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•Contribution of pelagic and terrigenous sediments to subarc mantle enrichment.•Continental fragments likely occur beneath both fossil and modern intra-oceanic arcs.•Continental ...fragments facilitate subduction initiation.•Continental fragments also account for continental signature of young island arc magmas.
Intra-oceanic arcs are potential candidates that generate Earth’s continental crust, and serve as active sites for the mixing of juvenile arc and mature continental material. However, whether continental fragments occur beneath intra-oceanic arcs, and how continental materials are involved in island arc magmas remain equivocal. Here we report zircon U-Pb-Hf-O isotopes and whole-rock elemental and Sr-Nd isotopic data on the Silurian island arc gabbro from the southeastern Jiamusi Block in NE China. The gabbro was emplaced at ca. 435–429 Ma associated with the northward intra-oceanic subduction of the Paleo-Asian Ocean, and belongs to the eastern extension of the Bainaimiao arc system. Elevated zircon δ18O (+5.5‰ to +7.9‰) and variable εHf(t) (-2.1 to +4.0) values, and negative whole rock εNd(t) (-1.1 to −1.6) values, suggest an enriched mantle component generated by pelagic and terrigenous sediments input into the mantle wedge. The contribution of terrigenous sediments, together with the recognition of Paleoproterozoic crustal rocks, proves for the first time that continental fragments existed beneath the intra-oceanic arc. In conjunction with published data, we propose that that Tarim Craton-type scattered continental fragments occurred within the Bainaimiao arc system, and that they not only facilitated the initiation of oceanic-oceanic subduction through increasing buoyancy, but also account for the old continental zircon and enriched isotopic variations of the young arc rocks. Our study highlights a fossil example that could help better quantify the scale and role of continental fragments in the origin and evolution of intra-oceanic arcs.
•The metabasic rocks in Cathaysia yield U–Pb ages of 969–984Ma and TDM(Hf) of 0.92–1.44Ga.•They originated from a subduction-modified MORB-like source linked to an arc–back-arc setting.•The South ...China Block was created by episodic amalgamation of a series of arc fragments between ∼970 and 820Ma.•The South China Block is an exterior accretionary orogen along the periphery of Rodinia rather than in the interior.
U–Pb geochronology along with elemental and Nd–Hf–Os isotopic data from the earliest Neoproterozoic metabasic rocks within the Cathaysia Block of the South China Block (SCB) constrain the tectonic setting and paleogeography of the block within the Rodinia supercontinent. The metabasic rocks give zircon U–Pb ages of 969–984Ma, ɛHf(t) values of +1.8 to +15.3 and Hf model ages of 0.92–1.44Ga. They are subalkaline basalts that can be geochemically classified into four groups. Group 1 has low Nb contents (1.24–4.33ppm), highly positive ɛNd(t) values (+4.3 to +5.2), and REE and multi-elemental patterns similar to fore-arc MORB-type basalt. Group 2 has Nb contents ranging from 3.13ppm to 6.48ppm, ɛNd(t) of +3.1 to +6.2, low Re and Os contents and high initial Os isotopic ratios, and displays an E-MORB geochemical signature. Group 3 has Nb=7.18–29.87ppm, Nb/La=0.60–1.40, Nb/U=5.0–37, Ce/Pb=1.1–6.6, ɛNd(t)=+2.9 to +7.0, 187Re/188Os=5.87–8.87 and γOs (t)=178–772, geochemically resembling to the Pickle Nb-enriched basalt. Group 4 has strong LREE/HREE and HREE fractionation and high ɛNd(t) values (+2.3 to +5.6), and is characterized by similar element patterns to arc volcanic rocks. Serpentinites coeval to Group 4 show 187Os/188Os of 0.1143–0.1442 and γOs (t) of −7.8 to +0.1. Groups 1 and 2 are interpreted to originate from the N-MORB and E-MORB-like sources with the addition of an arc-like component, genetically linked to fore- and back-arc settings, respectively. Groups 3 and 4 show inputs of newly subduction-derived melt and fluid in the wedge source. These geochronological and geochemical signatures fingerprint the development of an earliest Neoproterozoic (∼970Ma) arc–back-arc system along the Wuyi-Yunkai domain of the Cathaysia Block. Regional relationships indicate that the Wuyi-Yunkai arc–back-arc system was one of a series of separate convergent margin settings, which included the Shuangxiwu (∼970–880Ma) and Jiangnan (∼870–820Ma) systems that developed in the SCB. The formation and closure of these arc–back-arc systems resulted in the northwestwardly episodic amalgamation of various pieces of the Yangtze and Cathaysia to finally form the SCB. These signatures require the SCB to occupy an exterior accretionary orogen along the periphery of Rodinia during 990–820Ma, rather than to have formed through Mesoproterozoic Sibao orogenesis within the interior of Rodinia.
