A cummingtonite-porphyritic dacite with amphibole-rich xenoliths from the tertiary central volcano at Króksfjördur, NW Iceland

doi:10.1016/0024-4937(82)90006-8

Lithos

Volume 15, Issue 2, 15 April 1982, Pages 137-159

https://doi.org/10.1016/0024-4937(82)90006-8 Get rights and content

Abstract

An instrusive dacite and a salic pumice, emplaced late in the evolution of the Miocene (c. 10 m.y.) Króksfjördur volcano, NW Iceland, contain a varied assemblage of xenolithic metaigneous rocks. Mineral and rock chemistry shows that the dacite is very similar to calc-alkaline salic rocks from the SW Pacific. It contains phenocrystic plagioclase, quartz, pyroxene, cummingtonite, hornblende, biotite, two oxides, apatite and zircon in a rhyolitic glass. The rock equilibrated at 700 to 750°C. P ∼ 1.6 Kbar and P_H₂O ∼ 1 Kbar. The xenoliths are layered gabbros, granophyres and various fine-grained hornfelses and show that the dacite magma was residing within a gabbro intrusion capped by granophyre prior to the eruption. The hornfelses are amphibole-plagioclase, amphibole-pyroxene-plagioclase and pyroxene-plagioclase rocks formed during high-temperature metamorphism of basic dykes cutting the gabbro intrusion. The gabbros and hornfelses mostly record higher metamorphic temperatures (850–940°C) than the dacite, and indicate that they were equilibrated during the ascent of a magma body into a hydrous high-level region within the volcano. During a following thermal decline, the hydrated magma cooled to form the first cummingtonite-bearing low-T magma to be recorded from the ocean ridge systems.

References (60)

E.P Ambler et al.
Vermicular orthopyroxene-magnetic symplectites from the Wateranga layered mafic intrusion, Queensland, Australia
Lithos
(1977)
S.R Bohlen et al.
A critical evaluation of two-pyrotene thermometry in Adirondack granulites
Lithos
(1979)
A Ewart
Petrochemistry and feldspar crystallization in the silicic volcanic rocks, Central North Island, New Zealand
Lithos
(1969)
A Ewart
A review of the mineralogy and chemistry of Tertiary-Recent dacitic, latitic, rhyolitic and related salic volcanic rocks
J.R Holloway
The system pargasite-H₂OCO₂: a model for melting of a hydrous mineral with a mixed-volatile fluid. I. Experimental results to 8 kbar
Geochim. Cosmochim. Acta
(1973)
J.G Larsen
Glass-bearing gabbro inclusions in hyaloclastites from Tindfjalljökull, Iceland
Lithos
(1979)
J.M Allen et al.
Coexisting amphiboles from the Noranda area, Quebec: extension of the actinolite-hornblende miscibility gap to iron-rich bulk compositions
Am. Mineral.
(1978)
D.C Almond
Metamorphism of Tertiary lavas in Strathaird, Skye
Trans. R. Soc. Edinb.
(1964)
R.A Binns
The mineralogy of metamorphosed basic rocks from the Willyama Complex, Broken Hill District, New South Wales. Part I: Hornblendes
Min. Mag.
(1965)
R.A Binns
Hornblendes from some basic hornfelses in the New England region, New South Wales
Min. Mag.
(1965)

F.R Boyd

Hydrothermal investigations of amphiboles

E.H Brown

The crossite content of Ca-amphibole as a guide to pressure of metamorphism

J. Petrol.

(1977)

W.B Bryan

Cummingtonite-dacite drift pumice stranded at West Palm Beach, Florida

Bull. Geol. Soc. Am.

(1972)

A.F Buddington et al.

Iron-titanium oxide minerals and synthetic equivalents

J. Petrol.

(1964)

C.W Burnham et al.

Thermodynamic properties of water to 1000°C and 10,000 bars

Spec. Pap. Geol. Soc. Am.

(1969)

K.L Cameron

An experimental study of actinolite-cummingtonite phase relations with notes on the synthesis of Fe-rich anthophyllite

Am. Mineral.

(1975)

I.S.E Carmichael

The crystallization of feldspar in volcanic acid liquids

Q.J. Geol. Soc. Lond.

(1963)

I.S.E Carmichael

The petrology of Thingmuli, a Tertiary volcano in eastern Iceland

J. Petrol.

(1964)

I.S.E Carmichael

The mineralogy of Thingmuli, a Tertiary volcano in eastern Iceland

Am. Mineral.

(1967)

I.S.E Carmichael

The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates

Contrib. Mineral. Petrol.

(1967)

R.G Coleman et al.

