Island Arc Environment Research Articles

The Upper Permian volcanic-sedimentary succession in northern Qamdo Block, central Qinghai-Tibet Plateau and its sedimentary, paleogeographic and tectonic significance

A combined sedimentological and geochemical study has been carried out on a volcanic-sedimentary succession from a borehole newly drilled in the northern Qamdo Block (QMB, in central Qinghai-Tibet Plateau) in order to improve the knowledge of the sedimentary and tectonic environment. The lithologic data allow to separate four main lithofacies, including massive volcanic lithofacies (LF1), tuffaceous lithofacies (LF2), sandstone lithofacies (LF3) and mudstone lithofacies (LF4). LF1 likely reflects the products of felsic lava flows that were formed during the process of magma ascent onto the seafloor at an earlier stage. LF2 contains two sublifhofacies that respectively represents the proximal and distal accumulation of tuffaceous turbidity flows. LF3 could be the arc-sourced submarine fan deposits, and the sublithofacies LF3a and LF3b represent mid-fan distributaries and interchannel deposits in fan lobes. LF4 is mainly composed of suspension deposits, which represents the sedimentation of post-volcanic activities. Furthermore, the lithofacy associations reveal the evolution of a volcano-sourced submarine fan in the back-arc basin. Geochemically, LF1 and LF2 are characterized by LREE-enriched patterns with negative Eu anomalies, enrichment in large ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSEs), which correspond with the previously published island arc environment. However, LF2 shows more distinctive patterns, which represents a later evolution stage than that of LF1. This volcanic-sedimentary succession reveals that the sedimentary-tectonic setting could be consistent with the subduction of Jinshajiang oceanic slab in the late Permian period.

Petrogenesis of Jurassic Xietongmen intrusive rocks at the southern margin of the Lhasa terrane: implications for intra-oceanic arc evolution

The Gangdese batholith, Tibet, records the opening and closing of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. The Mesozoic magmatic rocks play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. This study focuses on Jurassic intrusive rocks in the Xietongmen area of the southern margin of the Lhasa terrane adjacent to the Yarlung–Tsangpo suture. Zircon U–Pb dating yielded Middle Jurassic dates for ca 170 Ma hornblende gabbro and ca 173 Ma granodiorite intrusions. All of the samples are medium- to high-K calc-alkaline, and the majority are metaluminous and enriched in the large ion lithophile elements and depleted in the high-field-strength elements. This indicates the magma was generated in a subduction-related tectonic setting. The intrusive rocks have high and positive εHf(t) values (hornblende gabbro: 13.3–18.7; granodiorite: 14.2–17.6) that yield Hf model ages younger than 312 Ma. These new data, combined with the results of previous research, suggest that the Jurassic igneous rocks were derived from a metasomatised region of an asthenospheric mantle wedge. Extremely depleted Sr–Nd–Pb–Hf isotope compositions are similar to the Yarlung ophiolite and igneous rocks within other intra-oceanic island arcs. Together with the existence of sandstone that is identified as the product of the oceanic island arc environment, we suggest formation in an intra-oceanic island arc.The new zircon U–Pb dating has yielded Middle Jurassic ages for the ca 170 Ma hornblende gabbro and ca 173 Ma granodiorite phases of the Xietongmen intrusion.Jurassic igneous rocks formed from a metasomatised asthenospheric mantle wedge by northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane.Late Triassic–Jurassic igneous rocks, which are characterised by highly depleted isotopic compositions within the Southern Lhasa sub-terrane, record residual intra-oceanic island arcs in the eastern Tethyan belt.

Origin of pre-mesozoic xenocrystic zircons in cretaceous sub-volcanic rocks of the northern Andes (Colombia): paleogeographic implications for the region

