Magmatic Garnets in the Cairngorm Granite, Scotland

Harrison, T. N.

doi:10.1180/minmag.1988.052.368.10

Cited by 39 publications

(26 citation statements)

References 23 publications

(23 reference statements)

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“…In general, garnets from the Leinster lithium pegmatites have higher spessartine contents than those from Galway or Thailand. Garnets in peraluminous granites from Cairngorm (Harrison, 1988) and the Arabian Shield (du Bray, 1988) contain lower amounts of spessartine than the Aclare pegmatite-and aplite-hosted garnets. These data show that the chemistry of the pegmatite-hosted garnets from Aclare are typical of those in Li pegmatites (cf.…”

Section: Garnet Chemistrymentioning

confidence: 99%

“…75-83 Copyright the Mineralogical Society a deeper insight into the origin and evolution of some granitic complexes (e.g. du Bray, 1988;Stone, 1988;Harrison, 1988). The presence and relative proportions of various garnet endmembers permit conclusions to be drawn about possible petrogeneses of granitic magma and also the approximate depth of formation.…”

Section: Introductionmentioning

confidence: 99%

“…Structural patterns now seen in the rocks reflect polyphase deformation determined during the initial D1 and D2 phases. Shearing characterised the D2 phase, as manifested by the activation of the East Carlow Deformation Zone (ECDZ), a major shear zone on the south-eastern (Harrison, 1988) = open diamond, Galway granite and aplite (Leake, 1967) = filled triangle, Thai aplite (Manning, 1983) -filled diamond, Thai pegmatite (Manning, 1983) = inverted triangle.…”

Section: Geological Settingmentioning

confidence: 99%

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Petrogenetic implications of garnets associated with lithium pegmatites from SE Ireland

Whitworth

1992

Mineral. mag.

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Spodumene pegmatites, aplites and schists from the Aclare Li prospect of SE Ireland host sporadically developed, small, red, euhedral garnets. Chemical data indicate that the pegmatite-and aplite-hosted garnets are rich in spessartine and are similar to garnets in many African Li-rich pegmatites. Partial melting as a source of the garnets is unlikely due to the large spessartine component, the abundance of inclusions within the garnets and the general P-T-X evolution of the pegmatite body. It is suggested that an aqueous fluid was exsolved during pegmatite crystallisation and complexed available Mn. Movement of this fluid to the intermediate zone of the pegmatite body produced areas of high Mn concentrations which permitted the crystallisation of spessartine-rich garnets at low pressures (<3 kbar).

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Section: Garnet Chemistrymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

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Petrogenetic implications of garnets associated with lithium pegmatites from SE Ireland

Whitworth

1992

Mineral. mag.

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“…On the other hand, many studies have shown that garnet in S-type granites can also be magmatic in origin (e.g. Allan and Clarke, 1981;Barnes et al, 2012;Dalquist et al, 2007;Harrison, 1988;Lackey et al, 2012;Villaros et al, 2009). In addition, when subjected to metamorphism at suitable conditions, granites may develop metamorphic garnet.…”

Section: Introductionmentioning

confidence: 99%

Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt (Brazil): Evidence for crystallization in the lower crust

Narduzzi

Farina

Stevens

et al. 2017

Lithos

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Magmatic garnet, together with epidote, is a rare mineral association in cordilleran-I-type granitoids and of special petrogenetic significance. The metaluminous to slightly peraluminous (ASI = 0.97-1.07) Galiléia batholith (Brazil) is a large (ca. 30,000 km 2 ), Neoproterozoic (ca. 632-570 Ma) weakly foliated calc-alkaline granitoid body, characterized by the widespread occurrence of garnet (grossular 25-43 mol%) and epidote (pistacite 9.3-22.7 mol%). Field, petrographic and mineral chemical evidence indicates that garnet, epidote, biotite as well as white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Galiléia granitoids and other garnet-free cordilleran-type granitoids worldwide. This evidence strongly suggests that the origin of the uncommon garnet + epidote parageneses is related to the conditions of magma crystallization, such as pressure, temperature and water content. Comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc-alkaline magmas, as well as within garnet-bearing metaluminous volcanic rocks and granitoids, indicates that the supersolidus coexistence of grossular-rich garnet, epidote and white mica is consistent with magma crystallization at pressures greater than 0.8 GPa (above 25 km depth) and at temperatures below 700°C, i.e. near the water saturated solidus. Furthermore, resorption textures around garnet (plagioclase ± quartz coronas) and epidote suggest that these minerals have been partially consumed prior to complete crystallization. These findings demonstrate that at 630 Ma the crust underneath the Araçuaí Orogen was already at least 25-30 km thick and relatively cool. However, this contrasts with the marked high heat flow registered from the neighbour Carlos Chagas Batholith located 50 km to the east. In fact such granitoids record granulite-facies metamorphism at the same pressure and time (ca. 570 Ma) of Galiléia granitoids crystallization. Thus, a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen. Eventually, field, petrographic and mineral chemical analogies with similar garnet-bearing granitoids located in the fore-arc settings of the British Columbia subduction zone, possibly imply that the Galiléia granitoids represent "rare" garnet-and epidote-bearing metaluminous Cordilleran-Itype granites which can only form in a fore-arc setting.

