High-silica Rhyolite Research Articles

The Role of Latent Heat Buffering in the Generation of High-Silica Rhyolites

Abstract The physical process of crystal-melt separation is responsible for the accumulation of small to very large volumes (>100 km3) of eruptible rhyolitic melt in the shallow crust. Granitic intrusions, although providing a terminal, time-integrated image of melt segregation processes, host an unmatched record of the physical properties controlling mechanisms and rates of interstitial melt extraction from a crystal-rich source. We applied mass balance calculations and thermodynamic modeling simulations to an extensive bulk rock geochemistry dataset (>150 samples) collected in a Permian upper-crustal granitoid intrusion of the Italian Southern Alps. Textural and geochemical evidence indicate that this intrusion constituted a single, zoned magma body, with a crystal-rich base and a thick (~2 km), high-silica cap (75–77 wt% SiO₂). The large compositional variability of the crystal-rich materials suggests variable degrees of melt extraction efficiency and corresponding terminal porosities. Specifically, the loosely bimodal distribution of porosity values (φ) indicates that at least two distinct melt segregation mechanisms were operating in this system, which produced both high (0.65–0.45) and low terminal porosities (0.45–0.25) in the crystal-rich, cumulate materials. Modeling of latent heat budget shows that coexistence of cumulate products with differing terminal porosity signature can be explained by melt segregation processes taking place at different depths across a thick, interconnected magmatic reservoir with an initial homogenous water content (~4 wt% H2O). Deep in the mush column, low water activities (aH₂O < 0.5) promoted thermal buffering of cooling magma at high crystallinities, enabling residual melt extraction by percolation through a crystalline framework accompanied by compaction. Instead, at shallower depths, high water activities (aH₂O > 0.5) ensured prolonged magma residence at porosities that promoted crystal melt separation via hindered settling. Distinct melt extraction processes, acting synchronously but at different depths in vertically extensive silicic mush columns, can account for the large volumes of residual, haplogranitic melt mobilized during the relatively short lifespan of upper crustal magma reservoirs (~105 years).

First Report of Tuff in Neoproterozoic Bhima Basin: Field Evidence, Petrography and Geochemical Studies

Abstract We report here the presence of tuffs from the Neoproterozoic Bhima basin unreported hitherto. These are exposed near Hotpet, Hulkal, Kanchankavi, and Ukinal area along two different stratigraphic horizons (1) within carbonates of Shahabad Formation and (2) above shale of Hulkal Formation. Tuffs are of two types: 1. Ash fall tuff and 2. Lapilli tuff, occurring as beds, matrix of a conglomeratic unit and patchy discontinuous layers. Physically ash fall tuff is thinly bedded (2-6 cm) and light-weight, lapilli tuff is hard, compact and individual beds are not discernible in the field. Presence of petrological features like bipyramidal quartz, fiamme structures, embayed quartz grains, blue CL colours of quartz grains, volcanic layering and banding, altered glass, and stretched and elongated volcanic glasses, are clear indications of its volcanic affinity. The tuff is anomalously high in silica content ranging from 71.4-91.2wt% and have low alumina value ranging from 0.75-4.11 wt % compared with that of the normal felsic tuffs, rhyolites and high silica rhyolites. REE in the tuffs can be grouped into two types, 1. Ash fall tuff with enriched LREE and fractionated HREE, 2. Lapilli tuff with flat LREE and HREE pattern. Tuff shows high Hf, overall enrichment of large ion lithophile elements (LILE Rb, Th, K and Pb) and depletion in HFSEs (Nb, Ta, Ti, P). The tuff is probably generated from continental crust source and the depositions are episodic and have more than one eruptive pulses of volcanic activity.

Compositional zoning of the Otowi Member of the Bandelier Tuff, Valles caldera, New Mexico, USA

Abstract The Otowi Member of the Bandelier Tuff erupted at ca. 1.60 Ma from the Valles caldera (New Mexico, USA). It consists of as much as 400 km3 (dense rock equivalent) of strongly differentiated high-silica rhyolite and shows systematic upward variations in crystallinity, mineral chemistry, and trace element concentrations through its thickness, but the major element composition is almost constant and is near the low-pressure granite minimum. Incompatible trace elements in whole pumice fragments and glasses show well-correlated linear covariations. Upward zoning to lower abundances of incompatible trace elements is accompanied by development of overgrowths on quartz and alkali feldspar, although earlier-formed interiors of quartz and feldspar have near-constant compositions throughout the tuff, modified by cation diffusion in the case of feldspar. Melt inclusions in remnant quartz cores show diverse Pb isotope ratios, pointing to a wide range of distinct protoliths that contributed rhyolitic melt to the Otowi magma. Mineral thermometers suggest a modest temperature gradient through the melt body, perhaps of 40 °C, at the time of eruption. Chemical, textural, and mineralogical variations and volume-composition relations through the tuff are consistent with an origin for zoning by melting of a high-crystallinity cumulate layer beneath cognate supernatant liquid to produce denser, remobilized liquid of accumulative composition (i.e., the “modified mush model”). Melting may have occurred in several episodes. The latest of these episodes, probably thousands of years prior to eruption, introduced new rhyolitic liquid into the system and was associated with a thermal excursion, recorded in core compositions of pyroxene, during which much of the earlier crystal mass was dissolved. This left inherited cores and interiors of accumulated quartz and feldspar mantled with new growth having less-evolved compositions (higher Ti, Sr, and Ba). Changing solubility of zircon during cumulate melting produced a reversal of Zr concentrations. There is no clear petrologic evidence of a recharge eruption trigger; nonetheless, compositional zoning resulted mainly from repeated recharge-induced remobilization of quartz-feldspar cumulate. The Otowi system was built, evolved, and modified by several events over the course of a few hundred thousand years.

