High Degrees Of Partial Melting Research Articles

The Late-Paleoarchean Ultra-Depleted Commondale Komatiites: Earth's Hottest Lavas and Consequences for Eruption

Abstract The c.3·3 Ga Commondale komatiites located south of the Barberton greenstone belt in the Kaapvaal Craton are different from other komatiites, possessing compositional and textural features unique to this occurrence. Unlike almost all other known komatiite occurrences, they are not associated with komatiitic basalts or basalts. The komatiite flows are 0·5–25 m thick and are made up of a marginal zone of spinifex-textured and fine-grained aphyric rocks (low-Mg group) and an inner zone of olivine cumulates (high-Mg group), arranged in such a way to give highly symmetrical compositional profiles for many flows. Olivine is the dominant phase in all rocks, but orthopyroxene occurs as spinifex and elongate laths in the marginal zone. Clinopyroxene and plagioclase are entirely absent. The olivine cumulates formed from Mg-rich magma (36·1% MgO, 6·8% FeO) which caused inflation of the thicker flows. The maximum observed olivine composition in cores (Fo 96·6) is the highest recorded for any komatiite worldwide. The high-Mg magma would have erupted at a temperature close to 1670°C, the highest inferred temperature for an anhydrous terrestrial lava. The marginal zone is enriched in incompatible elements compared with the inner zone and formed by fractionation of the parental melt. However, all rock-types in the marginal zone are depleted in FeO (some as low as 3·5%) which could not have been derived by any primary magmatic process. The marginal zone rocks were modified by assimilation and/or alteration by seawater (or brine) components causing migration of iron and strong enrichment of sodium (up to 1·6 wt % Na2O) and chlorine (up to 2400 ppm). Zirconium has an identical distribution to sodium, with both elements greatly enriched above what would result from fractional crystallization, and may result from speciation of these elements at high temperature followed by post-crystallization alteration. Rare earth elements, Y and Nb have contents commensurate with fractionation of the primitive parental magma. Dendritic-textured olivine-rich rocks with orthopyroxene spinifex spatially and compositionally transitional between the marginal zone and the olivine cumulates resulted from interaction of the high temperature parental magma in the centre of the flows with the fractionated melt at the flow margins. A further manifestation of this association is the development of highly regular fine-scale (5–15 cm) layering (up to 45 layers) of alternating olivine cumulate and spinifex near the base of thick flows. This is overlain by olivine cumulates in which the melt/crystal-mush became arranged into a 3-dimensional network controlled by re-distribution of the trapped melt manifest by a spectacular knobbly texture in outcrop. Over 200 flow units are recognized and detailed chemical and mineralogical studies were carried out on drill cores intersecting 375 m of stratigraphy. The parental magma was highly depleted (in ppm Nb 0·017, Zr 1·18, total REE 1·7 and Gd/YbN=0·3, La/YbN=0·038) and although generally regarded to fall into the rare category of Al-enriched komatiites (AEKs), it is considered that these lavas are a unique class of their own of ultra-depleted komatiites. Relative to other AEKs the Commondale komatiites are both enriched in Al as well as being markedly depleted in Ti (390 ppm), giving rise to the extremely high Al2O3/TiO2 (81). The high temperature and low viscosity of the magma resulted in emplacement processes previously unrecognized in komatiites. The primary melt was derived by melting of mantle peridotite in equilibrium with olivine and orthopyroxene. The initial source was depleted in incompatible elements by small degrees of melting (3–4%) followed by high degrees of partial melting (70%) of the subsequent refractory source at 5 GPa (∼150 km).

Mineralogy and Geochemistry Variation of Igneous Rocks from Ungaran and Muria Volcano and Its Processes related to Subduction Zone Magmatism of Sunda Arc

Sunda Arc represent Quaternary magmatism as one of the most significantly active volcanic arc in Indonesia resulted from subduction of oceanic plate beneath continental plate. Along this subduction system, there are systematic variations in subduction angle, depth, crust thickness, and composition contribute to geochemical characteristic of igneous rocks along and across the arc. This study compared mineralogy and geochemistry characteristics from two volcanoes which represent across-arc variation. Petrographic analysis have been conducted on rocks’ thin section samples from Ungaran and Muria Volcano to characterize mineralogy of rocks. Selected samples were analyzed using X-Ray Fluorescence (XRF) for geochemistry of major and trace elements. Mineralogy of Ungaran rocks are dominated by plagioclase, clinopyroxene, and hornblende phenocrysts with composition range from andesite to basaltic andesite of high-K calc alkaline series. Samples from Muria Volcano are from alkaline series characterized by feldspathoid and alkali feldspar rich rocks with calcic plagioclase and pyroxene phenocrysts. This mineral assemblages, supported by major elements data suggest fractionation process in magma. Geochemical trend is systematically consistent with other island arc setting, where there is significant increase of total alkali from Ungaran to Muria. Trace element data shows decreasing in LILE/HFSE ratio further from arc. This suggests that Ungaran has more crustal and subducted slab influence, while Muria has higher degree of partial melting from deeper Benioff Zone.

