Lithospheric Mantle Research Articles

Age and evolution of the lithospheric mantle beneath the Grib kimberlite (NW Russia)

Abstract The Grib kimberlite in NW Russia erupted into a poorly defined cratonic region along the White Sea coast. This region is part of the East European (or Baltica) Super-Craton that is comprised of several cratonic nuclei, including the Kola, Karelian and Murmansk. The eastern extent of the Kola-Karelian-Murmansk cratonic regions is uncertain due to a lack of exposed basement crust. We analysed olivine from a suite of 17 peridotite xenoliths from the Grib kimberlite in NW Russia for their Re-Os isotopic compositions and platinum-group elements (PGE) contents. Os isotopic compositions range from unradiogenic (below the present-day 187Os/188Os of the primitive upper mantle, i.e., < 0.1296) to values more radiogenic than any estimate of the primitive upper mantle (PUM). We observed no correlation between the amount of melt depletion reflected by olivine Mg# [100 × molar Mg/(Mg + Fe)] and the most unradiogenic Os isotopic composition measured in the inclusions within olivine. Samples with unradiogenic 187Os/188Os have similar PGEN patterns (where the subscript N indicates normalisation to CI chondrite), that are typical for depleted cratonic peridotites, with depletion in PdN and ReN compared to the iridium-group (I)-PGEN (Os, Ir, Ru). Only one Grib peridotite xenolith has an Archaean TRD age of 3.27 ± 0.37 Ga. The majority of Grib peridotite xenoliths have Proterozoic TRD ages between 2.5 and 1.2 Ga. Three olivines have 187Os/188Os between 0.136 and 0.316, more radiogenic than present-day PUM. Geothermobarometry of Grib peridotite xenoliths show that the local lithospheric mantle is between 210 and 240 km thick, with diamond stability below 130 km. Overall, the majority of mantle lithosphere beneath this portion of the East European (or Baltica) Super-Craton has a minimum age between 2.5 and 1.2 Ga suggesting significant lithosphere modification during Proterozoic rifting and subduction/accretion events.

Sulfide-rich continental roots at cratonic margins formed by carbonated melts

The cratonic crust contains abundant mineral deposits of metals such as gold, copper and rare earths1, 2, 3, 4–5 and is underlain by a thick mantle lithosphere rich in the volatiles carbon, sulfur and water6, 7–8. Although volatiles are known to be key components in metallogenesis9, how and where they are distributed in the cratonic lithosphere mantle and their role in the initial enrichment of metals have not been sufficiently explored. Here we compile sulfur and copper contents of global cratonic peridotites, identifying sulfide-rich and copper-rich continental roots at depths of 160–190 km at cratonic margins. Our new high-pressure experiments show that carbonated silicate melts originating from the asthenosphere lose silicate components during reaction with lithospheric peridotite, evolving to carbonatite melts that become concentrated at cratonic margins. Sulfur solubility in melts substantially decreases as the SiO2 content of melts decreases during this process, forcing sulfide precipitation and the formation of sulfide-rich continental roots at the base of the mantle lithosphere. The migration of carbonated melts towards cratonic margins replenishes the continental roots there with sulfur, explaining the co-location of magmatic metal deposits with carbonatites close to cratonic margins. These findings highlight the notable role of carbonated melts in metallogenesis and provide a potential platform for metal ore exploration.

Microstructural and Seismic Characteristics of Mantle Xenoliths From Damaping Area, and Their Geodynamic Implications of North China Craton Destruction

AbstractThe large‐volume lithospheric mantle xenoliths around the Damaping area provides valuable insights into the detailed destruction progression of the North China Craton (NCC). This paper presents a quantitative analysis of the microstructural and seismic properties of oriented mantle xenoliths (with distinct foliation and lineation). The selected peridotites all have coarse‐grained textures. The olivine crystallographic preferred orientations (CPOs) are predominantly axial‐[010] type (or AG‐type), with all xenoliths have girdled [100]OL and [001]OL characteristics. B‐type olivine CPO is newly discovered in the research area. Analysis of crystallographic vorticity axis (CVA) projections indicates that more than half of Damaping xenoliths have CVA maxima (sub)parallel to the lineation. These microstructure characteristics suggest that B‐type CPOs were formed before the onset of pure shear‐dominated transpression and pyroxene restriction, which significantly influenced the lithospheric mantle evolution. The upwelling asthenosphere beneath Eastern Block of the NCC not only delaminated its lithospheric mantle but also experienced rollback and flowed along the WNW‐SEE direction. This progression likely serves as the primary driving force of transpression. If foliation were vertical and lineation were horizontal, the valid S‐wave anisotropies range is 5%–12%. All selected samples are spinel facies, resulting in a maximum in situ depth of 90 km, and the calculated SKS splitting delay times (0.5–1.3 s) align with previous seismological observations. The SKS direction in the research area is predominantly oriented perpendicular (NNE‐SSW) to the flow direction of nearby asthenosphere. These characteristics are likely attributed to transpression. Therefore, the “fossil” anisotropy may have developed after the cessation of transpression.

