Back-arc Research Articles

Recognizing crystal accumulation in plutonic system: Evidence from mafic microgranular enclaves in Early Silurian Qingshan granodiorite, North Qilian Orogenic Belt, NW China

Granitoids are the primary constituents of the continental crust; recognizing and quantifying the effects of crystal accumulation in evolved magmas is a challenging task. Here we present whole-rock chemical, Sr-Nd isotopic, zircon U-Pb geochronological, and Hf isotopic data as well as EPMA and in-situ LA-ICP-MS analyses of amphiboles from Qingshan granodiorite and mafic microgranular enclaves (MMEs) from the North Qilian Orogenic Belt (NQOB). The MMEs and granodiorite have identical crystallisation ages (ca. 434 Ma), the same mineral assemblage, but different mineral modal proportions as the MMEs comprise more amphibole. The MMEs and granodiorite have indistinguishable and depleted zircon Hf (εHf (t) values of 6.1–8.3, and 6.4–9.1, respectively) and whole-rock Sr-Nd isotopic compositions, i.e. (87Sr/86Sr)i values of 0.70514–0.70913 and 0.70207–0.70552, respectively; and εNd(t) values of 0.27–2.23 and 0.21–2.39, respectively. The occurrence of idiomorphic amphibole grains, low MREEs and HREEs concentration, low Mg# values of equilibrium melts (AEMs), and decreasing Zr/Hf ratios indicate that Qingshan MMEs represent cumulates formed at a thermal boundary layer between the ascending conduits and the wall rocks. The Qingshan granodiorite was formed by partial melting of subducted oceanic crust with crustal materials during southward subduction of the North Qilian Ocean, followed by further amphibole-dominated fractionation by compaction without crystal deformation. We propose a model of southward subduction with the back-arc basin for the tectonic evolution of the western NQOB. Both the subduction- and collision-related granitoids of the NQOB have played a vital role in continental crust growth.

Geochronological and stratigraphical characterization of lower Jurassic tectonically-controlled felsic tuff ring volcanism in Northwestern Patagonia, Argentina

ABSTRACT The early Jurassic syn-rift phase generated a significant set of half-grabens, calderas, and pull-apart basins along the southwestern margin of South America. In particular, the Loncon Complex, located in the western Extra-Andean region of North Patagonia (∼40°S), forms part of such structures. In the present contribution, we characterize and define the Early Jurassic extensional architecture of the Loncon Complex, constituted of a small-scale pull-apart basin associated with polygenetic volcanism, based on outcrop-scale kinematic data from strike-slip faults, and stratigraphic analysis. Lithofacies analysis, texture, componentry, and structural data allow us to distinguish four stages during the evolution of the Complex; the three early stages are followed by a deformational phase, and crowned by a limited felsic monogenetic volcano; stages consist of a) A continental alluvial to fluvial sedimentation related with an early pull-apart basin phase; b) local phreatomagmatic explosions with syn-eruptive sedimentation periods of a medium life cycle (~104-105 years), b) effusive andesitic stage generated by strike-slip transtension, resulting in partial deformation and development of a local unconformity, and c) a final short-term silicic tuff ring volcano construction (days to months’ time span). The measured faulting pattern is synchronous with the Loncon Complex’s sedimentation and volcanism. Kinematic data allow us to characterize the Loncon Complex evolution under NNE-NNW minimum horizontal stress (SHmin). This local NNE-oriented extension is interpreted as a consequence of a regional process, in a similar way to the report for the Neuquén Basin and the surrounding region in Patagonia. New U-Pb data indicate the onset of volcanism began in the Sinemurian (193.08 ± 1.13 Ma) also supporting an equivalent timing with the vicinity half-grabens areas of the North Patagonia region and the Neuquén Basin (Figure S1a). The process that occurred during the Sinemurian, and probably during the Pliensbachian, affected the entire back-arc basin developed along the southwestern border of Gondwana.

