Acid Magmas Research Articles

Geology and Geochemistry of Aberma and Its Environ Sheet 54se Gusau, Zamfara State, Nigeria

The study area falls within the northwestern part of Nigeria and is underlain by crystalline rocks of the Basement Complex. Field mapping in Aberma and its environs were carried out using topographical map sheet of 54 SE Gusau on a scale 1: 25,000 with an area extends of about 30.8052km2. This field work was carried out using traverse method in conjunction with field equipment such as compass/clinometers, geologic hammer, sample bag and global positioning system. This study however, aims at producing a detail geological map, including the various rock geochemistry and petrography of the study area. The area is underlain by two major lithological rock type which is mica schist and granite gneiss. The granite gneiss is strongly foliated, caused by high temperature and pressure and the minerals observed in plane polarized light include quartz, orthoclase, plagioclase, muscovite, opaque mineral and biotite. The mica schists in the study area are mostly weathered in ditches below the ground surface, the rock is moderately weathered and show schistocyte. The minerals observed in plane polarized light include quartz and muscovite. The dominant structures observed include; joints, Quartz veins, foliation and pegmatite dyke. Structural analysis shows that the rocks have been affected by the Pan African orogeny with the joint’s orientation trending dominantly NNE-SSW and Quartz vein trending in somewhat E-W direction. The microscopic studies revealed that the mineralogical composition of the rocks types of the study area contain varying percentages of quartz, muscovite, plagioclase, orthoclase, opaque mineral and biotite as the major minerals. The geochemical analysis revealed the major and minor elemental distribution of the rock types of the study area. The chemical assay revealed that the granite gneiss and mica schist rock type of the study area are super-saturated with respect to silica, silica (SiO<sub>2</sub>) is the most abundant oxide with an average value of 78.12% in the rock sample. This implies that the rock analyzed is acidic in nature and must have formed from crystallization of an acidic magma because the silica content being greater than 65%.

Estimating the contribution of thermal processes to generation of seismicity in the junction zone of the Chuya Depression and the Kyrgyz Ridge of the Northern Tien Shan

This paper presents the attempt to estimate the contribution of thermal processes in the Earth’s crust to the generation of seismicity by the example of the seismically active region of the Chuya depression and the Kyrgyz ridge of the Northern Tien Shan. For this purpose, we use the models of temperature, petrophysical characteristics, and elastic moduli constructed in the previous works. The silica content model based on seismic tomography data is used to build a lithotype model. The constructed thermal conductivity model is utilized, along with the temperature model, to create a depth model of heat flow density. Based on the density, elastic moduli, and temperature models, the shear and thermoelastic stress models in the rocks are constructed. Their comparison with the distribution of earthquake hypocenters suggests that on the scale of the seismically active zone of the Chuya depression and the Kyrgyz ridge of the Northern Tien Shan, seismicity is mainly determined by the thermomechanical effect resulting from the hot ascending flow of acid magma from the upper mantle beneath the Muyunkum–Narat median mass.

Classification and mineralization of global lithium deposits and lithium extraction technologies for exogenetic lithium deposits

A reasonable classification of deposits holds great significance for identifying prospecting targets and deploying exploration. The world’s keen demand for lithium resources has expedited the discovery of numerous novel lithium resources. Given the presence of varied classification criteria for lithium resources presently, this study further ascertained and classified the lithium resources according to their occurrence modes, obtaining 10 types and 5 subtypes of lithium deposits (resources) based on endogenetic and exogenetic factors. As indicated by surveys of Cenozoic exogenetic lithium deposits in China and abroad, the formation and distribution of the deposits are primarily determined by plate collision zones, their primary material sources are linked to the anatectic magmas in the deep oceanic crust, and they were formed primarily during the Miocene and Late Paleogene. The researchers ascertained that these deposits, especially those of the salt lake, geothermal, and volcanic deposit types, are formed by unique slightly acidic magmas, tend to migrate and accumulate toward low-lying areas, and display supernormal enrichment. However, the material sources of lithium deposits (resources) of the Neopaleozoic clay subtype and the deep brine type are yet to be further identified. Given the various types and complex origins of lithium deposits (resources), which were formed due to the interactions of multiple spheres, it is recommended that the mineralization of exogenetic lithium deposits (resources) be investigated by integrating tectono-geochemistry, paleoatmospheric circulation, and salinology. So far, industrialized lithium extraction is primarily achieved in lithium deposits of the salt lake, clay, and hard rock types. The lithium extraction employs different processes, with lithium extraction from salt lake-type lithium deposits proving the most energy-saving and cost-effective.©2023 China Geology Editorial Office.

Geochronology, pyrite trace elements, and sulfur isotope geochemical characteristics of the Saibagou gold deposit in the eastern part of the northern Qaidam Basin

Introduction: The Saibagou gold deposit, located in the eastern part of the tectonic belt of the northern Qaidam Basin in western China, has its gold ore bodies strictly controlled by the regional fault system. Despite this understanding, there remain controversies surrounding the deposit’s metallogenic epochs, sources of ore-forming materials, and properties of ore-forming fluids. To address these controversy, the metallogenic process of the Saibagou gold deposit can be further determined by analyzing the U-Pb ages of hydrothermal zircons in the gold-bearing quartz veins and investigating the trace element and sulfur isotope compositions of pyrite in the gold deposit.Methods: This study does not focus on the mineral characteristics susceptible to interference by the metallogenic condition in gold ore bodies. Instead, it offers a detailed discussion on stable associated minerals and their indicative markers formed in the process of gold mineralization.Results: The results of this study showed that the metallogenic process of the gold deposit can be categorized into the following stages: 1) quartz–pyrite veins, 2) milky-white quartz—pyrite—native gold veins, 3) hoary quartz—native gold—polymetallic sulfide veins. As indicated by the U-Pb ages of hydrothermal zircons in the gold-bearing quartz veins, the Saibagou gold deposit has two metallogenic ages, namely, 423.91 ± 4.5 Ma (the Silurian) and 470.18 ± 4.92 Ma (the Ordovician).Discussion: The Silurian metallogenic age, predominates and nearly aligns with the expansion of the regional NWW-trending brittle-ductile shear zone, followed by the Ordovician metallogenic age. Data on the trace elements and sulfur isotopes of pyrite, show that the Saibagou gold deposit has similar pyrite compositions in the three metallogenic stages. Gold in the deposit primarily occurs as native gold or minor petzite inclusions and has a very low lattice gold concentration. As indicated by the concentrations of elements such as Co, Ni, and as in the pyrite, as well as the variation range of δ34S values, the ore-forming fluids were derived from low-temperature arsenic-bearing acidic magmas. In addition, the primary ore-forming materials appear to encompass mantle-derived materials from the deep earth.

Volcano-sedimentary interactions – A key to understand Cretaceous paleoenvironments in the Paraná basin (southern Brazil)

The Lower Cretaceous in the Paraná Basin record a large volcanic event (Serra Geral Formation) that covered an active sedimentary system (Botucatu Formation), before the fragmentation of the southern Gondwana. Therefore, the fluvial-aeolian deposits underlying aeolian deposits of residual dunes, were gradually covered by volcanic flows. Intense basic and minor acidic magmatism covered an extensive continental aeolian environment. The interface between the two units shows an aeolian environment during the early stages of volcanism forming deposits and volcano-sedimentary features overlying and underlying the flows. In six outcrops on the southern portion of the current border of Paraná Basin, in Rio Grande do Sul state, these deposits and features were recognized, showing the complexity of the environment and the processes associated with lava flows and sediments. They were formed by the interaction of consolidated or unconsolidated sediments, saturated or not in the water, with basaltic or dacitic flows. Interaction features and deposits synchronous to volcanism (volcanoclastic), later or in intervals between volcanic events (epiclastic) and only cover, with absence of interactions, were identified. Through the distinctive characteristics of the interaction processes between the volcanic rocks and the sediments analyzed in the volcanic-sedimentary features and deposits, the morphology of the aeolian environment of Botucatu Formation could be, by analogy, compared to the current aeolian environments. The proposed paleoenvironment indicates a sedimentary environment with different domains, categorized by their water saturation. The different siliciclastic components found in each domain allowed the generation of different features and volcanic-sedimentary deposits evidencing aspects related to the presence or absence of water among the proposed domains.

Sediment provenance and dispersal in the early Eocene Dongying Depression, Bohai Bay Basin, Eastern China: Evidence from detrital zircon geochronology, geochemistry and petrology

Comprehensive provenance studies of syn-rift basin fills are required to better understand possible sources of clastic detritus and sediment routing systems. The Eocene fill of the Bohai Bay Basin in eastern China represents a syn-rift succession, where subsurface datasets permit investigation of sediment sources and sinks. New detrital zircon U–Pb samples (441 detrital zircon grains) from six wells were combined with elemental geochemical analysis of siliciclastic sediment, sandstone petrography and palynology to investigate depositional ages and provenance. This study demonstrates the importance of integrating geochronometry, geochemistry, petrology and palynology datasets to fully unravel syn-rift sediment provenance and routing. Zircons of the Dongying Depression were derived principally from an active continental margin island-arc setting characterized by felsic acid magmas, and subordinately from a recycled orogenic belt. The Shicun and Binnan faults controlled the relationship between sediment routing systems and source areas, explaining spatial differences of provenance signals in the depression. The routing system around the Shicun fault is characterized by a dominance of late Paleozoic and Paleoproterozoic zircons and subordinate Mesozoic zircons. In contrast, increased Mesozoic zircons in samples from south of the Binnan fault provide evidence of Mesozoic magmatism in this area. The early Eocene sediments record the signal of early Cretaceous magmatism in the North China Craton, but lack a record of substantial syn-depositional magmatic activity since the Paleogene in the Dongying Depression. Zircons from early Cretaceous strata and pre-existing zircons from the Xing-Meng Orogenic Belt and Inner Mongolia paleo-uplift were transported to the Yanshan and Luxi areas together. These sediments entered the Dongying Depression in the early Eocene, following a period of recycling. In addition, a Precambrian basement signal indicates possible denudation of Neoarchean to Proterozoic rocks in the Luxi uplift and from the Binnan fault footwall. Overall, the provenance signals of the basin reflect zircon recycling from an Early Cretaceous succession, denudation of ancient magmatic rock masses and the absence of syn-sedimentary magmatism.

From Rodinian passive margin to peri-Siberian continental arc: Evidence from the multiphase Neoproterozoic–early Paleozoic magmatic record of the Zavkhan Block in the Mongolian Collage

Geochronological and geochemical study of the Neoproterozoic–early Paleozoic magmatic record was carried out in two sections of the Zavkhan Block, one of the largest continental fragments within the Mongolian Collage of the Central Asian Orogenic Belt. Our results show that two distinct magmatic events occurred in both studied domains, the Late Tonian (c. 840–800 Ma) and the Cambro–Ordovician (c. 510–470 Ma). Based on the variable geochemical signatures and two-stage Nd model ages (TNd.2stgDM=1.4–3.0 Ga), the late Tonian magmatism included both mantle-derived magmas that suffered a crustal contamination as well as partial melts of a heterogeneous continental crust that ranged from Paleo- to Mesoproterozoic (felsic metaigneous rocks or immature metagreywackes) to Neoarchean–Paleoproterozoic (metabasalts or metatonalites). The Cambro–Ordovician basic–acidic magmatism (TNd.2stgDM=1.3–2.2 Ga) reflected interplay between two components, mafic juvenile (depleted-mantle-derived) and felsic, generated by anatexis of Tonian, mainly metabasic crust. The late Tonian (c. 840–800 Ma) magmatic activity is interpreted as result of intracontinental rifting, whereas the Cambro–Ordovician (c. 510–470 Ma) magmatism is linked to a continental magmatic-arc system.The comparison of the data from the Zavkhan Block with other microcontinents of the Mongolian Collage shows that most of them experienced an identical two-stage magmatic evolution. The first stage is interpreted as a result of the Rodinia break-up, whereas the second one was associated with inception of the Paleo-Pacific Plate subduction. Based on similarities in the magmatic record, we conclude that the Mongolian Collage microcontinents were located at the Rodinia periphery, as a part of either the Siberian or Tarim cratons. They were separated in the late Tonian, established a passive margin and drifted towards southern Siberia, where they recorded arc magmatism related to the Cambro–Ordovician subduction of the Paleo-Pacific Plate.

Carbonatisation and overprinting mineralisation in Siah-Kamar porphyry molybdenum deposit, NW Iran

The Siah-Kamar porphyry Mo deposit (SKD) is located at the north-western termination of the Urumieh-Dokhtar magmatic arc and it is the only known porphyry Mo ore reserve in Iran. The SKD formed in two main mineralisation stages, at ~33–32 Ma (late-stage, disseminated) and ~ 29–28 Ma (late-stage, high-grade), within a context of a long-lived magmatism. We integrate results from the whole-rock geochemistry, the OC stable and Sr isotope systematics across the mineralised alteration zones with the available drilling data (ore element concentration) to define the ore-forming processes leading to Mo enrichment in the SKD. Our investigation shows that CO2 bearing magmatic fluids had the major role in both early and late-stage mineralisation. The Mo shows positive correlation with SiO2 content but a clear negative correlation with W, Cu, and S, due to a delayed precipitation of Mo during the early-stage mineralisation. The released fluids from the new acidic magmas and subsequent fluid-rock interaction in an open-system, primarily assisted by brittle fracturing, and accompanied by cooling and mixing with various amounts of meteoric water and fluid neutralisation, was responsible for the selective ore depletion and Mo-ore enhancement during carbonatisation along structurally-controlled pathways.

INVESTIGATION OF THE GEOTHERMAL RESOURCE POTENTIAL USING GEOELECTRICAL METHOD WITH THE WENNER CONFIGURATION: A CASE STUDY IN LEBONG REGENCY, INDONESIA

In geothermal reservoirs, hydrothermal distribution is significant. The most widely utilized geothermal reservoirs to date are those of hydrothermal systems, which are geothermal systems where the reservoir contains steam, water, or a mixture of both, depending on the pressure and temperature of the reservoir. One of geophysical method that is enough to map subsurface conditions to determine the hydrothermal distribution is the 2-Dimensional Geoelectric method using the Wenner configuration. This method aims to study the variation of rock resistivity below the earth's surface, resulting in a two-dimensional profile of the subsurface resistivity model used to describe the layered subsurface resistivity structure. The results are in the form of 2-Dimensional modeling of the distribution of hydrothermal manifestations with data processing using pyGIMLI (Python Library for Inversion and Modeling in Geophysics) software. In the study area Pungguk Pedaro Village, Bingin Kuning District, Lebong Regency, the Hulusimpang Formation has a high alteration intensity, as evidenced by the weathering in rocks and the micrographic appearance. The types of stones found in this research area include andesite rocks, including sandy gravel, volcanic breccia, tuff, andesite, basalt, basalt-andesite sand, silty sand, and clay. Or rocks derived from acidic magma.

Silurian to Cretaceous geological evolution of southern Mexico and its connection to the assembly and break-up of Western Equatorial Pangaea: geochronological constraints from the northern Sierra de Juárez Complex

Abstract The Sierra de Juárez Complex (SJC) of southern Mexico contains an extensive geological record from Precambrian to Cenozoic, involving Rodinia, NW Gondwana, western equatorial Pangaea, and eastern peninsular Mexico. It is thus critical for palinspastic reconstructions and lithotectonic correlations, mainly between the Mexican and NW South America terranes. In this contribution, we investigate the tectonic evolution of the northern SJC from Silurian to the Lower Cretaceous on the basis of fieldwork, petrography, and zircon U–Pb geochronology by laser ablation–inductively coupled plasma mass spectrometry. Our results allow us to constrain five main geological events: (1) Middle Paleozoic sedimentation along NW Gondwana during transtensional tectonics; (2) volcanosedimentary activity between 292 and 281 Ma in NW Gondwana during Rheic Ocean closure; (3) early-middle Permian metamorphism related to flat-slab subduction postdating Pangaea assembly; (4) Lower–Middle Jurassic anatexis and magmatism coeval with regional shearing at c. 175 Ma influenced by transtensional tectonics along eastern peninsular Mexico during Pangaea tenure; and (5) intermediate to acid magmatism between c. 136 and 129 Ma, correlated with the Zongolica continental arc in southern Mexico, followed by deep-crustal shearing related to either the formation of the extensional Chivillas basin or the Upper Cretaceous–Cenozoic contractional episode documented in the Cuicateco Terrane.

Petrogenesis of Early Cretaceous High Ba-Sr Granitoids in the Jiaodong Peninsula, East China: Insights into Regional Tectonic Transition

Element geochemistry, Sr and Nd isotope, and LA-ICP-MS zircon U-Pb isotope data have been obtained for the granitoids of Dazesan pluton in the Jiaodong Peninsula, East China, and their intermediate microgranular dark enclaves so as to reveal their petrogenesis and tectonic implications. These granitoids have high SiO2 (68.25–71.56 wt.%), K2O (3.44–5.50 wt.%), total alkalis (K2O + Na2O = 7.29–9.00 wt.%), Sr (451–638 ppm), Ba (1157–2842 ppm) and light rare earth elements (LREEs) (131.57–210.08), with strong depletion both in heavy rare earth element (HREE) and high field strength element (HFSE) concentrations as well as unclearly Eu anomalies, showing typical signatures of high Ba-Sr granitoids. They possess high (La/Yb)N (32–50) and Sr/Y (50–79) values and low MgO (0.76–1.11 wt.%), Cr (9.9–19.6 ppm) and Ni (4.51–7.04 ppm) concentrations. All the above geochemical compositions are similar to those of late Early Cretaceous granitoids, in combination with zircon LA-ICP-MS U-Pb ages of 119.6 ± 1.3 to 120 ± 1 Ma for these granitoids obtained in this study, indicating c. 120 Ma probably represents the lower limit of ages when late Early Cretaceous granitoids emplaced in the Jiaodong Peninsula. The microgranular dark enclaves, forming a linear trend with their host granitoids on the oxide against SiO2 plots, display higher MgO contents of 3.05–4.39 wt.% at lower SiO2 concentrations of 54.25–56.84 wt.% and possess a zircon LA-ICP-MS U-Pb age of 119 ± 2 Ma, identical to those of these granitoids, indicating the acid magma and intermediate magma were coeval. Furthermore, dark enclaves and their host granitoids have indistinguishable (87Sr/86Sr)i values of 0.709523–0.70972 and 0.709361–0.709858, respectively, and plot within a two-liquid immiscible field on the Greig pseudoternary phase diagram. In addition, they have markedly parallel REE patterns, with the dark enclaves having much greater REE and HFSE abundances than those of their host granitoids. Therefore, it is suggested that liquid immiscibility is a viable model to explain the chemical compositional variations between the Dazeshan granitoids and their dark enclaves. Based on the element geochemistry, geochronology and Sr- and Nd-isotope of the Dazeshan granitoids and their dark enclaves, it is envisaged the crust-derived acid melts due to partial melting of ancient continental lower crust in the Jiaodong Peninsula (mainly Neoarchean-Palaeoproterozoic basement in the Jiaobei terrane) containing a subduction-related material, resulting from the addition of the enriched subcontinental lithospheric mantle-derived melts, assimilated the lithospheric mantle-derived basic melts and formed the homogeneous magma chamber at the crust base, then split into two immiscible liquids, with one granitic liquid producing the Dazeshan granitoids and the other intermediate one forming the dark enclave during its ascent. Combined with previous studies, the identification of a lithospheric mantle-derived material in the Dazeshan granitoids suggests a catastrophic lithospheric thinning at c. 120 Ma, reflecting an abrupt change in the direction of Palaeo-Pacific plate subducting and the corresponding regional tectonic transition from E–W extension to NW–SE extension.

Distribution of Pyroclastic Deposits around Lake Maninjau Agam District, West Sumatera, Indonesia based on Magnetic Susceptibility

Lake Maninjau is an erupting volcano in West Sumatra Province. The results of the eruption have now settled in various places and scattered in all directions due to the transportation process. The process of transporting volcanic material is caused by several factors such as wind and rain. This study aims to analyze the distribution pattern of volcanic material that is scattered around Lake Maninjau, Agam Regency. We have 25 types of samples divided into 3 categories. There are pumice, lava, and ash. This research was conducted at the FMIPA UNP Laboratory using the rock magnetization method. In this study, a Bartington MS2 with MS2B sensor was used by calculating the magnetic susceptibility value based on the mass of the sample. The results of the magnetic susceptibility analysis obtained varied between 74.7×10-8 - 3956.7×10-8 m3/kg which is included in the ilmenite (FeTiO3) group. The highest value of vulnerability was 2800×10-8 - 4000×10-8 m3/kg found at the core of Lake Maninjau ± 5.0 km seen from the green contour map. The lowest magnetic susceptibility values ​​0 - 800×10-8 m3/kg were found in the western part of Lake Maninjau and the material was deposited at a distance of ± 13 km from the core of the Maninjau caldera. Deposits were also found in the eastern part of Lake Maninjau ± 22.4 km from the core of Lake Maninjau and to the Middle East ± 23 miles from the core of the Maninjau caldera. The white color represents 1600 - 2000×10-8 m3/kg which is included in the moderate magnetic susceptibility value, which is ± 4 km to the southeast of the Maninjau caldera. The benefit that can be taken from this research in the ink industry is the presence of a magnetic mineral distribution map based on suseptibility values ​​and makes it easier to find raw materials for making ink around Lake Maninjau.
 HIGHLIGHTS
 
 Lake Maninjau stores various elements contained in the released material. The elements that make up magnetic minerals are Fe, Si, Ca, Al, K, Ti. The collection of several elements forms the raw magnetic mineral
 Volcanic material contains the mineral ilmenite which in theory can be used as a raw material for making TiO2 pigments, ferrous metals and chemical compounds containing iron. In the industrial field TiO2 is widely used as a paint pigment, additives in the paper-making process, ceramic raw materials, pharmaceutical industry raw materials and TiO2 is also widely used for photoclinic materials
 Superparamagnetic minerals are present in volcanic rock samples resulting from an eruption, the magnetic sensibility values at high frequencies are slightly lower than the magnetic susceptibility values at low frequencies. If there are no superparamagnetic minerals (SP)
 It can be assumed that the ancient volcanic eruptions of Maninjau belong to the type of plinia eruption which is highly exposed from magma with high viscosity or acid magma, the composition of the magma is andesitic to rhyolitic
 
 GRAPHICAL ABSTRACT

AGE AND DURATION OF CRYSTALS GROWTH IN CHAMBERS OF VOLYN PEGMATITES (UKRAINIAN SHIELD)

Formation of chamber pegmatites is associated with tectonomagmatic activation of the region of their distribution. It should be expected that the granitic chamber pegmatites of Volyn were formed by products of degassing of acidic magma. However, in the growth of quartz of the late generation, the beginning of crystallization of which was about 200ºC, CO2-fluids were involved. They are products of degassing of basic magma, which also took part in the growth of crystals at higher temperatures (˃573ºC). Crystallization of minerals in chambers was long: from 1.75±0.10 billion years (age of protogenetic inclusions of zircon and uraninite in topaz crystals) to 1.39 billion years (age of galena inclusions in late-generation quartz). So, for the first time, it was substantiated that the duration of crystal growth in the chambers of Volyn pegmatites was at least 360±100 million years. Therefore, it is assumed that the growth of crystals in chambers Volyn pegmatites lasted for quite a long time.

PETROLOGY OF CENOZOIC DYKES ON THE ARGENTINE ISLANDS (WILHELM ARCHIPELAGO, WEST ANTARCTICA)

Dyke swarms that intrude Paleogene granitoids of the Argentine Islands near the Ukrainian Antarctic Station "Akademik Vernadsky" were studied. The field relations and mineralogical, petrographical and geochemical properties of the dykes allow their relative geological age and the geodynamic conditions to be clarified. The magmatic activity in the study area did not cease, at least until the end of the Miocene when the processes of orogenic uplift led to the erosive exposure of the Paleogene granitoids of the Barchans-Forge Massif. Tectonic exhumation of the latter was accompanied by the intrusion of dykes of different compositions at different depths. Cenozoic basalt and diabase dykes are the most common. Most are subvolcanic fractured intrusions formed after the complete exhumation of the host granitoids, which ended by the Miocene (11 Ma). The basaltic dykes are probably one of the youngest representatives of magmatism in the region. They have an intermediate position between high-LILE tholeiites and the calc-alkaline series. However, the nature of contamination of the basaltic dykes by crustal rocks requires additional research. Cenozoic microdiorites dykes are rarer than basaltic ones. They are hypabyssal fractured intrusions formed during the tectonic exhumation of the Barchans-Forge granitoids between the beginning of the Paleocene and the end of the Miocene (61-11 Ma). The microdiorites belong to the orogenic calc-alkaline series and they could be related to subduction processes. The discovery of only one dacite dyke indicates the rarity of acid magmatism during the Cenozoic period. It occurs as a fractured intrusion and formed after the exhumation of the host granitoids. The dacite dyke belongs to the calc-alkaline series, which, along with other compositional properties, suggests that dacite and microdiorite dykes are comagmatic.

Role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Island Ophiolite suite of India

The Mesozoic‐age Andaman Island Ophiolite located in the southeastern fringe of the Indian plate margin is one of the well‐preserved supra‐subduction‐zone oceanic lithospheric slices. It is tectonically associated with the Indonesian arc system and is an extension of the Phanerozoic Indonesian‐Myanmar orogenic belt. Thus, it offers a unique opportunity to study oceanic crust formation and specify magma evolution processes in the Tethyan subduction system. The volcanics of the Andaman Islands ophiolite display a wide compositional variation. Rock types vary from basic (basalts) to intermediate (basaltic‐trachyandesites, basaltic andesites) to acidic differentiates like dacites and rhyolites. This contribution presents the petrochemistry of the intermediate rock types—basaltic‐trachyandesites, trachyandesites and acidic—dacites and rhyodacites of Andaman Island Ophiolite (India). This study examines the role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Ophiolite suite. Plagioclases and pyroxenes are the major mineral constituents of these rocks on petrographic observation. Plagioclases of the intermediate rocks are of albite composition Ab98‐99. The acidic rocks show compositional variation from albite to oligoclase (Ab83–99). Analysed pyroxenes of intermediate and acidic rocks are augite (Wo46–49En34–41Fs13–17). The studied calc‐alkaline rocks show variable abundance of Large‐Ion Lithophile Elements and display strong positive Pb and negative Ti anomalies. The chondrite‐normalized Rare Earth Elements display an enriched pattern with varying degrees of enrichment. Clinopyroxene thermobarometry reveals that the intermediate magma and acidic magma becomes saturated with clinopyroxene at 1,062–1,133°C and 1,016–1,079°C, respectively, at 2 kbar and H2O = 2 wt%. Petrochemical findings and model calculations using PELE are consistent with a derivation of intermediate rocks from the basaltic melt by fractional crystallization. In contrast, acidic rocks are the product of partial melting of hydrothermally altered oceanic crust. The association of basaltic and intermediate rocks suggests a relatively stable magma chamber to generate differentiated intermediate magmas by fractional crystallization.

The volcanism of the Jurassic Chon Aike Silicic LIP influenced by Paleozoic inherited crustal structures in the northeastern Deseado Massif, Patagonia

An area of ∼40 km2 located in the northeastern sector of the Deseado Massif is detailed studied, in which Paleozoic metamorphic and Jurassic volcanic rocks crop out. Two genetically linked and compositionally different Jurassic volcanic events of the Chon Aike Silicic LIP are documented by geological mapping and lithofacies and petrographic analyses. The first event was only represented by andesitic lava flows assignable to the intermediate portion of the Bahía Laura Volcanic Complex (Bajo Pobre Formation). The second was represented by the acidic rocks of the Bahía Laura Volcanic Complex (Chon Aike and La Matilde formations) and it was constituted by low-grade and moderate grade ignimbrites, pyroclastic fall deposits (ash tuffs) and several lava bodies (rhyolitic lava domes and lava flows). Firstly, the effusive eruptions dominated during the intermediate magmatic eruptions, whereas the explosives mechanisms dominated during the first stage of the acidic magma extrusion, to end with effusive eruptions of previously degassed magma. According to the morphology of the NNW-elongated lava domes (feeder conduits plugs) and internal measurements carried out on these rhyolitic bodies, fissure conduits are interpreted, which were directly controlled by the Paleozoic inherited structures during the Jurassic extensional tectonic reactivation ascribed to the initial stages of the Gondwana breakup.

Products of an Early Cretaceous extensional mechanism in the central Great Xing'an Range: Felsic plutons in the Zhalantun area and their parental magma compositions and diagenetic processes

Late Mesozoic extensional mechanisms in the Great Xing'an Range (GXR) and relevant crust–mantle interactions are complicated, which result in widespread magmatisms with diverse parental origins and petrogenesises. In this paper, we present the lithology, whole‐rock geochemistry, and zircon U–Pb–Lu–Hf isotopes for granodiorite‐monzogranite rocks (GM) and their enclaves of quartz monzodiorites in the Zhalantun area, east of central GXR. Zircon U–Pb dating results reveal that both GM rocks and their enclaves of quartz monzodiorites yield a nearly identical crystallization age of ~125 Ma. The GM rocks exhibit weak peraluminous and high‐K calc‐alkaline compositions with high SiO2, Na2O + K2O, and Al2O3 contents, and low MgO, Mg#, Fe2O3T, and TiO2 contents, which are characterized by fractionated REE patterns, relatively strong negative Eu anomalies, enrichments of LILEs (e.g., Rb, Ba, and K), and depletions of Nb, Ta, P, and Ti. The parental origins for GM samples were juvenile crustal materials, with positive zircon εHf(t) values of +2.5 to +12.9. The whole‐rock and zircon geochemical simulations limit slightly lower GM magma crystallization temperatures (TTi‐Zrn) of 683–824°C and relatively more oxidizing conditions (log(fO2) = FMQ+7.6–FMQ‐6.8), and further confine that they were probably formed by ~10–35% degrees of fluid‐present partial melting of lower to middle crustal materials, with fractional crystallization and/or residual mineral phases of plagioclase, amphibole, clinopyroxene, and garnet. By contrast, the enclaves of quartz monzodiorites possess lower SiO2 and Na2O + K2O compositions, but higher MgO, Mg#, Fe2O3T, and TiO2 values, which are consistent with metaluminous–weak peraluminous and high‐K calc‐alkaline series. All quartz monzodiorites exhibit sub‐parallel right‐declined REE patterns, negligible to none Eu anomalies, enrichments of LILEs, and depletions of Nb, Ta, and Ti. Combined with mineral morphology and disequilibrium structures, the whole‐rock geochemical and Hf isotopic simulations imply that parental magmas for quartz monzodiorites were hybrid mixtures between ~50% host GM acid magmas and ~50% ancient crust compositions, with a wide range of zircon εHf(t) of +14.0 to −7.7, slightly higher TTi‐Zrn of 710–835°C, and lower log(fO2) of FMQ+8.2–FMQ‐10.5. Taken together, felsic plutons in the Zhalantun area correspond to an episode of extensional mechanism‐related crust–mantle interactions, which might be induced by either the Mongol‐Okhotsk domain or Palaeo‐Pacific tectonic domain, or both of them.

Provenance of Pleistocene Sediments in West Sarawak and Evidence for Pliocene Acid Magmatism in Central Borneo

Quaternary deposits in Borneo are commonly not assigned to any formation or group and are usually not studied in great detail but are important for understanding of the Pleistocene to Holocene climate and drainage evolution. This study presents a detrital zircon provenance analysis of two possible (Plio-) Pleistocene fluvial deposits in West Sarawak, indicating two very different source areas and paleo-river drainages. Those paleo-rivers resemble the present-day drainage but show much higher energy level deposits associated with higher sedimentation rates possibly as a function of (Plio-) Pleistocene climate and hinterland exposure. The deposits at Kampung Jangkar in western West Sarawak were entirely sourced by the uplifted Pueh batholith. In contrast, sediments in Petra Jaya district (northern Kuching city) were sourced by recycling of the Kayan Sandstone near the Bungo Range in the area of the town Bau. The Petra Jaya sediments have abundant Pliocene and some Late Miocene zircons. The Niut Volcanics south of West Sarawak are of this age, but are basic and could not yield significant zircons. It is therefore concluded that zircons came from acid igneous rocks of Pliocene age, as well as the Kayan Sandstone and Middle Miocene Bau Suite igneous rocks, where they formed a highland in the Bau-Bungo Range region which has been entirely removed by erosion.

Chemical and isotopic characterization of the thermal fluids emerging along the North\u2013Northeastern Greece

Hydrochemical and isotopic characteristics of fluids from major geothermal fields of middle/low temperature in N/NE Greece are examined [basins: Strymon River (SR), Nestos River Delta (ND), Xanthi–Komotini (XK), Loutros–Feres–Soufli (LFS) and Rhodope Massif]. The geodynamic context is reflected to isotopic/chemical composition of fluids, heat flow values and elevated CO2 concentrations in emitted fluids. B and Li are derived from leaching of the geothermal systems hosting rocks. δ18OH2O, δ18OSO4, δ13CCO2 values and chemical compositions of Cl, B and Li of geothermal discharges suggest two distinct source fluids. Fluids in SR exhibit high B/Cl and Li/Cl ratios, suggesting these constituents are derived from associated magmas of intermediate composition (andesitic rocks). Geothermal discharges in LFS exhibit low B/Cl and Li/Cl ratios, implying acid (rhyolitic) magmatism. δ13CCO2 and CO2/(CO2 + 105He) ratios in the west part, suggest fluids affected by addition of volatiles released from subducted marine sediments. For the eastern systems, these ratios suggest gas encountered in systems issued from mixing of crustal and mantle-derived volatiles. Isotopic geothermometers reflect, for the same direction, equilibrium processes more (LFS, XK) or less (SR) pronounced and discriminate the geothermal field from low to middle [SR, ND (Erasmio)] and middle to high enthalpy [ND (Eratino), LFS, XK].

Magmatic Water Content and Crustal Evolution Control on Porphyry Systems: Insights from the Central Asian Orogenic Belt

Abstract Porphyry systems, the most important reserves of Cu and Mo with significant Au, are genetically linked to the emplacement of hydrous and oxidized intermediate to acidic magmas, in response to temporal and geochemical evolution of crust in orogenic terranes. In this study, comprehensive whole-rock and zircon geochemical and isotopic datasets of intermediate to acid igneous rocks were integrated to characterize the crustal evolution and metallogeny of porphyry deposits in the Central Asian Orogenic Belt (CAOB). The ore-forming porphyries of Cu ± Au ± Mo and Mo deposits have higher but largely overlapped ΔFMQ (the proxy for oxygen fugacity) with those of the barren igneous rocks. However, the ore-forming porphyries of Cu ± Au ± Mo deposits are characterized by distinctly higher whole-rock V/Sc and zircon Eu/Eu* (both are proxies for water content) than barren rocks. Furthermore, the V/Sc ratios positively correlate with the Cu tonnages of calc-alkali porphyry Cu deposits, suggesting that magmatic water contents may yield the first-order control on metal endowment. The general decrease of V/Sc from the Paleozoic to Mesozoic, combined with the negative correlation of V/Sc with K2O and SiO2, also indicates the gradual evolution of crust in CAOB clearly controls the end members of porphyry-type systems (i.e., Cu‒Au and Mo deposits). The crustal residence age (TRes, the time difference between Nd depleted-mantle model age and the crystallization age) of c. 500 Ma from the Nd isotopes is proposed as the threshold distinguishing porphyry Cu ± Au ± Mo (TRes < 500 Ma) and Mo deposits (TRes > 500 Ma) in the CAOB. The coupled zircon Hf isotopes and crustal thickness reveal that the fundamental crustal architecture in the eastern and western CAOB had been built by the Late Permian and Late Carboniferous, respectively, highlighted by the converging trends of εHf(t) commencing at c.250 Ma in the eastern segment and c.300 Ma in the western segment of CAOB, indicating reworking and homogenizing of juvenile crust after collision. In the eastern CAOB, porphyry Cu ± Au ± Mo deposits were formed by juvenile materials in thin island arcs, while porphyry Mo deposits were formed by reworked materials in the thickened orogenic crust after c.250 Ma. In the western CAOB, porphyry Cu deposits in the Balkhash region during the Late Carboniferous were formed in thickened continental crust (generally > 40 km), genetically linked to the culmination of world-wide magmatic addition rates (MARs) triggered by accelerated production of the juvenile crust, in contrast to the porphyry Cu ± Au ± Mo deposits formed in thin island arc (generally < 40 km) during the Early Paleozoic. This study tests the zircon ΔFMQ as proxy for fO2, and zircon Eu/Eu* and whole-rock V/Sc ratios as proxy for water content. It highlights that whole-rock V/Sc ratio is a favorable index for the Cu tonnages of porphyry Cu ± Au ± Mo deposits, and that the distinct porphyry-type mineralization in the CAOB is controlled by the crustal evolution reflected by crustal composition and thickness.