Andesite Clasts Research Articles

Conduit armouring preceding explosive activity at an andesitic stratovolcano, an example from Taranaki Mounga, New Zealand

The strength and permeability of volcanic conduits can directly influence eruption dynamics via moderating the outgassing of ascending magma and the density of eruption plumes. Lithic clasts in pyroclastic ejecta can be used to understand the dynamic evolution of conduit walls because they are incorporated into the ascending melt-gas-particle mixture during volcanic eruptions. We examine the 1655 CE Burrell eruption of Taranaki Mounga, which transitioned from effusive activity to an explosive sub-Plinian phase and ended in unsteady columns. This episode was followed by a series of effusive eruptions of lower explosivity. Using textural analysis and physical properties, we distinguish five dominant lithic clast types within Burrell deposits that represent different regions of the shallow conduit and vent. Lithic types 1–3 represent juvenile (‘intrusive cognate’) and older (‘intrusive accessory’) conduit-filling plug materials. Lithic type 4 represents juvenile (‘extrusive cognate’) vent-filling lava dome extruded at the eruption onset, while Type 5 lithics (‘extrusive cognate’) represent sintered/compacted cognate material from the shallow vent accumulated during transitions in eruptive style. Crystalline andesite lithics (type 1) show a microlite-dominated groundmass. Hydrothermally altered andesite lithics (type 2) show breakdown of phenocrysts and increased seismic velocity relative to type 1 lithics. Brecciated andesite lithics (type 3) comprise fractured and sintered clasts of crystalline andesite. Glassy andesite lithics (type 4) show sub-rounded vesicles and glass-hosted microlites. Banded vitrophyre lithics (type 5) show bands of varying vesicularity, crystallinity and clast load. Physical property data reveals porosity, fracturing, sintering and alteration extent dictate dynamic changes in conduit permeability and potentially strength. Our results show how, during the explosive phase of the Burrell eruption, the conduit was lined with juvenile and remnant shallow plug material that was variably fractured, sintered and altered before being eroded and ejected. Comparison with previous work on Taranaki and dome-plug material from around the world shows how fracturing and sintering of conduit walls, combined with lining with dense juvenile material, cause overall permeability reduction and strengthening of the conduit. This inhibits outgassing and preserves conduit structure, facilitating the transition to explosive activity and the establishment of a stable eruption column.

SEDIMENTOLOGY OF THE PALEOGENE VOLCANICLASTIC GRAVITY FLOW DEPOSIT OF THE ULUKIŞLA FORMATION, SOUTH CENTRAL TÜRKIYE

The sedimentological study of volcaniclastic successions is essential for gaining insight into the intricate geological history of the Ulukışla Basin in south Central Anatolia. This study, based on a comprehensive facies analysis of two representative stratigraphic sections, attempts to reconstruct the depositional conditions associated with volcaniclastic sedimentation within the Ulukışla Formation. Facies assemblages dominated by debris-flow deposits and turbidites provide clear evidence for deposition under deep-water conditions related to the proximal environment. Sedimentary characteristics of these deposits made up of volcanogenic conglomerates with basaltic to andesitic clasts of variable size, sandstones, and subordinate mudstone are suggestive of derivation from the nearby Ulukışla volcanic rocks. Almost all the lithofacies display a high tuff content indicative of possible sediment contributions from sub-aerial volcanic sources. The main controlling factors responsible for deposition include submarine gravity flow processes thought to have been triggered by slope instability or the collapse of a volcanic edifice. Thus, the resulting volcaniclastic accumulations are interpreted to be linked to volcanic apron deposits. In the realm of prospective research, the acquisition of geochemical and geochronological data stands as a promising avenue, offering crucial insight into the temporal aspects and tectonic setting of deposition of the Ulukışla volcaniclastic sequence.

Badenian (middle Miocene) continental paleoenvironment in the Novohrad–Nógrád Basin (Central Paratethys): a volcano-sedimentary record from the Páris-patak Valley in Hungary

