scholarly journals The Late Carboniferous deeply eroded Tharandt Forest caldera–Niederbobritzsch granite complex: a post-Variscan long-lived magmatic system in central Europe

2021 ◽  
Author(s):  
Christoph Breitkreuz ◽  
Alexandra Käßner ◽  
Marion Tichomirowa ◽  
Manuel Lapp ◽  
Shan Huang ◽  
...  
Keyword(s):  
Central Europe ◽  
Late Carboniferous ◽  
Geochemical Data ◽  
Volcanic Complex ◽  
Magmatic System ◽  
Minimum Thickness ◽  
Rhyolitic Magma ◽  
Two Samples ◽  
Granite Complex ◽  
Radiometric Ages

AbstractSamples and documentation of outcrops and drillings, facies analysis, whole rock geochemistry and radiometric ages have been employed to re-evaluate the Late Carboniferous Tharandt Forest caldera (TFC) and the co-genetic Niederbobritzsch granite (NBG) in the eastern Erzgebirge near Dresden, Germany. The c. 52 km2 TFC harbours strongly welded ignimbrites with a preserved minimum thickness of 550 m. Composition of initial fallout tephra at the base of the TFC fill, comprising lithics of rhyolitic and basic lava, and of silica-rich pyroclastic rocks, suggests a bimodal volcanic activity in the area prior to the climactic TFC eruption. The lower part of the TFC fill comprises quartz-poor ignimbrites, overlain by quartz-rich ignimbrites, apparently without a depositional break. Landslides originating from the collapse collar of the caldera plunged into the still hot TFC fill producing monolithic gneiss mesobreccia with clasts ≤ 1 m in a pyroclastic matrix. Aphanitic and porphyritic rhyolitic magma formed ring- and radial dykes, and subvolcanic bodies in the centre of TFC. Whole rock geochemical data indicate a high silica (most samples have > 73 wt% SiO2) rhyolitic composition of the TFC magma, and a similar granodiorite–granitic composition for the NBG. Based on drillings and caldera extent, a minimum volume of 22 km3 of TFC fill is preserved, the original fill is assumed at about 33 km3. This estimate translates into a denudation of at least c. 210 m during Late Paleozoic to pre-Cenomanian. Telescopic subsidence of the TFC took place in two, perhaps three stages. A possible TFC outflow facies has been completely eroded and distal TFC tuff has not been recognized in neighboring basins. New CA-ID-TIMS measurements on two TFC samples gave mean zircon ages of 313.4 ± 0.4 Ma and 311.9 ± 0.4 Ma; two samples from NBG resulted in 318.2 ± 0.5 Ma and 319.5 ± 0.4 Ma. In addition, for one sample of the ring dyke an age of ca. 314.5 ± 0.5 Ma has been obtained. These ages, together with field relations, allow for a model of a long-standing evolution of an upper crustal magmatic system (~ 5 Ma?), where pulses of magmatic injection and crustal doming alternate with magmatic quietness and erosion. Together with the Altenberg–Teplice Volcanic Complex, located some 10 km to the southeast, the TFC–NBG Complex represents an early post-Variscan magmatic activity in central Europe.

scholarly journals Zircon geochronology suggests a long-living and active magmatic system beneath the Ciomadul volcanic dome field (eastern-central Europe)

2021 ◽  
Vol 565 ◽  
pp. 116965
Author(s):  
R. Lukács ◽  
L. Caricchi ◽  
A.K. Schmitt ◽  
O. Bachmann ◽  
O. Karakas ◽  
...  
Keyword(s):  
Central Europe ◽  
Zircon Geochronology ◽  
Magmatic System ◽  
Volcanic Dome

The first Permian scorpionfly from Germany (Insecta, Panorpida: Protomeropidae)

2021 ◽  
Vol 4 (3) ◽  
Author(s):  
MARKUS J. POSCHMANN ◽  
ANDRÉ NEL
Keyword(s):  
Central Europe ◽  
Black Shale ◽  
New Genus ◽  
Climatic Changes ◽  
Late Carboniferous ◽  
Early Permian ◽  
New Genus And Species