This paper reports the presence of xenocrystic zircons in sub-volcanic rocks (Guarco Andesite and Porphyritic Intrusives units) exposed in the northeast of the Western Cordillera of Colombia (WCC) and provides a new contribution to the understanding of the tectono-magmatic evolution of the area during the Upper Cretaceous associated with the geological history of the Caribbean Large Igneous Province (CLIP). The study area is located in an accretionary terrane constituted by a basement of oceanic affinity (Barroso Formation and San José de Urama Diabases unit), amalgamated against the northwestern South American margin and associated with the development of CLIP. The elemental and isotopic geochemical data (with an initial ratio of 87Sr/86Sr between 0.70334 and 0.70543 and εΝd between +6.25 and + 7.42) indicate an island arc environment for the formation of sub-volcanic rocks and a mantle wedge source area (and/or the arc itself), which presents subordinate levels of crustal material. U-Pb zircon ages by SHRIMP IIe and LA-ICP-MS show ages from the Mesoarchean to the Upper Cretaceous, with representative peaks of approximately 2700, 2100, 600 and 90 Ma. The younger age population is interpreted as the crystallization age of the studied rocks and indicates a spatial and temporal relationship with the basement rocks associated with the CLIP. The older zircons are interpreted as xenocrysts coming from terrigenous sediments that were dragged down into the subduction channel (reflecting crustal recycling processes through the mantle in subduction zones). Alternatively, the xenocrystic zircons could have come from the assimilation of crustal sediments that were previously deposited over the primitive arc. We suggest that the source areas of zircon xenocrysts probably are located close to the Arequipa Massif and the Marañon Complex inliers, and close to the ensialic Basement of the Central Cordillera of Colombia (CCCB). Using these source areas and paleogeographic/paleomagnetic models published by other authors, we propose two possible paleogeographic scenarios differentiated by the polarity of the subduction zone.

Electrically Anisotropic Crust From Three‐Dimensional Magnetotelluric Modeling in the Western Junggar, NW China

AbstractAn island arc environment related to intraoceanic subduction or ridge subduction from the Late Carboniferous to the Early Permian has been proposed for the formation of Western Junggar, NW China, situated in the southwest of Central Asian Orogenic Belt. However, the details and consequences of the subduction remain controversial. To further identify the relics of the Late Carboniferous subduction at depth, three magnetotelluric profiles were deployed. Phase tensors, real induction arrows, and three‐dimensional (3‐D) isotropic resistivity models indicate the presence of a 3‐D anisotropic resistivity structure in the crust. Consequently, a 3‐D anisotropic resistivity model was constructed by forward modeling. The 3‐D anisotropic resistivity model contains two azimuthally anisotropic middle‐upper crust anomalies with minimum resistivity striking N90°E at depths of 5 to 20 km, and an azimuthally anisotropic lower crust anomaly with minimum resistivity striking N20°E at depths of 20 to 46 km. The electrically anisotropic anomalies are closely related to relatively high shear‐wave velocities with significant negative radial anisotropy. The anisotropies in the middle‐to‐upper crust and the lower crust are related to the oceanic ridge (N90°E) subduction with a slab window in the Late Carboniferous and the remnant oceanic slab, respectively. The magnetotelluric observations support a NW70° subduction with an intraoceanic ridge‐transform system in the Late Carboniferous and a remnant oceanic slab trapped in Western Junggar.

Early Palaeozoic oceanic island–seamount assemblage in northern Fujian, South China: Implications for pre‐Devonian tectonic evolution of the Wuyi orogenic belt

The nature of the Precambrian basement in the Cathaysia Block of South China is controversial, which inhibits our understanding of the pre‐Devonian tectonic evolution of the region. In this study, a suite of amphibolites and marbles in northern Fujian area located in the north of the Cathaysia Block were subjected to LA–ICP–MS U–Pb zircon geochronological and geochemical analyses. Results show that the protolith of the amphibolite formed after ca. 459 Ma, and its metamorphism happened at ca. 429 Ma. The protolith of the marble was deposited after ca. 518 Ma, and its metamorphic age is ca. 437 Ma. The amphibolites are divided into two groups based on geochemical data. Group 1 has an OIB‐like feature that might have formed in an environment characterized by oceanic islands and seamounts. Magmas associated with Group 1 were derived from an enriched mantle source with OIB characteristics. The protolith of Group 2 shows IAB feature that formed in an island arc environment. Marbles are interpreted to represent metamorphosed marine carbonate rocks. Integration of geochronological and geochemical study indicates that the marble and amphibolite should be an early Palaeozoic oceanic island–seamount association. It suggests the Cathaysia Block underwent oceanic subduction during collisional orogenesis in the early Palaeozoic. This subduction is thought to have initiated prior to ca. 437 Ma. The U–Pb zircon ages of younger intrusive granites indicate that final amalgamation occurred before ca. 392 Ma. During this process, east and west Wuyi were amalgamated to form the unified Cathaysia Block. Thus, it suggests that there was no unified pre‐Devonian basement in the Cathaysia Block. The pre‐Devonian tectonic setting of the Cathaysia Block was an accretionary orogeny.

