedimentary rocks in the Kocaeli Peninsula. The volcanic activity is represented mainly by amygdaloidal basalt, rhyolite and minor trachydacite. The amygdaloidal basalt was derived from near-primary middle-K calc-alkaline mantle melts with negligible crystal fractionation. On the other hand, the rhyolite and trachydacite compositionally resemble A2-type rhyolites and underwent low-pressure crystal fractionation as indicated by the presence of a significant Eu anomaly. Initial ɛNd values of amygdaloidal basalt range from 0.0 to 1.5 and those of rhyolite-trachydacite are between −0.4 and −3.4. Amygdaloidal basalt and rhyolite-trachydacite are not directly related to each other by crystal fractionation. Amygdaloidal basalt probably represents the product of the near-primary mantle melts from low-degree melting of a spinel peridotitic source, and the rhyolite-trachydacite originated from highly-fractionated products of basic magmas that are slightly more alkaline than amygdaloidal basalt. However, basic and intermediate products of alkaline basic magmas are unknown in this region to date. U-Pb dating of zircons from a rhyolite sample yielded an igneous crystallization age of 261 ± 3 Ma (2σ), suggesting that the date of deposition of the continental red beds goes back to the latest Middle Permian. Based on the transgressive nature of the Permian-Triassic sequence that starts from the Middle Permian continental red beds and grades into Lower Triassic marine deposits, we suggest that the volcanism likely occurred in an extensional setting. This extension was concurrent with the northward subduction of the Palaeo-Tethys beneath the Sakarya and Istanbul zones after the Variscan orogeny. Therefore, the latest Middle to Late Permian volcanism might have occurred during the initial stage of a back-arc extensional setting

Contrasting origins of A-type granites in the Late Triassic-Early Jurassic Pitou complex, southern Jiangxi province: Implications for Mesozoic tectonic evolution in South China

The origin of A-type granites is purely circumstantial. Granitic complexes are quite common and generally composed of different type granites. I- and A-type granite association in a granitic complex is often described in the literature. Here we present a granitic complex, i.e. the Pitou complex in southern Jiangxi province, South China that is composed of different varieties of A-type granites. Detailed zircon LA-ICP-MS U Pb chronology and in situ Hf isotope, mineral chemistry and whole-rock element and Sr Nd isotope data of this granitic complex are used to explore their origin and to further constrain the geodynamics of Mesozoic magmatism in South China. Our new data indicate that the Pitou complex was emplaced during the Late Triassic (237 Ma) (central to northern part) and Early Jurassic (188–185 Ma) (southern part), respectively and that both the Late Triassic and Early Jurassic granites belong to A-type but show contrasting origins. The Late Triassic granites show an association of alkali-feldspar granite-syenogranite-monzogranite and the rock type of the Early Jurassic granites is alkali-feldspar granite. All the granites are composed mainly of K-feldspar, quartz, plagioclase and biotite (ferri-biotite or annite) with biotite occurring as anhedral crystal, interstitial to quartz and feldspar. Both the Late Triassic and Early Jurassic granites have high SiO 2 (>70 wt%) and total alkalis (K 2 O + Na 2 O > 8 wt%) with high FeO T /(FeO T + MgO) and low Mg#. They are enriched in rare earth elements (except Eu) and depleted in Sr and Ba. They display Zr + Y + Ce + Nb >350 ppm, 10,000 × Ga/Al >2.6 and have high zircon saturation temperatures (>800 °C). Biotites in the granites are Fe-rich with very low Fe 3+ /Fe 2+ , indicating fairly reducing conditions. However, the Late Triassic and Early Jurassic granites also show some geochemical differences. The Late Triassic granites have lower FeO T /(FeO T + MgO) and higher Mg# than the Early Jurassic granites, with the Mg# of the former consistent with pure crustal melts and the Mg# of the latter much lower than pure crustal melts. In addition, the Late Triassic granites have lower zircon saturation temperatures (up to 843 °C, average 823 °C) than the Early Jurassic granites (up to 946 °C, average 875 °C). Furthermore, the Late Triassic granites have high 87 Sr/ 86 Sr (t) (0.7132–0.7226) and negative ε Nd (t) (−10.3 to −10.9) and ε Hf (t) (in situ zircon) (−11.5 to −11.6) that are similar to those of the early Paleozoic granitoids in the region whereas the Early Jurassic granites have low 87 Sr/ 86 Sr (t) (down to 0.7034) and positive ε Nd (t) (up to +3.0) and ε Hf (t) (in situ zircon) (up to +8.3) that are similar to those of the Early Jurassic gabbroic rocks in the region. Geochemical data and major element modeling suggest that the Late Triassic A-type granites were formed by shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas and the Early Jurassic A-type granites were produced by extensive fractionation (ca. 90% to 95%) from gabbroic magmas by removal of plagioclase, amphibole, K-feldspar, clinopyroxene and magnetite. We further suggest the origin of the Late Triassic A-type granites was related to the transtension of the regional NE–trending strike-slip faults caused by the early Indosinian continental collisions and the Early Jurassic A-type granites were emplaced in a back-arc extensional setting coupled with the slab rollback and subsequent slab break-off of the subducted Palaeo-Pacific plate. Our new data confirm that the early Mesozoic magmatism in South China was controlled by continental collisions during the Late Permian to Triassic amalgamation in Southeast Asia whereas the late Mesozoic magmatism was related to the northwestward subduction of the Palaeo-Pacific plate. A transition in the dominant tectonic regime from the Tethyan to the Pacific tectonic domain most likely took place during the Rhaetian. • The Pitou granitic complex emplaced during the Late Triassic and Early Jurassic. • Both Late Triassic and Early Jurassic granites are A-type but show contrasting origin. • Late Triassic granites formed by shallow dehydration melting of Paleozoic granitoids. • Early Jurassic granites formed by extensive fractionation from gabbroic magmas. • Origin of both A-type granites suggests different geodynamic mechanisms.