Two levels of volcaniclastics, comprising conglomerates, sandstones and mudstones, are interbedded with upper middle Miocene (upper Badenian) andesite pyroclastics near the Hungarian-Slovakian border in the distal region of the Central Slovakian Neogene Volcanic Field. Based on the field sedimentological investigations, the facies of the volcaniclastics (e.g., lateral and vertical grain size changes, sedimentary structures, textures, clast composition), their geometry and field relationships are documented herein with the aim of reconstructing the depositional environment. The silica-cemented volcaniclastics are mostly andesite clasts with only ~ 5% being granitoid, quarzitic, and tuff clasts as well as charred fossil wood fragments. The coarse-grained facies association includes crudely stratified, tabular or lenticular, clast-supported pebble-cobble conglomerates with erosive basal surfaces, b-axis imbrication, alternating with sets of cross-bedding. The fine-grained facies association comprises cross-bedded pebbly to medium-grained sandstone and lenses of tuffaceous clayey siltstone with rare horizontal lamination and water-escape structures. Rip-up mudstone clasts, with diametre up to 1 m, are present in both facies associations, revealing the co-existence of abandoned silty palaeo-channel plugs. Facies associations are arranged in several 0.5-4-m-thick, fining-upwards successions that likely formed in shallow channels as downstream- to laterally accreting longitudinal bars, extensive gravel sheets and bars that migrated in peak flow during floods. Palaeocurrent indicators (i.e., clast imbrication, direction of planar cross-bedding, orientation of petrified wood logs) show bedload transport by traction currents, initially towards ~S, and later towards ~W. Intermittently debris flows also occurred. Cross-bedded sandstones formed as in-channel transverse bars during medium/low discharge. Variation of grain size shows frequent discharge fluctuations during permanently wet conditions in the late Badenian. The 4-5-m-deep, low-sinuosity channels were part of a high-energy, gravel-bed braided-river system on the south-eastern foothills of the Lysec palaeo-volcano. Here, pyroclastics were reworked and redeposited as volcaniclastics during inter-eruption, high-discharge episodes.

Multiphase Storm Deposits Eroded from Andesite Sea Cliffs on Isla San Luis Gonzaga (Northern Gulf of California, Mexico)

The 450-m long spit that extends westward from the northwest corner of Isla San Luis Gonzaga is one of the largest and most complex constructions of unconsolidated cobbles and boulders found anywhere in Mexico’s Gulf of California. The material source derives from episodic but intense storm erosion along the island’s andesitic cliff face with steep northern exposures. A well-defined marine terrace from the late Pleistocene cuts across the same corner of the island and provides a marker for the subsequent development of the spit that post-dates tectonic-eustatic adjustments. A total of 660 individual andesite clasts from seven transects across the spit were measured for analyses of change in shape and size. These data are pertinent to the application of mathematical formulas elaborated after Nott (2003) and subsequent refinements to estimate individual wave heights necessary for lift from parent sea cliffs and subsequent traction. Although the ratio of boulders to clasts diminishes from the proximal to distal end of the structure, relatively large boulders populate all transects and the average wave height required for the release of joint-bound blocks at the rocky shore amounts to 5 m. Based on the region’s historical record of hurricanes, such storms tend to decrease in intensity as they migrate northward through the Gulf of California’s 1100-km length. However, the size and complexity of the San Luis Gonzaga spit suggests that a multitude of extreme storm events impacted the island in the upper gulf area through the Holocene time, yielding a possible average growth rate between 7 and 8 m/century over the last 10,000 years. In anticipation of future storms, a system to track the movement of sample boulders should be emplaced on the San Luis Gonzaga spit and similar localities with major coastal boulder deposits.

Stratigraphy, provenance and localisation of the titanomagnetite placer at Waikato North Head, South Auckland, New Zealand

The sequence of coastal and river sands of Pleistocene to Holocene age at the north head of the Waikato River is ~80-m thick; the sands are located in a fault-angle depression at the southern end of the Awhitu Peninsula, a 40-km-long coastal sand barrier. The sequence at Waikato North Head (WNH) consists of three main formations: Awhitu Sands, Hood Sands and Mitiwai Sands. The Waiuku Black Sand member of the Hood Sands and the Entrican Sand member of the Mitiwai Sands contain titanomagnetite-rich dune sands, which are currently mined by New Zealand Steel and constitute a giant placer deposit that has a total resource of ~90 Mt Fe. A tephra at the top of the Awhitu Sands is correlated with the ~1000 ka Waiuku (Potaka) tephra. The Mitiwai Sands overlie the 1.85 ka Taupo Pumice. Comparison of electron probe microanalyses (EPMA) of titanomagnetites in heavy mineral separates of surface and drill-hole samples indicated that the bulk of the titanomagnetite in the Waiuku Black sand and the Entrican Sand is derived from andesitic rocks of the Taranaki Volcanoes 220 km to the south. The Waiuku Black Sand may correlate with an influx of andesitic clasts and mafic minerals in the cover beds of the ~400 ka Ararata Terrace in south Taranaki. There is also a contribution of titanomagnetite and minor ilmenite from the ignimbrites of the Taupo Volcanic Zone (TVZ), particularly in the Lower Hood and Entrican sands. Automated mineralogy data collected on selected sand samples contain silicate and resistate minerals that indicate other potential sources in addition to the Taranaki andesites and TVZ ignimbrites. Minor olivine was likely derived from the late Pleistocene basalts of the South Auckland volcanic field, whereas almandine and epidote were likely sourced from metasedimentary rocks of the Murihiku and Waipapa terranes. The formation of the giant placer at WNH was a multistage process involving (1) a supply of titanomagnetite-bearing sand eroded from the late Pleistocene–Holocene andesites of the Taranaki volcanoes that was transported northwards along the coast by the prevailing longshore drift; (2) concentration of titanomagnetite by wave action on beach faces and collection in a coastal embayment on the northern side of a headland of basement rocks at Port Waikato; (3) further concentration by wind action into dune sands during interglacial low sea levels; and (4) preservation by coeval subsidence in a fault-angle depression on the down-thrown north side of the Waikato Fault.