Petrography and Geochemistry of Dolerite Dykes from Cretaceous Bamwa Half Grabben (North Cameroon, Central Africa)

Petrography and geochemistry of dolerite dykes from cretaceous Bamwa half grabben of North Cameroon in Central Africa have been done. Field work study have shown that dykes swarms of Bamwa basin exceptionally occurred as well exposed dykes vertically overhanging the local granitoids of the basement along trending direction of N155E. Dykes are 1 to 26 m wide and extend on 400 to 500 m. Microscopic observations have revealed various textures, from ophitic, sub-ophitic, classical dolerite to microlitic porphyritic. All dolerites are petrographically composed of plagioclase, oxides, clinopyroxene, rare apatite, chlorite and amphibole crystals, marked by alteration signs. ICP-AES and ICP-MS geochemical analyses of Bamwa dolerites have distinguished the dolerites lavas of basaltic trachyandesite, andesite, dacite and rhyolite composition. Bamwa dolerites are giant dykes which exhibit the geochemical characters of continental tholeiites of low TiO2 composition (TiO2 < 2 wt. %). Petrographic and geochemical studies of the dolerite dykes of Bamwa basin have shown that studied dolerites have experienced a complex petrogenetic history through assimilation and fractional crystallization, fluid infiltration and varying degrees of crustal contamination processes, after a relatively high partial melting rate of enriched subcontinental lithospheric mantle source. Studied dolerites stand as fingerprints of post Pan African magmatic events of arc-back arc setting after subduction.

Identification and Geological Significance of Late Cambrian OIB-Type Volcanic Rocks in the Nailenggeledaban Area, Northern Yili Block

Paleozoic igneous rocks exposed in the northern Yili Block are thought to have resulted from the subduction of the North Tianshan oceanic crust. However, the exact timing of the transition of the northern margin of the Yili Block from a passive to an active continental margin remains unknown. In this paper, the petrological and geochemical features, zircon U-Pb chronology, Lu-Hf isotopes, and Sr-Nd isotopes of volcanic rocks in the Nailenggeledaban area on the northern margin of the Yili Block were studied. Zircon U-Pb dating results show that the crystallization ages of the volcanic rocks in the Nailenggeledaban area on the northern margin of the Yili Block are 491 ± 2 Ma and 500 ± 2 Ma, suggesting they were formed during the Late Cambrian. Geochemical features show that the volcanic rocks are alkaline basalts with rare earth and trace element distribution patterns similar to OIB, although they exhibit some degree of Zr and Hf depletion. The εHf(t) values of alkaline basalts in the Nailenggeledaban area at the northern Yili Block range from −3.48 to −1.00, with a TDM1 age of 1152 to 1263 Ma. The εNd(t) values range from −3.53 to −0.96, with a TDM1 age of 1471 to 2162 Ma. Combined with geochemical data, the alkaline basalt magma in the Nailenggeledaban area on the northern margin of the Yili Block may be derived from the Mesoproterozoic enriched lithospheric mantle. The composition of the mantle source area is potentially garnet lherzolite, and the magma appears to have been either unaffected or only minimally contaminated by crustal materials during the ascending process. On the basis of the research results of the Early Paleozoic tectonic evolution in the northern margin of the Yili Block, this paper proposes that the volcanic rocks in the Nailenggeledaban area, located on the northern margin of the Yili Block, were formed in a back-arc extensional environment resulting from the subduction of the North Tianshan Ocean (or Junggar Ocean) beneath the northern margin of the Yili Block during the Late Cambrian.