Re Variation Triggered from the Paleo-Pacific Plate Evolution: Constrains from Mo Polymetallic Deposits in Zhejiang Province, South China Mo Province

Although highly dispersed, critical Re metal has attracted lots of attention from geoscientists, the controlling factors of Re-content variation are not completely understood, especially with regards to the genetic relationship between Re-bearing Mo polymetallic deposits and plate subduction evolution. It is well documented that the South China Mo Province, in Zhejiang Province, is characterized by multi-stage Mo polymetallic mineralization associated with Paleo-Pacific plate subduction. The Xianlin Mo(Cu)–Fe deposit occurs in Western Zhejiang as porphyry mineralization or skarn mineralization between the granodiorite and limestone. Zircon U–Pb analysis of the ore-forming granodiorite yields a Concordia age of 150.8 ± 1.1 Ma. Six molybdenite samples have relatively high Re contents (128.9~155.7 ppm) and deliver a weighted mean model age of 149.6 ± 1.3 Ma. These geochronological data suggest the Xianlin polymetallic mineralization was genetically related to the granodiorite in the Late Jurassic. Moreover, a new compilation of reliable Re contents and Re–Os isotope age data in Zhejiang Province indicates a decreasing trend in Re contents of molybdenite from the Jurassic Fe-/Cu-dominated Mo mineralization stage to the Cretaceous PbZn-enriched Mo mineralization stage in the South China Mo Province. Based on previously proposed models relating tectonic, magmatic, and hydrothermal processes, it is suggested that the Jurassic Re-enriched Mo mineralization, associated with I-type granitoids, formed in a compressive setting during the low-angle subduction of the Paleo-Pacific slab, whilst the Cretaceous Re-poorer Mo/Mo–Pb–Zn mineralization, related to both I- and A-type granitoids, formed in an extensional back-arc setting triggered by the rollback of the Paleo-Pacific slab.

Late Mesozoic to Cenozoic exhumation of the SE South China Block: Constraints from zircon and apatite fission-track thermochronology

