Rhyolitic Lava Domes Research Articles

La Pacana caldera and the Atana Ignimbrite ? a major ash-flow and resurgent caldera complex in the Andes of northern Chile

The recently discovered La Pacana caldera, 60 × 35 km, is the largest caldera yet described in South America. This resurgent caldera of Pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the Central Andes of northern Chile at about 23° S latitude. Collapse of La Pacana caldera was initiated by the eruption of about 900 km3 of the rhyodacitic Atana Ignimbrite. The Atana Ignimbrite was erupted from a composite ring fracture system and formed at least four major ash-flow tuff units that are separated locally by thin air-fall and surge deposits; all four sheets were emplaced in rapid succession about 4.1 ± 0.4 Ma ago. Caldera collapse was followed closely by resurgent doming of the caldera floor, accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures. The resurgent dome is an elongated, asymmetrical uplift, 48.5 × 12 km, which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben. Later, postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3.5 Ma ago and persisting into the Quaternary. Hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome, but no ore deposits are known. A few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system.

Evidence for two-stage mixing in magmatic inclusions and rhyolitic lava domes on Niijima Island, Japan

Fine-grained inclusions are found in the Achiyama and the Mukaiyama rhyolitic lava domes on Niijima Island, Japan. The existence of fine-grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host. The chemical composition and the phenocryst assemblage of the inclusions in the Achiyama dome are identical with those of the effusive basaltic lava on Niijima Island (Wakago basalt). The inclusions of the Mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage (olivine, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, quartz and opaque minerals); whole-rock compositions lie on a mixing line between the host rhyolite and Wakago basalt. The dacitic magma is, therefore, a product of mixing of the basaltic and rhyolitic magmas. The occurrence of two different inclusions (basaltic and dacitic) in two different domes can be explained by a two-stage magma mixing model. The dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers. A small amount of rhyolitic magma is entrained into the lower basaltic magma layer, and two magmas can mix essentially in a liquid state. The mixed dacitic magma is formed in the lower layer. The second stage is mixing caused by forced convection during the ascent through a conduit just before eruption. The stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit. Under this condition, the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host. The mafic magma solidifies when the silicic endmember is predominant in the mixture; hence the equilibrium temperature is below the solidus of the mafic magma. Mafic inclusions are formed at this stage. Whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit (eruption) occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber.

Volcanic stratigraphy of the Los Azufres geothermal area, Mexico

The Los Azufres volcanic center, located 200 km northwest of Mexico City, is one of a number of Pleistocene silicic volcanic centers with active geothermal systems that lie north of the axis of the Mexican Neovolcanic Belt. Silicic volcanism began approximately 1 m.y. ago with eruption of the Agua Fria rhyodacite to high-silica rhyolite lava domes and flows. They are covered by rhyodacite and dacite lava domes and flows of the San Andres volcano, dated at 0.3 m.y. A north- to northeast-trending set of high-angle, normal faults is offset by a younger and more prominent east-trending set that parallels the principal regional structural trend. These faults cut all but the youngest volcanic rocks of the center, the Yerbabuena rhyodacites to high-silica rhyolites. These morphologically well-preserved lava domes are approximately 0.15 m.y. old and are probably associated with air-fall tuffs that blanket the western portion of the volcanic center. Two episodes of high heat flow suggested by studies of drill-core samples and fluids from the Los Azufres geothermal field may correspond to two different episodes of magma injection to high levels, the most recent one resulting in eruption of the Yerbabuena lavas.

Geology and footwall alteration of the South Bay massive sulphide deposit, northwestern Ontario, Canada

South Bay is an Archean volcanogenic massive Cu–Zn sulphide deposit having many features in common with the Kuroko deposits of Japan. The ore lenses overlie a quartz–feldspar porphyritic rhyolite (QFP) lava dome and are covered by or occur within rhyolitic tuff breccia that, together with rhyolite tuffs and lavas, is contained within a caldera-like structure.Footwall hydrothermal alteration at South Bay is detectable for several hundred metres from ore. "Unaltered" footwall QFP lava dome contains a mineral assemblage of quartz + two feldspars + two micas + epidote + calcite + ilmenite. Closer to the orebodies, K-feldspar, epidote, and biotite disappear first, followed by consumption of calcite, ilmenite, and albitized plagioclase. The most altered QFP has an assemblage of quartz + paragonite + phengitic muscovite + chlorite + dolomite + sphene + rutile. The ratio Fe/(Fe + Mg) in dolomite, muscovite (phengite), and chlorite decreases consistently towards the orebodies. Neither bulk chemistry (except for Na2O) nor oxygen isotopic ratio shows consistently systematic lateral changes within the alteration halo. Quartz from the stringer zone, from lenses in massive ore, and from ore-horizon chert all have a very narrow δ18O range of +9.0 to +11.3‰. The δ18O of the QFP is +9.3 to +9.4‰, regardless of the degree of alteration.The temperature of ore formation is estimated to have been around 300 °C based on the paragonite–muscovite geothermometer and the carbonate geothermometer. The δ18O value of the ore-forming solution at 300 °C would have been between +2.1 and +4.4‰, which is similar to that of the Kuroko deposits.

