Rock Assemblages Research Articles

Spatial and temporal reconstruction of postcollisional potassic rocks: Implications for mantle flow beneath the SE Tibetan Plateau

Postcollisional potassic magmatism occurs throughout the Alpine-Himalayan belt, providing a crucial record of crust-mantle interaction and associated ore-forming processes in a collisional orogen. Unravelling its spatial and temporal record is often complicated by intense tectonic deformation, including strike-slip faulting. New Hf-Nd isotope mapping of two Cenozoic potassic rock assemblages along the Ailaoshan−Red River shear zone from the SE Tibetan Plateau, together with Sr-Nd-Hf isotope evidence, establishes that these two segments of potassic magmatism were originally juxtaposed prior to strike-slip faulting. By reconstructing the spatiotemporal distribution of the potassic rocks, we identified a zone of mantle upwelling and lateral flow in response to India-Asia collision. This activity matches the distribution of porphyry Cu-Au and carbonatite-associated rare earth element deposits in the area, demonstrating the significant contribution of mantle materials to the metallogenic processes in collisional settings.

Influence of water saturation on mechanical characteristics and fracture evolution of coal rock assemblage with rough interfaces

To comprehensively investigate the influence of water content on the mechanical and crack propagation characteristics of coal rock assemblage (CRA) with a rough interface, uniaxial compression tests were conducted on specimens with varying water content. Nuclear magnetic resonance (NMR) and acoustic emission (AE) techniques were employed to monitor the water content and AE signals throughout the experiment. The physical and mechanical properties, as well as the extent of crack development and acoustic emission (AE) parameters, were comprehensively investigated under conditions of water erosion. The results demonstrate that a rough interface contributes to an enhancement in the compressive strength of the composite material. Moreover, the moisture content exerts a significant influence on various aspects of the composite specimen, including its compressive strength, time b value, crack development, and crack propagation. With the increase in water content, the initial single slope shear failure of the composite specimen gradually transitions into a multi-section shear failure mechanism. Under the influence of water-rock interaction, sandstone within the formation undergoes a metamorphosis from a densely cemented structure to an irregular honeycomb-like configuration. This transformative process engenders novel porosity and fractures, ultimately compromising the rock’s mechanical strength. The analysis focuses on the relationship between the AE parameter b and uniaxial stress and water content, with emphasis on its relevance to damage theory. A damage model based on water immersion rate was established to elucidate the correlation between damage variables and water content. This was achieved by considering the characteristics of water-rock coupling AE and constructing a structural model of the water absorption process in different pore throats, thereby providing valuable insights for stability design and evaluation of roadway rock masses.

Multiple episodes of late Neoproterozoic rifting in the Tarim Craton during its separation from the supercontinent Rodinia

In this paper, we review and evaluate the magmatic history, regional stratigraphy, structural patterns and sedimentary provenance features of the Late Neoproterozoic Tarim Craton to elucidate the nature and different modes of extensional tectonics during the Latest Neoproterozoic, a critical time coinciding with the disintegration of the supercontinent Rodinia. New detrital zircon U–Pb ages and geochemical data acquired from quartz schists and spatially associated granites in the Tugerming anticline along the northern margin of the Tarim Craton have revealed Cryogenian ages, as opposed to Early Neoproterozoic and older ages as reported in previous studies. Granitic intrusions are dated at 656–637 Ma, which involved remelting of the ancient crust following the decline of the initial (Cryogenian) rift stage. They are unconformably overlain by terrestrial clastic deposits. Based on the available detrital zircons from the Late Neoproterozoic sedimentary rock assemblages, we identify a major change in the provenance of the basinal strata at the end of the Cryogenian period. This inferred provenance change, associated with basement exhumation, overlaps in time with resumed magmatic activities, basin inversion and uplift along the northern margin of the craton. The initial, Cryogenian rifting stage was facilitated by pure shear deformation that produced rift basins in and across the Tarim Craton. The subsequent rifting stage was facilitated by simple shear deformation, confined to the northern edge of the craton as the break-up of the supercontinent Rodinia further progressed. Episodic rifting of the Tarim Craton and its separation from Rodinia were aided by thermal weakening of its lithosphere owing to the periodic impingement of mantle upwelling beneath it throughout the Cryogenian and Ediacaran periods.

