Arc-basin System Research Articles

Initial back-arc extension: Evidence from petrogenesis of early Paleozoic MORB-like gabbro at the southern Central Qilian block, NW China

As an indicator of extensional tectonic events, MORB-like magmas could provide great insights into tectonic evolution of subduction zones. In this study, we carry out an integrated study of geochronology, geochemistry and SrNd isotopic compositions for the Early Paleozoic MORB–like Lashuixia gabbro in Hualong, southern Central Qilian block. UPb zircon dating yields magma crystallization age of 506 ± 6 Ma for the gabbro. Geochemically, the gabbros display weakly fractionated REE patterns with (La/Yb)N = 1.17–1.61 and flat trace element patterns except for enrichment of large ion lithophile elements (LILEs) and depletion of Nb. Isotopically, the gabbros display pronouncedly positve εNd(t) values of +5.8 − +7.0, but their initial 87Sr/86Sr ratios ((87Sr/86Sr)i = 0.7046–0.7070) are slightly high because of post-magmatic alteration and metamorphism. Their geochemical and isotopic features indicate that several endmembers (i.e. MORB-sourced asthenospheric mantle, subcontinental lithospheric mantle and oceanic slab-derived fluid) were involved into their source region. Our modal batch partial melting calculation indicates that 7–17% partial melting of 90% asthenospheric mantle plus 10% subcontinental lithospheric mantle at spinel-stable depth accords well with most incompatible trace element compositions of the gabbros. In combination with regional data, we propose that rock association of arc-type pillow lava (ca. 494 Ma), Nb-enriched basalt (NEB)-like diabase (ca. 491 Ma), boninite (ca. 483 Ma) and later OIB-like basalt (ca. 468 Ma) in the Lajishan belt witnessed evolution of this belt from continental arc splitting to back-arc basin spreading due to rollback of the subducted North Qaidam-West Qinling Oceanic slab. We suggest that the Lashuixia gabbro, ca. 10 Ma earlier than the arc-type pillow lava, recorded the initial back-arc lithospheric extension. As with those in many modern arc-basin systems, MORB-like gabbro with overprinting of other endmembers in paleo-subduction zones could not only be an important indicator for back-arc extension, but also make a constraint on the time when the back-arc extension initiated.

Multi-stage metamorphic evolution and protolith reconstruction of spinel-bearing and symplectite-bearing ultramafic rocks in the Zheltau massif, Southern Kazakhstan (Central Asian Orogenic Belt)

Spinel-bearing ultramafic rocks, metamorphosed in high-pressure conditions, are described from the metamorphic complexes of the western part of the Central Asian Orogenic Belt (CAOB) in the Chu-Yili region in Southern Kazakhstan. They comprise small bodies of magnetite-bearing serpentinites, Cr-spinel-bearing serpentinized and amphibolitized dunites and peridotites enclosed by strongly retrogressed kyanite-bearing paragneisses. Apart from serpentinization, the ultramafic rocks were also overprinted by later rodingitization expressed in the development of the index-minerals hydrogrossular, prehnite and vuagnatite. Cr-spinel-bearing peridotites are characterized by extensive development of symplectitic and coronitic microtextures, which are interpreted to have been formed after garnet breakdown during decompression (i.e. exhumation). Calculated pressure (P) and temperature (T) obtained by phase diagram modeling and conventional geothermobarometry for the symplectites were P 11.5–14.5 kbar for a wide temperature range of T 600–850 °C, which followed the transition from garnet to spinel peridotite. Major and trace element whole-rock geochemical characteristics of the spinel-bearing ultramafic rocks as well as their structurally close relations indicate their mutual origin as parts of an oceanic cumulate complex of an arc-basin system. The protoliths were probably plagioclase-bearing (thus shallow, crustal) ultramafic rocks and troctolites, subducted to eclogite facies conditions and then exhumed along with other metamorphic complexes of the Zheltau massif. These spinel-bearing ultramafic rocks could be classified as crustal formations in accordance with the inferred depths of their origin (<5 kbar), in common with many HP ultramafic/mafic complexes in the CAOB. However, strongly depleted geochemical signatures of the rocks, together with the distinctive microtextural features observed in the spinel peridotites, are unique, and their occurrence in the western CAOB is described here for the first time.

