tephrite

Tephrite is a dark, fine- to medium-grained volcanic rock belonging to the alkaline basalt group. It is mafic and characteristically low in silica with relatively high alkali content, distinguishing it from subalkaline basalts. The essential minerals are plagioclase feldspar and a ferromagnesian phase, typically pyroxene; olivine is common in some occurrences but less widespread than in basanite. Accessory minerals may include magnetite or ilmenite, apatite, and, in some varieties, feldspathoids such as nepheline or leucite. Quartz is absent in tephrite.

Tephrite commonly forms as lava flows, fringing domes, or intrusive bodies derived from alkaline magmas. It often crystallizes under conditions that produce a porphyritic or aphanitic texture. In many regions tephrite is part of the broader alkaline basalt suite, and it is related to other rocks such as basanite and phonotephrite; it generally records lower silica activity and higher alkali contents than typical basalt.

Geologic context: Tephrite appears in continental rift zones, hotspot-related volcanism, and oceanic island settings where alkaline magmas are generated in the mantle. It is of interest in studies of mantle chemistry, mantle-derived magmas, and the evolution of alkaline volcanic provinces.