Minor Trachyte Research Articles

Makran ophiolitic basalts (SE Iran) record Late Cretaceous Neotethys plume-ridge interaction

ABSTRACT The Makran complex in southeast Iran provides a spectacular subduction-related accretionary complex to understand the mechanism of oceanic accretion and the evolution of subduction zones. In this paper, we present new major and trace element data as well as isotopic compositions of mafic volcanic blocks from the Makran ophiolitic mélange complex (OMC). Our aim is to assess the genesis of these rocks and discuss their implications on the evolution of Neotethys Ocean. These volcanic blocks are composed mainly of basalts with minor trachytes. The Makran lavas are occasionally interlayered with tuff layers. Zircons from these tuffs give U-Pb ages of 95 Ma, which is well in accordance with the reposted microfossil data for the interlayered pelagic limestones with pillow lavas. Makran basalts can be geochemically subdivided into four groups; normal to transitional MORB, enriched-MORB, Plume-type MORB and alkaline (-OIB-like) basalts. The OIB-like pillow lavas are represented by high values of Th/Tb (6.3–7.4) which are higher than other basalts (group 1 = 0.3–0.8; groups 2 = 0.7–1.6; group 3 = 1.58–1.36).143Nd/144Nd(t) ratios for basalts ranges from 0.51247 to 0.51292, whereas 87Sr/86Sr(t) isotopic composition of the OMC lavas varies from 0.704433 to 0.709466. The Pb isotopic composition of the lavas are quite high, ranging from 15.49–15.66 for 207Pb/204Pb(t), 18.09–19.12 for 206Pb/204Pb(t) and 37.80–39.23 for 208Pb/204Pb(t). The chemistry of these rocks suggests that they were formed most likely in an oceanic setting with clear plume-ridge interaction. These rocks can form from partial melting of a highly heterogeneous mantle source, which is extensively metasomatized with deep mantle OIB-type components. We suggest these rocks have been generated in an oceanic ridge with plume-ridge interaction, similar to the Iceland-Reykjanes Ridge, before being fragmented and accreted into the Makran accretionary complex.

Differential Fractionation of Rhyolites During the Course of Crustal Extension, Western Afar (Ethiopian Rift)

AbstractWe report field observation, age, chemical (major and trace elements), and isotope (Sr‐Nd‐Pb) data for felsic volcanic rocks from Central Afar and adjacent western margin. Investigated volcanic rocks are dominantly rhyolites with minor trachytes, and they are geochemically similar. Their ages range from ~30 Ma (prerift stage), ~20 Ma (early synrift), ~8–4 Ma (main thinning event) to ~2.5–0.1 Ma (late synrift/continental breakup), representing the entire volcanic‐tectonic events that occurred episodically. Major element variations are consistent with fractionation of gabbroic cumulates. Trace element and isotope data preclude an origin by crustal anatexis; the rhyolites are rather genetically linked to the associated basalts and variously contaminated by the crust during differentiation of magmas. Chemical and isotopic data of the rhyolites support an origin by open system differentiation at deep crustal levels (hot wall rock and high r = rate of assimilation/rate of crystallization) and shallow crustal levels (cold wall rock and low r) with a change in the composition of the assimilated material from lower crustal to upper crustal type. Assimilation appears to decrease in recent times with Quaternary rhyolites, emplaced nearby the active magmatic segments in Afar, which exhibit the isotopic compositions closest to original mantle signature. This is compatible with a crust below the active magmatic segments resulting from important addition of juvenile basic magmas. Such results and interpretations provide actual constrains to suggest that the present‐day stage is probably very close to continental breakup, which will be achieved once the continental crust will be entirely replaced by new magmatic crust.

Marco geológico y tectónico de la Isla del Coco y la región marítima circunvecina, Costa Rica

La Isla del Coco es la única isla oceánica y el único afloramiento subaéreo de la Cordillera Volcánica del Coco, el rasgo geográfico y geológico más extenso en aguas territoriales costarricenses. Desde el punto de vista geológico, está conformada por rocas volcánicas, predominantemente coladas de lavas basálticas y traquíticas en menor cantidad, con rocas piroclásticas y epiclásticas subordinadas. Posee suelos, coluvios y depósitos de playas (arenas y cantos) superficiales; su topografía es muy variable pero predomina el relieve quebrado y rugoso. La Isla del Coco es la parte emergida de un volcán submarino de evolución compleja, desarrollado a partir y durante el Pleistoceno Inferior (entre 2.2 y 1.5 millones de años, Ma), producto de una anomalía térmica en el manto a través de varias fisuras, que originaron varios alineamientos de volcanes submarinos. La Isla del Coco se encuentra en un estadio de erosión activo, y sus arcos y plataformas sumergidas (90-110m y 183m) son probablemente el producto de una erosión subaérea durante las dos últimas máximas glaciaciones, combinados quizás con la subsidencia debida al enfriamiento del escudo volcánico y de la corteza oceánica. Al ser la isla geológicamente joven, posee importantes implicaciones para la comprensión de la evolución y el endemismo de su biodiversidad. Las amenazas en la geodinámica principales identificadas son los deslizamientos, los tsunamis y, en menor grado, la sismicidad, poco frecuente y con magnitudes moderadas (≤ 5.8 Mw hasta la fecha) dentro de un radio de alrededor de 300km, predominantemente asociada con fallas dextrales de rumbo N-S. Los pocos datos obtenidos hasta el momento indican que hay un cierto grado de actividad sísmica en los alrededores de la Isla del Coco, relacionados con las fallas locales. Algunos pequeños tsunamis históricos y prehistóricos han afectado a la Isla del Coco y los sismos así como la precipitación pluvial elevada han generado deslizamientos. La licuefacción está restringida a las dos playas arenosas (bahías Chatham y Wafer).

