Composition Of Rocks Research Articles

The contribution and impact of shallow water platform as a potential source area on siliceous submarine fans

AbstractThe shelf‐margin deltas are the primary ‘sources’ of interest in siliceous submarine fan source‐to‐sink systems. However, less has yet to be discovered about the roles and effects of the shallow water platform situated on continental slopes. Using the Miocene sediments of the Ledong‐Lingshui Sag in the western part of the Qiongdongnan Basin (QDNB), this paper studies the YC35 coarse‐grained submarine fan that developed in the Meishan Formation (Fm.). The material sources, depositional characteristics, and developmental mechanisms of this fan were investigated. The restored palaeogeomorphology, based on high‐resolution 3D seismic data, shows that a shallow water platform exists. According to geochemical palaeoenvironmental data, the shallow water platform was ideal for forming carbonate deposits in the Meishan Fm., owing to the warm and humid palaeoclimate and the shifting trend in palaeowater depth. Based on the combined source tracing of heavy minerals, coherence slices, and seismic profiles, the shallow water platform and the Ningyuan River source on Hainan Island contributed to the YC35 coarse‐grained submarine fan. Compared to submarine fans formed during different periods, coarse‐grained fan stands out due to its distinct sedimentary structure, rock composition, and microlithological characteristics. Aside from not following the usual Bowmar sequence, there is an excess of gravel and no clear bedding or lamination. A large number of rock fragments, mostly granite and sedimentary rock (carbonatite), make up the rock composition. According to these results, the shallow water platform significantly affects submarine fan material composition. In addition, the attribute slice based on the root mean square reveals that shallow water platforms moderate peripheral deposition. We attribute the development and proximal supply of the shallow water platform to diapirism and forced regression. Our research provides novel insights and comprehension into the investigation of submarine fan sedimentary systems.

Alterations of High-Carbon (Shungite) Rocks by the Lake Onega Waters: Mineralogy and Geochemistry of the Process

Abstract ––Carbonaceous (shungite) rocks have high contents of trace elements, which can get to the environment through natural weathering. Shungite rocks are a group of Precambrian carbonaceous rocks of volcanic and sedimentary genesis in Karelia. In this work we present results of studying the mineral and geochemical compositions of shungite rocks at their outcrops on the shoreline of Lake Onega. The interaction of the Onega waters with shungite rocks led to: (1) the removal of most elements, except for K, Mn, Ba, and Mg, whose contents in the rocks increased; (2) the formation of an assemblage of secondary minerals, such as hematite, jarosite, goethite, chalcocite, anglesite, brookite, and Mn hydroxides. Based on the results obtained, we propose a model of the transformation of high-carbon (shungite) rocks by the Onega waters.

Facies of Quaternary Sediments In Bunga Village, North Luwuk District, Banggai Regency, Central Sulawesi

Abstract The distribution of Quaternary sedimentary rock deposits in Indonesia is quite extensive, and one of them is in the Quaternary coral reef units, which are evenly distributed along the eastern arm of Sulawesi. The research was conducted in Bunga Village, North Luwuk District, Banggai Regency, Central Sulawesi Province. This research was motivated by the lack of geological data, so the topic of developed facies became an interesting topic to be researched. Data was collected from a cliff at the coordinates 0053′57,57.1″N-122054′43′6″E. Stratigraphic measurements aim to determine the facies, depositional environment, and sedimentation mechanisms developed in the study area. The research was conducted using the method of field data collection and laboratory analysis. Collecting field data was carried out with measured stratigraphic with a 1:100 scale and taking several rock samples. Laboratory analysis was carried out in paleontological and petrographic analysis, which was carried out to determine the composition of the constituent rock and fossil identification, especially foraminifera in thin sections of rock that were selected. Based on the study’s results, the total rock layers measured in the study area were 15 m. Facies found in the stratigraphic path of the study area are floatstone facies, rudstone facies, allochemic sandstone facies, polymict conglomerate facies. The type of deposition environment of limestone and allochemic sandstone in the study area is on the foreslope, while the conglomerate facies is included in the fan delta deposition environment. Overall, the sedimentation dynamics in this area are in a high energy environment, indicating that the facies that developed were deposited with a sediment gravity flow mechanism.The results of the analysis of fossils found Cyclammina cancellata sp., Orbulina universa sp., Globigerina bulloides sp., and Trilobatus immaturus sp., which indicates the limestone in the study area was deposited in the Late Pleistocene to early Holocene.