Late Tithonian (Late Jurassic) palynological record from the Jaisalmer Basin (India)

ABSTRACT Late Tithonian (Late Jurassic) palaeobotanical records from the Jaisalmer Basin are infrequent. We report the first record of an age-diagnostic palynological assemblage of the sedimentary rocks in the Bhadasar Formation from the Jaisalmer Basin, India. The study is carried out to consider the palaeoenvironmental settings in this basin based on palynological and palynofacies investigations. A well-preserved assemblage of palynomorphs with 22 species belonging to 10 genera, including spores and pollen. The palynological assemblage is characterised by the dominance of coniferous pollen of Callialasporites spp. Araucariacites spp. along with some significant taxa viz. Cupressacites ramachandra, Microcachryidites antarcticus, Classopollis sp. Podocarpidites sp. Pityosporites sp. Ginkgoretectina spp. and Monosulcites sp. which suggest a Late Tithonian (Late Jurassic) age. The occurrence of conifer pollens (Araucariacites and Callialasporites) represents coastal vegetation and warm climate. However, some bisaccate pollen (Podocarpidites) indicates drier upland areas. Palynofacies records suggest two distinct Palynofacies Assemblages (PA – I and PA – II) correspond to the marginal oxic to dysoxic basin and the shelf to marginal transition.

The craft of rock climbing: exploring the implications of relational ontologies for learning to rock climb

AbstractThis paper explores how relational ontologies challenge the conception of skill development in rock climbing. Using especially the concepts of Tim Ingold and Phil Mullins, the paper suggests that pro-environmental behaviours, and relationships to place, can be fostered by direct involvement in skills development. This ontology is at odds with dualistic approaches that see climbing and pro-environmental behaviours as tensioned concepts. It is also contrary to historic practice theories that see climbing development as deriving from mastering oneself and conquering the climb. The paper proposes that climbing can be better understood as a craft and that each climb can be seen as co-created by the correspondence between the climber and the rock. This approach challenges the anthropocentric concept of the climb as a challenge to be overcome by skill and human endeavour, and instead suggests that a perspective of climbing as a craft better recognises the shared agency of the assemblage of rock and climber. Consequently, the paper suggests that completed rock climbs can be seen as co-evolved expressions of knowledge and action, and thus that climbing skills and pro-environmental behaviours derive from a creative process of engagement with the solid reality of the rock.

Three-Dimensional Subsurface Model of Luk-Ulo Melange Complex, Karangsambung, Indonesia: Insights from Gravity Modeling

The Luk-Ulo Melange Complex (LMC) is characterized by a chaotic assemblage of mixed rocks with a block-in-matrix fabric. The exposed blocks consist of various scattered rock types, trending in an ENE-WSW direction. In the case of Mt. Parang, the origin of the diabase remains uncertain, with ongoing debate as to whether it is associated with in situ volcanic activity or represents an exotic block within the melange deposit. Subsurface data obtained through geophysical investigation can aid in modeling the geometry of intrusive bodies using inverse modeling techniques. In this study, we conducted a gravity survey and performed 3D inverse modeling to investigate the subsurface beneath Karangsambung. A total of 818 gravity data points and 28 rock density measurements were integrated with existing geological data to construct an a priori 3D geological model. To ensure the results align with geological concepts, the 3D inversion utilized a stochastic approach, allowing for the incorporation of multiple geological constraints over fifty million iterative procedures. Ultimately, the inversion successfully reduced the misfit between observed and calculated data from 2.71 to 0.55 mGal. Based on the inverted 3D model, the diabase rock in Mt. Parang is identified as having an intrusive origin. The intrusion model exhibited minimal changes in density, volume, and shape during the inversion process. Additionally, the model suggests the presence of a solidified magma reservoir at a depth of approximately 3 km, potentially related to Dakah volcanism. The inverted model also reveals the block-in-matrix structure of the Luk-Ulo Melange Complex in the northern area.