Mid-ocean ridge vs. forearc and subduction settings: Clues from rodingitization of tectonic fragments in the Neoproterozoic ophiolites of the Eastern Desert, Egypt

Tectonic mélange fragments (metabasalt and metadiabase) in the ophiolitic serpentinites of the Egyptian Eastern Desert show peculiar mineral assemblages and they provide useful insights into metamorphism and metasomatism. New field and mineralogical data indicate the formation of rodingite at the expense of mid-ocean ridge basalt (MORB) at three localities, namely Wadi Sikait, Wadi Abu Rusheid and Hafafit (SRH belt). These rocks occupy a lower structural horizon in the tectonic mélange zone(s), whereas ocean-island and arc basalt and diabase (OIB) is relatively younger with no evidence of rodingite formation. Rodingitization starts at slow-ultraslow spreading centers and continued until subduction and exhumation at an accretionary wedge. It is a process that is characterized by Ca-rich fluids and the development of a complicated “blackwall” due to superimposed K+ and Mg2+-metasomatism. Oxygen fugacity is fluctuating where it is high for Hafafit and low for Wadi Abu Rusheid rodingites. The Ta/Yb-Th/Yb ratios suggest that rodingitization is not contemporaneous with serpentinization in an island-arc environment but with serpentinization near seamounts at the spreading centers. Combined field observations, whole-rock geochemistry and mineral chemistry data prove that rodingites are formed at the expense of a MORB (mid-ocean basalt) protolith and occupies a lower structural position than unrodingitized IAT (island-arc tholeiite) metadiabases.

Early Palaeozoic high-Mg basalt-andesite suite in the Duobaoshan Porphyry Cu deposit, NE China: Constraints on petrogenesis, mineralization, and tectonic setting

High-Mg basalt-andesite suites are extremely rare in the modern Earth but genetically important for indicating essential crust-mantle interactions, ascertaining critical geodynamic settings, and understanding the formation of porphyry copper deposits. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating and geochemistry document an early Palaeozoic high-Mg basalt-andesite suite from the Duobaoshan porphyry Cu deposit, eastern Central Asian Orogenic Belt (CAOB). Dating results reveal that the Duobaoshan high-Mg basalt and andesite erupted ca. 506 Ma and ca. 485 Ma, respectively. These high-Mg samples are typical subduction-related volcanic rocks with SiO2 contents of 47.73 to 55.16 wt%, high MgO (6.08 to 10.82 wt%), and high Mg# (58 to 67) and feature enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The samples have juvenile whole-rock initial 87Sr/86Sr ratios of 0.70272 to 0.70451, ɛNd(t) of 4.8 to 8.7 (average 7.23), and zircon ɛHf(t) of 7.3 to 15.9. Additionally, they have high Sr (average 619.36 ppm) and low Y (average 11.92 ppm) and Yb (average 1.21 ppm) contents that show affinity with adakitic high-Mg basalts and andesites worldwide. These high-Mg, depleted mantle-like isotopic and adakitic geochemical features imply a depleted mantle source variably assimilated by slab-derived melts under a sustained subduction tectonic setting. We also propose that the Duobaoshan high-Mg basalt-andesite suite, as the parental source magma, fertilized the overlying Duobaoshan porphyry Cu deposit by providing water, copper, and sulphur and high oxygen fugacity. A comprehensive comparison of the post-ore volcanic rocks shows that they might have originated from the slab-derived fluid metasomatized depleted mantle wedge, which had different properties from the mantle that produced the pre- and syn-ore volcanic intrusive rocks. The post-ore volcanic rocks underwent little crustal evolution en route to the surface during a reworked subduction event, which indicates a relatively immature island arc environment.