Volatile systematics in terrestrial igneous apatite: from microanalysis to decoding magmatic processes

Apatite has been recognized as a robust tool for the study of magmatic volatiles in terrestrial and extraterrestrial systems due to its ability to incorporate various volatile components and its common occurrence in igneous rocks. Most previous studies have utilized apatite to study individual magmatic systems or regions. However, volatile systematics in terrestrial magmatic apatite formed under different geological environments has been poorly understood. In this study, we filtered a large compilation of data for apatite in terrestrial igneous rocks (n > 20,000), categorized the data according to tectonic settings, rock types, and bulk-rock compositions, and conducted statistical analyses of the F–Cl–OH–S–CO2 contents (~ 11,000 data for halogen and less for other volatiles). We find that apatite from volcanic arcs preserves a high Cl signature in comparison to other tectonic settings and the median Cl contents differ between arcs. Apatite in various types and compositions of igneous rocks shows overlapping F–Cl–OH compositions and features in some rock groups. Specifically, apatite in kimberlite is characterized as Cl-poor, whereas apatite in plutonic rocks can contain higher F and lower Cl contents than the volcanic counterparts. Calculation using existing partitioning models indicates that apatite with a high OH (or F) content does not necessarily indicate a H2O-rich (or H2O-poor) liquid because it could be a result of high (or low) magma temperature. Our work may provide a new perspective on the use of apatite to investigate volatile behavior in magma genesis and evolution across tectonic settings, volatile recycling at subduction zones, and the volcanic-plutonic connection.

Genesis of the Ke’eryin Two-Mica Monzogranite in the Ke’eryin Pegmatite-Type Lithium Ore Field, Songpan–Garze Orogenic Belt: Evidence from Lithium Isotopes

Previous studies on the Ke’eryin pegmatite-type lithium ore field in the Songpan–Ganzi Orogenic Belt have explored the characteristics of the parent rock but have not precisely determined its magma source area. This uncertainty limits our understanding of the regularity of lithium ore formation in this region. In this study, to address the issue of the precise source area of the parent rock of lithium mineralization, a detailed analysis of the Li isotope composition of the ore-forming parent rock (Ke’eryin two-mica monzogranite) and its potential source rocks (Triassic Xikang Group metamorphic rocks) was conducted. The δ7Li values of the Ke’eryin two-mica monzogranite, Xikang Group metasandstone, and Xikang Group mica schist are −3.3–−0.7‰ (average: −1.43‰), +0.1–+6.9‰ (average: +3.83‰), and −9.1–0‰ (average: −5.00‰), respectively. The Li isotopic composition of the Ke’eryin two-mica monzogranite is notably different from the metasandstone and aligns more closely with the mica schist, suggesting that the mica schist is its primary source rock. The heavy Li isotopic composition of the two-mica monzogranite compared to the mica schist may have resulted from the separation of the peritectic garnet into the residual phase during the biotite dehydration melting process. Moreover, the low-temperature weathering of the source rocks may have been the main factor leading to the lighter lithium isotope composition of the Xikang Group mica schist compared to the metasandstone. Further analysis suggests that continental crust weathering and crustal folding and thickening play crucial roles in the enrichment of lithium during multi-cycle orogenies.

Позднемеловые-раннепалеогеновые вулканические породы Восточной Камчатки (особенности состава и изотопно-геохимическая гетерогенность)

New and previously published isotope-geochemical material on the Upper Cretaceous-Lower Paleogene volcanic complexes of the Ozernovsky-Valaginsky segment of the Achaivayam-Valaginsky paleoarc are presented. It is found that composition of rocks composing separate volcanic fields within the Achayvayam-Valagin paleoarc (terrane) is different. On Karaginsky Island, volcanics of the moderate-potassium calc-alkaline and shoshonite series are widespread. On Ozernaya Peninsula, low-potassium basalts of the tholeiitic series are predominantly developed. The rocks of the Kumroch Ridge include low-potassium basalt-basaltic andesite, medium-potassium calc-alkaline and high-potassium shoshonitic. Volcanic rocks of the Tumrok Ridge are represented by low-potassium picrites and high-potassium trachybasalt-trachyandesites. In the Valaginsky Ridge, low-potassium tholeiitic, moderate-potassium calc-alkaline and shoshonite-latite series, as well as picrites and lamproitoids are distinguished. Isotopic (Sr, Nd and Pb) and geochemical composition of the rocks indicate their formation from a depleted mantle source within an ensimatic volcanic arc. It is shown that in the absence of significant differences in the material composition of rocks, regional differences are manifested. It is suggested that large-ion lithophile elements enrichment of volcanics may relate to fluids introduced into the melts during the melting of the supra-subduction mantle wedge. Issues of geodynamics of the region are considered.

