The volcanogenetic significance of garnet-bearing minor intrusions within the Borrowdale Volcanic Group Eskdale area, Cumbria

Beddoe-Stephens, B.; Mason, I. M.

doi:10.1017/s0016756800018641

Cited by 14 publications

(10 citation statements)

References 21 publications

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“…Nonetheless, magmatic garnets have been reported from Phanerozoic calc-alkaline volcanic rocks of andesitic-dacitic-rhyolitic composition from various localities (e.g. Fitton, 1972;Bacon & Duffield, 1981;Barley, 1987;Gilbert & Rogers, 1989;Beddoe-Stephens & Mason, 1991;Gilbert, 1991;Branney, Kokelaar & McConnell, 1992;Gilbert, Bickle & Chapman, 1994;Harangi, 1999;Shinjoe et al 2002). Because increased MnO content allows a garnet to be stable at lower pressures, volcanic garnets tend to contain much more than 10 mol.…”

Section: A Garnet In Volcanic Rocksmentioning

confidence: 99%

“…We do see, however, Figure 5. Compositional domains of garnet phenocrysts in volcanic rocks from various parts of the world identified with the first-author names only (Beddoe-Stephens & Mason, 1991;Fitton, 1972;Harangi et al 2001;Kawabata & Takafuji, 2005;KSB: Kimata et al 1995, Sisson & Bacon, 1992and Bacon & Duffield, 1981; garnet reported by Mitropoulos, Katerinopoulos & Kokkinakis, 1999 is rich in spessartine). The fields in Ca-Mg-Fe space, calculated from cation ratios, are bounded at 1σ distances from the mean except for Harangi's data (see text).…”

Section: B Garnet (Al 787 Py 123 Gr 74 Sp 16 ) In the Sukhda Tuffmentioning

confidence: 99%

“…Although very rare in volcanic, hypabyssal or pyroclastic rocks, garnet phenocrysts have been described from basalts (Aydar & Gourgaud, 2002), andesites (Fitton, 1972;Gill, 1981;Barnes & Allen, 2006), dacites (Fitton, 1972;Gilbert & Rogers, 1989), rhyolites (Bacon & Duffield, 1981;Clemens & Wall, 1984;Smith & Cole, 1996;Friedman et al 2001;Mitropoulos, Katerinopoulos & Kokkinakis, 1999) and trachytephonolites (Dingwell & Brearley, 1985). They range in composition from the high-pressure almandine-pyrope variety (Barnes & Allen, 2006;Beddoe-Stephens & Mason, 1991;Kimata et al 1995) to a low-pressure (∼ 3 kbar) spessartine-rich variety (Frondel, 1970;Gauthier et al 1994). Some are zoned (Dingwell & Brearley, 1985;Beddoe-Stephens & Mason, 1991;Gilbert, Bickle & Chapman, 1994;Kimata et al 1995;Kawabata & Takafuji, 2005) and some are unzoned and uniform in composition (Reymer, 1983;Aydar & Gourgaud, 2002).…”

Section: C Larger Contextmentioning

confidence: 99%

“…They range in composition from the high-pressure almandine-pyrope variety (Barnes & Allen, 2006;Beddoe-Stephens & Mason, 1991;Kimata et al 1995) to a low-pressure (∼ 3 kbar) spessartine-rich variety (Frondel, 1970;Gauthier et al 1994). Some are zoned (Dingwell & Brearley, 1985;Beddoe-Stephens & Mason, 1991;Gilbert, Bickle & Chapman, 1994;Kimata et al 1995;Kawabata & Takafuji, 2005) and some are unzoned and uniform in composition (Reymer, 1983;Aydar & Gourgaud, 2002). Some have inclusions (Bacon & Duffield, 1981;Birch & Gleadow, 1974) and some are inclusion-free (Birch & Gleadow, 1974).…”

Section: C Larger Contextmentioning

confidence: 99%

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Almandine garnet phenocrysts in a ~1 Ga rhyolitic tuff from central India

