Intrusive Igneous Rocks, Part 1 Tonalite, Diorite, Gabbro, Norite, and Anorthosite

IUGS Intrusive Igneous Rock Chart - General

IUGS Intrusive Igneous Rock Chart

Plutonic Igneous Rocks Grain size is larger than extrusive rocks Grains are visible, usually identifiable Volume percent abundance is an important means of rock classification Total lack of glass - slow cooling allows matter to achieve the lowest energy state, which is as crystalline matter

Plutonic Igneous Rocks Continued Generally, intrusive rock field names are more reliable than the extrusive rock names because of the larger, more visible crystals Accurate determination of rock name demands a careful examination of thin sections of selected specimens

Gabbroic Layered Intrusions The rocks in this laboratory may occur in a variety of geologic settings. One setting in which these rocks are often associated is a gabbroic layered intrusion. These intrusions include some of the largest plutonic bodies in the world including Bushveld (South Africa), Skaergåard (East Greenland), Duluth Gabbro, Muskox (Northwest Territories, Canada), Great Dike (Zimbabwe), and the Stillwater Complex (Montana).

Gabbroic Layered Intrusions Gabbroic layered intrusions, as the name indicates, show distinct layers Layers may be quite complex Origin of layering is often at least partially due to fractional crystallization Early formed crystals segregate themselves from the main magma body by sinking (mafics) or rising (calcic plagioclase)

Layering Sequence in Gabbro Intrusions A typical sequence might be: Anorthosite Norite, feldspathic pyroxenites Harzburgite, Dunite (ultramafic rocks) Peridotite Fine-grained norite

Gabbro Intrusive igneous, plutonic to hypabyssal A phaneritic mafic rock with medium to coarse grains Gabbro is the intrusive equivalent of basalt The name may come from the gabbro region, Tuscany, Italy

Gabbro Mineralogy Essential: Mid to calcic plagioclase, commonly labradorite to bytownite (occasionally anorthite) Essential: Clinopyroxene, usually augite Accessory: Olivine and/or orthopyroxene Plagioclase grains range from equant to lath-shaped, and are almost always well-twinned Zoning is limited to the edge of the plagioclase grains, if present at all

Gabbro Mineralogy Continued Clinopyroxene is augite or diopside Some of these may be brown due to titanium and/or ferric iron content Twinning is often present, but zoning is very rare

Gabbro: IUGS Q = 0-5% Q P/(A + P) >90 pl/(pl + px + ol) is 10 - 90 Plagioclase composition > An50

Olivine Gabbro Contains olivine in addition to plagioclase and cpx Olivine gabbro is often richer in mafics than normal gabbro

Hornblende Gabbro Contains hornblende in place of the normal cpx Hornblende may be green or brown in thin section It may occur as independent prisms or as crusts on the pyroxene

Gabbro Photomicrographs CN views of gabbros The brightly colored minerals are clinopyroxene The white to gray minerals are plagioclase - note albite twinning The black minerals in both pictures are opaque grains of magnetite http://www.humboldt.edu/~jdl1/web.page.images/gabbro.html Photo #a_36.gif (upper) and a_37.gif (lower)

Gabbro Photomicrographs Olivine gabbro in CN Brightly colored and fractured olivine surrounded by plagioclase (upper photo) - small veins in olivine are serpentine Gabbro (lower photo) with cumulate texture, plagioclase poikilitically enclosed by clinopyroxene http://www.humboldt.edu/~jdl1/web.page.images/gabbro.html Photo #a_44.gif (upper) and a_47.gif (lower)

Ophiolites Pieces of oceanic plate that have been thrust (obducted) onto the edge of continental plates Provide information about processes at mid-ocean ridges Composed of an assemblage of mafic and ultramafic lavas and hypabyssal rocks found in association with sedimentary rocks like greywackes and cherts Found in areas that have complex structure http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/ophiolites.html

Ophiolite Obduction Ophiolites have been found in Cyprus, New Guinea, Newfoundland, California, and Oman http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/ophiolites.html Cross-sections simplified from R.C. Coleman, 1981, Journal of Geophysical Research, v. 86, p. 2497-2508. Phopt #op2.jpg

Typical Ophiolite Sequence Ophiolites are characterized by a classic sequence of rocks This sequence is well exposed at the Samail ophiolite Base of the sequence is sedimentary rocks of the Arabian shield, not part of the ophiolite, on which the oceanic plate was pushed From base to top the ophiolite is made of: peridotite, layered gabbro, massive gabbro, dikes, and volcanic rocks At Samail this entire sequence is 15 km thick. http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/ophiolites.html Sequence of rocks simplified from R.C. Coleman (1981). Phopt #op3.jpg Samail is a Wilayah (Town) located in the Ad Dakhiliyah Region (Interior Region) of the Sultanate of Oman close to the Samail Gap a break in the Hajar Mountains. Samail is known to have the warmest average nights for a city in the world, with an average July night dipping to just about 89 degrees Fahrenheit.