A new genus and species, Glanomerope virgoferroa gen. et sp. nov., the first Permian record of a scorpionfly from Germany, is described from the Niedermoschel black shale, Meisenheim Formation, Lower Rotliegend of the Saar-Nahe basin. It is assigned to the Protomeropidae, the oldest known family of the holometabolous superorder Panorpida, ranging from the Bashkirian-Moscovian (Late Carboniferous) to the Roadian. It confirms that this family was very diverse in Central Europe during the Early Permian. Protomeropidae possibly became extinct in the course of major climatic changes that progressively affected the supercontinent Pangea after the Artinskian, although generally these changes seem to have more severely affected some other insects such as the palaeopteran Dictyoneuridae than holometabolous groups.

scholarly journals P–T history of kimberlite-hosted garnet lherzolites from South-West Greenland

2007 ◽  
Vol 13 ◽  
pp. 45-48 ◽  
Author(s):  
Mark T. Hutchison ◽  
Louise Josefine Nielsen ◽  
Stefan Bernstein
Keyword(s):  
The Body ◽  
Mantle Xenoliths ◽  
Chemical Compositions ◽  
Equilibrium Pressure ◽  
West Greenland ◽  
Rock Types ◽  
South West ◽  
Three Phase ◽  
Two Samples ◽  
Granite Complex

Exploration for diamonds in West Greenland has experienced a major boost within the last decade following the establishment of world-class diamond mines within the nearby Slave Province of the Canadian Arctic. Numerous companies have active programmes of diamond exploration and increasingly larger diamonds have been discovered, notably a 2.392 carat dodecahedral stone recovered by the Canadian exploration company Hudson Resources Inc. in January 2007. The Geological Survey of Denmark and Greenland (GEUS) is currently carrying out several studies aimed at understanding the petrogenesis of diamondiferous kimberlites in Greenland and the physical and chemical properties of their associated mantle source regions (e.g. Hutchison 2005; Nielsen & Jensen 2005). Constraint of the mantle geotherm, i.e. the variation of temperature with depth for a particular mantle volume, is an important initial step in assessing the likelihood of such a volume to grow diamonds and hence the diamond potential of associated deep-sourced magmatic rocks occurring at surface. Cool geotherms are often present within old cratonic blocks such as West Greenland (Garde et al. 2000) and provide a good environment for the formation of diamonds (Haggerty 1986). This study aims to constrain the mantle geotherm for the southern extent of the North Atlantic Craton in Greenland by applying three-phase geothermobarometry calculations using chemical compositions of clinopyroxene, orthopyroxene and garnet from four-phase kimberlite-hosted lherzolite xenoliths. Xenoliths have been sampled from kimberlites from two areas in South-West Greenland: Midternæs and Pyramide- fjeld (Fig. 1). Kimberlites in the Pyramidefjeld area principally occur as sheeted sills hosted in the Pyramidefjeld granite complex of Palaeoproterozoic Ketilidian age. In contrast, Midternæs kimberlites occur as outcrops within a single, extensive and undulating sill hosted within pre-Ketilidian granodioritic gneiss and Ketilidian supracrustal rocks. Pyramidefjeld kimberlites have been shown to be Mesozoic (Andrews & Emeleus 1971), and work is currently being carried out to further constrain the ages of these and the Midternæs kimberlites and also xenoliths using modern methods. No attempt is made herein to provide a correct petrological classification of the rocks hosting the xenoliths; however, the abundance of clinopyroxene reported by Andrews & Emeleus (1971) suggests that further work may