TONALITE
Type Intrusive
igneous rock
Chemistry :
Intermediet
Compenents
Essentials :
plagioclase (Oligoclase or andesine), quartz (in amounts > 10
percent, i.e. excluding it from the family of granitic rocks),
hornblade, biotite. Accessories
: orthoclase, apatite, titanite, magnetite, ilmenite,zircon.
Accidentals
: allanite, clinopyroxene.
Appearance
: medium-gray rocks with frequent dark inclusions; massive structure,
sometime with fluidal areas; hypidiomorphic granular texture with
local transformations to porphyry, due in particular to xenomorphic
hornblade and biotite. Frequent mafic differentiation, less frequent
felsic differentiation.
Occurrence
: The type locality is Monte Tonale in the Adamello complex of the
Tyrol. It contains prevalent euhedral plagioclase, easily
identifiable in thin section (Specialy to microscope photo,you can
see) because it is zoned and twinned;with either euhedral or anhedral
amphibole with interference colors tending to brown-green due to
masking by the actual color ; large sheets of green biotite and light
colore interstitial quartz. All these minerals have a hint of
alteration identifiable by the presence of pleats filled with
fine-grained minerals. The quartziferous diorite of the val mazino
(Italy) and the val Bregaglia (Canton des Grizons, Switzerland) is
slightly oriented; it is known locally as ‘’serizzo’’ and
contains only orthoclase in the groundmass. This rock, which is
considerably darker and has a coarser grain size than the
above-describe tonalite, contains numberous light and dark colored
areas, because it is markedly hybrid in origin. In thin section
plagioclase has dense polysynthetic twining oriented more or less
like the biotite with interference colors tending toward green:
Amphibole has interference colors tending toward yellow: and anhedra
epidote has vivid interfrerence colors tanding toward red, violet and
orange. In the same pluton there is transition, by enrichment in
large crystals (up to 10 cm ; 4 in) of orthoclase, to a variety which
has an overall granodioritic composition in a dark, biotitic
groundmass in which the large orthoclase crystals are conspicuous.
Quartziferous diorites and tonalities are very common in the great
batholith of the sierra Nevada in southern California and the Cascade
Range in Oregon (USA) and British Columbia (Canada). They are also
found in the Caledonia in-trusives of Scotland, especially al loch
Awe and in Galloway. In Norway they occur with quartz-rich and rather
leucocratic varieties (trondhjemites).
Uses
: A building material, both in the natural state and as polised
slabs, sometimes also used to build steps, because of its rigidity.
Many mineral deposits, especially of pyrite and copper, are
genetically associated with tonalities and quartz-bearing diorites.
Geotectonic
Environment
: in granotype-type well defined batholiths and plutons, in which it
often forms an igneus core developed during early differentiation. It
is found in large masses that have been interpreted to be the result
of anatectic remelting at great depths, in veins formed during a
stage of compression and in environment with abundant water. As a
result it has the same petrological significance as andesite but has
remained at depth, crystallizing under plutonic condition and
evolving chemically ia a felsic sense. Quartz bering diorites are
virtually identical varieties from a chemical view point bu contain
no hornblade; some of these are considered to have been derived from
gabbro as a result of hybridization with granitic fluids. The term
quartz bearing or quartziferous diorite is often used to indicate all
rock of this type. The latest international agreements have given
preference to the term ‘’ tonalite’’ in order to avoid too
many attributive forms.
