Crystal-rich Tuff Research Articles

Rejuvenation of silicic magmatic systems: Insights from the formation of garnet-bearing, crystal-rich tuff in Southeast China

Crystal-rich volcanic rocks are important for understanding the evolution of large silicic magma systems. However, the magmatic evolution and rejuvenation processes that form the eruptible crystal-rich magma remain controversial. In this study, we conducted petrological, whole-rock major and trace element geochemical and zircon U–Pb and Hf–O isotopic analyses on the Banshan crystal-rich tuff from the large Cretaceous volcanic belt of southeast China. Systematic SIMS and LA-ICP-MS zircon U–Pb geochronology indicate that the Banshan tuff was formed at ∼114–112 Ma. Their zircon Hf isotopic compositions [εHf(t) = −10.9 − 4.2; TDMC = 2.18–1.68 Ga] indicate that the magma of Banshan tuff was derived from a combination of mantle-derived melts and partial melting of the Paleoproterozoic crust of southeast China. In addition, they have low zircon O isotopic compositions (δ18OVSMOW = 4.39–5.36‰) reflecting that the assimilation of wall-rocks which underwent high-temperature hydrothermally exchange with meteoric waters. Extremely Mn-rich garnets in the Banshan tuff (MnO = 30.48–35.82 wt%) suggest that the magma once reached a high degree of crystallization which was nearing consolidation. The subsequent mush rejuvenation caused by magma recharge is evidenced by partly resorbed alkali feldspar, quartz and garnet, as well as the CL-bright rims of zircon. Crystal-rich volcanic rocks are widely distributed in the lower sequences of Cretaceous volcanic strata from southeast China, but rarely developed in the upper sequences. We suggest this reflects that the magmas of lower sequences have higher volatile contents and were formed in a subduction tectonic setting.

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 (>300 km3) ignimbrite composed of 99% crystal poor (≤1%) high-silica rhyolite (HSR) and <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.

Astrochronology and radio-isotopic dating of the Alano di Piave section (NE Italy), candidate GSSP for the Priabonian Stage (late Eocene)

We have carried out an integrated chronostratigraphic analysis of the Alano di Piave section, proposed GSSP for the Bartonian/Priabonian boundary (late Eocene). Age constraints were derived independently from a floating cyclochronology based on carbon isotope and wt.% CaCO3 records, and 206Pb/238U dating of zircons from four volcanic tuffs. Orbital and radio-isotopic estimates of the duration of intervals bracketed by consecutive crystal-rich volcanic tuff layers are in good overall agreement, but discrepancies of 40–160 kyr are present between nominal volcanic tuff dates derived from astrochronology and 206Pb/238U. The degree to which these discrepancies are statistically significant at the 2σ level depends on the interpretation of the U-Pb zircon data, and the uncertainties assigned to the astronomical age model. The possible source and significance of these discrepancies are explored in detail. The resulting age model, combined with published bio-, and magnetostratigraphic data, is used to establish the duration of individual magnetochrons in the interval spanning the Bartonian/Priabonian transition, and the timing of late Bartonian-early Priabonian biostratigraphic and magnetostratigraphic events relative to the proposed GSSP, the regionally traceable crystal-rich tuff layer “Tiziano Bed”. The chronostratigraphic framework developed at Alano di Piave will facilitate the global correlation of the Priabonian GSSP based on both bio- and magnetostratigraphic criteria. The obtained astrochronology allows a precise correlation of the Alano di Piave δ13C record with oceanic successions confirming a strong dependency of the marine carbon cycle on astronomical forcing although with inter-site differences, which could be related to water mass organization.

Paleoproterozoic volcanic caldera in the Amazonian craton, northern Brazil: Stratigraphy, lithofacies characterization, and lithogeochemical constraints