Syncollisional gabbro in the Irtysh shear zone, Eastern Kazakhstan: Compositions, geochronology, and geodynamic implications

New data of petrography, mineralogy, and geochemistry from the Irtysh shear zone in Eastern Kazakhstan reveal three types of gabbro: (1) dolerite and gabbrodolerite, (2) biotite-bearing gabbronorite and gabbrodiorite, and (3) layered peridotite-gabbro intrusions. Rocks of types 1 and 2 occur mainly as small bodies and share similarity in major- and trace-element compositions. Layered gabbro (type 3) forms the largest intrusion in the area and has a different composition: higher CaO and Al2O3 but lower K2O, TiO2, P2O5, Zr, Nb, REE. Correspondingly, the gabbro of two first types may result from differentiation of the same tholeiite magma, whereas the layered gabbro rather derived from a magma intermediate between tholeiite and calc-alkaline series. According to geochemical and isotopic signatures, both types of primary magma originated from the same sublithospheric depleted mantle by low- and high-degree partial melting of spinel lherzolite. The obtained 317–313 Ma U-Pb and Ar-Ar ages of the gabbro fall within the time of collisional events in the area. The suggested model explains the origin of syncollisional gabbro as intrusion of ascending mafic melts from a mantle wedge at the transform boundary of colliding plates along the Irtysh shear zone where strike-slip faulting produced slab windows open to inputs of hot asthenospheric material.

Intense faulting and downwarping of Nanpu Sag in the Bohai Bay Basin, eastern China: Response to the Cenozoic Stagnant Pacific Slab

Abstract The Nanpu Sag (NPS), located in northwestern Bohai Bay Basin (BBB), East China, has significant oil discoveries in recent years. The effect of intense faulting and downwraping which generated the intense subsidence of NPS in the Dongying Formation (Fm.) (Ed) (28.5–23.8 Ma) of the Late Oligocene is presented in this paper. We analyzed the geochemical characteristics of the widespread Cenozoic basalts in NPS from drill core samples. Our results show that the magma experienced negligible crustal contamination and slight fractional crystallization, and the features of remarkable positive U, Pb, Sr and Ti anomaly, low Rb/Ba and Rb/Sr values of the NPS basalts were inherited from the source region. The NPS basalts in Ed have distinctive characteristics as sub-alkaline basalts with rare earth elements (REE) patterns parallel to enriched mid-ocean ridge basalts (E-MORB), and were generated from a mixture origin of garnet-lherzolite zone with relatively high degree of partial melting (30%–50%) and garnet + spinel-lherzolite transition zone with partial melting degree ranging from 3% to 15%. In contrast, NPS basalts of other Cenozoic periods or Ed basalts of other areas in eastern China are alkaline with light REE enrichment and oceanic island basalts (OIB)-like-pattern, their magmas originated from garnet + spinel-lherzolite transition zone with partial melting degree lower than 5%. Combined with previous studies of the mantle wedge in the mantle transition zone (MTZ) beneath northeastern Asia, forming around 30 Ma, we infer that the stagnating of the Pacific slab triggered the asthenosphere upwelling and increased the degree of partial melting in mantle source, and relatively weak areas of the North China Craton (NCC) like the Tanlu Fault (TLF) where NPS located on were reactivate and susceptible to be modified. Therefore, the intense faulting and downwarping of NPS in Ed, along with the igneous activities, was a response to the Cenozoic stagnant Pacific slab more than the result of tectonic effects from the Pacific subduction like other areas in northeastern China in Paleogene.

Geochronology, petrogenesis and tectonic implications of the newly discovered Cu–Ni sulfide-mineralized Yueyawan gabbroic complex, Kalatag district, northwestern Eastern Tianshan, NW China