Вулкан Иван Грозный (остров Итуруп, Курильские острова): вещественный состав продуктов извержений и современная активность

The paper presents summary data on the historical activity of Ivan Grozny volcano (Iturup Island, Kuril Islands), including the results of our own studies of solfatar-hydrothermal and eruptive activity of the volcano since 2004. Using modern analytical methods, we obtained data on the chemical composition of the volcano rocks and pyroclastics of the explosive eruption in 2012–2013. The composition of the volcano rocks varies from andesites to daciandesites of medium potassium series. Pyroclastics of the 2012–2013 eruption corresponds to andesites with medium potassium content in terms of bulk chemical composition. The material composition of glasses according to microprobe data is much more acidic. The obtained values are similar to the geochemical compositions of the products of the 1973 and 1989 eruptions. Taking into account the prevailing wind regime dominating in the central part of Iturup Island, the hazard from ash fall of probable future eruptions of Ivan Grozny volcano was assessed, indicating the predominant spread of ash in the north-western, western and south-western directions.

Analisis Pengaruh Mineral terhadap Nilai Kuat Tekan Batuan

To determine the mineral content and composition of a rock, a petrographic test is carried out using a thin section method. Petrographic test is known as an efficient test of time and energy with accurate results in obtaining the composition and mineral content of rocks. The Point Load test is an index test that has been widely used to predict the UCS value of a rock indirectly in the field. This is due to the simple test procedure, easy sample preparation and can be done in the field, so that the strength of the rock can be quickly known in the field before testing in the laboratory.

Results of the study of the section of the Takata formation "Yuzhnaya Rassolnaya" in the territory of the Poludovo-Kolchimsky anticlinorium of the Western-Ural folding zone (Krasnovishersky district, Perm territory)

Relevance. Relationship with the Takata formation of diamond reservoir rocks and previously discovered deposits and the presence of prospects for discovering new ones in the zone of the forward folds of the Urals. Aim. To reconstruct the conditions of sedimentation and composition of rocks, the source of the removal of terrigenous rocks of the Takata formation based on the results of studying the "Yuzhnaya Rassolnaya" section. Objects. Terrigenous rocks of the Takata formation in the "Yuzhnaya Rassolnaya" section located in the western part of the alluvial diamond deposit "Yuzhnaya Rassolnaya" in the Krasnovishersky district of the Perm Territory. Methods. Lithofacies analysis, petrographic study of thin sections in transmitted light using crossed nicol methods, determination of the content of rock-forming oxides by X-ray fluorescence on an S8 Tiger (Bruker) spectrometer, analysis of rock-forming elements on glassy disks. Impurity and microimpurity elements (with a content of <5%) were determined using a certified method for defining elements by inductively coupled plasma mass spectrometry according to the method, STO TGU 048-2012. The analysis was performed by ICP-MS on an Agilent 7500cx quadrupole mass spectrometer (Agilent Technologies Inc., USA) using an internal (In Internal standard, Inorganic Ventures, USA) and an external standard SGD-2A (GSO 8670-2005). Results. Lithological-facial, mineralogical, petrographic, lithogeochemical studies of Takata terrigenous deposits were carried out in the "Yuzhnaya Rassolnaya" section in the area of the Polyudovo-Kolchima anticlinorium of the Western Ural folding zone, which made it possible to clarify the sources of matter and sedimentation conditions. The formation of sediments occurred in a transitional sedimentation environment. A transgressive sequence is observed from bottom to top. Terrigenous rocks of the Takata formation include rocks of the first and second sedimentation cycles. They were cut off due to the destruction of primary igneous (acidic and basic compositions) and metamorphic rocks, as well as sedimentary rocks. The totality of the material characteristics of the terrigenous rocks of the Takata formation, taking into account the published data, allowed us to assume that sedimentation of the rocks occurred on the passive continental margin, with the supply of clastic material due to the erosion of internal raised blocks of the foundation of the East European Platform. The activation of the latter occurred in the Emsian time with the manifestation of general lithospheric folding. At the same time, water flows carrying weathering products from the East European Platform eroded sedimentary rocks of the Riphean, Vendian, Ordovician and Silurian, which fragments we observe in small quantities when describing thin sections.

