Rhyolitic Rocks Research Articles

Age and petrogenesis of the Yingyangguan volcanic rocks: Implications on constraining the boundary between Yangtze and Cathaysia blocks, South China

The Yingyangguan (YYG) Group is largely exposed in the possible suture zone between Yangtze and Cathaysia blocks, therefore crucial in understanding the amalgamation history of the South China Block. The LA-ICP-MS zircon UPb dating of andesitic and rhyolitic rocks from this Group yield crystallization ages of 822–816 Ma and ca. 765 Ma, respectively. The whole rock geochemistry indicates that the YYG andesitic rocks are analogous to the high-Mg sanukitoids from the Setouchi volcanic belt. They are significantly enriched in large ion lithophile elements (LILEs, eg., Rb, Ba, Pb) and light Rare Earth Elements (LREE), and depleted in high field strength elements (HFSEs, eg., Nb, Ta, Ti) and heavy Rare Earth Elements (HREE). They show enriched Nd isotopic signatures (εNd(t) = −7.3 to −0.9), and highly variable zircon εHf(t) characteristics (−12.4 to +6.8). High La/Ta, (Hf/Sm)N, Th/Yb and Th/La ratios underline derivation of the primary magma from a subduction melt metasomatized lithospheric mantle, indicating an arc-setting. Geochemical characteristics of ca. 765 Ma rhyolitic rocks point toward S-type granite affinity and enriched whole-rock Nd isotopic (εNd(t) = −7.9 to −6.9) and zircon Hf isotopic (εHf(t) = −12.9 to −5.0) signatures suggest an upwelled asthenosphere triggered partial melting of a recycled sedimentary source and intra-plate rift setting. Therefore, volcanic rocks from the YYG Group record a transition from a subduction setting during 822–816 Ma to a rift environment at ca. 765 Ma, coeval with the tectonic transition events recorded along the southeastern margin of the Yangtze Block, but distinct from the Cathaysia Block. Together with published geophysical data and the pieces of geochemical evidence, the similarity in the Neoproterozoic tectonic history between the YYG Group and southeastern Yangtze margin implies the existence of the southwestern boundary between Yangtze and Cathaysia blocks, to the east of the YYG Group.

Ammonium abundance and short-wave infrared absorption spectra of altered rocks

Rhyolitic rocks hosting the El Zapote epithermal Ag deposit are pervasively altered and contain ammonium ranging from 290 to 1750 ppm. High ammonium values are found in samples containing abundant fine-grained illite. This fine-grained illite shows overall low ratios of K/(Al + Fe), likely due to ammonium substituting K + . Samples containing high ammonium, greater than 1000 ppm in the proximity of Ag-bearing veins, show distinct absorption features of ammonium in short-wave infrared (SWIR) absorption spectra. Samples containing ammonium ranging from 500 to 1000 ppm show mixed absorption signatures; some show prominent absorption features related to ammonium, whereas others have no recognizable features. There is no discernible absorption feature related to ammonium for samples containing less than 500 ppm NH 4 . The data suggest that SWIR spectroscopy is useful in locating the proximal areas to mineralization, but the extent of ammonium alteration is much larger than that identified by SWIR spectroscopy. Nitrogen isotope compositions of ammonium are similar to those of sedimentary rocks, suggesting that the source of ammonium in altered rhyolite is sedimentary basement rocks in the area. Supplementary material : bulk rock composition is available at https://doi.org/10.6084/m9.figshare.c.5015663

Radiological hazard parameters of natural radionuclides for neoproterozoic rocks from Wadi Um Huytat in central eastern desert of Egypt

The aim of this study is to evaluate the radioactivity levels and radiological impacts of representative neoproterozoic rocks: monzogranite, dokhan volcanics, syenogranite, granodiorite, gabbro and metabasalt samples collected from Wadi Um Huytat region in central eastern desert of Egypt using HPGe detector and Alpha GUARD. Remotely sensed data (Landsat 8 OLI) also used in order to appear the high reflectance appearance of neoproterozoic rocks which contain high concentration of radioactive elements. Rhyolite rocks have the highest average concentration of radionuclides of 226Ra, 232Th, 40K and 222Rn. Our results aimed to shed more light on the spatial distribution of natural radionuclides in neoproterozoic rocks.

