Geochronologic and paleomagnetic evidence defining the relationship between the Miocene Hiko and Racer Canyon tuffs, eccentric outflow lobes from the Caliente caldera complex, southeastern Great Basin, USA

 Outflow sheets of the Hiko tuff and the Racer Canyon tuff, which together extend over approximately 16 000 km² around the Caliente caldera complex in southeastern Nevada, have long been considered to be products of simultaneous or near-simultaneous eruptions from inset calderas in the west and east ends, respectively, of the caldera complex. New high-precision ⁴⁰Ar/³⁹Ar geochronology and paleomagnetic data demonstrate that emplacement of the uppermost part of the Racer Canyon tuff at 18.33±0.03 Ma was nearly synchronous with emplacement of the single outflow cooling unit of the much larger overlying Hiko tuff at 18.32±0.04 Ma. Based on comparison with the geomagnetic polarity time scale derived from the sea-floor spreading record, we conclude that emplacement of the first of several outflow cooling units of the Racer Canyon tuff commenced approximately 0.5 m.y. earlier. Only one paleomagnetic polarity is found in the Hiko tuff, but at least two paleomagnetic reversals have been found in the Racer Canyon tuff. The two formations overlap in only one place, at and near Panaca Summit northeast of the center of the Caliente caldera complex; here the Hiko tuff is stratigraphically above the Racer Canyon tuff. This study demonstrates the power of combining ⁴⁰Ar/³⁹Ar and paleomagnetic data in conjunction with phenocryst compositional modes to resolve problematic stratigraphic correlations in complex ash-flow sequences where use of one method alone might not eliminate ambiguities. Received: 13 January 1997 / Accepted: 7 May 1997     

东昆仑闹仓坚沟组流纹质凝灰岩锆石U-Pb年龄及其地质意义

应用激光烧蚀多接收器电感耦合等离子体质谱(LA-MC-ICP MS)方法对东昆仑秀沟盆地闹仓坚沟组火山岩样品进行了锆石U-Pb定年研究。流纹质凝灰岩(DG25-4)中锆石的阴极发光图像具有振荡环带结构,属于典型的岩浆锆石。15个岩浆锆石206Pb/238U表面年龄集中在239~249 Ma之间,206Pb/238U加权平均值为243.5 ± 1.7 Ma,它记录了火山岩的形成年龄,说明该地区闹仓坚沟组形成于中三叠世早期。这一研究还在闹仓坚沟组火山岩中发现了早古生代和元古宇继承锆石,提供了昆南地体可能存在元古宇基底的信息。      

Age of the Yarrabee and accessory tuffs: implications for the upper Permian sediment-accumulation rates across the Bowen Basin

The Yarrabee Tuff is a stratigraphically significant marker across the Bowen Basin separating the Fort Cooper–Burngrove–Fair Hill formations from the overlying Rangal and equivalent coal measures. At least three to four persistent tuffs (referred here as accessary tuffs) beneath the Yarrabee Tuff were recognised in the Fort Cooper Coal Measures as suitable for regional stratigraphic correlations. In this study, we determined the ages of the Yarrabee and accessary tuffs across different morphotectonic zones of the basin through high-precision U–Pb dating of zircon with the CA-IDTIMS technique. The age of the Yarrabee Tuff is found to be 252.69 ± 0.16 Ma in the Duckworth 11 well, 253.07 ± 0.22 Ma in the Crocker Gully 2 well and <252.58 ± 0.23 Ma in the Peat 1 well. The age range of the Yarrabee Tuff coincides with the previously published date of the Kaloola Tuff Member in Meeleebee 5 suggesting that the tuffs are stratigraphically equivalent. The age range for the accessory tuff 1 is 253.12 ± 0.12 Ma to 252.85 ± 0.16 Ma, 253.45 ± 0.08 Ma for accessory tuff 2 and 253.77 ± 0.17 Ma to 253.57± 0.18 Ma for accessory tuff 3, placing them in the upper Changhsingian Stage. The age of the accessory tuff 6 (less laterally consistent in the basin) from the Fair Hill Formation is 254.03 ± 0.03 Ma, placing it in the lower Changhsingian Stage. The age-constrained intervals allow the estimation of sedimentation rates using decompacted coal and clastic sediment thickness. In the Taroom Trough, the temporal variation in sedimentation rates is found to be 902 m/Ma in the Fair Hill Formation decreasing to 234.5 m/Ma in the overlying Burngrove Formation, reflecting a decrease in accommodation or sediment supply upwards in the sequence. Across the basin, the sedimentation rates for the Burngrove Formation are consistently higher in the Taroom Trough ranging between 234.5 and 224.5 m/Ma and lower rates of 112 m/Ma in the Roma Shelf. This regional variation reflects areas of high sedimentation rates that are high accommodation sites recognised by split coal seams and increased interburden. Conversely, low sedimentation rates reflect low accommodation sites, such as the Roma Shelf and the Burunga Anticline that are characterised by coalesced coal seams. The results help to understand stratal relationships across variable accommodation sites, basin-fill history of the basin including extent of sediment supply and paleotopographic controls during the evolution of the Bowen Basin. We also discuss criteria for interpreting the results of CA-IDTIMS U–Pb dating and consider the possible geological uncertainties related to either the primary magmatic processes or secondary reworking of tuffs at the site of deposition.     