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“…Well-formed and unzoned garnets are generally inferred to be phenocrysts (e.g. Green and Ringwood, 1968;Harrison, 1988), although original grain shapes may be modified by reaction with magma, particularly in the case of garnets transported to shallow structural levels during emplacement. Distinction of garnet phenocrysts and xenocrysts on textural grounds is hampered by the fact that xenocrysts also tend to be partly resorbed.…”

Section: Introductionmentioning

confidence: 99%

Contrasting garnet parageneses in a composite Grenvillian granitoid pluton, Newfoundland

Owen

Marr

1990

Mineral. mag.

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Almandine-and grossular-rich garnet occurs as both a coronitic and non-coronitic phase in ferruginous, Grenvillian (c. 1050 Ma) granitoid rocks of the composite Potato Hill pluton of the Long Range Inlier, Newfoundland. The country rock includes garnetiferous gneiss, but garnets in the pluton are eompositionally distinct (higher Ca and Mn, lower Mg), so none are interpreted as xenocrysts from the Long Range gneiss complex.Coronal garnet, quartz and hornblende separate primary pyroxene, ilmenite and hornblende from feldspar in two-pyroxene charnockite. Balanced mass-transfer reactions based on microprobe data and modes for the pyroxene-centred corona structures'suggest that corona sites gained Fe and lost Na. The flux of Fe apparently controlled corona growth in the charnockite. The corona structures are attributed to subsolidus cooling of the pluton rather than to a metamorphic overprint because the coronas are obliterated in high strain zones cutting the charnockite. Temperature of formation is constrained at c. 775-630~ by two-pyroxene and garnet-hornblende thermometry. Compositionally-similar coronas in rare, Fe-rich enderbite of the Long Range gneiss complex probably formed during cooling after early, high-grade metamorphism or following the regional emplacement of the Grenvillian plutons.Non-coronitic garnets occur in equigranular and megacrystic hornblende-biotite granite. Garnets in the equigranular granite are large, well-formed, and in some instances are associated with compositional layering of probable igneous origin. These garnets are enriched in grossular ()(ca = 0.28), and are therefore interpreted as phenocrysts crystallized at high pressure (>9 kbar?). They would nevertheless have been stabilized to lower pressures by their moderate spessartine content (XMn = 0.13).Garnets in foliated megacrystic granite form tiny crystals depleted in Fe and Mg, and enriched in Mn and Ca, and in these respects are similar to garnet in deformed charnockite. These garnets are therefore interpreted to have formed (or re-equilibrated) during the late Grenvillian deformation. Garnet is absent in relatively magnesian Grenvillian granites (bulk XFe2+= 0.50--0.85) elsewhere in the inlier. The restriction of garnet to the Potato Hill pluton (bulk Xve2+ = 0.88-0.94) therefore testifies to bulk compositional controls on the formation of both magmatic and subsolidus garnet in this intrusion.

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Magmatic Garnets in the Cairngorm Granite, Scotland

Cited by 39 publications

References 23 publications

Petrogenetic implications of garnets associated with lithium pegmatites from SE Ireland

Petrogenetic implications of garnets associated with lithium pegmatites from SE Ireland

Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt (Brazil): Evidence for crystallization in the lower crust

Contrasting garnet parageneses in a composite Grenvillian granitoid pluton, Newfoundland

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