The late Mesozoic to Cenozoic tectonothermal history of the basin-and-range system within the Cathaysia Block in the southeastern South China Block remains poorly constrained, despite its significance in the tectonic and topographic evolution of eastern China. Zircon and apatite fission-track thermochronometers were applied to reconstruct the cooling histories of NE-trending mountain ranges mantled by late Mesozoic granites and older basement rocks in southwest Fujian. New zircon and apatite fission-track data record post-magmatic cooling of the sampled granites since their emplacement of ~164–118 Ma. Thermal modeling of the thermochronology data revealed two episodes of accelerated cooling events at ~110–85 Ma and ~40–25 Ma. The late Early Cretaceous initiation of an accelerated cooling event at rates of ~6 °C/Ma was considered to be the cause of the formation of the basin-and-range system in a back-arc extensional setting related to the rollback of the Paleo-Pacific Plate. The second cooling event at ~40–25 Ma at a rate of ~20 °C/Ma, reflected by the youngest apatite fission track ages (~25–20 Ma), was likely ascribed to headwater erosion along the drainage divide related to the intensification of the East Asian Monsoon and extensional tectonism. The sharp discrepancy in apatite fission track ages across the Zhenghe-Dapu Fault zone implies that the collapse of the late Mesozoic coastal magmatic arc in the early Eocene (51.4–45.6 Ma). This is attributed to the northward propagation of extension related to the rollback of the Paleo-Pacific Plate that formed the eastward-dipping topography. Furthermore, the long-lasting extension due to the rollback of the Pacific Plate since the ridge subduction of the Izanagi-Pacific at ~50 Ma likely drove the exhumation of the Cathaysia Block and opening of the South China Sea.

Late Cretaceous Nature of SW Borneo and Paleo-Pacific Subduction: New Insights from the Granitoids in the Schwaner Mountains

Abstract Cretaceous igneous rocks in SW Borneo are significant for investigating the Paleo-Pacific subduction and regional correlation. This paper presents new zircon ages and Sr–Nd–Pb–Hf–O isotopic and whole-rock geochemical data for the Cretaceous granitoids in the Schwaner Mountains. These granitoid samples yield formation ages of 92–80 Ma and can be geochemically subdivided into Group 1 biotite granite and granodiorite and Group 2 monzogranite. Group 1 has higher εHf t values of +1.2–+14.5 and lower δ18O values of 5.2–6.6‰ than Group 2 with εHf t and δ18O values of −1.5–+5.9 and 6.1–6.6‰, respectively. Group 1 samples show variable εNd t values of −1.9–+1.7, (206Pb/204Pb)i ratios of 18.63–19.29, (207Pb/204Pb)i ratios of 15.62–15.69, and (208Pb/204Pb)i ratios of 38.67–39.49 and are the products of partial melting of newly underplated mafic rocks with a component of crustal rocks. Group 2 with slightly negative εNd t values of −1.1–−1.5 was originated from partial melting of metasedimentary rocks with a juvenile mafic component. These granitoids were formed in a continental arc setting related to the Paleo-Pacific subduction during the Late Cretaceous and can compare with those contemporaneous I-type granites along the Kuching zone in NW Borneo. Those Late Cretaceous granitoids with A-type affinities from the easternmost Peninsular Malaysia and Singapore to the east of Sundaland might be formed in a back-arc extensional setting after the Paleo-Pacific subduction.

840–820 Ma Dahongshan bimodal volcanic rocks: new constraints on the Neoproterozoic arc–back-arc basin system along the northern margin of the Yangtze Block

ABSTRACT The newly identified earliest part of the Sanligang bimodal volcanic succession in the Dahongshan area of the northern Yangtze Block provides insight into the role of this block in the evolution of the Rodinia supercontinent. Field contact relationships and dating indicate that these rocks are contemporaneous and formed at 840–820 Ma. The Sanligang mafic rocks have low SiO2 (40.33–48.17 wt.%) and K2O (0.07–1.65 wt.%), high TiO2 (2.64–3.52 wt.%) contents, and low U/Th (0.33–0.38), Th/Zr (0.001–0.002), and Nb/Y (0.059–0.064) ratios with positive εNd(t) (+5.08 to +7.59). This suggests that they are formed from magmas derived from an asthenospheric mantle source that had not been modified by slab-derived materials, with a low degree of crustal contamination during their eruption. Contemporaneous felsic rocks in the study area have high SiO2 (73.1–80.0 wt.%), variable K2O (0.63–4.39 wt.%) and Na2O (1.73–6.54 wt.%), and low MgO (0.1–0.58 wt.%) contents and are light rare earth element enriched, with (La/Yb)N values of 1.37–2.65 and significantly negative Eu anomalies. They have A2-type granite affinities, as evidenced by their high 10,000 Ga/Al values (3.51–4.55) and Zr contents (285–620 ppm), and positive whole-rock εNd(t) (+6.16–+6.52), zircon εHf(t) (+9.69–+12.29) and δ18O (+4.82–+6.11) values, suggesting they formed by partial melting of juvenile crustal material caused by heating associated with contemporaneous mantle-derived mafic magmatism. Combining the data for these bimodal volcanic successions with other rock assemblages in the Dahongshan area indicates that this magmatism occurred in a back-arc extensional setting. This magmatism was caused by slab detachment and separation along a vertical tear during the Neoproterozoic evolution of the arc to arc–back-arc system of the northern margin of the Yangtze Block.