Geochemical and Sr-Nd-Pb isotopic characteristics of extrusive rocks within the Çetmi Ophiolitic Mélange, Biga Peninsula

The different geochemical characteristics of the extrusive rocks of the Cetmi Ophiolitic Melange (COM) a remnant of the Intra-Pontide Suture (IPS) in the Biga Peninsula, NW Turkey, evidence the episodic formation of oceanic crust. Geochemically, volcanic rocks are divided into three groups: ocean island basalts (OIB), enriched mid-ocean ridge basalts (E-MORB) and basalts with supra-subduction zone (SSZ) affinity. While the OIB type samples were produced by small degrees partial melting from an enriched source, the E-MORB type samples point to mixing of enriched and depleted sources. In the OIB-type samples, the depletion of Zr and Hf compared to Nd as well as enrichment of Nb-Ta are attributed to presence of the amphibole in the mantle source. Radiogenic Pb isotope compositions towards EMII support metasomatic processes related to fractionation in the mantle reservoir. The SSZ samples are characterized by enriched light rare earth elements (LREE) and depleted high field strength elements (HFSE). The trace element characteristics of the SSZ samples indicate that a depleted MORB mantle source interacted with subduction-derived fluids due to the slab dehydration processes. Besides, the Sr-Nd-Pb isotope characteristics suggest that these samples originated from a MORB-type mantle source metasomatized by fluids or melts from subducted sediments. The major, trace element and isotopic data of the volcanic rocks in the COM indicate that the oceanic lithosphere is generated along a SSZ-type spreading centre at an arc-basin system within the Intra-Pontide Ocean.

Preliminary geochemical data of the mafic rocks from the Ovacik and Pülümür Ophiolite Zone (Eastern Anatolia, Turkey): implications for the geodynamic evolution of the Northern Neotethyan Ocean

The Izmir-Ankara-Erzincan-Sevan-Akera Suture Belt (IAESASB) stretches from the Aegean Sea to the Lesser Caucasus and includes the remnants of the Northern Neotethys. The Ovacik and Pulumur Ophiolite Zone (OPOZ) to the north of Tunceli is less-known members of this belt. In this study, preliminary geochemical data from the basaltic and gabbroic rocks from this zone are presented to provide the first insights into their geochemistry. The studied mafic samples exhibit geochemical characteristics of an intra-oceanic subduction system. Three types of samples were differentiated on the basis of their major, trace and rare earth element (REE) geochemistry. The first one is boninitic and displays highly depleted elemental pattern (relative to N-MORB) and concave REE profile. The second type is akin to back-arc basin basalts (BABB) and generally reflects normal-MORB (NMORB)-like high field strength element (HFSE) patterns coupled with negative Nb anomalies. The last type is relatively similar to the BABB-type, but it lacks the marked negative anomaly in Nb and can be ascribed to the mid-ocean ridge basalt (MORB). The overall geochemical features suggest that these mafic rocks were predominantly generated in a supra-subduction setting from a subduction-modified mantle source. It is further suggested that the Ovacik and Pulumur Ophiolite Zone was formed in an oceanic arc-basin system within the northward subducting Northern Neotethys.

The Bangxi-Chenxing tectonic zone in Hainan Island (South China) as the eastern extension of the Song Ma-Ailaoshan zone: Evidence of late Paleozoic and Triassic igneous rocks

Mafic-intermediate igneous rocks in the Bangxi-Chenxing tectonic zone of Hainan Island, South China, can be used to constrain the Paleotethyan tectonic evolution in Southeast Asia. This paper presents petrologic, geochronological, elemental and Sr–Nd–Hf–O isotopic data for Carboniferous basalts, Permian andesites and Triassic gabbro-dolerites in this zone. Zircon grains from these rocks have U-Pb ages of ∼330 Ma, ∼257 Ma and ∼240 Ma, respectively. The ∼330 Ma basalts contain 1400 Ma captured zircon grains, and display MORB-like geochemical characteristics with positive εNd (t) values of +5.86–+9.85, reflective of a MORB source with the input of a slab-derived component in a continental back-arc basin setting. The ∼257 Ma andesites contain older captured zircon grains, and have arc-like geochemical signature with weakly positive εNd (t) values of +0.26–+0.38, zircon εHf (t) values of −0.5–+1.4 and δ18O values of 6.35–7.16‰, which were derived from a metasomatized mantle wedge modified by the sediment-derived component in a continental arc setting. The ∼240 Ma gabbro-dolerites exhibit enrichment in LILEs, depletion in HFSEs, negative εNd (t)-εHf (t) values (−8.45 to −1.05 and −5.9 to −2.7, respectively) and crustal-like δ18O values (7.26–8.70‰), indicative of a post-collisional environment in response to the asthenosphere upwelling shortly after the closure of the Bangxi-Chenxing back-arc basin. The Carboniferous-Triassic mafic-intermediate igneous rocks thus record the Carboniferous-Permian arc-basin system and its Triassic closure. The Bangxi–Chenxing tectonic zone separated South Hainan from North Hainan and westerly extended to the Song Ma–Ailaoshan suture zone.

