The Role of Manganese in the Paragenesis of Magmatic Garnet: An Example from the Old Woman-Piute Range, California

Small, euhedral Mn-rich garnets (32-52 mol. % spessartine) from the Cairngorm granite, Eastern Grampian Highlands, Scotland, are considered to be of magmatic origin and have not been derived from the assimilation of metasedimentary material, despite their occurrence largely at the margins of the pluton. Similar garnets also occur in a late cross-cutting aplite sheet. The garnets in the granite crystallized early in the sequence and are thought to have formed in response to the ponding of Mnrich fluids against the wall of the pluton. This Mn enrichment of the fluid phase continued throughout the evolution of the pluton, resulting in Mn-rich biotites and opaque oxides and the localized crystallization of Mn-rich garnets in aplite. Garnet contains up to 1.67 wt. % Y, but has not played a major role in the geochemical evolution of the Cairngorm granite, which has high SiO2 (72-77%) and is enriched in Y and HREE. Chemical analyses of garnets, biotites and rocks are given.

Section: Mgmentioning

confidence: 62%

Section: Discussionmentioning

confidence: 99%

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Magmatic Garnets in the Cairngorm Granite, Scotland

Harrison

1988

“…Even though experimental work indicates that lower crustal silicic calc alkaline melts can crystallize garnet and cordierite (Green and Ringwood, 1968a); filled squares--garnets from other granites (including leucogranites and associated aplites); filled triangles--Dartmoor (D) and Lundy (L) garnets; open diamonds--experimentally produced garnets (Green and Ringwood, 1968b). Most garnets from leucogranite0 aplite and pegrnatite, including those cited by Hall (1965), Leake (1968), Miller and Stoddard (1981), and Manning (1983, plot below the bottom right hand corner of the diagram, i.e. at spessartine-richer compositions.…”

Section: Origin Of Garnet In the Granites Of Dartmoor And Lundymentioning

confidence: 99%

The Significance of Almandine Garnets in the Lundy and Dartmoor Granites

Stone

1988

Almandine garnets in the cordierite-bearing granite of Sweltor Quarry, Dartmoor, contain < 10 mol. % of the spessartine end-member, whilst those in the Lundy granite have c. 10 mol. % spessartine. Experimental work indicates that such compositions can grow in equilibrium with siliceous melts at depths of 18-25 kin. This evidence, reaction rims, lack of marked zoning and comparison with garnets in other siliceous calc-alkaline siliceous rocks point to a genesis involving partial melting of the 'local' lower crust. A restite origin rather than direct crystallization from magma is favoured but the evidence is equivocal. The Dartmoor granite (Hercynian) is a typical peraluminous late-to posttectonic S-type granite. The S-type character of the Lundy (Tertiary) granite is revealed by the occurrence of garnet and topaz together with biotite enriched in Rb, Cs and F, despite its close association with Tertiary basic magmatism in an anorogenic setting.

“…Since garnet fractionates Mn more readily than most other common minerals, an increase in the spessartine content of garnet stabilizes this mineral to low pressures (Green, 1977;Miller andStoddard, 1978, 1981). On the other hand, experimental studies by Green (1977) demonstrate that grossular and spessartine concentrations in nearliquidus magmatic garnet vary antipathetically.…”

Section: Introductionmentioning

confidence: 99%

Contrasting garnet parageneses in a composite Grenvillian granitoid pluton, Newfoundland

Owen

Marr

1990

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.