PETROLOGICAL AND GEOCHEMICAL PECULIARITIES OF ALKALINE ROCKS IN THE NORTH-WESTERN REGION OF THE UKRAINIAN SHIELD

The data of chemical composition and content of trace elements in alkaline-ultrabasic rocks of the northwestern region of the Ukrainian Shield are summarized. The occurrences of alkaline rocks of this composition were discovered as results of drilling in four areas (Horodnytska, Hlumchanska, Bolyarkivska, and Hubkivska). They are presented by small hypabyssal intrusions, stem- and dike-like bodies. The rocks are represented by the jacupirangite-melteigite series, which belong to high-Mg (Mg# 0.7–0.8) varieties. Less common are alkaline and subalkaline gabbroids found in thin apophyses and veins near alkaline-ultrabasic bodies. It is assumed that these gabbroids were formed as a result of crust material contamination of the primary alkaline-ultrabasic melts. The alkaline-ultrabasic rocks of all occurrences have high Cr (233–2737 ppm) and Ni (95–1022 ppm), but are unusually depleted in incompatible elements (Nb, Ta, Zr, Hf, REE, Y), with moderate Ba and Sr. Their geochemical features sharply differ from similar rocks, carbonatite and alkaline complexes known as in the Ukrainian Shield and other regions of the world. Despite the primitive composition of alkaline-ultrabasic rocks and anomalous REE depletion, their chondrite-normalized patterns have negative slopes and are similar to differentiated rocks with a prevailing of LREE (La/Yb = 6–16). The petrogenesis of such primitive and trace-element depleted alkaline-ultrabasic rocks is enigmatic. We suppose that these unusual geochemical features are the result of primary melt(s) generation from mantle source and their evolution by a similar way. For the alkaline rocks of the Horodnytska and Hlumchanska intrusions, and possibly the Hubkivska occurrence, the primary melts arose through partial melting of garnet (with amphibole?) peridotites. At the same time, the primary melt for the Bolyarkivska intrusion was generated by partial melting of spinel peridotite, probably in a shallower environment. The anomalous depletion at HFSE and REE can be considered as the result of specific conditions of partial melting of depleted mantle source at low temperature and pressure, when refractory minerals concentrating these elements (rutile, amphibole) remained in the restite, or due to early crystallization and fractionation of concentrating minerals (perovskite, apatite).

THE U-PB AGE AND HAFNIUM ISOTOPE COMPOSITION OF ZIRCON FROM METAMORPHOZED ANDESITE OF THE CHORTOMLYK FORMATION AND RHYODACITE HYPABYSSAL INTRUSION OF THE SURA COMPLEX, CHORTOMLYK GREENSTONE BELT