Archean crust–mantle interaction processes provide keys to understanding crustal evolution in the Early Earth. The North China Craton (NCC) represents a natural laboratory to evaluate early ...Precambrian crustal evolution. The tectonic framework of the NCC is composed of two major crustal blocks, the Western Block (WB) and the Eastern Block (EB). The northwestern margin of the EB preserves voluminous metamorphosed supracrustal basaltic to andesitic rocks of ca. 2522–2640Ma in the Fuxin greenstone belt within the Western Liaoning Province (WLP). These were intruded by the magmatic precursors of ~2495–2521Ma granitoid gneisses, which were generally metamorphosed under ca. 2485Ma regional metamorphism up to granulite facies, followed by ca. 2401–2450Ma amphibolite facies retrogression. Geochemically, the metamorphosed volcanic rocks can be subdivided into five groups with affinities to MORBs, island arc tholeiites (IATs), calc-alkaline basalts (CABs), high-magnesium andesites (HMAs), and adakites. The petrogenesis and temporal–spatial distribution of these metamorphosed volcanic rocks and the intrusive granitoid gneisses in the WLP indicate that this basement terrane probably evolved as an intra-oceanic arc system, with major crustal growth at ca. 2.5–2.6Ga. These rocks record complex crust–mantle evolution history as follows. Partial melting of the depleted to slightly enriched asthenospheric mantle at a spreading ridge occurred at ~2600–2640Ma, generating both MORB-like rocks and juvenile oceanic lithospheric mantle sources. The mantle sources were gradually metasomatized by slab-derived fluids and melts during ~2506–2550Ma, and the partial melting of both the metasomatized lithospheric mantle and the descending oceanic slabs generated IATs, CABs, HMAs, and adakites (as well as tonalite–trondhjemite–granodiorite (TTG) gneisses of the high-magnesium group). Meanwhile, the coeval underplating of mantle-derived materials triggered the partial melting of the arc-root materials, yielding TTG gneisses of the low-magnesium group. Finally, the accretion of the intra-oceanic arc terrane to the ancient nucleus of the EB at ca. 2490Ma resulted in the regional granulite-facies metamorphism and crustal anatexis (resulting in the formation of potassium-rich granitoid rocks), possibly related to asthenospheric upwelling.
Based on a comprehensive analysis of the basement terranes in the EB, a late Neoarchean (~2.5–2.6Ga) intra-oceanic arc system with relict MORB-like basaltic rocks bordering the northwestern margin of the continental nucleus (ca. 2.7–3.8Ga) was established, extending from northern Liaoning Province, through western Liaoning Province, the Zunhua–Qinglong block of eastern Hebei Province, northern Hebei Province, and the Huai'an–Xuanhua complexes of northwestern Hebei Province, up to the Wutai complex. To the northwest of the proposed “Wutai–Zunhua–Majuanzi” boundary, the intra-oceanic arc system is characterized by the general lack of ancient crystalline basement (≥2.7Ga), and late Neoarchean metamorphosed basaltic rocks showing geochemical affinities to MORBs–IATs–CABs represent the oldest rocks yet identified.
Accordingly, the development of both plume-related komatiites and metamorphosed arc-related calc-alkaline volcanic rocks as well as granitoid gneisses within the interior of the EB to the east suggest early Neoarchean (ca. 2.7Ga) plume–craton and plume–arc interactions as the major crust–mantle geodynamic processes. In contrast, late Neoarchean (ca. 2.5–2.6Ga) slab–wedge interaction and arc–continent accretion processes are compatible with the intra-oceanic arc system as well as the asymmetric distribution of the basement rocks.
The Cadomian Orogeny produced a subduction‐related orogen along the periphery of Gondwana and configured the pre‐Variscan basement of the Iberian Massif. The architecture of the Cadomian Orogen ...requires detailed structural analysis for reconstruction because of severe tectonic reworking during the Paleozoic (Variscan cycle). Tectonometamorphic analysis and data compilation in SW Iberia (La Serena Massif, Spain) have allowed the identification of three Cadomian deformation phases and further constrained the global architecture and large‐scale processes that contributed to the Ediacaran building and early Paleozoic dismantling of the Cadomian Orogen. The first phase (DC1, prior to 573 Ma) favored tabular morphology in plutons that intruded during the building of a continental arc. The second phase (DC2, 573–535 Ma) produced an upright folding and contributed to further crustal thickening. The third phase of deformation (DC3, ranging between ∼535 and ∼480 Ma) resulted in an orogen‐parallel dome with oblique extensional flow. DC1 represents the crustal growth and thickening stage. DC2 is synchronous with a period of crustal thickening that affected most of the Gondwanan periphery, from the most external sections (Cadomian fore‐arc) to the inner ones (Cadomian back‐arc). We explain DC2 as a consequence of flat subduction, which was followed by a period dominated by crustal extension (DC3) upon roll‐back of the lower plate. The Ediacaran construction of the Cadomian Orogen (DC1 and DC2) requires ongoing subduction beneath Gondwana s.l., whereas its dismantlement during the Early Paleozoic is compatible with oblique, sinistral convergence.
