gabbro

Gabbro is a coarse-grained, mafic, plutonic igneous rock that forms at spreading centers in rift zones and mid-ocean ridges (so underlies oceanic crust). Gabbros can form as massive uniform intrusions or as layered ultramafic intrusions formed by settling of pyroxene and plagioclase (pyroxene-plagioclase cumulate).

As an essential component of the oceanic crust, gabbros are found in many ophiolite complexes in the sheeted dyke zone to massive gabbro zone (zones III and IV). Long belts of gabbroic intrusions are typical at proto-rift zones and around ancient rift zone margins, where they intrude into the rift flanks.

Gabbro is a dense rock that is greenish or dark-colored and comprises varied percentages of pyroxene, plagioclase feldspar, amphiboles, and olivine. Where olivine is present in large quantities, the rock is termed olivine gabbro.

A finer grained rock with the same composition as gabbro is termed diabase.

[images : layered gabbro, North Cascades : Salem gabbro-diorite cut by a a composite dike with felsic margins and a central core of basaltic rock : White Mountain Magma Series : pegmatitic gabbro : oceanic crust exposed on Cyprus : oceanic crust gabbro, 2 : thin section Oman Ophiolite gabbro : thin section of olivine gabbro - pyroxene and olivine show bright colours, striped grey rectangular crystals are plagioclase feldspar : thin section of gabbro with plagioclase and hypersthene (orthopyroxene) : hypersthene gabbro : thin section with pyroxene and (striped) plagioclases : thin section orthopyroxenes crystals surrounded by alteration (uralite) : thin section : thin section with twinned plagioclases :

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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, garnets, 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.

igneous rocks

Igneous rocks result when molten rock cools (± crystallization) from plutonic (intruded) magma or from volcanic (extruded) lava.

Left - click to enlarge image - texture and mineral composition of common igneous rocks:

Increasing to the left: SiO2 content, viscosity; Increasing to the right: darkness, mafic : felsic composition, (Fe, Mg, Ca) : (K, Na) ratio, temperature of melting:

◘ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava),
◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt
phaneritic texture, emplaced as magma (plutonic)
◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite
0-100: percentage mineral content:
◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite
◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine

Igneous rocks, predominantly plutonic, comprise about 95% of the Earth's crust, though their burial by sedimentary rocks and association with metamorphic rocks disguises their true extent.
The crystalline basement rock (shield) at the core of most continents is ancient, having arisen predominantly during a period from 3.0 to 2.5 billion years ago, which was the period of maximum continent formation. Most of the earliest rocks have been greatly altered through regional metamorphic processes, but later rocks (3.2-2.5 Ga) are mostly pillow-basalts that formed beneath the vast oceans. Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.

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peridotite

Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km.

Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine.

rocks

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

Petrology ◘ basalt ◘ breccia ◘ felsic (sial) ◘ gabbro ◘ granite ▪ greenstone belts ◘ hornfels ◘ igneous rocks ◘ lava ◘ mafic (sima) ◘ magma ◘ metamorphic rocks ◘ peridotite ▪ porphyry ▪ plutonic ◘ rocks ◘ sedimentary rocks ▪ texture ▪ xenoliths ▪

Mineralogy ◘ crystallization ▪ phenocryst

Tables Geology Section  Elemental Composition of Crust  Flood Basalts and Stratigraphic Boundaries  Minerals & Rocks: Carbonates  Minerals & Rocks: Evaporates, Sulphates  Minerals & Rocks: Metamorphic  Minerals & Rocks: Oxides  Radiometric Dating  Volcanoes  Weathering of Minerals 

images : Rock gallery :

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