Normal Mid-ocean Ridge Basalt Research Articles

Two-stages of plume tail volcanism formed Ojin Rise Seamounts adjoining Shatsky Rise

Many groups or chains of seamounts border, or are associated with, oceanic plateaus but their origin and relationship with plateau volcanism are debated. An example is Ojin Rise Seamounts, a group of ~80 seamounts located northeast of the 147–134 Ma Shatsky Rise, an oceanic plateau that was formed along the trace of a triple junction in the northwest Pacific Ocean. A notable topographic feature of Ojin Rise Seamounts is that they are distributed in such a way that they appear to be connected to another oceanic plateau in the east, the ~100 Ma Hess Rise. Many seamounts are arranged in a NW–SE direction (almost in the same absolute plate motion direction) and some are organized into seamount chains consisting of several volcanic cones connected by a common platform, reflecting a complex volcanic history. Here we report petrography as well as major and trace element, Sr, Nd, Pb, and Hf isotope, and 40Ar-39Ar age data for basalts dredged from five seamounts in the western part of Ojin Rise Seamounts. We found that the 40Ar-39Ar ages of the cones (123.7–120.7 Ma) are distinctly younger than the magnetic lineation age (M9-M7; 134–132 Ma) around the chain. The younger age suggests either a protracted development for the seamount chain, previously proposed to have formed near-ridge, or age difference between the platform and cones. The morphological and age differences are supported by petrography and geochemistry: platform basalts are aphyric, whereas cone basalts are slightly to highly plagioclase-phyric (up to 48 vol%); platform basalts are depleted in trace element and isotope composition, but cone basalts are enriched with high Nb/Ti and 87Sr/86Sr, and low 143Nd/144Nd and 176Hf/177Hf ratios. The depleted signature of the platform basalts is similar to that of the abundant normal type basalts of Shatsky Rise with an isotope composition that overlaps with that of normal mid-ocean ridge basalts (N-MORB), whereas trace elements are enriched (e.g., higher Nb/Zr and Nb/Ti than N-MORB). However, slight geochemical differences between the Ojin platform basalts and Shatsky normal type basalts exist and are expressed in higher 208Pb/204Pb isotope ratios of the former. The depleted signature implies shallow mantle dilution of the (enriched) Shatsky plume tail near a ridge. In contrast, the enriched signature of the cone basalts indicates little contribution from N-MORB source mantle as (enriched) plume volcanism moved away from the ridge. The enriched features (Nb/Zr and 176Hf/177Hf) of the Ojin Rise cone basalts increase with increasing distance from Shatsky Rise and conform with a geochemical evolution model that involves increasing contribution from a highly enriched Hess Rise source component with time.

Subduction and exhumation of Luliangshan eclogite in the North Qaidam, northern Tibet: Constraints from petrology, geochemistry and phase equilibrium modelling

Eclogites in the high‐pressure (HP) and ultrahigh‐pressure (UHP) belt record important information about the subduction process and evolution history of the orogenic belt. The Luliangshan eclogites surrounded by granitic gneiss or paragneiss as lenses are exposed in the western segment of the North Qaidam UHP metamorphic belt, northwestern China. Petrology, mineral chemistry, and P–T pseudosection modelling show that the eclogites have experienced a multi‐stage metamorphic process. The peak eclogite‐facies metamorphic stage, is characterized by omphacite in matrix and as inclusion in garnet, with the peak mineral assemblages of garnet + omphacite + rutile + quartz atT > 790°C andP > 25.5 kbar. The initial HP granulite‐facies retrogression is characterized by the symplectite of diopside + plagioclase around omphacite, with P–T conditions of 911°C and 16.9 kbar. The subsequent amphibolite‐facies stage is characterized by amphibole + plagioclase symplectite around the clinopyroxene, with the metamorphic conditions of 674–686°C and 6.4–6.9 kbar. Zircon U–Pb analyses yielded two metamorphic age clusters: (a) HP granulite‐facies metamorphic age of 422–425 Ma, and (b) amphibolites‐facies retrograde age of 397–420 Ma. The protolith of eclogite have geochemical characteristics similar to those of normal mid‐ocean ridge basalt (N‐MORB); and the varyingεNd(t) values (−6.3 to 2.1) indicate that the Luliangshan eclogites were derived from a mantle source with rare crustal contamination. Combining these data with previous studies, a multi‐stage tectonic model can be proposed: In the Early Neoproterozoic, the protolith of the Luliangshan eclogites were emplaced into ancient continental crust; during 460–430 Ma, following the oceanic subduction, the subduction of continental crust was dragged by the oceanic slab and continue to subduct towards the Qilian Block, and metamorphosed at the depth of at least 75 km. After a long and slow exhumation process, it returned to the shallow crust.