Samail Ophiolite The basal peridotite is made of a rock called harzburgite (made mostly of the minerals olivine and enstatite) Within the peridotite are many dikes of gabbro and dunite The peridotite is deformed and is overlain by dunite (an intrusive igneous rock made mostly of the mineral olivine) that grades upward to gabbro (an intrusive igneous rock made mostly of plagioclase and clinopyroxene - augite) Sequence is capped by dikes and volcanic rocks (pillow basalts that erupted on the ocean floor)

Mid-Ocean Ridge Cross-section From a tectonic perspective, peridotite is depleted mantle that was under the magma chamber at the mid-ocean ridge crest Gabbro layer is related, in some way, to the crystallization of the magma chamber (probably with repeated injections of magma) Dikes and volcanic rocks are formed by magma in transit to or at the surface http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/ophiolites.html Cross-section simplified from Pallister, J.S., and Hopson, C.A., 1981, Journal of Geophysical Research, v. 86, p. 2593-2644. Phopt #op1.jpg

Gabbro Dikes (Ophiolite) Oceanic detachment fault, Troodos ophiolite, Cyprus Slide shows the first recognized extensional detachment fault found within an ophiolite Detachment separates sheeted dikes (lower part of oceanic Layer 2) from the underlying gabbro of the plutonic complex (oceanic Layer 3) Dikes above the detachment are presently near-horizontal and were tilted by normal faulting to this position from originally vertical attitudes, as shown by paleomagnetic data Photo: Robert Varga, College of Wooster http://www.science.ubc.ca/~eoswr/cgi-bin/db_gallery/searchframe.html Photo #struc86m.jpg

Norite Intrusive igneous, plutonic to hypabyssal A gabbro with predominantly orthorhombic pyroxene (enstatite or hypersthene) rather than clinopyroxenes Opx may sometimes be identified in hand specimen by the presence of Schiller luster The name is for Norway, the original locality where it was first identified

Norite, IUGS Opx/(opx + cpx) > 95 Q = 0-5% P/(A + P) > 90 Pl/(pl + px + ol) is 10 - 90

Origin of Gabbro and Norite Found in sills, dikes, stocks, lopoliths, and other bodies. Mineralogy and texture of gabbros, norites, diabases, and basalts indicates that they are probably derived from the same type of magma crystallized under different conditions Diabase and gabbro are often found together Gabbroic layered intrusions may show considerable gradation in rock types between layers

Diorite Intrusive igneous, plutonic Essential: sodic plagioclase (oligoclase or andesine) Essential: A mafic, usually hornblende, or more rarely, biotite or pyroxene The name is from the Greek, diorizein, to distinguish, because the grains are large enough to be recognized in hand specimen

Diorite Continued These rocks are found in small bodies such as satellite stocks or batholiths, of the type associated with subduction zones Diorite is the intrusive equivalent of andesite

Diorite Photo Diorite cut by two veins dipping away from each other, and these in turn are cross cut by faults. From area in Quebec about 85 miles southwest of Chibougamau Photo: Richard Stenstrom http://www.science.ubc.ca/~eoswr/cgi-bin/db_gallery/searchframe.html Photo #struc75m.jpg

Quartz Diorite Intrusive igneous, plutonic Essential sodic plagioclase (oligoclase or andesine) Essential quartz, Q > 5% Usually, a mafic such as biotite or hornblende, or rarely pyroxene Quartz diorite is the intrusive equivalent of quartz andesite

Quartz Diorite, IUGS Q = 5 - 20 P/(A + P) >90 Plagioclase composition < An50

Quartz Diorite Photo Handspecimen of quartz diorite, location unknown http://www.science.ubc.ca/~eoswr/cgi-bin/db_gallery/searchframe.html

Diorite IUGS Q= 0 - 5 P/(A + P) >90 Plagioclase composition < An50 Mafics are generally 10-40% of the rock

Diorite Mineralogy Quartz is present as an interstitial, anhedral component, often not visible in hand specimen Hornblende is generally green, and may be replacing pyroxene (uralite) Biotite is very commonly found with the hornblende and is generally brown

Diorite Photomicrographs Upper photo, CN; lower, PP Large, twinned plagioclase crystals Hornblende grains Porphyritic texture http://www.humboldt.edu/~jdl1/web.page.images/diorite1.html Photo #a_34.gif (upper) and a_35.gif (lower)

Tonalite Intrusive igneous, plutonic Occurrence: Batholiths Sometimes mistakenly used as synonymous with quartz diorite

Tonalite Mineralogy: Essential Essential: Quartz, Q > 20 Quartz is almost always anhedral Essential: Plagioclase feldspar, usually andesine Zoning in plagioclase is common and often very strong Zoning may be oscillatory

Tonalite Mineralogy: Accessory Accessory : K-spar - if K-feldspar is present it should be less than 5% of the rock Typically it will be orthoclase or perthitic orthoclase Accessory: Mafics such as hornblende, biotite, and/or pyroxene Hornblende is the characteristic mafic mineral Biotite is usually brown to brownish-green

Tonalite, IUGS Q = 20 - 60 P/(A + P) >90 If M < 10, the rock may be called Trondhjemite (after Trondhjem, Norway, the type locality)

Tonalite Photomicrographs Upper photo, CN; Lower, PP Large grain of zoned, altered plagioclase in the lower right (partially altered to sericite and epidote) Large pleochroic crystals of biotite. The extinct (dark) areas in the upper left corner and lower left corner of the photo on the top are quartz crystals at extinction

Anorthosite Intrusive igneous, plutonic Composed of calcic plagioclase with less than 10% ferro-magnesium minerals - plagioclase may be labradorite, bytownite, or anorthite The name is from anorthose, an old name for triclinic feldspars Anorthosite is often associated with gabbro

Anorthosite: IUGS Q =0-5% P/(A + P) >90 M < 10 (M = Mafics)

Anorthosite Photo Garnets in a gabbroic anorthosite Location : Gore Mountain Garnet mine, Adirondack Mtn. NY Photo: R.L.Chase http://www.science.ubc.ca/~eoswr/cgi-bin/db_gallery/searchframe.html Photo #ric14m.jpg