Lithospheric mantle, asthenosphere, slab and crustal contribution to petrogenesis of Eocene to Miocene volcanic rocks from the west Alborz Magmatic Assemblage, SE Ahar, Iran

AbstractSignificant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ∼42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ∼19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.

ELEMENTS DISTRIBUTION FOR THE UPPER SANDSTONE MEMBER OF THE ZUBAIR FORMATION IN ZUBAIR OIL FIELD, SOUTHERN IRAQ

The upper sandstone member (Z/2) is one of the most important reservoirs in the Zubair Oilfield, south of Iraq. It belongs to the upper part of the Zubair Formation, the Lower Cretaceous age, and deposited in deltaic environments. The upper sandstone member consists of several sandstone units which are isolated by the shale units. Major trace and rare earth elements (REEs) of the upper sandstone member in Zubair Formation have been analyzed. Six core samples were used to determine the source of sediments, paleo-environment redox condition and elements concentrations. The geochemical analysis data indicate that the source of sediments rock was influenced by a low to moderate degree of weathering conditions in the Hammar and Safwan Domes. The results have shown that all samples were formed in an oxic environment, with humid paleoclimate. Geochemical analysis data indicate that all rock samples are fundamentally derived from rhyolitic and andesitic rocks more a felsic source. The results show that the samples are mainly of marine organic matters input. The values of Chemical Index of Alteration in the Hammar Dome range from 13.05 to 24.37, while from 23.72 to 77.22 in the Safwan Dome. These results may attribute to fine and medium clastic sedimentary rocks in the H sandstone unit, with a low degree of weathering in the Hammar Dome, and high to medium with slightly higher abundance of clayey silts in the Safwan dome.

Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.

Early Cretaceous bimodal magmatism related epithermal mineralization: A case study of the Gaosongshan gold deposit in the northern Lesser Xing’an Range, NE China

Low-sulfidation (LS) gold mineralization is partially associated with bimodal volcanism in origin. Sources of ore-mineralized fluids and relevant dynamic processes remain poorly understood. To better understand these issues, a study including deposit geology, petrology, zircon U–Pb dating, and element and Sr–Nd–Hf isotope geochemistry of bimodal volcanic rocks at Gaosongshan was conducted. The bimodal volcanic rocks are composed of andesitic rocks (basalt andesite–andesite–trachyandesite–andesitic pyroclastic) of the Meifeng Formation and overlying rhyolite lava of the Fuminhe Formation cut by equivalent porphyry stocks, which contain pyrite-bearing granular-textured quartz cavity with a sugary appearance, with trapped single-phase magmatic fluids. The andesitic suite formed at approximately 106–108 Ma, prior to eruption of the younger rhyolitic magmas (105.6 ± 1.7 Ma; n = 13, MSWD = 1.2). The andesites show calc-alkaline affinities, which is a subduction-related characteristic, and show moderate La/YbN (7.91–11.36) and weakly negative europium anomalies (0.80–1.02), indicating a source in the asthenospheric mantle with varying contamination by both subduction and continental materials. A-type rhyolitic rocks are geochemically comparable to the andesitic suite, and their higher La/YbN (13.01–30.05) and lower Eu/Eu* (0.66–0.73) indicate evolved products of andesitic magmas. The estimated oxygen fugacity of dry (＜1.2% water) rhyolite lava is FMQ −1.87, whereas rhyolite porphyry is more oxidized (FMQ + 0.12) and water rich (2.5–2.8%) with probably being water saturated at 2.0–2.4 km depths. Therefore, it is assumed that the essential constraints of bimodal magmatism on epithermal gold mineralization are sequential processes from ascending and gradual differentiation of crustal-level mantle-derived basaltic magmas to releases of hydrothermal solutions from the silica-rich magma reservoir at shallower levels. Most importantly, the oxygen fugacity of and the fluid fluxes from dry rhyolitic melts are critical restricting parameters for the limited reserves at Gaosongshan; thus, this research contributes objective evidence for investigations of deposit genesis and prospecting targets.