Andesites and felsic volcanic rocks are observed at all stratigraphic levels of the Konka and Bilozerka groups, which comprise greenstone structures in the Middle Dnieper Domain. Their nature and age are still poorly known. The youngest felsic volcanic rocks of the Solone Formation of the Konka Group and comagmatic with the tonalite-trondhjemite-granodiorite (TTG) association of the Sura Complex hypabyssal intrusions were pre-viously dated by the U-Pb zircon SHRIMP method at ca. 3.1 Ga. The purpose of this study is to determine the U-Pb zircon age and geochemical features of i) metamorphosed andesites of the Chortomlyk Formation and ii) low-alkaline metarhyodacite hypabyssal intrusions that cut the rocks of the Sura Formation of the Konka Group. In the Chortomlyk Greenstone Belt, the thickness of volcanogenic rocks of the Chortomlyk Formation (dacite-andesite-tholeiite association) reaches 2000 m. The youngest felsic volcanic rocks of the Solone Formation and comagmatic hypabyssal intrusions are located within three large volcanic fields located near the Novomykolaivka* massif. Using the LA-ICP-MS method, U-Pb ages of two zircon populations from metamorphosed andesites of the Chortomlyk Formation were dated. Twenty-three crystals of transparent colourless zircon crystals yielded a concordant age of (3222 ± 6) Ma. The U-Pb age of the second population of large, brown, opaque zircon crystals is 3132–3073 Ma. Interpretation of the obtained ages is not straightforward and at least two options can be proposed: 1). The studied metaandesites are differentiated mafic magmas and the age of their formation is defined by the older zircon population, and the young population corresponds to the time of superimposed thermal processes during later intrusion of plagioclase granitoids of the Novomykolaivka massif; 2). The age of the metaandesite is defined by the younger population, while the older population is inherited from the protolith. We consider the second option as being far more likely. The first option contradicts the stratigraphic position of the dated rock. The studied metaandesite is low in potassium and belongs to the sodium series. Relative to TTG, they have higher Nb (16.2 ppm) and Y (25.9 ppm). Rare earth elements are weakly differentiated, (La/Yb)N = 3.91 with a strongly negative Eu anomaly, Eu/Eu* = 0.44. The U-Pb zircon age from the low-alkaline rhyodacite hypabyssal intrusion that cuts the Sura Formation of the Konka Group is (3085 ± 6) Ma. It has a highly differentiated REE pattern, (La/Yb)N = 16.2 and a positive Eu anomaly, Eu/Eu* = 1.21. The Nb (6.7 ppm) and Y (10.8 ppm) contents are low. They chemically resemble TTGs of the Sura Complex. Based on our data, the andesites of the Chortomlyk Formation of the Konka Group and the low-alkaline rhyodacite hypabyssal intrusions have the same U-Pb age, but different origins. The former were produced by the melting of older crustal rocks, and the latter were formed due to the partial melting of metabasites with garnet-bearing restite. Hafnium isotope composition in zircon from both samples reveals their juvenile nature, i.e., they crystallized from partial melts of rocks with short crustal residence times. Our isotope data agrees with the neodymium isotope composition of the felsic volcanic rocks of the Sura greenstone belt, which yielded εHf values of +1.8. These values are lower than the depleted mantle isotope composition at this time (3200–3000 Ma). *Novomykolaivka massif was formerly known as Chkalove massif.

Predicting Mechanical Properties of Carbonate Rocks Using Spectroscopy Across 0.4–12 μm

Determining the mechanical characteristics of rocks is crucial in various civil engineering sectors. Traditionally, the mechanical properties of rocks are determined through on-site and laboratory tests carried out during geotechnical surveys. However, these extensive surveys require considerable time and resources. In contrast, hyperspectral remote sensing techniques offer a rapid and simple means to determine the mineral composition and crystallographic structure of rocks. These features, in turn, influence the rocks' mechanical properties. This study focuses on characterizing the mechanical properties of carbonate rocks in a laboratory setting, using hyperspectral sensors. Approximately 150 cylindrical carbonate rock samples, spanning a wide strength range, were collected from diverse Israeli rock outcrops. Employing a point spectrometer (0.4 to 2.5 µm) and a spectral image sensor (8.0 to 12.0 µm), we captured samples' light reflections and spectral emissivity. Mechanical attributes, including density, porosity, water absorption, and uniaxial compressive strength (UCS), were measured. Advanced data mining techniques identified statistical correlations between hyperspectral signatures and mechanical properties, pinpointing key wavelengths for prediction. The developed models exhibited excellent predictability for the specified properties, attributing accuracy to discernible mineralogy and internal crystalline structure through spectroscopy. However, predicting UCS showed slightly weaker results due to influences from internal flaws not entirely reflected in spectroscopic data. Nonetheless, outcomes regarding rock UCS were deemed satisfactory. These findings open avenues for non-destructive tools in assessing the mechanical properties of rocks in quarrying operations.

