granite

Granite is typically a medium to coarse grained felsic, intrusive igneous rock (plutonic) that is usually pink to dark gray, sometimes black, depending on its chemistry and mineralogy. Granites are the commonest basement rocks of the continental crust, many dating from the Precambrian.

In some granites, individual crystals are larger than the groundmass (porphyrys). Granites primarily comprises orthoclase and plagioclase feldspars, quartz, hornblende, muscovite and/or biotite micas, with minor accessory minerals such as magnetite, garnet, zircon and apatite. Rarely, a pyroxene is present. Very rarely, iron-rich olivine, fayalite, occurs.

Granites are classified according to the QAPF diagram for granitoids and phaneritic foidolites (plutonic rocks) that compares the percentages of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar.

As a plutonic rock, granite is often exposed in weathered tors, dykes and as massive batholiths.

| 0 Guide-Glossary

plutonic

Pluton is a general term for any large igneous intrusion of crystallized magma. Rocks that emplaced in large intrusive structures are termed "plutonic", whereas those that form in small intrusions are called "hypabyssal".

Plutonic igneous rocks cooled and crystallized from intrusive or irruptive magma within the Earth's crust. Plutonic rocks exhibit a fine-grained aphanitic texture if the magma cooled close to the surface in volcanic necks or feeder pipes, with grain size increasing through to coarse-grained, phaneritic textures for magmas that cooled very slowly at great depth in very large magma chambers. (image - click to enlarge - plutonic rock of North America)

Plutonic rock types include granite, diorite, gabbro, and rhyolite. Plutonic structures include huge solidified magma chambers called batholiths, laccoliths, stocks, bysmaliths, and lopoliths, while smaller hypabyssal structures include diapirs, dikes and dikelets, ring dykes, sills, volcanic necks and plugs, and cone sheets. Less commonly encountered plutonic structures include: phacoliths, cactoliths, ductoliths, harpoliths, sphenoliths, akmoliths, and ethmoliths. Chonoliths are intrusive igneous structure with shapes so irregular that they do not fall into the normal categories. Any intrusive structure may later become exposed at the surface by erosion.

(Veins are finite volumes within rocks, which are filled with crystals of minerals precipitated from hot (aqueous) fluids.)

intrusive emplacement / structures: ▪ batholith ▪ boudin ▪ bysmalith ▪ concordant ▪ conformable ▪ diapir ▪ diatreme ▪ dike ▪ discordant ▪ enclaves ▪ hypabassal ▪ laccoliths ▪ lopoliths ▪ pluton ▪ sills ▪ strata ▪ volcano ▪ vein ▪

structures reflecting flow/crystallization within magma chambers: ▪ cumulates ▪ enclaves ▪ schlieren ▪ xenoliths ▪

magmatic processes: ▪ assimiliation ▪ exchange of volatiles ▪ fractional crystallization ◘ igneous rocks ◘ igneous structures ◘ magma ▪ magmatic differentiation ▪ magmatic mixing ◘ ophiolite complexes ◘ tectonics extrusive: ◘ lava ◘ volcanoes ▪ vulcanism

igneous rocks: ◘ basalt ▪ cumulates ◙ dunites ▪ enclaves ◘ felsic (sial) ◘ gabbro ◘ granite ◙ granodiorite ◊ groundmass ◙ keratophyres ◘ lava ◘ mafic (sima) ◘ magma ◘ ophiolite complexes ◙ pegmatites ◘ peridotite ▪ peridotite ▪ porphyry ▪ schlieren ▪ tonalites ◊ xenocryst ▪ xenoliths

Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain.

[images : dike, sill, diapir, laccolith, batholith]

texture

The texture of a rock is determined by the size and configuration of its constituent minerals and any presence of gas bubbles. Rock fabric refers to the general appearance of a set of crystals that have grown together to produce a distinctive shape or texture. The term "fabric" is best applied to groups of crystals, or to invidividual crystals within groups, which have grown together so that their growing surfaces have encountered each other. Crystal habit is the general appearance of a crystal that results from the nature and prominence of crystal forms, such as faces or sets of faces. The term "habit" is best applied to invidividual crystals that have grown without their growing surfaces encountering any pre-exisiting solid (encounter = fabric). The microstructure of a rock is its set of structural features, such as grain boundaries, grain size and structure, which can be observed in thin-section (under a microscope).

igneous rocks:
aphanitic – all crystals are too small to be visible to the naked eye or with a hand lens, giving the rock a dull appearance; resulting from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments. Eg. basalt
phaneritic – all crystals sufficiently large to be clearly visible to the naked eye; resulting from slow cooling of magma deep underground in plutonic structures. Eg. granite, gabbro
porphyritic – larger phenocrysts embedded in a finer textured matrix; resulting from two-stage cooling of rising magma, first at depth and subsequently at the surface or shallow subsurface. Eg. porphyry
glassy – non-crystalline rocks; resulting from very rapid cooling of lava at or very near the Earth's surface. Eg. obsidian
vesicular – porous rock with vesicles (holes, pores, or cavities); resulting from gas expansion within rapidly cooling ejected lava. Eg. vesicular basalt
fragmental or pyroclastic – irregular grains welded together, sometimes with glassy shards; resulting from pyroclastic volcanic eruptions. Eg. volcanic tuff, ignimbrites

metamorphic rocks:
foliated – in which mineral constituents are oriented in a parallel or subparallel arrangement resulting from imposed pressure during regional metamorphism. From low to high metamorphic grades:
--slaty – parallel orientation of microscopic grains; Eg. slate
--phyllitic – parallel arrangement of platy minerals, usually micas, that are barely visible to the naked eye; Eg. phyllite
=-schistose – subparallel to parallel orientation of platy minerals such as chlorite or micas; Eg. schists
--gneissic – coarsely foliated texture in which minerals have segregated into discontinuous hands, each of which is dominated by one or two minerals; Eg. gneisses
...granuloblastic – typical of granulites, which have even-sized, granular mineral grains with weak preferred orientation. Microscopic structure reveals small, rounded grains forming a closely-fitted mosaic.
non-foliated – in which constituent minerals lack an ordered arrangement, retaining roughly the orientation of grains present in the country rock before thermal metamorphism; Eg. quartzite, marble, metaconglomerates, hornfels, anthracite coal.

Euhedral crystals are distinct, well-formed crystals with sharp, easily-recognized faces (almandine garnet in quartzitic gneiss at left).

Euhdral is opposite to the interlocked grains of anhedral textured rocks that have cooled in the crowded environment of magma chambers. Subhedral crystals are intermediate in character between distinct euhedral crystals and enmeshed anhedral textures.