THE 1915 ERUPTION OF LASSEN PEAK, CALIFORNIA: MASS, THERMAL, AND COMPOSITIONAL IMPLICATIONS OF THE MAGMA MIXING EVENT DOCUMENTED USING THE MAGMA CHAMBER SIMULATION

<p>Bal&#305;kesir Volcanites (BV) are included into the Bal&#305;kesir Volcanic Province and contain various products of Oligo-Miocene volcanic activity in NW Anatolia. BV are formed from trachyandesite, andesite and dacite lavas with associated pyroclastic rocks. In this study, we report the petrographical investigations, mineral chemistry results and geothermobarometry calculations of the Bal&#305;kesir Volcanites in order to deduce the magma chamber processes and crystallization conditions. Andesites present a mineral composition of plagioclase (An35&#8211;50) + amphibole (edenitic hornblende) +biotite &#177; quartz and opaque minerals. The major phenocryst phases in dacite lavas are plagioclase (An39&#8211;53), quartz, amphibole (magnesio-hornblende), biotite, sanidine and opaque minerals. The mineral composition of the trachyandesites, on the other hand, is represented by plagioclase (An38&#8211;57) + amphibole (pargasitic hornblende) + biotite + clinopyroxene (endiopside- augite) &#177; sanidine &#177; quartz &#177; opaque minerals. Bal&#305;kesir Volcanites present distinct textural properties such as rounded plagioclase phenocrysts with reaction rims, oscillatory zoning, honeycomb and sieve textures in plagioclase, reverse mantled biotite and hornblende crystals. The plagioclase- amphibole geothermobarometry calculations of Bal&#305;kesir volcanites indicate that, andesite and dacite lavas present similar crystallization temperature and pressures conditions of 798- 813&#176;C and 1,98- 2.17 kbar. Oppositely, trachyandesites were crystallized under 857&#176;C and 3,72 kbar temperature and pressure conditions. These results show that the andesite and dacite lavas were originated from the same magma chamber with the depth of 7km whereas trachyandesites were evolved in a deeper magma chamber with 13 km depth. Combined mineral chemistry, petrography and geothermobarometry studies indicate that the open system processes such as magma mixing/mingling and/or assimilation fractional crystallization (AFC) were responsible for the textural and compositional variations of the Bal&#305;kesir Volcanites.</p>

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Petrology, age, and tectonic setting of the rapakivi-bearing Margaree pluton, Cape Breton Island, Canada: evidence for a Late Devonian posttectonic cryptic silicic-mafic magma chamber

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2019-0220 ◽

2020 ◽

Vol 57 (9) ◽

pp. 1011-1029

Author(s):

Gabriel Sombini dos Santos ◽

Sandra M. Barr ◽

Chris E. White ◽

Deanne van Rooyen

Keyword(s):

Rare Earth ◽

Rare Earth Elements ◽

Magma Chamber ◽

Magma Mixing ◽

Tectonic Setting ◽

Mafic Magma ◽

Coarse Grained ◽

Late Devonian ◽

Cape Breton Island ◽

Cape Breton

The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U–Pb (zircon) ages of 363 ± 1.6, 364.8 ± 1.6, and 365.5 ± 3.3 Ma, respectively, consistent with field and petrological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm–Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen.

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Cryptic variation in the Rhum layered intrusion

Mineralogical Magazine ◽

10.1180/minmag.1978.042.323.04 ◽

1978 ◽

Vol 42 (323) ◽

pp. 347-356 ◽

Cited By ~ 49

Author(s):

A. C. Dunham ◽

W. J. Wadsworth

Keyword(s):

Magma Chamber ◽

Fractional Crystallization ◽

Electron Microprobe ◽

Magma Mixing ◽

Layered Intrusion ◽

The Other ◽

Layered Series ◽

Cryptic Variation ◽

High Calcium

SummaryElectron-microprobe analyses of cumulus olivine, chromite, pyroxene, and plagioclase from layered peridotites and allivalites of the Eastern and Western Layered Series of Rhum demonstrate the presence of cryptic variation. Olivine varies from Fo88-78 within individual units, and there are corresponding changes in the Mg/(Mg+Fe2+) ratios in the pyroxenes and chromites. Plagioclase changes are not so dramatic, but the An-content broadly follows the Mg/(Mg+Fe2+) ratio in the other minerals. The most Fe-(and Na-) rich phases do not occur at the top of lithological units, but some way below. The composition trend above them is reversed. The data are interpreted as the result of periodic infilling of a magma chamber, the new magma mixing with the remains of the previous pulse. Each pulse was followed by a period when fractional crystallization produced the layered rocks. New data on Ni in the olivines suggests that the ratio of the volume of initial magma to volume of layered rocks was about four to one, the initial magma being allied to the high-calcium low-alkali tholeiitic basalts of Skye.

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