Parental magma and mantle source compositions of chromitites in the Mesoarchean Nuggihalli greenstone belt, India: Evidence for Archaean subduction zone magmatism

We present comprehensive petrological and geochemical analyses of chromitites from the 3.3-3.1 Ga Nuggihalli greenstone belt in Western Dharwar Craton, India. Our findings provide insights into the primary magma and mantle source compositions associated with these Archean oceanic lithosphere chromitites. Major and trace element data from chromites and clinopyroxenes in the chromitites, combined with thermobarometry and hygrometry calculations, and numerical modeling, indicate that the Nuggihalli chromites are characterized by high-Cr chromites (Cr# = 68.1-78.6) with low TiO 2 (0.19-0.29 wt. %), Al 2 O 3 (9.10-13.13 wt. %) and Ga (4.24-10.37 ppm). These chemical signatures are comparable to those of chromites from high-Cr podiform chromitites and boninites. Clinopyroxenes are mainly augites with high Mg# (93-95). Equilibrated melts with the clinopyroxenes from the Nuggihalli chromitites have high Mg# values (79-86), similar to ancient komatiitic or picritic magmas (Mg# = 75-90). Thermobarometry and hygrometry calculations indicate that the clinopyroxenes likely formed at temperatures of 1194-1221℃ and pressures of 0.7-5.8 kbar with high water contents (∼2.4-2.6 wt. %), exceeding water contents of mid-ocean basalts (MORB) and oceanic island basalts (OIB). Numerical modeling of rare earth elements indicates moderate degrees of partial melting (1% to ∼10%) of a hydrated spinel lherzolite mantle source (water content: ∼478-5408 ppm). The parental magmas of the 3.3-3.1 Ga Nuggihalli chromitites are characterized by enrichment of water contents (∼2.4-2.6 wt. %) and depletion of high field strength elements (HFSEs, e.g., Nb and Hf), which are similar to boninites and arc magmas. Furthermore, the rock associations, including ultramafic rocks with chromitites, high-Mg basalts, and sheets of gabbro-anorthosites in the Nuggihalli greenstone belt, are similar to the rock assemblages of ophiolites, which could represent a relic of a Mesoarchaean oceanic lithosphere formed in a forearc setting.

Rodingitization of Neoproterozoic Al-Barramiya ophiolite metagabbro in the Eastern Desert of Egypt, Arabian-Nubian Shield