New zircon U-Pb LA-ICP-MS ages and Hf isotope data from the Central Pontides (Turkey): Geological and geodynamic constraints

The Central Pontides in northern Anatolia is located on the accretionary complex formed by the closure of Neotethyan Intra-Pontide Ocean between the southern Eurasian margin (Istanbul-Zonguldak Terrane) and the Cimmerian Sakarya Composite Terrane. Among other components of the oceanic lithosphere, it comprises not yet well-dated felsic igneous rocks formed in arc-basin as well as continent margin settings. In-situ U-Pb age results for zircons from the arc-basin system (öangaldağ Metamorphic Complex) and the continental arc (Devrekani Metadiorite and Granitoid) yield ages of 176 ± 6 Ma, 163 ± 9 Ma and 165 ± 3 Ma, respectively. Corresponding in-situ average (initial) 176Hf/177Hf initial ratios are 0.28261 ± 0.00003, 0.28267 ± 0.00002 and 0.28290 ± 0.00004 for these units and indicative of a subduction-modified mantle source. The new U-Pb ages and Hf isotope data from these oceanic and continental arc units together with regional geological constraints support the presence of a multiple subduction system within the Intra-Pontide Ocean during the Middle Jurassic.

Sedimentary provenance in response to Carboniferous arc-basin evolution of East Junggar and North Tianshan belts in the southwestern Central Asian Orogenic Belt

The North Tianshan and East Junggar belts in north Xinjiang, China, are two key terranes for the Paleozoic tectonic reconstruction of the Central Asian Orogenic Belt (CAOB). However, their Carboniferous paleogeographic setting and arc-basin relationships remain disputed. To better-constrain these problems, we carried out an investigation on the provenance of Carboniferous sandstones from the Yaomoliang Formation in East Junggar and the Qijiaojing Formation in North Tianshan. Modal and geochemical analyses reveal that the sediments are mainly derived from arc-related terranes. Detrital zircon U-Pb age spectra show distinct differences between the East Junggar and North Tianshan belts. The upper Yaomoliang Formation has a Late Carboniferous depositional age (ca. 315Ma) with a single peak spectrum of detrital zircon ages, which is similar with zircon ages from the igneous rocks and associated accretionary complexes in East Junggar belt, suggesting that it was probably derived from the East Junggar arc. The samples from the Qijiaojing Formation have Early Carboniferous depositional ages (ca. 330Ma to 354Ma). Their complicated detrital zircon age peaks resemble the detrital and igneous zircon ages from the Harlik-Dananhu arc, implying that their source terrane could be the Harlik-Dananhu arc, as revealed by the near-source petrographic and arc-derived geochemical features. In combination with our results and the published data, we suggest two distinctive Carboniferous arc-basin systems, the Kelamaili Ocean-related arc-basin which is characterized by the northward subduction beneath the East Junggar arc, and the Bogda intra-arc basin which evolved from splitting the previous Harlik-Dananhu arc by the rollback of the subducting North Tianshan oceanic plate. The sedimentary provenance evidence reveals that the Kelamaili Ocean has not closed until ca. 315Ma. The multiple arc-basin systems in North Xinjiang indicate an archipelago-type subduction and accretionary scenario in the evolution of southwestern CAOB during the Carboniferous.

Structures, uplift, and magmatism of the Western Myanmar Arc: Constraints to mid-Cretaceous-Paleogene tectonic evolution of the western Myanmar continental margin

Knowledge of Trans-Himalayan tectono-magmatic evolution is critical to understanding the complex pre-collisional history of southern Eurasia active continental margin. It has been proposed that magmatic rocks of the Trans-Himalayan batholith, extending from southern Tibet to Southeast Asia, are now exposed as the Western Myanmar Arc and Central Granite Belt in Myanmar, yet origin, emplacement, and relationships of the two juxtaposed belts remain poorly constrained. In this study, 2D seismic and drilling data for the Western Myanmar Arc, zircon U-Pb age and Hf isotope and whole-rock geochemical data for magmatic rocks from the arc have been applied. Our seismic profiles, borehole stratigraphic sequences and zircon U-Pb data show that a typical arc-basin system was well developed along the western Myanmar continental margin. The magmatic arc has experienced at least three igneous events in the mid-Cretaceous (110–90Ma), latest Cretaceous-Early Paleocene (69–64.5Ma) and Eocene (53–38Ma), as well as three associated uplift processes in the Late Cretaceous, Eocene and Late Oligocene. Whole-rock geochemical characteristics and zircons showing variable but predominately positive εHf(t) values, suggest a significant juvenile mantle source involving a proportion of ancient subducted sediments and juvenile crustal materials for these typical arc-related magmatic rocks. The identification of mid-Cretaceous to Paleogene magmatic rocks having positive εHf(t) values from the Western Myanmar Arc: 1) indicates that the magmatism can be correlated with the Gangdese arc within the Lhasa terrane of the southern Tibetan Plateau; 2) provides evidence for the proximal-derived model that Paleogene sediments in the Central Myanmar Basin were from the Western Myanmar Arc, but were not delivered by the paleo-Yarlung Tsangpo-Irrawaddy river system from the Gangdese arc; and 3) enables a model of eastward subduction of the Neo-Tethyan/Indian oceanic crust to reflect onset of the magmatism at the mid-Cretaceous and a long-existed back-arc extension in western Myanmar.

