Spinel Lherzolite Mantle Research Articles

Parental magma and mantle source compositions of chromitites in the Mesoarchean Nuggihalli greenstone belt, India: Evidence for Archaean subduction zone magmatism

We present comprehensive petrological and geochemical analyses of chromitites from the 3.3-3.1 Ga Nuggihalli greenstone belt in Western Dharwar Craton, India. Our findings provide insights into the primary magma and mantle source compositions associated with these Archean oceanic lithosphere chromitites. Major and trace element data from chromites and clinopyroxenes in the chromitites, combined with thermobarometry and hygrometry calculations, and numerical modeling, indicate that the Nuggihalli chromites are characterized by high-Cr chromites (Cr# = 68.1-78.6) with low TiO 2 (0.19-0.29 wt. %), Al 2 O 3 (9.10-13.13 wt. %) and Ga (4.24-10.37 ppm). These chemical signatures are comparable to those of chromites from high-Cr podiform chromitites and boninites. Clinopyroxenes are mainly augites with high Mg# (93-95). Equilibrated melts with the clinopyroxenes from the Nuggihalli chromitites have high Mg# values (79-86), similar to ancient komatiitic or picritic magmas (Mg# = 75-90). Thermobarometry and hygrometry calculations indicate that the clinopyroxenes likely formed at temperatures of 1194-1221℃ and pressures of 0.7-5.8 kbar with high water contents (∼2.4-2.6 wt. %), exceeding water contents of mid-ocean basalts (MORB) and oceanic island basalts (OIB). Numerical modeling of rare earth elements indicates moderate degrees of partial melting (1% to ∼10%) of a hydrated spinel lherzolite mantle source (water content: ∼478-5408 ppm). The parental magmas of the 3.3-3.1 Ga Nuggihalli chromitites are characterized by enrichment of water contents (∼2.4-2.6 wt. %) and depletion of high field strength elements (HFSEs, e.g., Nb and Hf), which are similar to boninites and arc magmas. Furthermore, the rock associations, including ultramafic rocks with chromitites, high-Mg basalts, and sheets of gabbro-anorthosites in the Nuggihalli greenstone belt, are similar to the rock assemblages of ophiolites, which could represent a relic of a Mesoarchaean oceanic lithosphere formed in a forearc setting.

Geochemistry of Volcanic Rocks in Ponelo Island, North Gorontalo, Indonesia

Ponelo Island is located in the northern part of Sulawesi, which is still an enigma regarding the genesis of the volcanic rocks found on this island. Therefore, the objective of this study is to understand the petrogenesis and tectonic implication of these volcanic rocks. Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to obtain trace and rare earth elements is the method of this study. The volcanic rocks found on Ponelo Island consist of basalt and basaltic andesite rocks with a calc-alkaline affinity. The transition data suggested a highly fractionated cause of low transition element (Ni=17-38 ppm; Cr=13-47) compared to primary magma concentration, anomalies negative of Ba, Sr, and Ti of spider diagrams, and negative anomaly of Eu (Eu/Eu*=0.88-0.99). Relationship between low concentration between Ce/Y (0.74-0.76) and La/Yb vs Sm/Yb ratio indicated ~5% spinel-lherzolite mantle source partial melting. On the other hand, incompatible element ratios, such as Ba/Nb (39.03-45.28), Ba/Th (75.52-82.67), Rb/Nb (3.93-6.22), K/Nb (1772.22-2703.45), Ba/La=13.67-14.57, Th/La (0.17-0.18), La/Nb (2.91-3.16), depleted Nb/U (6-6.74), and also lack of xenolith or enclaves indicate cryptic crustal contamination. The slab-derived fluid indicated by ratios of Rb/Y (0/019-0/05), Nb/Y (0.10-0.11), Th/Yb (0.52-0.61), and Ba/La ratio (13.29-14.57). Ponelo volcanic rocks shows typical calc-alkaline island arc tectonic setting particularly with enrichment in ion lithophile element (LILE) and light rare earth elements (LREE) along with depletion in high field strenght elements (HFSE) and heavy rare earth elemets (HREE), as shown by spider diagrams.