ABSTRACT The present work studies the rodingite that is recorded for the first time as being associated to Al-Barramiya Neoproterozoic ophiolite at Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different styles of alterations included carbonatization, listvenitization, chloritization and rodingitization. The Neoproterozoic ophiolitic rocks of Al-Barramiya district consists of serpentinized peridotite and metagabbros. Serpentinized peridotites are highly altered to assemblages of talc-carbonate rocks, listvenite and magnesite, while metagabbro is partly converted into rodingite. The field studies indicate that serpentinite and rodingite in Al-Barramiya area display high-strain cataclastic deformation. Rodingite consists of garnet, diopside, vesuvianite and chlorite with minor epidote, prehnite, actinolite and opaque minerals. The garnets and vesuvianite are mostly concentrated near the peripheries of the studied rodingite masses, while clinopyroxene, chlorite, epidote and prehnite gradually increase inwards due to progressive decreasing water/rock ratio as fluids originally buffered by serpentinite move towards equilibrium with gabbroic bulk chemistry. Some rodingite samples preserve a few relics of igneous textures of ophiolitic metagabbros. Garnets are distinguished into andradite, hydroandradite and grossular. Pyroxene is represented mainly by secondary clinopyroxene beside primary fresh relics of diopside; the latter has high Mg# (0.92–0.97) similar to ophiolitic clinopyroxene in the ANS. Rodingite is enriched in CaO, Fe2O3 and MgO but depleted in SiO2, Al2O3 and Na2O compared to associated metagabbro. The association of rodingite with serpentinites and metagabbros suggests that Al-Barramiya rodingite is genetically related to serpentinization of ultramafic rocks. The rodingitization occurred during ocean floor alteration contemporaneous with the serpentinization process. The superposition of cataclastic deformation on the altered rocks indicates that alteration preceded obduction and occurred on the seafloor of the suprasubduction zone where Al-Barramiya ophiolite was originally emplaced. During serpentinization the olivine in the peridotite broke down and released Mg2+ in the metasomatic fluids and clinopyroxene broke down producing Ca-rich fluids. These fluids react with the mafic protoliths and enrich them in MgO and CaO. Also, serpentinization fluids react with the plagioclase of the metagabbro, releasing Ca from the mafic source and concentrated it in the rodingitized samples. The positive Eu anomalies beside high Sr contents of the rodingite can be inherited from a plagioclase-rich protolith such as ophiolitic metagabbro. The enrichment of rodingite in Th and U indicates two distinct solutions affected the gabbroic protolith because the alteration of depleted peridotite to serpentinite cannot be an efficient source to enrich these elements. The present study indicates that the rodingitization process underwent lower greenschist facies conditions associating metamorphism that caused serpentinization and released Ca-rich metasomatic fluids.

Zircon Coupled Dissolution–Precipitation Replacement During Melt–Rock Interaction Modifies Chemical Signatures Resulting in Misleading Ages

ABSTRACTMelt migration through Earth's crust drives well‐documented melt–rock reactions, locally changing rock assemblage and geochemistry. However, melt–zircon interaction remains understudied. We report on three zircon‐melt interaction events from the Pembroke Granulite, New Zealand. Primary zircon from gabbroic gneiss which was subject to minor post‐emplacement melt migration and primary zircon from younger dykes exhibit straightforward microstructures, microchemistry, and age data. In contrast, zircon from melt‐mediated reaction halos adjacent to the dykes and from melt‐fluxed high‐strain zones display dissolution modification of grains, micro‐porosity and blurred or truncated internal zoning typical of replacement by coupled dissolution‐precipitation. Replaced zircon domains show changed rare earth element patterns and redistributed or lost radiogenic Pb that generates ambiguous apparent spot date arrays, smeared over tens of millions of years. We conclude that the metamorphism and three melt–rock interaction events were brief, and the arrays misrepresent the true age and duration of the metamorphism. Pb‐loss persisted beyond the metamorphism, with porosity and inclusions formed during coupled dissolution‐precipitation making replaced zircon domains more susceptible to subsequent Pb‐loss compared to the structurally intact, primary magmatic zircon in the host gabbroic gneiss or dykes. We recommend conducting high‐resolution microstructural investigations upon recognition of spot date arrays observed in single samples to rule out the possibility of spurious arrays resulting from coupled dissolution‐precipitation.

Transcrustal magmatic systems in NE China: Insights from Early Cretaceous metaluminous–peraluminous–peralkaline rock associations in the southern Great Xing’an Range