Rudny Altai VMS-polymetallic belt (Russia, Kazakhstan) and its formation factors

The paper presents a modern metallogenic overview of Rudny Altai and the results of the study of the volcanic rocks associated with contrasting basalt-rhyolite formation, manifested as a consequence of riftogenic processes. There are two linear metallogenic subzones within the Rudny Altai polymetallic belt that extend in a northwestern direction. The Zmeinogorsko-Zyryanovskaya subzone is the main one: it contains 2/3 of the belt's deposits, 3/4 of Zn, Pb, Cu, and 4/5 of Au and Ag, which are associated with Emsian-Givetian basalt-rhyolite formation. The Irtysh metallogenic subzons extends along the Irtysh Shear Zone, and is mainly composed of the Eifelian – Early Famennian basalt-rhyolite formation. Devonian bimodal volcanism occurred against a background of extension deformation with the formation of pull-apart basins. Taking into account the structure-kinematic characteristics of faults, the Devonian architecture of the Rudny Altai block can be considered as a 'negative flower' (tulip) structures. Based on the trace element characteristics of initial basic rocks, the original magmas were the product of partial melting of metasomatised lithospheric mantle. This is confirmed by Pb-Pb studies of galena monofractions from the Rudny-Altai volcanogenic massive sulfide (VMS-type) deposits. The magma source of the subsequent major phases probably corresponded to the asthenosphere, which may have risen to the depth of the preceding melting region. The generation of significant volumes of felsic volcanic series, to which the main VMS-type deposits are genetically related, was most likely associated with large-scale melting of thick terrigenous strata of the pre-Devonian palaeoshelf under the influence of mantle magmas. The sequence of Devonian mineralization types is considered to be a consequence of the change in the type of volcanism initiated by transtension tectonics. This is consistent with the concept that the formation of ore-forming systems VMS type is associated with periods of hydrothermal activity during the mantle upwelling under extensive tectonic settings. For this region, the antidromic nature of magmatic series caused a specific evolution trend of its metallogeny, expressed in the change of barite-polymetallic and polymetallic deposits of the Emsian-Eifelian stage (Zyryanovskoe, Tishinka, Ridder-Sokolnoe), pyrite-polymetallic at the Givetian stage (Belousovskoe, Talovskoe) and then copper-pyritic at the Frasnian-Early Famennian stage (Kamyshinskoe, Nikolaevskoe). The results obtained are consistent with the model of evolution of the marginal arc – back arc system, in which mantle uplift is associated with basin extension and plate rollback.

Petrogenesis of Paleoproterozoic Khalari Hornblende‐Pyroxenite Intrusion Within the Dongargarh Supergroup, Bastar Craton: Insights From Petrological and Geochemical Studies

ABSTRACTThis study investigates Paleoproterozoic hornblende pyroxenite, a lithological unit within the Khalari Ultramafic‐Mafic Complex (KUMC), which is intruded into the Neoarchean‐Paleoproterozoic Dongargarh Supergroup near Khalari village in the northern Bastar Craton. A comprehensive characterisation has been conducted through petrological analysis, bulk‐rock geochemistry, mineral chemistry, and platinum group elements (PGEs)‐Au geochemical studies to understand its petrogenesis and geotectonic implications. The presence of primary amphibole, specific pyroxene chemistry (low Ti and Cr), and enriched LILE, LREE and fluid‐mobile elements alongside negative Nb–Ta–Ti anomalies suggest these rocks were crystallised from a mantle melt originated from a metasomatized sub‐continental lithospheric mantle (SCLM) source. This metasomatization could be due to fluids derived from a subduction event predating the emplacement of the studied rocks. The proposed melt composition is estimated to have formed from 5% to 10% partial melting of a garnet‐rich peridotite mantle source. The crystallisation conditions are estimated to have occurred at an average pressure of 7.85 kbar and a temperature of 902°C, indicating moderately shallow depths influenced by fractional crystallisation and slow cooling rates. The emplacement of the KUMC is contemporaneous with several other magmatic activities in the Bastar Craton, around ca. 2.50–2.47 Ga, suggesting that mantle plume might have played a significant role in their formation. Low concentrations of PGEs in the studied samples indicate a PGE‐depleted mantle source.

DEEP-SEATED STRUCTURE OF THE LOWER AMUR PROVINCE AND EPITHERMAL GOLD MINERALIZATION THEREIN

The density and magnetic models of the Earth’s crust and lithospheric mantle were developed for the Lower Amur River gold-bearing area. Geological and geodynamical interpretations were made of the deep-seated inhomogeneities found there. It has been shown that the main differentiation of regional geophysical fields in the area is related to the subcrustal mantle inhomogeneities and magmatic bodies in the middle and upper crusts. These inhomogeneities resulted from the Late Cretaceous – Cenozoic magmatic processes occurring in this segment of the Paleoasian margin under subduction conditions, in the mode of strike-slip margin or continental rifting.The spatial relationship was shown between the epithermal gold deposits and some of the deep-seated structural features of the area. Consideration was given to the role of the mantle density boundaries and up to 20 km deep density inhomogeneities in the gold mineralization distribution. There were identified zones of development of deep-seated magmatic bodies which can be interpreted as basaltic intrusions tracing the inferred deep-seated magma-controlling faults, to which, in its turn, gold mineralization can be related.