Partial Melting of Lithospheric Mantle and Formation of the Early Cretaceous Alkaline Rocks in the Guandimiao REE Deposit, Luxi Terrane, Eastern China

The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO2, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe2O3 (Total), MgO, P2O5, TiO2, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.

Petrogenesis of Cretaceous volcanic rocks from islands off Fujian Province, East China Sea, and implications for episodic slab rollback of the paleo-Pacific plate

This study constrains the petrogenesis and regional tectono-magmatic evolution of volcanic rocks from Fuying and Dayushan Islands, northeast Fujian Province, southeast (SE) China, using petrological, zircon U–Pb geochronological, whole-rock geochemical and Sr–Nd–Pb isotopic data. New zircon U–Pb data show that volcanism occurred between 118 and 99 Ma. The volcanic rocks are dominantly alkalic to alkali-calcic, ferroan, and peraluminous in composition, and are characterized by strongly fractionated LREE and HREE, with depletion in Nb, Ta, Sr, and Eu. These features, together with their relatively enriched Sr–Nd–Pb isotopic compositions, imply that the volcanic rocks were derived from ancient lower crust and experienced extensive assimilation and fractional crystallization in the shallow crust. Taking into account previously published data, we conclude that the Fuying and Dayushan volcanic rocks were generated in a back-arc extensional setting in response to episodic slab rollback after flat subduction of the paleo-Pacific plate during the Cretaceous.

Late Paleozoic multi-stage subduction accretion of the southwestern Central Asian Orogenic Belt: insights from the Late Carboniferous-Early Permian granites in the southern West Junggar, NW China

ABSTRACT The Late Carboniferous to Early Permian tectonic evolution of the South West Junggar (SWJ), northwest China, a crucial constituent in the southwestern domain of the Central Asian Orogenic Belt (CAOB), has long been the subject of debate. Here, detailed petrographic studies, new nine LA-ICP-MS zircon U-Pb ages and forty-nine whole-rock geochemical results from six granitic plutons in the SWJ were combined with previously available data to elucidate their petrogenesis and re-decipher Late Palaeozoic tectonic evolution and accretionary process of the southwestern CAOB. The studied granites can be geochemically classified as I- and A-type granites, and the former is further subdivided into I-type and highly fractionated I-type granites. The calc-alkaline I-type amphibole-bearing granites (Group I) yield crystallization ages of ca. 317–315 Ma, and exhibit depletions in HFSEs and enrichments in LILEs with no to slight Eu anomalies and low zircon saturation temperatures, indicating that the formation of these granites may be closely related to the subduction of the Junggar oceanic slab. The Group II granite porphyries (ca. 312 Ma) contain negligible amphibole, and are characterized by evolved SiO2 but low zircon saturation temperatures with sharply negative Eu, Ba, Nb, Ta, Sr, P and Ti anomalies, belonging to highly fractionated I-type granites, and were probably produced in a tectonic transition from compressional to extensional setting. In contrast, the latest Carboniferous (ca. 304–298 Ma) alkaline A-type granites (Group III) are amphibole-absent, and are similar to Group II granites in major element compositions, but exhibit higher zircon saturation temperatures, indicating that they probably resulted in a back-arc extensional setting triggered by roll-back of the subducting Junggar oceanic slab since ca. 310 Ma. In combination with regional geology and re-interpretation of previous data, a multi-stage subduction-accretion model is suggested to account for the Late Palaeozoic tectonic evolution of the southwestern CAOB. This model can plausibly interpret the differences in duration of subduction between the northern and southern domains of West Junggar and the coexistence of A-type granites with subduction-related sanukites in the SWJ during Carboniferous to Permian.