Mush Architecture and Processes in the Reservoirs of a Supereruption-Scale Magma System, Permian Ora Ignimbrite (Northern Italy)

Abstract The Ora Ignimbrite (~275 Ma) is a crystal-rich (~40%), heterogeneous, supereruption-sized (~1300 km3) rhyolite in northern Italy and the final eruptive product of the Athesian Volcanic Group (289–274 Ma), the largest Permian volcanic district in Europe. Two key outcrops, an intracaldera vitrophyre and an outflow vitrophyre, contain well-preserved, glass-bearing juvenile material. We investigate at what scale heterogeneities existed in the Ora magmatic system by studying the geochemical and petrological characteristics of crystal-rich (20–50%) and crystal-poor (<20%) fiamme from the Ora vitrophyres, allowing us to reconstruct critical aspects of Ora's magmatic architecture and pre-eruptive history and granting us further insight into the storage and dynamic processes that occur in large, crystal-rich, silicic systems. Textural analysis of fiamme reveals four types: the intracaldera vitrophyre has very coarse-grained crystal-rich (VCCR) and fine-grained crystal-poor (I-FCP) fiamme, while the outflow vitrophyre has coarse-grained crystal-rich (CCR) and fine-grained crystal-poor (O-FCP) fiamme. The diversity of crystal-poor (I-FCP & O-FCP) fiamme glass trace-element compositions suggests that multiple small crystal-poor magma bodies were dispersed throughout the Ora magmatic system. Compositionally and texturally distinct glass types in single O-FCP fiamme preserve evidence of variable degrees of magma mixing and record different mingling to eruption timescales, revealing that the Ora magmatic system experienced multiple silicic recharge events. In the crystal-rich (VCCR & CCR) fiamme, high crystal contents (VCCR: 40–50%; CCR: 20–40%), large crystals (max grain size: VCCR: 4–10 mm; CCR: 3–4 mm), and interlocking growth textures suggest that these fiamme contain parcels of disaggregated magma mush. High-silica rhyolite glass compositions (77.2–78.3 wt.% SiO2) and extremely depleted Sr (VCCR: <20; CCR: <5 ppm) and Ba (VCCR: <70; CCR: <20 ppm) glass trace-element concentrations indicate that the crystal-rich fiamme glasses represent highly fractionated melts. Crystal-rich fiamme glass trace-element compositions also form discrete, tightly clustered populations, suggesting that multiple crystal-rich, melt-dominated magma bodies existed prior to eruption. We propose that the coexistence of large-scale heterogeneity (e.g. deposit-scale and outcrop-scale) with small-scale homogeneity observed in crystal-rich fiamme is resolved by a model in which at least two separate magma reservoirs, each containing multiple melt-dominant magma bodies, erupted to form the Ora Ignimbrite.

The effect of water on alkali trace element diffusion (Li, Rb, Cs) in silicic melts

Trace element diffusion is a powerful tracer of kinetic magmatic processes such as recharge and fluid exsolution. However, the dependence of diffusivities (D) on melt water contents is poorly quantified, and most trace element diffusivities were determined in water-free melts unrealistic for most natural magmatic systems. Here, we investigate the diffusion of the alkali trace elements Li, Rb, and Cs as a function of water content in rhyodacite, high-silica rhyolite, and peralkaline rhyolite. For the diffusion experiments, homogenous hydrous glass cylinders were produced from trace element-doped and undoped powdered glass with 1–8 wt% H2O. Glass cylinders with the same water content but different trace element concentrations were paired along polished contact surfaces. Diffusion experiments were conducted between 720 and 1100 °C at 100–700 MPa for 5–25 min using gas pressure vessels or a piston cylinder apparatus. Resulting diffusion profiles were analyzed by LA-ICPMS and evaluated by a Monte Carlo iterative fitting procedure for full error propagation. At a given experimental temperature, measured log10D values increase linearly with melt water content, with one order of magnitude increase for Li, two orders for Rb, and three orders for Cs from least hydrous (1 wt%) to wettest (8 wt%) experiment, due to a linear decrease in activation energies. Variations in major element composition only have a minor effect. We illustrate the impact of differential diffusion in hydrous silicic magmas on their trace element budget by coupling our data to a two-phase mechanical model of channelized fluid transport in magmatic systems. Quantifying water-dependent diffusivities provides an important tool to track the exsolution, mass and rate of fluid transport within long-lived shallow mushy magma reservoirs in the Earth’s crust.