Ages and origins of granitoids from the Kalatag Cu cluster in Eastern Tianshan, NW China: Constraints on Ordovician–Devonian arc evolution and porphyry Cu fertility in the Southern Central Asian orogenic belt

The pre-Carboniferous evolution of the Eastern Tianshan orogen, Xinjiang, NW China, an important Paleozoic tectonic component and Cu metallogenic belt in the southern Central Asian Orogenic Belt (CAOB), is controversial. New zircon UPb ages, whole-rock geochemistry, and zircon LuHf isotope data for three major granitic complexes (i.e., Kalatag, Dongerqu, Xierqu) from the Kalatag Cu cluster of Eastern Tianshan were combined with previous data to evaluate Ordovician–Devonian arc evolution and porphyry Cu fertility in the southern CAOB. The zircon UPb results reveal that the granitic intrusions and porphyry Cu mineralization in the Kalatag area formed during 453–427 Ma and 387–379 Ma, suggesting that along with Carboniferous intrusions, Ordovician–Silurian and Devonian intrusions were also very important targets for porphyry Cu mineralization in Eastern Tianshan. All intrusions are calc-alkaline I-type rocks with low K2O/Na2O ratios (mostly <1), moderate Mg# values (33.3–58.8), listric-shaped chondrite-normalized rare earth element (REE) patterns, typical arc magmatic trace element features (i.e., Ba, K and U enrichments relative to Nb, Ta and Th), and moderate Nb/Ta ratios (12.5–21.6). They have similar depleted source features (i.e., εHf(t) = 11.8–15.3) and likely formed by deep crustal melting, assimilation, storage, and homogenization (MASH) processes followed by minor variable degrees of fractional crystallization in an island arc environment (underlain by minor old relict crust) related to the northward subduction of the Kanggur Ocean. The mineralized porphyries likely derived from a hydrous pyroxene+amphibole±garnet magma source, as shown by adakitic features (e.g., Sr/Y > 40, Yb < 1.9 ppm), and were strongly metasomatized by slab fluids (with high Ba/Th = 200–800 and Ba/La = 20–100); these elements and ratios are thus important chemical indicators of the porphyry Cu fertility magma in Kalatag. The dry and barren arc intrusions are characterized by low Sr/Y ratios and show increased alkalinity and Sm/Yb and Sr/Y ratios with time, revealing crustal evolution from Ordovician–Silurian ca. 30–35-km-thick crust (plagioclase+pyroxene-dominant; Sm/Yb = 1–2) to Devonian thicker (<40 km; pyroxene+plagioclase±amphibole-dominant; Sm/Yb = 2–4) and mature crust.

Reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust of Zhongguai high area in the western Junggar Basin, China

Based on the drilling, cores, logs, seismic, laboratory analysis and so on, reservoir characteristics and hydrocarbon accumulation of Carboniferous volcanic weathered crust in Zhongguai high area are studied. Volcanic rocks were formed in an island arc environment. The lithology is mainly andesite and tuff; Reservoir spaces are mainly secondary pore, fracture and their combination forms, fractures have a better effect on reservoir seepage; There are four layer structures of volcanic weathered crust, weathered clay layer, strongly weathered zone, weakly weathered zone and unweathered zone and strongly weathered zone is the best, which is the main reservoir development zone; The development of reservoir is mainly affected by weathering-leaching, lithology and lithofacies, and fault (fracture); Effective reservoirs could reach to 480 m thickness (high quality reservoirs are within 240 m). Carboniferous volcanic reservoirs are distributed along three zones, which are near the fault zone, high structural part, favorable lithofacies development zone, and one plane, which is near the unconformity.

Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) – record of Middle to Late Jurassic arc-forearc system in the Tethyan subduction factory