more correctly conclude a classification as ‘orangeite’ after Mitchell (1995). Notwithstanding this, the term ‘kimberlite’ is employed throughout in order to be consistent with that adopted by previous authors. The Precambrian Pyramide fjeld granite complex and adjacent Archaean granod ioritic gneisses are host to several kimberlite sheets located at various levels between 400 and 900 m elevation (Fig. 1A; Andrews & Emeleus 1971, 1975). Kimberlites are mainly found as loose blocks in scree; however, these are almost always sourced locally from in situ bodies. Sheets can often be found deep within overhanging clefts, particularly in granitic walls. The kimberlite bodies are gently dipping, typically 20 degrees, and with a range of strikes. The maximum thickness of sills is approximately 2 m but thickness varies significantly over short distances. In many instances, the occurrence of kimberlite is seen to be controlled locally by structures in the country rocks. Field observations of the range of orientations of intrusive bodies do not appear to suggest a particular focal point which could be a likely location for an intrusive centre such as a pipe. This observation is in line with what is seen throughout West Greenland where kimberlite emplacement appears as dykes and sills (Larsen & Rex 1992) rather than the pipes and blows which are common in other world-wide settings. The occurrence of xenoliths amongst Pyramidefjeld kimberlites is highly variable with the most xenolith-rich localities being in the vicinity of Safirsø (Fig. 1A). The majority of xenoliths are dunites with occasional wehrlites and lherzolites (Emeleus & Andrews 1975). Of particular interest from the point of view of thermobarometry is the occurrence of garnet. This is rarely found, even in clinopyroxene-bearing samples, and the two samples chosen for thermobarometry (Fig. 1A) represent the majority of the garnet-bearing xenoliths identified within an estimated total population of 75 xenoliths collected. The Midternæs kimberlites are hosted in Archaean gneisses and Proterozoic supracrustal rocks (Fig. 1B; Andrews & Emeleus 1971, 1975). The style of kimberlite emplacement and occurrence of garnet-bearing xenoliths are closely similar to those of Pyramidefjeld. Contours of elevation between outcrops suggest that the kimberlites form parts of a largely contiguous single body dipping at approximately 30 degrees to the west-south-west. Individual outcrops as in Pyramidefjeld indicate that the body varies in thickness and undulates in response to local structure. The south-western portion of the body which outcrops near the glacier Sioralik Bræ, is considerably thicker than elsewhere (Fig. 2) and in some places is seen to have a true thickness in excess of 4 m. Xenoliths are less abundant on average than in Pyramidefjeld kimberlites, but a similar variety and proportion of rock types and infrequent occurrence of garnet is observed. The kimberlites from both areas were intruded along zones of platy jointing which likely were caused by degassing of the magma and formed just prior to the kimberlite intrusion. In contrast to some kimberlites in other cratons, very few xenoliths of local, lower crustal rock types have been recognised in the kimberlites from Pyramidefjeld and Mid ternæs. The intrusions are therefore believed to have been of a non-explosive nature, perhaps because of host-rock rheol - ogy or due to emplacement at relatively deep crustal levels. Here we report on calculations of equilibrium pressure and temperature using compositions of three-phase assemblages of garnet, orthopyroxene and clinopyroxene from Midternæs and Pyramidefjeld mantle xenoliths.