Subaqueous effusive and explosive phases of late Deccan volcanism: evidence from Mumbai Islands, India

About 30 m of massive basaltic lava sheets and pillows belonging to the Deccan Traps are exposed at Borivali in the Sanjay Gandhi National Park, Mumbai (India). These lavas are conformably overlain by ~ 100 m of volcaniclastic deposits exposed in the Kanheri Caves. The pillows are plagioclase-clinopyroxene-phyric, very poorly vesicular, tholeiite basalts and commonly develop one to few central large cavities. Pillow rinds are holohyaline with swallow-tailed lanceolate or slender needle-shaped plagioclase and fan-spherulitic or dendritic textures in clinopyroxene that indicate rapid cooling under water. The volcaniclastic succession lying on top of the pillow lavas is chiefly composed of non-vesicular to scoriaceous/pumiceous, basaltic and andesitic clasts. The numerous sediment dykes within the volcaniclastic succession and the conformably underlying pillow lavas imply that the entire sequence was emplaced in a submarine environment. The fluidal texture and quenched margins in some of the volcanic clasts strongly suggest that at least part of the volcanic clasts were produced by underwater explosive eruptions. This study describes subaqueous erupted pillow lavas in context of continental flood basalts and provides a significant analogy to recent submarine-erupted intraplate oceanic basalts. The occurrence of pillow lavas and volcaniclastic deposits well above the present-day high tide line (160 m) indicates either high eustatic sea level during the Danaian age (61.8–62.9 Ma), or post-rifting regional uplift in the western Deccan Traps.

Reconstruction of a 3700 Ma transgressive marine environment from Isua (Greenland): Sedimentology, stratigraphy and geochemical signatures

The rare preservation of a stratigraphy in the northwest of the Eoarchean Isua supracrustal belt (Greenland) demonstrates the world's earliest-recognised marine transgression. Stratigraphically lowest is the ~3710 Ma Solvang Volcanic Formation of picrites, basalts and basaltic-andesites with arc-like geochemical signatures. Close to its erosional top, this formation's volcanic rocks show increase in K2O with depletion of Na2O, depletion in Sr with no alteration in Eu abundance and δ18OVSMOW increase to +16‰, interpreted as subaerial weathering at ~3700 Ma. The sedimentary Snowpatch Formation was deposited unconformably on the Solvang Volcanic Formation. Locally, the unconformity is marked by the laterally discontinuous Conglomerate Member dominated by rounded quartz clasts, but also with clasts of basalt and andesite up to >10 cm across, layered chert and ultramafic rocks. The Conglomerate Member carries sparse detrital zircons, the youngest of which are ~3710 Ma. The conglomerates are succeeded by the Sandstone and Dolostone Member of layered quartz and dolomite clastic sandstones that locally preserve relict shallow marine cross-bedding with scoured layers, tempestite breccia beds and very rare stromatolite horizons. The Sandstone and Dolostone Member is succeeded by the ~3695 Ma BIF Member.The Snowpatch Formation quartz-dolomite sandstones and interlayered stromatolites have seawater-like REE + Y trace element patterns variably modified by a negligible to significant influx of a terrigenous component. Trace element signatures such as Ti/V ratios and normalising REE + Y data to a 3710 Ma weathered volcanic rock (rather than Post Archean Average Shale) suggest this component was derived from a juvenile arc source, resembling the basement Solvang Volcanic Formation. The detailed field and geochemical evidence such as positive correlation between Fe/Mg and Pr/Yb* in the Snowpatch Formation dolostones and BIF (banded iron formation) indicates that the dolomitic lithologies developed in a shallow water setting with clastic sedimentation, during a marine transgression over a weathered ~3710 Ma volcanic arc basement, followed by BIF deposition in deeper water. Thus by 3700 Ma, shallow-water, exposed, settings had already been established, which were the site of stromatolite development.