The tectonic setting of early Permian bimodal volcanism in central Inner Mongolia: continental rift, post-collisional extension, or active continental margin?

ABSTRACTThe petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive εNd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The εNd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.

Chapter 8 Regional comparisons, petrochemistry and petrogenesis

In a short review of the structure and volcanism of the southern part of the Gregory Rift, Dawson (1992) drew attention to differences in the contemporaneous volcanism in southern Kenya and northern Tanzania, the differences taking place abruptly at a latitude of c. 3°S. In Chapter 3 (Dawson 2008), differences were noted in the relationship of the volcanism to their setting relative to the buried cratonic margin and to the thermal structure of the crust and the upper mantle in the two areas. The composition of the mantle and its effect on the nature of the volcanicity are discussed in greater detail here. Table 8.1 summarizes the volcanic stratigraphy of southern Kenya and northern Tanzania, mainly from c. 8 Ma, i.e. the age of the oldest known activity in northern Tanzania at Essimingor. The period 8– c. 1.3 Ma (the time of the Older Extrusives) is dominated by central-type volcanoes characterized by effusion of basanites, basalts and minor trachytes, the last being reasonably interpreted as fractionates of the more voluminous basic rocks. There were some peralkaline volcanoes such as Essimingor, Sadiman and Mosonik, which, particularly in the case of Essimingor, the oldest of the northern Tanzania volcanoes, are similar to the peralkaline volcanoes (Mt Elgon, Napak, Yelele) that erupted prior to the onset of basaltic activity in Kenya (Baker et al. 1972). View this table: Table 8.1. Volcanic stratigraphy of S. Kenya and N. Tanzania In southern Kenya during this period, volcanic rocks were erupted from both major central volcanoes or from fissures and very minor volcanoes, the latter including the rift basalts and the Magadi flood trachytes. The Lengitoto trachytes (6.9–5.0 Ma) and the Kirikiti basalts (3.1–2.5 Ma) are exposed along the western flanks of the rift (Crossley 1979), whereas the faults along the eastern flank expose the Singaraini, Ol Keja Nero and …

Constructions et destructions du stratovolcan du Cantal

Constructions et destructions du stratovolcan du Cantal

Pliocene Intrusions in the Gunung Bijih (Ertsberg) Mining District, Irian Jaya, Indonesia: Major- and Trace-Element Chemistry

A group of small, hypabyssal Pliocene intrusions are associated with Cu-Au ore deposits in the Gunung Bijih (Ertsberg) Mining District of Irian Jaya (western New Guinea), Indonesia. Several skarn orebodies (GB, GBT, Dom, and Big Gossan) are located around the margins of the Ertsberg Intrusion. All but the Big Gossan deposit are related genetically to the Ertsberg Intrusion, at least in terms of calc-silicate formation. The supergiant Grasberg porphyry copper deposit is hosted in and related to the first two stages of intrusions in the Grasberg Complex. The intrusions are intermediate in composition, and can be divided into a high-K suite of latite to trachydacite and minor trachyte, and a low-K suite of andesite and dacite. All intrusions are enriched in large-ion lithophile elements (LILE) and have Nb and Ti depletions. The low-K suite tends to have higher La/Th and Ba/Th ratios than the high-K suite. Chemical variation in the Ertsberg Intrusion is largely the result of in situ fractional crystallization...

