Lithos最新文献

We document the formation of omphacitites in the Monviso Lago Superiore Unit (W. Alps) where eclogite-facies, serpentinite-bearing shear zones host ample evidence for intense intra-slab fluid flow. In the first locality studied, a serpentinite-hosted jadeitite block is rimmed by strained omphacitite (with lawsonite pseudomorphs and minor phengite). U-Th-Pb zircon dating yields a protolith age of c.150 Ma, thus pointing to a burial-related replacement of a former Tethyan seafloor material, and revealed a lack of alpine rims. Multi-mineral Rb-Sr dating of the minerals forming this omphacitite rind yields an isochron age of 41.6 ± 0.7 Ma, corresponding to the early exhumation path still within the eclogite-facies. In this case, omphacitization led to the nearly complete replacement of the former prograde jadeitite through solution-precipitation and was followed by crystal-plastic deformation processes.

In various Monviso localities, Fe-Ti metagabbro blocks (commonly exhibiting brecciated garnetite fragments) display evidence for intense fluid-rock interactions leading to the formation of decimeter-thick, weakly strained omphacite-dominated rinds (with minor amounts of lawsonite, clinochlore and talc) around the blocks at the contact with the surrounding serpentinite. Partly dissolved zircon crystals separated from omphacitite rimming an eclogitic Fe-Ti metagabbro block only yielded middle-Jurassic protolith ages. This indicates that omphacitite rinds did not overgrow the eclogite blocks but rather replaced metasomatically the garnet-rich eclogitic assemblage. The omphacitization overprint occurred independently of the nature of the protolith (namely, forming after a jadeitite or after an Fe-Ti eclogite) as a consequence of infiltration of externally-derived fluids and subsequent element sequestration from the incoming fluid phase. These observations highlight the extraordinary ability of eclogite-facies metasomatic fluids to nearly totally overprint rocks as resistant and impermeable as Monviso eclogite breccias or jadeitites.

The closure of the Paleo- and Neotethys resulted in a long history of subduction of oceanic crust and production of a large variety of Phanerozoic magmatic rocks in the region occupied by present day Iran. Adakitic rocks of varying ages are common in this area and have distinctive geochemical signatures that have been variably linked to slab break-off or melting of the lower continental crust. The geographic distribution and age of the adakitic rocks indicates a potential younging trend from northwest to southeast Iran, but this trend was interrupted by an older outlier in the area of Tafresh, in the central part of the Neotethys arc. We obtained new geochemical and U-Pb geochronological data for Eocene volcanic and Miocene intrusive rocks from this anomalous locality. Our results show that adakitic signatures are only present in younger Miocene intrusive porphyritic bodies and not in the main calc-alkaline Eocene volcanic succession as previously thought. The Tafresh adakitic porphyritic bodies have an age of 15.7 ± 0.1 Ma (n = 183, 2σ SE), which fits well with a regional younging of the adakites towards the southeast of Iran. The Tafresh adakitic rocks are classified as the high-silica variety, and show trace element signatures and isotopic values that are consistent with melting of the lower continental crust. We hypothesise that progressive slab break-off provided a mechanism for the formation of high-silica adakitic rocks along the former Neotethys arc.

Graphite is essential to characterize biogenic (organic) and abiogenic (inorganic) sources of metamorphic carbon. This study includes a detailed petrological, Raman spectroscopic, and stable carbon isotopic analysis of graphite from the high-pressure (HP) and ultrahigh-pressure (UHP) metapelites in the southwestern Tianshan orogenic belt, China. Graphite occurs as foliation-parallel bands, a feature described for the first time in Tianshan, and has relatively high δ¹³C values (δ¹³C_graphite of −14.8 to −12.5‰), indicating an abiogenic precursor. The structural characterization of the graphite morphologies using backscattered electron images and Raman spectroscopy reveals very high crystallinity. Thermodynamic modeling and zirconium in rutile thermometry constrained the P–T conditions of graphite formation during eclogite-facies peak metamorphism at approximately 27 kbar and 530 °C. The banded occurrence of graphite, the CH₄-rich fluid inclusions associated with graphite, and thermodynamic modeling (by GFluid) suggest that graphite precipitated oxidatively from a CH₄-rich fluid. This CH₄-rich fluid may stem from the (U)HP carbonate-bearing eclogites that have been reported in previous studies. The oxidative precipitation of the studied graphite from CH₄-rich fluid suggests that the ambient redox state is one of the key factors that control the fate of COH fluids in the deep carbon cycle.

