Geology topics

Colin J.N. Wilson

Publications and source records attributed to Colin J.N. Wilson.

4 recordsLinked to original sources

USGS2026

Ignimbrites of the Sour Creek dome, Yellowstone volcano, USA: Implications for the Lava Creek Tuff eruption and Yellowstone Caldera

Field observations plus sanidine compositional and 40 Ar/ 39 Ar age data collected on rocks from the Sour Creek dome (SCD) area of Yellowstone volcano (United States) serve to identify five new ignimbrite packages with ages (average 631.5 ± 0.9 ka) that are analytically indistinguishable from those of the Lava Creek Tuff (LCT), challenging the current framework of two LCT eruptive members. Newly recognized package 1 occurs as LCT-aged recycled clasts within a proximal lag breccia, reflecting a major time break in the LCT eruption sequence. Conformably above, the scoria-bearing ignimbrite of package 2 is extensively found in the dome, having been previously mis-mapped as Huck-leberry Ridge Tuff and LCT member A. To the south, package 3 is a crystal-rich, densely welded ignimbrite (inferred to be younger than package 2) that is conformably overlain by package 4, a fine-grained ignimbrite that contains abundant clasts of recycled tuff. Lastly, the topographically highest parts of the dome are made up of package 5, a multicomponent ignimbrite containing scoria, recycled clasts, and pumices, potentially correlative with package 2 and/or package 4. Cathodoluminescence imaging and zoning assessment was integrated with sanidine geochemistry from all five packages, including their recycled clasts. Four of the five packages were sourced from separate magmatic bodies within a larger plumbing system, each defined by their sanidine Ba distributions and componentry. Although the SCD was originally mapped as a resurgent dome and is actively deforming today, faults cutting the region show comparable offsets for NW-SE “resurgent faulting” as for orthogonal NE-SW faulting, the latter of which also cuts the 162 ka Elephant Back lava flow. We infer that faulting in the SCD well postdates eruption and that the SCD instead represents a domical accumulation of ignimbrite, not a resurgent structure. The abundance of recycled materials (and absence of Absaroka country rocks) in the SCD ignimbrites implies that vent locations were well inboard of the mapped caldera boundary, with some potentially along the line of the Yellowstone River, requiring a revision of the currently accepted LCT caldera outline. The LCT eruptive events were thus far more complex than currently recognized, with multiple deposits fed from multiple magma bodies over a time scale that cannot be resolved by radiometric dating.

Wyoming↗

USGS2026

Dismembering the Lava Creek Tuff, Yellowstone volcano

The youngest large explosive eruption of Yellowstone volcano generated the 0.63 Ma Lava Creek Tuff (LCT) ignimbrite and a voluminous, widespread fall deposit accompanying formation of Yellowstone Caldera. The tuff is conventionally divided into members A (LCT-A) and B (LCT-B), defined by a reversal in density/welding profiles and the presence of phenocrystic amphibole in LCT-A. Remapping within the northeastern caldera sector has, however, documented multiple ignimbrites that yield LCT eruption ages but are distinct from documented members A and B. Thus, the framework by which the LCT was originally subdivided is re-examined here using field observations, petrologic analyses, and sanidine geochemistry from LCT-A and LCT-B ignimbrite (as previously mapped) at reference localities around the caldera. Mineral modal abundances in both LCT-A and LCT-B outcrops are dominated by quartz, followed by sanidine, with sizes and abundances that generally increase up-section, as previously documented. Amphibole is found by us, however, only in western and southern sectors, in deposits mapped as both LCT-A and LCT-B, and only as a trace phase. Further, a locality reported as LCT-A with amphibole present yields a sanidine 40 Ar/ 39 Ar date of 0.775 ± 0.002 Ma, precluding any LCT correlation. Ignimbrite samples from mapped LCT-A and LCT-B exhibit variable crystal sizes, sanidine cathodoluminescence image populations and Ba concentrations, with no clear distinction between the mapped members. Collectively this research found no evidence for a definable LCT-A versus LCT-B, meaning that the traditional A and B subdivision does not accurately represent the eruptive architecture of the Lava Creek Tuff and warrants revision.

Idaho, Montana, Wyoming↗

USGS2025

High-precision 40Ar/39Ar dating of Quaternary basalts from Auckland Volcanic Field, New Zealand, with implications for eruption rates and paleomagnetic correlations

The Auckland Volcanic Field (AVF), which last erupted ca. 550 years ago, is a late Quaternary monogenetic basaltic volcanic field (ca. 500 km 2 ) in the northern North Island of New Zealand. Prior to this study only 12 out of the 53 identified eruptive centres of the AVF had been reliably dated. Careful sample preparation and 40 Ar/ 39 Ar analysis has increased the number of well-dated centres in the AVF to 35. The high precision of the results is attributed to selection of fresh, non-vesicular, non-glassy samples from lava flow interiors. Sample selection was coupled with separation techniques that targeted only the groundmass of samples with < 5% glass and with groundmass feldspars > 10 μm wide, coupled with ten-increment furnace step-heating of large quantities (up to 200 mg) of material. The overall AVF age data indicate an onset at 193.2 ± 2.8 ka, an apparent six-eruption flare-up from 30 to 34 ka, and a ≤ 10 kyr hiatus between the latest and second-to-latest eruptions. Such non-uniformity shows that averaging the number of eruptions over the life-span of the AVF to yield a mean eruption rate is overly simplistic. Together with large variations in eruption volumes, and the large sizes and unusual chemistry within the latest eruptions (Rangitoto 1 and Rangitoto 2), our results illuminate a complex episodic eruption history. In particular, the rate of volcanism in AVF has increased since 60 ka, suggesting that the field is still in its infancy. Multiple centres with unusual paleomagnetic inclination and declination orientations are confirmed to fit into a number of geomagnetic excursions, with five identified in the Mono Lake, two within the Laschamp, one within the post-Blake or Blake, and two possibly within the Hilina Pali.

Auckland Volcanic Field↗

USGS2018

New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand

Time–composition relationships in eruptive sequences at composite volcanoes can show how the ongoing intrusion of magmas progressively affects the lithosphere at continental convergent margins. Here, new whole-rock and microanalytical major and trace element data from andesite-dacite lava flows are integrated with previous studies and existing isotopic data, and placed within the framework of a high-resolution chronostratigraphy for Ruapehu volcano (southern Taupo Volcanic Zone, New Zealand). The geochemical evolution of lavas erupted over the ∼200 kyr lifetime of the exposed edifice reflects variable degrees of fractionation and systematic changes in the type of crustal assimilation in the Ruapehu magma system. Lavas erupted from ∼200–150 ka have previously been distinguished from those erupted <150 ka based on Sr-Nd isotopic characteristics, which indicate that the oldest lavas were sourced from magmas that assimilated oceanic crust. Such source rocks underlie the regionally widespread Mesozoic meta-sedimentary greywacke-argillite basement, which was conversely assimilated by <150 ka magmas. New results from this work reveal that since 150 ka, an upper limit of magma differentiation occurred from ∼50–35 ka. High K 2 O (∼6 wt%) and Rb contents (∼270 ppm) in melt inclusions, interstitial glass, and glass from in situ quenched melts of partially fused crustal xenoliths are reported for andesite-dacite lavas erupted during this period. In addition to crystal fractionation, selective partial melting and assimilation of K- and Rb-rich mineral phases (e.g., biotite, K-feldspar) that are significant components of the meta-sedimentary basement rocks is inferred to explain these geochemical characteristics. These processes coincided also with the effusion of high-MgO andesitedacite lavas that display petrological evidence for mixing between andesite-dacite and more mafic magmas. An influx of hotter mafic magma into the system explains why the extent of crustal assimilation recorded by Ruapehu lavas peaked during the ∼50–35 ka eruptive period. From 26 ka to the present, andesite lavas have reverted to more mafic compositions with less potassic melt inclusion and whole-rock compositions when compared to the ∼50–35 ka lavas. We suggest that the younger lavas assimilated less-enriched melts because fertile phases had been preferentially extracted from the crustal column during earlier magmatism. This scenario of bottom-up heating of the lithosphere and exhaustion of fertile phases due to the progressive intrusion of magma explains the geochemical evolution of Ruapehu lavas. This model may be applicable to other long-lived composite volcanoes of the circum-Pacific continental arcs.

Taupo Volcanic Zone↗