The Khangalas orogenic Au deposit, Yana-Kolyma metallogenic belt (Northeast Russia): structure, ore mineral and isotopic (O, S, Re, Os, Pb, Ar, He) composition, fluid regime and formation conditions

The Khangalas orogenic gold deposit is located in the central part of the Yana-Kolyma metallogenic belt. The structure of the deposit is determined by several mineralized crush zones with a thickness of up to 70 m and a length of up to 1400 m in the arch and on the southwest wing of the anticline of the northwest strike. Host rocks – Upper Permian terrigenous deposits. Ore bodies are characterized by massive, banded, veined, disseminated and breccia structures. The main vein minerals are quartz, carbonates, sericite is less common. The main ore minerals are pyrite, arsenopyrite; minor – galena, sphalerite, chalcopyrite, native Au; rare – Fe-gersdorffite, tetrahedrite, argentotennantite. Hypergenic minerals – sulfates, phosphates, arsenates and hydroxides – are widely manifested in the linear oxidation zone. Mineral formation occurred in two stages – gold-sulfide-quartz and silver-quartz ones. Quartz veins with visible Au were formed with the involvement of low-concentrated (about 5.0 wt.% eq. NaCl) of hydrocarbonate hydrotherms with CO2 in the gas component, at a temperature of 330–280 °C and a pressure of about 0.8 kbar. Disseminated gold-bearing pyrite-3 (up to 39.3 g/t Au) and arsenopyrite-1 (up to 23.8 g/t Au) from sericite-carbonate-quartz metasomatites have a non-stoichiometric composition, Fe excess and S lack (and As in Ару), Fe/(S+As)=0.47–0.52 (Py3) and 0.47–0.50 (Ару1). The predominant form of “invisible” gold in Py3 and Apy1 is structurally related Au+. Isotopic composition of oxygen δ18O quartz (from +15.2 to +16.1‰), oxygen in the fluid δ18OH2O (from +8.4 tо +9.2‰)‰), sulfur δ34S in sulfides (from –2.1 to –0.6‰); isotopic ratio 187Os/188Os (from 0.2212 to 0.2338) in native gold and Pb in galena (206Pb/204Pb=18.0214, 207Pb/204Pb=15.5356, 208Pb/204Pb=38.2216), as well as the geochemical features of Py3 and Apy1 suggest the participation in ore formation mainly of sources from the subcontinental lithospheric mantle and, to a lesser extent, crustal reservoirs. The formation of the gold ore bodies of the deposit is related to the completion of reverse and thrust fault progressive deformation of the stage D1, which occurred in the Valanginian of the Early Cretaceous (about 137 million years ago) during late-orogenic processes in the Yana-Kolyma belt with regional south-western transport of rocks. The results obtained are important for predictive metallogenic and prospecting work aimed at identifying large-volume gold mineralization of orogenic belts.

Geological characteristics of the Daduhe gold belt, western Sichuan, China: Implications for exploration

Abstract Geological characteristics, genetic model, and exploration implications in the Daduhe gold belt are studied. Three categories of gold deposits have been distinguished: “gold deposits in the basement (GDB),” “gold deposits in the cover layer (GDC),” and “gold deposits at the interface (GDI).” Among them, quartz vein type and quartz vein altered rock type are the main types of mineral deposits. The following features have been obtained: (1) inclusions of gold particles exist in pyrite, pyrrhotite, quartz, sellaite, and other sulfides; in fractures; or along cracks and grain margins. (2) The distribution curve of rare earth elements in pyrite and quartz indicates strong mineralization intensity in the GDB, (La/Sm)N fractionation characteristics, suggesting that pyrite may be more influenced by crustal materials and quartz by mantle materials. (3) The H–O and He–Ar isotopes manifested the ore-forming fluids derived from the mixing between crust and mantle. (4) The ranges of δ 34 S values of pyrite (−5.0 to 7.6‰) are consistent with those of metasomatized mantle lithosphere. (5) The homogenization temperatures and salinity of all quartz inclusions are concentrated from 150 to 210°C and 2.1–8.7 wt%, indicating a medium-low-temperature, low-salinity hydrothermal fluid. (6) The Daduhe orogenic gold deposits can be defined as epizonal subtypes. (7) The Daduhe gold deposits are medium-low temperature, low-salinity, and epizonal orogenic gold deposits, and fluid comes from a mixture of crust and mantle. In the future, efforts should be made to increase the exploration of deep gold deposits.