Pyroxenites of Kukesi Massif, Mirdita Ophiolite – geological record for magmatic system in SSZ environment – preliminary results

2021 ◽  
Author(s):  
Jakub Mikrut ◽  
Magdalena Matusiak-Małek ◽  
Jacek Puziewicz ◽  
Kujtim Onuzi
Keyword(s):  
Trace Elements ◽  
Major And Trace Elements ◽  
Intermediate Zone ◽  
Primitive Mantle ◽  
Magmatic System ◽  
Two Samples ◽  
Parental Melts ◽  
Lower Crustal ◽  
Mantle Section ◽  
Clinopyroxene Composition

<p>Kukesi massif is located in the eastern part of the Mirdita Ophiolite (northern Albania), which marks suture after Neo-Thetyan ocean closure. It is formed of well-preserved mantle and crustal sections which exhibit Supra-Subduction Zone affinity (e.g. Dilek and Furnes 2009, Lithos). Lower part of the mantle section of the Kukesi massif consist mainly of harzburgites, whereas dunites are located close to Moho. Crustal section records transition from lower part formed by peridotites and pyroxenites (so called intermediate zone after Hoxha and Boullier 1995, Tectonophysics) to gabbros. In this study we focus on composition and origin of pyroxenites occurring in the mantle and lower crustal parts of the Kukesi massif.</p><p>In this study we studied 9 samples. They have composition of olivine websterite, clinopyroxenite, orthopyroxenite, hornblende-clinopyroxenite and websterite. Five of the analyzed samples have mantle origin (M): we studied (M)-olivine websterites and (M)-clinopyroxenite from harzburgitic part, as well as two (M)-orthopyroxenitic veins (one with clinopyroxenitic central part - composite vein) with minor amphibole cross-cutting dunites from one locality. From intermediate zone in crustal (C) part we collected (C)-hornblende-clinopyroxenites and (C)-websterite. </p><p>Clinopyroxene composition is homogeneous in (M)-olivine-websterites (Mg#=84.5-87 and 88.8-90.5; Al=0.07-0.1 and 0.05-0.07, respectively), (M)-clinopyroxenite (Mg#=84-86, Al=0.04-0.08), (C)-hornblende-clinopyroxenites (Mg#=88.5-91, Al=0.08-0.12a.p.f.u.) and (C)-websterite (Mg#=87-88; Al=0.13-0.16a.p.f.u.). It differs widely between (M)-orthopyroxenitic veins: from Mg#=85-94 and Al=0.02-0.08 a.p.f.u  in clinopyroxenitic part of composite vein to Mg#=93.6-95 and Al=0.01-0.03 in the purely orthopyroxenitic one. Orthopyroxene from two samples of  (M)-olivine websterites have either Mg#=83 and Al~0.07 a.p.f.u (Fo<sup>olivine</sup>=81.5) or Mg#=87  and Al~0.04 a.p.f.u (Fo<sup>olivine</sup>=86). Orthopyroxene composition in composite(M)-vein varies in wide ranges (Mg#=83-89; Al=0.04-0.08 a.p.f.u.); the other vein is homogeneous (Mg#=90-91, Al=0.02-0.03 a.p.f.u, Fo<sup>olivine</sup>=86.8-90); in (C)-websterite orthopyroxene has Mg#=82.4-84 and Al=0.12-0.14 a.p.f.u. Amphibole has composition of tremolite-actinolite. Spinel, where present, is highly chromian (Cr#=0.59-0.80).</p><p>Clinopyroxene is LREE-depleted in most of the samples, the (La/Lu)<sub>N</sub>=0.03-0.08. It is also LREE-depleted in (M)-clinopyroxenite ((La/Lu)<sub>N</sub>=0.05-0.23), but the contents of trace elements are higher than in other samples (eg. Lu<sub>N</sub>=0.79-2.75 vs. 0.40-0.85). In (M)-veins the LREE contents are approximately at primitive mantle level ((La/Lu)<sub>N</sub>=0.28-1.66).  Clinopyroxene in all samples has positive Th-U, Pb and Sr anomalies and negative Ta and Zr anomalies, but concentrations of trace elements is significantly higher in (M) clinopyroxenite and veins.</p><p>The presence of tremolite and actinolite points to a retrogressive metamorphism which affected the rocks. The LREE-depleted nature of clinopyroxene forming all the pyroxenites and presence of orthopyroxene  point to crystallization of the rocks from tholeiitic melt, but variations in Mg# and REE content in clinopyroxene may reflect formation either from different generations of melts or from melts fractionated due to reactive percolation.  Variations in composition of the parental melts is visible even in a scale of one outcrop, which is demonstrated by (M)-orthopyroxenite veins with various modal composition and mineral major and trace elements compositions.</p><p>This study was financed from scientific funds for years 2018-2022 as a project within program “Diamond Grant” (DI 024748).</p>

scholarly journals Geochemical and Sr–Nd isotopic features of the Zaro volcanic complex: insights on the magmatic processes triggering a small-scale prehistoric eruption at Ischia island (south Italy)

2020 ◽  
Vol 109 (8) ◽  
pp. 2829-2849
Author(s):  
C. Pelullo ◽  
G. Cirillo ◽  
R. S. Iovine ◽  
I. Arienzo ◽  
M. Aulinas ◽  
...  
Keyword(s):  
Risk Mitigation ◽  
Mafic Magma ◽  
Small Scale ◽  
Volcanic Complex ◽  
Magmatic System ◽  
Mafic Enclaves ◽  
South Italy ◽  
Ischia Island ◽  
Magmatic Processes ◽  
Trace Element Contents