Convergent plate boundary environments for formation of ≥ 3800 Ma mafic–ultramafic assemblages (Isua area, Greenland): Implications for early global geodynamics

In the gneiss terrane on the south side of the Eoarchean Isua supracrustal belt, ultramafic rocks with relict abyssal peridotite mineralogy (Bennett et al., 2002; Friend et al., 2002; Nutman et al., 2007; Rollinson, 2007; van de Löcht et al., 2020), layered gabbros with cumulate ultramafic rocks, basalts and associated siliceous sedimentary rocks were tectonically-imbricated, prior to and during intrusion of ca. 3800 Ma tonalites. Together with ≥ 3800 Ma basalts in the Outer Arc Group of the nearby Isua supracrustal belt, the composition of all these mafic rocks (e.g., Th–Hf–Nb systematics, high Th/Yb, Ba/Nb, Ba/Yb ratios and negative Nb and Ti anomalies) shows affinity with modern suprasubduction rocks whose genesis involved fluid fluxing of the upper mantle. However, the majority of these samples have Ba/Nb and Ba/Yb values less than in modern island arc magmas, but similar to many backarc basin magmas (e.g., Pearce and Stern, 2006). It is unknown whether these ca. 3800 Ma mafic rocks are, (i) arc rocks where the Ba/Nb and Ba/Yb signatures reflect lower surficial Ba in Eoarchean oceanic settings, or (ii) in direct comparison with Phanerozoic suites, these signatures reflect a back-arc setting with interplay between fluid fluxing and decompressional melting. The tectonic intercalation of upper mantle with lower and upper crustal rocks, combined with the fluid-fluxing influences seen in chemistry of all the mafic rocks is best accommodated in a compressional Eoarchean convergent plate boundary setting within a mobile-lid regime. Thus stagnant lid scenarios of crust formation, if operative, must have co-existed or alternated with mobile-lid regimes by 3800 Ma.

From early Paleozoic subduction to end-Carboniferous post-orogenic collapse: New constraints on the tectonic evolution of the Central and South Tianshan (NW China)

Recognition of extensional events during orogenesis is critical for the reconstruction of geodynamic evolution of orogens, but is also a challenge due to multi-stage tectonic superimpositions. The South Tianshan is the southern margin of the Paleozoic Central Asian Orogenic Belt (CAOB) and a key for understanding the geodynamic evolution of the CAOB. Here we present new structural, geochronological and geochemical data for the Paleozoic sedimentary, igneous and metamorphic rocks in the Central and South Tianshan (NW China). Based on our new results and published data, we suggest that a neritic to terrestrial back-arc basin existed to the south of the Central Tianshan magmatic arc during the Early-Middle Devonian. We document a crustal anatectic event (∼304 Ma) associated with peraluminous granitic magmatism (∼305–295 Ma) in the South Tianshan. Multi-stage ductile deformation events witnessed the Paleozoic evolution from subduction-accretion to post-orogenic collapse of the Tianshan Orogen: (1) Late Devonian-Carboniferous regional top-to-the-north ductile shearing is related to the southward subduction of the South Tianshan back-arc basins; (2) late Carboniferous south-verging folds and brittle-ductile axial-plane cleavages correspond to back-thrusting resulted from the collision between Central Tianshan and Tarim; (3) latest Carboniferous ductile normal faults developed in migmatites define a dome-like structure linked with syn-anatectic exhumation of the middle crust during the post-collisional extension; and (4) regional dextral strike-slip faulting attributed to Permian lateral extrusion. Therein, two stages of crustal extension are key puzzle pieces to reconstruct the accretionary orogenic processes of the Paleozoic Tianshan.

РИФТОГЕНЕЗ В ОКРАИННЫХ МОРЯХ ЗАПАДНОЙ ПАЦИФИКИ

The paper presents a review of structural features of rifting during the formation of Cenozoic basins of marginal seas of the Western Pacific. The assumption that rifting always starts with a passive stage and is only occasionally interrupted by episodes of active rifting is fully confirmed by examples from the studied basins. Rifting occurred under a compressive regime with NE and NNE-directed shortening that resulted in the formation of either a chain of pull-apart basins or rifts scattered between major strike-slip faults in the great part of the South China Sea. The NE and NNE directions of horizontal compressive force are apparently related to the convective currents in the asthenospheric mantle that have extended from the spreading ridge of the Indian Ocean and have carried plate fragments deformed to some extent during their transportation. The north-northeastward drift of the Indian, Australian, and Eurasian plates associated with these mantle currents does not reveal any connection with subduction from adjacent Paleo-Pacific plates, which continue to move in the NW direction up to the present time. Thus, formation of the marginal basins of the Western Pacific were unaffected by subduction from the Pacific Ocean side and they can be regarded as back-arc basins only due to their geographical locations.

