Supra-subduction Environment Research Articles

Tracking magmatism and metasomatism in continental arc crust root: Significance from western Central Qilian belt in NE Tibet, China

Abstract Mantle peridotites and pyroxenites from arc crust root record vital information about petrogenetic processes in supra–subduction environments, which are commonly obscured by subsequent metamorphic and/or metasomatic overprints. The Heigou Complex from the western Central Qilian arc root (NE Tibet, China) consists of peridotites and pyroxenites. Petrography, bulk–rock and mineral geochemistry, zircon and titanite U–Pb ages, and bulk–rock Sr–Nd–Hf isotopes are presented to decipher its petrogenetic and metasomatic history. The peridotites display highly refractory compositions of low SiO2, Al2O3, CaO, but high MgO contents. Geochemical modeling indicates that these are the residues after up to ~22% melt extraction. The geochemical and isotopic data (87Sr/86Sr i = 0.7116–0.7179, εNd(t) = –4.2~–7.1 and εHf(t) = –5.3 ~ –5.4) suggest the pyroxenitic protoliths were derived from a subduction-metasomatized mantle source. Combining petrographic and fractional crystallization simulation, the pyroxenites formed as cumulates through 50–80% differentiation of primitive basaltic magma in the arc root at ca. 523 Ma. Elevated LILEs (Rb, Sr, Pb, etc.) and LREE in bulk-rock and mineral compositions, together with microstructure evidence, suggest that mantle peridotite experienced mantle metasomatism. Subsequent hydrous melt (fluid) metasomatism during exhumation then reacted with the preexisting clinopyroxenite, leading to local Mg-hornblende, titanite and epidote formation. The timing of retrograde melt infiltration is constrained by a titanite U–Pb crystallization age of ca. 485 Ma. The combined evidence reveals a picture of continental arc magmatism and metasomatism at arc crust roots. Dehydration of the Proto-Tethys oceanic slab could induce flux melting in the mantle wedge that had undergone metasomatism, resulting in the generation of hydrous arc magmas. These arc magmas intruded the overlying lithospheric mantle, segregating pyroxenite cumulates. The ultramafic rocks at the arc crust root were subsequently modified by hydrous melt (fluid).

Gabbro of the mantle part of the ophiolite section in the Alapaevsky dunite-harzburgite-gabbro massif (Eastern zone of the Middle Urals)

Research subject. An intrusive body of gabbroids breaking through mantle peridotites (dunites and harzburgites) of the Alapaevsky ophiolite massif (Eastern zone of the Middle Urals). Methods. The contents of petrogenic elements were determined by the X-ray fluorescence method; the contents of rare and scattered elements were studied by the ICP-MS method. The age of gabbroids was determined by 147Sm-143Nd ID-TIMS by isotope dating. Results. The gabbro and their containing ultramafic rocks were established to be of almost the same age of about 580 Ма, which indicates their belonging to a single ophiolite association of the Vendian age. At the same time, the gabbroids of the Alapaevsky massif differ sharply from both isotropic and stratified gabbro of the crustal part of the ophiolite section, fragments of which are observed within the Eastern zone of the Middle Urals, in terms of a significantly reduced content of light REE, rare alkalis, barium, uranium and thorium, as well as the absence of Pb maxima on spider diagrams. Conclusions. The established features in the composition and intrusive occurrence of the gabbro of the Alapaevsky massif among mantle ultramafic rocks indicate that the studied rocks cannot be identified with gabbroids of the crustal part of the ophiolite association. This suggests that this rather large intrusion of gabbro may be analogous to small veins and dikes of gabbroids observed among mantle peridotites in a number of ophiolite massifs, such as Voikar-Synyinsky massif in the Polar Urals and the Samail ophiolite in Oman. The distribution specifics of rare elements on the spider diagrams of the gabbroids of the Alapaevsky massif, i.e., the presence of Sr and Ba maxima and Nd and Th minima, as well as the Harzburgite composition of the mantle restite containing the gabbro body under consideration, indicate that the Vendian ophiolite association of the Eastern zone of the Middle Urals formed in a suprasubduction (pre-arc) environment. A model for the formation of large masses of gabbro in the mantle part of the ophiolite section is proposed.