Study of an innovative optimisation method for repair and renewal of production wells in the central section of the Mynkuduk field

ABSTRACT This study was aimed at identifying geological, geochemical, geotechnical and technical factors affecting the efficiency of repair and renewal of the Central section of the Mynkuduk field. The study included data collection on the technical parameters of the wells, including their design, filter sizes, as well as geological and hydrogeological features of the field. An analysis of the relationships between technical well parameters, geological factors and repair and renewal efficiency was carried out. The methods of operation efficiency assessment and their compliance with the actual results were also analysed. The study revealed that the mode of operation of technological wells depends on many factors, including both objective and subjective. One of them is the consideration of geological features such as well designs and rock composition. The study revealed that the methodology used at the mine is flawed due to the use of wells with different operating periods to estimate average values. The results emphasise the importance of an integrated approach to repair and renewal work on process wells in uranium deposits, considering their different characteristics. This will optimise the operation processes, minimise the risks of emergencies and increase the efficiency of uranium production.

Spinel chromianowy z dunitów masywu Inagli i ich termobarometria tlenowa (tarcza ałdańska, platforma syberyjska)

The Inagli dunite-peridotite-shonkinite zonal-ring intrusive with platinum-chromite mineralization is located on the Aldan shield of the Siberian Platform. Considering the structure, rock composition and ore mineralization, it is similar to the platinum-bearing zonal massifs of the "Ural-Alaskan" type, but this intrusive differs from the latter in its geological position. In order to clarify the physical and chemical conditions of formation of the Inagli massif, the mineral composition of rocks, especially сhromite-containing dunites, peridotites and shonkinites, as well as platinum-chromitite ore segregations, has been studied in detail. The rocks of the Inagli massif, from dunites to shonkinites, including peridotites, clinopyroxenites, and alkaline syenites, form a single continuous series. This is confirmed by a clear dependence of the composition of olivine, pyroxene, phlogopite and chromian spinel on the content of MgO in rocks. They were formed from the initial high-potassium picrite melt, which, during rising, underwent gradual decompression solidification and formed a cylindrical diapir-like body at the near-surface level in the Early Cretaceous. This occurred as a result of subduction processes related to the formation of the Mongol-Okhotsk orogenic belt along the southern framing of the Siberian craton. The values of oxygen volatility (lgfO2) for dunites, peridotites, shonkinites, chromitites and olivine-chromite inclusions in the isoferroplatinum of the Inagli massif, calculated using the method of the olivine-spinel oxygen thermobarometer of Ballhaus-Berry-Green (BBG), form a single trend FMQ+(2-4) in the range 620-11400C, i.e. along the band by 2-4 units of lgfO2 exceeding the fayalite-magnetite-quartz (FMQ) buffer. Such a rather narrow range of variation in the values of O2 fugacity in a wide interval of temperature indicates good comparability and reliability of the data obtained. At the same time, there is a natural decrease in temperature intervals for the formation of olivine-chromite parageneses (in 0С): with isoferroplatinum – (1140-680); in chromitite segregations – (980-710); in dunites – (930-620); peridotites – (890-770) and shonkinites – (840-710). The results obtained almost completely coincide with the field of values for dunites and chromitites from the Platinum-bearing belt of the Urals, given by other researchers. In terms of redox parameters, platinum-bearing zonal ultramafic-mafic massifs of the Ural-Alaskan and Aldanian types are close to more oxidized peridotites with a long history in the lithosphere. They differ significantly from the peridotites of the Beni-Bousera massif, as well as abyssal peridotites of the oceanic ridge systems, and others, which are formed under more reduced conditions corresponding to the range between FMQ, carbon-oxygen-CO (ССО) and iron-wustite (IW) buffers.

Understanding the links between volcanic systems and epithermal ore formation: A case study from Conical Seamount, Papua New Guinea

The Tabar-Lihir-Tanga-Feni (TLTF) island chain in northeastern Papua New Guinea formed by tectonic and alkaline to shoshonitic magmatic activity since the Pliocene. Several volcanic centers are CuAu mineralized including the world-class Ladolam Au deposit and Conical Seamount south of Lihir. The latter has been recognized as a juvenile analogue to the Ladolam deposit located on-shore. Whereas the mineralization at Conical Seamount is reasonably well studied, the specific magmatic processes that promote epithermal mineralization at this seamount but not at others are poorly understood. Here, we present new petrological and geochemical data from Conical Seamount, and compare them with those from the barren (unmineralized) Edison, Tubaf and New World seamounts nearby. We focus on whole rock compositions and major and trace element analysis of melt inclusions and minerals including clinopyroxene, sulfide and magnetite. We combine our observations with modelled constraints on mantle source composition and partial melting as well as magma evolution. A first-stage melting leaves a residual mantle source enriched in Au. Second-stage melting of a previously subduction-metasomatized mantle generally promotes the transfer and concentration of metals and volatiles in the ascending melts. These magmas are unlikely to control ore formation as all seamounts show evidence for similar mantle sources and parental melt composition. However, the presence of a shallow crustal magma chamber is unique to Conical Seamount. It is characterized by frequent melt replenishments and extensive magma fractionation leading to sulfide and magmatic volatile saturation. These specific magma chamber processes lead to the pre-enrichment of the magma in chalcophile elements including Au, while sulfide saturation coeval with magmatic volatile exsolution provide the way for an effective Au transfer from the magmatic to the epithermal system.