In the Vila Tancredo Neves locality, Amazonian craton, northern Brazil, occur Paleoproterozoic well-preserved volcano–plutonic centers. The lower 1.88 Ga Sobreiro Formation has overlapped lavas comprising andesite at the base; andesi-basalt and latite at middle; and quartz-latite and rhyolite at the top; besides associated pyroclastic and autoclastic lithofacies. Reveals high-K calc-alkaline and metaluminous signature. This unit shows K, Ba, Sr, and Rb enrichments; and Nb and Ta negative anomalies. Occurs an LREE-enriched pattern relative to HREE. Eu anomaly is absent in the most primitive rocks, whereas the evolved rocks reveal a minor negative Eu anomaly. These chemical characteristics are compatible with arc-related andesites. The upper, fissure-controlled, 1.87 Ga Santa Rosa Formation has peraluminous rhyolites and intrusions of rhyolitic porphyries and granites. These alkali rhyolites erupted in several vents within greater brittle faults, with outflows of ignimbrites and volcaniclastic deposits with variable textures. They reveal HFSE enrichment (Nb and Ta) and strong negative anomalies of LILE (Ba and Sr). Some evolved rhyolitic samples usually attributed to the Santa Rosa Formation are calc-alkaline and have geochemical constraints closer to those of the Sobreiro Formation andesitic rocks. Observations of the volcano–plutonic complex point to a Paleoproterozoic volcanic caldera related to several pulses of felsic and andesitic–rhyodacitic–rhyolitic lavas that comprise a stratovolcano evolution in the pre-caldera stage. Pyroclastic sin-caldera deposits with ignimbrite, ash tuff, crystal-rich tuff, and minor polymictic breccia are less preserved. Later evolved domes, dikes, and volcaniclastic rocks represent the recurrent magmatism in the post-caldera stage. This work supplies new support for more systematic stratigraphic and geochemical studies of these poorly studied, but promising, Proterozoic units.

The arc of the Snowball: U-Pb dates constrain the Islay anomaly and the initiation of the Sturtian glaciation

Author(s): MacLennan, S; Park, Y; Swanson-Hysell, N; Maloof, A; Schoene, B; Gebreslassie, M; Antilla, E; Tesema, T; Alene, M; Haileab, B | Abstract: In order to understand the onset of Snowball Earth events, precise geochronology and chemostratigraphy are needed on complete sections leading into the glaciations. While deposits associated with the Neoproterozoic Sturtian glaciation have been found on nearly every continent, time-calibrated stratigraphic sections that record paleoenvironmental conditions leading into the glaciation are exceedingly rare. Instead, the transition to glaciation is normally expressed as erosive contacts with overlying diamictites, and the best existing geochronological constraints come from volcanic successions with little paleoenvironmental information. We report new stratigraphic and geochronological data from the upper Tambien Group in northern Ethiopia, which indicates that the glacigenic diamictite at the top of the succession is Sturtian in age. U-Pb zircon dates obtained from two tuffaceous siltstones that are 74 and 84 m below the diamictite are 719.68 ± 0.46 Ma and 719.68 ± 0.56 Ma (2σ), respectively. We also report a U-Pb date of 735.25 ± 0.25 Ma from a crystal-rich tuff located 2 m above the nadir of a high-amplitude, basin-wide, negative δ13C excursion previously correlated with the Islay anomaly. This age for the anomaly agrees with Re-Os age constraints from Laurentia, suggesting that the δ13C signal is globally synchronous and preceded the Sturtian glaciation by ~18 m.y. The interval between the Islay anomaly and Sturtian glaciation is recorded in the Tambien Group as an ~600 m succession of predominantly shallow-water carbonates and siliciclastics with δ13C values recording a prolonged period at +5‰, followed by an interval of lower, but still positive, values leading up to the glaciation. Our data are consistent with synchronous global onset of the Sturtian glaciation at ca. 717 Ma. Shallow-water carbonates in strata directly below the first diamictite suggest that glacial onset was rapid in terranes of the Arabian-Nubian Shield.

Integrated U-Pb zircon and palynological/palaeofloristic age determinations of a Bashkirian palaeofjord fill, Quebrada Grande (Western Argentina)

This work presents a new age framework for the main Bashkirian glacio-eustatic transgression in Argentina, including the first absolute age for the Jejenes Formation, San Juan Province, based on radiometric dating of a crystal-rich tuff, supported by palynological and palaeofloristic studies, and presented within a revised palaeogeographic setting. The Jejenes Formation represents the glacial to postglacial fill of the Quebrada Grande palaeofjord carved in the Eastern Precordillera. The succession has been subdivided into five stages, the youngest of which suggests a previously unrecognised glacial event for this locality. Six productive levels for palynology were found within proglacial strata, and in the base and top of the succeeding interglacial stage. Palynoassemblages are characterized by poorly preserved trilete spores and monosaccate pollen grains along with a large amount of terrestrial phytoclasts. Main species indicating the Raistrickia densa-Convolutispora muriornata SubZone (DMa SZ) are Vallatisporites ciliaris, Cristatisporites rollerii, C. stellatus, C. chacoparanensis, C. inconstans and monosaccates such as Circumplicatipollis plicatus. This DMa SZ is estimated as Serpukhovian/Bashkirian and characterizes the glacial-related Guandacol Formation and equivalents units of the western Paganzo Basin. A tuffaceous level in the proglacial unit, bearing platyspermic seeds, plant remains and palynomorphs, yielded first-cycle volcanic zircons that were analysed by SHRIMP. An absolute age of 321.3 ± 5.3 Ma confirms a Bashkirian age for the main postglacial transgression in the Paganzo Basin, and offers a novel calibration for the palynoassemblages of DMa SZ that occurs elsewhere in Western Argentina.

Oxo-magnesio-hastingsite, NaCa2(Mg2Fe3+3 )(Al2Si6)O22O2, a new anhydrous amphibole from the Deeti volcanic cone, Gregory rift, northern Tanzania

AbstractOxo-magnesio-hastingsite, ideally NaCa2(Mg2Fe3+3)(Al2Si6)O22O2, is a new anhydrous amphibole from the Deeti volcanic cone in the Gregory rift (northern Tanzania). The mineral occurs as megacrysts up to 12 cm in size in crystal-rich tuff. Oxo-magnesio-hastingsite is brown with a vitreous lustre and has a perfect {110} cleavage. The measured density is 3.19(1) g/cm3. Ferri-kaersutite is biaxial (–), α = 1.706 (2), β = 1.715(2), γ = 1.720(2) (Na light, 589 nm). 2V (calc.) = 73°. Dispersion: r > v, weak; orientation: Y = b; Z ^ c = 8°; pleochroism: strong, Z: dark brown, Y: brown, X: light brown. The average chemical formula of the mineral derived from electron microprobe analyses, Mössbauer spectroscopy and direct water determination is (Na0.67K0.33)Σ1.00(Ca1.87Na0.14Mn0.01)Σ2.02(Mg3.27Fe3+1.25Ti0.44Al0.08)Σ5.04(Al1.80Si6.20O22)(O1.40OH0.60)Σ2.00. It has monoclinic symmetry, space group C2/m and unit-cell parameters a = 9.8837(3), b = 18.0662(6), c = 5.3107(2) Å, b = 105.278(1)o, V = 914.77(5) Å3, Z = 2. The five strongest powder-diffraction lines [d in Å, (I/Io), hkl] are: 3.383 (62) (131), 2.708 (97) (151), 2.555 (100) (), 2.349 (29) () and 2.162 (36) (261). The isotopic composition of H and O, as well as the concentration of trace elements in oxo-magnesio-hastingsite suggest its formation from a melt originated from a mantle source metasomatized by slab-derived fluids.

40Ar/39Ar age and correlation of the nonmarine Two Medicine Formation (Upper Cretaceous), northwestern Montana, U.S.A.

The age of the nonmarine Two Medicine Formation of northwestern Montana is currently based upon correlations with K–Ar-dated Western Interior ammonite zones. 40Ar/39Ar dating of biotite and plagioclase separated from four bentonites and one crystal-rich tuff permits for the first time direct determination of the age of Two Medicine strata. Biotite and plagioclase from a bentonite 10 m below the top of the Two Medicine Formation yield concordant 40Ar/39Ar ages of 74 Ma, while biotite and plagioclase from two bentonites and a crystal-rich tuff from approximately 100 m above the base of the formation cluster in age around 80 Ma. The total duration of Two Medicine deposition is estimated using these new radio-isotopic ages via extrapolation of an average rock accumulation rate.The new 40Ar/39Ar ages facilitate regional correlation of the dinosaur-dominated paleofauna recovered from the Two Medicine Formation, and help constrain the timing of the Claggett and Bearpaw transgressions. The ages support correlation of the richly fossiliferous upper lithofacies suite of the Two Medicine Formation with exposures of the Judith River Formation in Dinosaur Provincial Park, Alberta, Canada. Radioisotopically dated exposures of the Judith River Formation within Montana that include important Judithian "age" mammal localities correlate approximately with middle and lower parts of the middle lithofacies suite of the Two Medicine Formation. The new 40Ar/39Ar ages further indicate that the transgressions of the Claggett and Bearpaw seas culminated within northwestern Montana at ca. 79.6 and 74.0 Ma, respectively.