Geochemical characteristic of rocks and isotope dating of zircon from the andesite dyke of the rudyanskaya sequence (D2 rd) of the Middle Urals Eastern zone

Relevance. In the basin of the r. Pishma subvolcanic formations, localized in the fields of spreading of the Middle Devonian Rudyanskaya sequence, are presented by different on composition small bodies, sills and dykes, forming with volcanogenic rocks of the Rudyanskaya sequence a basalt-andesite-rhyolite complex, the formation of which is associated with volcanic activity, that took place in Middle Devonian in the environments close to the modern island arcs. Their isotope dating and geochemical characteristic are rather relevant for specification of the composition, volume and paleogeodynamic environment of the vulcanite formation. In the paper new data are presented on geochemistry of andesites of the dyke from the Diviy Kamen rock, considered as part of the Rudyanskaya sequence, as well as the age of zircon from them. Methods. Chemical analyses of vulcanites have been performed in the Centre of collective usage “Geoanalyst” of the Institute of Geology and Geochemistry of the UB of RAS (Ekaterinburg) by X-ray fluorescent method and ICP–MS method. Zircons were studied with scanning electron microscope JSM-6390L of the Jeol firm and with electrone-probe microanalyser Cameca SX100. The data on the U and Pb isotopes, as well as on elements-impurities in zircon grains were obtained by mass-spectrometry method with inductively coupled plasma with laser ablation (LA–ICP–MS) at the quadrupole ISP–MS NexION 300S with the attachment for laser ablation LA NWR 213. Purpose of researches. Getting new data on geochemical composition and age of the andesites of dyke in the section along the r. Pishma. Results. The andesite dyke intersects medium clastic unstratified tuffs and dipply occurring fragment of the lava flow of porphyry dacite. Andesites present themselves porphyry rocks with 20–30% of plagioclase phenocrysts. At the normalized many-component diagram the Ta–Nb minimum is displayed. For zircon grains are characteristic differentiated spectra, demonstrating the growth of compositions from light to heavy REE, positive Ce and negative Eu anomalies, typical of the zircon of magmatic genesis. For the first time geochronological study of zircons from andesites has been maded by method of LA–ICP–MS. 4 age groups of zircons, Ma: 390–387, 369–362, 337–345, 327–329 were revealed. Conclusions. The data obtained testify that andesites were formed under suprasubduction environments, and magmatic stage of rock formation corresponds to the upper Devonian varieties of zircons. Early Carboniferous datings can be associated with the thermal impact caused by the intrusion of a later dolerite dyke.

Vodorazdelny Granite Massif (Subpolar Urals) and Correlation Problems of the Pre-Ordovician Granitoids and Volcanic Rocks from the Northern Part of the Lyapinsky Anticlinorium

Petrogeochemical features of the Vodorazdelny massif granites (Subpolar Urals, Lyapinsky anticlinorium) indicate these rocks are close to I-granites and were formed in suprasubduction environment. The ratios of the key elements (Rb, Ba, Th, Sr, Y, Nb) suggest that the basites of the melting slab and the fluid separated during their dehydration could participate in the granite generation. The U–Pb age of the main population of magmatogenic zircons is 593 ± 4 Ma and corresponds to the Vendian (Ediacarian). It coincides with the age of the granites from the nearby Vangyr massif (598 ± 5 Ма), as well as with the age of zircon cores from the granites of the Kozhim massif located to the north. Values of εHf(t) from –2 to 0 in magmatogenic zircons with an age corresponding to the age of Vodorazdelny granites indicate a heterogeneous source of melts. The zircons of these granites also contain ancient cores with U–Pb ages from 2200 to 700 Ma, where the values of εHf(t) from +0.8 to +13 indicate the presence of a crust component in the substrate (the substance of the ancient platform basement). Petrogeochemical and isotope-geochronological parameters of granites (and their zircons) do not confirm the validity of attributing of Vodorazdelny granite massif (and its analogues – the Vangyr and Kozhim massifs) to the Cambrian Salner-Mankhambo complex. We consider there are the possibility of separating of independent complex (about 598 Ma, Vendian?) during geological mapping. The presence in the Lyapinsky anticlinorium of several stages of granite generation (Middle Riphean–Vendian–Cambrian), accompanied by metamorphism, and complicate composition of ancient metamorphic strata from this structure basement led to varying isotopic parameters characterizing the heterogeneity of the source of melts, on the one hand, and the convergence of a number of geochemical features, on the other.

A record of metasomatism and crustal contamination of the Mediterranean lithosphere in chromitites of the Orhaneli Ophiolite Complex (NW Türkiye)