Abstract The prehistoric (< 7 ka) Zaro eruption at Ischia island (Southern Italy) produced a lava complex overlaying a pyroclastic deposit. Although being of low energy, the Zaro eruption might have caused casualties among the neolithic population that inhabited that area of Ischia, and damages to their settlements. A similar eruption at Ischia with its present-day population would turn into a disaster. Therefore, understanding the magmatic processes that triggered the Zaro eruption would be important for volcanic hazard assessment and risk mitigation, so as to improve a knowledge that can be applied to other active volcanic areas worldwide. The main Zaro lava body is trachyte and hosts abundant mafic and felsic enclaves. Here all juvenile facies have been fully characterized from petrographic, geochemical and isotopic viewpoints. The whole dataset (major and trace element contents; Sr–Nd isotopic composition) leads to rule out a genetic link by fractional crystallization among the variable facies. Thus, we suggest that the Zaro mafic enclaves could represent a deep-origin mafic magma that mingled/mixed with the main trachytic one residing in the Ischia shallow magmatic system. The intrusion of such a mafic magma into a shallow reservoir filled by partly crystallized, evolved magma could have destabilized the magmatic system presumably acting as a rapid eruption trigger. The resulting processes of convection, mixing and rejuvenation have possibly played an important role in pre- and syn-eruptive phases also in several eruptions of different sizes in the Neapolitan area and elsewhere in the world.

Illite-smectite from the North Sea investigated by scanning tunnelling microscopy

Clay Minerals ◽  
1992 ◽  
Vol 27 (3) ◽  
pp. 331-342 ◽  
Author(s):  
H. Lindgreen ◽  
J. Garnaes ◽  
F. Besenbacher ◽  
E. Laegsgaard ◽  
I. Stensgaard
Keyword(s):  
Thickness Distribution ◽  
Particle Diameter ◽  
Upper Jurassic ◽  
Scanning Tunnelling Microscopy ◽  
Minimum Thickness ◽  
Two Samples ◽  
Scanning Tunnelling ◽  
Tunnelling Microscopy ◽  
Dimensional Growth ◽  
Particle Thickness

AbstractTwo samples of illite-smectite (I-S) isolated from Upper Jurassic clays in well 2/7-3 (3365 m) and well 2/11-1 (4548 m) have been investigated by scanning tunnelling microscopy (STM), and the particle shape and dimensions have been correlated to results from transmission electron microscopy (TEM) on shadowed specimens and results from atomic-force microscopy (AFM). By STM, lath-like and equant particles were observed in both samples, some of the particles having sharp edges. In the I-S from well 2/11-1, spiral-shaped particles were also seen. For both samples, the most frequent particle diameter was 100–200 Å. Particle-thickness distributions from STM and TEM were similar for the I-S from well 2/7-3, being dominated by 10 Å thick particles. For I-S from 2/11-1, the STM particle-thickness distribution has a predominance of 20 Å thick particles, but the TEM particle-thickness distribution is broad, with about equal amounts of 20, 30, 40, 50 and 60 Å thick particles. The AFM particle-thickness distribution for this I-S resembles the TEM thickness distribution. It is concluded that mainly thin (10 and 20 Å) particles are seen by STM. Failure of AFM to show sharp particle edges (seen in STM and TEM) might be attributed to the AFM tip movement or tip shape. In STM, I-S particles from well 2/7-3 have peaks along edges, whereas I-S particles from well 2/11-1 have rims. These rims are also seen in AFM and are therefore real geometrical features, probably a result of two-dimensional growth, whereas the spirals in the I-S from well 2/11-1 demonstrate three-dimensional growth. The minimum thickness of most particles is 10 Å.

