Precambrian coronitic metagabbro

--- Grenville province of southern Ontario, Canada

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Figures 1-2. A garnet-studded metagabbro from the Muskoka district, near Huntsville, east of Georgian Bay, Lake Huron. Left: hand specimen, showing large irregular masses of granular red garnet. Right: two polished thin sections of the rock, seen in transmitted light. Vertical dimension approximately 40 mm. Sample 1092. The rock is dominated by almandine garnet and intermediate plagioclase feldspar, plus clinopyroxene and secondary amphibole. Ilmenite is the main Fe-Ti oxide. There are traces of apatite and zircon, as well as a late, post-peak metamorphism assemblage of chlorite, talc, pyrite and chalcopyrite (Wilson, 1994, pp.79-80,176). The hand specimen in the figure weighs 1140 g and is weakly magnetic (mean bulk magnetic susceptibility 5.2x10-3 SI units, n=5).

"Rock of the Month #150, posted for December 2013" ---

Coronitic metagabbro is preserved in small basic intrusions within the Central Gneiss Belt of the Grenville province. Here we see sample 1092, garnet-rich coronitic metagabbro from Muskoka Road 2, on a roadside outcrop in hilly terrain roughly 3 km northwest of the town of Huntsville. The sample was taken from a variably sheared coronite outcrop, cut by granitic pegmatite. Corona textures are visible in the weathered outcrop. A pyroxene- garnet- plagioclase rock with abundant irregular clots of granular red garnet. The bulk of the rock displays a grain size of 1-2 mm, with scattered coarse pyroxene crystals to 10 mm. It is cut by sparse, late chloritic fractures, which may entrain traces of sulphides.

The Grenville province shows a wide range of metamorphic facies, and inferred crustal depths, from localised greenschist through predominant amphibolite to minor (in Ontario at least) granulite. The range of metamorphic grades and structural style (e.g., degree of deformation) is well displayed by the mafic to (sparsely) ultramafic intrusions, of which 30 or so, including the coronite featured here, are summarized by Wilson (1994). Whereas this coronite is a high-grade metagabbro (granulite with late deformation and partial retrogression to amphibolite facies), the Hastings basin around Cordova Mines represents the opposite end of the Grenville metamorphic spectrum, in the upper greenschist facies.

Corona structures are reaction rims separating two or mineral phases which form an unstable assemblage during regional metamorphism (e.g., Barker, 1990; Yardley et al., 1990). The core phase around which coronas are developed in metabasites is most often olivine, but the nucleus may also be a pyroxene, oxide or other mineral (see 25 tabulated examples in Wilson, 1994, pp.188-190). Some of the best-developed examples of granulite-facies metagabbros and corona textures in the Grenville occur in the area, beside and near Georgian Bay, e.g., Honey Harbour, Huntsville and Lake of Bays (Davidson, 1998; also Davidson and Grant, 1986; Schau et al., 1990; Davidson, 1991; Bussy et al., 1995). These include coronitic olivine gabbro and related rocks. The metagabbros contain corona structures around primary olivine and Fe-Ti oxides (e.g., Davidson and van Breemen, 1988). Corona textures also occur in other gabbroic to noritic rocks in the region (e.g., Friedman, 1955). Coronas formed in both the 1240 Ma Sudbury metadiabase to the north (Bethune and Davidson, 1988; Bethune, 1991; Davidson,1997), and in the 1170-1150 Ma coronitic olivine metagabbros (Nadeau and van Breemen, 1998; Ketchum and Davidson, 2000).

In the Adirondack mountains of upstate New York, garnet reaction rims in the Grenville massif anorthosites (p.38) are well-known (e.g., Roesler, in Kemp, 1921, pp.39-46). The Adirondacks and surrounding Grenville terrain are noted for Fe-Ti oxide deposits and garnet deposits. In the latter, garnet coronas may arise in olivine gabbro (Bartholome, 1960). Oxide exsolution may result in clouding in the feldspars (Poldervaart and Gilkey, 1954).

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Figures 3-4. Two photomicrographs of sample 1092. Left: recrystallized plagioclase and amphibole against a coarse crystal of clinopyroxene displaying exsolution textures. Right: polygonal garnet separated from granular recrystallized amphibole by pale plagioclase. Both images were taken in transmitted plane-polarized and (right) cross-polarized light, nominal magnification 50X, approximate long-axis field of view 1.7 mm.