Key Points
Three Cadomian deformation phases affected the upper plate of a peri‐Gondwanan arc
Crustal thickening and thinning resulted from flat subduction and roll back
The Cadomian Orogeny was ruled by sinistral plate convergence in Iberian Gondwana
•The 800-Ma Shimian mafic-ultramafic mélange is an SSZ-type ophiolite.•South China was not located in the centre of Rodinia.•A giant Andean-type arc system existed along the NW margin of Rodinia.
The ...configuration of the supercontinent Rodinia has long been a matter of debate; the key controversy is the position of South China in Rodinia. We report an incomplete Neoproterozoic ophiolite sequence, including gabbros and serpentinized peridotites intruded by mafic dikes, near Shimian (Sichuan Province), along the western margin of the Yangtze Block in South China. Serpentinized peridotites have very low REE (0.14–1.16ppm) and trace element concentrations, and are interpreted as parts of a depleted mantle sequence. Chromites from the serpentinized peridotites have low TiO2 (<0.3wt%) and Fe2O3 (<7.6wt%), and moderate Cr/(Cr+Al) (0.48–0.67) and Mg/(Mg+Fe2+) (0.42–0.67) ratios and are clearly indicative of strong depletion of the host peridotites, similar to those of supra-subduction zone (SSZ). Mafic dikes and gabbros contain zircon grains with similar U-Pb ages of ∼800Ma, and are chemically akin to MORB-like tholeiitic basalts and boninite-series volcanic rocks reported in other SSZ ophiolites and in the Izu-Bonin-Mariana arc system. Therefore, the serpentinized peridotite, mafic dikes and gabbros together appear to form a SSZ-type ophiolite assemblage preserving the accretion of oceanic lithosphere to the western Yangtze Block. The age and location of the Shimian ophiolite rule out the possibility that South China was located in the centre of Rodinia. Instead, the Neoproterozoic arc-affinity igneous rocks at the western margin of the Yangtze Block are well correlated with those in Greater India and Madagascar. These three blocks thus together formed a giant Andean-type arc system along the NW margin of Rodinia that existed for more than 100Mys.
The tectonic setting in which Jurassic igneous rocks of the Sanandaj-Sirjan Zone (SaSZ) of Iran formed is controversial. SaSZ igneous rocks are mainly intrusive granodiorite to gabbroic bodies, which ...intrude Early to Middle Jurassic metamorphic basement; Jurassic volcanic rocks are rare. Here, we report the age and petrology of volcanic rocks from the Panjeh basaltic-andesitic rocks complex in the northern SaSZ, southwest of Ghorveh city. The Panjeh magmatic complex consists of pillowed and massive basalts, andesites and microdioritic dykes and is associated with intrusive gabbros; the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex. Igneous rocks belong to a metaluminous sub-alkaline, medium-K to high-K calc-alkaline mafic suite characterized by moderate Al2O3 (13.7–17.6wt%) and variable Fe2O3 (6.0–12.6wt%) and MgO (0.9–11.1wt%) contents. Zircon U-Pb ages (145–149Ma) define a Late Jurassic (Tithonian) age for magma crystallization and emplacement. Whole rock compositions are enriched in Th, U and light rare earth elements (LREEs) and are slightly depleted in Nb, Ta and Ti. The initial ratios of 87Sr/86Sr (0.7039–0.7076) and εNd(t) values (−1.8 to +4.3) lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle. Immobile trace element (Ti, V, Zr, Y, Nb, Yb, Th and Co) behavior suggests that the mantle source was enriched by fluids released from a subducting slab (i.e. deep-crustal recycling) with some contribution from continental crust for andesitic rocks. Based the chemical composition of Panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the Ghorveh area we offer two interpretations for these (and other Jurassic igneous rocks of the SaSZ) as reflecting melts from a) subduction-modified OIB-type source above a Neo-Tethys subduction zone or b) plume or rift tectonics involving upwelling metasomatized mantle (mostly reflecting the ~550Ma Cadomian crust-forming event).