Stratigraphy and geochronology of the Nacientes del Teno and Río Damas Formations: Insights into Middle to Late Jurassic Andean volcanism

Author(s): Junkin, William DeForest | Advisor(s): Gans, Phillip B | Abstract: The Nacientes del Teno Formation and the lowermost strata of the Rio Damas Formation in Maule, Chile (35.10°S) comprise a sequence of sedimentary and volcanic rocks deposited along the western margin of the Andean backarc basin during the Middle to Late Jurassic (174-145 Ma). Revised stratigraphy and new U-Pb geochronological data from this classic succession provide insights into the location, composition, and timing of coeval arc volcanic systems.U-Pb geochronological data from detrital and primary volcanic zircons represent the first absolute ages (ca. 172-161 Ma) reported for the Nacientes del Teno Formation. Detrital zircon ages include a tightly clustered population ca. 165 Ma that coincides with a Middle Jurassic trough present in frequency histogram and relative probability plots of published datasets of U-Pb detrital zircon ages, challenging the notion of a lull in volcanic arc activity at this time.The dominantly marine deposits of the Nacientes del Teno Formation interfinger with subordinate subaerial strata. The latter include channelized conglomerate with sub-rounded to sub-angular andesitic clasts up to 40 cm in diameter, suggesting a proximal alluvial fan and fan delta depositional environment with alternating continental and marine deposition. The Nacientes del Teno Formation includes a quartz rhyolite ignimbrite that yielded a U-Pb zircon age of ca. 160 Ma, the first Jurassic rhyolite reported for this portion of the Andes in central Chile, and in stark departure from the commonly assumed Miocene and Triassic ages for local rhyolites. Paleocurrent indicators from the W ± ~90° together with a general eastward decrease in grain size, decrease in volcanic content, and increase in carbonate content of marine deposits, suggest a sediment source to the west. The proximal volcaniclastic stratigraphy and tight distribution of U-Pb detrital zircon ages of the Nacientes del Teno Formation suggest nearby andesitic stratovolcanoes in the Middle Jurassic. These hypothesized volcanoes are located much further east than traditional models for the location of the “main magmatic arc” during the Jurassic, challenging the notion that Jurassic arc volcanism at this latitude was restricted to a single, narrow (a few tens of kilometers wide) volcanic chain located in the present day Coastal Cordillera.Sedimentological features of lowermost Rio Damas Formation strata at 35.10°S are consistent with deposition in an alluvial fan environment. A maximum depositional age of ca. 151 Ma calculated from U-Pb detrital zircon ages agrees with previously published maximum depositional ages for the Rio Damas Formation. Sedimentological and geochronologic evidence suggest that the Late Jurassic regression represented by the formation resulted from tectonic uplift.

Explosive destruction of a Pliocene hot lava dome underwater: Dogashima (Japan)

Transition from effusive to explosive volcanism is common during subaerial eruptions, and here we demonstrate that this behavior is also possible underwater. The pyroclastic facies produced underwater are distinctive and can be used to distinguish subaqueous from subaerial eruptions and depositional settings. The Pliocene Dogashima Formation (Izu Peninsula, Japan) is a pumice-rich succession originally deposited in an open-marine, below wave-base setting (Jutzeler et al., 2014a). A thick, clast-supported, gray andesite breccia composed of very coarse, dense andesite clasts with quenched margins was sourced from disintegration of an active lava dome. Overall, the gray andesite breccia is gradationally to sharply overlain by thick, graded, clast-supported white pumice breccia chiefly composed of angular pumice clasts and free broken crystals. Regional setting and distinctive facies show that this succession was produced by a fully underwater, magmatic volatile-driven, pumice-forming explosive eruption. The gradational contact between the two breccias, compositional similarities, rare mingled clasts, and fluidal textures in the gray andesite clasts suggest that the explosive eruption destroyed a hot lava dome and generated an eruption-fed, high-concentration density current. In most places, the coarsest hot lava dome fragments were deposited first, followed by the lower density white pumice clasts. The low amount of fine (<2mm) components, well-developed hydraulically controlled grading and sorting, clast angularity, and very coarse dome-derived clasts, some including well-defined quenched margins and common fluidal textures, distinguish the products of subaqueous effusive-to-explosive eruptions from their subaerial counterparts.