Geochemistry and Sr–Nd–Hf isotopes of Middle Devonian igneous rocks of the Sarsuk polymetallic Au deposit: implications for understanding the tectonic evolution of the south Altay Orogenic Belt, Northwest China

ABSTRACTThe medium-tonnage Sarsuk polymetallic Au deposit is located in the Devonian volcanic–sedimentary Ashele Basin of the south Altay Orogenic Belt (AOB), Northwest China. Within the deposit, the rhyolite porphyries and diabases are widespread, emplaced into strata. The orebodies are hosted by the rhyolite porphyries. We studied the petrography, geochemistry, and Sr–Nd–Hf isotopes of the rhyolite porphyries and diabases, in order to understand the petrogenesis of these rocks and their tectonic significance. They display typical bimodality in geochemistry compositions. The diabases are characterized by SiO2 contents of 44.84–59.77 wt.%, high Mg# values (43–69), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb and Ta, low (87Sr/86Sr)i (0.706687–0.707613) values, positive εNd(t) (4.8–6.8) values, and positive and high εHf(t) (7.15–15.19) values, suggesting a depleted lithosphere mantle source that might have been metasomatized by subduction-related components. The rhyolite porphyries show affinity to sanukitoid magmas contents [high SiO2 (78.6–81.82 wt.%) and MgO (3.38–5.94 wt.%, one sample at 0.61 wt.%), and enrichments in LILE and LREE], they were derived from the equilibrium reactions between a mantle source and subducted oceanic crust materials. Those characteristics together with the positive εNd(t) (4.1–8.4) and εHf(t) (2.88–15.17) values indicate that the diabases and rhyolite porphyries were generated from the same mantle peridotite source. But the rhyolite porphyries underwent fractional crystallization of Fe–Ti oxides, plagioclase, and apatite due to their negative Eu (δEu = 0.21–0.28) and P anomalies. According to the geochemical and isotopic data, the Sarsuk Middle Devonian igneous rocks are considered to be the products of the juvenile crustal growth in an island arc setting. The Sarsuk polymetallic Au deposit formed slightly later than the Ashele Cu–Zn deposit in the Ashele Basin, but they have the same tectonic setting, belonging to the trench–arc–basin system during extensional process in the south AOB.

Petrogenesis and geodynamics of plagiogranites from Central Turkey (Ekecikdağ/Aksaray): new geochemical and isotopic data for generation in an arc basin system within the northern branch of Neotethys

In the Late Cretaceous, throughout the closure of the Neotethys Ocean, ophiolitic rocks from the Izmir–Ankara–Erzincan ocean branch were overthrusted the northern margin of the Tauride-Anatolide Platform. The ophiolitic rocks in the Ekecikdag (Aksaray/Central Turkey) region typify the oceanic crust of the Izmir–Ankara–Erzincan branch of Neotethys. The gabbros in the area are cut by copious plagiogranite dykes, and both rock units are intruded by mafic dykes. The plagiogranites are leucocratic, fine- to medium-grained calc-alkaline rocks characterized mainly by plagioclase and quartz, with minor amounts of biotite, hornblende and clinopyroxene, and accessory phases of zircon, titanite, apatite and opaque minerals. They are tonalite and trondhjemite in composition with high SiO2 (69.9–75.9 wt%) and exceptionally low K2O (<0.5 wt%) contents. The plagiogranites in common with gabbros and mafic dykes show high large-ion lithophile elements/high-field strength element ratios with depletion in Nb, Ti and light rare-earth elements with respect to N-MORB. The plagiogranites together with gabbros and mafic dykes show low initial 87Sr/86Sr ratios (0.70419–0.70647), high ƐNd(T) (6.0–7.5) values with 206Pb/204Pb (18.199–18.581), 207Pb/204Pb (15.571–15.639) and 208Pb/204Pb (38.292–38.605) ratios indicating a depleted mantle source modified with a subduction component. They show similar isotopic characteristics to the other supra-subduction zone (SSZ) ophiolites in the Eastern Mediterranean to East Anatolian–Lesser Caucasus and Iran regions. It is suggested that the Ekecikdag plagiogranite was generated in a short time interval from a depleted mantle source in a SSZ/fore-arc basin setting, and its nature was further modified by a subduction component during intra-oceanic subduction.