The early Permian Cu–Ni sulfide-bearing mafic-ultramafic intrusions from North Xinjiang in NW China help to provide constraints on the late Paleozoic crustal growth in the Central Asian Orogenic Belt (CAOB) and improve the understanding of the genesis of magmatic sulfide mineralization in orogenic settings and the identification of future prospecting targets. The Cu–Ni sulfide-mineralized Yueyawan gabbroic complex in Kalatag represents a new breakthrough for Cu–Ni prospecting in covered areas in the northwestern part of the Eastern Tianshan orogenic belt, North Xinjiang. Rocks from this intrusion have similar LA-ICP-MS zircon U–Pb ages (282.4 ± 1.1–281.3 ± 1.2 Ma) to other mafic-ultramafic intrusions containing magmatic sulfide mineralization in North Xinjiang. These early Permian gabbroic intrusions are thus important targets for Cu–Ni mineralization exploration in this region. The rocks of the Yueyawan gabbroic complex are a ca. 1 km2 elongated NW-trending crescent-shaped at the junction of two E-W- and NW-SE-trending faults. The intrusion has the morphology of a conduit tube. The rocks include troctolite, olivine gabbro, hornblende gabbro, and gabbro, and the Cu–Ni sulfides occur as steeply dipping (65–70°) lenses with disseminated textures in the contact zone between the olivine gabbro and hornblende gabbro, indicating that sulfide precipitation was coeval with the crystallization of the magmas giving rise to these rocks. The rocks investigated in this study are gabbroic in composition; they are characterized by moderate concentrations of SiO2 (44.8–48.8 wt%) and low to moderate concentrations of MgO (1.69–13.4 wt%, Mg# values of 39.6–73.3), Ni, and Cr. Furthermore, the rocks have high positive εHf(t) values (+13.1 to +15.9) and flat rare earth element patterns ((La/Yb)N = 2.01–2.89), with marked positive Eu anomalies, high Ti/Zr ratios (70–164), and low Ce/Pb (3.65–14.83) and Nb/Ta values (9–16), indicating very restricted amounts (5–10%) of crustal assimilation. This complex was likely derived from 10 to 20% partial melting (as evidenced by the low Sm/Yb values of 1.48–2.02 and La/Sm values of 1.61–2.23) of weakly slab melt-metasomatized (as evidenced by the low Ba/Nb and Ba/Zr ratios) subcontinental lithospheric mantle (with 2% garnet and 2% spinel lherzolite) at depths greater than 85 km in a post-collisional extensional source that contained sufficient Ni and Cu for the saturation and segregation of immiscible sulfides to concentrate the metals. The magma underwent sulfide saturation and segregation together with olivine fractionation (as evidenced by Ni and TFe2O3 increase and SiO2, CaO, and Zr decrease with increasing MgO content) in the conduit-type intrusion. Therefore, regional fault structures, ore-bearing rock types, and geochemical ratios that reflect high degrees of partial melting are features of the Cu–Ni-mineralized gabbroic rocks in adjacent areas that help support exploration.

Fractionation of Ruthenium from Other Highly Siderophile Elements (HSEs) in Forearc Mantle

AbstractThe geochemical characteristics and behaviors of highly siderophile elements (HSEs) in forearc peridotites remain poorly constrained due to the scarcity of data. Here, we report HSE abundances of mantle peridotites from the New Caledonia ophiolites, a classical ophiolite generated in a forearc setting. Those peridotites show non‐chondritic, strongly fractionated HSE patterns and can be classified into two distinct types (namely Group I and Group II). Group I peridotites have higher HSE contents than Group II peridotites, which might be because intergranular sulfides were completely removed but sulfide inclusions were retained during partial melting of peridotites in a forearc environment, and meanwhile the distribution of sulfide inclusions are not uniform in mantle. Moreover, Group I peridotites display flat patterns from Os to Pt but strongly depleted in Pd, which resemble those patterns of some mantle wedge xenoliths. The Pt–Pd decoupling can be attributed to high degrees of partial melting. However, Group II peridotites are characterized by strongly positive Ru anomaly with highly super‐chondritic Ru/Os and Ru/Ir ratios. Such characteristics are the first reported cases for forearc peridotites. The fractionation of Ru from other HSEs might reflect the stability of refractory Ru‐rich phases in mantle wedge peridotites during different processes, e.g., partial melting and melt/fluid‐rock reactions.

Extreme intensity of fluid-rock interaction during extensive intraplate volcanism