Geochemical constraints on the provenance and depositional setting of Neoproterozoic volcaniclastic rocks on the northern margin of the Yangtze Block, China: Implications for the tectonic evolution of the northern margin of the Yangtze Block

Neoproterozoic volcanic and sedimentary rocks, cropping out along the Chengkou–Fengxi Fault on the northern margin of the Yangtze Block in China, consist primarily of volcaniclastic deposits and sandstone–conglomerate successions. The volcaniclastic deposits (termed the Longtanhe Formation) are composed of volcanogenic sedimentary rocks with several primary volcaniclastic rock interlayers. The Longtanhe Formation provides a good opportunity to investigate the Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block. In this paper, geochemistry and zircon U–Pb geochronology are used to infer the provenance and tectonic setting of the Longtanhe Formation volcaniclastic rocks. Two volcanic sandstone samples, collected from the base and top part of the Longtanhe Formation respectively, and one tuff sample were studied for zircon U–Pb dating. They show three consistent age populations with peaks at ∼825Ma, ∼770Ma and 703Ma, indicating that the Longtanhe Formation was deposited after 703Ma and there were no changes in the sources during the period of sedimentation. Chondrite-normalized REE patterns are uniform with slight LREE enrichment (LaN/YbN, average 3.8) and flat HREE patterns without a distinct Eu anomaly (Eu/Eu*, average 0.99), indicating a magmatic arc source. Sc–V enrichment, high La/Th (average 9.4), Cr/Ni (average 2.1) and low Th/Sc (commonly <0.1) suggest that the volcaniclastic rocks were mainly derived from basic to intermediate rocks. Large volumes of basaltic to andesitic clasts in volcaniclastic rocks also suggests that the source rocks were predominantly mafic rocks. A few samples have higher Th/Sc (0.12–0.24) ratios and Hf (4.70–4.90) contents implying minor felsic rocks in the source area. A small amount of quartz clasts in matrix for these six samples also supports a minor contribution from felsic rocks. Not only the lapilli in tuff but also the pebbles and lithic grains in volcanic sandstones are characterized by poor rounding, indicating that the Longtanhe Formation was deposited near-source. Both major- and trace-element discrimination diagrams suggest an oceanic island arc-related setting for the Longtanhe Formation. All these lines of evidence suggest that a contemporaneous (703Ma) magmatic arc, primarily consisting of basalt to andesite, was the main source for the Longtanhe Formation volcaniclastic rocks. The older and minor felsic components were probably derived from mafic to felsic rocks with an arc signature that cropped out widely in the Hannan Complex (840–746Ma) and/or Yaolinghe Group (845730 Ma). Based on its arc-related geochemical affinity, the Longtanhe Formation is proposed to have been deposited in an oceanic arc-related setting, indicating that oceanic crust subduction beneath the northern margin of the Yangtze Block had not ended before 703Ma.

Lithofacies architecture of basaltic andesite lavas and their interaction with wet-sediment: Port a’ Chroinn, Kerrera, NW Scotland

Synopsis Lavas of the Lorne Plateau Volcanic Succession (LPVS) crop out on the island of Kerrera, NW Scotland, and were emplaced during the Devonian Period, synchronous with Old Red Sandstone (ORS) terrestrial sedimentation. These flows are discontinuously exposed for c . 3 km at Port a’ Chroinn, Kerrera, and typically comprise blocky, basal breccias, coherent, crystalline cores, and clinker flow tops, interpreted as basaltic andesite ’a’a lavas. Gradational zones are observed from coherent core, through highly vesicular core, to semi-coherent lava and normally graded clinker. Prismatically jointed, inflated pahoehoe lavas are also present, but have few characteristic morphological features. Localized domains of magma–sediment mingling occur at the bases of several lavas, with clasts of basaltic andesite within a sandstone matrix. These domains are interpreted as peperites, formed as lava flowed over and mingled with unconsolidated, wet sediment that had been deposited on the surfaces of underlying lavas during periods of volcanic quiescence.

Plate tephra: Preserved bubble walls from large slug bursts during violent Strombolian eruptions