Geochemistry of Cenozoic mafic potassic and sodic volcanic rocks in southwestern Madagascar: Long-lived lithospheric mantle heterogeneities in an extensional tectonic setting

Cenozoic (≥ 43 Ma) silica undersaturated (potassic) trachybasalts and trachyandesites in southwestern Madagascar (Tsianihy-Manja, southern Morondava Basin) form a small monogenetic volcanic field emplaced above Paleogene detritic sedimentary rocks, along a NE-SW-trending fault system. These olivine-chromite±clinopyroxene-phyric primitive lavas (Mg# = 69; MgO = 10–11 wt%; Cr = 450 ppm, Ni = 200 ppm; K2O = 3–4 wt%) have highly peculiar trace element and isotopic composition (e.g., Ba/Nb = 18.4; 87Sr/86Sri = 0.70529–0.70555, 143Nd/144Ndi = 0.51262–0.51263, 206Pb/204Pbm = 18.415–18.424, 207Pb/204Pbm = 15.576–15.579, 208Pb/204Pbm = 38.799–38.813). A hitherto undescribed plug of primitive (sodic) basanite of the 11–12 Ma-old Ankililoaka district south of Tsianihy-Manja (hosting spinel lherzolite mantle xenoliths) has noticeable different geochemistry (Ba/Nb = 8–9.2; 87Sr/86Sri = 0.70346–0.7036, 143Nd/144Ndi = 0.51281–0.51282, 206Pb/204Pbm = 19.079–19.374, 207Pb/204Pbm = 15.621–15.645, 208Pb/204Pbm = 39.115–39.424).The relatively low CaO, Sc, V, Fe2O3t, MnO at high MgO, Cr and Ni, and the potassic affinity of the Tsianihy-Manja trachybasalts, all indicate that the mantle source is relatively clinopyroxene-poor (i.e., depleted by previous melt extractions), in the same way as the source of lamproitic (or boninitic) magmas, but the primitive nature, the concentration of high field strength elements, the incompatible element patterns and their isotopic ratios indicate their unequivocal within-plate setting and indicate a derivation by low-degree partial melting of an incompatible element-enriched mantle and insignificant role for crustal contamination. In terms of incompatible element concentrations, and thus also Sr-Nd-Pb-isotopic composition, we find no evidence in favour of a mid-ocean ridge basalt (MORB)-mantle component, or for a MORB-mantle strongly enriched by ocean island basalt-like components, to form the mantle source regions of the Tsianihy-Manja and Ankililoaka mafic alkaline rocks. The significant isotopic change from the northernmost Cenozoic volcanic rocks of Madagascar and those in the central and southern part of the island (which range in composition from sodic to potassic, and from tholeiitic basalt to olivine melilitite) implicates a distinct source heterogeneity, and ultimately assess the role of the old continental lithospheric mantle as source region.

The 2.03–2.02 Ga arc–back-arc mafic magmatism from the Zhongxiang Complex, northern Yangtze Block: Implications for Columbia supercontinent assembly

To better understand Paleoproterozoic tectonic processes in the Yangtze Block during the assembly of the Columbia supercontinent, an integrated geochronological and geochemical study of amphibolite facies mafic rocks exposed in the Zhongxiang Complex in the northern Yangtze Block is presented. Based on trace element and rare earth element (REE) characteristics, these mafic rocks can be divided into two coeval groups. The MORB-like mafic rocks show flat REE patterns and are enriched in large-ion lithophile elements (e.g. Rb, Ba, K, Sr). Their zircon ɛHf(t) values range from + 3.43 to + 4.90. The other mafic rocks are arc-like and have elevated REE contents and display relatively light REE enriched patterns, as well as depletion in high field strength elements (e.g. Nb, Ta, Ti). The zircon ɛHf(t) values of the latter group range from + 0.17 to + 2.68. Zircon U-Pb dating yielded a weighted mean 207Pb/206Pb age of 2029 ± 15 Ma for the MORB-like mafic rocks, and 2023 ± 14 Ma for the arc-like mafic rocks, respectively. The elemental and isotopic characteristics indicate that the MORB-like mafic rocks may be derived from a depleted asthenospheric mantle source, whereas its counterpart with arc-like signatures originated from a lithospheric mantle source. The low (La/Yb)CN (1.10–2.84) and Sm/Yb (0.99–1.29) ratios indicate that the mafic rocks are formed by partial melting of spinel lherzolite mantle. Combined with other geological observations, these mafic rocks are inferred to constitute part of a mid-Paleoproterozoic continental arc–back-arc basin system. The 2.03–2.02 Ga mafic rocks from the northern Yangtze Block indicate that the continental margin experienced subduction in response to assembly of the Columbia supercontinent.

Petrogenesis of submarine volcanic arc rocks from Andaman subduction zone, Northeast Indian Ocean: Constraints from slab components and mantle wedge characteristics

The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm(N), LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The 87Sr/86Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the 143Nd/144Nd-La/Sm(N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust.