The initial formation and temporal evolution of large igneous plateaus and the extent to which these large enigmatic igneous features impact on their immediate and distant ecological surrounding remains a matter of active research. The compositional variability in large igneous provinces has mainly been attributed to changes in the melting regime and shallow crustal processes and commonly ranges from depleted tholeiitic basaltic to enriched alkaline lavas. Large igneous provinces erupted in the submarine environment however, may also experience intense hydrothermal alteration during their formation resulting from an increased exchange between seawater and the erupting lavas during their eruptive history. The submarine Azores Plateau in the Central Northern Atlantic has generally been treated to represent such large igneous province formed since ∼10 Ma by widespread volcanism and the unique tectonic regime which results in large fault systems exposing the erupted lavas. Here, we present new seismic, petrological and major element, trace element and isotope geochemical data from a ∼1000 m stratigraphic section of submarine lavas exposed at the western Princessa Alice bank. The 22 samples recovered from the near-vertical rift wall provide evidence for intense water-rock exchange not observed anywhere in oceanic crust sampled to date. Fluid-immobile incompatible trace elements show that the samples formed from higher degrees of partial melting of a mantle source that is less enriched than the source that gives rise to the islands today. The extents of melting today are very small, implying a change in melting regime since initial formation of the Princessa Alice Plateau basalts that correspond to a melting anomaly in the Azores. Our observations indicate that the extreme levels of alteration may result from a combination of intensified magmatic activity during initial formation of the Azores Plateau and the tectonic regime providing pathways for the fluids. Our results impact on the interpretation of shallow level crustal magmatic processes, in which the contribution of crustal sources to the ascending melt may be different to what had previously been suggested. We propose that hydrothermal alteration during submarine igneous plateau forming events can drastically change the compositions of the igneous crust. The associated elemental and isotopic exchange between the oceanic crust and hydrosphere may substantially change the chemical fluxes between oceans and crust during the emplacement of oceanic plateaus.

Petrogenesis of the southern Qiangtang mafic dykes, Tibet: Link to a late Paleozoic mantle plume on the northern margin of Gondwana?

AbstractThis study presents 13 new U-Pb zircon ages obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) together with whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data for a mafic dike swarm in the southern Qiangtang area of Tibet. These data provide the basis for a new model of the late Paleozoic evolution of the Tethys. Combined with the results of previous zircon U-Pb dating, the magmatic zircon grains extracted from mafic dikes yield latest Carboniferous to Early Permian ages (317–279 Ma). The geochemistry of the southern Qiangtang mafic rocks indicates the presence of low-Ti (QLT) and high-Ti (QHT) suites. The magmas that formed the QLT suite underwent higher degrees of partial melting (>5%) and display evidence of crustal contamination, whereas the QHT suite was derived from magmas generated by low-degree (1%–5%) partial melting of a garnet-bearing mantle source, with a greater extent of fractional crystallization than the QLT suite, and no evidence of crustal contamination. We propose that the QHT and QLT suites may have been derived from magmas from different parts of a single mantle plume. The formation of the southern Qiangtang mafic dikes (latest Carboniferous to Early Permian; 317–279 Ma) may have been related to the northward drift of the Cimmerian continent from the northern Gondwana margin, which resulted in the opening of the Meso-Tethys Ocean.

Mafic whole-rock geochemistry and neodymium isotopes, Green Mountain and Rowe/Prospect Rock slices, Vermont Appalachians

Mafic rocks containing sodic-calcic amphibole in the Green Mountain slice (GMS) and Rowe/Prospect Rock slice (R/PRS), Vermont Appalachians were originally subalkaline basalts emplaced as melts during Neoproterozoic rifting of Rodinia that led to the formation of the Iapetus Ocean basin. Relatively high degrees of partial melting of asthenosphere that was highly depleted in incompatible elements produced R/PRS magma(s), which may have been contaminated locally by continental crust and/or fluids. Trace element chemistry suggests that mafic bodies from different locations in the R/PRS may be fractionated magmatic equivalents. Mafic rocks in the GMS formed from magmas produced by relatively low degrees of partial melting of mantle that was relatively enriched in incompatible elements compared to depleted mantle. These enriched melts may have been derived from a plume and/or enriched lithospheric components, potentially continental crust, embedded in depleted upper mantle. A depleted mantle signature preserved locally in the GMS probably reflects increasing asthenospheric input during crustal thinning, opposed to crustal or fluid contamination. Whole-rock minor and trace element data from the GMS and R/PRS are distinct from analyses of glaucophane schist from the Tillotson Peak Complex in northern Vermont, which may be exhumed Iapetan ocean floor. Exhumed mafic rocks in the GMS and R/PRS were formed as lower plate melts prior to relatively high pressure subduction zone metamorphism, providing evidence for subduction of the Laurentian margin, not subduction erosion. Low pressure greenschist facies mafic rocks that occur structurally between the GMS and R/PRS were sourced from depleted or highly depleted mantle in a supra-subduction zone environment, potentially a forearc or backarc basin; an ophiolitic origin is equivocal. Geochemical and isotopic data and interpretations are compatible with rift-related tectonomagmatic models for the peri-Laurentian realm of the northern Appalachians.