Rifting and subduction records of the Paleo–Tethys in North Laos: Constraints from Late Paleozoic mafic and plagiogranitic magmatism along the Song Ma tectonic zone

Abstract In Southeast Asia, the Jinshajiang–Ailaoshan suture in Southwest China and the Song Ma zone in Vietnam were generally accepted as the amalgamation boundary of the South China and Simao/Indochina blocks. However, the tectonic location of the Paleotethyan suture in North Laos and its tectonic affinity remain ambiguous. To address this issue, we present the geochemical and geochronological data of a set of mafic and plagiogranitic rocks along the Song Ma suture zone in North Laos. Three groups of rocks with distinctive petrological, geochronological, and geochemical signatures were identified. The ca. 370 Ma Group 1 plagioclase amphibolite was preserved as xenoliths in the ca. 260 Ma granitoids. It exhibits a normal mid–oceanic–ridge basalt (N–MORB)-like, multi-elemental pattern with highly positive εNd(t) (+3.3 to +10.7) and εHf(t) (+6.68 to +7.41) values and low δ18O values (5.3‰−5.5‰), that are interpreted as products of the Late Devonian − earliest Carboniferous continental rift setting. The ca. 270 Ma Group 2 gabbro/diabase–diorite exhibits arc-like geochemical affinity with the εNd(t), εHf(t), and δ18O values of –5.85 to –3.76, –3.8 to +3.2, and 6.6‰–7.6‰, respectively. It is derived from the metasomatized wedge. The ca. 260 Ma Group 3 plagiogranite shows Nb-Ta and Ti negative anomalies and positive εNd(t) (+3.21 to +4.21), εHf(t) (+8.4 to +13.6), and δ18O (5.9‰−6.8‰) values, which are indicative of its derivation from mafic oceanic crust in a subduction setting. In combination with the published data, these results support the development of the Carboniferous–Permian Paleotethyan branch in North Laos, which represents the tectonic location of the Song Ma suture zone that spatially connects the Jinshajiang–Ailaoshan suture zone in Southwest China and the Chenxing–Bangxi suture zone in Hainan Island, South China.

Transition from the lithospheric to asthenospheric mantle-derived magmatism in the Early Jurassic along eastern Bangong–Nujiang Suture, Tibet: Evidence for continental arc extension induced by slab rollback

Abstract The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

Geochronology, geochemistry, origin, and tectonic implications of high‐pressure mafic granulites of the Amdo region, Central Tibet

The Amdo Terrane, Central Tibet, was previously located in the middle of the Bangong–Nujiang Ocean (BNO), and is key to understanding the subduction history of the BNO. However, the high‐pressure (HP) mafic granulites of the Amdo Terrane remain poorly studied. Here, we report the petrology, geochemistry, and zircon U–Pb geochronology of the Amdo HP mafic granulites. The granulites occur mainly as enclaves or deformed dikes in granitic gneisses of the Amdo Terrane. Zircons from the granulites yield two clusters of ages, at 193–192 and 180 Ma. The former represents the best estimate of the age of peak metamorphism, and the latter represents the age of retrograde amphibolite‐facies metamorphism. Although the contents of mobile elements in the granulites, including the large‐ion lithophile elements (Rb, Sr, Cs, Pb, K), were significantly altered during metamorphism, the contents of the high‐field‐strength elements (HFSEs; Th, Nb, Zr, Ti) and rare earth elements (REEs) remained largely unchanged. The granulites belong to the tholeiitic basalt series, with SiO2 contents of 47.05–51.03% and Mg# of 35–50. The REE contents of the granulites are characterised by flat chondrite‐normalised patterns (LaN/YbN = 0.96–1.92; LaN/SmN = 1.0–1.59) and weak Eu anomalies (Eu/Eu* = 0.84–0.99). Most of the granulite samples have high TiO2 contents, clear negative Nb and Ta anomalies, and high Pb enrichment, with positive zircon εHf(t) values (+8.14 to +16.02). The granulites exhibit both normal mid‐ocean ridge basalt (N‐MORB) and island‐arc basalt (IAB) characteristics. Using the data presented here and previous data, we suggest that the protolith of the granulites were back‐arc basin basalts derived from high‐degree partial melting of depleted mantle in the spinel stability field, which records the early evolution of the BNO. Northward subduction of the BNO oceanic plate led to back‐arc spreading and the formation of back‐arc basin basalts on the northern margin of the Amdo Terrane. The back‐arc basin extended further to form an ocean basin, which separated the Amdo Terrane and the Southern Qiangtang–Baoshan (SQB) Terrane, resulting in the Amdo Terrane becoming an isolated microcontinent in the BNO.