Research Article| January 01, 2014 Plate tephra: Preserved bubble walls from large slug bursts during violent Strombolian eruptions Dawn C.S. Ruth; Dawn C.S. Ruth * 1Department of Geology, 411 Cooke Hall, State University of New York University at Buffalo, Buffalo, New York 14260, USA *E-mail: dcs34@buffalo.edu. Search for other works by this author on: GSW Google Scholar Eliza S. Calder Eliza S. Calder 1Department of Geology, 411 Cooke Hall, State University of New York University at Buffalo, Buffalo, New York 14260, USA2School of Geosciences, Grant Institute, University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh EH9 3JW, UK Search for other works by this author on: GSW Google Scholar Geology (2014) 42 (1): 11–14. https://doi.org/10.1130/G34859.1 Article history received: 20 Jun 2013 rev-recd: 09 Sep 2013 accepted: 10 Sep 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Dawn C.S. Ruth, Eliza S. Calder; Plate tephra: Preserved bubble walls from large slug bursts during violent Strombolian eruptions. Geology 2014;; 42 (1): 11–14. doi: https://doi.org/10.1130/G34859.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We describe unusual “plate tephra” particles that provide key information about rarely observed processes that occur during volcanic eruptions. The tephra formed during the 2008–2009 eruption of Llaima volcano, Chile, and dispersed as far as 9 km from the vent. The plates are angular clasts of vesicular basaltic andesite ranging in size from 1 to 14 cm and in thickness from 2 to 5 mm. External features such as ridges, varying degrees of curvature, and adhered material are present. Internal textures include strong crystal alignment and deformed enclaves. We propose that the plates are wall fragments formed during the rupture of large gas slugs associated with unsteady fire fountaining during the violent Strombolian phase of the eruption. The presence of plate tephra may be a diagnostic feature of highly unsteady activity where slug rupture is concurrent with the formation of a sustained eruption column. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetic study of juvenile basaltic–andesite clasts from Andean pyroclastic density current deposits

Additional paleomagnetic data are necessary to improve geomagnetic models of secular variation during the Holocene, especially from the southern hemisphere. In most of the Andean volcanoes from Ecuador to the Chilean central volcanic zone, very well dated lava flows are rare. In contrast, andesitic to basaltic pyroclastic density current (PDC) deposits commonly contain charcoal, facilitating their age determination with 14C. In this study we present the magnetic properties and the paleomagnetic results obtained from three PDC deposits of basaltic to andesitic composition. One is the 2006 PDC deposit from the Ecuadorian Tungurahua volcano and the two others are well-dated PDC deposits from Chilean volcanoes Osorno and Villarica. Although most paleomagnetic studies in pyroclastic rocks deal with the estimation of emplacement temperatures from bulk deposits or accessory and accidental (non-juvenile) clasts, we show that juvenile clasts embedded in PDC deposits provide well-grouped paleomagnetic directions indicating post-emplacement in situ cooling below Curie points. Moreover, the rapid cooling of the juvenile clasts yields an abundance of single domain titanomagnetite grains providing low unblocking temperatures and a reliable material for paleointensity determination.

Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming

Research Article| May 01, 2012 Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming John P. Craddock; John P. Craddock * 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA *E-mail: Craddock@Macalester.edu. Search for other works by this author on: GSW Google Scholar Jesse Geary; Jesse Geary 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA Search for other works by this author on: GSW Google Scholar David H. Malone David H. Malone 2Department of Geography, Geology, Illinois State University, Campus Box 4400, Normal, Illinois 61790, USA Search for other works by this author on: GSW Google Scholar Author and Article Information John P. Craddock * 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA Jesse Geary 1Geology Department, Macalester College, St. Paul, Minnesota 55105, USA David H. Malone 2Department of Geography, Geology, Illinois State University, Campus Box 4400, Normal, Illinois 61790, USA *E-mail: Craddock@Macalester.edu. Publisher: Geological Society of America Received: 10 Aug 2011 Revision Received: 03 Jan 2012 Accepted: 05 Jan 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (5): 463–466. https://doi.org/10.1130/G32734.1 Article history Received: 10 Aug 2011 Revision Received: 03 Jan 2012 Accepted: 05 Jan 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John P. Craddock, Jesse Geary, David H. Malone; Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming. Geology 2012;; 40 (5): 463–466. doi: https://doi.org/10.1130/G32734.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Carbonate ultracataclasite (CUC) is found as a veneer along the bedding plane portion of the Eocene Heart Mountain detachment (Wyoming, United States). At White Mountain, where the CUC is thickest, we report the discovery of six dikes, as much as 1 m wide, that were intruded vertically ∼120 m from the detachment into the overlying Madison and Bighorn Formations. The horizontal basal detachment and injectite material is texturally, petrographically, geochemically, and isotopically identical, containing sparse, sand- to pebble-sized andesitic clasts, various quartz-calcite melt spherules, and armored lapilli in a matrix (95%) of fine-grained calcite. The relationship of hanging wall displacement to fault gouge generation, and the presence of vertical, 120 m, fault gouge injectites, makes the Heart Mountain slide anomalous to all fault systems, especially as this fault system had only one episode of motion. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Post-Eocene volcanics of the Abazar district, Qazvin, Iran: Mineralogical and geochemical evidence for a complex magmatic evolution