Disclosing the most intense magmatic flare-up in southern Tibet: Insights from study of the Pangduo volcanic-plutonic complex

The Gangdese arc underwent the most intense magmatic flare-up of southern Tibet in the early Eocene (peaked at ∼50 Ma), generating widespread plutonic and volcanic rocks both along and across the arc. However, the dynamic mechanism for the magmatic flare-up is mysterious. This paper systematically analyzed the petrology, mineralogy, geochronology, and geochemistry of the early Eocene volcanic and intrusive rocks in the Pangduo basin in the eastern Gangdese arc. These rocks yielded identical zircon UPb ages of ∼50 Ma and similar isotopic compositions. The low-silica (< 60 wt%) rocks, including basalt andesite and trachyandesite, result from low degree (2–3%) partial melting of garnet-bearing lithospheric mantle. Based on textural observations and geochemical studies, coupled with Rhyolite-MELTS modeling and zircon trace elements Rayleigh fractionation models, the following research results were obtained. The high-silica rhyolite, ignimbrite, and granite porphyry are the product of limited melt extraction, while the quartz monzonite belongs to the associated residual silicic cumulates. The quartz monzonitic porphyry and trachydacite show a widespread dissolution structure and high formation temperature of ∼850 °C. Whole-rock geochemical mixing model results indicate that they are generated by mixing products of mafic magma, high-silica melts, and remelting melts of quartz monzonite. The volcanic and intrusive rocks in the Pangduo basin are considered as a volcanic-plutonic complex, resulting from the synergistic effect of melt extraction and cognate magma mixing. Both the complex magmatic processes and the early Eocene magmatic flare-up are attributed to mafic magma injection.

Precaldera Mafic Magmatism at Long Valley, California: Magma-Tectonic Siting and Incubation of the Great Rhyolite System

The iconic volcanic center at Long Valley has released ∼820 km3 of rhyolite in at least 110 eruptions. From 2.2 Ma until 0.23 Ma, products were exclusively rhyolitic, and ∼ 700 km3 were high-silica rhyolite severely depleted in Sr, Ba, and Eu. The rhyolitic interval was preceded by an interval from 3.9 to 2.6 Ma with numerous basalt-andesite-dacite eruptions accompanied by no rhyolite at all. We have now mapped the circumcaldera products of this interval, defined 107 eruptive units, characterized them all chemically and petrographically, and dated many by 40Ar/39Ar. Here we display and describe them by sector around the caldera, interpret the nature of the transcrustal magma system that eventuated in the 35-km-wide Long Valley granite-rhyolite pluton, and analyze regional tectonic factors that did or did not contribute to siting the system.Nine Miocene (12–6 Ma) eruptive units close to Long Valley were followed by a Pliocene flare-up that released >300 mafic eruptions in a SW–NE swath 170 km long, centered across the later site of Long Valley. The basalts and their fractionates are intraplate alkalic products dominated by a continental lithosphere that had long been fluxed by Mesozoic subduction. Tertiary arc volcanism had not impinged on the area of Long Valley. Volumes estimated for the 107 Neogene precaldera eruptive units (only 40 of which exceeded 0.1 km3) total ∼ 27 km3 ± 50%—only ∼3% of the subsequent volume of rhyolite erupted. Such a volume of high-silica rhyolite with ultra-low Sr and Eu is not a product of partial melting but requires as proximate parent a leucogranitic crystal mush that is itself the upper level of a long-lived plutonic reservoir that extends to the lower crust. The 27 km3 of Neogene magma that erupted was a small contingent of the mantle-derived basaltic flux needed to energize (and contribute its fractionated melt to) a 30-km-deep compositionally graded crustal column, which culminated in ∼10,000 km3 of granitoid mush from which 820 km3 of Quaternary high-silica rhyolitic melt escaped and erupted.Pliocene basaltic eruptions ceased at ∼2.6 Ma, probably because the basaltic flux intensified sufficiently to render the mushy upper crust impenetrable. The 2.6–2.2 Ma quiescent interval represented culmination of thermal activation of the plutonic column and refinement of its leucogranitic mushy upper layer, from which extreme melts escaped for the next 2 Myr. The Pliocene mafic swath crosses the Sierran rangefront fault zone coincident with a left-stepping extensional reentrant that also began developing at ∼3 Ma. Moreover, Long Valley overlies a dextral offset in the initial Sr-isotope 0.706 line, which may reflect the rifted or attenuated edge of Proterozoic crust and mantle lithosphere. Concatenation of these three influences may account for siting of intensified edge-focused magmatism that produced the great Quaternary pluton.

Volcano-pluton connections at the Lake City magmatic center (Colorado, USA)

Abstract Exposed at the Lake City caldera (Colorado, USA) is the ca. 23 Ma reversely stratified (rhyolite to trachyte) Sunshine Peak Tuff and post-collapse syenite and monzonite resurgent intrusions. Existing models for this system suggest that the rhyolites are related to the trachyte and resurgent syenite through fractional crystallization, separation, and remobilization (crystal mush model), and that multiple magma batches were involved in the system (Hon, 1987; Kennedy et al., 2016; Lubbers et al., 2020). We use U-Pb zircon CA-ID-TIMS-TEA and zircon trace-element modeling to further probe age and geochemical relationships between the extrusive and intrusive units. Zircon ages and compositions from the erupted units and the syenite overlap, suggesting these magmas were related and may have mixed prior to eruption. Results from the monzonite suggest it was a contemporaneous but distinct magma batch that mixed with parts of the larger system. Trends in zircon geochemistry are decoupled from time, reflecting a complex history of accessory mineral saturation and mixing of magma batches, and a distinct high-Hf population of zircon grains hints at the existence of an additional, independent batch of rhyolitic magma in the system. The new ages we present shorten the lifetime of the Lake City magmatic system from 80 to 300 k.y. (Bove et al., 2001) to 60 to 220 k.y. and suggest the high-silica rhyolite magma crystallized over a minimum of ~160 k.y. This latter timescale likely reflects a protracted history that includes differentiation of a parent melt prior to extraction of eruptible high-silica rhyolite magma.

