Biotite leucogranite

--- employed as a decorative stone, displaying superb igneous textures

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Figure 1-2. A polished face of granite composed largely of pale creamy to pinkish feldspars, translucent and somewhat smoky grey quartz, and coarse flakes of black biotite mica, plus a few small grains of red garnet. The rock is coarse-grained to pegmatitic, and locally contains masses of quartz-feldspar intergrowths of dinner-plate size, approaching 30 cm in diameter.


"Rock of the Month #169, posted for July 2015" ---

Leucogranite

is a term describing a granitic rock with a low proportion of dark minerals (such as hornblende, biotite mica, magnetite or tourmaline) thus presenting a pale ("leucocratic") appearance. This month's specimen (Figures 1-4) is a handsome rock displayed to advantage in a newly refurbished hotel foyer, opened 22 May 2015. These photographs were taken on 14 June 2015 at the Heritage Inn Hotel & Conference Centre, Cardinal Crescent, Saskatoon. The source quarry and polisher are unknown to the writer, but such rocks, which occur in Canada, were quite probably imported from elsewhere, perhaps China, or India or Brazil.

Leucogranites are well developed by partial melting within the thick sedimentary sequences of the Himalayas, as in Nepal and Sikkim, for example (Basu, 2013). The Himalayan leucogranites commonly contain two micas (i.e., silvery muscovite and brown-black biotite) and accessory phases such as tourmaline and cordierite (Shrestha et al., 1993). Such granites may form by anatexis, the melting of precursor rocks such as shales and siltstones. The mineralogy is generally very simple, and the rarer chemical elements such as rare earths may be concentrated into accessory minerals such as garnet (e.g., Harris et al., 1992) or in phosphates such as monazite (Sevigny, 1993). Garnets formed in granitic melts tend to be rich in manganese (the Al,Mn-rich spessartine molecule, typically combined with the Al,Fe-rich almandine component). A study of garnets in the Galway granite of Ireland showed that the Mn generally increases from granite to aplite to pegmatite, and that the latter (fine- and coarse-grained late-stage variants of the parental granite) tend to also become enriched in the rare metal yttrium (Whitworth and Feely, 1994).

Leucogranites are common in other orogenic belts where the crustal rocks have been deeply buried, metamorphised and later exhumed, such as the Proterozoic Grenville province (Van Berkel, 1987). Similar leucogranites occur in the Proterozoic of northern Canada (Peterson et al., 1989). Younger, Paleozoic and Mesozoic examples of anatectic melts may be intruded along shear zones (Strong and Hanmer, 1981; Tobisch and Cruden, 1994).

In northwest Ontario, the Archean Quetico subprovince is composed mainly of metamorphosed turbiditic sediments, largely derived from (and deposited during and after) the volcanic climax in the neighbouring Wawa, Wabigoon and Abitibi subprovinces, 2700-2690 Ma (Williams, 1991). Leucogranites are common in the Quetico, whether as small (<1 km) discrete stocks of coarse, sparsely garnet-bearing rock, or as outcrop-scale partial melts evolved in migmatization. These leucogranites and associated pegmatites may be enriched in alkali elements and other uncommon elements such as zirconium, tin, tantalum and niobium (Kissin et al., 1986).

Intimate intergrowths of quartz and alkali feldspar, known as graphic granite, are well displayed in the counter top illustrated here, and may have formed by the simultaneous growth of the two minerals (Fenn, 1986).

One problem with identification of ornamental stones is the long tradition in the industry of applying attractive but unscientific and often misleading trade names. The classic example is "black granite": there is no such thing! Examples of so-called black granites, which may indeed be black or dark grey, are typically variants gabbro, gabbroic anorthosite or syenite. Examples of trade names (Wilson, 1990) traced to their sources in Brazil and India include: a biotite leucogranite dubbed Indian Multicolour; Lila Gerais (a leucogranite from Minas Gerais); Tropical Violet, another Brazilian leucogranite, with only 1% dark minerals, dominated by biotite; and Jupparrana Gold, a tawny leucogranite, again with only 1% of biotite-dominated ferromagnesian minerals.