Fate of an oceanic plate in the Neo-Tethys intra-oceanic subduction system: Evidence from elemental and Rb/Sr – Sm/Nd isotopic systematics

We report the coexistence of normal mid-ocean ridge basalt (NMORB)-type and alkaline ocean island basalt (OIB)-type mafic volcanics from the Zildat Ophiolitic Mélange (ZOM), Indus Suture Zone (ISZ), eastern Ladakh Himalaya. Trace element modeling portrays chemically heterogeneous mantle sources, wherein blueschists and sub-alkaline mafic volcanics depict almost flat REE-patterns [(La/Sm)N = 1.04–2.9] and respectively <20 % and >20 % degrees of partial melting from a depleted spinel-lherzolite mantle source. In contrast, alkaline mafic volcanics depict enriched LREE-patterns [(La/Yb)N = 35–48] with <5 % degrees of partial melting from a transitional garnet-spinel lherzolite enriched mantle source. Since the prevalent hypothesis considers seamounts and island arc rocks of the subducting oceanic plate within the Neo-Tethys Ocean to be the protolith for the contemporaneous Shergol Ophiolitic Melange (SOM) blueschists. In contrast, ZOM blueschists depict different protolith signatures. The εNd(t = 140 Ma) values for ZOM blueschists (+3.7 to +7.4) and sub-alkaline mafic volcanics (+9.2 to +9.3) are analogous to present-day Indian Ocean NMORB, while for OIB-type mafic volcanics (+3.7 to +4.3) are comparatively lower and analogous to Hawaii and Kerguelen OIB. Therefore, the observed protolith decoupling in the contemporaneous blueschists suggest the involvement of different protolith contenders, which were partially off-scraped from of the subducting plate during prolonged deep subduction followed by exhumation along the ISZ.

The ca. 1.13–0.92 Ga magmatism in the western Yangtze Block, South China: Implications for tectonic evolution and paleogeographic reconstruction

The tectonic regime of the Yangtze Block in the South China and its palaeogeographic location in the Rodinia supercontinent during the late Mesoproterozoic to early Neoproterozoic remain enigmatic due to limited geological records. In this study, we report late Mesoproterozoic basalts and granodiorites and early Neoproterozoic plagioclase amphibolites in the western Yangtze Block. Zircon LA-ICP-MS U-Pb dating yield concordant ages of 1130–1128 Ma for basalts, 1026 ± 1.6 Ma for granodiorites, and 917 ± 1.4 Ma for plagioclase amphibolites. The basalts are tholeiitic in chemical composition and are slightly enriched in LREEs with (La/Yb)N ratios of 2.87–3.99 and exhibit E-MORB-like trace element patterns. They are depleted in whole-rock Nd (+3.3 to +5.9) and zircon Hf isotopes (+11.1 to +12.8). Geochemical compositions showed that the basalts were derived from high-degree (7%–20%) partial melting of spinel lherzolite mantle. The granodiorites belong to the A-type granites and have negative whole rock εNd(t) (−5.5 to −7.8) and zircon εHf(t) (−15.3 to +0.6) values. We suggested that these A-type granites were mainly generated by the partial melting of felsic crustal rocks under low-pressure conditions. The plagioclase amphibolites are tholeiitic and exhibit arc-like geochemical affinities. They show depleted isotopic compositions (zircon εHf(t) = +11.0 to +13.6; whole rock εNd(t) = +0.6 to +2.5) and were interpreted as derived from a lithospheric mantle source that was metasomatized by subduction-related fluids. Geochemical compositions show that basalts were formed in an intracontinental rift setting, whereas granodiorites and plagioclase amphibolites were emplaced into a subduction background. Integrated with published work, we suggest that the late Mesoproterozoic and early Neoproterozoic magmatism in the western Yangtze Block record a tectonic switch from a passive margin to a convergence setting. In addition, we also proposed that the South China may be located at the western periphery of the Rodinia supercontinent.

Zircon U Pb geochronology and petrology of the tholeiitic gabbro from the Kovanlık (Giresun) area: Constraints for the Late Cretaceous bimodal arc magmatism in the Eastern Pontides Orogenic Belt, NE Turkey