Reconstructing multiple arc-basin systems in the Altai–Junggar area (NW China): Implications for the architecture and evolution of the western Central Asian Orogenic Belt

The Altai–Junggar area in northwestern China is a critical region to gain insights on the tectonic framework and geological evolution of the western Central Asian Orogenic Belt (CAOB). In this study, we report results from integrated geological, geochemical and geophysical investigations on the Wulungu Depression of the Junggar Basin to determine the basement nature of the basin and understand its amalgamation history with the Chinese Altai, within the broad tectonic evolution of the Altai–Junggar area. Based on borehole and seismic data, the Wulungu Depression is subdivided into two NW-trending tectonic units (Suosuoquan Sag and Hongyan High) by southward-vergent thrust faults. The Suosuoquan Sag consists of the Middle–Late Devonian basaltic andesite, andesite, dacite, tuff, tuffaceous sandstone and tuffite, and the overlying Early Carboniferous volcano-sedimentary sequence with lava flows and shallow marine sediments from a proximal juvenile provenance (zircon εHf(t)=6.0–14.9), compared to the Late Carboniferous andesite and rhyolite in the Hongyan High. Zircon SIMS U–Pb ages for dacites and andesites indicate that these volcanics in the Suosuoquan Sag and Hongyan High erupted at 376.3Ma and 313.4Ma, respectively. The Middle–Late Devonian basaltic andesites from well LC1 are calc-alkaline and exhibit primitive magma-like MgO contents (7.9–8.6%) and Mg# values (66–68), with low initial 87Sr/86Sr (0.703269–0.704808) and positive εNd(t) values (6.6–7.6), and relatively high Zr abundance (98.2–116.0ppm) and Zr/Y ratios (5.1–5.4), enrichment in LREEs and LILEs (e.g., Th and U) and depletion in Nb, Ta and Ti, suggesting that they were probably derived from a metasomatized depleted mantle in a retro-arc extensional setting. The well LC1 andesitic tuffs, well L8 dacites, well WL1 dacitic tuffs and well L5 andesites belong to calc-alkaline and metaluminous to peraluminous (A/CNK=0.8–1.7) series, and display low Mg# values (35–46) and variably positive εNd(t) (4.5–8.5) and εHf(t) (10.2–16.8) values, as well as young isotopic model ages. These Devonian–Carboniferous intermediate–felsic volcanics are interpreted as the products of partial melting of a juvenile lower crust with some contributions from mantle components in an evolved island arc setting from immature to mature island arc. The basin filling pattern and the distribution of arc volcanics and their zircon Hf model ages with the eruptive time suggest that the Wulungu Depression represents an island arc-basin system with the development of a Carboniferous retro-arc basin. In combination with previous work, we propose that the northern Junggar area comprises three arc-basin belts from south to north: the Darbut–Luliang–Karamaili, Wulungu–Yemaquan, and Saur–Fuhai–Dulate. Such tectonic subdivisions are consistent with the regional gravity and magnetic anomaly data. The recognition of the Wulungu arc-basin system demonstrates that the Junggar Basin is likely underlain by juvenile continental crust rather than ancient Precambrian basement, and also implies that the CAOB was built by amalgamation of multiple linear arcs and accretionary complexes.

Early Neoproterozoic (∼840 Ma) arc magmatism: Geochronological and geochemical constraints on the metabasites in the Central Jiangnan Orogen

The Jiangnan Orogen is traditionally considered the suture zone, along which the Yangtze and Cathaysia blocks collided to form the united South China Block. There are still major controversies surrounding the pre-collisional tectonic pattern and amalgamation process along the Orogen and the relationship of the united South China Block (SCB) with the Rodinia. Our recent field investigations identified the presence of the volcano-sedimentary unit of the Lengjiaxi Group at Jianxichong, with the thickness of over 1700m. A set of new zircon U–Pb geochronological, whole-rock elemental and Sr–Nd isotopic data for the metabasites have been obtained and presented in this paper. The layered diabase and tuffaceous samples from the Jianxichong volcano-sedimentary unit yielded the weighted mean ages of 837±5Ma and 835±6Ma, respectively. The U–Pb ages of detrital zircons from the phyllite interlayer form a main age-cluster at the peak age of 870Ma with the youngest apparent age of 845±23Ma. This indicates that the metabasites in the Jianxichong volcano-sedimentary unit formed at ∼845–835Ma rather than in the previously-thought Neoarchean in the Central Jiangnan Orogen. The metabasitic samples have SiO2 ranging from 48.78wt.% to 53.72wt.% and MgO from 7.71wt.% to 12.74wt.% with mg-number of 63–71, and hence can be classified as tholeiitic basalt and basaltic andesite. They demonstrate a sub-parallel and right-sloping chondrite-normalized REE pattern with slightly negative Eu anomalies and sub-parallel spiky PM-normalized patterns with enrichment in LILEs, depletion in HFSEs and P–Ti negative anomalies, similar to those of arc volcanic rocks. Their ɛNd(t) values range from −1.64 to −0.37 and initial 87Sr/86Sr values from 0.7046 to 0.7081, respectively. Such crust-like geochemical characteristics might have been inherited from a sub-arc source modified by fluid/melt released from the subduction sedimentary component. In conjunction with available data and geological observations, we herein propose the development of the early Neoproterozoic (∼840Ma) continental arc-basin system along the Central Jiangnan Orogen in response to an arc-continent accretionary assembly along the Rodinia exterior.