The Sieggraben Complex (SC) in Eastern Austria provides comprehensive information on the pre-Permian, Permian, and Alpine (Cretaceous) evolutional stages of the crustal and mantle sections included into the Austroalpine basement HP/UHP belt. The SC in the Sieggraben–Schwarzenbach area consists of three, top-to-bottom tectono-stratigraphic units: (1) the Paragneiss unit, (2) the Marble–Eclogite unit, which contains MORB and OIB types of mafic rocks, and (3) the Micaschist–Calcschist unit. The N-MORB type mafic rocks yield a UPb age of 369.7 ± 2.0 Ma, have ε_Nd(370) = +8.5 to +7.4 and TDM_(2st) of 0.42–0.50 Ga, while the E-MORB types (UPb ages around 400 Ma) show ε_Nd(400) = +9.4 to +5.6 and TDM_(2st) of 0.37–0.67 Ga, both indicating rather juvenile mantle sources during the formation of the inferred oceanic Devonian basin. The rocks of these units revealed Cambrian, Proterozoic, and Archean zircon sources. The ages of 285.8 ± 9.2 and 265.7 ± 2.1 Ma from eclogite and amphibolite, respectively, and 265.5 ± 1.4 Ma from paragneiss indicate an important Permian metamorphic overprint. Permian granitic dykes dated from ∼265 to 260 Ma, exceptionally at 273 and 253 Ma, crosscut the lithological units. A clinopyroxenite dyke, which crosscuts harzburgite of the underlying mantle fragment, yielded a magmatic age of 253.3 ± 2.9 Ma. Overall, these ages suggest a strong Permian extension, overheating, and melting of the crustal and mantle rocks. The eclogite ages from ∼100 Ma, but mainly between 92 and 90 Ma, constrain the Late Cretaceous metamorphic event terminated by an 88.4 ± 0.8 Ma pegmatite intrusion in the Paragneiss unit. The Permian and Cretaceous metamorphic events often caused resetting of the relatively older zircon ages, together with a decrease of zircon ε_Hf(t) values, and an increase of whole-rock ⁸⁷Sr/⁸⁶Sr values. A Carboniferous metamorphic age of ∼340 Ma was rarely detected. The SC finally represents a part of an early Late Cretaceous intra-continental subduction zone, which formed within a strongly-thinned crystalline basement due to the Devonian and Permian extensions. A similar Cretaceous suture zone with HP amphibolite, dated at ∼130–100 Ma by white mica ArAr ages (100–90 Ma exhumation), has been described around the Lubeník line between the Gemeric and Veporic tectonic units of the Inner Western Carpathians in the framework of the Cenozoic AL–CA–PA (Alpine–Carpathian–Pannonian) microplate.

The Varena Iron Ore deposit in the SW East European Craton is a significant ore body that occurs within metamorphosed and hydrothermally reworked Paleoproterozoic dolostones. We have performed microstructural investigations supplemented with mineral chemistry and geochronological investigations (LA-ICP-MS) to obtain age constraints on the ore-forming event(s) and improve the understanding of the conditions during mineralization process. Mineral chemistry and textures suggest a drop in pressure after the event of peak metamorphic skarn formation. Influx of oxidized, iron-rich H₂O fluids resulted in (1) Mg mobility that caused secondary dolomitization of calcite, (2) dissolution of metamorphic magnetite and formation of a new, inclusion-rich (Mag-1) and inclusion-poor (Mag-2) magnetite, and (3) replacement of the peak skarn assemblages. During these fluid-related processes, accessory phases of monazite, baddeleyite, and zircon were formed. Their UPb dating yield individually robust ages of 1721 ± 9 Ma (monazite, 23 spots), 1703 ± 10 Ma (baddeleyite, 18 spots) and 1706 ± 54 Ma (zircon, 14 spots), respectively. The weighted mean age of 1713 ± 7 Ma (2σ internal) is considered to represent the best age estimate of the iron-ore mineralization in the Varena Iron Ore deposit, and possibly also dates influx of P, REEs etc. into the system. This mineralization event is contemporaneous with ca. 1.73–1.70 Ga metamorphic reworking of the host rocks in the region and may be linked to regional continental-margin type Transscandinavian Igneous Belt (TIB) magmatism in south-central Sweden.

The West Qinling Orogen (WQO) of the northeastern region of the Tibet Plateau occupies a key tectonic position at the junction between the Pan-Asian and the Tethyan tectonic domains. However, there are ongoing debates regarding the exact timing of final continental collision in the WQO and closure time for the Paleo-Tethyan ocean. Here, we present LA-ICP-MS zircon UPb dating results along with major-trace elemental and Sr-Nd-Hf isotopic data for four granitic plutons across the western WQO, from north to south including Xiangyu, Tongren, Heri, and Wenquan. Notably, both the Tongren and Wenquan plutons contain abundant mafic microgranular enclaves (MMEs). Our new data indicate that the Xiangyu granites were emplaced at 249 Ma in a continental arc setting. These rocks are high-K calc-alkaline, formed by partial melting of Precambrian basement within the normal lower crust (< 40 km) followed by fractional crystallization of primary crustal melts. The Heri granitoids were emplaced at 237 Ma in a continental arc setting as well. They are also high-K calc-alkaline, but generated by partial melting of Precambrian basement at depths of 40–50 km. The Wenquan granitoids along with MMEs were emplaced at 234–233 Ma in a continental arc setting. The enclave magmas were formed through orthopyroxene-dominant fractional crystallization of primary mantle-derived melts originating from a phlogopite-bearing lherzolitic lithosphere at depths no more than 60 km, while the host granitoids were formed through mixing of the crustal melts resembling the Heri granitoids with approximately 40 to 60% enclave magmas. The Tongren granitoids along with MMEs were emplaced at 231–228 Ma in a back-arc setting. The enclave magmas originated from a phlogopite-bearing lherzolitic lithosphere at depths of 60–70 km, while the host granitoids were formed through mixing of the crustal melts resembling the Xiangyu granites with approximately 18 to 40% enclave magmas. We suggest that the Paleo-Tethyan (Anemaqen) oceanic crust had subducted northward approaching the northernmost WQO by 249 Ma followed by the initiation and gradual southward migration of the oceanic slab rollback. The slab subduction did not cease at least before 228 Ma.

Cenozoic (≥ 43 Ma) silica undersaturated (potassic) trachybasalts and trachyandesites in southwestern Madagascar (Tsianihy-Manja, southern Morondava Basin) form a small monogenetic volcanic field emplaced above Paleogene detritic sedimentary rocks, along a NE-SW-trending fault system. These olivine-chromite±clinopyroxene-phyric primitive lavas (Mg# = 69; MgO = 10–11 wt%; Cr = 450 ppm, Ni = 200 ppm; K₂O = 3–4 wt%) have highly peculiar trace element and isotopic composition (e.g., Ba/Nb = 18.4; ⁸⁷Sr/⁸⁶Sr_i = 0.70529–0.70555, ¹⁴³Nd/¹⁴⁴Nd_i = 0.51262–0.51263, ²⁰⁶Pb/²⁰⁴Pb_m = 18.415–18.424, ²⁰⁷Pb/²⁰⁴Pb_m = 15.576–15.579, ²⁰⁸Pb/²⁰⁴Pb_m = 38.799–38.813). A hitherto undescribed plug of primitive (sodic) basanite of the 11–12 Ma-old Ankililoaka district south of Tsianihy-Manja (hosting spinel lherzolite mantle xenoliths) has noticeable different geochemistry (Ba/Nb = 8–9.2; ⁸⁷Sr/⁸⁶Sr_i = 0.70346–0.7036, ¹⁴³Nd/¹⁴⁴Nd_i = 0.51281–0.51282, ²⁰⁶Pb/²⁰⁴Pb_m = 19.079–19.374, ²⁰⁷Pb/²⁰⁴Pb_m = 15.621–15.645, ²⁰⁸Pb/²⁰⁴Pb_m = 39.115–39.424).