Non-mantle-plume process caused the initial spreading of the South China Sea

The mantle plume process is thought to be the prevailing dynamic mechanism for the South China Sea opening, but controversy persists due to the lack of critical evidence of magma in the initial seafloor spreading. International Ocean Discovery Program (IODP) Expedition 367 successfully recovered at Site U1500 the mid-ocean ridge basalt (MORB) representing the magma activity of the initial spreading of the South China Sea during the earliest Oligocene. Here we present the whole-rock and olivine phenocryst geochemistry of the basalts to constrain the potential influence of the Hainan mantle plume on the evolution of the South China Sea. Major and trace elemental compositions indicate that the basalts were mainly influenced by fractional crystallization of olivine and formed by melting of a spinel peridotite source without any pyroxenite in mantle source. The calculated mantle potential temperature of those most primitive basalts is much lower than plume-related MORB of Iceland, but similar to normal MORB elsewhere. Both lithological composition and mantle potential temperature clearly contradict with the mantle plume model, signifying that the mantle plume didn’t exist at the earliest Oligocene. Therefore, the initial spreading of the South China Sea should be caused by non-plume processes, most likely by the westward subduction of the Pacific Plate.

Slab-controlled elemental–isotopic enrichments during subduction initiation magmatism and variations in forearc chemostratigraphy

Supra-subduction zone (SSZ) ophiolites formed in forearc settings and the Izu–Bonin–Mariana (IBM) in-situ forearc show conversions from normal mid-ocean ridge basalt (MORB) to boninitic affinities, which are widely interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism (SIM). Nevertheless, forearc sequence generated by subduction initiation need not always include boninitic rocks. Here we reconstruct a forearc chemostratigraphy without boninitic affinity based on the Central Palawan and Amnay (CPA) ophiolites of the west Philippines to show diversity in melt evolution of SIM. Zircon U–Pb dating on the MORB-type Amnay gabbros reveal a 10 m.y. of seafloor spreading from 33 to 23 Ma. These ages together with the progressively enriched zircon in-situ oxygen isotopes (δ18O= 1.62–10.1‰) and whole rock elemental and Nd–Hf isotopic signatures ([La/Yb]N= 0.19–3.45; Nb/Yb= 0.07–5.24; εNd= 2.1–10.2; εHf= 11.2–24.1) indicate that the depleted MORB mantle (DMM)-type source of the Amnay ophiolite was getting enriched by fluxing of aqueous slab fluid in the early stage followed by addition of adakitic and Nb-enriched slab melts in the later stage. This distinct mantle evolution together with higher εHf at a given εNd of the Amnay mafic samples than the contemporaneous South China Sea (SCS) oceanic samples do not support previous interpretation that the Amnay ophiolite was a fragment of the SCS. However, the 33–23 Ma Amnay ophiolite can be the younger part of the 34 Ma forearc-type Central Palawan ophiolite, and both (i.e. the CPA ophiolites) represent on-land fragments of the NW Sulu Sea. We propose that the 34–23 Ma CPA ophiolites and the NW Sulu Sea constitute forearc basin of the late Oligocene–middle Miocene island arc–backarc basin duo of the Cagayan Ridge and SE Sulu Sea, associated with subduction of the Cretaceous to Eocene Proto-SCS since 34 Ma. The CPA ophiolites lack boninite but show geochemical progression from normal to enriched MORB affinities, whose melt evolution differs from that of the IBM SIM. We attribute the differences in SIM and variations in forearc chemostratigraphy to the subduction of a younger slab with thick terrigenous sediment cover of the Proto-SCS versus an older slab with thin pelagic sediment cover of the Pacific Plate.

Early Carboniferous seafloor spreading recorded by volcanic rocks in the western segment of the Changchun–Yanji Suture Belt, NE China