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•Panjeh complex is a unique Jurassic submarine volcano in the Sanandaj-Sirjan Zone.•Panjeh mafic and andesitic rocks crystallized at 145Ma.•Panjeh melts came from mantle moderately affected by subduction-related fluids.•Both intra oceanic arc system and continental plume are considered for this complex.
The Kuching zone, the most important tectonic boundary in Borneo, separates the Sibu zone in Central Sarawak from the Schwaner Mountains granitoids in SW Borneo. It potentially provides a key window ...for probing the paleo-Pacific subduction process. This paper presents new geochronological, elemental and SrNd isotopic data for the Pakong-Serabang and Serian mafic rocks, along with detrital zircon UPb age-data for the associated greywacke in the Sarawak Kuching zone. These mafic rocks are dated at ~77–98 Ma and show distinctive geochemical signatures. The Pakong- Serabang mafic rocks are subalkaline basalt with SiO2 = 46.22–51.75 wt%, TiO2 = 0.49–2.42 wt% and MgO = 6.30–10.54 wt%. They exhibit MORB-like PM-normalized patterns with depletion in LILEs and HFSEs. Their 87Sr/86Sr(t) and εNd(t) range from 0.70285 to 0.70728 and + 7.4 to +9.4, respectively, originating from a slab-derived fluid-modified MORB-like source. The late Cretaceous Serian Volcanics are marked by SiO2 of 51.52–57.90 wt%, MgO of 3.50–5.06 wt% with mg-number = 36–46 and show arc-like geochemical signatures with (Nb/La)n = 0.37–0.47. They have 87Sr/86Sr(t) = 0.71188–0.71417 and εNd(t) = −11.1 ~ −7.3, and were derived from a mantle wedge with the input of recycled sedimentary components. The detrital zircon grains from the associated sandstones give similar age-spectra to those in East Peninsular Malaysia and SE Vietnam. Our data revealed a Cretaceous Kuching trench-arc system. In combination with other geological observations, it is concluded that NW Borneo to the southwest of the Sarawak Kuching zone was a part of the Indochina/East Peninsular Malaysia fragment in SE Asia prior to the Jurassic and re-activated as an active continental margin in the Cretaceous. Widespread accretionary orogenesis along the Kuching zone likely initiated at or before the early Cretaceous and ended no earlier than the latest Cretaceous in response to the westward subduction of the paleo-Pacific slab.
•The mafic rocks in the Kuching zone of Sarawak Malaysia were dated at 77–98 Ma.•The Pakong-Serabang basalts originated from the slab fluid-modified MORB-like source.•The Serian Volcanics were derived from the recycled sediment-modified wedge source.•The Kuching Cretaceous accretionary orogenesis results from paleo-Pacific subduction.
The early Paleozoic tectonic evolution of the Xing’an–Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing’an Block, i.e., the ...Xinlin–Xiguitu Ocean and the Nenjiang Ocean. However, the early development of the Nenjiang Ocean remains unclear. Here, we present zircon U–Pb geochronology and whole-rock elemental and Sr–Nd isotopic data on the gabbros in the Xinglong area together with andesitic tuffs and basalts in the Duobaoshan area. LA-ICP-MS zircon U–Pb dating of gabbros and andesitic tuffs yielded crystallization ages of 443–436 Ma and 452–451 Ma, respectively. The Early Silurian Xinglong gabbros show calc-alkaline and E-MORB affinities but they are enriched in LILEs, and depleted in HFSEs, with relatively low U/Th ratios of 0.18–0.36 and εNd(t) values of −1.6 to +0.5. These geochemical features suggest that the gabbros might originate from a mantle wedge modified by pelagic sediment-derived melts, consistent with a back-arc basin setting. By contrast, the andesitic tuffs are characterized by high MgO (>5 wt.%), Cr (138–200 ppm), and Ni (65–110 ppm) contents, and can be termed as high-Mg andesites. Their low Sr/Y ratios of 15.98–17.15 and U/Th values of 0.24–0.25 and moderate (La/Sm)n values of 3.07–3.26 are similar to those from the Setouchi Volcanic Belt (SW Japan), and are thought to be derived from partial melting of subducted sediments, and subsequent melt-mantle interaction. The Duobaoshan basalts have high Nb (8.44–10.30 ppm) and TiO2 contents (1.17–1.60 wt.%), typical of Nb-enriched basalts. They are slightly younger than regional adakitic rocks and have positive εNd(t) values of +5.2 to +5.7 and are interpreted to be generated by partial melting of a depleted mantle source metasomatized by earlier adakitic melts. Synthesized with coeval arc-related igneous rocks from the southeastern Xing’an Block, we propose that the Duobaoshan high-Mg andesitic tuffs and Nb-enriched basalts are parts of the Late Ordovician and Silurian Sonid Zuoqi–Duobaoshan arc belt, and they were formed by the northwestern subduction of the Nenjiang Ocean. Such a subduction beneath the integrated Xing’an–Erguna Block also gave rise to the East Ujimqin–Xinglong igneous belt in a continental back-arc basin setting. Our new data support an early Paleozoic arc–back-arc model in the northern Great Xing’an Range.