Early mesozoic bimodal volcanic sequences of the Central Mongolia: implications for evolution of Khentey segment of the Mongol-Okhotsk Belt

Volcanic sequences of bimodal basalt–trachyte–alkaline-rhyolite character with alkaline granites are widespread in the Central Mongolia. They outcrop within small sublatitudinal grabens scattered along the southern and western frame of the Khentey part of the Mongol-Okhotsk Belt. According to geochronological data, the bimodal magmatic activity occurred from the end of Triassic to start of Jurassic (220–195 Ma). Many rocks of bimodal sequences have high alkali content and rare metal signatures. Fractionation crystallization was the leading process causes an attainment of ore-level rare elements concentrations in the most differentiated melts. Mafic magmas enriched relative to the OIB in the majority of incompatible trace elements were primary melts for all rocks of these associations. At the same time, they show increased Ba and depleted Ta and Nb contents testify to participation of lithospheric mantle component in their source. The Nd and Sr isotopic ratios of the rocks corresponds to at least two magma sources identified as enriched asthenospheric mantle and subduction-modified lithospheric mantle. Bimodal magmatism in the Khentey segment of the Mongol-Okhotsk belt appeared ~30 Ma after the collision caused by the closure of the Ada-Tsag branch of the Mongol-Okhotsk Ocean at about 250 Ma. Rifting occurred along the entire frame of the Khentey segment of the belt and controlled this magmatism. It was initiated by collapse of the orogen with delamination of its keel caused the involvement of asthenospheric mantle in the Late Triassic–Early Jurassic magmatism of the region.

Zircon U-Pb geochronology and Sr-Nd-Pb-Hf-O isotope signatures of Middle Eocene high-K plutons from the Eastern Pontides Orogenic Belt, NE Turkey: implications for parental magma sources and magmatic processes

ABSTRACT The Eastern Pontides Orogenic Belt (EPOB) in northeastern Turkey is a well-preserved continental magmatic arc in the Alpine-Himalayan Orogenic Belt and contains many Eocene I-type high-K plutons. Employing LA-ICP-MS U-Pb zircon geochronology, the crystallization ages of the investigated Yaylalıköy and Taşbaşı plutons were established to be Lutetian (Middle Eocene), ranging from 47 Ma to 46 Ma. These plutons exhibit a compositional spectrum ranging from diorite to granodiorite, characterized by I-type, metaluminous, and high-K geochemical signatures. Notably, they are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), displaying pronounced negative Eu anomalies (EuN/Eu* = 0.58–0.96). Isotopic signatures reveal homogeneous and moderately low initial 87 Sr/86 Sr(i) ratios (0.704643–0.705034) and positive εNd(i) values (0.24 to 1.67) for the investigated plutons. These isotopic signatures are consistent with mantle origin, as evidenced by their position within the mantle array on isotope ratio diagrams. Lead isotope compositions exhibit limited variations, with initial ratios of 206 Pb/204 Pb(i) (18.55–18.64), 207 Pb/204 Pb(i) (15.61–15.64), and 208 Pb/204 Pb(i) (38.46–38.75). Zircon εHf(i) values are positive (2.32–6.20), plotting between the depleted mantle and chondritic evolution lines. Additionally, these rocks display low zircon δ18 O values (+4.54 to + 6.53‰), comparable to mantle composition. The petrographic characteristics of the investigated plutons suggest that fractional crystallization with minimal assimilation and/or magma mixing played a dominant role during their solidification. The integration of geochemical and isotopic data, alongside regional geological constraints, advocates for enriched lithospheric mantle as the source for the parental magmas of the studied plutons. These magmas subsequently underwent differentiation within a deep-seated magma chamber, followed by emplacement into the crust.

Mantle xenoliths in Northeast China record deep carbon cycling triggered by subduction of the Pacific Plate