The relatively low CaO, Sc, V, Fe₂O_3t, MnO at high MgO, Cr and Ni, and the potassic affinity of the Tsianihy-Manja trachybasalts, all indicate that the mantle source is relatively clinopyroxene-poor (i.e., depleted by previous melt extractions), in the same way as the source of lamproitic (or boninitic) magmas, but the primitive nature, the concentration of high field strength elements, the incompatible element patterns and their isotopic ratios indicate their unequivocal within-plate setting and indicate a derivation by low-degree partial melting of an incompatible element-enriched mantle and insignificant role for crustal contamination. In terms of incompatible element concentrations, and thus also Sr-Nd-Pb-isotopic composition, we find no evidence in favour of a mid-ocean ridge basalt (MORB)-mantle component, or for a MORB-mantle strongly enriched by ocean island basalt-like components, to form the mantle source regions of the Tsianihy-Manja and Ankililoaka mafic alkaline rocks. The significant isotopic change from the northernmost Cenozoic volcanic rocks of Madagascar and those in the central and southern part of the island (which range in composition from sodic to potassic, and from tholeiitic basalt to olivine melilitite) implicates a distinct source heterogeneity, and ultimately assess the role of the old continental lithospheric mantle as source region.

Granite formation and evolution are influenced by various physical and chemical processes in magmatic systems, resulting in diverse textures and compositions. While the generation of granitic rock through dehydration melting of hydrous minerals, which requires higher temperatures, is widely accepted, the formation and evolution of cold granite remains a topic of considerable debate. Our experiments aim to understand how small changes in granitic magma composition influence late-stage crystallization, providing insights into the chemical and textural evolution of granitic rocks and determining the significance of these conditions in shaping the diversity observed in nature. We conducted crystallization experiments at 1 and 2 kbar, between 680 and 815 °C, using two granitic compositions: a natural I-type granitoid (MA) and a slightly more mafic, synthetic analogue (FC). In these relatively low-pressure and low-temperature experiments, orthopyroxene remains stable at 1 kbar regardless of the starting material. At 2 kbar, its stability is limited to the more mafic sample FC when containing 3.7 wt% water. Hornblende only crystallizes in the more mafic rock FC at 2 kbar across all temperatures tested when water concentration exceeds 4 wt%. Plagioclase spans a broad temperature range (700–815 °C) at both 1 and 2 kbar in the FC material, whereas in the MA material at 2 kbar, it appears only at temperatures ≤725 °C. At 2 kbar few crystals were observed in the MA run at 750 °C, indicating that the liquidus temperature was nearly reached, contrasting with the FC starting material. Additionally, temperature cycling proved more efficient in promoting crystal growth in FC samples. Under low-temperature conditions, only local equilibrium was achieved, highlighting the significant role of late-stage crystallization processes and kinetic limitations in shaping the diversity observed in cold granites. Our experiments demonstrate that even small changes in CaO and FeO content can markedly alter the solidus/liquidus temperature of melts, significantly influence phase stability, melt production, and texture in granitic systems. This study surmises the need to further pursue systematic investigations of the influence of major elements on crystallization sequences in granitic systems, and we compare our findings to previous observations made on the Gentio metagranitoids of the Mineiro belt, Brazil.