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Figures 3-4. Two more close-ups of the graphic granite texture, showing the hieroglyphic appearance of the intergrowth. For scale, the red cap on the pen is 40 mm long. A general paucity of magnetic minerals (e.g., magnetite, common in some other granite types) typically makes for extremely low levels of intrinsic magnetism in leucogranites. Magnetic susceptibility measured on these polished surfaces (away from underlying metal supports and electrical wiring!) is <0.05x10-3 SI units. This is typical also for the Quetico leucogranites in northwest Ontario, and lower than a typical value of circa 0.2x10-3 for older biotite schists, the foliated clastic metasediments in the same belt, from which the leucogranites likely formed in anatexis.


References

Basu,SK (2013) Geology of Sikkim State and Darjeeling District of West Bengal. Geol.Soc.India, 256pp. plus map.

Fenn,PM (1986) On the origin of graphic granite. Amer.Mineral. 71 nos. 3-4 (R.H. Jahns Memorial Issue), 325-330.

Harris,NBW, Gravestock,P and Inger,S (1992) Ion-microprobe determinations of trace-element concentrations in garnets from anatectic assemblages. Chem.Geol. 100, 41-49.

Kissin,SA, Zayachkivsky,B and Branscombe,LA (1986) Genesis of pegmatites in the Quetico gneiss belt of northwestern Ontario - granitoids, "barren" pegmatites, and metasediments, with additional data on rare-element pegmatites, from the Georgia Lake pegmatite field. Ontario Geol.Surv. Misc.Pap. 130, 65-78.

Peterson,TD, LeCheminant,AN and Rainbird,RH (1989) Preliminary report on the geology of northwestern Dubawnt Lake area, district of Keewatin, N.W.T. Geol.Surv.Canada Pap. 89-1C (Canadian Shield), 413pp., 173-183.

Sevigny,JH (1993) Monazite controlled Sm/Nd fractionation in leucogranites: an ion microprobe study of garnet phenocrysts. Geochim.Cosmochim.Acta 57, 4095-4102.

Shrestha,SB, Maskey,ND, Sharma,T and Bashyal,RP (editors) (1993) Geology and Mineral Resources of Nepal. Atlas of Mineral Resources of the ESCAP Region, volume 9. United Nations, New York, 107pp. plus two 1:1,000,000 scale maps.

Strong,DF and Hanmer,SK (1981) The leucogranites of southern Brittany: origin by faulting, frictional heating, fluid flux and fractional melting. In `Peraluminous Granites' (Clarke,DB editor), Can.Mineral. 19 part 1, 216pp., 163-176.

Tobisch,OT and Cruden,AR (1994) Small component of arc-scale right-lateral shear during 102-96 Ma in the central Sierra Nevada batholith, California, constrained by AMS patterns and zircon ages of two plutons. Geol.Soc.Amer. Abs.w.Progs. 26 no.7, 533pp., 133, Seattle.

Van Berkel,JT (1987) Geology of the Dashwoods Pond, St. Fintan's and Main Gut map areas, southwest Newfoundland. Geol.Surv.Canada Pap. 87-1A, 946pp. in 2 vols., 399-408.

Whitworth,MP and Feely,M (1994) The compositional range of magmatic Mn-garnets in the Galway granite, Connemara, Ireland. Mineral.Mag. 58, 163-168.

Williams,HR (1991) Quetico subprovince. In `Geology of Ontario' (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), Ontario Geol.Surv. Spec.Vol. 4, part 1, 709pp., 382-403.

Wilson,GC (1990) Untitled (dual-format, popular and technical descriptions of seven building stones from India and Brazil). Turnstone G.S.L. Report for Sacerdote Marble, Toronto, 14pp.

Graham Wilson, 01 July 2015.

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