• Orhaneli chromitites include Middle Devonian to Mesoproterozoic zircon and rutile . • Chromite trace elements points a metasomatic signature in relics within chromitites. • More data should be sought for ultrahigh-pressure history of Orhaneli chromitites. This study provides the first-ever report of U-Pb isotopic ages of zircon and rutile from high-Cr chromitites of the Orhaneli Ophiolite Complex (OOC). The chromites within the OOC chromitites yield Cr-numbers between 0.77 and 0.86, Mg-numbers between 0.46 and 0.71 and TiO 2 < 0.26 wt%. They exhibit V, Ga and Ni depletion and Sc, Ti, Zn, Co and Mn enrichment relative to MORB spinels, with significant Sc and Ti positive anomalies. This, and the Al 2 O 3 and TiO 2 compositions of the parental melts, suggest that the studied chromitites formed from island arc melts of boninitic affinity, in a supra-subduction environment. In addition to zircons, chromites host other solid mineral inclusions, including Os- and Ir-dominant platinum-group minerals (PGM), base-metal sulfides (BMS), alloys, ubiquitous silicate (some as micron-sized oriented lamellae), and two poorly defined Fe-Cr-O and Ti-Si-C grains. The zircons show Th/U ratios typical for magmatic zircons from felsic melts, and present Middle Devonian to Mesoproterozoic U-Pb isotopic ages. Although the reworked/resorbed forms of the zircons suggest a xenocrystic origin, coupling of U-Pb data from some zircons and rutile with the Ti positive anomaly in the chromites raises the possibility for an earlier Neoproterozoic metasomatic event affecting the relics now included within the chromitite bands. The current state of knowledge about the silicate micro-lamellae within the chromites has prompted reassessment of their early interpretation as direct evidence of deep mantle recycling. We currently suggest a more cautious approach on this issue by seeking additional evidences for an ultrahigh-pressure (UHP) history.

Water availability controls crustal melting temperatures

Although the capacity for water to lower the solidus during crustal melting is well recognised, a modern consensus is that most granitic magmas are generated under fluid-absent melting conditions at temperatures ≥850 °C, and that water has little influence on crustal melting temperatures. By contrast, we show that granite magmas produced in different tectonic settings equilibrated in the crust at significantly different temperatures, ranging between 700 and 1000 °C, depending on water availability during melting. On a molecular scale, the process that lowers the solidus is similar to hydroxylation, i.e. the depolymerization of the aluminosilicate framework melt by complexing of OH− ions with the melt network formers, Si4+ and Al3+. On a larger scale, the process is “water fluxing”, which is consistent with the growing realisation that many granitic magma reservoirs form under protracted, cool-storage conditions, and consistent with the typically low (100–200 ppm) Zr content (hence low zircon saturation temperatures) of granitic magmas. Global compilations of common igneous and sedimentary rocks also show that Zr contents are consistently in the range of 100–200 ppm, indicating that average Zr contents do not change significantly during crustal differentiation. Such low average contents are most easily reconciled with models of low-T silicic melt generation and entrainment of refractory or antecrystic zircon in the magma. Low-K, I-type (Cordilleran) granitic magmas are cooler and much more voluminous than A-type magmas because they are hydrous, although transiently heated by mafic magma infusions. In turn, water availability is tectonically controlled, occurring in most suprasubduction (SSZ) environments where mantle-derived water drives melting of orogenic crust already elevated in heat flux (>75 mWm−2), typical of upper amphibolite facies conditions. However, within intraplate environments, where melting proceeds by high-T, hydrate-breakdown melting, crustal temperatures are not buffered during water fluxing and rise to granulite facies and ultrahigh temperature (UHT) conditions, particularly for refractory protoliths, producing ferroan, alkalic A-type granites and charnockites in deep crustal hot zones (DCHZ). The combination of tectonic setting, contrasting source rocks, and variation in water availability, controls the compositional diversity of granitic magmas.

Ophicalcites from the Upper Tectonic Unit on Tinos, Cyclades, Greece: mineralogical, geochemical and isotope evidence for their origin and evolution

Ophicalcites exposed on the island of Tinos, Greece, occur as ellipsoidal bodies within greenschist-facies phyllites of the Upper Cycladic Unit. Close to their outcrops, blocks of serpentinites, metabasic rocks and metasediments were identified, implying a tectonically dismembered ophiolitic sequence in the study area. The ophicalcites comprise brecciated serpentinites cemented by calcite. Based on textural, mineralogical and deformation features, five ophicalcite varieties were discriminated, reflecting calcite precipitation, sedimentary features and increasing brecciation. Serpentinitic fragments comprise antigorite, while Cr-spinel, magnetite, talc and chlorite are accessory minerals. Carbonate veins consist of calcite and minor dolomite, talc, chlorite, and rarely epidote. Bulk rock chemical compositions and Cr-spinel mineral composition point towards a supra-subduction environment. Carbon and oxygen isotope ratios of calcite imply precipitation from mixed marine and hydrothermal fluids, followed by isotope exchange due to late, greenschist-facies overprint. The Tinos ophicalcites record intraoceanic exhumation of the ultramafics at the seafloor, where faulting and serpentinization caused an extensive network of fractures, healed by carbonates. Such intraoceanic deformation can be attributed either to obduction tectonics expressed by thrusting of oceanic piles, or to transpressional(?) transform faults, or more probably to slip along detachment fault of an oceanic core complex.

PETROLOGICAL CONSTRAINTS ON THE ORIGIN OF PYROXENITE DYKES IN THE LITHOSPHERIC MANTLE OF THE CHESHMEH-BID OPHIOLITIC MASSIF, SOUTHERN IRAN

The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopyroxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low Al2O3, CaO, Na2O and TiO2 abundances coupled to relatively high MgO and SiO2 contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P > 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a suprasubduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.