Earth science studies have focused on the deep mantle carbon cycle and its geodynamic effects. However, the way in which carbonated silicate melts modify the mantle remains poorly constrained. In this study, we report petrographic, mineral major and trace element, and Sr isotopic data for a suite of peridotite and pyroxenite xenoliths from the Shulan and Yitong areas of Northeast China. These data are used to investigate how carbonated silicate melts modify the mantle. The olivine, orthopyroxene, and clinopyroxene in the Shulan harzburgite and lherzolite xenoliths have relatively high Mg# values (90−93), and the clinopyroxenes have low (La/Yb)N ratios (0.49−0.61), implying that they may be post-partial melting residues of mantle previously modified by silica-rich melts, which is consistent with the light rare earth element (LREE)-depleted REE patterns of the clinopyroxenes. The olivine, orthopyroxene, and clinopyroxene in the Shulan (olivine) websterite xenoliths also have high Mg# values (90−94) that are indicative of a mantle origin. However, the clinopyroxenes have high (La/Yb)N (1.43−65.8), Ti/Eu (3504−5255), and Ca/Al (5.13−9.59) ratios, a positive correlation between Sm/Hf and Zr/Hf ratios, and high LREE contents, which suggest carbonated silicate metasomatism. This inference is also consistent with the replacement of olivine and orthopyroxene by clinopyroxene. In contrast, the Mg# values (78−86) of the olivines in the Yitong orthopyroxenite, wehrlite, websterite, and clinopyroxenite xenoliths are lower than those of the corresponding minerals from peridotites. This result, combined with the variably LREE-enriched and high field strength element (HFSE)−depleted patterns (e.g., Nb, Ta, Zr, and Hf) of the clinopyroxenes, suggest that these pyroxenites could have crystallized from mantle-derived melts. Compared with the Shulan peridotites, the Yitong xenoliths record higher temperatures (1028−1310 °C) and higher (La/Yb)N (1.05−5.30) and Ti/Eu (2624−6567) ratios, and a positive correlation between La and Sr, which reflect the occurrence of carbonated silicate melts in the lithospheric mantle. This is also supported by the estimated equilibrium melt compositions obtained from clinopyroxene. Thus, we propose that the different types of Yitong xenoliths are the crystallized products of silica-rich to carbonated silicate melts. The low 87Sr/86Sr ratios (0.70345−0.70488) of the clinopyroxenes in the wehrlites and pyroxenites from Shulan and Yitong, along with the timing of formation of the Northeast Asian big mantle wedge (ca. 20 Ma) and geochemical characteristics of the Cenozoic basalts and high-Mg andesites in Northeast China, indicate that the carbonated silicate melts were derived from the partial melting of recycled carbonatized oceanic crust and ancient recycled crustal material in the mantle transition zone. This implies that the lithospheric mantle along the Yilan−Yitong faults experienced not only crystallization and the accumulation of silica-rich and carbonated silicate melts that formed the Yitong wehrlite and pyroxenite xenoliths, but also subsequent modification by carbonated silicate melts that formed the Shulan (olivine) websterite xenoliths. These mantle xenoliths record deep carbon cycling triggered by the subduction of the Pacific Plate.

Halogen composition and heterogeneity in the continental lithospheric mantle

Halogen composition and heterogeneity in the continental lithospheric mantle

Osumilite-bearing lavas of the Keli highlands (Greater Caucasus): petrological and geochemical characteristics, mineral composition and conditions of magmatic melts formation

Comprehensive petrological, geochemical and mineralogical studies of osumilite-bearing andesite-dacitic lavas of the volcano Kordieritoviy (Keli Highland, Greater Caucasus), erupted at the end of the Pleistocene (about 200 Ka), were carried out. The results of petrographic study of thin sections and microprobe analysis showed that the rocks contain three paragenetic mineral associations: (1) “xenogenic” (metamorphogenic) – garnet (XPrp = 0.42, XAlm = 0.51–0.53, XGrs = 0.04–0.05) + hercynite + sapphire + bronzite + pargasite + ilmenite; (2) early magmatic – hypersthene + hercynite + garnet (XPrp = 0.21–0.31, XAlm = 0.52–0.71, XGrs = 0.04–0.13) + ferro-kersutite + ilmenite; (3) late magmatic – hypersthene-ferrohypersthene + labradorite + garnet (XPrp = 0.04–0.14, XAlm = 0.65–0.81, XGrs = 0.06–0.18) + osumilite-(Mg) + phlogopite + tridymite + ilmenite + apatite. Osumilite-(Mg) (phenocrysts, xenomorphic aggregates in the rock matrix and crystals in miarolic cavities), the average formula of which for dacites of the Kordieritoviy volcano can be written as (K0.73Na0.06Ca0.02□0.20)1.00(Mg1.06Fe2+0.90Mn0.04)2.00(Al2.75Fe2+0.18Fe3+0.06Ti0.01)3.00(Si10.34Al1.66)12O30, formed mainly at late magmatic stages – in intermediate chambers immediately before the rise of the melt to the surface or after its eruption. Accordingly, this mineral in the studied lavas has purely magmatic origin. Thermobarometric calculations and petrological modeling showed that the deep magma chamber of the Kordieritoviy volcano was located at a level of 45–53 km from the surface in near the Moho boundary. The temperature of the melt at the early magmatic stage was no less than 1100°C at 17–23 kbar. Crystallization of osumilite-(Mg) in intermediate magmatic chambers (at depths of 30–40 km) and during the process of lava outpouring occurred at 1030–870°C and pressure progressively decreasing from 14–9 to 1 kbar. A petrogenetic model has been proposed to explain the reasons for the formation of exotic osumilite-containing lavas of the Kordieritoviy volcano. Its main provisions include: (1) enriched upper mantle source (lithospheric mantle metasomatized as a result of permanent interaction at the Moho boundary with the overlying lower crust composed of metamorphosed terrigenous-volcanogenic formations); (2) generation of “dry” basaltic magmas in the source; (3) crystallization differentiation in the source (fractionation of olivine and chrome spinels) with the formation of a “dry” superheated andesitic melt; (4) limited-scale assimilation by highly differentiated andesitic melts rising to the surface of the material of the lower crust, directly below the volcano, composed of leucocratic granulites, with simultaneous fractionation of garnet, orthopyroxene and ilmenite from the melt.

