![[276 kb]](lg01.jpg)
![[179 kb]](lg02.jpg)
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.
A likely source has come to light recently: see discussion of trade (varietal) names, below.
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 (Figs. 1,3,4), 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.
Continuing the theme of popular (albeit unscientific)
trade, commercial or "fan" names, traits common
to building stones and gemmy, semi-precious stones alike....
Juparana (the usual spelling) is a Brazilian granite term,
varying from leucogranites to metamorphosed and deformed variants,
grading to othogneiss. Marketers are quick to adopt a viable
name, and so some "Juparana" actually comes from Australia.
Looking at material on the market for countertops in 2023,
the featured "rock of the month"
indeed appears to be of Brazilian origin.
It is marketed as Juparana Delicatus or Delicatus White granite.
This comes from a quarry in the Pedra Branca area, in
central-southern Ceara state, in northeast Brazil. The port of Fortaleza is a logical shipping point on the northeast coast.
How old is this granite? That's hard, not knowing the local geology.
Given how well the igneous textures are preserved, the lack of deformation, it is probably relatively young, for Brazil. latest Neoproterozoic to Cambrian perhaps, emplaced in the Brasiliano / Pan-African orogeny, circa 600-500 Ma.
Older Neoproterozoic suites, 900-1000 Ma, are now augen gneiss and migmatite.
A stunning ornamental stone, regardless of the details.
![[143 kb]](lg03.jpg)
![[164 kb]](lg04.jpg)