Lawsonitebearing

Lawsonitebearing refers to rocks or mineral assemblages that contain lawsonite, a hydrous calcium aluminum silicate with the approximate formula CaAl2Si2O7(OH)2·H2O. Lawsonite is a characteristic mineral of high-pressure, low-temperature metamorphism and is a key indicator of subduction-zone processes. Consequently, lawsonitebearing assemblages are most commonly associated with blueschist facies metamorphism, though they can occur in other HP–LT settings.

Lawsonite is stable at relatively high pressures and relatively low temperatures, conditions typical of subduction zones. As metamorphism proceeds to higher temperatures, lawsonite breaks down, releasing water and driving dehydration reactions that alter mineralogy toward higher-temperature assemblages such as epidote- or garnet-bearing rocks.

Geological significance: The presence of lawsonite constrains pressure–temperature conditions and helps reconstruct P–T paths in subduction zones. Because lawsonite contains water in its structure, its dehydration during metamorphism contributes to fluid release that can influence metamorphic reactions, metasomatism, and hydration of the mantle wedge.

Occurrence: Lawsonitebearing rocks have been reported in blueschist and related high-pressure belts worldwide, including the Franciscan Complex of California, Alpine and Himalayan belts, Japan, New Zealand, and Taiwan. The mineral lawsonite was named after Andrew Lawson in recognition of his work on California blueschists.

See also: Blueschist facies, subduction zone metamorphism, glaucophane, dehydration reactions.

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