Paleoenvironmental conditions and sedimentation dynamics in Central Europe inferred from geochemical data of the loess-paleosol sequence at Süttő (Hungary)

2018 ◽  
Vol 196 ◽  
pp. 21-37 ◽  
Author(s):  
Jörn Profe ◽  
Lena Neumann ◽  
Ágnes Novothny ◽  
Gabriella Barta ◽  
Christian Rolf ◽  
...  
Keyword(s):  
Central Europe ◽  
Geochemical Data ◽  
Sedimentation Dynamics

The ’Little’ Fish Canyon Tuff in Romania: Rejuvenation of granodioritic crystal mush resulting in homogeneous dacite recorded by the Haramul Mic lava dome (Ciomadul)

2020 ◽  
Author(s):  
Emese Pánczél ◽  
Maurizio Petrelli ◽  
Réka Lukács ◽  
Szabolcs Harangi
Keyword(s):  
Lava Dome ◽  
Thermal Evolution ◽  
Bulk Composition ◽  
Volcanic Complex ◽  
Bulk Rock ◽  
Crystal Mush ◽  
Magmatic System ◽  
Zircon Core ◽  
Accessory Zircon ◽  
Zircon Crystallization

&lt;p&gt;Long-dormant volcanoes (quiescence time is several 100&amp;#8217;s to 10&amp;#8217;s thousand years between eruptions) pose a particular hazard, since the long repose time decreases the awareness and there is mostly a lack of monitoring. The Haramul Mic, a pancake-shaped flat dacitic lava dome is part of the Ciomadul Volcanic Complex in eastern-central Europe (Romania) and serves as an excellent example of such volcanoes. The Haramul Mic lava dome is the earliest product of the Young Ciomadul Eruption Period (YCEP), when the activity recrudesced in the area after about 200.000 years quiescence time. Eruption age of the dome determined by (U-Th)/He dating on zircon gave 154 +/- 16 ka that is in agreement with the youngest zircon U-Th outer rim date (142 +18/-16 ka). In the YCEP zircon crystallization dates record typically long, up to 350-400 kyr lifetime of the magmatic plumbing system, in case of &amp;#160;Haramul Mic the oldest zircon core is 306 +/- 37 ka old.&lt;/p&gt;&lt;p&gt;The 880.7 m high lava dome covers an area of 1.1 km&lt;sup&gt;2&lt;/sup&gt; and has a volume of ~0.15 km&lt;sup&gt;3&lt;/sup&gt;. It is composed of crystal-rich homogeneous high-K dacite. The average crystal content is 35-40% and consists of plagioclase, amphibole, biotite and accessory zircon, apatite, titanite and Fe-Ti oxides. The groundmass is mainly built up by perlitic glass with some microlites. The dacite includes mafic enclaves having plagioclase and amphibole besides a large amount of biotite crystals, that eventuates K-rich, shoshonitic bulk composition. The dacite contains abundant felsic crystal clots which comprise plagioclase, amphibole, biotite and interstitial vesicular glass.&lt;/p&gt;&lt;p&gt;Amphiboles are relatively homogeneous in chemical composition. They are low-Al hornblendes suggesting 700-800 &lt;sup&gt;o&lt;/sup&gt;C crystallization condition at 200-300 MPa compared with experimental data. Al-in-hornblende geobarometer and amphibole-plagioclase geothermometer calculations give results reproducing these temperature and pressure ranges. Although the Kis-Haram dacite is fairly rich in 25-45 anorthite mol% plagioclase, no negative Eu anomaly can be observed in the bulk rock and the glass. Similarities between Fish Canyon Tuff and Kis-Haram rocks can be strikingly noted regarding the major and trace element contents of mineral phases, glass and bulk rock that all refer to a wet oxidised calc-alkaline magmatic system. The relatively small volume Kis-Haram lava dome represents a rejuvenated low-temperature granodioritic crystal mush having similar features as the large volume silicic eruption of Fish Canyon Tuff. Their recorded almost similarly long zircon crystallization intervals give an interesting aspect with regard to the thermal evolution of the magmatic system and eruption volumes.&lt;/p&gt;&lt;p&gt;This research was financed by the Hungarian National Research, Development and Innovation Fund (NKFIH) within No. K116528 project and was supported by the &amp;#218;NKP-19-1 New National Excellence Program of the Ministry for Innovation and Technology.&lt;/p&gt;

scholarly journals The Huckleberry Ridge Tuff, Yellowstone: evacuation of multiple magmatic systems in a complex episodic eruption