Molybdenum isotopic evidence for the initiation of a big mantle wedge beneath eastern Asia

The subduction of the Paleo-Pacific plate and the formation of a big mantle wedge have influenced the tectonic-magmatic evolution of eastern Asia. However, the timing and mechanism of big mantle wedge formation remain obscure. Here we report molybdenum (Mo) isotope compositions of Mesozoic-Cenozoic mafic rocks in eastern China, which provide constraints on the geodynamics of Paleo-Pacific subduction. The >125 Ma rocks have high δ98Mo values of −0.27‰ to 0.17‰ and arc-like features, whereas the <125 Ma rocks have low δ98Mo values of −0.67‰ to −0.04‰ and ocean-island basalt (OIB)-like features. The mantle sources of these two types of rocks contain subducted Paleo-Pacific slab components at sub-arc and mantle transition zone depths, respectively. Therefore, the Paleo-Pacific subduction involves shallow subduction yielding a small mantle wedge in the early stage and deep subduction yielding a big mantle wedge in the late stage. The dramatic change in Mo isotopes reveals that the big mantle wedge beneath eastern Asia was initiated at ∼125 Ma.

Late Jurassic granitoids in Mufushan complex and their significance for the Mesozoic tectonic evolution of eastern South China

The Mesozoic tectono-thermal evolution of eastern South China plays an important role in forming the abundant magmatic rocks and associated giant polymetallic deposits. The middle-late Mesozoic granitoids in Mufushan and adjacent regions (e.g., Lianyunshan, Wangxiang, Taohuahsan, etc.) represent the western front of the southeast China magmatic province. However, the tectonic regimes and petrogenesis of the magmatic rocks are still elusive. Here, we report zircon UPb ages, geochemistry, and Sr-Nd-Hf-Pb isotopic data for the granitic rocks in the Mufushan region, in combination with data available from the literatures, to discriminate the middle-late Mesozoic tectono–magmatic history of the Mufushan complex in north-central South China Block. New zircon UPb dating suggests that the Mufushan two-mica monzogranites were emplaced at 149–144 Ma, a short time after the 153–151 Ma granodiorites. These late Jurassic felsic magmas share broad similarities in geochemical-isotopic characteristics, with significant enrichment in LREEs and LILEs (e.g., Ba, Rb, Th, and K), depletion in some HFSEs (e.g., Nb, Ta, Ti, and P) and positive Pb anomalies, similar to those of arc-type rocks. The common existence of Neoproterozoic zircons within the magmatic rocks indicates a certain amount of ancient material involved in the genesis of the magma. The two-mica monzogranites have εHf(t) values ranging from −13.4 to +3.5, overlapping with those of the granodiorites (−11.3 to +6.6), but both are lower than contemporaneous diorites (−2.4 to +0.59), suggesting a greater incorporation of enriched materials into the source for the granitoids. The two-mica monzogranites and granodiorites have εNd(t) values ranging from −7.9 to −8.8 and − 10 to −7.9, with corresponding two-stage Nd model ages of 1.7–1.6 Ga and 1.6 Ga, respectively, falling within their two-stage Hf model ages of 2.0–1.4 Ga and 1.8–1.4 Ga, respectively. Compared with the geochemical and isotopic compositions of the coeval magmatic rocks in eastern South China, we favor that the late Jurassic Mufushan granodiorites evolved from variable mixing and differentiation of the diorites and granitoids, accompanied by a continuous magma assimilation. The presence of late Jurassic highly fractionated granites suggests an extensive magma differentiation in eastern South China and the well-developed early Cretaceous A-type granites in eastern South China reflect a dominant extensional tectonic regime induced by slab roll-back of the Izanagi plate. The involvement of subduction-related melts facilitated the underplating of mantle-derived magma and crustal heating, triggering intensive partial melting of the lithosphere and magma enrichment, as well as the polymetallic deposits in eastern South China.