2008

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We report on the newly discovered almandine garnet phenocrysts in rhyolitic ignimbrites (Sukhda Tuff) in the Precambrian Churtela Shale Formation of the Chhattisgarh Supergroup in central India. SHRIMP ages of igneous zircon from the ignimbrites range from 990 Ma to 1020 Ma. These ignimbrites exhibit characteristic eutaxitic texture with compacted curvilinear glass shards with triple junctions. Quartz (commonly embayed; bluish cathodoluminescence) and albite (altered but retaining ghosts of twinning) are common phenocrysts; others are apatite, ilmenite, rutile, magnetite, zircon, monazite and garnet. There are no metamorphic or granitic xenoliths in the ignimbrites. Garnet grains occur as isolated broken isotropic crystals with sharp or corroded boundaries in a very fine-grained groundmass of volcanic ash that consists principally of albite, quartz, magnetite and glass. They do not have any systematically distributed inclusions. A few have penetratively intergrown phenocrysts of apatite, ilmenite, rutile and zircon, which we interpret as subophitic texture. Extensive SEM-BSE imaging of more than 100 grains and electron microprobe traverses across about 30 grains showed no zoning or systematic compositional variability. Common (metamorphic) garnets are usually zoned with respect to Fe–Mg–Mn and typically have mineral inclusions. We infer, therefore, that these observed garnets are not metamorphic xenocrysts. The average major oxide composition of analysed garnets from five different horizons within the Sukhda Tuff, spanning approximately 300 m of the stratigraphic section, have very small standard deviation for each element, which is suggestive of a single magmatic source. Phenocrysts of quartz, including those in contact with coexisting garnets, show blue scanning electron CL, indicating rapid cooling from high temperature; this suggests that adjacent coexisting garnets are not slowly cooled restites. We conclude, on the basis of texture, mineral chemistry and absence of any indicative xenoliths or xenocrysts, that these almandine garnets (Al78.7Py12.3Gr7.4Sp1.6) are phenocrysts within the Sukhda Tuff. Almandine of such composition is stable under high pressure. We infer that almandine crystallized at lower crustal depths in a magma that ascended very rapidly and may have erupted explosively.

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Section: A Garnet In Volcanic Rocksmentioning

confidence: 99%

Section: B Garnet (Al 787 Py 123 Gr 74 Sp 16 ) In the Sukhda Tuffmentioning

confidence: 99%

Section: C Larger Contextmentioning

confidence: 99%

Section: C Larger Contextmentioning

confidence: 99%

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Almandine garnet phenocrysts in a ~1 Ga rhyolitic tuff from central India

2008

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“…Adjusting their data for the slightly higher Nd isotopic ratio they obtained for standard BCR-1, their samples have initial e Nd values of À6.2 to À5.8 for the andesites and À5.6 for the dacite. Strong textural and geochemical evidence exists that the garnets crystallized in their host rocks (Fitton 1972;Thirlwall & Fitton 1983;Beddoe-Stephens & Mason 1991).…”

Section: Nd Isotopic Compositionsmentioning

confidence: 99%

Andesite petrogenesis in the Ordovician Borrowdale Volcanic Group of the English Lake District by fractionation, assimilation and mixing

McConnell

Menuge

Hertogen

2002

JGS

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A suite of composite basalt-andesite dykes in the Ordovician Borrowdale Volcanic Group of the English Lake District records magmatic evolution from basalt to andesite. Although altered, the rocks preserve textures that indicate magma mixing. Primitive basalts were derived from a fertile, subduction-modified mantle source. Evolution by fractional crystallization is suggested by rapid decrease of compatible trace element concentrations and similar incompatible trace element and REE profiles throughout the sample range. Recharge of the evolving magma with fresh batches of basaltic composition is indicated by some samples that have compatible trace element concentrations much higher than expected from their otherwise evolved compositions. Geochemical evidence of magma mixing is provided by linear trends between pairs of incompatible trace elements not at constant ratio. e Nd values indicate addition of a crustal component, but samples do not conform to a simple mixing equation. The combined evidence points to magma evolution by assimilation of wallrock during fractional crystallization (AFC) with periodic recharge. We have modelled an AFC process using a Skiddaw Group metasediment contaminant that can explain trace element and isotopic variation through variable amounts of assimilation, crystal fractionation and recharge, to produce a spectrum of compositions that was sampled periodically by the dykes. There is evidence from other parts of the Borrowdale Volcanic Group that similar processes operated throughout its history.

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