Longlin-Ruili subduction-accretionary complex belt in the southeastern Gaoligong orogen, and its relationship with the evolution of the Meso-Tethyan Ocean

龙陵-瑞丽增生杂岩带的构造属性对确定高黎贡构造带作为腾冲-保山地块的边界及班怒带南向延伸十分关键。本文在地质填图的基础上，通过岩石学、矿物学、岩石地球化学、同位素年代学和同位素示踪等方法，查明混杂岩带主要由蛇纹石化橄榄岩、玄武岩/辉长岩、硅质岩、碳酸盐岩、含放射虫层状硅质岩和锰结核的深海沉积岩等岩块呈规模不等的团块状、透镜状分布于浊积岩基质中，具有典型的俯冲增生杂岩岩石组合特征。蛇纹石化橄榄岩原岩由方辉橄榄岩和少量纯橄岩组成，具有轻稀土轻微富集、Mg^#值高（88~92），铬尖晶石Cr^#、Mg^#值分别在60~70和20~26区间，橄榄石Fo值为90~95。在铬尖晶石Cr^#-Mg^#指数图解和橄榄石Mg^#-铬尖晶石Cr^#图解上样品都落在弧前SSZ型橄榄岩区，说明研究区内的地幔橄榄岩是经历了高度部分熔融（>30%）和熔体抽离后的残留相，形成于弧前构造背景。玄武岩和辉长岩地球化学特征类似，具有富钛（TiO₂>2.26%）、高Mg^#值特征（49~57），其稀土配分模式和微量元素蛛网图、构造环境判别图解、ε_Nd（t）值（+2.2~+5.1）、以及岩石中含少量富钛角闪石和黑云母等，表明它们属于洋岛/海山型基性岩类，其岩浆来源于富集地幔。橄榄岩中脉状辉石岩锆石U-Pb年龄为183~185Ma，浊积岩中杂砂岩最小碎屑锆石U-Pb年龄为212~241Ma。此外，增生杂岩带中存在含早白垩世流纹岩/凝灰岩夹层的弧前/弧间沉积，并被晚白垩世陆相沉积岩不整合覆盖，这些特点说明俯冲增生杂岩带形成于晚三叠-早白垩世。研究区内混杂岩带的构造属性和时代与班怒带（拉萨-南羌塘地块间）和缅甸境内密支那蛇绿混杂岩带完全一致，是中特提斯洋演化的产物。中生代，高黎贡东南缘混杂岩带北连班怒带、南东连密支那蛇绿混杂岩带，随后，在新生代印度板块向北俯冲过程中，密支那蛇绿混杂岩带被Sagaing断裂带分支——八莫断裂带右行走滑位移到现今位置。;The tectonic setting of the Longlin-Ruili mélange belt is a key to identify the Gaoligong tectonic zone as the suture zone between Tengchong and Baoshan blocks, and the southward extension of the Bangonghu-Nujiang suture. Here we present new geological, petrochemical, mineral chemical, geochronological and isotopic data for the Longlin-Ruili mélange belt in the southeastern Gaoligong orogen. The slices inside this mélange belt include serpentinization peridotite, basalt/gabbro, chert,carbonite and pelagic sedimentary rock with radiolarian bearing layered siliceous rocks and manganese nodules distributed in the turbidites, showing typical rock assemblage of a subduction-accretionary complex. The peridotite consists of the harzburgite and a small amount of dunite. It is characterized by the slightly enriched LREE, high values of Mg^# (88~92), high values of Fo (90~95) for olivine, and Cr^# and Mg^# values for chrome-spinel are 60~70 and 20~26, respectively. The Mg^# (spinel) vs. Cr^# (spinel) and Mg^# (olivine) vs. Cr^# (spinel) diagrams show the peridotite is the residual phases after high degree partial melting (>30%) and extraction of melt in supra-subduction environment. The basalt and gabbro are oceanic island or seamount type whose mafic magmas are derived from a metasomatized mantle, with features of the enriched TiO₂(>2.26%) and high values of Mg^# (49~57), and their REE patterns and trace element spider lines are consistent with that of OIB, with low ε_Nd(t) (+2.2 to +5.1). Furthermore, a small amount of Ti-enriched biotite and hornblende occurred in mafic rocks. The U-Pb dating of the zircons from pyroxenites within harzburgite yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 183~185Ma. The U-Pb dating of the detrital zircons from greywackes in turbidite yielded the youngest group ²⁰⁶Pb/²³⁸U ages of 212~241Ma. There are forearc and inter-arc sedimentary rocks with Early Cretaceous rhyolite or tuff in mélange belt. The Upper Cretaceous continental sedimentary rocks unconformably onlapped on the mélange zone. All these features suggest that this mélange belt was formed in a supra-subduction zone during Late Triassic to Early Cretaceous. The tectonic setting and age of the Longlin-Ruili mélange belt are consistent with that of the Bangonghu-Nujiang and Myitkyina ophiolitic mélange belts which are related with the subduction of the Meso-Tethyan Ocean. As a result, the Longlin-Ruili mélange belt should be connected with Bangonghu-Nujiang suture zone in the north, while the Myitkyina ophiolitic mélange belts in the southeast during Mesozoic. After that, the Myitkyina ophiolitic mélange belt was displaced to present position by Sagaing dextral slip-shearing in Cenozoic.