Latest Mesoproterozoic provenance shift in the southwestern Yangtze Block, South China: Insights into tectonic evolution in the context of the supercontinent cycle

The Huili Group, one of the most representative late Mesoproterozoic strata in the Yangtze Block, bears key information for the tectonic history of the South China Block in the context of the supercontinent cycle. We present an integrated dataset involving field observation, petrology, detrital zircon U-Pb dating, and Hf isotope analyses for the Huili Group. Our new detrital zircon U-Pb ages, in combination with available magmatic age data, confirm that the Huili Group was mainly deposited at ca. 1.1–1.0 Ga. The lower Huili Group is characterized by subrounded to rounded detrital zircon grains with age intervals of 2.80–2.40 Ga (~21%), 2.34–1.72 Ga (~64%), and 1.68–1.19 Ga (~14%). In contrast, zircon grains from the upper Huili Group have variable morphologies and yield two pronounced age populations of 2.00–1.25 Ga (~50%; subrounded to rounded grains) and 1.2–1.0 Ga (~46%; euhedral to subhedral grains). The majority of zircon grains have εHf(t) values and depleted mantle two-stage (TDM2) model ages comparable to those of magmatic rocks or older sediments in the Yangtze Block. Multiple sources are suggested to have contributed older detrital zircon grains to the lower Huili Group, whereas the upper part received greater proportions of zircon grains with ages close to or equal to the depositional age from proximal magmatic sources. This observation indicates a drastic provenance shift most likely occurred within the Huili Group at ~1.04 Ga. Through reappraising our detrital zircon results and available geological datasets, we suggest that the terrigenous clastic and carbonate rocks of the lower-middle Huili Group were deposited on a passive continental margin, but the upper volcano-sedimentary sequences were formed in a back-arc extensional setting. We further propose that the Yangtze Block was spatially separated from (i.e., not yet part of) the Rodinia supercontinent during the sedimentation of the Huili Group, supporting that a global supercontinent did not exist yet in the late Mesoproterozoic.

The Triassic Duobaoshan appinite-granite suite, NE China: Implications for a water-fluxed lithospheric mantle and an extensional setting related to the subduction of the Mongol-Okhotsk Ocean

The appinite suite of rocks offers a unique opportunity to reveal the properties of subarc lithospheric mantle, indicate essential crust-mantle interactions, and ascertain critical geodynamic settings. In this study, we present zircon U–Pb ages, Hf–O isotopes, and whole-rock geochemical data for the newly identified Duobaoshan appinite-granite suite in the eastern Central Asian Orogenic Belt (CAOB). Field and petrographic investigations show that this suite of rocks is composed of hornblende gabbro, quartz monzonite, and granite porphyry. The appinites are abnormally hornblende-rich, and contain spectacular hornblende phenocrysts, pegmatitic textures, and miarolitic cavity structures that indicate an anomalous water-rich origin. The quartz monzonite and granite porphyry show intrusive contact relations with the appinites. Secondary ion mass spectrometry (SIMS) zircon dating reveals that the Duobaoshan appinite-granite suite was emplaced in a very short time interval of 224–223 Ma. These samples show typical subduction related arc magmatic geochemical characteristics that are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (REEs), depleted in high field strength elements (HFSEs) and middle and heavy REEs. All samples exhibit comparative total REEs and no significant evolutionary relationships. The appinites show high Sr/Y ratios (reaching ~100) and positive or zero Eu anomalies, which reveal amphibole fractionation promotion and plagioclase fractionation suppression in a water-rich magma system. The quartz monzonite possesses high Sr/Y and La/Yb ratios that exhibit adakite-like properties and were derived from thickened lower continental crust. The granite porphyry is silica-rich and non-adakitic. All samples have juvenile whole-rock initial 87Sr/86Sr ratios (~0.7038) and ɛNd(t) values. Zircons have depleted mantle-like ɛHf (t) (to +12.0) and δ18O (~ + 5.65‰) values. Combining these geochemical features with observations of the regional geology and tectonic setting, we propose that the Duobaoshan appinite-granite suite was generated under a Triassic back-arc extensional setting related to the southward subduction of the Mongol-Okhotsk Ocean. The appinites were derived from an unusual water-rich mantle source, which was previously metasomatized by slab-derived fluids. The quartz monzonite and granite porphyry were generated from partial melting of the juvenile lower continental crust triggered by coeval appinitic magma underplating.