Petrology, geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite, LongmuCo–Shuanghu–Lancang suture zone, northern Tibet: evidence for an intra-oceanic arc–basin system

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai–Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo–Shuanghu–Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo–Shuanghu–Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo–Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo–Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai–Tibet Plateau.

Late Paleozoic subduction system in the southern Central Asian Orogenic Belt: Evidences from geochronology and geochemistry of the Xiaohuangshan ophiolite in the Beishan orogenic belt

The Xiaohuangshan ophiolite of the Beishan (Inner Mongolia) is located in the southern margin of the Central Asian Orogenic Belt. It consists of several blocks composed dominantly of serpentinized ultramafic rocks, cumulative gabbros and basalts. The geochemical data of gabbros and basalts obtained from the Xiaohuangshan ophiolite are similar to tholeiitic rocks. They all have low TiO2 and high Al2O3 contents. Their light rare earth elements are slightly enriched, (La/Yb)N=3.62–6.80, similar to the typical enriched mid-ocean ridge basalts. The mafic rocks display enrichments in large ion lithophile elements and depletions in high field strength elements, as well as significantNb–Ta–Ti negative anomalies, similar to subduction-derived rocks. All these geochemical characteritics indicate that the Xiaohuangshan ophiolite would form in a subduction zone from a slightly enriched mantle source. Ion microprobes (SHRIMP) U–Pb dating were conducted on zircons from the basalt and gabbro. The weighted mean ages are 336.4±4.1Ma and 345±14Ma, which are considered as the crystallization ages of the basalt and gabbro, respectively. Together with other two units, the Dongqiyishan arc belt and the Yueyashan–Xichangjing ophiolite, the Xiaohuangshan ophiolite forms a Late Paleozoic arc-basin system, indicating that the Paleo-Asian Ocean did not close in the early Carboniferous. Based on the geochemical characteristics of adjacent geological bodies and their settings, the Xiaohuangshan ophiolite is considered as an indicator of a suture zone between the different epicontinental belts in the Beishan region.

Geochemistry of Permian Mafic Igneous Rocks from the Napo‐Qinzhou Tectonic Belt in Southwest Guangxi, Southwest China: Implications for Arc‐Back Arc Basin Magmatic Evolution

AbstractThe Napo‐Qinzhou Tectonic Belt (NQTB) lies at the junction of the Yangtze, Cathaysia and Indochina (North Vietnam) Blocks, which is composed of five major lithotectonic subunits: the Qinzhou‐Fangcheng Suture Zone (QFSZ), the Shiwandashan Basin (SB), the Pingxiang‐Nanning Suture Zone (PNSZ), the Damingshan Block (DB) and the Babu‐Lingma Suture Zone (BLSZ). On the basis of geochemical compositions, the Permian mafic igneous rocks can be divided into three distinct groups: (1) mafic igneous rocks (Group 1) from the Longjing region in the PNSZ and Hurun region in the BLSZ, which are characterized by intermediate Ti, P and Zr with low Ni and Cr contents; (2) mafic igneous rocks (Group 2) from the Naxiao and Chongzuo region in the DB, characterized by low‐intermediate Ti, P and Zr with high Ni and Cr concentrations; and (3) mafic igneous rocks (Group 3) from the Siming region in the Jingxi carbonate platform of the northwestern margin of the NQTB, with intermediate‐high Ti, P and Zr and low Ni and Cr contents. The Group 1 rocks yield a weighted mean 206Pb/238U age of 250.5±2.8 Ma and are geochemically similar to basalts occurring in back‐arc basin settings. The Group 2 rocks exhibit geochemical features to those basalts in island arcs, whereas the Group 3 rocks show geochemical similarity to that of ocean island basalts. All three groups are characterized by relatively low εNd(t) values (–2.61 to +1.10) and high initial 87Sr/86Sr isotopic ratios (0.705309–0.707434), indicating that they were derived from a subduction‐modified lithospheric mantle and experienced assimilation, fractional crystallization, and crustal contamination or mixing during magmatic evolution. Accordingly, we propose the existence of an arc‐back arc basin system that developed along the NQTB at the border of SW Guangxi Province (SW China) and northern Vietnam, and it was formed by continued northwestward subduction of the Cathaysian (or Yunkai) Block under the Yangtze Block, and northeastward subduction of the Indochina Block beneath the Yangtze Block during Permian time.