Effect of water on the rheology of the lithospheric mantle in young extensional basin systems as shown by xenoliths from the Carpathian-Pannonian region

Incorporation of hydrogen as structural hydroxyl (commonly referred to as water) in nominally anhydrous mantle minerals is known for the ‘hydrolytic weakening’ effect, which decreases the strength and electrical resistivity of the rock. Recent models have provided means of calculating rheological properties from geochemical data of upper mantle xenoliths. In the Carpathian-Pannonian region, upper mantle xenoliths can be found on the surface at five locations, both at marginal and central regions of the basin system. In this study we present a comprehensive overview of water contents in these xenoliths, including previously lacking data from two outcrops (Füzes-tó and Tihany) from the Bakony-Balaton Highland Volcanic Field for the first time. The Tihany xenoliths have significantly higher water contents (< 1–5, 116–353 and 327–1394 ppm in olivine, orthopyroxene and clinopyroxene, respectively) than the Füzes-tó xenoliths (0–2.7, 6.2–114 and 3.1–213 ppm in olivine, orthopyroxene and clinopyroxene, respectively). This can be explained with the older eruption age of their host basalt and greater depth of origin, as they likely represent an asthenospheric layer that became part of the lower lithosphere during thermal relaxation. In contrast, the Füzes-tó xenoliths represent a lot dryer mantle portion and are assumed to have been more affected by decompression-induced water loss resulting from decreased water activity during the extension. In general, the marginal xenolith locations of the Carpathian-Pannonian region, associated to prior supra-subduction environment, contain more water than xenoliths from central locations (with the exception of Tihany locality) which were significantly affected by extension-related lithospheric thinning. To reveal the differences in rheology inferred from the different water contents, we calculated effective viscosities and electrical resistivities for the xenoliths of the Bakony-Balaton Highland and other locationss in the Carpathian-Pannonian region using previously published data for input. Based on the results, the central locations have higher effective viscosities (1.4∙10 20 –2.2∙10 21 Pa s) and electrical resistivities (48–913 Ωm) compared to the marginal locations (9.3∙10 19 –6.8∙10 20 Pa s and 36–182 Ωm, respectively), suggesting that the lithospheric mantle is more rigid in the former than in the latter areas. This may be a common feature for extensional basins, as the extension leads to the ‘drying’ of the upper mantle, whereas subduction zones keep hydrating their overlying mantle wedge. However, for more accurate estimations, other factors such as regional differences in strain rate or the potential presence of melts or fluids in the mantle need to be considered as well. • Water lowers effective viscosity and electrical resistivity in mantle xenoliths • Water contents of xenoliths from different tectonic environments were compared • Central regions of the back-arc basin are drier compared to marginal areas • Rheological differences are present in distinct parts of extension-subduction systems • Decompression related water loss results in strengthening of the lithosphere

Silurian Granitoid Magmatism of the Rassokha Terrane (North-East Russia)

U–Pb dating of zircons from granitoids located within the Rassokha terrane show a Silurian age of their formation. Younger dates from one of the samples are related to radiogenic lead loss during secondary alteration of the rocks. Based on REE distributions as well as well-defined positive anomalies of K and Pb and negative anomalies of Ba, Sr, Nb, Ta, and Ti, the granitoids are comparable to rocks from supra-subduction environments. Combined with intrusive ages from adjacent areas, Silurian granitoid magmatism of the Rassokha terrane supports the existence of a back-arc basin and a long island arc in Ordovician-Silurian time.

Transformation of PGM in supra subduction zones: Geochemical and mineralogical constraints from the Veria (Greece) podiform chromitites