This study reports zircon U–Pb geochronological, geochemical, and zircon Hf–O isotopic data for metavolcanic rocks from the Shitoukoumen and Yongji areas in central Jilin Province, northeast China, to reveal their petrogenesis and tectonic setting. The metavolcanic rocks collected from the Shitoukoumen and Yongji areas are composed of metabasaltic trachyandesite, metatrachyandesite, and metarhyolitic tuff. Zircon U–Pb dating results indicate that the metavolcanic rocks were erupted during 359–355 Ma. Metavolcanic rocks in the Shitoukoumen area can be divided into two groups according to their geochemical characteristics. Group‐I rocks (metabasaltic trachyandesite and metatrachyandesite) show geochemical features similar to those of ocean island basalt (OIB), with slightly lower zircon δ18O values (4.06 ± 0.42‰ to 5.16 ± 0.28‰) than those of mantle‐derived zircons, and depleted εHf(t) values (7.84–15.4). Group‐II rocks (metabasaltic andesite) show similar geochemical characteristics to those of normal mid‐ocean ridge basalt (N‐MORB). Group‐I rocks may have been derived from partial melting of enriched mantle involving high‐temperature altered oceanic crust, whereas Group‐II rocks originated mainly from partial melting of depleted mantle. Metarhyolitic tuffs from the Yongji area have high SiO2 and K2O contents, as well as high Ga/Al ratios, and show similar geochemical characteristics to those of A‐type rhyolites. The results of the study, together with published data, indicate that the eastern segment of the northern margin of the North China Craton was a passive continental margin setting during the early Carboniferous, and that the Paleo‐Asian Ocean remained open along the Changchun–Yanji suture belt until the early Carboniferous.

The origin of the Pailin Crystalline Complex in western Cambodia, and back-arc basin development in the Paleo-Tethys Ocean

The Pailin Crystalline Complex, which consists of amphibolites, metagabbros, and felsic igneous rocks, is located in western Cambodia. We used the geochemistry of the amphibolites and the zircon UPb ages of felsic igneous rocks to constrain the origin of the Pailin Crystalline Complex and to gain insights into the tectonic framework of Southeast Asia. Geochemical analyses indicate that the amphibolites are normal (N-type) mid-ocean ridge basalts, similar to the Sa Kaeo Back-arc Basin basalts of southeastern Thailand. The zircon UPb ages are 283 ± 2 Ma (anorthosite dike), 280 ± 2 Ma (aplite dike), and 275 ± 2 Ma (plagiogranite). The geochemistry and detrital zircon UPb ages of sandstones adjacent to the Pailin Crystalline Complex suggest that they are sediments that filled the Sa Kaeo Back-arc Basin during the Triassic. We infer that the Pailin Crystalline Complex is part of a dismembered ophiolite related to the back-arc basin, and that the igneous rocks formed during the Early Permian. Most of the metamorphism occurred during continental collision in Southeast Asia. However, the amphibolites of the Pailin Crystalline Complex also experienced ocean-floor metamorphism at a back-arc spreading center, in a tectonic setting similar to that of the mafic metamorphic rocks of the Nan Back-arc Basin, northern Thailand. We conclude that the Pailin Crystalline Complex is the southernmost extension of a back-arc basin related to subduction of Paleo-Tethys oceanic crust and the Sukhothai Arc.

Late Paleozoic tectonic transition from subduction to post-collisional extension in Eastern Tianshan, Central Asian Orogenic Belt

Abstract Late Paleozoic large-scale transcurrent tectonics and synkinematic intrusions are prominent features in the Eastern Tianshan segment of the southwestern Central Asian Orogenic Belt. However, the spatial and temporal relationship between synkinematic intrusions and crustal-scale shear zones remains unclear. Here we report petrology, geochemistry, and geochronology of the Qiziltag pluton associated with the Kanggur-Huangshan Shear Zone (KHSZ) with a view to characterize the spatial and temporal relationship between synkinematic intrusions and large-scale transcurrent shearing. Field relations and zircon U-Pb ages indicate that the Qiziltag pluton was formed through two stages of magmatism, with earlier stage granitoids (gneissic biotite granite: 288.9 ± 1.9 Ma, biotite monzogranite: 291.5 ± 1.7 Ma, K-feldspar granite: 287.9 ± 3.1 Ma), and later stage bimodal intrusions (biotite quartz monzonite: 278.5 ± 1.8 Ma, gabbro: 278.1 ± 2.3 Ma). The earlier stage granitoids are high-K calc-alkaline, enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their depleted isotopic compositions (εNd(t) = +6.29 to +7.48) and juvenile model ages (TDM2 = 450–610 Ma), we infer that the granitoids were derived from juvenile lower crust in a post-collisional tectonic transition (from compression to extension). The structural and temporal features indicate that the earlier stage (ca. 290 Ma) granitoids formed prior to the regional large-scale dextral strike slip. The later stage bimodal intrusions are dominated by biotite quartz monzonite as the felsic member and gabbro as the mafic component. The biotite quartz monzonite is high-K calc-alkaline with enriched LREEs and LILEs (e.g., Rb, Th, and U), and depleted HFSEs (e.g., Nb, Ta, and Ti), whereas the gabbro is subalkalic with depleted LREEs and HFSEs (e.g., Nb and Ta), resembling normal mid-ocean ridge basalt features. The bimodal intrusions show similar isotopic compositions (εNd(t) = +6.41 to +6.72 and εHf(t) = +9.55 to + 13.85 for biotite quartz monzonite; εNd(t) = +9.13 to +9.69 and εHf(t) = +4.80 to +14.07 for gabbro). These features suggest that the later stage (ca. 280 Ma) bimodal intrusions were derived from partial melting of depleted mantle and anatectic melting of lower crust materials induced by synchronous underplating of basaltic magma in a post-collisional extension. The structural features of the bimodal intrusions indicate that the later stage (ca. 280 Ma) magmatism was coeval with the development of the KHSZ. In conjunction with spatial and temporal evolution of magmatism and sedimentary records of Eastern Tianshan, we infer that transition between the northward closure of the North Tianshan Ocean and subsequent collision between the Central Tianshan Massif and the Qoltag Arc belt occurred at ca. 300 Ma.