Tectonic evolution of the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau, composed of several continental slivers within the eastern Tethyan domain, is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on Earth. This plateau is generally inferred to be a collage of several continental blocks that rifted from Gondwanaland and subsequently accreted to the Asian continent. However, recent recognition of over twenty ophiolite mélange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision. Based on 177 recently finished 1:250,000 scale geological maps and related studies, we summarize the main tectonic context of the Qinghai-Tibet Plateau and propose a new integrated model to account for the new findings. The complex orogen of the immense Qinghai-Tibet Plateau, consisting of multiple island arc-basin systems that developed at different stages while surrounded by the North China, Yangtze, Tarim, and Indian plates, is emphasized. The entire orogen, surrounded by suture zones that mark the locations of oceanic closure, is investigated by examining (I) the first-order tectonic units and ophiolitic mélanges (including arc–arc/continent collision zones) and (II) their internally enclosed blocks as the second-order tectonic units. Therefore, the Qinghai-Tibet Plateau is divided into three major orogenic systems, namely, from northeast to southwest, the Early Paleozoic Qinling–Qilianshan–Kunlunshan (Qin–Qi–Kun), the Late Paleozoic–Triassic Qiangtang–Sanjiang, and the Late Paleozoic to Cenozoic Gangdese–Himalaya orogenic systems, which are separated by the Kangxiwa–Muzitagh–Maqin–Mianxian and the Bangong–Shuanghu–Changning–Menglian sutures, respectively. We propose that the formation and evolution of the Qinghai-Tibet Plateau to have been intrinsically related to those of the eastern Tethys, recorded by the Longmu Co-Shuanghu ophiolite mélange zone, the Southern Qiangtang Paleozoic accretionary arc-basin system, the Bangong–Nujiang suture zone, and their associated, composite island arc-basin systems. The present-day Bangong–Shuanghu–Changning–Menglian suture system marks the final closure of the Tethyan Ocean. The Yarlung Zangbo Ocean opened as a back-arc basin in response to the southward subduction of the Tethyan Ocean lithosphere in the Middle Triassic and closed as a result of the India–Asia collision at the end of Cretaceous, followed by the northward indention of the Indian plate that resulted in significant intra-continental deformation and plateau uplift in the Cenozoic.

Geochemistry of mafic dykes from the Southeast Anatolian ophiolites, Turkey: Implications for an intra-oceanic arc–basin system

The Late Cretaceous–Tertiary accretionary prism in Eastern Turkey includes several ophiolitic megablocks and/or tectonic slivers (Mehmetalan, Mollatopuz and Alabayir) within a mélange complex, mainly comprising harzburgite, dunite and cumulate-textured gabbro. The diabases, which are the main focus of this study, cut across the ophiolites as parallel and variably thick dyke-swarms. Geochemistry of the diabases reveals three distinct groups, including a) supra-subduction zone (SSZ) type, which is characterized by marked Nb-anomaly and normal mid-ocean ridge basalt (N-MORB) like HFSE distribution, b) enriched MORB (E-MORB) type, showing some degree of enrichment relative to N-MORB, c) oceanic-island basalt (OIB) type with characteristic hump-backed trace element patterns, coupled with fractionated REE distribution. Among these groups, SSZ- and E-MORB-type signatures are acquired from the Mehmetalan and Mollatopuz suites, whereas OIB-type characteristics are found in the Alabayir suite. The melting models indicate involvement of both depleted and enriched sources for the genesis of the studied dykes. The close spatial relationship, similar ages (based on Ar–Ar dating) and the presence of variable subduction component displayed by Mehmetalan and Mollatopuz suites may indicate melt generation in an intra-oceanic SSZ within the southern branch of Neotethys. In spite of the solely OIB-like character of the Alabayir suite, the similar age obtained from these dykes may suggest their formation in a similar SSZ setting. Alternatively, the Alabayir suite may have represented an oceanic island or seamount formed in an intra-plate setting with or without plume influence. We suggest that decompression melting triggered by slab roll-back mechanism during the closure of the southern branch of the Neotethys during the Late Cretaceous may have been the main process that led to generation of magmas of both depleted and enriched characteristics.

Metallogenic Province and Large Scale Mineralization of Volcanogenic Massive Sulfide Deposits in China

Volcanogenic massive sulfide (VMS) deposits are one of the most important base–metal deposit types in China, are major sources of Zn, Cu, Pb, Ag, and Au, and significant sources for Co, Sn, Se, Mn, Cd, In, Bi, Te, Ga, and Ge. They typically occur at or near the seafloor in submarine volcanic environments, and are classified according to base metal content, gold content, or host-rock lithology. The spatial distribution of the deposits is determined by the different geological settings, with VMS deposits concentrated in the Sanjiang, Qilian and Altai metallogenic provinces. VMS deposits in China range in age from Archaean to Mesozoic, and have three epochs of large scale mineralization of Proterozoic, Palaeozoic and Mesozoic. Only Hongtoushan Cu–Zn deposit has been recognized so far in an Archaean greenstone belt, at the north margin of the North China Platform. The Proterozoic era was one of the important metallogenic periods for the formation of VMS mineralization, mainly in the Early and Late Proterozoic periods. VMS-type Cu–Fe and Cu–Zn deposits related to submarine volcanic-sedimentary rocks, were formed in the Aulacogens and rifts in the interior and along both sides of the North China Platform, and the southern margin of the Yangtze Platform. More than half of the VMS deposits formed in the Palaeozoic, and three important VMS–metallogenic provinces have been recognized, they are Altai–Junggar (i.e. Ashele Cu–Pb–Zn deposit), Sanjiang (i.e. Laochang Zn–Pb–Cu deposit) and Qilian (i.e. Baiyinchang Cu–Zn deposit). The Triassic is a significant tectonic and metallogenic period for China. In the Sanjiang Palaeo–Tethys, the Late Triassic Yidun arc is the latest arc–basin system, in which the Gacun-style VMS Pb–Zn–Cu–Ag deposits developed in the intra-arc rift basins, with bimodal volcanic suites at the northern segment of the arc.

Accretionary Orogenesis in the Active Continental Margins

This paper reviews the research history and progress in the study of accretionary orogenesis, and concludes that accretionary orogenesis is the fundamental process of continental evolution throughout Earth's history. According to the authors' point of view, the formation and evolution of orogenic belts can be explained by the evolution of composite island arc-basin systems along active continental margins. The formation of still larger orogenic systems is closely related to composite island arc-basin systems formed on one or two sides of an ocean, and controlled by the subduction of long-lived oceanic lithosphere. Accretionary orogenic systems in active continental margins are built up by a series of collisional and accretionary orogenies, including arc-arc and arc-continent collision, and the accretion of forearc and backarc subduction complexes, tectonic collages of oceanic plateaus/islands/seamounts, and exotic blocks (terranes). Convergent crustal consumption zones (sutures) are formed when the long-lived ocean lithosphere is eventually eliminated. Large orogenic complexes formed due to the amalgamation of orogenic systems transformed from composite island arc-basin systems on both sides of long-lived ocean. Subsequently such large orogenic complexes are overprinted by the effects of continent-continent collision and intracontinental compression/extension, which make the orogenic processes still more complex. The convergent crustal consumption zone, therefore, is the key to understand the formation and evolution of orogenic systems. All kinds of orogenic processes of composite island arc-basin systems in active continental margins are actually the consequences of accretionary orogenesis. The study of composite island arc-basin systems is essential to understand the components, textures and structures of orogenic systems. Postorogenic intracontinental tectonic evolution is also controlled by the evolutionary histories of composite island arc-basin systems. Composite island arc-basin systems and continental margin systems provide more comprehensive records of Wilson cycle than fossil oceanic lithosphere. According to the ideas of composite island arc-basin systems, a new model for the evolution of the Tethyan ocean is proposed, specifically that the Gangdese belt (Lhasa terrane) was initially controlled by the southward subduction of the Tethyan oceanic lithosphere in the Carboniferous, that an accretionary orogeny took place in the Triassic, and that the Tethyan ocean vanished by the end of Early Cretaceous.

Tectonic evolution of the Hengshan–Wutai–Fuping complexes and its implication for the Trans-North China Orogen

The Hengshan–Wutai–Fuping complexes comprise the central segment of the Trans-North China Orogen (TNCO) in the North China Craton (NCC). The tectonic evolution of these complexes is regarded as representative for the entire TNCO, although the timing of the events has been controvesial. On the basis of new Nd isotope and structural data, and a re-appraisal of published geochemical and geochronological data, a refined model for the tectonic evolution in the Neoarchean to Paleoproterozoic has been proposed. A major ocean, named the Proto-North China Ocean, developed between the Western and Eastern Blocks prior to ∼2565 Ma. Northwest-ward intra-oceanic subduction resulted in the formation of a proto-arc, as represented by the ∼2565–2540 Ma Wutai granitoids. The proto-arc was rifted, and a back-arc basin propagated and extended to form an intra-oceanic arc-basin system at about ∼2540–2525 Ma. The main products during the development of this system were a MORB-arc-back arc basalt–adakite association in the Wutai greenstone belt and synchronous granitoids in the Wutai Complex. The main subduction–accretion process involving multiple subduction zones occurred at ∼2525–2475 Ma, resulting in closure of the Wutai back-arc basin and the formation of diverse arc magmas. Although the tectonic history between ∼2.5 and 1.8 Ga remains uncertain, final collision between the Eastern and Western Blocks and the buildup of the TNCO most likely took place at ∼1.9–1.8 Ga. The tectonic evolution of the TNCO is characterized by subduction–accretion processes and the development of arc-basin systems, thus it is more likely a Cordilleran-type orogen than a simple continent–continent collisional orogen.