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•A ~452–451 Ma association of high-Mg andesitic tuffs and Nb-enriched basalts were identified in the Duobaoshan.•Such an association revealed continental arc magmatism related to northwestern subduction of the Nenjiang Ocean.•The subduction gave rise to ~443–436 Ma Xinglong gabbros in a back-arc basin setting.•An early Paleozoic arc–back-arc system existed in the northern Great Xing’an Range.
The Khantaishir Magmatic Complex (KMC) (south–central Mongolia) exposes a section of a magmatic system consisting of deep crustal, ultramafic cumulates (coarse-grained Amp gabbros and hornblendites; ...c. 0.35–0.5GPa) to shallower crustal levels dominated by Amp–Bt tonalites (c. 0.1–0.2GPa). The magmatic rocks were emplaced during most of the Cambrian (c. 538–495Ma) and are mostly geochemically primitive (Mg#~50), Na-rich and metaluminous. The (normal-) calc-alkaline signature and characteristic trace-element enrichment in hydrous-fluid mobile large-ion lithophile elements (LILE) relative to high-field strength elements (HFSE) suggest an origin within a magmatic arc. Multiple intrusions of basic magma derived from a subduction-modified depleted mantle developed by fractional crystallization and/or accumulation of (Ol, Cpx) Amp+Bt, later joined by Pl. Magma mixing with, or without, exchange of xenocrysts between compositionally dissimilar melt batches was also important. Over time, partial melting of older, lower crustal metabasic rocks became increasingly significant, again with a strong subduction signature. The lack of zircon inheritance in the magmatic products and rather high zircon εHft values (all >+3, but for most samples>+8) as well as whole-rock Sr–Nd isotopic compositions imply that the arc was not founded on mature continental crust. It was probably located at the margin of the Baydrag microcontinent, dominated by accreted metabasic rocks of an older (early Tonian?) island arc covered by a thin layer of subordinate metasediments containing detrital zircons with Tonian and ill-defined Palaeoproterozoic ages. The KMC represents a small vestige of an extensive Cambrian–Ordovician subduction system (termed here the Ikh-Mongol Arc System), bordering the western margin of a chain of Precambrian microcontinents (Tuva-Mongolia, Zabkhan and Baydrag) that, together with accreted Neoproterozoic marginal basins (the Lake Zone), defines the external part of the Mongolian orocline.
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•The Khantaishir arc developed at the Baydrag continental margin at c. 538–495Ma.•Magma originated from subduction-modified depleted mantle, less so accreted early Tonian arc.•>1800km Cambro–Ordovician Ikh-Mongol Arc System bordered Tuva–Mongolian ribbon continent.
A high‐resolution seismic velocity model is presented for the crust and upper mantle of the Mariana arc–back‐arc system (MABS) based on active source seismic profiling. The major characteristics are ...(1) slow mantle velocity of <8 km s−1 in the uppermost mantle, especially, and deep reflectors under the Mariana arc (MA) and the West Mariana Ridge (WMR), (2) a deep reflector in the upper mantle beneath the relative thick crust of the Mariana Trough (MT) axis, (3) distribution of lower‐velocity lower crusts (6.7–6.9 km s−1) beneath the volcanic front and adjacent to the MT, and (4) high‐velocity lower crust (7.2–7.4 km s−1) beneath the boundary regions between the MA and MT, and between the WMR and the Parece Vela Basin (PVB), adding to structural characteristics of crust and upper mantle beneath the MABS. Of the characteristics described above, characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the Moho, considering that the WMR is extinct arc currently. On the other hand, characteristic 2 suggests that the origin of deep reflectors beneath the MT axis might be lower velocity materials due to the diffractive signals with strong amplitudes, characteristic 3 suggests that the lower‐velocity lower crust advanced crustal growth and characteristic 4 suggests that the high‐velocity lower crust beneath arc–back‐arc transition zone is composed of mafic/ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite.
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