Age and composition of the lithospheric mantle beneath Jiande, Zhejiang Province: Implications for crust-mantle co-evolution of the South China Block

Fresh mantle peridotite xenoliths entrained in the Eocene Jiande alkaline basalts from western Zhejiang Province, South China Block (SCB), offer an opportunity to elucidate the compositional and temporal relationships between the subcontinental lithospheric mantle (SCLM) and its overlying crust. The Jiande peridotites (20 lherzolites + one harzburgite) exhibit tightly correlated and relatively fertile whole-rock major element compositions (WR Mg#=89.0–90.8), indicating mild to moderate degrees (mostly ≤ 10 %) of mantle partial melting and limited metasomatism experienced by the Jiande SCLM. The above mantle processes are further underscored by: 1) the nearly unfractionated highly-siderophile-element (HSE) relative abundances ((Pd/Ir)N = 0.81–1.37, normalized to primitive-upper-mantle values); 2) the robust positive correlations observed between 187Os/188Os (0.1196–0.1283) and 187Re/188Os ratios (0.112–0.451; R2 = 0.79), and Al2O3 contents (1.5–3.8 wt%; R2 = 0.85) across the majority of high-187Os/188Os samples. Both Re-Os (initial 187Os/188Os = 0.1172) and Al-Os isochrons (initial 187Os/188Os = 0.1181) also yield an age of ∼ 1.4 Ga for the predominant melt depletion event that shaped the formation of the Jiande SCLM, aligning well with the peak of two-stage depleted mantle Hf model ages of the SCB crustal rocks. This age-coupled crust-mantle differentiation event occurring in the Mesoproterozoic may have been controlled by tectonic-magmatic processes during the breakup of the Columbia supercontinent. In addition, two low-187Os/188Os (0.1108–0.1128) samples with ∼ 15–20 % melting degrees ((Pd/Ir)N = 0.15–0.30) show the oldest Re-depleted Os model ages (2.2 and 2.4 Ga) among all the reported SCB mantle peridotites. These older SCLM materials may represent remnants linked to earlier crust-forming events that were incompletely reworked or replaced. Alternatively, they could represent mantle heterogeneity inherited by the newly formed lithospheric mantle during the Mesoproterozoic era.

The Inherited Crustal Structure and Lithospheric Thermal Field Beneath the Sea of Marmara (NW Türkiye): Observations From 3D Gravity Modeling and Seismic Tomography Analysis

AbstractThe North Anatolian Fault (NAF) extends for over 1,000 km across Türkiye and poses significant seismic hazard in the region. The Main Marmara Fault (MMF) segment of the NAF in the Sea of Marmara (NW Türkiye), exhibits along‐strike segmentation in its interseismic strain accumulation. Constraining the lithospheric configuration below the MMF is critical to understand its segmentation and assessing seismic hazard in the area. We present a new 3D lithospheric‐scale density of the Sea of Marmara, that combines gravity modeling and seismic tomography analysis. Using forward and inverse gravity modeling with free‐air gravity data and available constraints of geological units we derived the intra‐crustal density structure. Shear‐wave velocity tomography models provided insights into the temperature and density configuration of the uppermost mantle, and the geometry of the 1330°C isotherm. Our results highlight significant crustal density variations: lower‐density crust in the Sakarya Zone and Strandja Massif, and denser crust below the Istanbul Zone, which overlies a relatively hotter lithospheric mantle. This lithospheric configuration reflects both ongoing tectonic processes and inheritance from past geological events, including the drifting of the Istanbul Zone crustal block and the signature of past subduction events. The extent of the Istanbul Zone denser crust spatially correlates with the locked segment of the MMF. The bimaterial nature of the fault segment likely influences its interseismic and coseismic behavior. The denser, stiffer Istanbul Zone crust would promote interseismically locked conditions in contrast to the adjacent, more compliant crustal block and could result in asymmetric rupture with a preferred directivity.