Rhyolite domes and pyroclastic socks (Whitianga Group) of the Hahei Area, Coromandel Peninsula

Abstract Acidic volcanic rocks (Whitianga Group) in the Hahei area consist of breccia, lapilli breccia and tuff (Coroglen Subgroup) overlain by rhyolite lava domes (Minden Rhyolite Subgroup). The remnants of three or more domes are preserved, composed of biotite, biotite-hornblende, and pyroxene rhyolite.

Petrogenesis of the felsic rock units of the late-Precambrian Sinclair Group, South West Africa

The Sinclair Group is a late-Precambrian succession of basic to intermediate and felsic lava flows and intrusives, volcaniclastic and clastic units constituting part of an extensive volcanic, plutonic and clastic arc — the Rehoboth Magmatic Arc — which is regarded as having developed on the western and north-western margins of the sialic Kalahari Plate as a result of the subduction of oceanic crust. The major felsic units comprise high-level granites, rhyolitic lava flows, plugs, domes, dykes and various volcaniclastics including ash-flow tuffs. Average major and trace element analyses of these units are presented and on this basis they can be separated into high-Ca and low-Ca groupings each with distinctive Ca content ranges and trace element distribution patterns. The genesis of the felsic units is discussed and it is considered that the magmas were produced by partial fusion of sialic crust; the heat source for fusion can be found in the repeated injection of basic magma during evolution of the Group. The high-Ca magmas resulted from melting at the deepest crustal levels mainly involving a quartzofelspathic host rock and hornblende whereas the low-Ca melts were derived from the partial fusion of a similar host rock and biotite at shallower levels in the crust. Water content of the magmas, which was probably in the region of 1–2 per cent and not more than 4 per cent, could have been provided by these hydrous minerals and partial melting probably took place between depths of 18 km and 36 km in the continental crust at temperatures of about 750° C to 950° C. Ash-flow tuffs resulted from vesiculation of the felsic magma at depths between 0 and 4 km, at temperatures between 800° C and 950° C and with water contents restricted to a range of about 1,5 to 4 per cent.

Emplacement and Thermal History of a Rhyolite Lava Flow near Fortymile Canyon, Southern Nevada

The rhyolite of Comb Peak is one of a series of related rhyolitic lava flows and domes that postdate formation of the Timber Mountain caldera in southern Nevada. The Comb Peak body consists of bedded rhyolitic tuffs overlain by a rhyolitic lava flow that has a foliated interior with fluidal struc- ture, enveloped by breccia. The vent for the lava flow and the pre-eruption topography have been exposed by erosion and indicate the general condi- tions and direction of flow. These indications have been corroborated by study of the foliation pattern and by a semiquantitative statistical analysis of flow folds and related features. Geologic relationships in- dicate that brecciation of the lava flow occurred mainly during periods when the flow was spreading, whereas the body eroded its floor and walls much as does a glacier during periods when the flow was con- fined. Tuffs adjacent to parts of the lava flow have been conspicuously modified. Local compaction and in- duration (fusion) by the hot lava caused the bed- ded tuffs to weld so that they resemble welded ash- flow tuffs except for characteristic bedding and sorting indicative of their ash-fall origin. The zone of fusion is everywhere parallel to the contacts of the lava flow and intersects bedding at high angles. Locally fusion affected the tuffs for a thickness of 75 m or more from the contact of the flow. Approxi- mate calculation of the heat flow necessary to ac- complish the observed alteration indicates that sim- ple conduction from the margins of the flow would not have been adequate. Heat transfer was greatly increased, probably by convection of superheated steam from the edges of the flow through porous tuffs. Simultaneous cooling and crystallization of the fused tuffs and the lava flow produced a series of zones that encompass the entire Comb Peak body and cross contacts between emplacement units. The basal vitrophyre zone, for example, is in the lowest part of the lava flow in many places, but elsewhere it is entirely within fused tuffs and has been mapped as much as 300 m beyond the present posi- tion of the flow contact.