GEOCHEMISTRY AND PETROGRAPHY OF METAMORPHIC SOLE AMPHIBOLITES FROM THE SLATINA QUARRY, MT. ZRINSKA GORA, CROATIA

The investigated Mt. Zrinska Gora amphibolites are a part of the metamorphic sole of the Banovina ophiolite complex. Mineral composition and other petrographic characteristics of amphibolites and adjacent peridotite were investigated by polarised microscopy and chemical analyses of rocks were obtained by a combination of inductively coupled plasma mass and emission spectrometry. The main minerals in these garnet-bearing amphibolites are amphibole and plagioclase with accessory garnet, sphene, spinel, opaque mineral ± clinopyroxene ± quartz ± actinolite ± zoisite-clinozoisite ± prehnite ± pumpelyite and ± clay minerals. Kelyphitic corona is developed around garnet. The amphibolites have nematoblastic, nematogranoblastic, porphyroblastic and porphyroclastic textures and homogenous to foliated structures. The presence of clinopyroxene in some of the investigated amphibolites points to their possible formation under P‒T conditions of upper amphibolite facies. The greenschist facies retrograde metamorphism and subsequent surface weathering of amphibolites is also evident. The chemical composition of rocks indicates that protoliths of most amphibolites were tholeiitic basalts, initially formed in island arcs and the adjoining back-arc basins. During the intraoceanic subduction in Jurassic Neotethys Ocean basalts/basaltic tuffs of island arc tholeiite affinity and back-arc basin basalts, positioned on the top of the down-going oceanic slab, were welded to the base of the hot over-riding mantle wedge, metamorphosed to amphibolite rocks and obducted together with ophiolites on the Adria continental margine. The new described amphibolite type having peridotite protolith characteristics originated as the consequence of the hydration of the bottom part of the overlying mantle wedge.

Bringing the Submarine Mariana Arc and Backarc Basin to Life for Undergraduates and the Public

ABSTRACTThis paper aims to better teach about submarine arc and backarc basin volcanic and hydrothermal activity using the ~1400 km long Mariana convergent margin as an example. Four US National Oceanographic and Atmospheric Administration (NOAA) expeditions (2004–2016) equipped with a remotely operated vehicle (ROV) have discovered and explored many of submarine volcanoes and associated hydrothermal fields and generated many short (~1 min long) videos about them. Some of these videos would be very useful for teaching about these processes if they were organized and context provided, which is done here. Eighteen short videos about nine sites generated by NOAA are presented and discussed here. These are organized into three categories: volcanic eruptions, magmatic degassing, and hydrothermal activity. Volcanic eruption videos include two about glassy pillow lavas erupted in 2013–2015 and a rare example of a submarine eruption. Four videos about magmatic degassing include an example of sulfur produced by disproportionation of magmatic sulfur dioxide associated with a submarine eruption, two rare examples of molten sulfur lakes, and liquid carbon dioxide venting. Four videos about hydrothermal activity are provided. Suggestions for how this material might be used in the classroom are also given.

Detrital zircon U–Pb and Hf analyses of Silurian–Devonian sediments in the Sukhothai Terrane, northern Thailand: implications for the Mid-Paleozoic arc belt

The Sukhothai Terrane in northern Thailand consists of continental basement and a Permo-Triassic magmatic arc related to the subduction of the main Palaeo-Tethys Ocean. The Donchai Group represents the oldest sedimentary sequence of the Sukhothai Terrane and consists mainly of meta-sandstone, quartzo-feldspathic schist, phyllite and silty slate. We present new detrital zircon U–Pb geochronology, Lu–Hf isotope data and geochemical data for the sedimentary rocks of the Donchai Group to elucidate the depositional age, provenance and nature of the group. The youngest detrital zircon ages (433–403 Ma) suggest that the Donchai Group accumulated between 433 and 368 Ma, revealing Silurian–Devonian arc magmatic rocks on the western margin of the Sukhothai Terrane. Sediments of the Donchai Group were sourced from both the continental basement and a Silurian–Early Devonian magmatic arc, suggesting a depositional setting on the continental slope of a back-arc basin along the western flank of the Sukhothai Terrane. The Silurian–Devonian arc belt in SW China probably extends to the Chiang Rai region to the west of the Sukhothai Terrane, northern Thailand, revealing the northwards subduction of the Proto-Tethys Ocean along the western Simao and Sukhothai margin during the mid-Paleozoic. The inferred arc and back-arc configuration of the Proto-Tethys Ocean in northern Thailand is comparable with that recently established in Yunnan, SW China. Supplementary material: Detrital zircon U–Pb ages and trace element analyses, zircon in-situ Lu–Hf isotopic compositions and whole-rock geochemical data are available at https://doi.org/10.6084/m9.figshare.c.6858373