Multi-stage evolution of the lithospheric mantle beneath the Hessian Depression (Germany): Peridotite xenoliths from Stöpfling

Multi-stage evolution of the lithospheric mantle beneath the Hessian Depression (Germany): Peridotite xenoliths from Stöpfling

Correlation of Crustal Kinematic Characteristics and the Uplift of the Southeastern Tibetan Plateau During the Paleogene

AbstractThe mechanism of the uplift of the southeastern Tibetan Plateau during the Cenozoic is much debated. To address this problem, we conducted a paleomagnetic study of the Eocene sedimentary strata in the Relu Basin, in the southeastern Tibetan Plateau. Two primary magnetic components of Ds = 24.0°, Is = 26.8°, k = 156.8, α95 = 3.5° acquired during ∼48.0–43.0 Ma and Ds = 172.2°, Is = −38.0°, k = 38.6, α95 = 9.8° during ∼43.0–35.0 Ma were isolated. The results combined with previous Paleocene and Eocene paleomagnetic and paleo‐altimetry data sets from the southeastern Tibetan Plateau, revealed a spatio‐temporal correlation between crustal movement and the uplift process of the southeastern Tibetan Plateau. The results showed that the final collision between India and Asia at ∼53.0–47.0 Ma immediately triggered the latitudinal crustal convergence and rotational deformation within the southeastern Tibetan Plateau. Since ∼47.0 Ma the southeastern Tibetan Plateau underwent a coherent crustal rotational pattern, and by at least ∼43.0 Ma, it finished the substantial latitudinal crustal convergence. The crustal movement transformations coincided with the major uplift of the southeastern Tibetan Plateau during ∼47.0–35.0 Ma and the eruption of extensive mafic magmatic rocks across the southeastern Tibetan Plateau and its margin during ∼43.0–33.0 Ma. These spatio‐temporal relationships suggest that the integral uplift of the southeastern Tibetan Plateau was mainly associated with the asthenospheric mantle upwelling and convection resulting from the tearing of the mantle lithosphere of the India Plate beneath the southeastern Tibetan Plateau.

Re-Os isotopic evidence for ancient melt depletion in refertilized Neo-Tethyan suboceanic mantle domain

Ancient mantle domains with unradiogenic Os isotopes have been well documented in the Yarlung-Tsangpo ophiolites (YTO), southern Tibet. Nonetheless, previous Os isotope studies have yielded relatively young Re-depletion model (TRD) ages of <1.0 Ga for mantle peridotites of the Zedong ophiolite in the eastern segment of the YTO. In this study, a comprehensive petrological and geochemical study has been conducted on a suite of mantle samples including harzburgites, lherzolites and one pyroxenite. Our results indicate that the Zedong harzburgites have been subjected to low to moderate degrees (∼6–20 %) of partial melting and late-stage melt-rock interaction with basaltic melts. In contrast, both lherzolites and pyroxenite were generated by melt refertilization. The Zedong lherzolites display flat patterns of highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re), whereas the harzburgites show depletion in Pt, Pd and Re relative to Os, Ir and Ru. The Re-Os isotopes of the Zedong lherzolites have been modified by melt refertilization and thus cannot provide reliable age information. Three Zedong harzburgites with the lowest PdN/IrN and ReN/IrN ratios have the least radiogenic 187Os/188Os of 0.11497–0.11537, giving the old TRD ages of 1.89–1.98 Ga. These ages are considerably older than previously reported TRD ages for the Zedong peridotites (< 1.0 Ga), but comparable to the oldest TRD age for the Luobusa peridotites. This supports the occurrence of similarly old mantle domains in both Zedong and Luobusa ophiolites in the eastern part of the YTO. The Zedong pyroxenite has a radiogenic 187Os/188Os ratio of 0.158 that is similar to the 187Os/188Os ratios (0.156–0.180) of previously reported for the YTO pyroxenites, which can be explained by selective assimilation of interstitial sulfides during the percolation of migrating melts through the mantle lithosphere. Grain boundary sulfides may play a significant role in the high 187Os/188Os of the YTO pyroxenites.