Petrology and petrogenesis of a tertiary bimodal dolerite‐peralkaline/subalkaline trachyte/rhyolite dyke association from Lundy, Bristol Channel, UK

AbstractThe island of Lundy forms the southernmost igneous complex of the British Tertiary Volcanic Province (BTVP) and consists of granite (≈ 90%) emplaced into deformed Devonian sedimentary rocks (Pilton Shale) and associated with a swarm of dykes of dolerite/basalt, minor trachyte and rhyolite composition. The dolerites are of varied olivine basalt composition and are associated with peralkaline trachyte and subalkaline/peralkaline rhyolite with alkali feldspar and quartz ± alkali amphibole ± pyroxene mineralogy. The dyke swarm is therefore an anorogenic bimodal dolerite/basalt–trachyte/rhyolite BTVP association. Although the dyke association is bimodal in major element terms between dolerite/basalt and minor trachyte/rhyolite, the mineralogy and trace element geochemistry indicate that the dykes may be regarded as a cogenetic dolerite—peralkaline trachyte/rhyolite association with minor subalkaline rhyolites. Sr and Nd isotope data indicate derivation of these magmas from a similar BTVP mantle source (with or without minor contamination by Pilton Shale, or possibly Lundy granite).The petrogenesis of the Lundy dyke association is therefore interpreted in terms of extensive fractional crystallization of basaltic magma in a magma chamber of complex geometry below the (exposed) Lundy granite. Fractional crystallization of a representative dolerite magma (olivine ± clinopyroxene ± plagioclase) yields trachyte magma from which the crystallization of alkali feldspar (anorthoclase) ± plagioclase (oligoclase) + Fe–Ti oxide + apatite results in peralkaline rhyolite. Rarer subalkaline rhyolites result from fractionation from a similar dolerite source which did not achieve a peralkaline composition so allowing the crystallization and fractionation of zircon. The basalt–(minor trachyte)/rhyolite bimodality reflects rapid crystallization of basalt magma to trachyte (and rhyolite) over a relatively small temperature interval (mass fraction of melt, F = ≈ 0.15). The rapid high level emplacement of basalt, trachyte and rhyolite dyke magmas is likely to have been associated with the development of a substantial composite bimodal basalt–(minor trachytel)/rhyolite volcano above the BTVP Lundy granite in the Bristol Channel.

Volcanic geology and eruption frequency, S�o Miguel, Azores

Six volcanic zones comprise Sao Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoacao caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoacao caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on Sao Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150–250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau.

Recent volcanic history of pantelleria: A new interpretation

The island of Pantelleria is located in the Pantelleria Rift between Sicily and Africa. This rift is floored by a 20 km thick continental-type crust that is characterized by horsts and grabens formed by NW-SE tensional faults and NE-SW shear faults. Several faults and fractures that intersect the island follow these regional trends. Prominent volcano-tectonic features on the island include two calderas, the younger being related to the eruption of the green ignimbrite and the uplifted Montagna Grande block. Pantelleria mostly consists of peralkaline silicic rocks with minor trachytes and basalts. Volcanism on the island is characterized by both explosive and effusive activity. Reconstruction of the volcanic history of the island since the eruption of the green ignimbrite is based on geological and geochronological data. The eruption of the green ignimbrite (50,000 y.b.p.) was followed by collapse of the Monastero caldera that has been the site of most of the subsequent acid volcanism. The Montagna Grande volcanic complex (35,000 y.b.p.) fills the central part while the lower pantelleritic lava flows (16,000 y.b.p.) erupted along its rim. Very probably the young Mt. Gelfiser and Khaggiar domes are also located on the caldera rim. The Montagna Grande block rose within the caldera at an estimated average rate between 1.0 and 1.5 cm/y for about 20,000 y. Subsequent pumice cones (9,000 y.b.p.), endogenous domes, and lava flows also erupted inside this caldera. Among the main acid volcanic units younger than the green ignimbrite, only the Mt. Gelkhamar endogenous dome is not linked to this caldera. In the last 50,000 years basaltic activity only occurred in the northern part of the island. The basaltic vents are aligned parallel with the main tensional trends of the rift. At least two phases have been recognized; the older is about 29,000 y.b.p. and the younger is a few thousands of years old. A change occurred in the eruptive style after the eruption of the green ignimbrite and collapse of the Monastero caldera. Explosive activity diminished and extrusion of lava became dominant.

REE-fractionated trachytes and dacites from Papua New Guinea and their relationship to andesite petrogenesis

Minor trachyte and dacite temporally associated with, but spatially separated from, arc-trench type volcanoes in Papua New Guinea have distinctive REE abundances similar to experimentally produced and theoretically predicted partial melts of eclogite. However, modelling based on small amounts of equilibrium partial melting indicates that only fractionation involving a garnet-dominated residuum can account for the observed REE patterns if the source rock was subducted oceanic basalt. If the source was geochemically evolved, other mineral phases (e.g., amphibole) are possible in the residuum, and there is no necessity to postulate that the downgoing slab was the source for these magmas. The REE fractionated trachytes and dacites appear to be a part of the nearby late Cenozoic volcanic provinces, and possibly represent minor partial melts which only in unusual tectonic situations arrive unmodified at the Earth's surface.