The generation and evolution of crustal-scale magmatic systems are important in revealing the continental crust differentiation. In the Bayangeer-Aliwula area of Inner Mongolia, metaluminous–peraluminous–peralkaline rock assemblages provide a rare opportunity to evaluate Early Cretaceous magmatic systems. This study presents new geochemical data, including whole-rock and zircon geochemistry, zircon U–Pb dating, and Hf–O isotopic analysis of rocks formed between 132 and 121 Ma. The metaluminous rocks exhibit low SiO2 (55.35–65.65 wt%), low Rb/Sr (0.08–0.24) and (La/Yb)N (4.88–7.58) ratios, and positive εHf(t) values (+4.9 to +8.7). These features, along with enrichment in large-ion lithophile elements and depletion in Nb and Ta, suggest formation via partial melting of fluid-metasomatised lithospheric mantle. By contrast, peraluminous rocks have high SiO2 (69.37–80.58 wt%), a high differentiation index (DI = 88–95), and Fe-index (0.84–0.95), resembling highly fractionated I-type granites. They show high Rb/Sr (0.77–3.35) and (La/Yb)N (5.79–22.75) ratios, and positive εHf(t) values (+6.7 to +10.8), combined with the modelling results, indicating origin from partial melting of K-rich mafic lower crust followed by magma fractionation. Peralkaline rocks display typical ferroan A-type granite characteristics (Zr + Nb + Ce + Y = 906–4292 ppm; Fe-index = 0.96–0.99), with high SiO2 (74.02–77.50 wt%), high Rb/Sr (6.17–121.38), and (La/Yb)N ratios (2.19–17.72), and positive εHf(t) values (+3.1 to +9.6). Zircon geochemistry characteristics suggest that peralkaline and peraluminous felsic melts are different batches extracted from the same magma reservoir. Further analysis, including hyperbola diagrams and zircon oxygen isotope compositions, suggests peralkaline magma formation through the mixing of altered oceanic crust fluids and peraluminous melts after melt extraction. During Early Cretaceous, the transcrustal magmatic system provides a reasonable explanation for the petrogenesis of various contemporaneous rocks in the study area in southern Great Xing’an Range.

Channel evolution in the lower reach of palaeo Fen River: evidence from detrital apatite petrochronology and zircon geochronology

This study provides a comprehensive analysis combining multi-element (Sr, Y, Th, U and REE) geochemistry and U–Pb geochronology of single-grain apatite with zircon U–Pb ages from channel sand deposits of the palaeo Fen River, collected at the Chaizhuang and Weijiazhuang sites, corresponding to sediments from the Late Pleistocene and late Middle Pleistocene, respectively. The results unveil that the analysed apatites originated from a varied assemblage of source rocks, encompassing mafic–ultramafic rocks, intermediate to felsic I-type plutons, highly fractionated S-type granites and pegmatites, as well as assorted metamorphic rocks. U–Pb age dating of detrital zircons and apatites identifies three predominant age peaks at 2.4, 2.2 and 1.8 Ga, consistent with known tectonothermal episodes within the North China Craton (NCC). The similarity in U–Pb age spectra for both zircon and apatite, along with comparable apatite geochemical characteristics across both the Chaizhuang and Weijiazhuang sections, indicates stable provenance with little to no change from the late Middle Pleistocene to the Late Pleistocene. This continuity in provenance signatures suggests an absence of shift in drainage patterns or sediment sources, indicating that the current location and pattern of Fen River lower reach drainage was not formed at least by then. Consequently, it is inferred that the detrital material composing the palaeo Fen River sediments during this period predominantly originated from the NCC.

Palynology of strata associated with the Hercynian Unconformity across the Arabian Plate, from the Levant to southern Arabia

Abstract A survey of the palynostratigraphy of the strata above and below the Hercynian Unconformity indicates that the range of its associated hiatus, due to either non-deposition or erosion (or both), varies considerably across the Arabian Plate depending on the position of the succession in relation to the three main highs or arches, the Levant, Al Batin and Oman-Hadhramaut arches, and any Hercynian faulting or other uplift in basinal regions, for example in the Ghawar region. As expected, the succession spanning the Hercynian Unconformity is most complete in basinal areas, for example in the Faydah-Jafurah and Nafud-Ma'aniya basins. The well with the shortest hiatus, corresponding approximately to a maximum duration of part of the Serpukhovian and Bashkirian, appears to be KH-5/1, close to the axis of the Nafud-Ma'aniya Basin. Palynological assemblages in rocks above the Hercynian Unconformity indicate the diachronous onset of the basal Khuff clastics NW through the Arabian Plate over a period of c. 15 Myr. The oldest strata above the Hercynian Unconformity across the Arabian Plate also indicate systematic variation, probably due to palaeoclimate, with assemblages of similar age in Iraq and northern Saudi Arabia showing some similarities to those of Oman, but differences probably due to a strong climatic gradient at the time.

