Figure 1. Epidote (with bright interference colours) and coarser hornblende (at lower edge: near extinction position, cleavage traces visible) concentrated along grain boundaries between albite-twinned plagioclase feldspar crystals. Nominal magnification 100X, crossed-polarized transmitted light, long-axis field of view 0.8 mm.

Figure 2. View of twinned plagioclase crystals, in a typical area of the anorthosite. Nominal magnification 50X, crossed-polarized transmitted light, long-axis field of view 1.6 mm.

"Rock of the Month # 50, posted for August 2005" --- The 800 km² Shawmere anorthosite is centred 15 km west of Foleyet, and contains a significant resource of calcic plagioclase feldspar (Veldhuyzen, 1993a,b). This is sample 2442, donated by Pam Sangster, Resident Geologist with the Ministry of Northern Development and Mines in Tweed, Ontario. Two digital images, 2005.

Nature of the sample

The hand specimen is a pale grey, coarse, granular, massive anorthosite, not appreciably magnetic, showing no effervescence in dilute HCl. The estimated modal mineralogy for this particular specimen includes some 96 vol.% plagioclase (crystals up to 3.5 mm in diameter, optically estimated composition An₈₂ [bytownite]); hornblende (3%); epidote (1%); plus traces of hematite and zircon. There is no evident layering, neither magmatic fabric nor tectonic overprint. The ferromagnesian minerals tend to be closely associated (hornblende, epidote and trace oxide) and are often along plagioclase grain boundaries. The predominant calcic plagioclase (bytownite) crystals are equant, polygonal, with ubiquitous twinning but no evidence of concentric zonation.

Overview of the anorthosite

The Shawmere anorthosite is a large (circa 800 km²) late-Archean anorthosite complex located in the Chapleau-Foleyet area of northern Ontario, east of Lake Superior (Thurston et al., 1976, 1977), centred some 15 km west of Foleyet. The 15x50 km main body of the Shawmere intrusion is anorthosite with a border zone of migmatitic garnetiferous amphibolite (Costanzo-Alvarez et al., 1993). The region is part of a lower-crustal domain subject to high-grade regional metamorphism, known as the Kapuskasing structural zone (Percival et al., 1983; Rudnick et al., 1983), reaching conditions of 700-800°C, 6-8 kbar. The ages and cooling histories of the rocks (Lopez Martinez and York, 1986, 1990), revealed in minerals such as hornblende and biotite mica, indicate slow cooling for the deeply-buried domain. Krogh (1993) determined that, while volcanism and batholith formation had ceased by 2700 and 2680 Ma respectively, granulite formation occurred 2660-2640 Ma and ductile deformation continued until 2585 Ma. Granulites (and local Au mineralization) formed in a slow episode of ductile underplating, between 2670 and 2585 Ma.

Anorthosites contain as much as 34% alumina (Dolan et al., 1991). The anorthosite is relatively calcic, certainly when compared to younger Grenville anorthosites in southeast Ontario, southern Quebec and northern New York state. The economics of both sodic and calcic anorthosites in Ontario have been studied (Veldhuyzen 1992, 1993a,b, 1994), with an eye to possible applications as fillers in the plastics and paper industries, in cement manufacture and elsewhere. The plagioclase in the Archean bodies is more calcic than An₇₀, whereas the Grenville equivalents are An_40-60.

Although fresh in gross terms, the Shawmere anorthosite is cut by linear alteration zones, with minerals assemblages including calcite, sericite, epidote, zoisite and clinozoisite, sericite, margarite and paragonite, scapolite, quartz, pyrite and pyrrhotite, evidence of `post peak-metamorphic fluid infiltration' (Morrison and Valley, 1987; Lamb and Morrison, 1992).

References

COSTANZO-ALVAREZ,V, DUNLOP,DJ and PESONEN,LJ (1993) Paleomagnetism of alkaline complexes and remagnetization in the Kapuskasing structural zone, Ontario, Canada. JGR 98 B2, 4063-4079.

DOLAN,ML, HAINS,DH and ASH,DR (1991) High-Alumina Rocks in Ontario: Resources and Process Technology. Ontario Ministry of Northern Development and Mines, Industrial Mineral Background Paper 10, 130pp.

KROGH,T (1993) High precision U-Pb ages for granulite metamorphism and deformation in the Archean Kapuskasing structural zone, Ontario: implications for structure and development of the lower crust. EPSL 119, 1-18.

LAMB,WM and MORRISON,J (1992) Retrograde fluids in the Kapuskasing structural zone: late formation of CO₂-rich fluid inclusions and isobaric cooling. GSA Abs.w.Progs. 24 no.7, Cincinnati, 340.

LOPEZ MARTINEZ,M and YORK,D (1986) A ⁴⁰Ar/³⁹Ar age study of the Kapuskasing structural zone. GAC/MAC Prog.w.Abs. 11, 96.

LOPEZ MARTINEZ,M and YORK,D (1990) A comparative ⁴⁰Ar/³⁹Ar study of the Kapuskasing structural zone and the Wawa gneiss terrane: thermal and tectonic implications. CJES 27, 787- 793.

MORRISON,J and VALLEY,JW (1987) Late fluids in the Kapuskasing structural zone. EOS 68 no.44, 1527-1528.

PERCIVAL,JA, CARD,KD, SAGE,RP, JENSEN,LS and LUHTA,LE (1983) The Archean crust in the Wawa-Chapleau-Timmins region. In `Workshop on a Cross Section of Archean Crust' (Ashwal,LD and Card,KD, editors), LPI Tech.Rep. 83-03, 172pp., pp.99-169.

RUDNICK,RL, ASHWAL,LD and HENRY,DJ (1983) Metamorphic fluids and uplift-erosion history of a portion of the Kapuskasing structural zone, Ontario, as deduced from fluid inclusions. In `Workshop on a Cross Section of Archean Crust' (Ashwal,LD and Card,KD, editors), LPI Tech.Rep. 83-03, 172pp., pp.76-80.

THURSTON,PC, SAGE,RP and SIRAGUSA,GM (1976) Chapleau- Foleyet. OGS Map 2221, 1:253,440 scale.

THURSTON,PC, SIRAGUSA,GM and SAGE,RP (1977) Geology of the Chapleau area, districts of Algoma, Sudbury, and Cochrane. ODM Geosci.Rep. 157, 293pp. plus 3 maps.

VELDHUYZEN,H (1992) Site evaluation and market testing of Ontario anorthosite deposits, anorthosite mapping and sampling, northeastern Ontario. OGS Misc.Pap. 160, 253-254.

VELDHUYZEN,H (1993a) Calcic anorthosite: multiple market potential, chemicals, coatings, fillers, and cement. MS presented at Industrial Minerals '93, Toronto, 35pp.

VELDHUYZEN,H (1993b) Site evaluation and market testing of Ontario anorthosite deposits, anorthosite mapping and sampling, northeastern Ontario. In `Summary Report 1992-1993' (Wood,N, Shannon,R, Owsiacki,L and Walters,M editors), OGS Rep., 180pp., 150-154.

VELDHUYZEN,H (1994) Calcic anorthosite: an aluminum chemical source with possible coatings, fillers and cement by-products. In `NODA (Northern Ontario Development Agreement) Summary Report, 1993-1994', Ontario Ministry of Northern Development and Mines, 183pp., 148-153.