Basalts

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Basalt

- Basalts exist in variety of tectonic environments such as Mid-ocean ridges, island arcs, back arc, basins, intra-plate oceanic islands, large igneous provinces, and intra- continental rifts).

-Basalt a fine grained, mafic igneous rock consisting augite + calcic plagioclase. Augite is a high-Ca pyroxene and Calcic Plagioclase is a plagioclase which contain crystals of more anorthite than albite.

basalts could be aphyric, but contain more commonly phenocrysts or microphenocrysts of olivine and of pyroxene and plagioclase.

basalts only enriched in olivine crystals of phenocrysts described as Picrite.

basaltic rock rich in olivine and augite phenocrysts known as ankaramite.

Basalt contain low-Ca pyroxene(enstatite or pigeonite) is known as tholeiite.

Nepheline + olivine crystals in basalt known as alkali basalt.

In porphyritic texture, the phenocrysts formed in slow cooling, nucleation is sparse so conditions favouring large crystals(euhedral)in magma chamber . while on the other hand when the eruption occur which favour high supercooling and a large nucleation rate, generate a finer groundmass comprising large number of crystals.

basalts contain two generation of phenocrysts and microphenocrysts before eruption, microphenocrysts are intergranular texture, in which equatic olivine and pyroxene fill spaces within a framework formed by tabular plagioclase microphenocrysts.

Aphanitic are those, where all crystals are too small to be distinguished by naked eyes.

Aphyric are fine grained rocks that lack phenocrysts and they called non- porphyritic.

Alkali basalt contain identifiable nepheline and olivine.

Tholeiite contain low- Ca Pyroxene with or without olivine.