The style of volcanism of post-Eocene volcanism in the Alborz zone of northern Iran is different to that of Eocene volcanism (Karaj Formation). Indeed, the volcanic succession of the Abazar district, located in a narrow volcanic strip within the Alborz magmatic assemblage, is characterized by distinct mineralogical and chemical compositions linked to a complex magmatic evolution. The succession was produced by explosive eruptions followed by effusive eruptions. Two main volcanic events are recognized: (1) a thin rhyolitic ignimbritic sheet underlain by a thicker lithic breccia, and (2) lava flows including shoshonite, latite, and andesite that overlie the first event across a reddish soil horizon. Plagioclase in shoshonite (An 48–92) shows normal zoning, whereas plagioclase in latite and andesite (An 48–75) has a similar composition but shows reverse and oscillatory zoning. QUILF temperature calculations for shoshonites and andesites yield temperatures of 1035 °C and 1029 °C, respectively. The geothermometers proposed by Ridolfi et al. (2010) and Holland and Blundy (1994) yield temperatures of 960 °C and 944 °C for latitic lava, respectively. The samples of volcanic rock show a typical geochemical signature of the continental arc regime, but the andesites clearly differ from the shoshonites, the latites and the rhyolites. The mineralogical and chemical characteristics of these rocks are explained by the following petrogenesis: (1) intrusion of a hot, mantle-depth mafic (shoshonitic) magma, which differentiated in the magma chamber to produce a latitic and then a rhyolitic liquid; (2) rhyolitic ignimbritic eruptions from the top of the magma chamber, following by shoshonitic and then latitic extrusions; (3) magma mingling between the latitic and andesitic magmas, as indicated by the occurrence of andesite clasts within the latite; and (4) andesitic effusions. The youngest volcanic events in the Alborz zone show a close chemical relationship with continental arc rocks, indicating that they formed in a continental collision setting.

Lower Cretaceous strata of the Sonora Bisbee Basin: A record of the tectonomagmatic evolution of northwestern Mexico

Lower Cretaceous strata of the Sonora Bisbee Basin record four discrete Paleozoic and Mesozoic magmatic episodes and indicate a previously unrecognized Early Cretaceous regional tectonic event along the southwestern margin of North America. Strata of the Bisbee Group in the Sonora Bisbee Basin overlie Upper Jurassic strata on an angular unconformity. Cretaceous strata are predominantly continental fluvial deposits. Bisbee Group sandstones are sublitharenites and litharenites rich in paleovolcanic lithic fragments. Detrital-zircon age populations form five main groups—Proterozoic, Early Permian–Early Triassic, Late Triassic–Jurassic, earliest Cretaceous, and late Early Cretaceous (Aptian). U-Pb zircon ages from three types of conglomerate clasts in the Morita Formation match these age groups: (1) Quartzite clasts are dominated by Proterozoic ages; (2) rhyolite clasts are Middle Jurassic (ca. 172 Ma); (3) andesite clasts are earliest Cretaceous (ca. 140 Ma). Proterozoic zircons and quartzite clasts were recycled from strata of the Caborca block, whereas Permian and Mesozoic grains and andesite and rhyolite clasts were derived from volcanic arc rocks to the west and northwest. We identify two separate episodes of Early Cretaceous volcanism recorded in the basin, the first between 145 and 133 Ma and the second beginning ca. 125 Ma and continuing into the Albian. The presence in the Sonora Bisbee Basin of near-source fluvial detritus from the first of these episodes requires either the presence of a continental arc along the North American margin during the earliest Cretaceous or the accretion of similar-aged material to the margin prior to ca. 130 Ma. Neovolcanic zircon grains in the upper part of the Morita Formation record the initiation of volcanism in the Alisitos arc, an Early Cretaceous volcanic arc presently exposed in the Peninsular Ranges of Baja and southern California. Structural, stratigraphic, and detrital-zircon data suggest that a heretofore unrecognized episode of regional shortening, uplift, and incision occurred to the southwest of the study area in the interval 148–130 Ma.