Subduction-zone contributions to axial volcanism in the Oman–U.A.E. ophiolite

Over four decades of research on the Semail ophiolite (Oman–U.A.E.) has greatly in uenced our understanding of processes occurring at fast-spreading ocean ridges. While the well-developed sheeted dike complex and comagmatic lower pillow lavas indicate that the early Semail crust formed at a spreading axis, the precise tectonic setting of this axis—whether true mid-ocean ridge, back-arc or “proto”-arc— is contentious. This is largely because the tectonic implications of the geochemistry of the main axial volcanic unit (Geotimes/V1) are disputed. We bypass this hurdle by focusing on intercalations of primitive lavas that are depleted relative to mid-ocean-ridge basalt and that are deeply intercalated within the early Geotimes axial volcanostratigraphy throughout the northern ophiolite. Our analyses of these intercalations show a clear trace-element in uence from a subducting slab. We interpret the depleted axial melts to have formed by localized, high-degree partial melting assisted by a high-Th/Nb slab uid. These results con rm a deep subduction in uence on the entire axial spreading phase of the world’s largest ophiolite. Considered in the context of later hydrous and boninitic Alley volcanism and of insight from modern tectonic environments, our observations support a proto-arc, subduction-initiation setting for the origin of the Semail ophiolite.

Mineralogical Evidence for Partial Melting and Melt-Rock Interaction Processes in the Mantle Peridotites of Edessa Ophiolite (North Greece)

The Edessa ophiolite complex of northern Greece consists of remnants of oceanic lithosphere emplaced during the Upper Jurassic-Lower Cretaceous onto the Palaeozoic-Mesozoic continental margin of Eurasia. This study presents new data on mineral compositions of mantle peridotites from this ophiolite, especially serpentinised harzburgite and minor lherzolite. Lherzolite formed by low to moderate degrees of partial melting and subsequent melt-rock reaction in an oceanic spreading setting. On the other hand, refractory harzburgite formed by high degrees of partial melting in a supra-subduction zone (SSZ) setting. These SSZ mantle peridotites contain Cr-rich spinel residual after partial melting of more fertile (abyssal) lherzolite with Al-rich spinel. Chromite with Cr# > 60 in harzburgite resulted from chemical modification of residual Cr-spinel and, along with the presence of euhedral chromite, is indicative of late melt-peridotite interaction in the mantle wedge. Mineral compositions suggest that the Edessa oceanic mantle evolved from a typical mid-ocean ridge (MOR) oceanic basin to the mantle wedge of a SSZ. This scenario explains the higher degrees of partial melting recorded in harzburgite, as well as the overprint of primary mineralogical characteristics in the Edessa peridotites.

Paleoproterozoic volcanic rocks in the southern margin of the North China Craton, central China: Implications for the Columbia supercontinent

The volcanic rocks of the Xiong'er Group are situated in the southern margin of the North China Craton (NCC). Research on the Xiong'er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic. In this study, to constrain the age of the Xiong'er volcanic rocks and identify its tectonic environment, we report zircon LA-ICP-MS data with Hf isotope, whole-rock major and trace element compositions and Sr–Nd–Pb–Hf isotopes of the volcanic rocks of the Xiong'er Group. The Xiong'er volcanic rocks mainly consist of basaltic andesite, andesite, dacite and rhyolite, with minor basalt. Our new sets of data combined with those from previous studies indicate that Xiong'er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism. These volcanics have extremely low MgO, Cr and Ni contents, are enriched in LREEs and LILEs but depleted in HFSEs (Nb, Ta, and Ti), similar to arc-related volcanic rocks. They are characterized by negative zircon εHf(t) values of −17.4 to −8.8, whole-rock initial 87Sr/86Sr values of 0.7023 to 0.7177 and εNd(t) values of −10.9 to −6.4, and Pb isotopes (206Pb/204Pb = 14.366–16.431, 207Pb/204Pb = 15.106–15.371, 208Pb/204Pb = 32.455–37.422). The available elemental and Sr–Nd–Pb–Hf isotope data suggest that the Xiong'er volcanic rocks were sourced from a mantle contaminated by continental crust. The volcanic rocks of the Xiong'er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean. Thus, we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic, rather than in an arc setting.

Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction

Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon UPb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon UPb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns to E-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock εNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock εNd(t) (+2.9 to +3.4) and zircon εHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.

Evidence for rutile-bearing eclogite in the mantle sources of the Cenozoic Zhejiang basalts, eastern China

Cenozoic intraplate basalts are widely distributed along the southeastern margin of Eurasia. Both carbonated and eclogitic/pyroxenitic components, which are considered related to oceanic crust recycling, have been identified in the mantle source of these basalts. However, the origin of the recycled oceanic crust and the genetic relationship between different source lithologies remain unclear. Here, we assess the source lithologies of late Miocene intraplate basalts from Ninghai, Zhejiang Province, eastern China, using new elemental and Sr–Nd–Hf isotopic compositional data. These data are also compared with those of the nearby Xinchang–Shengzhou flood basalts and other Cenozoic basalts in Zhejiang Province. New Mg isotopic compositions of the Ninghai and Xinchang–Shengzhou basalts are used to evaluate the potential influence of carbonated components during their formation. Nephelinites and basanites from Ninghai and Xinchang–Shengzhou have negative Ti and Hf anomalies, superchondritic Ca/Al ratios, and light Mg isotopic compositions (δ26Mg = −0.35 to −0.38‰), indicating a carbonated component in the mantle source. By comparison, other Ninghai basalts have positive Ti and Nb anomalies, with Sm/Yb and Lu/Hf ratios that generally correlate with Ti/Ti* values. These observations indicate that rutile-bearing eclogite should also play an important role in the formation of the Ninghai basalts, in addition to mantle peridotite. The terrestrial mantle-likeMg-isotopic compositions of the Ninghai basalts further support their formation by high-degree partial melting of such mixed source. In the plots of Sr-Nd isotopes and Ti/Ti*, the nephelinites and basanites from Ninghai and Xinchang–Shengzhou show different trends compared with other Ninghai basalts, and the carbonated component in the mantle source of the former has similar and depleted Sr-Nd isotopes to the rutile-bearing eclogite source of the latter. Additionally, samples of these basalts show negative ΔεHf values similar to those of Pacific MORB. Subducted Pacific oceanic crust is therefore considered an appropriate source for both the carbonated and rutile-bearing eclogitic components. The Ninghai basalts provide an important insight into oceanic crust recycling in eastern China, with data indicating a genetic relationship between the carbonated component and rutile-bearing eclogite in the mantle sources of intraplate basalts.

The first record of ultramafic cumulates from the Mt. Kalnik ophiolite mélange in the SW part of the Zagorje-Mid-Transdanubian Zone (NW Croatia): mineralogy, petrology, geochemistry and tectono-magmatic affinity

Ultramafic cumulate rocks represent the rarest allochthonous fragments of the Mesozoic oceanic lithosphere observed today in the Upper Jurassic to Lower Cretaceous melange of Mt. Kalnik, located in the SW part of the Zagorje‒Mid-Transdanubian Zone (ZMTDZ). Poikilitic heteroadcumulate ultramafic rocks of Mt. Kalnik are represented by amphibole lherzolites/ harzburgite sand plagioclase lherzolites. Both were formed by in-situ processes within a magma chamber following the general crystallization sequence of: Al-chromite → Mg-rich olivine → enstatite ± augite→ Ca-amphibole (pargasite ± edenite ± magnesiohornblende) → Ca-plagioclase (An82.6-87.4). Cumulate minerals are spinel and olivine as well as orthopyroxene and clinopyroxene which areusually enclosed in intercumulate phases such as amphibole and/or plagioclase that render aninterstitial mesostasis. Rocks’ textural characteristics, mineral crystallization order and their phase chemistry are all suggestive of low-pressure sub-solidus crystallization in an open system. The low Ti content in augite and scant HFSE abundances suggest the studied rocks may have formed from a depleted mantle source. In addition to the medium to high degree of partial melting of the source, the parental process that gave rise to the Mt. Kalnik ultramafic cumulates also included a low degree of fractional crystallization. The segregation of oxidized Al-chromite and oikocrysts of pargasite and edenite in an early crystallization stage illustrates the formation of a cumulate sequence from volatile-rich magmas. These magmas usually have a high oxidation potential and are exclusively found in intra-oceanic subduction zones, predominantly in island arcs. The overall whole-rock geochemistry [e.g. (Nb/La)n = 0.25-0.34; (Ti/Gd)n = 0.49-0.89;(Th/Nb)n = 5.29-8.63; (La/Lu)cn = 0.57-0.68] together with a record of Ca-rich plagioclase (up toAn87.4) and low Ti clinopyroxene (≤0.54 wt%) corroborate the supra-subduction tholeiitic nature of the magma source. Ultramafic cumulates from the ophiolitic melange of Mts. Kalnik and Medvednica show common genetic features and geotectonic provenance. Comparison with analogous ultramafic lithotypes of the north-eastern segment of the ZMTDZ (the Szarvasko Complex,Hungary), the ultramafic cumulates of Mts. Kalnik and Medvednica portray some subtle differences that may indicate their distinctive geotectonic provenance. Mts. Kalnik and Medvednica ultramafic cumulates represent the vestiges of a single Upper Jurassic intra-oceanic arc system formed in the western branch of the Meliata-Maliak segment of the Neotethyan oceanic realm.