Geochronology and geochemistry of Neoproterozoic Hamamid metavolcanics hosting largest volcanogenic massive sulfide deposits in Eastern Desert of Egypt: Implications for petrogenesis and tectonic evolution

The Gebel Abu Hamamid area forms the middle part of Shadli Metavolcanics Belt of the South Eastern Desert, which hosts the largest volcanogenic massive sulfide deposits in Egypt. The host-rock succession belongs to the Younger Hamamid Metavolcanics (YHM) group, consisting of mafic and felsic lavas with variable volcaniclastics. Although several studies have attempted to clarify the crustal evolution of the Neoproterozoic basement rocks of the South Eastern Desert, researchers have yet to develop a deep understanding of the petrogenesis, tectonic setting, and ages of these host rocks. In the current study, we newly report whole-rock geochemistry, mineral chemistry, and in-situ U–Pb zircon dating from basaltic to rhyolitic rocks of the YHM group to describe their magmatic evolution. Results indicate that the mafic rocks are characterized by tholeiitic affinity and arc-like geochemical signatures with significantly enriched large-ion lithophile elements and depleted high field strength elements. The parental magma was likely generated by partial melting (~10–15%) of a spinel-lherzolite source mantle that was metasomatized by fluids from a former subduction event, followed by magma ascent under low pressure and low oxygen fugacity conditions. The felsic lavas (~695 Ma) are characterized by subduction-related geochemical characteristics with significant enrichments in Zr, Hf, and Sm. They formed from lithosphere-derived magma enriched by melts from the ancient subducted slab, and later experienced an assimilation-fractional crystallization (AFC) process. The YHM group was formed under extensional geodynamic conditions (back-arc basin setting). The zircon grains display hydrothermal overgrowth at the rims, most likely due to regional metamorphic and hydrothermal changes. This characteristic is likely the effect of hydrothermal fluids associated with the Jurassic-Cretaceous tectonothermal event in the South Eastern Desert.

Bio-weathering of a uranium-bearing rhyolitic rock from Xiangshan uranium deposit, Southeast China

Uranium mining and processing has left a serious problem in terms of waste disposal. Microbially mediated U release from crystalline rocks has made the problem even more challenging, but the specific mechanisms for such U release have remained elusive. In this work, bio-weathering experiments of a U-bearing rhyolitic rock were conducted to investigate the rates and mechanisms of microbially-induced U mobilization. Four bacterial isolates from Xiangshan uranium deposit, one of the largest volcanic rock-hosted U deposits worldwide, were able to grow in an oligotrophic medium (OM) in the presence of a U-bearing rhyolitic rock. These bacteria liberated U from the rock through a combination of acidolysis, metabolite-promoted complexation, and oxidative dissolution, among which U mobilization via complexation with citrate was one of the most significant mechanisms. Metabolite secretion was strain-specific, which accounted for the differences in U mobilization efficiency among different bacterial isolates. In particular, Microbacterium sp. 6-1 selectively colonized the surface of U-bearing minerals and mobilized a substantially higher amount of U than other isolates, suggesting a potentially active, mineral-specific attachment and bio-weathering mechanism. Our results demonstrated a high potential and capacity of native microorganisms to mobilize U from U-bearing rock, posing a great challenge for long-term U waste disposal, but also providing an opportunity for U recovery from mine tailings.