Geochemical correlation of Ordovician flow tuffs in North Wales

Ordovician rhyolitic tuffs in North Wales are correlated on the basis of their whole-rock trace element composition. Multivariate statistics verify that major ignimbrite sheets are chemically unique, with Zr, Y and Nb being the most useful elements to characterize individual deposits. Fractionation processes during transport and emplacement of the pyroclastic flows results in little lateral variation in the whole-rock geochemistry of deposits; different diagenetic histories in subaerial and subaqueous environments did not mask recognition of primary ignimbrite units. A partially welded vitric ash flow tuff is correlated with the Pitts Head Tuff ash flow sheet, and a tuff horizon previously assumed to be reworked Pitts Head Tuff is shown to be chemically most similar to the fourth member of the Capel Curig Volcanic Formation. Validity of these correlations is upheld by biostratigraphic and sedimentological evidence. The new correlations necessitate a revision of the stratigraphy in North Wales and indicate a significant lapse of time (perhaps one million years) between emplacement of the Pitts Head Tuff and eruption of the Lower Rhyolitic Tuff of the overlying Snowdon Volcanic Group.     

Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry

 Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas. Received: 13 February 1997 / Accepted: 9 August 1997     

Buried oblique‐slip faults in the Irish Caledonides

Despite over a century of geological investigation, the Ordovician evolution of South Mayo, western Ireland, is still imperfectly understood. An example of this is the supposed lateral equivalence of two formations within the succession, the Rosroe and Derrylea Formations of Arenig age, exposed on opposite limbs of a major east–west syncline. These formations exhibit characteristics which suggest that they were not deposited in the same basin. Both formations contain tuff horizons. Geochemical analysis of these tuffs shows that each formation contains chemically distinct volcanic signatures suggesting deposition in separate sub‐basins. Previously the Rosroe Formation on the south limb of the syncline was considered the coarse‐grained proximal equivalent of the finer‐grained Derrylea Formation, both being deposited in a deep‐water fan environment. Previously published palaeocurrent data together with new data show the Rosroe Formation to have been derived from the northeast and therefore it cannot be the proximal equivalent of the Derrylea Formation. Additionally, the two formations show different and distinct associations of heavy mineral assemblages. It is suggested that one explanation for these data is that both formations were deposited in separate sub‐basins controlled by oblique slip sinistral faults, similar in some respects to the Cenozoic basins of the Gulf of California. In the Irish case these faults would have been largely buried by later Ordovician sedimentation. Some models for the Ordovician evolution of this area postulate the presence of an initial oceanic arc situated above a southward directed subduction zone. The presence of thick proximal submarine tuffs derived from an arc environment in the Rosroe Formation suggest that at least by this time the subduction zone was in fact northward directed and outboard of the arc. Copyright © 2002 John Wiley & Sons, Ltd.     

A Fissure-Controlled Acidic Volcanic Centre (Ordovician) at Yr Arddu,North Wales

The Yr Arddu Tuffs (Ordovician) are a sequence of predominantly welded acid ash-flow tuffs in the outlier centred on Yr Arddu (North Wales). The tuffs accumulated above a NNE-trending fracture, the Yr Arddu Fracture, which had previously influenced sedimentation. The heterogeneity of the tuffs and their restricted development suggest that they represent proximal accumulations from small or suppressed (boiling-over) eruption columns. The eruptions and emplacement developed, at least in the initial stages, in a submarine environment. Lithological, geochemical and palaeontological evidence indicates that the tuffs represent the earliest eruptive phase of the Lower Rhyolitic Tuff Formation. They were later intruded by comagmatic rhyolite domes, whose alignment reflects the continued influence of the early fracture.