Magmatic genesis, hydration, and subduction of the tholeiitic eclogite-facies Allalin gabbro (Western Alps, Switzerland)

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones.

Strontium isotopic composition of the Mesozoic sedimentary formations of the southern Central Andes (33°–34°S) and its paleogeographic implications

Strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr) in sedimentary and igneous rocks may provide valuable interpretations that allow constraining the geologic evolution of sedimentary basins. We present a new ⁸⁷Sr/⁸⁶Sr dataset from sedimentary units in the southern Central Andes (33°–34°S)—coupled to available isotopic information of igneous rocks—to examine the paleogeographic evolution of the Andes at the western margin of South America during the Mesozoic. These new data was obtained from shales, limestones and fine-grained sandstones from Mesozoic formations in the Coastal Cordillera and the Principal Cordillera of the main Andes. The new Sr-isotopic data on the Mesozoic sedimentary rocks can be grouped by both rock type and location: a) Continental red sandstones: 0.7038–0.7124; b) Calcareous rocks of the main Andes (Principal Cordillera): 0.7070–0.7081; and c) Calcareous rocks of the Coastal Cordillera: 0.7041–0.7067. The ⁸⁷Sr/⁸⁶Sr signatures in most of the clastic units in both Cordilleras are related to an evolved or enriched magmatic source for clasts and matrix indicating different sources. On the other hand, the Sr-isotopic composition of calcareous rocks differs between the units in the Andean and the Coastal mountain belts: shales and limestones in the Principal Cordillera of the Andes display ⁸⁷Sr/⁸⁶Sr ratios corresponding to the contemporary ocean in the Late Jurassic and Early Cretaceous; whereas the carbonatic units in the Coastal Cordillera do not show a relation with any oceanic signatures. The Sr-isotopic compositions of the Andean carbonatic units in the Principal Cordillera support the interpretation of their marine depositional environment in the paleo Pacific Ocean. In contrast, carbonate rock samples from the formations in the Coastal Cordillera suggest that they formed in brackish-water depositional basins made by topographic highs partially disconnected from the ocean. In the case of clastic rocks, their Sr-isotopic compositions provide insights into the contributions derived from the magmatic arc to the clastic units. Therefore, the Sr-isotopic composition of the sedimentary rocks provides a new dataset which can improve our understanding of the geological evolution of the southern Central Andes.

Petrogenesis of A-type leucocratic granite magmas: An example from Delbegetei massif, Eastern Kazakhstan

The genesis of leucogranite magmas is important issue of geodynamics, petrology and ore geology because leucogranites are associated with collisional belts, partial melting of sedimentary source rocks, and may host rare metal (Sn, W, Li, NbTa, and Be) mineralization. To establish the petrogenesis of large leucogranite intrusions, detailed studies of petrography, mineralogy, fluid regime, and the material and isotopic composition of rocks are required. The paper reports results of the studies of the Delbegetei massif of the intrusion in Eastern Kazakhstan. The massif is composed predominantly of leucocratic granites, while syenogranites are subordinate. Rocks of the massif belong to shoshonitic and high-K calc-alkaline series; demonstrate a predominance of K over Na, high ferroan and high contents of LREE and HFSE; which allows them to be classified as A-type granites. The age of Delbegetei massif, estimated by the UPb zircon dating, varies in the range 249–240 Ma, which correspond the Early-Middle Triassic. The differences in rock composition and in temperatures of zircon saturation allow supposing that syenogranites and leucogranites formed from different parental magmas. Syenogranite magma formed as a result of partial melting of metamorphosed volcanic rocks (andesidacites or dacites) with possible influence of mafic magmas. Leucogranite magma formed as a result of fluid-present partial melting of metaterrigenous sedimentary rocks. Leucogranite magma underwent the feldspars differentiation in the fluid-present conditions. This led to composition variations of leucogranites. Analysis of the geological position, age and composition of the rocks allows concluding that the Delbegetei massif formed at the Early Triassic in an intraplate geodynamic setting and that the activity of the Siberian mantle plume is the most probable reason for their formation.