Mafic intrusions of the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey are found rarely in varying ages, sizes and compositions. Of these, tholeiitic gabbro stocks, namely Kovanlık gabbro, consist of pyroxene gabbro and minor hornblende gabbro with a mainly granular texture. The pyroxene gabbro contains plagioclase, clino- and ortho-pyroxene, olivine, and Fe Ti oxides, whereas the hornblende gabbro includes mainly plagioclase, hornblende, rarely biotite and Fe Ti oxides. The clinopyroxene compositions of the pyroxene gabbro exhibit arc-related tholeiitic magmas. The Kovanlık gabbroic rocks have high U/Yb zircon ratios, implying continental arc-related tectonic settings. Zircon U Pb dating on the Kovanlık gabbro yielded a concordia age of ~92 Ma (Turonian). Petrochemically, the gabbroic rocks have a tholeiitic character and show arc- and MORB-like signatures. The gabbroic rocks show positive Rb, Ba, K, Pb, Sr, Ti and negative Nb, Ce, Nd, Hf, Zr anomalies in N-MORB normalized diagrams. The chondrite-normalized REE plots of the rocks show flat to feeble right-dipping REE patterns (La N /Lu N = 0.94–2.56) with positive Eu anomalies (Eu N /Eu* = 1.15–2.14; except for two pyroxene gabbro samples). The gabbroic rocks have 87 Sr/ 86 Sr (i) ratios (0.704533 to 0.706265), εNd (i) values (4.04–4.85), 206 Pb/ 204 Pb (18.56–18.68) and 207 Pb/ 204 Pb (15.61–15.64) ratios. Low Nb/La and Nb/Y ratios, as well as high Sr/Yb and Ba/Rb ratios, indicate that source regions were modified by slab-derived fluids and/or melts. The obtained zircon dating, isotopic and geochemical data, as well as regional geology, revealed that the tholeiitic mafic magma derived from a spinel-lherzolite mantle at a relatively shallow depth, and the Kovanlık gabbro in the EPOB represents one of the first mafic intrusions of Late Cretaceous bimodal arc magmatism related to the roll-back of the northwards subducting Tethys oceanic plate. • The Kovanlık tholeiitic gabbro formed in a continental arc with a MORB-like source. • The zircon U Pb dating of the gabbro gave a concordia age of ~92 Ma (Turonian). • The parent magma originated by partial melting of a spinel lherzolite mantle source. • The gabbro evolved by fractional crystallization and lesser crustal assimilation. • Cretaceous bimodal arc magmatism is related to the rollback of the subducting slab.

The Early Paleozoic Subashi ophiolite in the West Kunlun Orogenic Belt (northwestern Tibetan Plateau): Implication for the tectonic evolution of the Proto-Tethys

The West Kunlun Orogenic Belt (W-KOB) in the northwestern Tibetan Plateau retains critical information about the tectonic evolution of the Proto- and Paleo-Tethys during the Early Paleozoic to Mesozoic. Although a trench-arc-back arc basin model was used to address the tectonic evolution of Proto-Tethys, records about the evolution of this ocean basin remain ambiguous, particularly no studies about ages and tectonic affinities of ophiolites in the W-KOB are available. In this paper, we present new results on whole-rock elemental and Sr-Nd isotopic composition of basalts, and U-Pb zircon ages of gabbro, diabase and basalt from the Subashi ophiolite. The basalts are characterized by low REE concentrations, depleted LREEs in the chondrite normalized REE patterns, and unfractionated HFSEs in the primitive mantle normalized trace element spider diagrams, resembling normal mid-ocean ridge basalts. They have constant 87Sr/86Sr(i) values of 0.70472–0.70590 and positive εNd values of +7.5 to +7.9. Accordingly, it can be concluded that the Subashi ophiolite was derived from a depleted spinel lherzolite mantle source in a mid-ocean ridge setting. Zircon grains separated from basalt, diabase and gabbro have 206Pb/238U ages of 455 ± 4 Ma, 451 ± 6 Ma, and 446 ± 6 Ma, respectively, which are taken to be the crystallization ages of these rocks. Together with previously published data, our new data propose that the Mazha-Kangxiwa-Subashi ophiolitic mélange zone marks the existence of the Proto-Tethys ocean basin during the Early Paleozoic.

Episodic bimodal magmatism in Central Inner Mongolia, China: Insights from geochronological, geochemical, and Sr-Nd isotopic evidence

This study combines zircon U-Pb ages, whole-rock major and trace elements, and Sr-Nd isotope data to investigate the petrogenesis and tectonic implications of late Paleozoic magmatic rocks in central Inner Mongolia of the southeastern Central Asian Orogenic Belt. The results indicate the presence of two stages of bimodal magmatism from the late Carboniferous to early Permian. The late Carboniferous (ca. 323 Ma) bimodal associations consist mainly of gabbroic and granitic intrusive rocks, whereas the early Permian (ca. 273 Ma) bimodal suites are dominated by dolerite and granite porphyry dikes. The gabbroic intrusive rocks and dolerite dikes are calc-alkaline, slightly enriched in light rare earth elements and depleted in Nb, Ta, Ti, and have low initial 87 Sr/ 86 Sr ratios (0.70317–0.70510) and high ɛ Nd (t) values (+3.1 to +7.4). They are both interpreted to result from partial melting of a depleted spinel-lherzolite mantle source that was metasomatized by subduction-related components. The granitic intrusive rocks show I-type granite affinity and similar geochemical features to coeval gabbroic rocks, suggesting they were derived from a melt-induced anatexis of basaltic lower crust. The early Permian granite porphyry dikes show characteristics of LREE enrichment, negative Nb-Ta anomalies and positive Zr-Hf anomalies. They have higher La/Yb and Th/Zr ratios and ɛ Nd (t) values, and lower initial 87 Sr/ 86 Sr ratios than the coeval dolerite dikes. These features suggest that the early Permian felsic dikes were most likely generated by partial melting of a juvenile lower crust mixing with minor mantle-derived components. An integrated study with regional geological data suggests that the episodic bimodal suites in central Inner Mongolia formed in a post-collisional to within-plate rift setting after the closure of the Paleo-Asian Ocean. • Two stages of bimodal magmatism are identified in central Inner Mongolia. • The mafic rocks were derived from a depleted spinel-lherzolite mantle source. • The felsic rocks show a compositional variation from calc-alkaline to alkaline. • The bimodal suites formed in a post-collisional to within-plate rift setting.

Melt generation and trace element fractionation of intermediate arc magma from Andaman subduction zone

The submarine volcanoes, located in the southern part of Andaman Sea, north eastern Indian Ocean, result from the subduction of the Indo-Australian Plate beneath the Southeast Asian Plate and represent one of the less studied submarine volcanism among the global arc systems. The present study provides new petrological and geochemical data for the recovered rocks from the submarine volcanoes and documents the petrogenetic evolution of Andaman arc system. Geochemical attributes classify the studied samples as basaltic andesite, andesite, dacite to rhyodacite reflecting sub-alkaline, intermediate to acidic composition of the magma. Petrographic studies of the basaltic andesites and andesites show plagioclase [An38-An57 in basaltic andesites; An27-An28 in andesites] and clinopyroxene as dominant phenocrystal phase in a cryptocrystalline groundmass. Plagioclase (An25-An45) marks the principal phenocrystal phase in dacite with sub-ordinate proportion of biotite and amphibole of both primary and secondary origin along with minor amount of K-feldspar. The submarine volcanic rocks from Andaman arc system exhibit pronounced LILE, LREE enrichments and HFSE (negative Nb, Ta and Ti anomalies), MREE and HREE depletion thereby endorsing the influence of subduction zone processes in their genesis. Elevated abundances of Th with relatively higher LREE/HFSE than LILE/HFSE, LILE/LREE suggest significant contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids towards mantle wedge metasomatism thereby modifying the sub-arc mantle. Partial melting curves calculated using the non-modal batch melting equation suggest primary magma generated due to ~31–35 % degree of partial melting of spinel lherzolite mantle beneath the arc system. Fractional crystallization model suggests fractionation of 45 % plagioclase, 40 % clinopyroxene, 5–10 % amphibole and 5–10 % biotite which is consistent with the petrographic observations. Further, the assimilation-fractional-crystallization (AFC) model for the studied rocks indicates nominal crustal contamination. Therefore, this study infers that the melt evolution history for the Andaman arc volcanic rocks can be translated in terms of (i) generation of precursor magma by ~31–35 % partial melting of a spinel lherzolite mantle wedge, metasomatized predominantly by subducted slab sediments and (ii) the parent magma generation was ensued by fractionation dominated melt differentiation with nominal input from arc crust.

New U[sbnd]Pb zircon and geochemical constraints on Late Devonian Back-arc basin origin of eclogite protoliths from northeastern Hainan Island, South China

Geochronological, geochemical, and SrNd isotopic data are used to constrain the age, origin, and tectonic setting of the protoliths of newly discovered eclogites from Chaotanbi, northeastern Hainan Island, South China. New sensitive high-resolution ion microprobe (SHRIMP) UPb zircon dating reveals that the protoliths of the Chaotanbi eclogites formed at 364 ± 2 Ma and were metamorphosed at 297 ± 1 Ma. All samples collected from a coherent area of ~1.8 km2 in the intertidal and subtidal zones on the island exhibit tholeiitic compositions, with initial 87Sr/86Sr and εNd values of 0.705–0.707 and − 1.1 to +7.4, respectively. The samples can be further categorized into three groups based on incompatible trace elements. Group #1 samples have a normal mid-ocean ridge basalt (N-MORB) affinity and were originated from relatively high-degree partial melting of a depleted spinel lherzolite mantle source slightly modified by slab-derived fluids. Group #2 samples have an enriched mid-ocean ridge basalt (E-MORB) affinity and were derived from an enriched spinel and spinel–garnet lherzolite transition mantle source at depths greater than that of Group #1. Group #3 samples have an island arc basalt (IAB) affinity and were generated by partial melting of an enriched spinel–garnet lherzolite mantle source modified by fluid- and sediment-related recycled components. The association of these diverse basalt types, with MORB-type samples accounting for ~90% of all samples, suggest a Late Devonian mature back-arc basin existed on northeastern Hainan Island, as a part of the Jinshajiang–Ailaoshan–Song Ma–Hainan back-arc basin system. Therefore, the Chaotanbi eclogites are a fingerprint of the opening and closure of the eastern Paleo-Tethyan Ocean.

Mesoproterozoic magmatism redefines the tectonics and paleogeography of the SW Yangtze Block, China

The geodynamic setting of the Yangtze Block and its paleogeography in the reconstructed Rodinia supercontinent during the late Mesoproterozoic is loosely constrained due to the limited geological information reported. In this contribution, we identify a new suite of amphibolites in the SW Yangtze with an 1175 ± 5 Ma emplacement age. Geochemical modeling demonstrates they were derived from low-degree (3.5%–6.5%) partial melting of spinel lherzolite mantle. Additionally, three suites of late Mesoproterozoic granites in the SW Yangtze Block are revisited by providing new zircon U-Pb-Hf-O isotopes and whole-rock geochemical data. The granites have been dated to emplace during 1060–1042 Ma and have Zr + Nb + Y + Ce and Tzr values mostly at 304–576 ppm and 831–886 ℃, respectively, similar to A-type granites. Zircon Hf-O isotopic data indicate two subgroups of granites, in which one group has suprachondritic εHf(t) values of + 2.4 to + 11.4 with δ18O values of 6.5‰ to 7.4‰ while the other group has εHf (t) values of −3.9 to +1.5 with δ18O values of 10.0‰–11.6‰. The first group of granites was mainly derived from partial melting of mafic tholeiitic rocks while the second group of granites was mainly formed by partial melting of metasedimentary rocks. Integrated with published work, we propose that an intra-continental rift basin was developed in the SW Yangtze Block during ∼ 1.2–1.04 Ga, arguing against a late Mesoproterozoic orogenic event in the SW Yangtze Block. Statistical analyses based on compiled zircon U-Pb-Hf isotopes from Laurentia, Yangzte, and Amazonia show that the Yangtze has an isotopic affinity with west Nuna. These continents likely remained united during the transition from Nuna/Columbia to Rodina along the western periphery of the Rodinia supercontinent.

Major and trace element compositions of basaltic lavas from western margin of central main Ethiopian rift: enriched asthenosphere vs. mantle plume contribution

Major and trace element data are presented for basaltic lavas from western rift margin of central Main Ethiopian Rift located at Kella area to investigate the processes involved in the petrogenesis of the erupted magmas and the nature of mantle source compositions. Kella area is composed of Quaternary (<1.6 Ma), Miocene (10.6–8.3 Ma) and Oligocene basalts (30-29 Ma) ranging from alkaline to tholeiitic in composition. The geochemical variations of basaltic samples from Kella area exhibit two compositionally distinct basaltic groups. The Oligocene tholeiitic basalts display low MgO (5.29–6.11 wt.%), TiO2 (2.15–2.47 wt.%), P2O5 (0.28–0.34 wt.%), and high ratio of CaO/Al2O3 (0.68–0.72) and La/Nb (0.89–1.10). Whereas Quaternary and Miocene alkaline basalts display high MgO (7.40–8.86 wt.%), TiO2 (2.4–2.53 wt.%), P2O5 (0.44–0.52 wt.%) and low ratio of CaO/Al2O3 (0.62–0.66) and La/Nb (0.71–0.76). The contrasting incompatible element ratios (e.g., K/Nb, La/Nb, Rb/Zr and Zr/Nb) between tholeiite and alkaline basalts reflect differences in their mantle sources. Major and trace element variations, therefore, reflect the involvement of two geochemically distinct mantle sources in the petrogenesis of Kella basaltic lavas: i) the Oligocene tholeiite basaltic melts derived from enriched asthenosphere mantle source (E-MORB) and ii) the Miocene and Quaternary alkali basaltic melts show a close similarity with ocean island basalts (OIBs) geochemistry, and this end member ascribed to the arrival of Afar plume head. The geochemical modeling reveals that the Oligocene tholeiite basaltic melts produced by an equilibrium melting with 3–5 % degree of partial melting in spinel lherzolite mantle source, whereas the alkali basalts were produced with ∼2% degree of partial melting within spinel-garnet lherzolite transition zone mantle sources.

Coexisting arc and MORB signatures in the Sonakhan greenstone belt, India: late Neoarchean – early Proterozoic subduction rollback and back-arc formation

Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains. The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites. Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in the tectonic evolution of the Sonakhan greenstone belt, which may have been common in other late Neoarchean – early Proterozoic greenstone belts.