Petrogenesis and geodynamic setting of the Late Carboniferous granodiorite porphyry in Miaoergou pluton, southern West Junggar

位于西准噶尔南部的庙尔沟岩体主体由碱长花岗岩和少量紫苏花岗岩组成。本文在前人工作基础上，以岩体东南边缘新发现的花岗闪长斑岩为研究对象，开展岩石学、年代学和Hf同位素以及全岩地球化学研究，确定花岗闪长斑岩形成时代、揭示岩石成因类型及源区属性、探讨其与碱长花岗岩和紫苏花岗岩岩浆演化成因联系及其形成的深部动力学过程。锆石U-Pb定年结果显示，花岗闪长斑岩形成于317.4±1.9Ma，为晚石炭世早期岩浆活动的产物，明显早于紫苏花岗岩（~307Ma）和碱长花岗岩（~303Ma）。岩石地球化学数据表明，花岗闪长斑岩具有较高硅、中等铝，贫钙、铁、镁，富集Rb、K、Th、U，强烈亏损Nb、Ta、Ti的特征，为钙碱性弱过铝质I型花岗岩；紫苏花岗岩更多的表现出钙碱性-高钾钙碱性镁质I型紫苏花岗岩特征；碱长花岗岩为碱性准铝质-弱过铝质A型花岗岩。锆石Hf同位素分析结果表明，花岗闪长斑岩、紫苏花岗岩和碱长花岗岩均具有高正的ε_Hf（t）值（+11.6~+15.8）和年轻的二阶段模式年龄（325~600Ma），表明其原始岩浆主要起源于亏损地幔新衍生的年轻地壳物质。综合分析认为，庙尔沟岩体花岗闪长斑岩形成于晚石炭世早期洋壳俯冲背景，由底侵的、受流体交代的幔源基性岩浆与其诱发的年轻下地壳酸性岩浆在深部混合而成。紫苏花岗岩和碱长花岗岩形成于弧后伸展背景，前者是伸展初期继续底侵于下地壳的幔源玄武质岩浆降温释放大量的水和热诱使早期侵位于下地壳的镁铁质岩石再次发生部分熔融的产物，后者是伸展后期大规模软流圈地幔上涌底垫加热年轻中下地壳使其部分熔融而成。;The Miaoergou pluton, located in southern West Junggar, is mainly composed of alkali-feldspar granites with small amount of charnockites. On the basis of previous researches, this paper mainly focuses on the newly discovered granodiorite porphyry in southwestern margin of the Miaoergou pluton. A comprehensive study of petrographical, zircon U-Pb geochronological and in-situ Hf isotopic, whole-rock geochemical analyses were carried out to determine its diagenetic age and reveal the petrogenesis, the nature and composition of the magma source of this porphyry, and furthermore, to discuss the genetic relationships among the granodiorite porphyries, alkali-feldspar granites and charnockites and their geodynamic processes. LA-ICP-MS zircon U-Pb dating for the granodiorite porphyry yielded crystallization age of 317.4±1.9Ma, defining an early Late Carboniferous magmatic event in southern West Junggar, which is significantly earlier than the ages of charnockites (~307Ma) and alkali-feldspar granites (~303Ma). The geochemical characteristics indicate that the studied granodiorite porphyries belong to cal-alkaline weakly peraluminous I-type granite, and are characterized by slightly high silicon, medium aluminum, but low ferrum, calcium and magnesium, and also enrichment in Rb, K, Th, U with pronounced negative Nb, Ta and Ti anomalies. The charnockites possess calc-alkaline to high-K calc-alkaline magnesian I-type charnockite affinities, which were previously considered A-type granites by some geologists, while alkali-feldspar granites belong to alkaline metaluminous to weakly peraluminous A-type granites. In-situ zircon Hf isotope analyses show that these granites have high positive ε_Hf(t) values (+11.6~+15.8) and very young two-stage Hf model ages of 325~600Ma, indicating that the magma was sourced from juvenile crust originated from depleted mantle during the Late Neoproterozoic to Early Carboniferous period. In association with previous analysis, it is proposed that the Miaoergou granodiorite porphyries were formed in an oceanic crust subduction setting during early Late Carboniferous. They are probably the products of the acidic magma formed by partial melting of the juvenile lower crust and mixed with the underplated basaltic magma derived from the fluid metasomatic mantle wedge in the deep crust to some extent. Both the charnockites and alkali-feldspar granites were formed in a back-arc extensional setting related to the northwestward subduction of the Junggar oceanic crust. The charnockites may be generated by re-melting of juvenile lower crust mafic rocks, probably triggered by addition of water and heat released from cooling of underplated depleted mantle-derived basaltic magma during the incipient rifting stage of the back-arc basin, and the alkali-feldspar granites may be generated by partial melting of the juvenile middle-lower crust induced by the large-scale upwelling of asthenosphere during the later stage of the back-arc extension.