Dynamic rockfall risk analysis

Rockfall is a dangerous hazard on steep and susceptible slope. However, it is not easy to identify potential rockfalls on cliff mountains. Traditionally, rockfall risk analysis ignores the temporal evolution of risk due to the changes of the elements at risk, or changes in the annual probability. This may lead to an underestimation of risk in time. In this paper, we present an innovative approach for a dynamic risk analysis, and we demonstrate this approach for the ShenXianjJu scenic area case study, where rockfall represent a threat for thousands of tourists each year. Moreover, we studied the role of rock mass quality in controlling the rockfall potential and rockfall volume, since this may lead to significant changes of rockfall risk in space. We describe the use of Unmanned Aerial Vehicle, Terrestrial LiDAR and detailed field surveys to identify 34 potential rockfalls on slopes where historical rockfalls have occurred. Within ignimbrite and rhyolite rock masses rockfall blocks range in size between large and very large. Andesite derived rockfalls are characterized by medium-size to large block size. Faulted and fractured ignimbrite and rhyolite rock masses within the fault damage zone exhibit small-very small rockfall block sizes. Based on the identified potential rockfalls in the study area, we quantified the dynamic risk by considering the temporal–spatial changes of tourist activity. To demonstrate the methodology, two potential rockfalls on two heavily-used tourist routes were selected. For these scenarios we quantified the annual probability of occurrence, the reach probability, the dynamic temporal-spatial probability, and the vulnerability of tourists. The dynamic temporal probability was also calculated considering different visiting periods (e.g., working days, weekends, holidays), showing significant changes in the risk level among the different visiting periods and with time. Some of the investigated scenarios were within the ALARP zone in the first years (i.e. PLL on workday in 2017–2019), and subsequently became unacceptable (i.e. PLL on workday in 2020). As a conclusion, the research demonstrates that the traditional static risk analysis would lead to a significant rockfall risk underestimation.

Towards Identification of Zircon Populations in Permo-Carboniferous Rhyolites of Central Europe: Insight from Automated SEM-Mineral Liberation Analyses

Zircon is a main mineral used for dating rhyolitic magmas as well as reconstructing their differentiation. It is common that different populations of zircon grains occur in a single rhyolitic sample. The presence of both autocrystic and antecrystic zircon grains is reflected in their strongly varied chemical compositions and slight spread of ages. However, postmagmatic processes may induce lead loss, which is also recorded as a spread of zircon ages. Therefore, new approaches to identify different zircon populations in rhyolitic rocks are needed. In this study, we suggest that detailed examination of zircon positions in the thin sections of rhyolitic rocks provides valuable information on zircon sources that can be used to identify autocrystic and antecrystic zircon populations. Automated Scanning Electron Microscope (SEM) analyses are of great applicability in determining this, as they return both qualitative and quantitative information and allow for quick comparisons between different rhyolite localities. Five localities of Permo-Carboniferous rhyolites related to post-Variscan extension in Central Europe (Organy, Bieberstein, Halle, Chemnitz, Krucze) were analyzed by automated SEM (MLA-SEM). The samples covered a range of Zr whole rock contents and displayed both crystalline and glassy groundmass. Surprisingly, each locality seemed to have its own special zircon fingerprint. Based on comparisons of whole rocks, modal composition and SEM images Chemnitz ignimbrite was interpreted as containing mostly (or fully) antecrystic zircon, whereas the Bieberstein dyke was shown to possibly contain both types, with the antecrystic zircon being associated with disturbed cumulates. On the other hand, Organy was probably dominated by autocrystic zircon, and Krucze contained dismembered, subhedral zircon in its matrix, whereas Halle zircon was located partly in late veins, filling cracks in laccolith. Both localities may, therefore, contain zircon populations that represent later stages than the crystallization of the main rhyolitic body.

Basement faults deduction at a dumpsite using advanced analysis of gravity and magnetic anomalies