Mashhad komatiitic rocks in NE Iran: Origin and implications for the evolution of the Paleo‐Tethyan Ocean

AbstractMashhad ultramafic volcanic rocks, located within the Paleo‐Tethys suture zone in the north‐eastern Iran, are composed mainly of komatiitic rocks with interbedded picrobasalts. The Mashhad komatiitic rocks are characterized by low Al2O3/TiO2 ratios (9.85–11.44) and depleted HREE patterns with slightly high (Gd/Yb)N ratios (1.48–1.74), similar to those of the Al‐depleted Barberton‐type komatiites. These rocks have ƐNd (t) values ranging from +9.0 to +10.4 and initial 87Sr/86Sr ratios ranging from 0.7039 to 0.7072, indicating they were derived from a depleted mantle source. They have a total 187Re/188Os and 187Os/188Os ratios ranging from 0.278 to 72.254 and from 0.12741 to 0.14012, respectively, and display consistent PGE patterns similar to that of the primitive mantle. The picrobasalts have high contents of SiO2 (45.08–48.87 wt.%), MgO (7.28–11.51 wt.%), nearly flat REE pattern, and ƐNd (t) values ranging from +6.5 to +7.9 and initial 87Sr/86Sr ratios focused on 0.7040 and 0.7070, illustrating that they were derived from a relatively enriched source compared with the komatiitic rocks. The thermobarometric calculations suggest that the Mashhad komatiitic rocks were generated by high degree partial melting between 20% and 30% at 1,604–1,681°C and 3.20–4.65 Gpa, essentially consistent with those of the Hawaiian mantle plume (1,500–1,600°C). These new data demonstrate that the Mashhad komatiitic rocks are of a mantle plume origin, which might have caused the opening of the Paleo‐Tethyan Ocean, and the mantle plume was likely located beneath or nearby the mid‐ridge of the Paleo‐Tethyan Ocean when the Mashhad komatiitic lava erupted, a scenario resembling the present Iceland and Ontong Java.

Primitive Magmas in the Early Central American Volcanic Arc System Generated by Plume-Induced Subduction Initiation

Primitive lavas with Mg# ≥60 have been identified in the literature from the Sona-Azuero, Golfito and Chagres-Bayano forearc segments of southern Costa Rica and Panama of the early (75-39 Ma) Central American Volcanic Arc system (CAVAS). Primitive CAVAS basalts are remarkably similar in terms of major and many trace elements to primitive MORB and primitive basalts of the Mariana Arc. Significant differences however, are apparent between primitive lavas of Chagres-Bayano relative to those of Sona-Azuero and Golfito. Primitive Chagres-Bayano lavas record much higher degrees of partial melting and higher amounts of shallow and deep subduction additions (e.g., higher Ba/Th, Th/Nb) than those of the older Sona-Azuero and Golfito arc segments which reflects the early petrologic and tectonic evolution of the CAVAS which is similar to other forearc systems. As shown in previous studies, early CAVAS lavas are exceptionally similar in radiogenic isotopic composition to those of the Caribbean Large Igneous Province (CLIP) oceanic plateau at 90 Ma, the approximate timing of onset of the volumetrically dominant phase of CLIP magmatism. This recognition is consistent with subduction initiation models which require plume-subduction interaction. Such models also dictate that an elevated sub-arc thermal anomaly was present at inception of the CAVAS. It is postulated that in addition to providing an anomalously high temperature regime in the sub-arc mantle at inception of the CAVAS, that extrusion of the CLIP also resulted in lithospheric weakening and extension which allowed for production and rapid ascent of a high percentage of primitive lavas.

Contrasting Ba-Sr Granitoids from Bamenda Area, NW Cameroon: Sources Characteristics and Implications for the Evolution of the Pan African Fold Belt