Pre-Eruptive Magma Configurations and Petrogenetic Relationships of the Rattlesnake Tuff, Oregon–Insights From Spectacularly Banded High-Silica Rhyolite Pumices

The 7.1 Ma Rattlesnake Tuff (RST) of eastern Oregon is a widespread and voluminous (&amp;gt;300 km3) ignimbrite composed of 99% crystal poor (≤1%) high-silica rhyolite (HSR) and &amp;lt;1% dacites. Basaltic andesitic to basaltic inclusions within dacites are samples of underpinned mafic magmas. The RST HSR is comprised of five increasingly evolved compositional Groups (E–A), and HSR pumices range from white to dark grey, often co-mingled in spectacular banded pumices. Previously, Groups were interpreted as rhyolites generated by crystal fractionation within a single reservoir, where more evolved rhyolite melts formed from relatively less evolved rhyolite parents. To reassess compositional HSR Groups and their implications for tapping a single or multiple rhyolite reservoirs as well as reevaluating the petrological relationships among groups, we focus on large banded pumices for geochemical analysis. Statistical analysis of existing and new data verified these five compositional Groups and gaps, best characterized by variations in Ba, Eu/Eu*, Eu, FeO*, Hf, and Zr. Wet-liquidus temperatures, storage temperatures, and storage pressures calculated for all HSR Groups indicate similar pre-eruptive conditions (∼6.1–7.5 km depth; storage temperatures of ∼805–895°C). Differentiation trends, trends in storage pressure and temperature, and lack of crystal-rich tuff or country rock corroborate existing models for HSRs that involve a single, density-stratified magma reservoir prior eruption. Density differences are sufficient to prevent convection between layers of HSRs in a single reservoir when water content increases from 2–4 wt% from Groups E–A. However, if HSRs do not represent a liquid line, it is possible to generate HSRs through batch melting of various regional country rock. Yet, HSRs would still accumulate within the same storage zone, where density variations kept HSRs from mixing until eruption when these banded pumices formed. In either scenario, our study underscores the significance of water content and density variations for accumulating rhyolite magmas in a contiguous magma body without mixing. This has implications for other compositionally heterogenous rhyolitic ignimbrites where natural samples do not provide comparable evidence to argue for pre-eruptive confocal storage of different rhyolite magmas as is the case for the Rattlesnake Tuff.

The Petrogenesis of Icelandic High-Silica Rhyolites

The Petrogenesis of Icelandic High-Silica Rhyolites

Lithofacies architecture and paleoenvironment of a Paleoproterozoic volcano-sedimentary sequence: Insight into rift-related volcanism during supercontinent assembly

The greenschist to lower-amphibolite facies, Proterozoic (Orosirian: ca. 1883–1848 Ma) Aillik Group in Labrador, Canada, comprises a well-preserved example of subaqueous to intermittently subaerial volcanism and sedimentation. The group consists of interlayered sedimentary, volcaniclastic, pyroclastic and bimodal volcanic units that exhibit well-preserved depositional and structural relationships. Cumulatively, these lithofacies significantly demonstrate the evolution of Paleoproterozoic volcanism during the assembly of the supercontinent Nuna. A minimum stratigraphic thickness of 14.9 km of the Aillik Group is exposed along the coast of Labrador and is divided into two successions. The western stratigraphic succession consists of subaerial to subaqueous sandstone, siltstone, conglomerate, resedimented tuffaceous sandstones, and tuffaceous conglomerate with lesser rhyolite flows and felsic tuff. The eastern stratigraphic succession is dominated by felsic tuff and rhyolite, lesser pillowed to massive mafic lavas, and in situ and resedimented mafic tuff. Mafic rocks are basaltic, whereas felsic rocks comprise high-silica rhyolites and tuffs. Primary subaqueous pyroclastic deposits are indicative of explosive activity, an observation also supported by the presence of accretionary lapilli and an abundance of lithic fragments in epiclastic deposits. The volcano-sedimentary lithofacies of the Aillik Group are consistent with that observed in modern intra-arc or arc-rift settings, and the succession is interpreted to have formed in a localized back-arc rifting environment within an overall complex convergent margin.