Major and Trace Element Compositions of Basaltic Lavas From Western Margin of Central Main Ethiopian Rift: Enriched Asthenosphere vs. Mantle Plume Contribution

Major and trace element data are presented for basaltic lavas from western rift margin of central main Ethiopian rift located at Kella area to investigate the processes involved in the petrogenesis of the erupted magmas and the nature of mantle source compositions. Kella area is composed of Quaternary (<1.6 Ma), Miocene (10.6-8.3 Ma) and Oligocene basalts (30-29 Ma) ranging from alkaline to tholeiitic in composition. The geochemical variations of basaltic samples from Kella area exhibit two compositionally distinct basaltic groups. The Oligocene tholeiitic basalts display low MgO (5.29-6.11 wt.%), TiO2 (2.15-2.47 wt.%), P2O5 (0.28-0.34 wt.%), and high ratio of CaO/Al2O3 (0.68-0.72) and La/Nb (0.89-1.10). Whereas Quaternary and Miocene alkaline basalts display high MgO (7.40-8.86 wt.%), TiO2 (2.4-2.53 wt.%), P2O5 (0.44-0.52 wt.%) and low ratio of CaO/Al2O (0.62-0.66) and La/Nb (0.71-0.76). The contrasting incompatible element ratios (e.g., K/Nb, La/Nb, Rb/Zr and Zr/Nb) between tholeiite and alkaline basalts reflect differences in their mantle sources. Major and trace element variations, therefore, reflect the involvement of two geochemically distinct mantle sources in the petrogenesis of Kella basaltic lavas: i) the Oligocene tholeiite basaltic melts derived from enriched asthenosphere mantle source (E-MORB) and ii) the Miocene and Quaternary alkali basaltic melts show close similarity with ocean island basalts (OIBs) geochemistry, and this end member ascribed to the arrival of Afar plume head. The geochemical modeling reveals that the Oligocene tholeiite basaltic melts produced by equilibrium melting with 3-5 % degree of partial melting in spinel lherzolite mantle source, whereas the alkali basalts are produced with ~2% degree of partial melting within spinel-garnet lherzolite transition zone mantle sources.

Mesoarchean continental intraplate volcanism and sedimentation: The case of the Simlipal basin, Singhbhum Craton, eastern India

The origin of continental intraplate basalts, their role in crust-mantle interaction, and processes of formation of the associated sedimentary rocks have remained important research problems. The poorly-studied Simlipal basin hosts a well-preserved, weakly-metamorphosed ensemble of three bands of mafic volcanic rocks intercalated with shallow marine sandstones and minor phyllites, occurring unconformably over a Paleoarchean continental basement of the Singhbhum Craton. Little is known about the age of the basin, the nature of sediment source and source weathering, petrogenesis of the volcanic rocks, and the tectonic setting of the volcano-sedimentary assemblage. To address these issues, we present whole-rock geochemical data on the Simlipal sediments and volcanic rocks, detrital zircon U-Pb ages and mineral chemical analyses. The sandstones and phyllites are characterized by high K2O/Na2O and chemical index of alteration. The source-indicating elemental ratios and REE patterns (negative Eu anomaly and flat HREE) suggest that felsic rocks, especially the granitoids resulting from shallow crustal melting, were the dominant source. The detrital zircons are mostly rounded with U-Pb ages ranging from 3.63 to 3.17 Ga. All these features suggest that the Simlipal basin formed during the Mesoarchean Era on a slowly subsiding continental platform undergoing extension, which received reworked and recycled sediments from a diverse but granitoid-dominated, intensely-weathered, mostly Paleoarchean source. The mafic volcanic rocks are low-Ti, tholeiitic basalts showing low CaO/Al2O3 and TiO2/Yb values, and flat HREE pattern indicating derivation from a spinel lherzolite mantle at shallow depth (<80 km). The presence of xenoliths of crustal rocks, quartz-normative nature, wide range of magnesium number (39–80), primitive mantle-normalized negative Nb, Ta and Ti anomalies, and high U/Nb and Th/Yb ratios reflect that the magma underwent significant fractional crystallization and crustal assimilation. The Simlipal volcanic rocks are consanguineous with the spatially close and coeval gabbro-anorthosite bodies, layered mafic–ultramafic rocks, and A-type Mayurbhanj Granite of the Singhbhum Craton, together forming an intraplate bimodal volcanic-plutonic association. We construe that the widespread Mesoarchean magmatism and sedimentation was linked to mantle upwelling possibly related to an event of lithospheric delamination and crustal extension.