Neoarchean granitoid magmatism and geodynamic process in the northeastern North China craton

Abstract The composition of Archean granitoid rocks changed from predominantly tonalite-trondhjemite-granodiorite (TTG) gneisses in the early Archean (4–3 Ga) to diversified granitoid rock assemblages in the late Archean (3.0–2.5 Ga), marking a crucial transformation in the geodynamic processes of early Earth. However, the reason for this major transition remains enigmatic because the petrogenetic features of different granitoid assemblages and their crust-mantle interactions during different periods are poorly understood. We use variations in the spatial-temporal distribution, lithological association, chemical composition, and petrogenesis of Neoarchean (2.7–2.5 Ga) granitoids and inferred correlative crust-mantle interactions in the Eastern Liaoning Range (ELR) of the northeastern North China craton to explore this geodynamic transition. The early Neoarchean (ca. 2.7 Ga) ELR granitoids were dominated by TTG gneisses, and the late Neoarchean (2.6–2.5 Ga) ELR granitoid typology and compositions became more complex, changing into TTGs and more K2O-rich granitoid rocks. The TTGs can be subdivided into a high-Ca group and a low-Ca group: The 2.71–2.68 Ga high-Ca group TTG magma originated from partial melting of subducted juvenile oceanic crust, and the low-Ca group TTG magma was derived from fractionation crystallization of the high-Ca group TTG magma. The chemical composition of the magmatic sources played a dominant role on the 2.60–2.50 Ga TTG magmatism: the high-Ca and low-Ca group TTG magmas came from low-K mafic rocks and tonalites, respectively. The 2.58–2.49 Ga K2O-rich granitoids can be divided into three petrogenetic series: (1) The high-Ca-Mg group K2O-rich granitoid magma originated from partial melting of high-K mafic rocks, (2) the low-Ca-Mg group K2O-rich granitoid magma was derived from partial melting of sedimentary rocks, and (3) the transition group K2O-rich granitoid magma was sourced from metagreywackes. The 2.71–2.68 Ga TTGs were generated in an island arc belt, and subducted slab melting and subsequent magmatic differentiation were the dominant mechanisms of the TTG magmatism. The 2.60–2.50 Ga diversified granitoids were formed in the oceanic-continental subduction process under the active continental margin; the complicated oceanic slab subduction and arc-arc and arc-continent collisions contributed to the diversity of late Neoarchean granitoid magmatism.

The use of ASTER and landsat remote sensing data for exploring hydrothermal mineral in Arabian-Nubian Shield

Multispectral remote sensing data provided valuable information for hydrothermal minerals exploration. In this article, Landsat-8 Operational Land Imager (OLI) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were processed and analyzed using band ratio and principal component techniques to demarcate areas of hydrothermal mineral resources by highlighting the most influenced zone of hydrothermal alteration (high-grade) in Egypt. The area is a part of the Arabian-Nubian Shield, covering 1068 km2, and comprises wide varieties of Pre- and Pan-African rock assemblages. Preparing and integrating thematic layers consisting of the grade of hydrothermal intensity in a GIS technique supported generating a hydrothermal mineral predictive map (HMPM) using a multi-criteria decision-making technique. The plausible zone of very high mineral resources covers about 6.57 %. Analysis of ASTER data revealed that this zone is reliable with phyllic and argillic minerals and iron-oxides. The results of OLI data helped in demarcating potential areas of hydrothermal alteration areas (Al–OH) mixed with iron oxides. Field observations and investigations enabled the validation of the HMPM. The spatial distribution of hydrothermal zones throughout the study was reliable to that observed in the mine areas. Field and ore microscopic examination verified the existence of Au-sulfide minerals connected with hydrothermal alteration zones (HAZs) in different areas e.g., Abu-Marawat, Gidami, Semna, Abu-Gaharish, and Hamama mines. Overall, integrating multi-spectral data through overlay analysis techniques can provide valuable information about hydrothermal mineral resources in an accurate and cost-effective approach.

Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit

Abstract Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39–2.28 Ga magmatism and 1.93–1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452–439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.

A New Insight into the Genesis of Graphite Deposits in Madagascar Using U–Pb Zircon Dating and Electron Microprobe Analysis

Madagascar is globally recognized as an important producer of high-quality flaky graphite. However, current research on graphite deposits in Madagascar remains insufficient. Previous studies have linked the genesis of Madagascan graphite deposits to the metamorphism of sedimentary organic matter. Here, we provide a case study of graphite deposits in Madagascar, combining new data from the Ambahita graphite deposit (AMG) in southern Madagascar with data from the Antanisoa graphite deposit (ANG) in central Madagascar and the Vohitasara graphite deposit (VOG) on the east coast of Madagascar. We note that the mineral assemblages of graphite-bearing rocks in the AMG, ANG, and VOG are not typical of metamorphic mineral assemblages but rather the results of filling and metasomatism by mantle-derived fluids that occurred after peak metamorphism. Electron microprobe analysis indicates that the graphite of the AMG, VOG, and ANG is usually associated with phlogopite or Mg-biotite; the phlogopite shares a common genesis with other widespread phlogopite deposits across Madagascar. We reveal that the distribution of graphite deposits in Madagascar is primarily controlled by ductile shear zones between blocks. Ductile shear zones that extend deep into the mantle can provide an ideal migration channel and architecture for the emplacement of mantle-derived fluids. The graphite mineralization formed no earlier than the peak metamorphism (490 Ma) and no later than the intrusion of pegmatite veins (389 ± 5 Ma). The distribution of graphite deposits, graphite orebody morphologies, mineral associations, and geochemical data suggest that the genesis of graphite deposits in Madagascar is linked to mantle-derived fluid filling rather than the metamorphism of sediments, as previously suggested. These findings have important implications for similar deposits in Madagascar and potentially globally.

Metamorphic Mineral Reactions and Mineral Assemblages in Rocks of the Meyeri Tectonic Zone, Southeastern Fennoscandian Shield, Russia

Metamorphic Mineral Reactions and Mineral Assemblages in Rocks of the Meyeri Tectonic Zone, Southeastern Fennoscandian Shield, Russia

Geochemical Characterization and Genetic Analysis of Rocks of the Dianzhong Formation in Geda Township Area,South-Central Tibet

The study on the genetic and geochemical characteristics of volcanic rocks of the Cretaceous-Paleogene Dienzhong Formation in central and southern Tibet is of great significance for the establishment of magma evolution sequence in the working area. The volcanic rocks of Dianzhong Formation in Geda area can be roughly divided into dite, andesite and rhyolite. Zircon U-Pb isotopic dating results show that the weighted average age of Pb206/U238 of oligopleural rhyolite and Andesite samples is 65. 5±2. 3Ma (MSWD=2. 0) and 68. 7±3. 1Ma (MSWD=2. 0) , respectively. This indicates that the middle acid volcanic rocks of Dianzhong Formation erupted from Late Cretaceous to Paleocene. Geochemical analysis of rocks shows that the acidic volcanic rocks in Dianzhong Formation are a set of meta-aluminous-peraluminouscalc- alkaline andesite-dacite-rhyolite rock assemblages. Large ion lithophile elements and light rare earth elements are enriched, and high field strength elements are deficient. In the process of magmatic evolution, there is a large amount of crystallization of plagiase or a large amount of refractory residue of plagiase, and there are a lot of continental crust materials in the volcanic rocks of the Dian Zhong Formation. Comprehensive analysis shows that the volcanic rocks of Dianzhong Formation have the chemical characteristics of continental arc volcanic rocks, and may be magmatic products in the transition stage from subduction mechanism to continental collision mechanism. Zircon U-Pb dating, in situ Lu-Hf isotope and whole rock principal and trace analysis were carried out on the acidic volcanic rocks in Dianzhong Formation, Geda Area. The aim is to study the geochemical characteristics and evolutionary origin of rocks in this area. It provides the geochemical data support for the discussion on the division of tectonic evolution stages in the study area since Jurassic.