ABSTRACTA study of the fracture systems in the basement of the Matatlan urban waste dump of Guadalajara City, near the western edge of the Rio Grande de Santiago canyon (southwestern Mexico), is reported. The Matatlan dumpsite was developed on top of andesitic and rhyolitic rocks. Measurements at outcrops indicated that N–S (0°–15° and 165°–180°), N45–90E and N105–135E fracture systems affect the basement. Statistical analysis (micromagnetic method) of magnetic lineament azimuths indicated the presence of two lineament groups. The major one (N60–120E) has a maximum at N90–105E, and a secondary group (N120–165E). The gravity anomalies are featured by a major group (N60–120E, with maximums at N60–75E, and N105–120E) and a secondary group (N120–180E). Thus, a good to fair correlation exists between the measured fracture orientations and the lineaments featuring the gravity and magnetic anomalies. Horizontal derivative and tilt angle depicted horizontal limits of anomaly‐causative bodies. Results of the micromagnetic method are consistent with fracture orientations and with the limits established through edge‐detection techniques. Lineaments established by means of the gravity and magnetic Euler deconvolution are also consistent. Gravity and magnetic Euler deconvolution helped to estimate the depth extent of the limits of gravity and magnetic common sources. Euler deconvolution indicated the fault or contact nature of these lineaments. Furthermore, Euler deconvolution indicated slopes of the limits of gravity and magnetic sources. The fault or fracture nature of the major lineaments was finally confirmed by forward modelling (forward 2D cooperative modelling of gravity and magnetic profiles). For example, lineaments delimiting the northern depression could be characterized as normal faults. In general, this study indicates that it is possible, based on gravity and magnetic data, to characterize fractures affecting the basement of a dumpsite, a key parameter in hydrogeophysical studies. In particular, through this study, the inferred structures could be associated with regional tectonic faulting, which might provide migration paths to unhealthy lixiviate outside the dumpsite, giving rise to the contamination of the neighbouring environment. In particular, the study explains the presence of an unhealthy leachate waterfall to the northeast of the dumping site. Heavy metal contents in samples from this waterfall support that lixiviates originate at the dumpsite.

Magmatic clasts in the Saldanha ignimbrites, and Trekoskraal beach pebbles: missing pieces from the volcanic puzzle in the Cape Granite Suite

Abstract Previous studies have shown that the 542 Ma Saldanha eruption centre, situated on the west coast of South Africa, consists of the basal Saldanha Ignimbrite, which is partly intermingled with and partly overlain by the Jacobs Bay Ignimbrite, both having S-type characteristics. Together, the Saldanha eruption centre and the Postberg eruption centre (to the south, across Saldanha Bay) form part of the volcanic phase of the Cape Granite Suite. The lowermost parts of the Jacob’s Bay Ignimbrite contain magma clasts that are chemically dissimilar to their host ignimbrites. Some clasts are recrystallized ignimbrites that are chemically distinct from any unit that has outcrop expression, and are inferred to form part of a previously unrecognised volcanic event. Other clasts are non-fragmental, hypabyssal rocks that were evidently intruded prior to the explosive intracaldera eruptions that formed the Saldanha ignimbrites. Beach cobbles and pebbles, sampled from the Trekoskraal coastal area, include three texturally and chemically distinct groups – rhyolitic ignimbrites, rhyolitic hypabyssal rocks and dacitic hypabyssal rocks or lavas. Only a minority of these rocks (from the rhyolitic ignimbrite group) show some chemical affinities with the Saldanha Bay ignimbrites. The other pebble types show neither chemical nor textural similarities with the rocks of either the Saldanha or the Postberg eruption centres. The pebbles and cobbles also have no chemical affinities with any of the granitic intrusive rocks of the region. Their chemical and isotopic characteristics suggest that a variety of different magma batches were formed through partial melting of heterogeneous Malmesbury Group metamorphic rocks, at depth. LA-ICP-MS dating of igneous zircon crystals from two of the pebbles (a low-silica rhyolite ignimbrite and a dacite) yielded magmatic ages of 540 ± 4 Ma and 533 ± 4 Ma, respectively. Taking uncertainty brackets into account, these new dates suggest that there may have been a 3 Myr hiatus in eruptive activity, between the eruptions responsible for the exposed Saldanha ignimbrites and the eruptions that produced the volcanic units from which the pebbles were derived. This confirms the inference that there was a previously unidentified, later, volcanic event associated with the Cape Granite Suite in the Saldanha area.

U–Pb ages and Hf isotopic compositions of zircon from the Early Miocene Kestanbol Magmatic Complex in NW Anatolia (Turkey): Implications for crustal contribution in the post-collisional magmatism

The Tertiary post-collisional magmatic activity in NW Anatolia occurred after the Late Cretaceous to Early Tertiary collision of the Sakarya and Anatolide-Tauride blocks and the closure of the Izmir-Ankara Ocean. The magmatism resulted in coeval emplacement of granitic plutons and andesitic volcanic rocks along the Izmir-Ankara-Erzincan Suture Zone (IAESZ). The Kestanbol Magmatic Complex (KMC) on the Biga Peninsula (W of the IAESZ), is one of the representative magmatic complexes in the Sakarya Zone situated north of the IAESZ. Monzo-dioritic (syeno-diorite) plutonic rocks show close relationships with dacitic, trachyandesitic and rhyolitic rocks in space and time which are characterized by enrichment in large-ion lithophile elements (LILE) and depletion in Nb, Ta, P and Ti, interpreted as reflecting an arc-related origin.Trace element concentrations and isotope data indicate that the KMC (the Kestanbol pluton, and the Ayvacık and the Balabanlı volcanic rocks) was formed by differentiation of mantle-derived mafic melts via assimilation of ancient continental materials coupled with mineral fractionation. The zircon ages for the Kestanbol pluton and the Ayvacık volcanic rocks are 22.24 and 21.58 Ma, respectively, whereas zircon U-Pb age data suggest that the Balabanlı volcanic rocks (19.33 ± 0.30 Ma) is ~3 million years younger than the Kestanbol pluton and the Ayvacık volcanic rocks. Hf isotopes in zircon (εHf(t) ranging from −5.7 to −0.7), combined with U-Pb ages, support the hypothesis that the mantle source of the Kestanbol magmatism was severely modified by crust-derived metasomatic fluids during the previous subduction.

Zeolites in acid vitreous rocks, southern Mendoza, Argentina

A mordenite-rich deposit located in the south of the province of Mendoza (Argentina) was studied. The host rock in this area has not been previously described and corresponds to rhyolitic lavas of high-K calc-alkaline series. Petrographic, mineralogical and geochemical analyses of fresh and altered rocks are shown, and a proposal for the alteration process and the stratigraphic position of the protolith is presented. The main mordenite-rich sectors developed in zones of autobreccias of high permeability, with minor clinoptilolite, smectites, secondary K-feldspars, opal-CT, and colloform silica. The proportion of relict glass vs. alteration minerals is variable and mainly depends on the texture (permeability) of the host rock. The studied rhyolites overlie the Upper Cretaceous sediments of the Neuquén Group and are covered by Tertiary basalts of the Molle Eruptive Cycle and Quaternary sediments. Geochemically, they present similarities with rhyolitic rocks of Cordón del Burrero Volcanic Complex of Lower to Middle Miocene age.

An overview of the Muara Laboh geothermal system, Sumatra

The Muara Laboh geothermal system is a liquid-dominated, fracture-controlled reservoir showing some characteristics of both intrusion-related and fault circulation systems. The developed reservoir has moderate to high-temperature (230–310 °C) with a long NNW outflow (160–230 °C) extending to the Sapan Malulong boiling chloride springs. Reservoir fluids generally have low salinity (∼400-1600 ppm Cl) with benign chemistry and low non-condensable gas (NCG) content (∼0.5 to 2 wt% in steam). The proven reservoir is divided into distinct SW and NE sectors. The principal deep upflow zone (270 to 310 °C) is located in the SW and is associated with Patah Sembilan volcano and satellite vents (Anak Patah Sembilan). The SW upflow is characterized by relatively low salinity (∼400-600 ppm Cl), whereas fluids in the NE emerge from a second upflow along faults and fractures at 240–250 °C (1200–1600 ppm Cl) and ascend to an initial-state steam cap near the Idung Mancung fumarole. The shallow NE part of the geothermal system is hosted mainly by Quaternary to Miocene Age andesitic to rhyolitic rocks, whereas the deeper SW part of the system occurs in a Mesozoic to Cenozoic Age plutonic complex and its host rocks that are cut by younger dikes. Rock porosity and permeability trends can be related to reservoir age, volcanic deposit type, and alteration history. Fracture permeability is high, whereas matrix porosity (<1-7%) and permeability are moderate to low and decrease with depth. The volcanic sequence is capped by tuffs dated at ∼34,000 to 41,000 years BP, incapsulated in debris flows. This sequence may have formed during and shortly after likely sector collapse episodes of the Patah Sembilan volcano. The shallow clay cap of the system wraps around the crater, suggesting that it was excavated during crater formation.Permeable fracture patterns and their origins differ somewhat between the deep SW and the shallow NE reservoir. In the deep SW reservoir, fluids ascend along fractured margins of stock and dike intrusions. Steeply dipping intrusions and open fractures strike from WNW to NNW, subparallel to the main Great Sumatra Fault trend, with a secondary set of N to NE fractures and intrusions parallel with the inferred maximum horizontal stress direction (SHMax). In the shallower NE steam cap and outflow, N to NE steeply dipping fractures dominate, with bounding N to NNW faults. Bulk rock alteration includes shallow argillic and transitional clay zones overlying phyllic and propylitic zones. High-T propylitic alteration (secondary amphibole) and rare potassic alteration (secondary biotite) are associated with intrusion. Vein paragenesis in the SW reservoir indicates that portions of the initially permeable propylitic zone have been sealed by calcite and quartz ± prehnite. Late calcite veins suggest ingress of steam-heated bicarbonate waters contributed to resealing of the system. These relationships suggest that the SW system underwent extensive boiling, fluid loss, and inflow of dilute bicarbonate waters, possibly related to sector collapse and partial excavation of the topseal of the system (now Patah Sembilan crater).

Early Paleozoic magmatism and metallogeny related to Proto-Tethys subduction: Insights from volcanic rocks in the northeastern Altyn Mountains, NW China

Remnants of the Proto-Tethys are mainly preserved in the region between south of the North China-Tarim Block and north of Qiangtang-Sibumasu/Baoshan Blocks. Magmatic-metallogenic events related to the Proto-Tethyan subductions were rarely reported, and the subduction history and polarity of the Proto-Tethyan are still under debate. Here, we presented new data of zircon UPb ages, whole-rock Sr–Nd–Pb isotopes, major and trace elements and zircon Hf isotopes for the volcanic rocks in the northeastern Altyn Mountains. Information of over 14 volcanic-hosted deposits/prospects in the region has been compiled. These volcanic ore hosts consist mainly of basaltic andesite, andesite, dacite and rhyolite rocks. The andesite and rhyolite rocks are newly zircon UPb dated to be Late Cambrian-Early Ordovician (andesite: 490.5 ± 5.2 Ma; rhyolite: 492.6 ± 2.9 Ma and 491.6 ± 5.6 Ma), representing the timing of volcanism and VMS (Volcanogenic Massive Sulfide) mineralization. All the volcanic rocks belong to the high-K calc-alkaline and shoshonite series: the andesite rocks from the Kaladawan area in north of the region display arc geochemical affinities and contain (87Sr/86Sr)i (0.7082–0.7083) and εNd(t) (−9.7 to −7.6), indicating that they were likely formed by partial melting of the mantle wedge with subducted sediment inputs. The rhyolite rocks from the Kaladaban area in south of the region are characterized by high SiO2 (64.46–78.55 wt%), low alkali (Na2O + K2O, 3.46–7.17 wt%), and contain (87Sr/86Sr)i (0.7063–0.7095), εNd(t) (−6.6 to −1.5), and zircon εHf(t) (−5.5 to 5.4), indicating that they were likely derived from partial melting of the lower crust with depleted mantle inputs. Rock assemblage and geochemistry suggest that volcanic rocks in the northeastern Altyn Mountains may have formed in a continental arc setting. Their spatial distributions with respect to the ophiolites in the region suggest that the subduction was likely south-dipping. This subduction-related arc magmatism may have formed the many important VMS and porphyry–skarn deposits in the region.

