I-type granites are a category of
granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...
s originating from igneous sources, first proposed by Chappell and White (1974).
They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics that indicate, for example, magma hybridization in the deep crust.
I-type granites are saturated in
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
but undersaturated in
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
; petrographic features are representative of the chemical composition of the initial magma. In contrast
S-type granites are derived from partial melting of supracrustal or "sedimentary" source rocks.
Petrographic characteristics
Primary minerals
Minerals that crystallized from the silicate melt are considered primary minerals. They are grouped into "Major", "Minor", and "Accessory" minerals based upon their modal percentages in the rock.
Major mineralogy
Primary minerals in I-type granites are
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
,
potassium feldspar Potassium feldspar refers to a number of minerals in the feldspar group, and containing potassium:
*Orthoclase ( endmember formula K Al Si3 O8), an important tectosilicate mineral that forms igneous rock
*Microcline, chemically the same as orthocla ...
, and
quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
as in
S- and
A-type granites. I-type granites have less quartz then their S-type granite color index equivalents. Plagioclase displays zonation and
albite twinning. Potassium feldspar can show
perthite
Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar (near K-feldspar, KAlSi3O8, in composition) includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar (near albite, N ...
textures,
carlsbad twinning, and, in
microcline
Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slo ...
, tartan twinning. Quartz and potassium feldspar scarcely show
granophyric textures.
Minor minerals
Biotite is the most common minor mineral in I-type granites. The biotites in I-type granites are greener in general than those in S-type, both in hand sample and in plane polarized light. More
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...
composition granites, those with a higher color index, contain more
hornblende
Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rock ...
and biotite.
Hornblende is a typical I-type granite mineral which never occurs in S-type granite. Hornblende crystals can be twinned and compositionally zoned.
Accessory minerals
Zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...
and
apatite can occur in both I- and S-type granites, whereas
titanite
Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals ...
(sphene) and
allanite
Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rock ...
are considered diagnostic accessory minerals for I-type granites.
Allanite is typically surrounded by radial fractures, caused by the
subsolidus increase in volume of allanite as a result of
metamict alteration due to
radioactive decay. While apatite inclusions are common, they are not as abundant or large as those in S-Type granites. Primary
muscovite
Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...
can occur in weakly
peraluminous
Peraluminous rocks are igneous rocks that have a molecular proportion of aluminium oxide higher than the combination of sodium oxide, potassium oxide and calcium oxide.Blatt, Harvey and Robert J. Tracy, ''Petrology,'' Freeman, 2nd ed., 1995, p. ...
fractionated I-type granites.
Therefore, the presence of muscovite alone is not diagnostic of S-type granites.
Subsolidus and alteration minerals
Minerals that form in the rock as a result of chemical reactions that take place between primary minerals and
hydrothermal fluid
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
s are classified as subsolidus minerals. They form below the temperature and pressure conditions of the solidus in the absence of a silicate melt. Other alteration minerals may form at surface conditions from interaction of the minerals present in the rock with groundwater and the atmosphere.
Alteration of biotites can produce
fluorite
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
The Mohs sca ...
,
chlorite, and iron oxides such as
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
and
ilmenite
Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...
. Sericitic alteration is seen within feldspars. In more evolved I-Type granites,
calcite occurs as a late stage and/or a subsolidus mineral. Fluorite, like calcite, is rare and where observed it is associated with the more evolved I-type granites. It can form as a late stage product of crystallization. It is commonly observed as part of the subsolidus alteration of biotite along with chlorite and opaque oxides.
Muscovite
Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...
occurs as an alteration of feldspars and biotite.
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral.
Description
Well developed crystals of epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in hab ...
can be found, especially on the edges of allanite.
Color index
Color index, or the modal abundance of minerals other than quartz, plagioclase and alkali feldspar (e.g., mafic silicates, oxides, sulfides, phosphates, etc.), can be used to infer the maturity of a granite. Juvenile I-type granites have a higher color index.
Amphibole, biotite,
sphene
Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals in ...
, allanite, and oxides are typically more abundant. In contrast, more evolved (i.e. fractionated) I-type granites have a lower color index, and may contain minerals such as muscovite that are indicative of their fractionated nature.