Zircon geochronology and geochemistry of Quaternary rhyolite domes of the Coso volcanic field, Inyo County, California

The Quaternary Coso volcanic field (CVF) is a compositionally bimodal volcanic field located within a releasing bend along the eastern range-front Sierra Nevada fault zone in California's southern Owens Valley. The erupted products of CVF silicic magmatism since ~1 Ma comprise 38 high-silica rhyolite domes, with the volumetric majority (~99%) of rhyolite emplaced within the past ~300 ka. The CVF hosts an economically important geothermal field driven by heat associated with a shallow (~5 km) igneous intrusion. The CVF is potentially an immature analog to the nearby Long Valley system, which culminated in generation and eruption of the voluminous and widespread Bishop Tuff. As such, the CVF represents a considerable volcanic hazard, making a detailed understanding of the eruptive history and pre-eruptive conditions of the system critically important. We present uranium-series isochron dates from zircon ± allanite crystal surfaces and zircon trace element geochemical data on the youngest 17 rhyolite domes at Coso, which represent ~60% (by volume) of the silicic magma erupted by the system. These data suggest: (1) a shorter emplacement duration than previously recognized for these domes, with a duration of 20 ± 5 ka; (2) 4 shorter-duration eruption pulses within this interval, all of which occur during the marine isotope stage (MIS) 5 interglacial period; (3) an uptick in the volume of CVF magma erupted between ~200 ka and ~ 78 ka relative to that emplaced over the lifetime of the system; (4) near-coeval eruption of geochemically distinct magma in close geographic proximity, either sourced from different portions of the same magma system at depth or from discrete, uncommunicating bodies; (5) ambiguity with respect to whether or not CVF magmatism is time-predictable, as previously suggested, or erupted as a series of punctuated episodes; (6) no rhyolite volcanism in the past ~78 kyr.

Geology, petrogenesis, and geochronology of the Rio Salitre Complex: Implications for the Paleoproterozoic evolution of the northern São Francisco Craton, Brazil

Greenstone belts and volcano-sedimentary sequences are essential features to understand the tectonic framework of Precambrian terranes worldwide. The São Francisco Craton, eastern Brazil, hosts several of these supracrustal belts related to its Archean-Paleoproterozoic evolution. The Rio Salitre Complex (RSC) is a Paleoproterozoic greenschist-metamorphosed volcano-sedimentary sequence deposited over Archean orthogneisses of the northern São Francisco Craton. Here, we present a tectonic model for the Rio Salitre Complex, integrated with regional and global contexts, based on geology, structures, whole-rock geochemistry, U–Pb zircon geochronology, and Lu–Hf isotopes. The RSC comprises the Baixo Vale do Rio Salitre (BVRS), formed by Lower and Upper Subunits, and the Sobradinho Unit. The BVRS Lower Subunit is composed of ultramafic, mafic, and felsic metavolcanic rocks, chemical, and minor clastic metasedimentary rocks. The BVRS Upper Subunit comprises clastic metasedimentary rocks, while the Sobradinho Unit consists of chemical and clastic metasedimentary rocks. The RSC units display evidence of shallow to deep marine platformal deposition in an intra-cratonic rift (abundance of pelitic facies, presence of phosphorites, iron formations, graywackes, arkoses, and cross-bedded quartzites). RSC phosphorites are a register of a global Paleoproterozoic phosphogenic event. The volcanics of the Rio Salitre Complex compose a bimodal suite. The mafic and ultramafic metavolcanics exhibit komatiitic, tholeiitic, and alkaline affinities. Enriched REE, trace elements patterns, and incompatible element ratios indicate a continental rift environment for the mafic and ultramafic metavolcanic rocks. These features also suggest enriched mantle-derived melts, with a variable crustal contribution. The porphyritic rhyolites and aphyric high-silica rhyolites have subalkaline to alkaline features, with enriched REE and trace element profiles and negative Eu, Nb, and Ti anomalies. U–Pb SHRIMP dating of porphyritic rhyolite resulted in a concordia age of 2161.1 ± 4 Ma (MSWD: 1.1). Archean Lu–Hf TDM ages with negative εHf(t) values (from −8.90 to −2.70), and an isolated Paleoproterozoic TDM age (2440 Ma) with positive εHf(t) value (+3.96), related to the geochemical features, indicate that RSC felsic metavolcanics were originated from melting of multiple Archean crustal sources (continental and slab), with minor contributions of Paleoproterozoic juvenile material, and emplaced in a continental rift environment. 2.0 Ga foliated metagranites and metagranodiorites (locally augen) intrude the RSC rocks, resulting in local hornblende-hornfels metamorphism. The RSC supracrustal lithotypes register D1’, D1″, and D2 structures. D1′ structures are mainly represented by N–S high-dip angle schistosity, N and SSE plunge folds, and high-rake mineral stretching lineation (Lx’); while D1″ corresponds to NE-SW transpressional shear zones, NE-SW low-dip angle crenulation cleavage, and medium to low-rake mineral stretching lineation (Lx1″). The D2 structures are related to, locally quartz-filled, fractures, in addition to NE-SW and ENE-WSW oriented faults. The structural framework reflects a two-phase deformation, with a ductile to ductile-brittle, W-E frontal to NW-SE oblique D1 phase, and a late brittle, NE-SW D2 phase. Late hydrothermal activity resulted in quartz veins and pyrite-pyrrhotite-(chalcopyrite) concentrations. The documented aspects of the Rio Salitre Complex are consistent with evolution in a Rhyacian-Orosirian continental rift basin, inverted by the influence of orogenic belts, approaching from northwest and southeast.