Oceanic plagiogranite

J. Geophys. Res.

(1975)

A Ewart et al.

Voluminous low temperature rhyolitic magmas in New Zealand

Contrib. Mineral. Petrol.

(1971)

A Ewart et al.

Quaternary acid magmas in New Zealand

Contrib. Mineral. Petrol.

(1975)

R.J Floran et al.

Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: Correlation of compositional and assemblage variations at low to moderate grade

J. Petrol.

(1978)

R.H Grapes et al.

The actinolite-hornblende series in metabasites and the so-called miscibility gap: A review

Lithos

(1978)

K Grönvold

Structural and petrochemical studies in the Kerlingarfjöll region, central Iceland

K Grönvold

Variation and origin of magma types in the Namafjall area, North Iceland

Bull. Soc. géol. France

(1976)

N Hald et al.

The Króksfjördur central volcano in north-west Iceland

Acta Nat. Islandica

(1971)

R.T Helz

Alkali exchange between hornblende and melt: a temperature-sensitive reaction

Am. Miner.

(1979)

T.J.B Holland et al.

Thermodynamic parameters of CaMgSi₂O₆Mg₂Si₂O₆ pyroxenes based on regular solution and cooperative disordering models

Contrib. Mineral. Petrol.

(1979)

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The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (∼225 km³). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO₂) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An_10-40), Na-rich anorthoclase (<Or₂₃), clinopyroxene (Fs_37-20), FeTi oxides (Usp_32-60; Ilm_93-88), hornblende (edenitic–ferroedenitic) and olivine (Fo_22-37), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The $δ^{18} O$ values of the silicic lavas are in the range 3.6–4.4, and these are lower than the $δ^{18} O$ values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations, $^{18} O$ depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low- $δ^{18} O$ , hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust.
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A cummingtonite-porphyritic dacite with amphibole-rich xenoliths from the tertiary central volcano at Króksfjördur, NW Iceland

Abstract

Lithos

Lithos

Lithos

Geochim. Cosmochim. Acta

Lithos

Coexisting amphiboles from the Noranda area, Quebec: extension of the actinolite-hornblende miscibility gap to iron-rich bulk compositions

Am. Mineral.

Metamorphism of Tertiary lavas in Strathaird, Skye

Trans. R. Soc. Edinb.

The mineralogy of metamorphosed basic rocks from the Willyama Complex, Broken Hill District, New South Wales. Part I: Hornblendes

Min. Mag.

Hornblendes from some basic hornfelses in the New England region, New South Wales

Min. Mag.

Hydrothermal investigations of amphiboles

The crossite content of Ca-amphibole as a guide to pressure of metamorphism

J. Petrol.

Cummingtonite-dacite drift pumice stranded at West Palm Beach, Florida

Bull. Geol. Soc. Am.

Iron-titanium oxide minerals and synthetic equivalents

J. Petrol.

Thermodynamic properties of water to 1000°C and 10,000 bars

Spec. Pap. Geol. Soc. Am.

An experimental study of actinolite-cummingtonite phase relations with notes on the synthesis of Fe-rich anthophyllite

Am. Mineral.

The crystallization of feldspar in volcanic acid liquids

Q.J. Geol. Soc. Lond.

The petrology of Thingmuli, a Tertiary volcano in eastern Iceland

J. Petrol.

The mineralogy of Thingmuli, a Tertiary volcano in eastern Iceland

Am. Mineral.

The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates

Contrib. Mineral. Petrol.

Oceanic plagiogranite

J. Geophys. Res.

Voluminous low temperature rhyolitic magmas in New Zealand

Contrib. Mineral. Petrol.

Quaternary acid magmas in New Zealand

Contrib. Mineral. Petrol.

Mineralogy and petrology of the Gunflint Iron Formation, Minnesota-Ontario: Correlation of compositional and assemblage variations at low to moderate grade

J. Petrol.

The actinolite-hornblende series in metabasites and the so-called miscibility gap: A review

Lithos

Structural and petrochemical studies in the Kerlingarfjöll region, central Iceland

Variation and origin of magma types in the Namafjall area, North Iceland

Bull. Soc. géol. France

The Króksfjördur central volcano in north-west Iceland

Acta Nat. Islandica

Alkali exchange between hornblende and melt: a temperature-sensitive reaction

Am. Miner.

Thermodynamic parameters of CaMgSi2O6Mg2Si2O6 pyroxenes based on regular solution and cooperative disordering models

Contrib. Mineral. Petrol.

Thermodynamic parameters of CaMgSi₂O₆Mg₂Si₂O₆ pyroxenes based on regular solution and cooperative disordering models