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Department of Mineralogy and Petrology
Faculty of Natural Sciences
Comenius University in Bratislava

Plutonic rocks

Plutonic rocks are magmatic rocks crystallized from magma, which is emplaced in depth or close to surface. Hence, the plutonic rocks are sometimes designated as intrusive ones.

Plutonic rocks are named after Pluto – the Roman god of underworld. They are characteristic of phaneritic, massive and confining textures. Rarely, mineral grains exhibit preferred orientation, parallel texture is exceptional. Very specific is the orbicular (spherical) texture typical only of plutonic granitoid rocks. Some plutonic rocks may have parallel texture with recurrent, unequally sized layers of different colour, granularity and composition. Plutonic rocks usually have holocrystalline, equally grained and hypauthomorphic granular structures. Symplectitic structures, such as myrmekitic, graphic and granophyric ones are diagnostic of more acidic plutonic rocks. Poikilitic, ophitic and sub-ophitic structures are typical of mafic rocks, whereas cumulate structure occurs in ultramafic rocks.

Main rock-forming minerals of plutonic rocks are as follows: quartz, alkali feldspar, plagioclase, micas (muscovite and biotite), foids (nepheline, leucite, sodalite, haüyne), alkalic pyroxenes and amphiboles (in alkalic plutonic rocks only), olivine, pyroxenes and amphiboles (mainly in more mafic plutonic rocks). Melilite is additional rock-forming mineral in ultramafic plutonic rocks.

Important accessory minerals are as follows: Fe-Ti oxides, apatite, zircon, titanite, garnet, sulphides. Acidic plutonic rocks contain monazite, xenotime, REE-oxides, beryl, topaz and allanite. Corundum and minerals of the spinel group (spinel, chromite, hercynite) occur in mafic plutonic rocks, whereas phlogopite is typical for ultramafic rocks. Carbonatites are specific group of rocks, containing magmatic carbonatites (calcite, dolomite, ankerite) as main rock-forming minerals besides diopside, fluorite, fluorapatite, magnetite, pyrochlore, acmite, aegirine-augite, akermanite, Ti-Zr garnets, barite, bastnäsite, calzirtite, celestite, forsterite, columbite, monticellite, baddeleyite and strontianite. Many of these minerals create economically important accumulations. Plutonic carbonatites contain also perovskite, aragonite, brucite, titanite, uranopyrochlore, periclase. Less abundant are accessoric ilmenite, anatase, stibnite, betafite, burbankite, apatite with CO32-, cerite, diamond, euxenite, fergusonite, geikielite, rutile, vibianite, siderite, wollastonite, vermiculite, nepheline, zircon and monazite.

Plutonic rocks are subdivided into 5 major groups:

1. Plutonic rocks with less than 90 vol. % mafic minerals saturated/undersaturated with quartz: alkali feldspar granite, granite, orthopyroxene granite – charnockite, granodiorite, orthopyroxene granodiorite – charno-enderbite, pincinite, tonalite, orthopyroxene tonalite – enderbite, alkali feldspar quartz syenite, quartz syenite, quartz monzonite, quartz monzodiorite, quartz diorite, quartz gabbro, alkali feldspar syenite, syenite, monzonite, orthopyroxene monzonite – mangerite, monzodiorite, monzogabbro, diorite, anorthosite and gabbro-group rocks (gabbro s.s., norite, troctolite, gabbronorite, orthopyroxene gabbro, clinopyroxene norite, hornblende gabbro). All these rocks are classified in the upper triangle of the QAPF diagram for plutonic rocks, except for grabbroic rocks having individual classification scheme.

2. Plutonic rocks with less than 90 vol. % of mafic minerals, with foids, undersaturated with respect to quartz: foid-bearing alkali feldspar syenite, foid-bearing syenite, foid-bearing monzonite, foid-bearing monzodiorite, foid-bearing monzogabbro, foid-bearing diorite, foid-bearing gabbro, foid-bearing anorthosite, foid syenite, foid monzosyenite, foid monzodiorite, foid monzogabbro, foid diorite, foid gabbro and foidolite. All these rocks are classified in the lower triangle of the QAPF diagram for plutonic rocks.

3. Plutonic ultramafic rocks with more than 90 vol. % of mafic minerals: dunite, harzburgite, lherzolite, wehrlite, olivine orthopyroxenite, olivine websterite, olivine clinopyroxenite, orthopyroxenite, websterite, clinopyroxenite, hornblendite. The rocks are classified using modal classification for ultramafic rocks.

4. Plutonic rocks with more than 50 vol. % of primary carbonates: calcite-carbonatite, dolomite-carbonatite, ferrocarbonatite. Exceptional is natrocarbonatite which is known from the only active carbonatite volcano - Oldoinyo Lengai in Tanzania. The rocks are classified according to mineral composition. If modal composition is unknown or cannot be determined, the rocks are classified according to chemical classification for carbonatites which is valid for the carbonatites with less than 20 wt. % SiO2. The carbonatite rock with more than 20 wt. % SiO2 corresponds to silicocarbonatite.

5. Plutonic ultramafic rocks with more than 10 vol. % melilite and foids are melilitolites. These rocks are classified according to their mineral composition.