Reheating and Magma Mixing Recorded by Zircon and Quartz from High-Silica Rhyolite in the Coqen Region, Southern Tibet

Abstract Understanding the formation of high-silica rhyolites (HSRs, SiO2 = 75 wt%) is critical to revealing the evolution of felsic magma systems and magma chamber processes. This paper addresses HSR petrogenesis by investigating an integrated data set of whole-rock geochemistry, geochronology, and mineral composition of the ~74 Ma Nuocang HSR (SiO2 = 74.5–79.3 wt%) from the Coqen region in southern Tibet. Cathodoluminescence (CL) images show that zircons from the Nuocang HSRs can be divided into two textural types: (1) those with dark-CL cores displaying resorption features and overgrown by light-CL rims, and (2) those comprising a single light-CL zone, without dark-CL cores. In situ single-spot data and scanning images demonstrate that these two types of zircon have similar U-Pb ages (~74 Ma) and Hf isotopic compositions [εHf(t) = –9.09 to –5.39], indicating they were generated by the same magmatic system. However, they have different abundances of trace elements and trace element ratios. The dark-CL cores are likely crystallized from a highly evolved magma as indicated by their higher U, Th, Hf, Y, and heavy rare earth elements concentrations, lower Sm/Yb ratio, and more negative Eu anomalies. In contrast, the uniformly light-CL zircons and the light-CL rims are likely crystallized from less evolved and hotter magma, as indicated by their lower U-Th-REE abundances and higher Ti-in-zircon temperatures. This is consistent with the Ti-in-quartz geothermometer in quartz phenocrysts that reveals that the light-CL zones are hotter than dark-CL cores. We propose that the composition and temperature differences between cores and rims of zircons and quartz record a recharge and reheating event during the formation of the Nuocang HSRs. This implies that HSR is a result of mixing between a hotter, less evolved silicic magma and a cooler, highly evolved, and crystal-rich mush. This study shows that zircon and quartz with distinct internal textures can be combined to disentangle the multi-stage evolution of magma reservoirs, providing critical insights into the origin of HSRs.

Repetitive Duality of Rhyolite Compositions, Timescales, and Storage and Extraction Conditions for Pleistocene Caldera-forming Eruptions, Hokkaido, Japan

Abstract During the Early Pleistocene, numerous caldera-forming eruptions occurred in the southernmost Kurile arc (central Hokkaido, Japan), building an extensive pyroclastic plateau with an area &amp;gt;1600 km2. The arc remains active today, and proximity to populations and infrastructure makes understanding these magmatic systems a critical endeavor. We investigate three major caldera-forming ignimbrite eruptions: Biei (c. 2·0 Ma), Tokachi (c. 1·2 Ma), and Tokachi–Mitsumata (c. 1·0 Ma), with an emphasis on constraining the pressures of magma extraction and storage and the timescales of crystallization. Although all pumice glass compositions from the three eruptions are high-silica rhyolites (77–78 wt% SiO2), hierarchical clustering analysis of major and trace element glass data indicates that the Tokachi and Tokachi–Mistumata ignimbrites each have two distinct pumice populations (Type-1F and Type-2F). We find that these two distinct pumice types record pre-eruptive temperatures, extraction pressures, and crystallization timescales that are strikingly similar between the two eruptions. Using the rhyolite-MELTS geobarometer, we estimate that although all magma types from all three eruptions had storage pressures of 50–150 MPa (∼2–6 km), Type-1F magma was extracted from a deeper mush reservoir (200–450 MPa) compared with Type-2F (100–200 MPa). Pre-eruptive temperatures, constrained by plagioclase–liquid equilibration thermometry and rhyolite-MELTS, suggest that Type-1F magma in both eruptions was hotter (800–820 °C) compared with Type-2F (780–800 °C), but that both reached thermal equilibrium upon eruption (760–780 °C). Because zircon is observed only as inclusions and rarely in contact with glass, we conclude that all magmas were zircon-undersaturated, and thus zircon saturation temperatures, which are 60–100 °C lower than those estimated by the other three independent thermometers, underestimate magmatic temperatures. Using these temperatures as minimum estimates, diffusional relaxation times of Ti zonation in quartz, as revealed by cathodoluminescence (CL), give absolute maximum quartz residence times of &amp;lt;1800 years for Type-2F samples and &amp;lt;600 years for Type-1F for all samples; residence times are &amp;lt;300 years for all samples if the more reasonable Fe–Ti oxide temperature is used instead (∼770 °C). Our modelling therefore suggests that the melt-dominated rhyolite magmas that fed these caldera-forming eruptions were ephemeral features that crystallized within the shallow crust for centuries to several millennia. Rapid rim growth occurred in all magma types in all three eruptions, with a majority of quartz rims (10–200 µm) having grown in less than 2 years. Using plagioclase textures and major and trace element data, we conclude that the bright-CL rims of quartz resulted from decompression and subsequent rapid growth, rather than by a recharge-driven heating event. Thus, decompression occurred within 2 years prior to eruption. Remarkably, the two distinct magma types are statistically similar in terms of composition, crystallization timescales, magma storage conditions, and extraction depths, despite being from eruptions that occurred 240 kyr apart, and from calderas that are separated by 35 km. This suggests magma assembly and storage processes that are spatiotemporally repetitive in this region of Hokkaido.