Extremely abundant PGE-minerals (PGM) hosted in chromitites from the Veria ophiolite complex in Macedonia (N. Greece) may be unique among ophiolite complexes. This study focuses on differences between the low- and high-PGE chromitites. New textural, mineralogical and geochemical constraints from those ores are presented, aiming to define factors controlling the PGE enrichment in a supra subduction environment, in the light of post-magmatic processes. The whole ore analyses for mmajor and trace elements indicated an unusually high-IPGE content (up to 25 ​ppm) and higher Fe, Ca, Mn, Zn and V contents in high-PGE compared to low-PGE in massive chromitites. The wide compositional variation of chromite, even in the same polished section, the occurrence of very fine PGM (less than 20 ​μm) as inclusions within chromite and extremely large (>1000 ​μm), angular or fine-grained PGM aggregates ones within a matrix of highly fragmented chromite - Cr-garnet matrix, may indicate crystallization/recrystallization of chromite from more than one precursor phases. Laurite (RuS2) is very limited, occurring as remnants surrounding by Ru–Os–Ir oxides/hydroxides, of a wide compositional variation. Irarsite occurs as euhedral crystals up to 200 ​μm, surrounding by chromite, as anhedral exsolutions 1–200 ​μm within laurite, or creating segregates with platarsite and relics of (Ru, Pt, Rh, Os) sulfarsenides. Platinum–Ru–Rh–Pd-minerals occur commonly as relatively fine-grained assemblages, up to 50 ​μm, along with irarsite and other relics of (Ru, Pt, Rh, Os) sulfarsenides. Pt-alloys show a variation ranging from tetraferroplatinum to Pt–Ir–Fe–Ni alloys. The presence of laurite relics in large IPGM, awaruite, heazlewoodite, and carbon-bearing material reflecting a super-reducing environment, and the transformation of primary PGM into Os–Ir–Ru-alloys and oxides/hydroxides in association with Fe-chromite and Fe3+-bearing garnet (andradite-uvarovite solid-solution series) may reflect changes of the redox conditions from reducing to oxidizing. The relatively high Na content in hydrous mineral inclusions within high-PGE chromitites suggest a hydrous mantle source and provide the possibility for estimation of the P (average 3.0 ​kbar) and T (average 874 ​°C), indicating formation at a shallow mantle environment.

Eoarchean contrasting ultra-high-pressure to low-pressure metamorphisms (1000 °C/GPa) explained by tectonic plate convergence in deep time

Greenland’s Itsaq Gneiss Complex (IGC) shows Eoarchean (>3600 Ma) 250–400 °C/GPa (low T/P – high pressure) and ≥1000 °C/GPa (high T/P) metamorphic regimes, demonstrating a similarity of contrasting metamorphic T/P regimes from the Phanerozoic back to the start of Earth’s rock record. Low T/P metamorphism produced: (i) Deep crustal eclogitised mafic rocks which upon partial melting formed the tonalites dominating the IGC; (ii) ~550 °C ≥ 2.6 GPa conditions (≤250 °C/GPa) demonstrated by an olivine + antigorite + titano-chondrodite/titano-clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment, that were exhumed into the crust by 3712 Ma; (iii) rare vestiges of 3658 Ma high-pressure (garnet + clinopyroxene) granulite; and (iv) Barrovian-style kyanite + staurolite assemblages. High T/P metamorphism is shown by 3669 Ma crustal melts equilibrated with orthopyroxene. This was coeval to the youngest juvenile tonalitic crust in the complex (latter derived by anatexis under low T/P conditions), and a 3670–3570 Ma history of deep crust migmatisation under low pressure, garnet-free conditions. Structural geology of the IGC indicates its low T/P regimes coincide with crustal imbrication by compression of arc-like tholeiites, boninite-like lavas, andesites, felsic-intermediate volcano-sedimentary rocks and chemical sedimentary rocks, whereas post-3660 Ma high T/P metamorphism was marked by late-orogenic extension/exhumation and deep crustal flow with mafic underplating and partial melting generating granites. Thus the diversity of Earth’s earliest-recorded geodynamic settings resembles more those of modern geodynamics, than the lithological and structural relationships expected from theoretical non-uniformitarian scenarios like drip tectonics in a stagnant lid regime. The recognition of an ultra-high-pressure ≤250 °C/GPa metamorphic regime at >3700 Ma in the IGC removes the last argument against a form of plate tectonics operating throughout the Archean. Hence since the start of the rock record, a mobile lid plate tectonic regime contributed to interior heat loss, facilitating chemical communication and feedbacks between Earth’s surface and its deep interior.

From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N &gt; 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

Genesis of pyroxenite veins in supra-subduction zone peridotites: Evidence from petrography and mineral composition of Egiingol massif (Northern Mongolia)

Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain (Central Asian Orogenic Belt, Northern Mongolia). The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals, calculations of closure temperatures and composition of equilibrium melt. The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho- and clinopyroxene due to melt-rock interaction. Relics of peridotite olivines are found in pyroxenites, Cr^# of spinel increases from peridotites to pyroxenites, and compositions of ortho- and clinopyroxene change from peridotite to pyroxenite. The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain. Therefore, formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites. Orthopyroxenite veins formed at first, followed by websterite veins. Thus, the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment.

