Back-arc Basin Basalts Research Articles

Submarine basaltic fountain eruptions in a back-arc basin during the opening of the Japan Sea

Basaltic rock generated during the middle Miocene opening of the Japan Sea, is widely distributed on the back-arc side of the Japanese archipelago. Few studies have investigated on submarine volcanism related to opening of the Japan Sea. The present study aimed to reconstruct details of the subaqueous volcanism that formed the back-arc basin basalts (BABB) during this event, and to discuss the relationship between volcanism and the tectonics of back-arc opening, using facies analyses based on field investigation. The study area of the southern Dewa Hills contains well-exposed basalt related to the opening of the Japan Sea. Five types of basaltic rock facies are recognized: (1) coherent basalt, (2) massive platy basalt, (3) jigsaw-fit monomictic basaltic breccia, (4) massive or stratified coarse monomictic basaltic breccia with fluidal clasts, and (5) massive or stratified fine monomictic basaltic breccia. The basaltic rocks are mainly hyaloclastite. Based on facies distributions, we infer that volcanism occurred along fissures developed mainly at the center of the study area. Given that the rocks contain many fluidal clasts, submarine lava fountaining is inferred to have been the dominant eruption style. The basaltic rocks are interpreted as the products of back-arc volcanism that occurred by tensional stress related to opening of the Japan Sea, which drove strong tectonic subsidence and active lava fountain volcanism.

Early Carboniferous ophiolite in central Qiangtang, northern Tibet: record of an oceanic back-arc system in the Palaeo-Tethys Ocean

ABSTRACTZircon U–Pb dating of two samples of metagabbro from the Riwanchaka ophiolite yielded early Carboniferous ages of 354.4 ± 2.3 Ma and 356.7 ± 1.9 Ma. Their positive zircon εHf(t) values (+7.9 to +9.9) indicate that these rocks were derived from a relatively depleted mantle. The metagabbros can be considered as two types: R1 and R2. Both types are tholeiitic, with depletion of high-field-strength elements (HFSE) and enrichment of large-ion lithophile elements (LILE) similar to those of typical back-arc basin basalts (BABB), such as Mariana BABB and East Scotia Ridge BABB. Geochemical and isotopic characteristics indicate that the R1 metagabbro originated from a back-arc basin spreading ridge with addition of slab-derived fluids, whereas the R2 metagabbro was derived from a back-arc basin mantle source, with involvement of melts and fluids from subducted ocean crust. The Riwanchaka ophiolite exhibits both mid-ocean ridge basalts- and arc-like geochemical affinities, consistent with coeval ophiolites from central Qiangtang. Observations indicate that the Qiangtang ophiolites developed during the Late Devonian–early Carboniferous (D3–C1) in a back-arc spreading ridge above an intra-oceanic subduction zone. Based on our data and previous studies, we propose that an oceanic back-arc basin system existed in the Longmuco–Shuanghu–Lancang Palaeo-Tethys Ocean during the D3–C1 period.

Permian back-arc basin basalts in the Yushu area: New constrain on the Paleo-Tethyan evolution of the north-central Tibet

The Paleo-Tethyan tectonic evolution of north-central Tibet remains controversial, particularly regarding the nature and extension of associated sutures. The Yushu mélange, located in the junction part of the western Jinshajiang, the southern Jinshajiang and the Ganzi–Litang sutures, is a key area to reveal the Paleo-Tethyan tectonics of this area. This study presents new geochronological and geochemical data for the Zhimenda volcanic suite of the Yushu Mélange. These rocks preserve a record of back-arc magmatism along the northern margin of the North Qiangtang Block. The Zhimenda volcanics are composed of ignimbrite, basalt, and porphyritic tuff intercalated with terrigenous clastic rocks. Zircon 206Pb/238U ages of the ignimbrites indicate they formed at ~254Ma. The basalts are relatively enriched in large-ion lithophile elements and depleted in high field strength elements. Their high MgO, Ni, and Cr contents, relatively high Sm/Yb and Rb/Nb values, and positive zircon εHf(t) and bulk εNd(t) values suggest they were derived from the partial melting of a depleted subcontinental lithospheric mantle source metasomatized by hydrous fluids. The Zhimenda basalts are geochemically similar to back-arc basin basalts in the Okinawa Trough. They were erupted related to subduction of the Longmuco–Shuanghu Paleo-Tethyan oceanic plate beneath the North Qiangtang Block. We propose that the Yushu Mélange converges with the Ganzi–Litang Suture Zone to the east, rather than with the Jinshajiang Suture Zone to the southeast, and that a huge Permian trench–arc–back-arc system developed in north-central Tibet.

