Classification of Igneous Rocks Questions and Answers

Answer: C

The rock is mafurite.

Mafurite is a rare ultrapotassic volcanic rock from the Toro-Ankole field of south-west Uganda. It consists of olivine and pyroxene phenocrysts in a groundmass of clinopyroxene and kalsilite, with minor perovskite and biotite. The presence of kalsilite, a potassium feldspathoid, with no feldspar, is its defining feature.

Katungite, from the same region, is an olivine melilitite in which melilite takes the place of kalsilite. Kimberlite is an olivine-rich breccia with phlogopite and carbonate but no kalsilite groundmass, and pyroxenite is a plutonic rock of pyroxene alone.

Answer: B

The rock is basanite.

Basanite is a silica-undersaturated basaltic rock containing calcic plagioclase, a feldspathoid (nepheline or leucite), olivine and clinopyroxene. The olivine distinguishes it from tephrite, which has the same minerals without olivine.

Nephelinite has the feldspathoid but essentially no plagioclase. Basalt has plagioclase, pyroxene and often olivine but no feldspathoid. Coppaelite is a rare kalsilite-melilite rock with neither plagioclase nor the assemblage described.

Answer: D

The correct matching is (a)-(i), (b)-(ii), (c)-(iii), (d)-(iv).

1. Monzonite has roughly equal potash feldspar and plagioclase.
2. Norite is a gabbroic rock of calcic plagioclase (labradorite) with orthopyroxene such as hypersthene.
3. Picrite is an olivine-rich basaltic rock with augite and calcic plagioclase.
4. Rhyolite is the volcanic equivalent of granite: quartz with alkali feldspar exceeding plagioclase.

Option A gives monzonite the norite assemblage, option B gives rhyolite the monzonite assemblage, and option C gives monzonite the rhyolite assemblage.

Answer: B

The rock is Basanite.

Basanite is a silica-undersaturated basaltic rock containing plagioclase, clinopyroxene, a feldspathoid (usually nepheline) and more than 10% olivine, exactly the assemblage given.

The near misses are distinguished by olivine and nepheline. Tephrite has the same minerals but little or no olivine. Troctolite is a plutonic rock of plagioclase and olivine with little pyroxene and no nepheline. Olivine norite is a gabbroic rock with orthopyroxene and olivine, also without nepheline.

Answer: A

Amba Dongar is known for its Carbonatite complex.

Located in Chhota Udaipur district of Gujarat, Amba Dongar is a ring complex of carbonatite and alkaline rocks emplaced at about 65 Ma, linked to Deccan volcanism. It is India's main fluorite deposit and has been studied for its rare earth element potential.

India's anorthosite complexes lie in the Eastern Ghats and Oddanchatram, its chromite deposits mainly in Sukinda (Odisha), and its kimberlites in the Wajrakarur field of Andhra Pradesh and at Majhgawan near Panna.

Answer: B

An orthopyroxene-bearing granitoid is called Charnockite.

Charnockite is the name for granitic rocks containing orthopyroxene (hypersthene), formed under the high-temperature, low-water-activity conditions of the granulite facies. It is named after Job Charnock, whose tombstone in Kolkata is cut from it, and it is widespread in South India.

Enderbite is the orthopyroxene-bearing tonalite member of the charnockite series, a narrower term than the one asked for. Plagiogranite and tonalite are defined by their feldspar and quartz content and do not imply orthopyroxene.

Answer: A

They are called sanukitoids.

Sanukitoids are late Archaean granitoids, about 2.95-2.5 billion years old, with unusually high magnesium, nickel and chromium together with high potassium, barium and strontium. They are explained by melting of mantle that had been enriched by slab melts or fluids at convergent plate margins, which ties them to subduction and collision events in the late Archaean.

Tonalites (TTG suites) are the typical Archaean granitoids, but they are sodic and low in magnesium. Adakites are high-Sr, low-Y rocks formed mainly by slab melting, and the term is used mostly for modern examples. Anatectic granites form by melting of crust and are not magnesium-rich.

Answer: A

Harzburgite is made of olivine + orthopyroxene.

Peridotites are ultramafic rocks with more than 40% olivine and are named by their pyroxenes: harzburgite contains olivine and orthopyroxene with little or no clinopyroxene, lherzolite contains olivine with both orthopyroxene and clinopyroxene, and wehrlite contains olivine with clinopyroxene. Harzburgite is typical of depleted mantle from which basalt has been extracted, as in the mantle sections of ophiolites.

Olivine alone (more than 90%) is dunite. Orthopyroxene with clinopyroxene is websterite, and pyroxene alone is pyroxenite.

Answer: B

The condition for peraluminous ('Pre-aluminous') rocks is Al₂O₃ > (CaO + Na₂O + K₂O), in molar proportions.

Shand's alumina saturation scheme compares molar alumina with the alkalis and lime:

1. Peraluminous: Al₂O₃ greater than CaO + Na₂O + K₂O; the excess alumina appears as muscovite, cordierite or garnet, as in S-type granites.
2. Metaluminous: Al₂O₃ less than CaO + Na₂O + K₂O but greater than Na₂O + K₂O; the third option.
3. Peralkaline: Al₂O₃ less than Na₂O + K₂O; the first option, with sodic amphiboles or pyroxenes.

Equality of alumina and alkalis is only the boundary between metaluminous and peralkaline.