Early Permian subduction-related transtension in the Turpan Basin, East Tianshan (NW China): implications for accretionary tectonics of the southern Altaids

AbstractThe interaction of the Palaeo-Pacific and Palaeo-Asian Oceans is an enigmatic issue as their temporal and spatial features are controversial. To address this issue, we present a systematic study of large volumes of early Permian volcanic rocks and intrusions developed in the East Tianshan. The represented samples of basaltic andesites and rhyolites yield zircon crystallization ages of 285.1 ± 5.9 Ma and 275.3 ± 1.8 Ma, respectively. The basalts have normal mid-ocean ridge basalt (N-MORB) and arc-related geochemical signatures with high TiO2 contents, negative Rb, Th, U, Nb and Ta anomalies and positive Eu anomalies. Basaltic andesites and andesites have arc-related geochemical characteristics with moderate TiO2 contents and relatively negative Nb, Ta and Ti anomalies, together with slightly negative to positive Eu anomalies. The rhyolites show an affinity with A2-type granite with high SiO2, K2O + Na2O, Fe/Mg, Ga, Zr, Nb, Y, HFSE, REE and Y/Nb levels (>1.2). These geochemical data suggest that the rocks formed in a supra-subduction zone. The presence of high ϵNd(t) values of +4.6 to +8.2 and low (87Sr/86Sr)i (0.70342–0.70591) values indicates that the volcanic rocks originated from a depleted mantle. We propose that oblique subduction with slabs breaking off gave rise to transtension and to the emplacement of large volumes of mantle-derived melts in the early Permian in the East Tianshan, serving as an important record of the subduction zone of the Palaeo-Pacific Ocean.

Tectonic Implications and Petrogenesis of the Various Types of Magmatic Rocks from the Zedang Area in Southern Tibet

In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158–161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cretaceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive eHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of eHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.

Geochemistry and Mineralogy of Basalts from the South Mid-Atlantic Ridge (18.0°–20.6°S): Evidence of a Heterogeneous Mantle Source

The South Mid-Atlantic Ridge is a typical slow-spreading ridge that represents a modern example to understand mantle composition and the evolution of mid-ocean ridge magmatism. In this paper, we investigate basalt samples dredged from four locations along the South Mid-Atlantic Ridge ranging from 18.0° to 20.6°S. The basalts belong to the tholeiitic series and exhibit normal mid-ocean ridge basalt (N-MORB) geochemical features with variable enrichments of Rb, Th, U, and Pb and depletions of Ba and Sr relative to primitive mantle. Some samples have negative Nb–Ta anomalies whereas others have positive Na–Ta anomalies to average N-MORBs. Plagioclase phenocrysts, microphenocrysts, and microlites occur in the in the matrix; phenocrysts and microphenocrysts are bytownite and labradorite in composition. Olivine phenocrysts are forsterite (Fo87 to Fo96). Chemical zoning in phenocrysts are interpreted to record crystal fractionation and magma mixing. Cores of plagioclase phenocrysts have higher anorthite values (An72–83) and estimated crystallization temperatures (~1180–1240 °C) that may suggest a xenocrystic origin. The lower anorthite proportions of rims of plagioclase phenocrysts (An65–71) and microphenocrysts (An54–72) yield lower estimated crystallization temperatures of ~1090–1120 °C and ~980–1060 °C, respectively. Rims of plagioclase phenocrysts and microphenocrysts may be generated in different environments such as magma chambers or magma channels, respectively. The basalt samples probably originated from partial melting of a depleted mantle spinel lherzolite source with a minor contribution of enriched materials possibly derived from the Saint Helena plume and subcontinental lithospheric mantle in the asthenosphere. Variable compositions of the basalt samples suggest heterogeneous mantle that includes depleted and enriched components at the South Mid-Atlantic Ridge between 18.0°–20.6°S.