Trace Element Distribution in Magnetite Separates of Varying Origin: Genetic and Exploration Significance

Magnetite is a widespread mineral, as disseminated or massive ore. Representative magnetite samples separated from various geotectonic settings and rock-types, such as calc-alkaline and ophiolitic rocks, porphyry-Cu deposit, skarn-type, ultramafic lavas, black coastal sands, and metamorphosed Fe–Ni-laterites deposits, were investigated using SEM/EDS and ICP-MS analysis. The aim of this study was to establish potential relationships between composition, physico/chemical conditions, magnetite origin, and exploration for ore deposits. Trace elements, hosted either in the magnetite structure or as inclusions and co-existing mineral, revealed differences between magnetite separates of magmatic and hydrothermal origin, and hydrothermal magnetite separates associated with calc-alkaline rocks and ophiolites. First data on magnetite separates from coastal sands of Kos Island indicate elevated rare earth elements (REEs), Ti, and V contents, linked probably back to an andesitic volcanic source, while magnetite separated from metamorphosed small Fe–Ni-laterites occurrences is REE-depleted compared to large laterite deposits. Although porphyry-Cu deposits have a common origin in a supra-subduction environment, platinum-group elements (PGEs) have not been found in many porphyry-Cu deposits. The trace element content and the presence of abundant magnetite separates provide valuable evidence for discrimination between porphyry-Cu–Au–Pd–Pt and those lacking precious metals. Thus, despite the potential re-distribution of trace elements, including REE and PGE in magnetite-bearing deposits, they may provide valuable evidence for their origin and exploration.

Geochemistry and petrogenesis of the early Archean mafic crust from the Saglek-Hebron Complex (Northern Labrador)

The Saglek-Hebron Complex located in Northern Labrador, Canada, is one of the oldest granite-greenstone terrains on Earth. It is dominated by granitoids as old as 3.9 Ga and several enclaves of supracrustal rocks. We present here the largest whole-rock major and trace element geochemical dataset (over 100 samples) on the mantle-derived rocks from the Saglek-Hebron Complex to constrain their petrogenesis. The mafic amphibolites are metavolcanic rocks basaltic in composition, have tholeiitic affinities, and exhibit relatively homogeneous compositions with flat rare-earth element profiles. They include incompatible element enriched and depleted compositions, both which appear to be related by fractional crystallization of a gabbroic assemblage within volcanic flows. The enriched mafic rocks exhibit higher Ti contents compared to the more depleted mafic rocks, representing, respectively, evolved liquids and pyroxene-rich cumulate-liquid mixtures. Previous work has suggested the occurrence of two distinct supracrustal units in the Saglek-Hebron Complex, the Eoarchean Nulliak assemblage and the Mesoarchean Upernavik assemblage. Here we report that the petrological and geochemical composition of both assemblages is indistinguishable. This suggests either that the Nulliak and Upernavik metavolcanic rocks represent a single unit, or that they were formed by identical processes in a comparable context, despite an age difference of ∼400 Ma. Contrary to some other early metavolcanic belts, such as the Isua supracrustal belt and Nuvvuagittuq greenstone belt, none of the metavolcanic rocks from the Saglek-Hebron Complex studied here exhibit geochemical compositions that could be reminiscent of suprasubduction environments. The Saglek-Hebron Complex also includes two compositionally distinct groups of ultramafic rocks characterized by different Al/Ti ratios, Fe contents and controlled by olivine fractionation with different Fo contents. Ultramafic rocks from both groups appear to be cumulates derived from distinct parental magmas with a komatiitic basalt composition. One of these parental magmas may be genetically linked through fractional crystallization with the mafic metavolcanic rocks, whereas the other group is more difficult, at least geochemically, to relate to the Saglek-Hebron basaltic rocks.

The Crust–Mantle Transition of the Khantaishir Arc Ophiolite (Western Mongolia)

AbstractThe crust–mantle transition of the Khantaishir ophiolite in western Mongolia is well exposed. The mantle section shows an up to 4 km thick refractory harzburgitic mantle with local dunite channels and lenses. Towards its top, the mantle is increasingly replaced by discrete zones of pyroxenite, which form a kilometre-wide and hundreds of metres-thick horizon at the contact with the overlying crustal section. The plutonic crustal section is composed of gabbros, gabbronorites, tonalites and minor plagiogranites. The lower part of the crustal section is intercalated with pyroxenite lenses, forming a layered sequence, whereas the upper part is cut by volcanic dykes associated with the overlying basalt–andesitic volcanic section. Most of the ultramafic rocks and gabbronorites show a depletion in high field strength elements and positive anomalies for Sr and Pb, whereas gabbros, tonalites and plagiogranites are enriched in large ion lithophile elements and have slightly enriched rare earth element patterns. Non-modal fractional melting models indicate that the most depleted harzburgites of the ophiolite originated after 20–25% of melt extraction from the mantle. Leached minerals and whole-rocks from the crust–mantle transition of the Khantaishir ophiolite define a Sm–Nd isochron at 540 ± 12 Ma, which is interpreted as the formation age of the crust–mantle transition. Additionally, minerals and whole-rocks display a restricted εNd(t=540 Ma) composition (+3·5 to +7·0) and a large scatter in εSr(t=540 Ma) (–19·8 to +14·2). Clinopyroxenes in the crust–mantle transition rocks indicate that they were in equilibrium with a boninitic-like melt, consistent with the lavas observed in the volcanic section of the ophiolite. It is therefore inferred that the Khantaishir ophiolite represents a slice of an incipient oceanic island-arc formed in a suprasubduction environment.