In the Late Jurassic to Early Cretaceous ophiolite melange from the Mt. Medvednica (Vardar Ocean) blocks of boninite rocks have been documented. They emerge as massive lavas made of augite, spinel, albite and secondary hydrous silicates (e.g., chlorite, epidote, prehnite, and pumpellyite). An established crystallization sequence (spinel→clinopyroxene→plagioclase±Fe-Ti oxides) was found to be typical for the boninite series from the suprasubduction zones (SSZ). Augite crystallization temperatures and low pressures of ~1048 to 1260 °C and ~0.24 to 0.77 GPa, respectively, delineated the SSZ mantle wedge as a plausible source of boninite parental lavas. Their whole-rock geochemistry is characterised by low Ti, P2O5, Zr, Y, high-silica, and high Mg# and Cr# values. Low and U-shaped REE profiles are consistent with the negative Nb-Ta, P and Ti anomalies indicative for SSZ. Thorium and LILE enrichment, and very low initial Nd-isotopic values (eNd(T = 150 Ma) + 0.49 to +1.27) act as vestiges of mantle-wedge metasomatism. The mantle source was likely depleted by the MORB and IAT melt extraction and was contemporaneously affected by subduction fluids, prior to the large-scale adiabatic melting of the mantle hanging wall. This eventually gave rise to boninite lavas and an ultra-refractory harzburgite residiuum. The genesis of boninites is related to the Tithonian mature forearc setting that evolved from an intra-oceanic, Callovian to Oxfordian, island-arc environment. The Mt. Medvednica boninite rocks stand for the youngest SSZ-related Jurassic oceanic crust from the NW segment of the Dinaric-Vardar Tethys that are nowadays obducted onto the passive margins of Adria. Taking into account the existence of similar rocks in the ophiolite zones of Serbia, Albania and Greece, the boninites of Mt. Medvednica strongly favours the single Tethyan oceanic basin that existed in this part of Europe during the Late Jurassic.

Processes of primary and re-equilibration mineralization affecting chromitite ore geochemistry within the Vourinos ultramafic sequence, Vourinos ophiolite (West Macedonia, Greece)

The Vourinos ophiolite is a complete ophiolite sequence covering an area of ∼450 km2, that hosts the largest chromite mining district in Europe. Chrome ore deposits range in size from several millions of tonnes of Xerolivado-Skoumtsa mine to some thousand tonnes of the smallest exploited ores. Widespread mining activity closed in the 1990s, leaving behind good exposures of chromitite bodies within the closed mines. Samples from 10 of these mines, 4 located in the Northern part of the massif and 6 located in its Southern part, were collected in order to study the spatial variability of chromite and olivine mineral chemistry in these ores at different scales, and to infer the roles of primary magmatic and secondary post-magmatic re-equilibration processes. Chromites from chromitites show higher XCr (0.75–0.87) than scattered spinels in peridotites and serpentinites (0.45–0.70) and largely variable XMg (0.40–0.78). Chromite compositions of North and South Vourinos ores largely overlap, while they show differences at the scale of single deposits, with a remarkable higher XMg at Xerolivado-Skoumtsa. High variability internal to each deposit is due to different re-equilibration conditions at variable olivine/spinel volume ratios. Olivine and chromite XMg values are positively correlated to chromite modal content, while XCr of chromite is not significantly affected by re-equilibration. Calculated re-equilibration temperatures are higher for the larger Xerolivado-Skoumtsa deposit (777–837 °C) than for the other smaller ones (715–746 °C). The inferred magmatic temperature is 1243 °C for Xerolivado-Skoumtsa and 982 °C for the other chromite deposits. Calculated melt compositions are very consistent for the 10 studied mines that all show a boninitic affinity. The whole set of data shows that Vourinos chromitites were formed in an island arc environment from a heterogeneous mantle source that underwent different degrees of melting. Subsolidus re-equilibration occurred during cooling at temperatures as low as 700 °C, partially obliterating primary magmatic features.

The Marbat metamorphic core-complex (Southern Arabian Peninsula): Reassessment of the evolution of a Neoproterozoic island-arc from petrological, geochemical and U-Pb zircon data