Infant intra-oceanic arc magmatism due to initial subduction induced by oceanic plateau accretion: A case study of the Bangong Meso-Tethys, central Tibet, western China

The Meso-Tethyan oceanic plateaus are becoming conspicuous as giant units on the oceanic floor and have played important roles in both continental marginal orogenesis and Tethys oceanic evolution. In this study, we present mineralogical, geochronological, geochemical and Sm–Nd isotopic data for basaltic lavas from the Namco ophiolite and a high-Mg pillow lava–dyke–gabbro association from the Pengco ophiolite in central Tibet. Zircon U–Pb and Ar–Ar dating reveals that the Namco lavas erupted at ∼181 Ma while the Pengco boninitic association formed at ∼164 Ma. The Namco lavas display nearly flat rare-earth element (REE) patterns with no Nb–Ta depletions as well as high εNd values, characteristic of oceanic plateau lava. In contrast, the Pengco high-Mg rocks exhibit low REE concentrations below the normal mid-ocean ridge basalt (N-MORB), ubiquitous Nb–Ta depletions and low εNd values, and the dykes and gabbros are characterized by U-shape REE patterns, indicating that they could have derived from a depleted mantle source that was contaminated by sedimentary flux and marking a mid-Jurassic initial intra-oceanic arc magmatism erupted on the Early Jurassic Meso-Tethyan oceanic plateau represented by the Namco ophiolite. Our Pengco boninitic rocks, along with the literature data, indicate a 167–160 Ma boninitic-like initial intra-oceanic arc within the Bangong Meso-Tethys, running from the Shiquanhe area to the Naqu area with a length of ∼1000 km, which was uniformly built on the Early Jurassic Meso-Tethyan oceanic plateau. Our literature investigation also indicates a ∼175 Ma accretionary orogeny with distinct signature of the oceanic plateau involvements along the southern Qiangtang continental margin, which is manifested by regional metamorphic, magmatic and depositional records. We thus suggest that the accretion of the Early Jurassic Meso-Tethyan oceanic plateau onto the southern Qiangtang continental margin resulted in the extensive orogeny along the continental margin, jammed the subduction zone at ∼175 Ma and induced intra-oceanic subduction initiation as well as the intra-oceanic infant arc magmatism in the Meso-Tethys at ∼164 Ma.

Geochemistry of axial lavas from the mid- and southern Mariana Trough, and implications for back-arc magmatic processes

The Mariana Trough, a relatively simple intra-oceanic back-arc basin, is ideal for investigating magmatic processes and mantle-crust interaction in a subduction setting. We present new major- and trace element compositions for 31 basaltic lava and glass samples from the Mariana Trough back-arc spreading center. The studied lavas include phenocrysts of plagioclase, olivine and pyroxene. Major element compositions show that these lavas range from tholeiitic basalt to basaltic andesite, and belong to a sub-alkali tholeiitic series produced by fluid-influenced fractional crystallization of primary basaltic melts. Trace element compositions show that these lavas are transitional between typical normal mid-ocean ridge basalts (MORB) and island arc basalt (IAB), and are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs). Trace element ratios, e.g., Ba/Th, Pb/Ce, Th/Nd, La/Sm, Th/Nb, Ba/Nb and Th/Nb, indicate that the mantle from which these lavas were derived underwent modification resulting from the addition of multiple subduction components. Some typical trace element ratios (e.g., Ba/Nb- total subduction component, Ba/Th- shallow subduction, and Th/Nb-deep subduction component) from our new data and the literature suggest that a latitudinal variation exists in addition to subduction components, and indicates a more complex and heterogeneous distribution of subduction components in the Mariana back-arc region. We suggest that, (1) compared to back-arc locations at 18° N and 15.5° N, lavas from back-arc locations at 17° N indicate higher levels of modification by hydrous fluid released from the subducted slab, and (2) compared to back-arc locations at 17° N and 15.5° N, petrogenesis of lavas from back-arc locations at 18° N indicates a greater influence of sediment melt.

Petrology and Geochemistry of Mafic Intrusive Rocks from the Sapi-Shergol Ophiolitic Mélange, Indus Suture Zone, Western Ladakh: Constraints on Petrogenesis and Tectonic Setting