Canteen Koppie at Barkly West: South Africa's first diamond mine

Canteen Koppie, a national monument of historical value, occurs next to Barkly West on the north bank of the Vaal River. Diamonds were discovered there in 1869 and it became the first alluvial diamond diggings in South Africa. The digging continued, albeit on and off in the years leading up to 1948 when the site was proclaimed a national monument. The sediments occur in a structurally controlled and glacially modified depression within the andesitic lavas of the Archaean Ventersdorp Supergroup. The fluvial gravels were deposited, and locally mixed with the colluvium, in the downstream end of a palaeo-loop of the Vaal River as a splay deposit where the channel abruptly widens as it exits this narrow loop. The gravel accumulation has been described as the 12 m to 16 m terrace package linked to the Younger Gravels of the Vaal Basin and correlated with the Pleistocene Rietputs Formation. There are two gravel facies associations and one sand facies within the splay unit. Colluvial facies are dominant particularly in the upper part and are composed of large andesite fragments which are mostly sub-angular and lacking obvious abrasion features suggesting that these are of local derivation. The gravel of the fluvial facies are crudely cross-bedded and consist of small to medium sized exotic sub-rounded pebbles that have been mixed with the local andesite boulders in the toes of the scree deposits. These facies are more prominent in the lower part of the succession. The red sand facies occurs as thin cover particularly in the distal part of the gravel units and increases in thickness in the lee of the gravel splay. The exotic clasts in the fluvial gravel are derived from the palaeo-Vaal, erosion of nearby Dwyka sediments which can still be found along the north bank of the loop, and by reworking of higher level and older gravels, remnants of which are still present on top of the hill at Canteen Koppie. The input of the coarse andesite clasts is linked to scree slope deposits fed by exfoliation of local bedrock from this hill. The upward coarsening trend of this infill reflects the gradual abandonment of the loop by the palaeo-Vaal and its inability to remove the coarse colluvium during those latter stages of its occupation of this palaeo-loop. A climatic change to more arid periods might have had some influence. Canteen Koppie has also produced an abundance of Acheulian Stone Age artefacts. These are present in both sedimentary facies suggesting that this splay deposit is at least Late Pliocene to Lower and Middle Pleistocene in age. Recent dating of the overlying sands indicates that these are at least 125 000 years old. Finally an analysis of the mining records suggests that this splay deposit might have produced between 10 000 carats and 15 000 carats of diamonds which would have expressed itself in the region of three to five carats per hundred tonnes. The oversize clasts of the scree deposits would have acted as important traps for the diamonds.

Jurassic volcaniclastic – basaltic andesite – dolerite sequence in Tasmania: new age constraints for fossil plants from Lune River

Jurassic plants excavated from a 12 × 5 m site, at Lune River, southern Tasmania, include an araucarian tree and numerous pteridophytes, belonging to the orders Osmundales, Filicales and Bennettitales. The fossils occur in 2 – 3 m of immature volcanilithic sandstone beds. The sandstone consists primarily of clasts from granitic basement rocks underlying much of southeast Tasmania and mafic clasts containing feldspathic microliths, and primary, phreatomagmatic quartz crystals. Detrital zircons from the sandstones are mostly Early Jurassic (Toarcian) in age (182 ± 4 Ma) with minor Triassic (226 Ma), Devonian (380 – 360 Ma) and Proterozoic populations. Basaltic andesite, hereafter referred to as andesite, caps the volcanilithic units and displays similar ratios of fluid-immobile trace elements (e.g. Zr/Nb, Ti/V), to the Jurassic dolerite found in Tasmania, indicative of a common source. The andesites are correlated with the Jurassic Kirkpatrick Basalts (Trans-Antarctic Mountains, Antarctica) based on their field relationships with bounding strata, age, and distinctive similarities in major-element composition and fluid-immobile trace-element ratios. The andesite is interpreted as an extrusive equivalent of the Tasmanian dolerite. Importantly, drillcore from Lune River contains stoped clasts of andesite in fine-grained dolerite, indicating that the andesite pre-dates the dolerite. Thermal alteration index of microfossils (3 – 3.3) and reflectance of organic material within the sediments (0.54 – 0.77 Ro) resulted from contact metamorphism associated with the emplacement of this basalt. The sedimentology and stratigraphy of the depositional environment, plus the presence of hydrophilic pteridophytes and gymnosperms, indicates that the Toarcian environment was temperate to warm and humid, with an abundant supply of water.

Prehistoric Stratigraphy of the Soufrière Hills–South Soufrière Hills Volcanic Complex, Montserrat, West Indies

Abstract The Soufriere Hills and South Soufriere Hills volcanoes are morphologically distinct but petrologically similar. The history of the complex is subdivided into seven episodes. The Soufriere Hills subunit I (≤175 ka) has deposits of Pelean style with ignimbrites of Plinian style abundant near its top. This is followed by deposits from the South Soufriere Hills (∼130 ka) that represent open‐crater St. Vincent–style activity. Soufriere Hills subunits II–V (∼112,000–∼400 BP) return to more characteristic Pelean activity; at ∼22,000 BP, block‐and‐ash flow deposits containing abundant clasts of vesicular andesite dominate. Age dating shows that renewed activity began just before European settlement in 1632 CE. This activity, including the ongoing eruption, is typical of the dominant eruptive style of this volcano for the last 175 ka.

Early stages in the evolution of Izu–Bonin arc volcanism: New age, chemical, and isotopic constraints

A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40Ar/ 39Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74 ± 0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0 ± 0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84 ± 0.14 to 43.88 ± 0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply west-dipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench.