Evolution of young andesitic–dacitic magmatic systems beneath Dominica, Lesser Antilles

Hekla is a Holocene volcanic ridge in southern Iceland, which is notable for the link between repose periods and the composition of the first-erupted magma. The two largest explosive silicic eruptions, H4 and H3, erupted about 4200 and 3000 years ago. Airfall deposits from these eruptions were sampled in detail and analysed for major and trace elements, along with microprobe analyses of minerals and glasses. Both deposits show compositional variation ranging from 72 % to 56 % SiO 2 , with mineralogical evidence of equilibrium crystallization in the early erupted rhyolitic component but disequilibrium in the later erupted basaltic andesite component. The eruptions started with production of rhyolitic magma followed by dacitic to basaltic andesite magma. Sparse crystallization of the intermediate magma and predominant reverse zoning of minerals, trending towards a common surface composition, indicate magma mixing between rhyolite and a basaltic andesite end-member. The suggested model involves partial melting of older tholeiitic crust to produce silicic magma, which segregated and accumulated in deep crustal reservoir. Silicic magma eruption is triggered by basaltic andesite dyke injection, with a proportion of the dyke magma contributing to the production and eruption of a mixed hybrid magma. Both the volume of the silicic partial melt, and the proportion of the hybrid magma depend on the pre-eruptive repose time.

Silicic

Basaltic andesite

Fractional crystallization (geology)

Peléan eruption

Magma chamber

Igneous differentiation

Dense-rock equivalent

Caldera

10.1017/s0016756807003470

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Citations (48)

The Southeastern Saurashtra dyke swarm, Deccan Traps: Magmatic evolution of a tholeiitic basalt–basaltic andesite–andesite–rhyolite suite

Lithos (2020)

Ciro Cucciniello Hetu Sheth Raymond A. Duraiswami Wencke Wegner Christian Koeberl

Fractional crystallization (geology)

Andesites

Basaltic andesite

Flood basalt

Silicic

Igneous differentiation

10.1016/j.lithos.2020.105759

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Citations (22)

Evolution of Silicic Magma Chambers and their Relationship to Basaltic Volcanism

Geophysical monograph (2011)

J. C. Eichelberger R. Gooley

Andesitic and rhyolitic volcanism is commonly preceded by the eruption of basalt. Similarly, the earliest phases of granitic plutonic complexes are often gabbro. Thus basaltic magma apparently plays a role in the initiation of a large silicic magma system. Three lines of evidence suggest that basaltic magma also enters silicic chambers and influences their further evolution: (1) Contemporaneous basalt vents flank silicic volcanic centers. (2) Thermal models of silicic magma bodies suggest that their heat must be replenished to maintain them in the upper crust for their observed life-span. (3) Petrologic data indicate that 'cognate' mafic clots and xenoliths common in granodiorite and andesite represent basaltic magma quenched within active silicic magma chambers. The presence of abundant basaltic material in intermediate volcanic and plutonic rocks, phase assemblages in volcanic rocks, and the bulk composition of volcanic and plutonic rocks of the basalt-andesite-rhyolite and gabbro-granodiorite-granite suites show that these rock associations form by mixing of basaltic and rhyolitic magmas. It is proposed that basaltic magma from the upper mantle provides heat for generating rhyolitic melt in the lower crust and that the resulting rhyolitic magma body continues to receive injections of basaltic magma as it rises through the crust. Implications of this model are that (1) the development of a silicic system depends on the intensity of basaltic volcanism and the ability of the lower crust to produce a rhyolitic melt and (2) the relative volume of intermediate versus basaltic and rhyolitic end products depends on the extent of mixing. It appears that the influence of tectonic setting on crustal residence time of silicic chambers controls to what degree the system is homogenized.

Silicic

Magma chamber

Basaltic andesite

Igneous differentiation

Peléan eruption

Fractional crystallization (geology)

Caldera

10.1029/gm020p0057

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Citations (63)

Mineralogical Constraints on the Magma Mixing Beneath the Iheya Graben, an Active Back-Arc Spreading Centre of the Okinawa Trough

Journal of Petrology (2020)

Yuxiang Zhang Zhigang Zeng G. A. Gaetani Le Zhang Zhiqing Lai

Abstract The Iheya Graben is a back-arc spreading centre in the middle part of the Okinawa Trough. It is also located in the centre of an anomalous volcanic zone (volcanic arc migration phenomenon, or VAMP) and is characterized by bimodal volcanism, unusually high heat flow and active hydrothermal circulation. The subvolcanic magma plumbing system and the magmatic processes related to the formation of rare erupted intermediate lavas in this area remain uncertain. In this study, we conducted systematic mineralogical analyses (in situ major element, trace element and Sr isotopes) and whole rock geochemical analyses (major element, trace element and Sr–Nd isotopes) on an andesite (T5-2; type C andesite) and a rhyolite (C11; type 2 rhyolite), and present evidence for magma mixing in the origins of these lavas. Andesite T5-2 contains a mafic mineral assemblage and a silicic mineral assemblage, which are derived from a basaltic melt and a type 2 rhyolitic melt, respectively. A 4:6 mixture of basalt and type 2 rhyolite from the Iheya Graben reproduces the whole-rock major element, trace element, and Sr–Nd isotope compositions of T5-2. Rhyolite C11 contains a group of disequilibrium minerals that crystallized from a less evolved rhyolitic melt with relatively more enriched Sr–Nd isotope compositions, suggesting mixing of this melt with a more evolved and isotopically more depleted rhyolitic melt. This mixing process could produce a series of rhyolitic melts with a negative correlation between SiO2 concentrations and 87Sr/86Sr ratios (or a positive correlation for 143Nd/144Nd ratios), which are recorded by the whole group of type 2 rhyolites. The results from mineral-based thermobarometers suggest that the premixing storage temperatures of the basaltic and rhyolitic melts are ∼1100 °C and 870–900 °C, respectively. The hybrid andesitic melt has temperatures of ∼950 to ∼980 °C. The magma storage pressures are not well constrained, ranging from ∼400 MPa to ∼100 MPa. We show that magma mixing plays a significant role in the origins of diverse volcanism in the middle Okinawa Trough; more specifically, two of the three types of andesites (types B and C) and one of the two types of rhyolites (type 2) are associated with magma mixing. We thus propose a complex magma plumbing system with multichamber magma storage and frequent magma mixing beneath the Iheya Graben.

Silicic

Igneous differentiation

Trace element

Fractional crystallization (geology)

Felsic

Dacite

Andesites

Magma chamber

10.1093/petrology/egaa098

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Citations (15)

The Petrology and Geochemistry of the Aniakchak Caldera-forming Ignimbrite, Aleutian Arc, Alaska

Journal of Petrology (2005)

Scott T. Dreher J. C. Eichelberger J. F. Larsen

Aniakchak caldera, Alaska, produced a compositionally heterogeneous ignimbrite ∼3400 years ago, which changes from rhyodacitic at the base to andesitic at the top of the eruptive sequence. Interpretations of compositionally heterogeneous ignimbrites typically include either in situ fractional crystallization of mafic magma and generation of a stratified magma body or replenishment of a silicic magma chamber by mafic inputs. Another possibility, silicic replenishment of a more mafic chamber, exists. Geochemical characteristics of the caldera-forming rhyodacite and several late pre-caldera rhyodacites indicate independent origins for each, within a maximum of ∼5000 years prior to caldera formation. Isotopic considerations preclude derivation of the caldera-forming rhyodacite from the caldera-forming andesite. However, the caldera-forming rhyodacite can be explained as the residual liquid of a mostly crystallized basalt, with addition of crustal material. The Aniakchak andesite probably formed in a shallow chamber by successive mixing events involving small volumes of basalt and rhyodacite, together with contamination. The pre-caldera rhyodacites represent erupted portions of intruding silicic magma, whereas another portion homogenized with the resident mafic magma. The caldera-forming event reflects a large influx of rhyodacite, which erupted before significant mixing occurred and also triggered draining of much of the andesitic magma from the chamber.

Caldera

Silicic

Magma chamber

Basaltic andesite

Fractional crystallization (geology)

10.1093/petrology/egi032

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Citations (44)

The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes

Journal of Volcanology and Geothermal Research (2000)

James G. Brophy Scott T. Dreher

Fractional crystallization (geology)