Geochemical and zircon U-Pb and Lu-Hf isotopic constraints on the origin of supracrustal rocks from the mid-Qilian terrane: A comparison between supracrustal rocks on the two sides of the eastern segment of the Altyn Tagh Fault

Comparisons between the tectonic units on two sides of the sinistral ENE-striking Altyn Tagh Fault (ATF) are important tracers to find out the corresponding relationships among different terranes. This study focuses on the tectonic units along the eastern segment of the ATF. The Paleoproterozoic Beidahe Group in the mid-Qilian terrane, south side of the eastern segment of the ATF, is mainly composed of metasedimentary rocks and amphibolites. These rocks are petrologically and mineralogically similar to those from the Dunhuang Group, north side of the eastern segment of the ATF. The detrital zircon U-Pb ages indicate that the protolith sediments of the Beidahe metasedimentary rocks were mainly sourced from a provenance dominated by magmatic rocks in ages of 2.85–2.55Ga, 1.70–1.55Ga and 1.42–1.03Ga. This sedimentary sequence was deposited no earlier than 1.03Ga as passive continental margin deposits. Geochemical and zircon U-Pb and Lu-Hf isotopic data suggest that the Neoarchean-Mesoproterozoic basement rocks from the Tarim Craton most likely provide the majority of source materials for their protolith. Amphibolites from the Beidahe Group have high total Fe2O3 and Al2O3 contents but low Mg# values, indicating a high degree of compositional evolution and contamination of the older crust. They have flat REE patterns but positive LILE and negative Nb-Ta and Ti anomalies, suggesting a back-arc basin basalts (BABBs) affinity for their protolith. The amphibolites have a crystallization age of 1.61Ga and zircon εHf(t) values ranging from −7.7 to +10.6. The zircon εHf(t) values and whole-rock εNd(t) values indicate an involvement of juvenile and pre-existing old materials in the formation of their parental magmas. Comparisons between the Beidahe and Dunhuang Groups indicate that the metasedimentary rocks from these two groups are both sourced from the Tarim Craton but have different depositional ages, and that the amphibolites have similar crystallization ages but different tectonic settings for their source rocks. The field relationships and geochemical and geochronological characteristics of the metasedimentary rocks and amphibolites from the Beidahe Group support the interpretation that the observed recent proximity of these two rock complexes might be the result of tectonic juxtaposition rather than a primary intrusive contact.

Arc-generated metavolcanic rocks in the Anshan–Benxi greenstone belt, North China Craton: Constraints from geochemistry and zircon U–Pb–Hf isotopic systematics

The Anshan–Benxi granite-greenstone belt (ABGB) in the Eastern Block of the North China Craton mainly comprises supracrustal rocks of amphibole plagioclase gneisses, plagioclase amphibolites, chlorite schists, biotite schists, quartzites and banded iron formations (BIFs), and intrusive monzogranitic to syenogranitic gneisses. Based on geology, spatial distributions, lithologies and zircon U-Pb geochronology, the metavolcanic rocks of the southern ABGB can be divided into the ∼2527Ma southern plagioclase gneiss group, ∼2536Ma southern amphibolite group, ∼2574 to 2560Ma northern chlorite schist group and northern amphibolite group from south to north. These rocks outcrop as stratified and stratoid beds, whereas some occur as enclaves within later intrusive K-rich granitic gneisses. The southern plagioclase gneiss group exhibits calc-alkaline andesite compositions with high MgO, Cr, and Ni, fractionated REE patterns and depletion in Nb–Ta, which resemble high magnesian andesite (HMA) and are derived from the partial melting of sub-arc mantle peridotite that was previously metasomatized by slab melts. The southern amphibolite group has tholeiitic basalt compositions with slight enrichment in LREEs and high Nb–Ti concentrations but low (Nb/La)PM ratios, which coincide with Nb-enriched basalt and are generated by the remelting of residues of sub-arc mantle after the extraction of HMA melts. The northern schist group shows high-K calc-alkaline andesite-dacite compositions and have highest silica and potassium, and obvious enrichment in LREEs and LILEs and depletion in Nb–Ta, which are comparable to the sanukitoid series rocks. These rocks probably formed by low-degree partial melting of a sub-arc mantle wedge that was metasomatized by slab-derived fluids. The northern amphibolite group exhibits tholeiite compositions with unfractionated REE patterns and low (Nb/La)PM ratios, which are consistent with back-arc basin basalts (BABB) and are produced by partial melting of depleted mantle.Combined with lithological associations of the HMA, NEB, sanukitoid-like rocks and BABB-like rocks, the Anshan–Benxi granite-greenstone belt was produced under a convergent plate environment. Both the zircon εHf values and petrogenesis indicate that the four groups of the metamorphic volcanic rocks directly contributed to continental crustal growth. Integrating with the widespread ∼2.5Ga syenogranites and subordinate K-rich granitoids that were derived from reworking of ancient crust, we propose that the ABGB experienced a significant episode of continental growth dominated by a subduction-related lateral accretion accompanied with contemporaneously strong reworking of ancient crustal materials.

Geochemistry of metabasites from the North Shahrekord metamorphic complex, Sanandaj-Sirjan Zone: Geodynamic implications for the Pan-African basement in Iran

Metabasic rocks (amphibolites, garnet-amphibolites and eclogites) of the North Shahrekord Metamorphic Complex (NSMC) of the Sanandaj-Sirjan structural Zone in southwest Iran are part of a ductile shear zone forming lenses or blocks associated with orthogneiss, paragneiss and metagranite. Geochemical characteristics (immobile trace elements and Sr-Nd isotope ratios) of these metabasites reveal similar tholeiitic protoliths. The rocks are slightly altered during metamorphism but no evidence for sea floor alteration was found. Rare earth elements (REE) display relatively flat patterns or a slight enrichment of light REEs. The (La/Yb)cn ratios range from 1.42 to 4.91, the Nb/La ratio from 0.50 to 1.07, and the Sm/Nd ratio from 0.24 to 0.31 resembling to a continental back-arc basin. The (87Sr/86Sr)t isotopic ratios display a large range between 0.7041 and 0.7090, while 143Nd/144Nd(t) varies from 0.5120 to 0.5122 and between εNd(t) 2.5 and 7.2. Immobile trace element patterns and Sr-Nd isotopic characteristics reveal mixed sources of depleted mantle and enriched mantle type II with a contribution from a supra-subduction zone and some effects of crustal contamination. Therefore, the data indicates formation of metabasites in a supra-subduction zone setting during late Neoproterozoic to early Cambrian times. The generation and emplacement of OIB-like magmas in a back-arc environment is likely the result of rollback of oceanic subducting slab. The formation of NSMC Ediacaran to Cambrian basement rocks is approximately contemporary with other arc and back-arc basin basalts reported from Alborz, Central Iran and Turkey. These developed as a result of Proto-Tethys subduction beneath the Gondwana supercontinent during Ediacaran to early Cambrian times.

Geochemistry and isotopic signatures of metavolcanic and metaplutonic rocks of the Faina and Serra de Santa Rita greenstone belts, Central Brazil: Evidences for a Mesoarchean intraoceanic arc

The Archean-Paleoproterozoic Terrane of Goiás, Central Brazil, is an allochthonous block within the Neoproterozoic Tocatins Province and consists of an association of Archean TTG complexes and gold-bearing Archean-Paleoproterozoic greenstone belts. The Faina and Serra Santa Rita greenstone belts, located in the southern portion of the terrane, are investigated using geochemistry and isotope geology to establish the time of magmatism and tectonic environment. Our data show that the ultramafic rocks have some chemical characteristics similar to modern boninites, whereas the amphibolites are subdivided into two groups: the type 1 basalts group are tholeiites with flat REE patterns and are similar to back-arc basin basalts; the type 2 basalts group have high Nb contents and are comparable to Nb-enriched basalts. Felsic to intermediate rocks present some of the main chemical diagnostic features of adakites, in which the metandesites and metatonalites are comparable to high-SiO2 adakites, and the metadiorites, characterized by very high MgO, Cr and Ni contents, are comparable to low-SiO2 adakites or high-Mg andesites. Metavolcanic and metaplutonic rocks show two main periods of magmatic crystallization ages with juvenile and slightly crustal contaminated rocks. The first occurred at 2.96–2.92Ga with positive ƐNd (t) values of +2.16 to +2.77, while the second formed at 2.8Ga with slightly negative ƐNd (t) value of −0.15. The volcanic and plutonic protoliths of the two greenstone belts were formed in an intraoceanic forearc-arc-back-arc system. The initial stage corresponds to ultramafic lava eruption in the forearc region of a proto-island arc, at 2.96Ga. The evolution of the island arc and subduction progression led to oceanic slab-melting and generation of adakites. At 2.92Ga, the adakitic melt was totally consumed by peridotite mantle and the subsequent melting of these hybridized mantle wedge generated high-Mg andesites that lodged in the crust as dioritic intrusions with high MgO, Cr and Ni contents. The late-stage corresponds to a continental arc formation at 2.8Ga, marked by tonalitic magmatism and amalgamation with other island arcs and continental arcs of the TTG complexes of the Archean-Paleoproterozoic Terrane of Goiás.

Petrogenesis of meta-volcanic rocks from the Maimón Formation (Dominican Republic): Geochemical record of the nascent Greater Antilles paleo-arc

Metamorphosed basalts, basaltic andesites, andesites and plagiorhyolites of the Early Cretaceous, probably pre-Albian, Maimón Formation, located in the Cordillera Central of the Dominican Republic, are some of the earliest products of the Greater Antilles arc magmatism. In this article, new whole-rock element and Nd–Pb radiogenic isotope data are used to give new insights into the petrogenesis of the Maimón meta-volcanic rocks and constrain the early evolution of the Greater Antilles paleo-arc system. Three different groups of mafic volcanic rocks are recognized on the basis of their immobile element contents. Group 1 comprises basalts with compositions similar to low-Ti island arc tholeiites (IAT), which are depleted in light rare earth elements (LREE) and resemble the forearc basalts (FAB) and transitional FAB-boninitic basalts of the Izu-Bonin-Mariana forearc. Group 2 rocks have boninite-like compositions relatively rich in Cr and poor in TiO2. Group 3 comprises low-Ti island arc tholeiitic basalts with near-flat chondrite-normalized REE patterns. Plagiorhyolites and rare andesites present near-flat to subtly LREE-depleted chondrite normalized patterns typical of tholeiitic affinity. Nd and Pb isotopic ratios of plagiorhyolites, which are similar to those of Groups 1 and 3 basalts, support that these felsic lavas formed by anatexis of the arc lower crust.Geochemical modelling points that the parental basic magmas of the Maimón meta-volcanic rocks formed by hydrous melting of a heterogeneous spinel-facies mantle source, similar to depleted MORB mantle (DMM) or depleted DMM (D-DMM), fluxed by fluids from subducted oceanic crust and Atlantic Cretaceous pelagic sediments. Variations of subduction-sensitive element concentrations and ratios from Group 1 to the younger rocks of Groups 2 and 3 generally match the geochemical progression from FAB-like to boninite and IAT lavas described in subduction-initiation ophiolites. Group 1 basalts likely formed at magmatic stages transitional between FAB and first-island arc magmatism, whereas Group 2 boninitic lavas resulted from focused flux melting and higher degrees of melt extraction in a more mature stage of subduction. Group 3 basalts probably represent magmatism taking place immediately before the establishment of a steady-state subduction regime. The relatively high extents of flux melting and slab input recorded in the Maimón lavas support a scenario of hot subduction beneath the nascent Greater Antilles paleo-arc. Paleotectonic reconstructions and the markedly depleted, though heterogeneous character of the mantle source, indicate the rise of shallow asthenosphere which had sourced mid-ocean ridge basalts (MORB) and/or back-arc basin basalts (BABB) in the proto-Caribbean domain prior to the inception of SW-dipping subduction. Relative to the neighbouring Aptian-Albian Los Ranchos Formation, we suggest that Maimón volcanic rocks extruded more proximal to the vertical projection of the subducting proto-Caribbean spreading ridge.

A Neoarchean subduction recorded by the Eastern Hebei Precambrian basement, North China Craton: Geochemical fingerprints from metavolcanic rocks of the Saheqiao-Shangying-Qinglong supracrustal belt

The Saheqiao-Shangying-Qinglong supracrustal belt (SSQB) in the northern Eastern Hebei Precambrian basement (EHPB) is located in the northern margin of the Eastern Block (EB) of the North China Craton (NCC). The Shangying terrane constitutes the middle segment of the SSQB and contains primarily metamorphic volcanics and plutonic tonalitic gneisses. The metamorphic volcanics mainly consist of pyroxene plagioclase amphibolites, garnet plagioclase amphibolites, biotite plagioclase amphibole gneisses, and amphibole plagioclase gneisses. Zircon U-Pb-Lu-Hf isotopic analyses reveal that the metavolcanic rocks from the Shangying terrane crystallized at ∼2506–2613Ma with TDM (Hf) values of ∼2541–2944Ma. These metamorphic volcanic rocks are subdivided into four groups based on their lithological and chemical features. Group I consists chiefly of tholeiites that are characterized by slightly light rare earth element (LREE) depleted patterns and flat multi-element spider diagrams, which are similar to back-arc basin basalt (BABB)-like rocks and were derived from the partial melting of the depleted mantle. The tholeiites in Group II have slightly fractionated rare earth element (REE) patterns without Nb anomalies, exhibit an affinity to Nb-enriched basalt (NEB)-like rocks, and were produced by the partial melting of HFSE-enriched mantle peridotites. Group III is composed of slightly LREE-enriched tholeiites with negative Nb-Ta anomalies that resemble island arc tholeiites. Group IV comprises calc-alkaline basalts and andesites with highly enriched LREEs and evident Nb, Ta and Ti depletions that are geochemically similar to the products of island arcs. The island arc tholeiites and calc-alkaline basalt-andesites originated from the partial melting of sub-arc mantle peridotites that were previously metasomatized by slab-derived fluids/melts with the fractional crystallization of ferromagnesian minerals. Collectively, the BABB-like rocks, the NEBs, arc tholeiites and calc-alkaline basalt-andesites recorded magmatism from a subduction regime. Similarly, NEB, arc tholeiite-andesite-dacites and back-arc related rocks were recognized in the Saheqiao and Qinglong areas in the western and eastern parts of the SSQB (R. Guo et al., 2013, 2015), which further supports the existence of a Neoarchean convergent margin in the Eastern Hebei Precambrian basement (EHPB). Compared to the northwestern EHPB, the southeastern EHPB underwent lower metamorphism, contains arc dacites/rhyolites and greater contributions of the ancient continental crust, implying that the southeastern EHPB might have been or near a back-arc setting. Thus, we conclude that the SSQB experienced a southeastward-dipping subduction, which is consistent with studies of the adjacent regions (e.g., Western Liaoning Province). When combining the ∼2.5–2.6Ga supracrustal rocks and plutonic granitoid gneisses with depleted mantle origins along the northern margin of the EB of the NCC, we concluded that ∼2.5–2.6Ga was a significant period for juvenile crustal growth and that lateral accretion played a dominant role in crustal growth.

Chromian spinels from the Magura Unit (Western Carpathians, Eastern Slovakia) – their petrogenetic and palaeogeographic implications

Detrital chromian spinels in sedimentary rocks provide much information concerning the tectonics of their parental ultrabasic rocks. Chromian spinels occurring in the Eocene to Oligocene deposits from the Magura Nappe were examined to provide some constraints on the history of the Magura Basin. The Magura Nappe is a part of the Flysch Belt belonging to the External Western Carpathians. The Magura Nappe is separated by a narrow zone associated with the Pieniny Klippen Belt and is divided into three principal tectono-lithofacies units (from the S to N): the Krynica, Bystrica and Rača units. Cr-spinel is a common accessory mineral (2.3–5.9 vol% of heavy mineral spectra) in the siliciclastic rocks of the Rača and Krynica units. In terms of texture and chemical composition, two types of Cr-spinels were recognized: unaltered and altered. Unaltered spinels were found to contain silicate inclusions such as chromio-pargasite, enstatite, diopside, pargasite, plagioclase and olivine (forsterite). The chromian spinels show wide variations in compositional parameters such as Cr# (0.3–0.7), Mg# (0.3–0.7), TiO2 (<0.03–1.9 wt.%) and Fe 2+ /Fe 3+ (2.5–13) whereas the differences between the Rača and Krynica units are insignificant. These parameters suggest a peridotitic and volcanic origin of the spinels, respectively. The ophiolite source consisting of harzburgitic mantle peridotites was developed mainly in a supra-subduction zone setting; volcanic spinels indicate an origin in mid-ocean ridge basalts, back-arc basin basalts and sporadically in ocean-island basalts. Concerning their geochemical features, we propose that during the Eocene to Early Oligocene, the ophiolitic detritus in the eastern part of the Magura Basin deposits may have been derived from a source area located in the Fore-Marmarosh Suture Zone (Eastern Carpathians) that is considered an equivalent of the Black Flysch and Ceahlau units. Some Cr-spinels found in the Eocene sedimentary successions may have resedimented from older Late Cretaceous-Paleocene formations of the Magura Unit, which are considered as reworked sedimentary material from the Pieniny Klippen Belt.

Petrogenesis of the Dalongkai ultramafic-mafic intrusion and its tectonic implication for the Paleotethyan evolution along the Ailaoshan tectonic zone (SW China)

Layered ultramafic-mafic intrusions are usually formed in an arc/back-arc or intra-plate tectonic environment, or genetically related to a mantle plume. In this paper, we report on an ultramafic-mafic intrusion, the Dalongkai intrusion in the Ailaoshan tectonic zone (SW China), whose occurrence is closely associated with arc/back-arc magmatic rocks. The Dalongkai intrusion is composed of plagioclase-lherzolite, hornblende-peridotite, lherzolite and wehrlite at the bottom, cumulate plagioclase-pyroxenite at the middle part, changing to fine-grained gabbro towards the upper part of the intrusion, forming layering structure. Zircons from the plagioclase-pyroxenites and gabbros yielded U-Pb ages of 272.1±1.7Ma and 266.4±5.8Ma, respectively. The plagioclase-pyroxenites show cumulate textures, and are characterized by high MgO (25.0–28.0wt.%; mg#=80.6–82.3), Cr (1606–2089ppm) and Ni (893–1203ppm) contents, interpreted as early cumulate phases. By contrast, the gabbros have relatively lower mg# values (56.3–62.7), and Cr (157–218ppm) and Ni (73–114ppm) concentrations, and may represent frozen liquids. The plagioclase-pyroxenites and gabbros share similar chondrite-normalized REE patterns and primitive mantle-normalized trace element profiles which are analogous to those of typical back-arc basin basalts. The εNd(t) values for both rock types range from +2.20 to +4.22. These geochemical and isotopic signatures suggest that the Dalongkai ultramafic-mafic rocks originated from a MORB-like mantle source metasomatized by subduction-related, sediment-derived fluids. Our data, together with other geological evidence, indicate that the emplacement of the Dalongkai ultramafic-mafic intrusion most likely occurred in a back-arc extensional setting associated with subduction of the Ailaoshan Paleotethyan branch ocean during the Middle Permian, thus ruling out the previously speculated linkage to the Emeishan mantle plume, or to an intra-continental rift.

Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: Examples at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and other localities

The felsic-dominant Gawler Range Volcanics and cogenetic Hiltaba Suite granitoids constitute the ca. 1590Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. The province includes minor occurrences of olivine-phyric basalt at Kokatha and Mount Gunson. In this study, we describe additional olivine-phyric basalts and dykes intersected by drill holes at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and the Wirrda Well Cu–Au prospect. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb dating results (un-anchored) of apatite in the basalts and dykes at Mount Gunson (1576±33Ma), Wirrda Well (1596±17Ma), and Olympic Dam (1621±20Ma) confirm their effective temporal correlation with the ca. 1590Ma Gawler silicic large igneous province.Compositions of Cr-spinel inclusions enclosed in former olivine phenocrysts (forsterite number >80) in basalts at Mount Gunson, Wirrda Well and Olympic Dam imply derivation from a heterogeneous mantle source that may have been modified by subduction. Least-altered olivine-phyric basalts at Mount Gunson and Kokatha are characterized by negative Nb and Ta anomalies, which are typical of arc basalts but are also common in back-arc basin basalts, in accordance with the result suggested by the Cr-spinel source indicator. The mafic components of the Gawler Range Volcanics generally have higher Zr contents and Zr/TiO2 ratios than those of high-Mg basalts and picrites produced in variable tectonic settings worldwide, possibly reflecting continental crustal components involved in their mantle source.High abundances (typically ∼20vol.%) of former olivine phenocrysts in the basalt at Olympic Dam imply high-Mg whole-rock compositions and a high temperature of the primary magma. This result is consistent with previous models in which the heat flux from mantle magmas caused large-scale partial melting of crustal rocks that subsequently gave rise to silicic magmas erupted and intruded to form the Gawler silicic large igneous province.

Petrogenesis and Geodynamic Evolution of the Paleoproterozoic (~1878 Ma) Trout Lake Volcanogenic Massive Sulfide Deposit, Flin Flon, Manitoba, Canada

The Trout Lake deposit (1878 ± 1 Ma) is a rhyolite-hosted, bimodal-mafic, volcanogenic massive sulfide (VMS) deposit. It is one of the largest VMS deposits in the Flin Flon mining district, containing 21.8 Mt of Cu-Zn rich ore. The deposit underwent greenschist facies metamorphism and polyphase synvolcanic to postvolcanic hydrothermal alteration. Footwall and hanging-wall lithostratigraphic units of the Trout Lake deposit occur as an upright, steeply NE dipping, homoclinal succession of thrust-repeated volcanic and siliciclastic sedimentary units. The deposit consists of several massive sulfide ore lenses hosted within a felsic volcanic unit that represents a localized felsic eruptive center composed of flows, synvolcanic sills, and volcaniclastic lithofacies with an FIIIb-type tholeiitic to transitional rhyolite composition (Zr = 213–385 ppm; Ybcn = 47–100; La/Ybcn = 0.97–2.33). The felsic unit occurs along strike of an andesitic unit comprising an intercalation of high-MgO andesite (MgO > 5.0 wt %) and Nb-enriched andesite (Nb = 10–13 ppm) flows that are underlain by a footwall basaltic unit composed of flow lithofacies with a low-Ti tholeiitic basaltic composition (TiO2 < 0.48 wt %; Zr = 15–17 ppm; Al2O3/TiO2 = 37–45). Postvolcanic gabbros with back-arc basin basalt composition intrude the volcanic units, which are unconformably covered by a younger argillite unit deposited at ca. <1843 ± 9 Ma. The felsic volcanic and argillite units were repeated and structurally interleaved during west-directed thrusting. The presence of low-Ti tholeiitic basalts indicates high-temperature melting (1,200°–1,500°C) of strongly depleted refractory mantle sources (Nb/Yb 1.45) variably affected by slab-derived hydrous melts. These petrologic characteristics suggest a geochemically heterogeneous mantle wedge that underwent metasomatism by slab-derived components. In addition, zircon saturation thermometry indicates that FIIIb-type rhyolites formed at temperatures >900°C during partial melting of mafic lower crust. Collectively, the occurrence of high-temperature melts indicates that the Trout Lake VMS deposit formed in a hot, intraoceanic, extensional-arc geodynamic setting. Intra-arc rifting, asthenospheric upwelling, and subduction of young and hot oceanic crust are mechanisms that explain the high-temperature magmatism and are features consistent with the formation of the massive sulfide ore lenses. The back-arc basalt geochemical signatures of the postvolcanic gabbros suggest that the tectonic environment of the Trout Lake area evolved from an intra-arc rift, during the deposition of the ore lenses, to an incipient back-arc basin during the intrusion of the gabbros.

Ca. 830 Ma back-arc type volcanic rocks in the eastern part of the Jiangnan orogen: Implications for the Neoproterozoic tectonic evolution of South China Block

The volcanic rocks in the eastern part of Jiangnan orogen provide an important suite to investigate magma petrogenesis and tectonic evolution of South China Block. We carried out a combined study of geochronology and geochemistry for the bimodal association of basalts and rhyolitic tuffs from the Shuangqiaoshan Group in south Anhui Province. Zircons from a basalt and two rhyolitic tuff samples yield crystallization ages of 829±12Ma, 832±9Ma and 832±10Ma, respectively. The basalts are characterized by left-sloping REE distribution patterns, enriched large iron lithosphere elements (LILEs, e.g. Rb, Ba, K) and depleted high field strength elements (HFSEs, e.g. Nb, Ta) relative to those in N-MORB, resembling those of modern Central Lau back-arc basin basalts (BABB). In addition, they have positive ɛNd(t) values (+4.6 to +5.7) with initial 87Sr/86Sr ratios in the range of 0.70706–0.70719. These geochemical and isotopic signatures indicate that the basalts were derived from an N-MORB-like source modified by slab-derived fluids. The rhyolitic tuffs have high SiO2 (72.9–78.3wt %) contents and high 10,000*Ga/Al ratios (mostly >2.6). Taking into account their aluminous nature (A/CNK>1.1), they are affinitive to aluminous A-type granites. They invariably show a relative enrichment in light-REE, and strong negative Eu anomalies (Eu/Eu*=0.16–0.31) and negative anomalies in HFSE, Sr and P compared with the neighboring elements in their trace element spidergrams. Their ɛNd(t) values range from −2.1 to +0.1. Detailed elemental and isotopic data suggest that the rhyolitic tuffs formed by partial melting of mafic lower crust triggered by asthenosphere upwelling. The occurrence of bimodal volcanic rocks with the basalts showing geochemical affinity to intra-oceanic BABB appears to support the development of a Neoproterozoic back-arc basin in the eastern part of Jiangnan orogen at ca. 830Ma. Combined with the published data from central and western part of Jiangnan orogen, it is inferred that the back-arc basin developed in the entire Jiangnan orogen during 850–810Ma. Considering the age of the strata overlying the folded basement sequence, we suggest that the final collision of the Yangtze and Cathaysia blocks happened during 820–810Ma along the Jiangnan orogen.

The water content and hydrogen isotope composition of continental lithospheric mantle and mantle-derived mafic igneous rocks in eastern China

The water contents of minerals and whole-rock in mantle-derived xenoliths from eastern China exhibit large variations and are generally lower than those from other on- and off-craton lithotectonic units. Nevertheless, the water contents of mineral and whole-rock in Junan peridotite xenoliths, which sourced from the juvenile lithospheric mantle, are generally higher than those elsewhere in eastern China. This suggests that the initial water content of juvenile lithospheric mantle is not low. There is no obvious correlation between the water contents and Mg# values of minerals in the mantle xenoliths and no occurrence of diffusion profile in pyroxene, suggesting no relationship between the low water content of mantle xenolith and the diffusion loss of water during xenolith ascent with host basaltic magmas. If the subcontinental lithospheric mantle (SCLM) base is heated by the asthenospheric mantle, the diffusion loss of water is expected to occur. On the other hand, extraction of basaltic melts from the SCLM is a more efficient mechanism to reduce the water content of xenoliths. The primary melts of Mesozoic and Cenozoic basalts in eastern China have water contents, as calculated from the water contents of phenocrysts, higher than those of normal mid-ocean ridge basalts (MORB). The Mesozoic basalts exhibit similar water contents to those of island arc basalts, whereas the Cenozoic basalts exhibit comparable water contents to oceanic island basalts and backarc basin basalts with some of them resembling island arc basalts. These observations suggest the water enrichment in the mantle source of continental basalts due to metasomatism by aqueous fluids and hydrous melts derived from dehydration and melting of deeply subducted crust. Mantle-derived megacrysts, minerals in xenoliths and phenocrysts in basalts from eastern China also exhibit largely variable hydrogen isotope compositions, indicating a large isotopic heterogeneity for the Cenozoic SCLM in eastern China. The water content that is higher than that of depleted MORB mantle and the hydrogen isotope composition that is deviated from that of depleted MORB mantle suggest that the Cenozoic continental lithospheric mantle suffered the metasomatism by hydrous melts derived from partial melting of the subducted Pacific slab below eastern China continent. The metasomatism would lead to the increase of water content in the SCLM base and then to the decrease of its viscosity. As a consequence, the SCLM base would be weakened and thus susceptible to tectonic erosion and delamination. As such, the crust-mantle interaction in oceanic subduction channel is the major cause for thinning of the craton lithosphere in North China.

Al-and Cr-rich chromitites from the Eastern Havana-Matanzas ophiolites (Western Cuba)

Chromite deposits in the Havana-Matanzas ophiolites of Western Cuba include both Al- ([Cr/(Cr+Al)=0.44-0.46]) and Cr-rich ([Cr/(Cr+Al) = 0.65-0.74] chromitites that commonly occur in the upper part of mantle ectonites near the mantle-crust transition zone. These chromitites generally form small lenticular bodies spatially associated with strongly serpentinized dunite. Platinum group minerals (PGM) are found only in the Cr-rich chromitites, as euhedral or subeuhedral, very small (less than 10 μm long) crystals. The most abundant PGM is laurite, accompanied by minor osmium, cuproiridsite, iridium and an unnamed phase composed of Ir-Ni-Fe-S, all of which are magmatic in origin. These magmatic PGMs and amphiboles crystallized at high temperatures (~1300 °C) and at relatively high sulphur fugacities from fluid-rich magmas. The chromite mineral chemistry suggests that melts that were in equilibrium with Cr-rich and Al-rich chromitites had island-arc tholeiite - boninite and backarc basin basalt (BABB) affinities, respectively.

Geochemical constraints on the protoliths of eclogites and blueschists from North Qilian, northern Tibet

An integrated study of whole-rock geochemistry, mineral O isotope geochemistry and zirconology was carried out for low-temperature eclogites and blueschists from the North Qilian orogen in northern Tibet. The results demonstrate that these oceanic-type eclogites and blueschists were produced by metamorphism of backarc basin basalts rather than mid-ocean ridge basalts as commonly thought. These high pressure metabasites show significant heterogeneity in their major and trace element compositions, varying from island arc basalts-like, mid-ocean ridge basalts-like to oceanic island basalts-like, respectively, in trace element distribution diagrams. Such variable compositions are comparable with those of backarc basin basalts at different stages. Whole-rock Nd isotope analyses indicate that the compositional difference between the metabasite protoliths can be ascribed to involvement of different amounts of subducted sediment-derived melts in the backarc mantle. Oxygen isotope fractionations between coexisting minerals are not at equilibrium; garnet O isotope analyses indicate that whole-rock δ18O values are either higher or lower than normal mantle value, which is attributable to seawater-hydrothermal alteration during seafloor spreading to open the backarc basin. Relict magmatic zircon domains show oscillatory zoning, high Th and U contents, high Th/U ratios, and steep HREE patterns, with protolith U–Pb ages of 486–496Ma. Metamorphic zircon domains exhibit lowered Th, U and HREE contents and Th/U ratios than the relict magmatic domains and contain mineral inclusions of omphacite, rutile and phengite, giving concordant U–Pb ages of 463±10Ma for eclogite-facies metamorphism. Zircons have markedly depleted Hf isotope compositions with Hf model ages slightly or significantly older than the protolith ages, suggesting incorporation of crustal components into the magma sources of these metabasite protoliths. Most zircons have δ18O values different from normal mantle values, and the distinct zircon O isotopes in different samples are in accordance with garnet δ18O values for the same sample, pointing to a localized fluid source for the metamorphism. Therefore, the different metabasites with different compositions were metamorphosed from different basaltic rocks generated in a backarc basin in the early Paleozoic. Their occurrence in the same region indicates that a backarc basin was transformed to an oceanic subduction zone where eclogite-facies metamorphism occurred.

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.

Early Neoproterozoic multiple arc–back-arc system formation during subduction–accretion processes between the Yangtze and Cathaysia blocks: New constraints from the supra-subduction zone NE Jiangxi ophiolite (South China)

The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic mélange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous greywacke. A combined geochemical and geochronological study of the NE Jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation. The basalts were geochemically subdivided into three groups with different FeOt and TiO2 contents. Group 1 basalts have the lowest FeOt (12.17–13.07 wt.%) and TiO2 (1.48–1.62 wt.%) contents and the lowest Nb/Yb (0.80–0.88) and Th/Nb (0.02–0.03) ratios. Furthermore, they have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns, suggesting derivation from an N-MORB-type mantle source without subduction input. Group 2 Fe–Ti basalts have the highest FeOt (15.52–16.30 wt.%) and TiO2 (3.06–3.23 wt.%) contents, Nb/Yb and Th/Nb ratios from 1.75 to 1.89 and from 0.11 to 0.15, respectively, and trace element patterns similar to those of back-arc basin basalts. The geochemical characteristics suggest that Group 2 basalts were derived from a slightly enriched MORB-type mantle source with a minor subduction contribution. In contrast, Group 3 Fe–Ti basalts have moderate FeOt (12.98–13.40 wt.%) and TiO2 (2.37–2.71 wt.%) contents, and Nb/Yb and Th/Nb ratios from 1.28 to 1.45 and from 0.27 to 0.30, respectively. These basalts further display markedly negative Nb–Ta anomalies and show a geochemical affinity to island-arc basalts (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U–Pb dating on gabbros gave ages of 995±22Ma and 993±12Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon εHf(t) (+8.8 to +13.8) values for the gabbros and whole-rock εNd(t) (+5.5 to +6.6) values for the basalts indicate that the NE Jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe–Ti basalts favor a formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990Ma. Both the present and previous studies imply that multiple arc–back-arc systems formed during long-lasting subduction–accretion processes between the Yangtze and Cathaysia blocks during the early Neoproterozoic.

Late Triassic island-arc–back-arc basin development along the Bangong–Nujiang suture zone (central Tibet): Geological, geochemical and chronological evidence from volcanic rocks

A major debate related to the evolution of the Tibetan Plateau is centered on whether or not an island arc–back-arc basin system occurred along the Bangong–Nujiang suture zone, central Tibet. Here we present new zircon U–Pb geochronology, rare earth elements (REEs) and bulk-rock geochemistry of these magmatic rocks in the Amdo area, the middle Bangong–Nujiang suture zone, central Tibet, to identify significant and new records of Mesozoic tectonomagmatic processes. Zircon U–Pb dating using LA-ICP-MS techniques yields a concordant age with a weighted mean 206Pb/238U age of 228.6±1.6Ma (n=7, MSWD=1.19) for the Quehala basalts, and a mean age of 220.0±2.1 (n=8, MSWD=1.5) for the Amdo pillow lavas. On the normalized REE patterns of zircon, significant Ce enrichment indicates that the magma sources of these magmatic rocks have been subjected to modification of slab-derived fluid. Geochemical features suggest that the Quehala basalts (ca. 228Ma), displaying an island arc tholeiites (IATs) affinity, resulted from partial melting of an relatively enriched mantle wedge in the subduction zone, whereas the Amdo pillow lavas (ca. 220Ma) characterized by both arc-like and N-MORB-like geochemical characteristics occurred as associated back-arc basin basalts (BABBs) at the spreading center of back-arc basin after the formation of island arc tholeiites. In conclusion, the volcanic rocks in the Amdo area have documented the magmatic processes from early-stage subduction to development of associated back-arc basin, confirming the occurrence of intra-oceanic subduction within the Bangong–Nujiang Tethys during the late Triassic. Furthermore, the spatial relationships among the Quehala formation, Tumengela formation and Amdo pillow lavas indicate likely northward subduction of the Bangong–Nujiang Tethyan Ocean during the Late Triassic to middle Jurassic.