This article reports the existence of subduction-related tholeiitic, normal mid-ocean ridge basalt (N-MORB)-type mafic intrusives emplaced within the Sapi-Shergol ophiolitic mélange of the Indus Suture Zone, western Ladakh. The Shergol mafic intrusives show Fe enrichment with basalt to basaltic-andesite composition. Based on their mineralogy and textures, these mafic rocks can be identified as fine- to medium-grained gabbros that have undergone greenschist-grade metamorphism. These rocks have N-MORB-type geochemical characteristics, exhibiting nearly flat to depleted light rare earth element patterns ((La/Sm)N=0.66–1.05). Petrogenetic modeling suggests <20% partial melting of a depleted MORB-type mantle source, within the spinel peridotite stability. The presence of slightly negative anomalies of high field strength elements like Nb, Zr, and Ti in multielement patterns reflect the influence of subduction zone magmatism. The presence of low-Ti clinopyroxene (En38–50Fe11–25Wo31–43; enstatite-ferrosilite-wollastonite), Ca-rich plagioclase (An2–36; andesine), and pargasitic amphibole also reflects their subduction-related depleted-mantle origin. The MORB–island arc tholeiite signature displayed by the Shergol gabbros intrusive in Shergol peridotites reflect their generation in a mantle wedge associated with the Early Cretaceous intraoceanic subduction within the Neo-Tethys Ocean. They are similar to Spongtang ophiolite gabbros intrusive in the Spongtang ophiolite mantle peridotites from south Ladakh, ophiolitic mélange gabbros from southern Tibet, and Muslim Bagh ophiolite gabbros from Pakistan. Based on this study, we offer a geodynamic model suggesting that the Sapi-Shergol ophiolitic slice was intruded by mafic intrusive rocks that represent the relict of the intraoceanic substratum of the Cretaceous Dras arc complex.

Geochemistry and Geochronology of Ophiolitic Rocks from the Dongco and Lanong Areas, Tibet: Insights into the Evolution History of the Bangong-Nujiang Tethys Ocean

The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet hosts a series of dismembered Jurassic ophiolites that are widely considered as remnants of the vanished Meso-Tethys Ocean. In this study we present new compositional, isotopic, and geochronological data from anorthosites and gabbros of the Dongco and Lanong ophiolites in order to test several hypotheses about the nature of subduction in the Bangong-Nujiang Tethys Ocean (BNTO) during the Mesozoic era. Uranium–Pb dating of magmatic zircons separated from the Dongco anorthosites yielded an (average) age of 169.0 ± 3.7 Ma. Zircons separated from the Lanong anorthosites and gabbros yielded U–Pb ages of 166.8 ± 0.9 Ma and 167.3 ± 1.1 Ma, respectively. Zircons separated from the Dongco and Lanong anorthosites have positive εHf(t) values (5.62–15.94 and 10.37–14.95, respectively). The Dongco anorthosites have moderate initial 87Sr/86Sr (0.703477–0.704144) and high εNd(t) (+6.50 to +7.91). The Lanong anorthosites have high (87Sr/86Sr)i (0.706058–0.712952) and εNd(t) in the range of −1.56 to +2.02. Furthermore, the Lanong gabbros have high (87Sr/86Sr)i (0.705826–0.706613) and εNd(t) in the range of −0.79 to +4.20. Most gabbros from Dongco and a few gabbros from Lanong show normal mid-ocean ridge basalt (N-MORB)-like primitive mantle (PM)-normalized multi-element patterns. In contrast, most gabbros from Lanong show U-shaped chondrite-normalized rare earth element (REE) profiles. The investigated gabbros are characterized by wide ranges of δEu {(Eu)N/[(Sm)N*(Gd)N]1/2} values (0.83–2.53), indicating that some of them are cumulative rocks. The trace element contents of all anorthosite samples imply that their composition was controlled by cumulative processes. The geochemical and isotopic compositions of the non-cumulative gabbros from Dongco (δEu: 0.95–1.04) and Lanong (δEu: 0.83–1.03) indicate that their parental melts were derived from melting of heterogeneously depleted, juvenile mantle reservoirs. These rocks have arc-related affinities, indicating that their mantle sources were influenced by minor inputs of subducted lithospheric components. Our preferred hypothesis for the origin of the non-cumulative gabbros from Dongco is that they were formed in a transient back-arc basin (BAB) setting in the middle-western segment of the BNTO, whereas our preferred scenario about the origin of the non-cumulative gabbros from Lanong is that they were generated in a forearc setting in the middle part of the BNTO. We conclude that both geotectonic settings were developed in response to the northward subduction of the BNTO during the Middle Jurassic.