Geochemistry and petrogenesis of Paleoproterozoic rhyolite-hosted zinc-rich metamorphosed volcanogenic massive sulfide deposits in the eastern Betul Belt, central India

The Palaeoproterozoic Betul Belt in central India contains several small base metal (Zn-Cu and Zn-Pb-Cu) deposits that are hosted by felsic volcanics which form part of a bimodal volcanic suite. Although a volcanogenic massive sulfide (VMS) origin has been attributed to these deposits, they are not well-defined based on modern classification criteria. Regionally, the volcanic facies association around the Bhuyari zinc prospect includes massive rhyolite, pillowed basalt, rhyolitic autobreccia, hyaloclastite and peperite that indicate volcanism and sedimentation in relatively deep water. At the deposit-scale, the mineralization is hosted by intensely altered massive quartz-porphyritic rhyolite and associated tremolite-carbonate rock. Alteration facies mapping show a zoned alteration system with an inner phlogopite-biotite-chlorite-garnet-rich core (BPG schist; interpreted as having a chlorite-rich protolith) and an outer quartz-muscovite-K-feldspar zone (QMK schist; sericite-rich protolith). Metamorphism was largely isochemical and the mineral assemblages are amphibolite facies equivalents of hydrothermally altered protoliths. Tremolite-carbonate rocks are spatially close to mineralization and are interpreted either as metaexhalites or intensely altered rhyolite and probably had talc-dolomite protoliths. Carbon and oxygen isotope signatures of carbonate show derivation from seawater-dominant hydrothermal solutions. Tremolite-carbonate and some BPG schist show enrichment in HREE indicating hydrothermal mobilization of REE. Geochemically, the least-altered rhyolite has transitional magmatic affinities with moderate negative Nb anomaly and strong negative Sr, and Ti anomalies. Mantle upwelling and the associated high-heat source generated high-temperature, high-silica rhyolites with elevated HSFE and REE contents. Emplacement of felsic melts at depth provided heat that drove hydrothermal fluid circulation and formation of zinc-rich mineralization and associated hydrothermal alteration zones. Pb-model ages of galena give ~1800 Ma using the Stacey and Kramers (1975) model for VMS mineralization in Bhuyari. The Bhuyari deposit possibly formed in a continental back-arc rift environment during the amalgamation of Bastar and Bundelkhand cratons along the Central Indian Tectonic Zone. The high HFSE-REE contents of Bhuyari rhyolites are similar to other highly prospective VMS districts formed by rifting of evolved continental crust. The recognition of metamorphosed VMS deposit at Bhuyari has implications for further exploration of such deposits along the Paleoproterozoic Betul Belt and similar-aged deformed terrains in India and elsewhere.

At the crossroads of the Lesser Caucasus and the Eastern Pontides: Late Cretaceous to early Eocene magmatic and geodynamic evolution of the Bolnisi district, Georgia

The Bolnisi district is a distinct tectonic zone of the Lesser Caucasus, which is considered to represent the eastern extremity of the Turkish Eastern Pontides. Late Cretaceous, low-K, calc-alkaline to high-K rhyolite of the Mashavera and Gasandami Suites is the predominant rock type of the district, and is accompanied by subsidiary dacite, and rare high-alumina basalt and trachyandesite of the Tandzia Suite. The Mashavera and Gasandami rhyolite and dacite have yielded U-Pb LA-ICP-MS and TIMS zircon ages between 87.14 ± 0.16 and 81.64 ± 0.94 Ma, which are in line with the Coniacan-Santonian ages of radiolarian fauna of the Mashavera Suite. The felsic rocks of the Mashavera and Gasandami Suites were deposited during a ~6.6 m.y.-long silicic magmatic flare-up event, which together with the Tandzia Suite mafic rocks, documents Late Cretaceous bimodal magmatism in an extensional tectonic setting. Trace element data indicate that high Y-Zr, low- to high-silica rhyolite and dacite, and low Y-Zr high-silica rhyolite have been erupted, respectively, from coeval deep and shallow crustal reservoirs. The rocks of the bimodal magmatic event are overlain by high-K volcanic rocks of the Campanian Shorsholeti Suite, which have been erupted during slab roll-back and steepening, from magmas produced by deep melting of a metasomatised mantle. Eocene postcollisional felsic intrusions crosscut the Late Cretaceous rock.The Coniacian to early Campanian bimodal magmatism, and the subsequent high-K magmatism of the Bolnisi district are contemporaneous and share geochemical characteristics with the Late Cretaceous magmatism of the Eastern Pontides. It documents the existence of a Late Cretaceous regional silicic magmatic province, and subsequent high-K magmatism during slab steepening. This regional magmatic evolution coincided with the opening of the Black Sea and the Adjara-Trialeti basins. This evolution was coeval with the wanning stages of northern Neotethyan subduction, after a ~40 m.y.-long magmatic lull along the southern Eurasian convergent margin. Early Eocene adakite-like magmatism affected both the Bolnisi district and the Eastern Pontides, demonstrating a common postcollisional magmatic evolution.