The Marbat basement (Sultanate of Oman) belongs to the Neoproterozoic accretion domain of the Arabian-Nubian shield. We present new geochronological, petrological and geochemical data as an extension of our previous study (Denèle et al., 2017) re-interpreting this basement as a metamorphic core complex (MCC). We showed that this MCC consists of a metamorphic unit (Juffa complex) separated by an extensional detachment from a plutonic unit (Sadh complex and Tonalite plutons). Geochemical data show that the Juffa metasediments correspond to volcanogenic graywackes, suggesting deposition in front of a juvenile magmatic arc. New in situ U–Pb zircon data show that the protolith of the Juffa paragneisses is Tonian in age (960–830 Ma). The Juffa complex recrystallized under amphibolite facies conditions (950 MPa, 630 °C) corresponding to thermal gradient of 17–18 °C/km, i.e. close to that observed in fore-arc environment subjected to tectonic underplating. It was then retrogressed (<400 MPa, <400 °C) during exhumation of the MCC.Phase assemblages of the Sadh complex and Tonalite plutons record magmatic temperatures estimated at 700–840 °C and 610–840 °C from hornblende and zircon-saturation thermometry, respectively; pressures are grossly estimated at ca. 250 MPa. These data and structural evidence led us to consider that the Sadh complex is a plutonic unit emplaced during exhumation of the MCC. Hence, we re-interpret the previously published zircon core U–Pb ages of the Banded gneisses (ca. 860–830 Ma) as inherited, and zircon rim ages (ca. 815 Ma) as the age of their intrusion. This suggests that the parent magma of all these granitoids could have interacted with volcanic material equivalent to the Juffa complex.Overall, the Marbat area developed during the Tonian and involved (i) deposition of volcanosediments (Juffa complex) in an island-arc environment (960–830 Ma); (ii) metamorphism under middle pressure conditions during tectonic underplating at ca. 820–815 Ma, followed by emplacement of plutonic rocks in the upper crust during initiation of the MCC at ca. 815–810 Ma; and (iii) late-tectonic intrusions associated with MCC amplification at ca. 800–790 Ma (Mahall intrusives and Tonalite plutons).

A newly discovered Early Paleozoic ophiolite in Dagele, Eastern Kunlun, China, and its geological significance

This study reports the discovery of an ophiolite located in the Eastern Kunlun Orogen (Dagele, Eastern Kunlun, China) that is mainly composed of isotropic and serpentinized dunite, gabbro, and eclogites. According to the geochemical results, the ophiolite unit is characterized by the depletion of Nb, Ta, and Ti and a slight enrichment in large‐ion lithophile elements, which suggests that the Dagele ophiolite was formed in an island arc environment. To constrain the time of formation of the ophiolite, gabbro samples were selected for sensitive high‐resolution ion microprobe U–Pb zircon dating and yielded an age of 445 ± 4.8 Ma. By integrating the regional geological and geochronological data, we propose that the Dagele ophiolite is a suprasubduction zone‐type. Further studies on this and other ophiolites in the region will provide crucial constraints on the evolution of the Eastern Kunlun Orogen as well as the Proto‐Tethys Ocean in central western China.

Late Neoproterozoic to Carboniferous genesis of A-type magmas in Avalonia of northern Nova Scotia: repeated partial melting of anhydrous lower crust in contrasting tectonic environments

Avalonian rocks in northern mainland Nova Scotia are characterized by voluminous 640–600 Ma calc-alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc. However, after the cessation of arc activity, repeated episodes of felsic magmatism between ca. 580 Ma and 350 Ma are dominated by A-type geochemical characteristics. Sm–Nd isotopic data, combined with zircon saturation temperature estimates, indicate that these magmas were formed by high temperature (800–1050 °C) melting of the same anhydrous crustal source. Regional tectonic considerations indicate that A-type felsic magmatism was produced (1) at 580 Ma in a San Andreas-type strike slip setting, (2) at 495 Ma as Avalonia rifted off Gondwana, (3) at 465 and 455 in an ensialic island arc environment and (4) at 360–350 Ma during post-collisional, intra-continental strike-slip activity as Avalonia was translated dextrally along the Laurentian margin. These results attest to the importance of crustal source, rather than tectonic setting, in the generation of these A-type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane. They also suggest that the shallow crustal rocks in northern mainland Nova Scotia were not significantly detached from their lower crustal source between ca. 620 Ma and 350 Ma, a time interval that includes the separation of Avalonia from Gondwana, its drift and accretion to Laurentia as well as post-accretionary strike-slip displacement.