Origin of Mesozoic ophiolitic mélanges in the western Yarlung Zangbo suture zone, SW Tibet

The petrogenesis and tectonic evolution of the Mesozoic ophiolitic mélanges in the western section of the Yarlung Zangbo suture zone (YZSZ) remain controversial. In this paper, we present the results of whole-rock geochemical and SrNd isotope analyses, zircon UPb ages and in situ LuHf isotopic data obtained from mafic rocks of the northern and southern sub-belts of the western YZSZ Mesozoic ophiolitic mélanges to help us understand these controversial issues. Diabases and dolerites from the northern sub-belt and gabbros from the southern sub-belt exhibit variable fore-arc basalt (FAB)-like geochemical compositions and have zircon UPb ages of ∼126.4–120.3 Ma. In addition, gabbro-diabases from the northern sub-belt have boninite series affinities and yield a zircon UPb age of ∼125.7 Ma. These results, along with previous studies on the YZSZ Mesozoic ophiolitic mélanges and the Gangdese arc, reveal that the western YZSZ Mesozoic ophiolites were likely generated over multiple stages in the epicontinental Gangdese fore-arc basin as the Yarlung Zangbo Neo-Tethyan Ocean subducted northward in front of the Lhasa terrane. The Early Cretaceous FAB-like and boninite series mafic rocks were formed by the reinitiation of subduction, which was followed by a retreat of the subduction zone and the creation of the fore-arc basin and strong hyperextension, accompanied by asthenosphere upwelling at ∼130–120 Ma. During this process, the upwelling asthenosphere underwent decompressional melting with limited penetration of slab-derived fluids and gave rise to the N-MORB (normal mid-ocean ridge basalt)-like basaltic magmas that intruded the overlying, previously generated depleted mantle as FAB-like gabbro, diabase and dolerite sills or dykes. Then, boninitic magmas represented by boninitic gabbro-diabases were generated by remelting the extremely depleted residual mantle source, which was metasomatized by a small amount of slab-derived fluids, following previous extractions of FAB-like magma.

Pillow lava basalts with back-arc MORB affinity from the Usagaran Belt, Tanzania: relics of Orosirian ophiolites

The Paleoproterozoic Usagaran Orogenic Belt in East Africa is an exemplary site of preserved relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic crust. This paper describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group). Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist-facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid-immobile elements. The (La/Sm) N ratios (0.62–1.09) and REE patterns point to normal mid-ocean ridge basalt (N-MORB) and transitional MORB (T-MORB) mantle source. The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts are an analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian. The emplacement of the Konse pillow basalts is older than the neighbouring 1920–1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Thus, the Konse pillow basalts might have been displaced from their suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Mineral and bulk–rock chemistry of Shadli bimodal metavolcanics from Eastern Desert of Egypt: Implication for tectonomagmatic setting and Neoproterozoic continental growth in the Arabian–Nubian Shield

The Neoproterozoic Shadli metavolcanics in the Eastern Desert of Egypt have been conventionally categorized as a bimodal island-arc association. The present study deals with Shadli metavolcanics of three areas: Hamama in the central and Darhib and Atshan in the southern Eastern Desert of Egypt. They contain both mafic and felsic compositions and display a bimodal nature. Geochemical classification indicates that the mafic end-members are tholeiitic basalt and basaltic andesite whereas the felsic ones comprise calc-alkaline dacite and rhyolite.Mafic end-members show flat to slightly LREE-depleted patterns comparable to normal MORB produced from a depleted mantle source. Major element data indicates affinity to arc-related non-cumulative mafic rocks. The obvious negative Nb and positive Pb anomalies, the nearly flat REE patterns together with pyroxene chemistry are characteristic of island-arc tholeiitic suite. Accordingly, we suggest that their primary magma was derived from partial melting of a depleted mantle source metasomatized by subduction-related slab fluids/melts.Felsic end-members exhibit magnesian character consistent with a relatively hydrous, oxidizing magma and subduction-related source. Their weakly peraluminous character and calc-alkalic to calcic compositions are characteristics of arc-related magmas. Moreover, the negative Nb and Ti anomalies suggest an influx of subduction-related slab fluids/melts. Their SiO2 contents are too high to represent magmas directly derived by partial melting of a mantle source. So, the high SiO2 and Al2O3 and the low MgO contents suggest melting crustal source. The high Y/Nb ratios (> 1.2) confirm that they were generated from crustal source. Actually, their Ce/Pb ratios (average of 2.66) are affiliated to those of continental crust and contrast typical mantle values. The weakly peraluminous and sodic nature is a typical feature of I-type magma. Therefore, their magmas were most likely derived by partial melting of thickened low-K mafic lower crust.Several lines of evidence let us speculate a genetic relationship between mafic and felsic end-members and that the felsic rocks were possibly evolved from the mafic melts that derived from metasomatized mantle. These comprise the close proximity and a coeval nature; the subalkaline character with approximately continuous trend; the similarity of trace element patterns; the arc-like signature, and the well fitness of major element data of the felsic rocks with melts derived from low-K mafic lower crust. The general tendency of silica increase with alkalis from mafic to felsic end-members may indicate that the felsic rocks were generated by fractional crystallization processes of the mafic magmas in the area.