Lithospheric extension in response to subduction of the Paleo-Pacific Plate: Insights from Early Jurassic intraplate volcanic rocks in the Sk2 Borehole, Songliao Basin, NE China

Subduction of the Paleo-Pacific and Mongol–Okhotsk oceanic plates is widely believed to have caused extensive Mesozoic magmatism and lithospheric deformation in East Asia. However, it remains unclear how the two tectonic domains affected NE China, and whether a boundary exists between them, because the Songnen Block in central NE China is covered by sedimentary rocks and the related mafic magmatism has been rarely discovered. Here we report a suite of Early Jurassic (ca. 175 Ma) mafic volcanic rocks (including basalt and basaltic andesite) from the Sk2 Borehole, Songliao Basin, NE China. The Sk2 mafic rocks are characterized by high Th/U, Zr/Y and Ti/Y ratios, and their geochemical signatures are consistent with formation in a back-arc extensional setting, rather than an arc setting. The rocks show depleted zircon εHf(t) and whole-rock εNd(t) values, which are comparable to mafic rocks located on the east of the Songliao Basin. This suggests that they both have a depleted mantle source, unlike the mafic rocks on the west of the Songliao Basin with relatively enriched Nd and Hf isotopes. Early–Middle Jurassic mafic rocks are distributed in the Erguna Block, >500 km to the west from the Sk2 Borehole, whereas coeval mafic rocks are widely distributed in the nearby eastern Heilongjiang and Jilin provinces. These intraplate-like mafic rocks and the Early Jurassic bimodal rocks within the Lesser Xing'an and Zhangguangcai ranges likely formed in response to lithospheric extension that was triggered by subduction of the Paleo-Pacific Plate. The Early Jurassic mafic rocks directly underlie the oldest sedimentary rocks (the Huoshiling Formation) in the Songliao Basin, which suggests that basin formation was probably initiated by lithospheric extension.

Slab roll-back triggered back-arc extension south of the Paleo-Asian Ocean: Insights from Devonian MORB-like diabase dykes from the Chinese Altai

To better understand the origin of mid-ocean ridge basalt (MORB)-like mafic magmas and its geodynamic implications for the subduction zone system, we present an integrated study of petrology, mineralogy, geochronology and geochemistry of newly-discovered diabase dykes from the Chinese Altai, southwestern Central Asian Orogenic Belt. The diabases have zircon UPb ages of ~386 Ma, and are mainly composed of clinopyroxene (Wo43–55En17–27Fs26–34) and albite (An0–1Ab92–99Or0–7). Although the clinopyroxenes have consistent Mg# values (up to 75) that appear to be in equilibrium with bulk compositions, they can be divided into two groups based on their differences in other geochemical variables: type ǀ has high SiO2 and CaO but low TiO2 and Al2O3 contents while type ǁ displays low SiO2 and CaO but high TiO2 and Al2O3 contents. This indicates that the two types of clinopyroxenes may share a common source but crystallized at different depths under different P-T conditions (e.g., type ǀ: 6.3 kbar (~20 km), 1227 °C; type ǁ: 15.5 kbar (~49 km), 1332 °C). The dykes have low SiO2 (44.5–48.2 wt%) and K2O (0.17–0.33 wt%), but high (Fe2O3)T (11.2–13.6 wt%) and MgO (7.16–9.90 wt%) contents, placing them within the low-K tholeiitic series. With both N-MORB- and arc-like geochemical characteristics, including weakly fractionated rare earth element (REE) patterns ((La/Yb)N 1.43–1.62), enrichment in large-ion lithophile elements (LILE) (e.g., Pb) and slightly depletion of high field strength elements (HFSE) (e.g., Nb, Ta and Ti), and highly depleted SrNd isotopic compositions (87Sr/86Sri 0.7039–0.7042, ɛNd(t) +8.3 to +8.6), the diabase dykes were probably derived by partial melting of a N-MORB-like asthenospheric mantle source metasomatized by subduction-related fluids under spinel-facies conditions. Melting temperatures and pressures for the primary magmas were estimated at 13711394 °C and 2.22.4 GPa, respectively. The dykes underwent insignificant crustal contamination, but trace-element modeling indicates that minor subcontinental lithospheric mantle (SCLM) components could have been involved in their source region. We suggest that the MORB-like magmatism was triggered by asthenospheric upwelling in a back-arc extensional setting, in response to the roll-back subduction of the Paleo-Asian oceanic slab. Combined with the coeval arc magmatism in the study area, we envisage that an active arc–back-arc basin system developed in the Chinese Altai during Devonian time, linked to the northward subduction of the Paleo-Asian Ocean. This study emphasizes that MORB-like mafic rocks in paleo-subduction zones could be an important tectonomagmatic process-indicator (e.g., back-arc extension), which offers a new perspective in reconstructing past tectonic regimes.