Evidence for Ordovician subduction-related magmatism in the Truong Son terrane, SE Laos: Implications for Gondwana evolution and porphyry Cu exploration potential in SE Asia

The Truong Son Fold Belt (TSFB) is characterised by Late Carboniferous-Late Triassic metamorphic, volcanic and plutonic rocks, the product of accretion of the Indochina Terrane onto the South China Terrane and a range of composite subduction, collision and extensional events. This study discusses geochronological and geochemical data obtained from a dioritic intrusion and rhyolitic tuff mapped in the Donken area of SE Laos, and previously assigned to the Permian Antoum Granodiorite rock suite within the TSFB. Magmatic zircon U-Pb Q-ICP-MS dating undertaken in this study suggests ages of ca 470±2Ma for the diorite and ca 476±1.5Ma for a proximal rhyolitic tuff.Whole-rock geochemistry of both units suggests a subduction-related island arc environment, with calc-alkaline and tholeiitic affinities for the diorite and tuff respectively. The intrusion also exhibits an adakitic signature (high Sr, low Y and HREE contents) suggesting that Ordovician magmatism also occurred within the Indochina Terrane, associated with an enigmatic, early Gondwana subduction event. This intrusion appears part of a broader, bilateral Early Ordovician magmatism, newly linked to the south-east subduction of the Tamky-Phuoc Son Ocean underneath the Kontum terrane, and a north-west subduction beneath the Truong Son terrane. Significantly, sub-economic hydrothermal Cu mineralisation observed within the dioritic intrusion, hints at the presence of local Ordovician, porphyry-style base metal enrichment.

Geochemistry of late Palaeozoic granitoids of the Balkhash metallogenic belt, Kazakhstan: implications for crustal growth and tectonic evolution of the Central Asian Orogenic Belt

ABSTRACTThe Balkhash metallogenic belt (BMB) in Kazakhstan is a famous porphyry Cu–Mo metallogenic belt in the Central Asian Orogenic Belt (CAOB). The late Palaeozoic granitoids in the BMB are mainly high-K calc-alkaline and I-type granites, with shoshonite that formed during a late stage. Geochemical analyses and tectonic discrimination reveal a change in the tectonic environment from syn-collision and volcanic arcs during the Carboniferous to post-collision during the Permian. The late Palaeozoic granitoids from the Borly porphyry Cu deposit formed in a classical island-arc environment, and those from the Kounrad and Aktogai porphyry Cu deposits and the Sayak skarn Cu deposit are adakitic. The εNd(t) values for the late Palaeozoic granitoids are between −5.87 and +5.94, and the εSr(t) values range from −17.16 to +51.10. The continental crustal growth histories are different on either side of the Central Balkhash fault. On the eastern side, the εNd(t) values of the granitoids from the Aktogai and Sayak deposits are very high, which are characteristic of depleted mantle and suggest that crustal growth occurred during the late Palaeozoic. On the western side, the εNd(t) values of the granitoids from the Borly and Kounrad deposits are slightly low, which suggests the presence of a Neoproterozoic basement and the mixing of crust and mantle during magmatism. The granitoids have initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.335–20.993, 15.521–15.732, and 38.287–40.021, respectively, which demonstrate an affinity between the late Palaeozoic magmatism in the BMB and that in the Tianshan, Altai, and Junggar orogens.

High-alumina basalts from the Bogda Mountains suggest an arc setting for Chinese Northern Tianshan during the Late Carboniferous

Considerable debate persists as to the tectonic setting of the Tianshan Orogen during the Late Paleozoic, with active subduction system and intraplate large igneous provinces as two dominant schools. With aims of providing constraints on this issue, geochronological and geochemical analyses have been carried out on the Late Carboniferous high-Al basaltic lava (HAB) from the Bogda Mountains. These lavas, in conformable contact with the felsic rocks, belong to the Upper Carboniferous Liushugou Group. Zircon SHRIMP U–Pb dating of two felsic ignimbrites further suggest that they were mainly erupted during 315–319Ma. The Bogda basaltic lava is classified as HAB given their high Al contents >16% and their chemical resemblance to those from modern arcs such as Aleutian and Kamchatka. They are characterized by strong enrichment in large ion lithophile elements (LILE), strong negative Nb–Ta and Ti anomalies, and distinct positive Pb anomalies. Hence, they are significantly different from the mantle plume-related basalts, as exemplified by those from Siberian, Emeishan, and Tarim large igneous provinces. Instead, their MORB-like Nd–Hf–Pb isotopes and arc-like trace elements indicate that the Bogda HABs may have been generated from a mantle wedge metasomatized by sediment-derived melts. The sector and oscillatory zoning in clinopyroxene phenocrysts in the Bogda HABs is attributable to rapid dynamic crystallization during magma ascent. High Al content is due to delayed plagioclase nucleation likely by the high crystallization pressure rather than water content. Collectively, our data lend support to an island arc environment during the Late Paleozoic, probably related to southward subduction of the Paleo-Tianshan Ocean.