The identification of lithology and characteristics of the mantle sources plays a pivotal role in understanding the origin of mantle-derived magmas. In this study, we present new zircon U-Pb ages, geochemical and Sr-Nd-Pb isotopic data for the Tugurige basalts and gabbros, at the intersection of the Xing'an Mongolia Orogenic Belt (XMOB) and the northern North China Craton (NNCC). We combined this new evidence with previous data on the Permian mafic rocks of XMOB and NNCC to unravel the petrogenesis of these rocks and shed light on the tectonic evolution of the XMOB and NNCC during the Late Paleozoic. Zircon U-Pb isotopic data constrained the magmatic emplacement of the Tugurige basalts and gabbros at ca. 288 and 277 Ma, respectively. The low SiO₂, high MgO, E-MORB-like trace element patterns and isotopic compositions (ε_Nd(t): +3.5 to +3.9, (²⁰⁶Pb/²⁰⁴Pb)_i: 18.00–18.54) of the basalts are comparable to that of the Permian mafic rocks in the XMOB. The various Ba/Nb, La/Nb, La/Ta, Nb/Th, and Th/La ratios of the Permian mafic rocks from the XMOB demonstrate the contributions of two distinct mantle sources beneath the XMOB – asthenosphere and young lithosphere that were metasomatically enriched by the Paleo-Asian Ocean subduction pre-Permian. Conversely, the gabbros exhibit high SiO₂, low MgO, a typical enrichment in light rare earth elements (LREEs), large ion lithophile elements (LILEs), Th, and U relative to the high field strength elements (HFSEs), and an enriched isotopic composition (⁸⁷Sr/⁸⁶Sr_(t): 0.7069–0.7071, ε_Nd(t): −4.2 to −3.6, (²⁰⁶Pb/²⁰⁴Pb)_i: 18.13–18.23), analogous to the Permian mafic rocks of the NNCC. These observations are attributed to the partial melting of pyroxenite and peridotite from the ancient lithospheric mantle beneath the North China Craton, which had been significantly modified by multistage subduction-related metasomatism before the Permian. We suggest that the arc-like trace element signatures of the Permian mafic rocks from the XMOB and NNCC were supposed to be a remarkable degree of inheritance from the enriched lithospheric mantle without requiring the involvement of contemporaneous subduction environment or deep-Earth water cycling process as previously suggested. The mixed, asthenospheric and lithospheric mantle source of the XMOB's Permian mafic rocks suggests that the Permian magmatic activities probably developed in a post-collisional extensional setting. This implies that the final closure of the Paleo-Asian Ocean occurred prior to the Permian, at least in the western part of the XMOB.

The East Kunlun Orogen (EKO), a major component of the Greater Tibetan Plateau, provides an excellent natural laboratory to investigate the tectonic evolution of the Paleo-Tethys Ocean (also known as the A'nyemaqen Ocean in the EKO). We present a combined study of in situ zircon UPb ages and LuHf isotopic compositions, and whole-rock major and trace element and Sr–Nd–Hf isotopic compositions of the post-collisional Xiangjia granodiorite at the eastern end of the EKO. Zircon UPb dating indicates that the Xiangjia granodiorites were emplaced at 225.0–217.1 Ma. Relict zircon cores yield ages of 267.5–239.5 Ma. These granodiorites are calc-alkaline to high-K calc-alkaline and metaluminous to weakly peraluminous, enriched in large-ion lithophile elements and light rare earth elements, and depleted in high field strength elements (e.g., Nb and Ta) and heavy rare earth elements. They yield trace element characteristics that are typical of adakitic rocks, including high Sr (493–590 ppm) and low Yb (0.74–1.12 ppm) and Y (8.83–12.24 ppm) contents, high Sr/Y (40.47–63.64) and (La/Yb)_N (11.81–30.91) ratios, and positive Eu anomalies. The UPb ages and Hf isotopic compositions of the relict zircon cores and whole-rock SrNd isotopic compositions of the Xiangjia adakitic rocks are similar to those of the mafic arc magmatic rocks formed during the subduction of the Paleo-Tethys Ocean, suggesting they originated from the reworking of juvenile mafic lower crust. Phase equilibrium modelling suggests that the contemporaneous adakitic rocks from Xiangjia and other areas in the EKO are likely the products of partial melting of the juvenile mafic arc rocks under granulite-facies conditions at 10–11 kbar and 934–951 °C, possibly related to slab break-off in a post-collisional setting.