Geochemistry and petrogenesis of mafic volcanic rocks from Kahak area (Urumieh-Dokhtar magmatic arc)

The Kahak mafic volcanic rocks in the central part of the Urumieh-Dokhtar Magmatic Arc are composed of ‎basalts‏ ‏and‏ ‏basaltic ‎andesite and show sub alkaline to transitional affinity. They are calc-alkaline based on the tholeiitic index (THL). U-‎Pb zircon dating ‎yields almost 60‎‏ ‏‎(Middle Paleocene) and 24 to 19 Ma (Late Oligocene–Early Miocene) for andesitic basalt and basaltic ‎rocks ‎respectively. These rocks ‎are identified by LREE and LILE enrichment and HFSE depletion with relatively negative or without ‎Eu ‎anomalies and E-MORB like ‎pattern in multiple spider diagrams that.attributed to the subduction of the Neotethyan oceanic ‎slab ‎beneath the central Iranian microcontinent. Based on petrography, trace and rare earth elements, and isotopic features, ‎fractional ‎crystallization played a significant role ‎during magma evolution in these rocks. Trace element modeling suggests that the ‎studied mafic ‎rocks were derived by partial melting ‎within the spinel‏ ‏lherzolite mantle. Isotopic ratios also show that they resulted from ‎lithospheric ‎mantle metasomatized by ‎released fluids from subducted slab sediments. The studied samples might have formed in the ‎extensional regime ‎followed ‎by slab rollback and undergone a continental arc to back-arc basin transition during the Paleocene to ‎Miocene. This basin might ‎have ‎been closed in the‏ ‏middle Miocene.

Petrology and geochemistry of South Mid-Atlantic Ridge (19°S) lava flows: Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge (SMAR) 19°S segment, approximately located along the line of Saint Helena volcanic chain (created by Saint Helena mantle plume), is an ideal place to investigate the issue whether the ridge-hotpot interaction process affected the whole MAR. In this study, we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible trace elements (e.g., Ni ​= ​239–594 ​ppm and Cr ​= ​456–1010 ​ppm) and low Fe/Mn (54–67) and Ce/Yb (0.65–1.5) ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source. Using software PRIMELT3, this study obtained mantle potential temperatures (Tp) beneath the segment of 1321–1348 ​°C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of ~63 ​km and ceases melting at ~43 ​km with a final melting temperature of ~1265 ​°C. The extent of partial melting is up to 16%–17.6% with an average adiabatic decompression value of 2.6%/kbar. The correlations of major oxides (CaO/Al2O3) and trace elements (Cr, Co, V) with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.87Sr/86Sr (0.702398–0.702996), 143Nd/144Nd (0.513017–0.513177) and 206Pb/204Pb (18.444–19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle, PREMA mantle, and HIMU mantle materials. The simple, binary mixing results among components from plume-free SMAR MORB, Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched materials. The abovementioned mantle potential temperatures, together with the low Saint Helena (<10%) and Tristan (<5%) components remaining in the asthenospheric mantle at present, show that the physically ridge-hotspot interactions at SMAR 19°S segment may have ceased. However, the trace element and Sr-Nd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.

Geotectonic significance of the Neoproterozoic ophiolitic metagabbros of Muiswirab area, South Eastern Desert, Egypt: constraints from their mineralogical and geochemical characteristics

Petrological and geochemical studies of Neoproterozoic metagabbros were carried out in the Muiswirab area, South Eastern Desert of Egypt. The Muiswirab area comprises of ophiolitic metagabbroic rocks (MOM), which are tectonically thrusted over a thick pile of metavolcanic rocks and intruded by syn- and post-tectonic granitoid rocks. The whole-rock geochemical variations coupled with chemical compositions of mineral constituents are used to attain the genesis and tectonic evolution of the studied metagabbros. The geothermobarometric investigation of the analyzed amphiboles from (MOM) revealed that these metagabbros underwent regional metamorphism under lower to upper greenschist facies (biotite zone) conditions (at a temperature of 450 to 500 °C and pressure of 1–3 kbar). Geochemically, the metagabbros (MOM) show tholeiitic affinity and exhibiting both arc- and MORB- like characters as evidenced by their clinopyroxene compositions and the Ti/V ratios (11.84–31.65), which considered as prominent features of forearc tectonic regime. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The parental magma of the investigated MOM rocks seems to be developed in a sub-arc mantle wedge setting due to the enrichments of LILE (e.g., Rb, Ba, Sr, Pb) over HFSE (e.g., Ti, Nb, Y, Zr, Hf, Ta). The studied MOM rocks have lower values of Nb/U relative to MORB and OIB indicating that their geochemical variation produced due to the enrichment of a lithosphere mantle by OIB-like components. The ratios of Zn/Fet, La/Sm, Sm/Yb, Th/Yb and Nb/Yb indicate that the MOM rocks represent a fragment of oceanic crust originated at a supra-subduction zone environment and their parental magma developed by 5–30 % partial melting of a spinel lherzolite mantle rather than pyroxenite in an island arc setting and conformable with most of the Egyptian ophiolitic metagabbros.