Early Paleozoic monzodiorite–granodiorite association in the northeastern flank of the South Mongolia–Khingan orogenic belt (Nora–Sukhotinsky Terrane): Age and tectonic setting

The paper presents the results of U–Pb geochronological and geochemical studies of the rocks of the monzodiorite–granodiorite association in the northeastern flank of the South Mongolia–Khingan orogenic belt, which composes a tectonic block among the provisionally Lower Paleozoic volcanosedimentary complexes of the Nora–Sukhotinsky Terrane. It is shown that the studied rocks have similar petrographic features (with the presence of transitional varieties) and form common trends in the petrographic diagrams. This suggests that they are members of a single magmatic association. The geochemical features of the monzodiorites, quartz monzodiorites, and granodiorites, in particular their enrichment in large ion lithophile elements (LILE) and depletion in some HFSE, indicate their similarity with island-arc magmatic rocks. The presence of monzonites and quartz monzonites in the studied monzodiorite–granodiorite association along with high K, Rb, Th, and Pb concentrations gives reasons to believe that it formed in active continental margin or ensialic island-arc environments. The granodiorites of the monzodiorite–granodiorite associations of the Nora–Sukhotinsky Terrane are dated at 440 ± 10 Ma and may be considered as a fragment of the early Silurian active continental margin or ensialic mature island arc in the structure of the South Mongolia–Khingan orogenic belt.

SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan: Implications for late Paleozoic granitic activity in Japanese Islands

AbstractThe new result of SHRIMP U–Pb zircon dating of the Kinshozan Quartz Diorite from the Kanto Mountains, Japan, provides 281.5 ± 1.8 Ma. The age is 30 m.y. older than the available age of the Kinshozan Quartz Diorite obtained by hornblende K–Ar method. The new U–Pb zircon age represents the time of crystallization of the Kinshozan Quartz Diorite. The hornblende K–Ar age indicates the time that the Kinshozan Quartz Diorite cooled down to 500 °C which is the closure temperature of the systematics. Permian granites are found in small exposures in Japan, and frequently referred to as 250 Ma granites. The Kinshozan Quartz Diorite is considered as a type of the 250 Ma granites, and the age was influential in establishing a model of Paleozoic tectonic evolution for the Japanese Islands. The new age of the Kinshozan Quartz Diorite provides the opportunity to re‐examine the model. The Kinshozan Quartz Diorite and other Permian granites in the south of the Median Tectonic Line of Japan were constituents of the Paleo‐Ryoke Belt. The geochemical characteristics of the granitic rocks in the Paleo‐Ryoke Belt indicated that the granitic rocks were formed in a primitive island arc environment, and the new trace element data also support this interpretation. Examination of the available data and results of the present study suggests the late Paleozoic granitic activity in Japan as follows. At about 310–290 Ma, arc magmatism generated adakitic granites and other granites in the South Kitakami Belt. Quartz diorite and tonalites of primitive characteristic, such as the Kinshozan Quartz Diorite and granites in the Maizuru Belt appear to have been formed at the immature island arc, and accreted to the Japanese Islands at the end of Paleozoic or early Mesozoic era. During 260–240 Ma, granitic activity took place in the Hida and Maizuru Belts as a part of the Asian continent.

Zircon U‐Pb Geochronology and Geochemical Characteristics of the Volcanic Host Rocks from the Tongyu VHMS Copper Deposit in the Western North Qinling Orogen and Their Geological Significance

AbstractPrecise in situ zircon U‐Pb dating and Lu–Hf isotopic measurement using an LA‐ICP‐MS system, whole‐rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo‐Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle‐normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE (e.g. Th, U, Pb and La) and depletion of HFSE (e.g. Nb, Ta, Ti, Zr and Hi). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr (0.703457 to 0.708218) and ∊Nd(t) (–2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr–∊Nd(t) diagram. The involvement of crustal‐derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon ∊Hf(t) values, which range from –3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores (2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn‐volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U–Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo‐Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore‐forming elements. The magma and ore‐bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS‐type copper deposit.