2019 ◽  
Vol 60 (7) ◽  
pp. 1371-1426
Author(s):  
Elliot J Swallow ◽  
Colin J N Wilson ◽  
Bruce L A Charlier ◽  
John A Gamble
Keyword(s):  
Mafic Magma ◽  
Geochemical Data ◽  
Magmatic System ◽  
Isotopic Data ◽  
Magmatic Complex ◽  
System 2 ◽  
Compositional Diversity ◽  
System 1 ◽  
Glass Compositions ◽  
Fall Deposits

Abstract The 2·08 Ma, ∼2500 km3 Huckleberry Ridge Tuff (HRT) eruption, Yellowstone, generated two fall deposits and three ignimbrite members (A, B, C), accompanying a ∼95 x 65 km caldera collapse. Field data imply that the pre-A fall deposits took weeks to be erupted, then breaks of weeks to months occurred between members A and B, and years to decades between B and C. We present compositional and isotopic data from single silicic clasts (pumice or fiamme) in the three ignimbrite members, plus new data from co-eruptive mafic components to reconstruct the nature and evacuation history of the HRT crustal magmatic complex. Geochemical data, building on field characteristics, are used to group nine silicic clast types into seven compositional suites (A1-A3; B1; C1-C3) within their respective members A, B and C. Isotopic data are then added to define four magmatic systems that were tapped simultaneously and/or sequentially during the eruption. Systems 1 and 2 fed the initial fall deposits and then vented throughout member A, accompanied by trace amounts of mafic magma. In member A, volumetrically dominant system 1 is represented by a rhyolite suite (A1: 73·0–77·7 wt % SiO2, 450–1680 ppm Ba) plus a distinct low-silica rhyolite suite (A2: 69·2–71·6 wt % SiO2, >2500 ppm Ba). System 2 yielded only a low-Ba, high-silica rhyolite suite (A3: 76·7–77·4 wt % SiO2, ≤250 ppm Ba). Glass compositions in pumices from systems 1 and 2 show clustering, indicative of the same multiple melt-dominant bodies identified in the initial fall deposits and earliest ignimbrite. Member B samples define suite B1 (70·7–77·4 wt % SiO2, 540–3040 ppm Ba) derived from magmatic system 1 (but not 2) that had undergone mixing and reorganisation during the A: B time break, accompanying mafic magma inputs. Mafic scoriae erupted in upper member B cover similar compositions to the member A clasts, but extend over a much broader compositional range. Member C clast compositions reflect major changes during the B: C time break, including rejuvenation of magmatic system 2 (last seen in member A) as suite C3 (75·3–77·2 wt % SiO2, 100–410 ppm Ba), plus the appearance of two new suites with strong crustal signatures. Suite C2 is another rhyolite (74·7–77·6 wt % SiO2, with Ba decreasing with silica from 2840 to 470 ppm) that defines magmatic system 3. Suite C2 also shows clustered glass compositions, suggesting that multiple melt-dominant bodies were a repetitive feature of the HRT magmatic complex. Suite C1, in contrast, is dacite to rhyolite (65·6–75·0 wt % SiO2, with Ba increasing with silica from 750 to 1710 ppm) that defines magmatic system 4. Compositions from magmatic systems 1 and 2 dominantly reflect fractional crystallization, but include partial melting of cumulates related to earlier intrusions of the same mafic magmas as those syn-eruptively vented. Country rock assimilation was limited to minor amounts of a more radiogenic (with respect to Sr) evolved contaminant. In contrast, systems 3 and 4 show similar strongly crustal isotopic compositions (despite their differences in elemental composition) consistent with assimilation of Archean rocks via partial melts derived from cumulates associated with contrasting mafic lineages. System 3 links to the same HRT mafic compositions co-erupted in members A and B. In contrast, system 4 links to olivine tholeiite compositions erupted in the Yellowstone area before, sparsely during, and following the HRT itself. All four magmatic systems were housed beneath the HRT caldera area. Systems 1 and 2 were hosted in Archean crust that had been modified by Cretaceous/Eocene magmatism, whereas systems 3 and 4 were hosted within crust that retained Archean isotopic characteristics. The extreme compositional diversity in the HRT highlights the spatial and temporal complexities that can be associated with large-volume silicic magmatism.

Sign in / Sign up

Export Citation Format

Share Document