Late Mesozoic intraplate rhyolitic volcanism in the North China Craton: Far-field effect of the westward subduction of the Paleo-Pacific Plate

AbstractThe Late Mesozoic was characterized by extensive volcanism, crustal extension, lithospheric thinning, and craton destruction in the North China Craton (NCC). Here we investigate the petrology, whole-rock geochemistry, zircon U-Pb geochronology, and Lu-Hf isotope of rhyolitic rocks from the Chicheng region of China along the northern margin of the NCC to constrain their petrogenesis, magma evolution, and associated geodynamic processes. The newly obtained zircon U-Pb age data constrain the eruption age of rhyolitic rocks at ca. 144–114 Ma during the Early Cretaceous with multiple magmatic pulses at ca. 141, ca. 137, and ca. 130 Ma as defined by the age peaks. Zircon Hf isotopic data show markedly negative εHf(t) values of –23.0 to –11.8, and corresponding Hf crustal model ages (TDMC) are in the range of ca. 2650 to 1944 Ma, suggesting magma derivation through melting of Paleoproterozoic crustal materials with minor input of reworked Neoarchean components. Geochemically, the rhyolitic rocks correspond to A-type granites, with a mixed arc- and subduction-related signature, although generated in an extensional intraplate setting through partial melting of the mafic lower crust and upper crustal fractional crystallization. We correlate the late Mesozoic intraplate volcanism to the westward subduction of the Paleo-Pacific Plate and its far-field effect. Lithospheric extension and slab rollback of the Paleo-Pacific Plate are considered as the main triggers for the multiple eruptions. The late Mesozoic volcanism in the study area and adjacent regions also broadly coincide with the tectonic transition from the Paleozoic Paleo-Asian to Mesozoic Paleo-Pacific subduction realm with concomitant compressional to extensional tectonic regime.

The discovery of high-Mg rhyolitic rocks in Peng Tso area, Tibet and its significance for evolution of Bangong-Nujiang Ocean

The discovery of high-Mg rhyolitic rocks in Peng Tso area, Tibet and its significance for evolution of Bangong-Nujiang Ocean

Genesis of the Yaguila Pb-Zn-Ag-Mo skarn deposit in Tibet: Insights from geochronology, geochemistry, and fluid inclusions

The Yaguila skarn deposit with reserves of 52 Mt at 4% Pb, 2% Zn, and 64.3 g/t Ag is located in the eastern segment of the central Lhasa subterrane, Tibet. The stratabound skarn Pb-Zn-Ag orebodies are hosted in the Upper Carboniferous-Lower Permian Laigu Formation, which consists of meta-quartz sandstone, slate, and marble. Molybdenum occurs as quartz-molybdenite ± pyrite veinlets, veins, and partial disseminations in the Paleocene quartz porphyry, slate, and garnet skarn. Zircon U-Pb dating suggests that three major magmatic events occurred in the Yaguila area, producing Cretaceous rhyolitic rock (ca. 128 Ma), Paleocene quartz porphyry (65.6 ± 1.2 Ma), and Miocene granite porphyry (17.8 ± 0.4 Ma). Combined with the published data, the quartz porphyry has variable εHf(t) values of −13.9 to +12.1, (87Sr/86Sr)i values of 0.71718–0.72805, and εNd(t) values of −5.2 to −13.5, whereas the granite porphyry displays relatively restricted εHf(t) values of +2.3 to + 6.5, (87Sr/86Sr)i values of 0.70798–0.71678, and εNd(t) values of −2.8 to −5.5. Geochemical and isotopic (Sr-Nd-Pb-Hf) signatures indicate that the Cretaceous rhyolitic magma was derived from the melting of continental lower crust of the ancient Lhasa subterrane with a strong fractional crystallization. The emplacement of the Paleocene quartz porphyry probably resulted from the magma mixing process between the dominant Lhasa middle-upper crust-derived magma and varying contributions of mantle-derived materials during the collision and Neo-Tethyan subduction slab rollback. The Miocene granite porphyry was likely formed by the melting of thickened Lhasa lower crust, which was triggered by upwelling of hot asthenosphere. Our molybdenite Re-Os data from garnet skarn, together with published data from quartz porphyry and slate, display an excellent isochron age of 64.3 ± 1.6 Ma. Fluid inclusion data from this study reveal that the ore-forming fluids associated with diverse types of molybdenum mineralization were a common magmatic-hydrothermal fluid with moderate to high temperatures (290–400 °C) and moderate salinities (9–22 wt% NaCl equiv.). Our study suggests that the Yaguila Pb-Zn-Ag and Mo mineralization occurred under the same ore system associated with the Paleocene quartz porphyry.