Barker,AJ (1990) Introduction to Metamorphic Textures and Microstructures. Blackie, Glasgow, 162pp.

Bartholome,P (1960) Genesis of the Gore Mountain garnet deposit, New York. Econ.Geol. 55, 255-277.

Bethune,KM (1991) Fate of the Sudbury dykes in the Tyson Lake area south of Sudbury, Ontario: implications for Grenville Front history and the origin of coronites of high-grade terranes. GAC/MAC Prog.w.Abs. 16, 11, Toronto.

Bethune,KM and Davidson,A (1988) Diabase dykes and the Grenville Front southwest of Sudbury, Ontario. GSC Pap. 88-1C, 151-159.

Bussy,F, Krogh,TE, Klemens,WP and Schwerdtner,WM (1995) Tectonic and metamorphic events in the westernmost Grenville province, central Ontario: new results from high-precision U-Pb zircon geochronology. Can.J.Earth Sci. 32, 660-671.

Culshaw,N, Corrigan,D, Ketchum,J and Wallace,P (1990) Georgian Bay geological synthesis: Twelve Mile Bay to Port Severn, Grenville province of Ontario. GSC Pap. 90-1C, 107-112.

Davidson,A (1991) Metamorphism and Tectonic Setting of Gabbroic and Related Rocks in the Central Gneiss Belt, Grenville Province, Ontario. GAC Field Trip A9 Guidebook, 57pp.

Davidson,A (1997) New developments in Grenville Front geology, Sudbury area, Ontario. Institute on Lake Superior Geology, Field Trip Guidebook, vol. 43 part 3, Sudbury, 38pp.

Davidson,A (1998) Metamorphic Mineral Assemblages in the Central Gneiss Belt, Grenville Province, Ontario, with Emphasis on Mafic Rocks and their Tectonic Setting. International Mineralogical Association 17th General Meeting, Toronto, Field Trip A1, 67pp.

Davidson,A and Grant,SM (1986) Reconnaissance geology of western and central Algonquin Park and detailed study of coronitic olivine metagabbro, central Gneiss Belt, Grenville province of Ontario. GSC Pap. 86-1B, 837-848.

Davidson,A and Van Breemen,O (1988) Baddeleyite-zircon relationships in coronitic metagabbro, Grenville province, Ontario: implications for geochronology. Contrib.Mineral.Petrol. 100, 291-299.

Friedman,GM (1955) Petrology of the Memesagamesing Lake norite mass, Ontario, Canada. Amer.J.Sci. 253, 590-608.

Kemp,JF (1921) Geology of the Mount Marcy quadrangle, Essex county, New York. New York State Museum Bull. 229-230, 86pp. plus 1:62,500 scale map.

Ketchum,JWF and Davidson,A (2000) Crustal architecture and tectonic assembly of the Central Gneiss belt, southwestern Grenville province, Canada: a new interpretation. Can.J.Earth Sci. 37, 217-234.

Nadeau,L and van Breemen,O (1998) Plutonic ages and tectonic setting of the Algonquin and Muskoka allochthons, Central Gneiss Belt, Grenville province, Ontario. Can.J.Earth Sci. 35, 1423-1438.

Poldervaart,A and Gilkey,AK (1954) On clouded plagioclase. Amer.Mineral. 39, 75-91.

Schau,M, Davidson,A and Carmichael,DM (1986) Granulites and granulites. GAC/MAC Field Trip Guidebook 6, 36pp.

van Breemen,O and Davidson,A (1990) U-Pb zircon and baddeleyite ages from the Central Gneiss Belt, Ontario. In `Radiogenic Age and Isotope Studies: Report 3', GSC Pap. 89-2, 191pp., 85-92.

Wilson,GC (1994) Mafic-Ultramafic Intrusions, Base-Metal Sulphides, and Platinum Group Element Potential of the Grenville Province in Southeastern Ontario. OGS OFR 5880, 196pp.

Yardley,BWD, MacKenzie,WS and Guilford,C (1990) Atlas of Metamorphic Rocks and their Textures. Longman Group UK Ltd, 120pp.

Graham Wilson, 12,13,16-17,27 October 2013.

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