An overview of the evolution of the internal hellenides (Albania, Republic of North Macedonia, Greek Central Macedonia): obductions, collisions and North Aegean extension

The geological analysis of the Albanian-Macedonian transect constraints a framework of the Internal Hellenides in which 3 or 4 Jurassic oceanic basins opened at 174–160 Ma; they bordered the Western European Continent. During the Late Jurassic, the oceanic crusts subducted eastward, the Mirdita basin below the Pelagonian continent, the Almopias basin below the Malarupa-Veles continent and the Thessaloniki ophiolitic basin below the Continental Margin of the basin. A Paikon Volcanic tholeiitic Island Arc formed above the Almopias subduction and subsequently subducted below the rhyolitic volcanic centre of the Arc; the Guevgueli ophiolitic back-arc basin opened above the subduction of the Paikon Volcanic Arc (164–160 Ma). The top-to-the-W Late Jurassic obduction of the oceanic crusts (D1 event) is followed by a gravitational exhumation (D2 event). A new Mid-Late Cretaceous Almopias marine basin formed; the Late Cretaceous-Palaeocene continental subduction (=collisions, D3-4 events) re-activated the Jurassic subductions and a dacitic Paikon Volcanic Arc formed in the Palaeocene-Early Eocene (56–45 Ma). The Vardar Trough opened at 45 Ma in the back of the dacitic Paikon Volcanic Arc, above the subduction of Vardar and Apulia units; it was submitted in the Late Priabonian to a tectonic event (D5 event, 36–34 Ma) and to the Early Miocene compression (D6 event). The N. Aegean WNW-ESE extension began at ∼21–20 Ma; change from the WNW to the NE-SW/N-S extensional directions occurred between the Tortonian (∼12 Ma) and the Pliocene (∼6 Ma); it was probably driven by hundreds kms of NE-SW/N-S stretching of the Central and Southern Aegean basins.

Correction to “Bringing the Submarine Mariana Arc and Backarc Basin to Life for Undergraduates and the Public”

Correction to “Bringing the Submarine Mariana Arc and Backarc Basin to Life for Undergraduates and the Public”

Implications of high-grade metamorphism on detrital zircon data sets: A case study from the Fraser Zone, Western Australia

Detrital zircon grains provide a useful tool to determine the maximum age of sedimentary strata and can elucidate tectonic setting and basin evolution. However, high-grade metamorphism can overprint the mineral chemistry of the detrital cargo and reset both primary age and compositional information. Incomplete resetting of the U–Pb isotopic system, fluid alteration, and complex internal structures due to granulite-facies metamorphism makes dating and interpretation of the obtained zircon ages challenging. Here, we use trace element chemistry, U–Pb geochronology (LA-ICP-MS and SHRIMP), textural observations, and inclusion characterization from detrital zircon crystals in the Snowys Dam Formation (Albany–Fraser Orogen; AFO) to deconvolve its strongly overprinted sedimentary history, and to evaluate the likely geodynamic setting for basin genesis. Detrital zircons that survived metamorphic overprinting show that the main detrital load was sourced from Paleoproterozoic lithologies from neighbouring regions, with rare Archean Yilgarn crystals. The youngest detrital grains correlate with the age of plagiogranites in the Madura Province to the east of the AFO and imply a maximum depositional age of c. 1390 Ma, indicating that the Fraser Zone temporally correlates to an oceanic arc and thus best fits a back-arc basin setting. High-grade metamorphism during Stage I of the orogeny (c. 1330–1260 Ma) promoted recrystallization of pre-existing detrital zircons together with neoblastic growth of metamorphic zircon rims. At c. 1245 Ma, during early Stage II of the AFO, anatexis of the metasedimentary rocks produced rare neocrystallized magmatic zircon. Late-stage fluids are indicated by elevated LREE for these grains, which also contain a hydrous inclusion assemblage. High volume sampling techniques such as LA-ICP-MS often drilled through multiple age domains that can hinder geological interpretation. Thus, a lower volume sampling technique (e.g., SHRIMP) and verification with mineral chemistry is highly instructive in accurately resolving primary age groups in high-grade terrains.

Recycled crustal components in the depleted mantle source: Evidence from Early Cretaceous mafic rocks in the southern margin of the North China Craton

Abundant subducted materials are recycled into the deep mantle during the plate subduction process and are further reflected in the postcollision mafic rocks. The Early Cretaceous Mid-Oceanic-Ridge-Basalt (MORB)-like mafic rocks in the southern margin of the North China Craton (SNCC) preserve significant information on recycled materials. These rocks provide a window for discussing the crust-mantle interaction and tracing the history of subduction. The studied rocks comprise the Jingwan metadiabase, Putaozhuang, Nianpanzhuang, and Niujiaowan metahornblende gabbro, with Early Cretaceous ages (140–129 Ma). They show large ion lithophile element enrichment in both the Enriched-MORB and Normal-MORB rare earth element patterns. They are characterized by slightly depleted isotopic compositions, high MgO, Cr, Ni and Mg# values as well as the low SiO2, suggesting a depleted mantle source affinity. The enriched NdHf (εNd(t) = −14.50 to −13.47; εHf(t) = −9.27 to −8.82) and unradiogenic Pb isotopes imply that Jingwan samples originated from a depleted mantle source metasomatized by lower continental crustal materials. Their relatively enriched zircon εHf(t) values (−15.8 to −1.6) and δ18O values (4.82‰–8.77‰) along with their Paleoproterozoic to Neoproterozoic inherited zircons also argue for the contribution of low continental crustal materials from the North China Craton. In contrast, Putaozhuang, Nianpanzhuang, and Niujiaowan samples have positive whole-rock εNd(t) values (4.08–5.20) and εHf(t) values (3.23–5.62). Combined with the existence of Paleozoic zircons, high zircon εHf(t) values, and high Nb/U ratios, these materials suggest that they originated from a depleted mantle source metasomatized by juvenile crust and terrigenous sediments. This is corroborated by variable zircon δ18O values (5.57‰–9.63‰) and mantle-like δ18O values (5.08‰–5.60‰) of Paleozoic inherited zircons from Niujiaowan samples. The distinct geochemical characteristics of Early Cretaceous mafic rocks are induced by the incorporation of different components from subducted crust and sediments into the depleted mantle source. Paleo-Pacific Plate far-field effects controlled the intense lithospheric extension and thinning of the Early Cretaceous MORB-like rocks in the SNCC, which were placed in an immature back-arc basin setting.

Subsurface delineation of Sukadana Basalt Province based on gravity method, Lampung, Indonesia

Research subject. The geological profile of Sukadana Basalt Province as a large basalt outcrop in the back arc of Sumatra remains to be unclear. This also concerns the geological structures and their relationship with the Sundaland regional geology. Aim. To reveal the type and pattern of geological structures that controlled the Sukadana Basalt Province (SBP) to the surface, the distribution of Sukadana Basalt on subsurface and its relationship with Sundaland regional tectonics. Materials and Methods. A reprocessed bouguer anomaly map of Tanjungkarang quadrangle 1991 was used. Results. We found that the main eruption was located in the center of SBP. The forward modeling data show the thickness of SBP to reach 3,200 m. There are two Northwest-Southeast striking normal faults and one fissure controlling the development of SBP. These fractures served as the primary conduit for magma to ascend from the mantle to the Earth’s surface. We also found North-South striking normal faults and West-East dextral strike-slip fault that formed at Early Oligocene and indirectly supported the magma ascend to the surface. Conclusions. The North-South striking normal faults were correlated with the Sundaland oroclinal bending. These faults developed through the extrados zone, a large pull-apart area that caused the continental lithosphere to become thinner. Meanwhile, Quaternary-Northwest-Southeast striking fractures are correlated with the development of the Great Sumatra Fault. The formation of Northwest-Southeast striking fractures was affected by the Great Sumatra Fault movement, and the thinning of the back-arc crust affected by multi-extensional structures was implicated in the ascend of SBP’s magma to the surface.

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.

Long-lived shallow crustal magma reservoirs beneath Ulleung Island volcano (South Korea) and their rejuvenation recorded in zircon

The microanalytical data obtained from zircons collected on Ulleung Island, an emergent peak of a Quaternary alkaline magma system in the southwestern East Sea (Sea of Japan) back-arc basin, provide new insights into the origin of magma, magmatic processes, and the trigger of explosive eruptions. The U-Th ages of zircons extracted from a pumice layer formed during the intense Plinian eruption at ∼11 ka, known as the Ulleung-Oki tephra, show a wide range spanning the pre-eruption equilibrium state (>350 ka) up to the eruption event itself. Zircon grains derived from syenite and gabbro fragments entrained within the tephra yield weighted mean U-Th and U-Pb ages of 34.7 ± 2.3 ka and 1378 ± 22 ka, respectively. Under cathodoluminescence, zircons from the syenite reveal dark, featureless, or oscillatory-zoned cores, containing numerous inclusions of britholite-like Y-rich phases and pyrochlore. These cores are partially or entirely replaced by inclusion-free, brighter domains, suggesting that in situ dissolution and precipitation processes significantly influenced the transformation of zircon. The consistently low δ18O values (5.4–2.6‰) of zircons from Middle Pleistocene trachyte lavas, the Ulleung-Oki tephra, and lithic fragments indicate the persistent presence of shallow crustal magma reservoirs beneath Ulleung Island. These reservoirs have undergone substantial assimilation of hydrothermally altered rocks, with assimilation rates typically around 10–30% and reaching a maximum of approximately 50%. Zircon core-to-rim variations in oxygen isotopic composition, along with textural observations, suggest the rejuvenation of magma reservoirs through the injection of new melt, which likely served as a trigger for the explosive eruptions at Ulleung Island. The similarity in the range of εHf values between the core and rim (+7.4 to −4.3) implies the Hf isotopic consistency of the recharge melt.

Genesis and tectonic setting of an Early Palaeozoic skarn in the East Kunlun Orogenic Belt, China: constraints from adakitic rocks and garnet megacrysts

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) in the northern Tibet Plateau has undergone a long multi-stage history of drifting, accretion, and collision of the Proto- and Palaeo-Tethyan oceans. It has been addressed a trench-arc-back-arc basin system of the Proto-Tethys in the E-KOB, and the tectonic history of this major ocean has been well constrained by previous studies. However, the evolution of the back-arc basin still remains unclear. In this study, the Yugusayi monzodiorite, quartz-bearing syenite, and gradually contacted skarn rocks characterized by garnet megacrysts, in the North Qimantagh Belt were investigated. To reveal the tectonic setting and skarnization mechanism, geochemical and geochronological studies were conducted on the intrusions and garnet skarn, and the elemental compositions of the garnet megacrysts were also analysed. These monzodioritic rocks have high MgO (6.87–8.31 wt.%), Sr and La contents, low Y and Yb contents, and relatively high La/Yb (31–55) and Sr/Y (41–51) ratios, resembling to high-Mg adakites. They display right-sloping chondrite-normalized REE patterns, enriched in LILEs, and depleted in Ti with significant Nb-Ta troughs, which are similar to those of continental arc rocks. Zircons from monzodiorite and skarn samples have contemporaneous U-Pb ages of 459 ± 3 Ma and 454 ± 4 Ma with εHf(t) values of − 8.53 to − 4.64 and − 7.7 to + 12.8, respectively. In addition, the chemical zoning garnet megacrysts are composed mainly of grossular and andraditic grossular, and are characterized by spoon-shaped REE patterns. Based on our new data and the regional geology, it can be concluded that the Yugusayi intrusive rocks formed in a subduction-related tectonic setting and resulted in the skarnization. These data not only provide key constraints on the initial subduction of the Qimantagh back-arc basin’s oceanic crust, but also offer a unique opportunity to explore the Caledonian skarnization in the Qimantagh Belt.

Thermodynamic modelling of continental arc-adjacent magmatism: the Loicas Trough, N. Patagonia, Argentina

Continental arcs are associated with volcanism concentrated into two main belts—the main arc and back arc, often separated by fold and thrust belts. The Loicas Trough, Argentina, is a post-orogenic extensional feature that obliquely cuts the fold and thrust belts. The trough hosts large Pliocene–Holocene volcanic centres, including Domuyo and Tromen, that lie between the main arc and back arc and thus provide a rare window into this setting. We present major and trace element data for the Loicas Trough, which we combine with geochemical modelling using the Magma Chamber Simulator (MCS) to explore the origin and evolution of the volcanism. The lavas display a wide continuous range from alkaline basalts to subalkaline rhyolites. Trace elements reveal variable extents of arc enrichment (2 < Nb/U < 28), which correlate with proximity to the trench and differentiation indices. Our results and MCS models indicate that the Loicas Trough parental magmas formed from compositionally zoned mantle. Best-fit models indicate that the differentiation occurs at middle and upper crustal levels, in sharp contrast to lower crustal hot zones beneath main arcs. Assimilation of partial crustal melts drives compositional evolution and obscures source signatures. Pure or high fraction end-member partial crustal melts are also identified at Domuyo based on their low Ba (~ 250 ppm) and moderate Sc contents (~ 8 ppm). We find evidence of similar lavas in transtensional settings adjacent to continental arcs worldwide, which do not adhere to the main versus back arc volcanism binary. We suggest the term arc-adjacent magmatism, where compositions are mainly controlled by extensive assimilation and reworking in the middle to upper crust.

Nature and Origin of a Massive Sulfide Occurrence in the Karrat Group: Evidence for Paleoproterozoic VMS Mineralization in Central West Greenland

Abstract Mafic volcanic rocks of the Kangilleq Formation of the Paleoproterozoic Karrat Group host volcanogenic massive sulfide (VMS) mineralization in the area of central Kangiusap Kuua, central West Greenland. The mafic volcanic rocks display evidence of subaqueous, effusive eruption and redeposition by mass debris flows generated along fault scarps on the sea floor. A zone of semiconformable quartz alteration and disconformable chlorite alteration within hydrothermal breccias and mafic tuff breccias near the top of the volcanic sequence is interpreted to reflect a synvolcanic hydrothermal system. Conformable, massive to semimassive, and discordant, stringer-style sulfide mineralization is hosted within the quartz- and chlorite-altered volcanic rocks. The massive to semimassive sulfide mineralization is ~10 m thick and crops out along strike for ~2,000 m. The stringer zone is ≤10 m thick with individual sulfide stringers ranging in width from 5 to 90 cm. All sulfide zones are dominated by coarse pyrrhotite and pyrite, with trace amounts of sphalerite and chalcopyrite. The pillow lavas are subalkaline with geochemical characteristics typical of modern transitional to tholeiitic mid-ocean ridge or back-arc basin basalt. Trace element and Nd isotope data suggest that these lavas erupted in an epicratonic, back-arc basin. Characteristics of the host rocks indicate a period of localized rifting, volcanism, and VMS formation during genesis of the Karrat Group, which is dominated by siliciclastic rocks.

The accumulation characteristics and exploration potential of oil and gas in the back-arc basin of Japan under the background of high heat flow

The Sea of Japan is located in the southeast margin of Eurasia, in the triangle area of the western Pacific Ocean. Due to the interaction of the Pacific plate, Eurasian plate and Philippine plate, its tectonic environment is complex, forming a typical trench-arc-basin system. At present, 148 oil and gas fields have been discovered in Japan, with an oil and gas resource of 255.78×106 t, showing a good prospect for oil and gas exploration. Based on the previous research and the recently collected geological and geophysical data, the characteristics of tectonic-sedimentary evolution and geothermal field in the basins around the Sea of Japan are analyzed. The results show that the tectonic evolution of the basin is mainly controlled by plate subduction and back-arc oceanic crust expansion, and it mainly undergone four tectonic-sedimentary evolution stages: Subduction period, basin development period, subsidence period and compression deformation period. The overall heat flow value of Japan Sea is high, and it is distributed annularly along Yamato Ridge. The geothermal heat flow value is about 50–130 MW/m2, and the average heat flow is 75.9±19.8 MW/m2, which has a typical “hot basin”. The high heat flow background provides unique thermal evolution conditions for hydrocarbon generation, which leads to the high temperature and rapid evolution. The authors summarized as “early hydrocarbon generation, rapid maturity and shallow and narrow hydrocarbon generation window”. The type of oil and gas is mainly natural gas, and it mainly distributed in Neogene oil and gas reservoirs. The trap types are mainly structural traps, lithologic traps and composite traps. In addition, the pre-Neogene bedrock oil and gas reservoirs also show a good exploration prospect. The resource prospecting indicates that Niigata Basin, Ulleung Basin and kitakami Basin are the main target areas for future exploration and development.©2023 China Geology Editorial Office.

Reproductive biology and population structure of three hydrothermal gastropods (Lepetodrilus schrolli, L. fijiensis and Shinkailepas tollmanni) from the South West Pacific back-arc basins

Reproductive biology and population structure of three hydrothermal gastropods (Lepetodrilus schrolli, L. fijiensis and Shinkailepas tollmanni) from the South West Pacific back-arc basins