Evolution of Subduction Dynamics beneath West Avalonia in Middle to Late Ordovician Times

Abstract Middle to Upper Ordovician volcanic rocks in the Arisaig area of Nova Scotia, Canada, constitute the only known record of volcanism in West Avalonia during that interval. Hence, they have been extensively studied to test paleocontinental reconstructions that consistently show Avalonia as a drifting microcontinent during that period. Identification of volcanic rocks with an intermediate composition (the new Seaspray Cove Formation) between upper Darriwilian bimodal volcanic rocks of the Dunn Point Formation and Sandbian felsic pyroclastic rocks of the McGillivray Brook Formation has led to a reevaluation of magmatic relationships in the Ordovician volcanic suite at Arisaig. Although part of the same volcanic construction, the three formations are separated by significant time-gaps and are shown to belong to three distinct magmatic subsystems. The tectonostratigraphic context and trace element contents of the Dunn Point Formation basalts suggest that they were produced by the high-degree partial melting of an E-MORB type source in a back-arc extensional setting, whereas trace element contents in intermediate rocks of the Seaspray Cove Formation suggest that they were produced by the low-degree partial melting of a subduction-enriched source in an arc setting. The two formations are separated by a long interval of volcanic quiescence and deep weathering, during which time the back-arc region evolved from extension to shortening and was eventually onlapped by arc volcanic rocks. Based on limited field constraints, paleomagnetic and paleontological data, this progradation of arc onto back-arc volcanic rocks occurred from the north, where an increasingly young Iapetan oceanic plate was being subducted at an increasingly shallow angle. Partial subduction of the Iapetan oceanic ridge is thought to have subsequently generated slab window magmatism, thus marking the last pulse of subduction-related volcanism in both East and West Avalonia.

Geochronology and geochemistry of early Carboniferous basalts from Baogutu Formation in West Junggar, Northwest China: evidence for a back-arc extension

ABSTRACT Early Carboniferous magmatism of the southern West Junggar (SWJ) region is essential for understanding the accretionary processes and crustal growth in the southwestern segment of the Central Asian Orogenic Belt. A comprehensive study of geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic compositions was carried out in this study for basalts from the Baogutu Formation in Sulushuoke region, aiming to provide constraints on the Early Carboniferous tectonic evolution of the SWJ, NW China. LA-ICP-MS zircon U-Pb dating of four basalt samples yielded crystallization ages ranging from 339 Ma to 347 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The studied tholeiitic basalts are geochemically characterized by nearly flat REE patterns, enriched large iron lithosphere elements (e.g. K) and depleted high field strength elements (e.g. Nb, Ta, Zr, Hf) relative to those in N-MORB, resembling those of modern Mariana Trough back-arc basin basalts. In addition, they have high positive εNd(t) values (+5.86 to +6.59) with initial 87Sr/86Sr ratios in the range of 0.704783–0.705315 and variable εHf(t) values (+3.99 to +15.6). The rocks cover restricted ranges in initial Pb isotope ratios with (206Pb/204Pb)i from 17.529 to 18.289, (207Pb/204Pb)i from 15.509 to 15.627, and (208Pb/204Pb)i from 37.628 to 38.397. These signatures indicate that the tholeiitic basalts were derived from 10% to 20% partial melting of a spinel-lherzolitic N-MORB-like depleted mantle source modified by fluids released from the subducted lithosphere in a back-arc extensional setting. From our new data presented in this study and in conjunction with those from previous works, we, therefore, suggest that a back-arc basin related to the northwestward subduction of the Junggar oceanic crust was developed in the Hatu-Baogutu region during the early Carboniferous time.