Sources and timing of pyroxenite formation in the sub-arc mantle: Case study of the Cabo Ortegal Complex, Spain

Pyroxenites exposed in ophiolites and orogenic peridotite massifs may record petrogenetic processes occurring in mantle domains generated and/or transferred in supra-subduction environments. However, the timing of their formation and the geochemical characteristics of their source region commonly are obscured by metamorphic and metasomatic overprints. This is especially critical in arc-related environments, where pyroxenites may be formed during the differentiation of primitive magmas. Our approach combines Sr- and Nd-isotope geochemistry and geochronology, and modelling of REE diffusion, to further constrain the origin of a well-characterized set of pyroxenites from the arc-related Cabo Ortegal Complex, Spain. In the light of petrological constraints, Sr- and Nd-isotope systematics consistently indicate that cpx and amphibole have acquired disequilibrium during two main episodes: (1) a magmatic/metasomatic episode that led to the formation of the pyroxenites, coeval with that of Cabo Ortegal granulites and corresponding to the incipient stage of a potential Cadomian arc (459–762 Ma; isochron and second-stage Nd model ages); (2) an episode of metamorphic amphibolitization upon the percolation of relatively unradiogenic and LREE-enriched hydrous fluids, subsequent to the delamination of the pyroxenites from their arc-root settings during Devonian subduction. Calculations of diffusional timescale for the re-equilibration of REE are consistent with this scenario but provide only poor additional constraints due to the sensitivity of this method to grain size and sub-solidus temperature. We thus emphasize the necessity to combine isochron ages and Nd model ages corrected for radiogenic ingrowth to put time constraints on the formation of subduction- and/or collision-related pyroxenites, along with petrological and geochemical constraints. Homogeneous age-corrected 143Nd/144Nd of 0.5121–0.5125 (εNd between 0 and +7.5) and 87Sr/86Sr of 0.7037–0.7048 provide information on the sources of the metasomatic agents involved (and potentially the parental melts) and notably indicate the contributions from enriched mantle components (EM I and/or II). This suggests the involvement of an old crustal component, which is consistent with the derivation of the pyroxenites and granulites from an ensialic island arc, potentially built on the northern margin of either Gondwana or a pre-Gondwanan continental block. This case study thus documents the role of melt–rock reactions as major pyroxenite-forming processes in the sub-arc mantle, providing further constraints on their sources and timing in the Cabo Ortegal Complex.

Exotic island arc Paleozoic terranes on the eastern margin of Gondwana: Geochemical whole rock and zircon U–Pb–Hf isotope evidence from Barry Station, New South Wales, Australia

Early Paleozoic intra-oceanic terranes crop out along the Peel-Manning Fault System, in the southern New England Orogen, NSW Australia. These are the Cambrian ophiolitic Weraerai terrane and the Siluro-Devonian island arc Gamilaroi terrane. There has been debate whether these terranes formed at the Gondwana margin or if they are intra-oceanic, and were accreted to Gondwana later in the Paleozoic. Major-trace-REE elemental data indicate Weraerai terrane formed in a supra-subduction environment. Rare zircons extracted from Weraerai terrane gabbro-plagiogranite suites at Barry Station yield a U–Pb zircon date of 504.9±3.5Ma with initial εHf values of +11.1 indicating a juvenile source. Amphibole-bearing felsic dykes and net-vein complexes are also found within the gabbro with a U–Pb zircon date of 503.2±5.7Ma and initial εHf values of +11.6. These are coeval in age with their host rocks and we propose they represent partial melts of the mafic crust during the circulation of seawater. The Gamilaroi trondhjemites of prehnite-pumpellyite–greenschist metamorphic grade terrane yielded very few zircons with an age of 413±8.7Ma. Zircon initial εHf values range from +5.0 to +2.9, indicating an input from an evolved crustal source, unlike the purely oceanic Weraerai terrane. Gamilaroi terrane trondhjemites are enriched in LREE have low K2O and K2O/Na2O ratios and strong negative Nb anomalies consistent with supra-subduction zone environments. Multiple subduction zones may well have existed within the Panthalassa Ocean during the early-mid Paleozoic with the Weraerai-Gamilaroi being accreted onto the Gondwanan margin during the latest Devonian.