Continental Arc Accumulation Mafic Rocks in the Mid-Upper Crust: Constraints From the Early Eocene Hornblende Gabbro–Diorite in the Tengchong Block, Southeastern Extension of Tibet

Abstract Magmatic activity in the syn-collision stage is key for net crustal growth. To understand the mechanism of accretion–differentiation and compositional change of the continental crust, it is important to focus on the magmatic activity during the syn-collision stage. Early Eocene mafic–ultramafic rock assemblages found in the western part of the Tengchong Block resulted from a continuous series of arc magmatic evolution, thoroughly recording the continental arc magmatic system during the subduction of the Neo-Tethys Ocean and syn-collision of the Indian-Asian continents. Early Eocene hornblende gabbro–diorite in the Tengchong Block formed at 53 Ma, and the primitive magma was derived from an enriched mantle source due to the enriched Nd–Hf isotopes. The amphibole and biotite thermobarometer measurements indicate that the mafic magma reservoirs in the Tengchong Block occurred at a mid-upper crust. Petrography, amphibole Fe/Mg exchange coefficient (KD), Rayleigh fractionation, and equilibrium melt calculation indicate that the Early Eocene hornblende gabbro–diorite in the Tengchong Block was created due to plagioclase-dominated accumulation at the mid-upper crust level. Based on the calculation, the corresponding amphibole equilibrium melt is more silicic (dacitic–rhyolitic in composition) than the bulk rocks, indicating a more evolved composition in the mid-upper crust. Three types of plagioclases reveal the multi-recharging and dissolution–reprecipitation promoting the further evolution of these mafic rocks. Therefore, this study concludes that magma recharge and plagioclase-dominated accumulation processes may be important mechanisms for the formation and evolution of mafic magma and the further crustal differentiation at the mid-upper crust level in a continental margin arc.

Current state and long-term changes in the taxocene of marine polychaetes of the Karadag Nature Reserve (Republic of Crimea, Russian Federation)

Taxonomic and trophic structure and quantitative development of polychaetes are examined in the marine coastal areas of the Karadag Nature Reserve in 2021–2022. 40 species belonging to 30 genera and 15 families are discovered. From 1938 to 2022, 75 species have been recorded in this area. According to the nutritional spectrum, polyphages are the most represented, phytophages are the least represented. The average number of polychaetes in 2021–2022 is 100 ± 53 ind./m2, the average biomass is 3.55 ± 0.023 g/m2; S. papillocercusthe prevails in the total abundance and biomass, 98 and 99%, respectively. The change in the species composition, abundance and biomass of marine polychaetes of the Karadag Nature Reserve, inhabiting hard bottom substrates (1984–2022) and soft soils (1938–2021), is considered. From 1984 to 2021, 29 species of polychaetes have been recorded on soft substrates, and only two species in 2021–2022: Namanereis littoralis and Saccocirrus papillocercus. In underwater rock assemblages, on the contrary, the largest number of species (38) has been found in 2021–2022, and the smallest (14) in 1938–1940 (a total of 65 species found during 1938–2022). A transformation of the species composition and trophic structure of polychaetes occurred in the coastal biotopes of soft and hard bottom substrates in the marine area of the Karadag Nature Reserve from 1938 to 2022, with a decrease in the number of phytophages, as well as a decrease in quantitative characteristics. Presumably, the de crease in the number and diversity of polychaete worms on soft substrates is associated with a decrease in the macrophyte algae biomass. The results obtained, supplemented by data on the structure of phytocenoses and characteristic of bottom substrates, can be used in environmental monitoring of protected water areas of the Black Sea.