The basement rocks of Bamenda town are mainly covered by volcanic rocks and are made up of two distinct types of granitoids: granites and leucogranites. These basement rocks belong geochemically to the same granite field. The studied granitoids are silica rich, with concentrations ranging from 70 to 73% in granite and 73 to 76% in leucogranite. Both belong to high-K calc-alcaline series with granite being magnesian and more potassic (Na2O/K2O 2O/K2O >1). Leucogranites are low Ba-Sr granitoids, strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, while granites are high Ba-Sr granitoids, slightly peraluminous and plot in the field of I-type granitoids. The chondrite-normalized REE patterns show more LREE enrichment in granites (>100*chondrite) than in leucogranites (>10*chondrite). Major and trace element compositions of the leucogranites and granites indicate crustal derivation from the partial melting of metapelite and metagreywacke respectively. The REE patterns display pronounced negative Eu anomalies (Eu/Eu* = 0.23-0.36) in leucogranites due to low degree of partial melting of a plagioclase depleted crustal source, and weakly negative to null Eu anomalies (Eu/Eu* = 0.81-1.08) in granites due to high degree of partial melting. The estimated temperatures of Bamenda granitoids magma range between 800°C and 950°C. Bamenda I-type granites are syn-tectonic and are similar to the other granitoids of the central domain of the Pan-African North-Equatorial Fold belt in Cameroon. Bamenda S-type peraluminous leucogranites are post-tectonic and chemically similar to the Himalayan peraluminous leucogranites. Furthermore, decompression model may be related to Bamenda leucogranite formation. Tectonically, the studied granitoids are related to crustal delamination characterizing the post-collisional event within the Pan African Fold Belt.

Mineralogy and geochemistry of peridotites and chromitites in the Aladag Ophiolite (southern Turkey): melt evolution of the Cretaceous Neotethyan mantle

The Aladag Ophiolite (also known as the Pozanti-Karsanti Ophiolite) is one of the largest Neotethyan ophiolites in Turkey and represents Cretaceous oceanic lithosphere derived from the Inner Tauride seaway of Neotethys. Its peridotites consist of spinel harzburgites, dunites and chromitites. The harzburgites display a depleted mineral and whole-rock geochemistry, indicating high degrees of partial melting. They contain Pd- and Pt-rich interstitial sulphides, which crystallized from S-saturated melts during melt–rock interactions. Their spoon-shaped rare earth element patterns indicate metasomatism by fluids, derived from a subducting slab, which were enriched in light rare earth elements. The chromitites in both the mantle and mantle–crust transition zone rock units show similar geochemical compositions, typical of high-Cr chromitites. They also show similar chondrite-normalized platinum group element patterns characterized by Os, Ir, Ru and Rh enrichments relative to Pt and Pd. The parental melts of the Aladag Ophiolite chromitites were S-undersaturated and boninitic in composition. The presence of microdiamond, moissanite, zircon and needle-shaped clinopyroxene exsolutions in chromite (chr) grains suggests that their formation might have originally involved high-temperature and high-pressure conditions within the mantle transition zone before their participation in further melting and the formation of oceanic crust at shallow depths in a forearc tectonic setting during the initiation of intra-oceanic subduction within the Inner Tauride seaway. Supplementary material: Sampling and analytical methods, representative microprobe analyses and whole-rock major and trace element compositions are available at https://doi.org/10.6084/m9.figshare.c.4153631 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

Petrology and geochemistry of mafic and ultramafic rocks in the north Tianshan ophiolite: Implications for petrogenesis and tectonic setting

New petrological and geochemical data for the mafic (pillow basalt, massive basalt, gabbro) and ultramafic rocks (serpentinite, serpentinized harzburgite) in the north Tianshan ophiolite cropping out along the Duku road are combined to constrain the geological evolution of the north Tianshan ophiolite, which marks the boundary between the Yili-central Tianshan and the Junggar terranes. Pillow basalt, characterized by significant enrichments of light rare earth elements (REE) as well as high Nb/Yb and TiO2/Yb ratios, probably represents the seamount fragments accreting in the north Tianshan accretionary prism. Massive basalt and gabbro, displaying flatten REE patterns with low Ti/V ratios, are similar to forearc basalts. Serpentinite and serpentinized harzburgite, belonging to the mantle section of the ophiolite, are depleted in incompatible elements compared to the primitive mantle. The compositions of primary chromian spinel, olivine, and pyroxene in these mantle rocks are comparable to those in forearc peridotites. Such characteristics indicate that the protoliths of serpentinite and serpentinized harzburgite have undergone high degrees of partial melting. They were modified by island arc tholeiitic to boninitic melts during and/or after melt extraction as suggested by the enrichment of Ti in chromian spinel coupled with the occurrence of amphibole inclusions in chromian spinel. These new petrological and geochemical data suggest that the north Tianshan ophiolite was generated in a forearc setting. Retreat and rollback of the north Tianshan subducted slab triggered the upwelling of asthenosphere with minor mass transfer from the slab, leading to the formation of massive basalt and gabbro. Increasing flux of slab-derived fluids/melts largely enhanced partial melting degree of the mantle peridotites, producing island arc tholeiitic to boninitic melts. These melts interacted with refractory mantle residues and formed the mantle rocks (i.e., the protoliths of serpentinite and serpentinized harzburgite) in the north Tianshan ophiolite.