High-silica rhyolites in the terminal stage of massive Cretaceous volcanism, SE China: Modified crustal sources and low-pressure magma chamber

Late Cretaceous felsic volcanism erupted as calderas in the terminal stage of the massive Cretaceous volcanism in SE China. This paper reports comprehensive geochemical and isotopic data for high-silica rhyolites from the Shiniushan and Xiaoxiong calderas in eastern Fujian–Zhejiang provinces, SE China. Zircon U-Pb dating constrains these rhyolites eruptions to 94–92 Ma and 93–89 Ma in the sites of the two calderas. All the samples possess high-silica contents with peraluminous characteristics and are enriched in light rare earth elements (LREEs), Rb, and Th-U and depleted in Ba, Nb-Ta, Sr and Eu. These samples have high whole-rock F (114–596 ppm) but low Cl (3.3–54.8 ppm) contents. Apatite minerals separated from the high-silica rhyolites are classified as fluorapatite and show F-rich (F > 3.56 wt%), Cl-poor (Cl < 0.02 wt%), low (La/Sm)N and high (Gd/Yb)N ratios, with a negative Eu anomaly. The high-silica rhyolites yield initial (87Sr/86Sr)i ratios of 0.70405 to 0.70789, negative whole-rock εNd(t) (−3.3 to −6.3), negative apatite εNd(t) (−5.2 to −6.6) and negative zircon εHf(t) values (−0.1 to −9.6). Our data suggest that their primary magma was derived from modified crustal sources that could contain a mixture of ancient and juvenile components, and it then evolved, was injected into a low-pressure magma chamber, and finally erupted with the release of volatiles at the magma chamber roof. We infer that SE China was mainly under a setting of lithospheric extension from the Early to early Late Cretaceous because of slab roll-back and steepening of the subducted Paleo-Pacific Plate.

Titanium diffusion profiles and melt inclusion chemistry and morphology in quartz from the Tshirege Member of the Bandelier Tuff

AbstractMany rhyolites contain quartz crystals with relatively Ti-rich rims and Ti-poor cores, with a sharp interface between zones, attributed to partial dissolution followed by overgrowth following a heating event due to mafic recharge of the system. Quartz crystals in the compositionally zoned, high-silica rhyolite Tshirege Member of the Bandelier Tuff erupted at 1.26 Ma from the Valles caldera, New Mexico, show a range in zoning styles with Ti-rich rims becoming more abundant upward in the ignimbrite sheet among progressively less evolved magma compositions. Here we compare times between quartz overgrowth and eruption obtained by applying Ti diffusion coefficients to Ti concentration profiles in Tshirege Member quartz crystals with those from cathodoluminescence (CL) brightness profiles and show that panchromatic CL provides only a crude proxy for Ti in quartz in this unit. Titanium concentrations are measured to detection limits of ~1.2 ppm with small analytical errors (&amp;lt;5%) using MAN backgrounds, blank corrections, and oblique corrected transects to resolve diffusion-relaxed zone boundaries as thin as ~10 μm. Timescales derived from Ti profiles using the widely applied Tiin-quartz diffusion coefficients of Cherniak et al. (2007) range from 60 to 10 000 years, suggesting heating and mobilization events at different times prior to the eruption. However, the use of the newer Ti diffusivities reported by Jollands et al. (2020) yields timescales up to three orders of magnitude longer, including results that are geologically unreasonable for the Bandelier system. We suggest that assumptions commonly made in diffusion modeling, specifically about the form of the Ti zoning profile prior to diffusive relaxation, may be invalid.Melt inclusions in the Ti-poor cores of late-erupted quartz are chemically akin to early erupted melt compositions, while adhering and groundmass glasses more closely reflect the composition of the host pumice. Heating and mobilization events identified from quartz Ti zoning are thus linked to overall compositional zoning of the tuff, which may have been produced by repeated episodes of melting of a crystal cumulate cognate to the early-erupted, evolved rhyolite. Quartz-hosted melt inclusion faceting suggests the development of a crystal mush over a minimum time frame of 1000–10 000 years prior to the recharge events that produced the erupted Tshirege magma at 1.26 Ma.

Breadcrust bubbles: Ash particles recording post-fragmentation brittle-ductile deformation

Abstract Breadcrust bubbles are a previously undescribed pyroclast providing unique insights into post-fragmentation conduit dynamics. We describe these particles from the ca. 7 Ma Rattlesnake Tuff deposits (Oregon, USA) and ca. 15 ka deposits at Laguna del Maule (Chile). The clasts comprise discrete, ∼1-mm-diameter, fully intact hollow spheroids composed of high-silica rhyolite glass with a series of cross-cutting cracks on the shell exterior akin to breadcrust texture on volcanic bombs. We interpret the spheroids as resulting from fragmentation of a thick-walled magmatic foam. As the pyroclasts accelerated up the conduit, breadcrust textures recorded cycles of brittle and ductile deformation to relieve overpressure. We exploit this texture to quantify parameters inaccessible through direct observation including strain accommodation and the time scale necessary for it to accrue. Breadcrust bubbles have additional potential to better describe and quantify the nature of fragmentation and transport conditions in high-silica rhyolite eruptions.