Textures
I-type granites can have variable textures. I-type granites, like other granite types, can vary in crystal size from
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geolog ...
to
phaneritic
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided human eye. In contrast, the crystals in an aphanitic rock are too fine-grained to be identifiable. Phaneritic texture fo ...
; crystal size distributions include porphyritic, seriate, and rarely equigranular textures. Like other granites,
phenocryst
300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
s in I-type granites are commonly feldspars, but can also be
hornblende
Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rock ...
. Amphibole is a diagnostic feature on the hand sample scale between S-type and I-type granites.
Geochemistry
Major elements
I-type granites are rich in silica, calcium and sodium but contain lesser amounts of aluminium and potassium when compared to S-type granites. I-type granites are typically metaluminous to weakly peraluminous. This is expressed mineralogically by the presence of
amphibole and accessory minerals such as
sphene
Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals in ...
and
allanite
Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rock ...
in the metaluminous I-type granites. Note that weakly peraluminous fractionated I-type granites may crystallize primary muscovite and rare
spessartine
Spessartine is a nesosilicate, manganese aluminium garnet species, Mn2+3Al2(SiO4)3.Gemological Institute of America, ''GIA Gem Reference Guide'' 1995, This mineral is sometimes mistakenly referred to as ''spessartite''.
Spessartine's name is a ...
-rich
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
.
Trace and rare earth elements
The rare earth element diagrams of I-type granite suites tend to be flatter than those of S-type granites, which has been inferred to be caused by the lesser amounts of apatite in I-type granites. I-type granites have lower
rubidium/
strontium (Rb/Sr) ratios than S-type granites.
Isotopic characteristics
Initial strontium isotopic ratios (
87Sr/
86Sr)''
i'' are a good differentiator between I- and S-type granites, with I-type granites having lower initial strontium isotopic ratios than S-type granites.
Interpretation(s)
Source characteristics
I-type granites are interpreted to be generated from the melting of igneous rocks. The “I” in I-type in fact stands for igneous. This interpretation was made by Chappell and White in their 1974 paper based on their observations in the
Lachlan Fold belt The Lachlan Fold Belt (LFB) or Lachlan Orogen is a geological subdivision of the east part of Australia. It is a zone of folded and faulted rocks of similar age. It dominates New South Wales and Victoria, also extending into Tasmania, the Australia ...
of southeastern
Australia.
The I-S line
The I-S line is an observed contact between I- and S-type granites in an igneous terrane. This contact is usually clearly defined; one example of this occurring is within the Lachlan fold belt of Australia. The I-S line is interpreted to be the location of a paleo-structure in the subsurface that separated the generation zones of the two different melts.
Suites and supersuites
Granite plutons can be grouped into suites and super suites by their source regions, which in turn are interpreted by comparing their compositions. This interpretation comes from the plotting of different element concentrations against the level of evolution of the granite, usually as percent silica or its magnesium to iron ratio. Igneous rocks with the same source region will plot along a line in silica to element space.
Restite unmixing
Granites traced to the same source region can often have very variable mineralogy; color index for example can vary greatly within the same batholith. In addition, many minerals resist melting and would not melt at the temperatures known to create the magmas that form I-type granites. One model that explains this mineralogic anomaly is restite unmixing. In this model, minerals that are resistant to melting, such as the color index minerals, do not melt but are rather brought up by the melt in solid state. Melts that are farther from their source regions would therefore contain fewer color index minerals, while those closer to their source regions would have a higher color index. This model supplements the models of partial melting and
fractional crystallization.
Other models
Other models include
magma mixing,
crustal assimilation, and
source region mixing. More recent studies have shown that the source regions of I-type and S-type magmas cannot be homogeneously igneous or sedimentary, respectively. Instead, many magmas show signs of being sourced from a combination of source materials. These magmas can be characterized by having a series of
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishe ...
and
hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
isotope characteristics that can be thought of as a combination of both I- and S-type isotopic characteristics. Magma mixing is another aspect of granite formation that must be taken into account when observing granites. Magma mixing occurs when magmas of a different composition intrude a larger magma body. In some cases, the melts are
immiscible and stay separated to form pillow like collections of denser mafic magmas on the bottom of less dense dense felsic magma chambers. The mafic
pillow basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
s will demonstrate a felsic matrix, suggesting magma mingling. Alternatively, the melts mix together and form a magma of a composition intermediate to the intrusive and intruded melt.
References
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Igneous rocks