DEFINITION OF SERPENTINITE

A metamorphic rock consisting almost entirely

of minerals from the serpentine group (commonly antigorite,

chrysotile, and lizardite), typically with minor talc, chlorite,

and magnetite. Most serpentinites are derived through the

alteration and hydration of ultramafic and less commonly

mafic rocks, through the conversion of olivine, pyroxenes, and

amphiboles to serpentine minerals. Serpentinites are typically

massive to strongly foliated rocks with a greasy or slippery

feel, a concoidal fracture, and may be granular or fibrous.

Depending on the dominant serpentine mineral, the color of

serpentinites may range from green, yellowish brown, to grayish

blue, or a mixture of the three. The serpentine minerals

have the general formula (Mg,Fe)3Si2O5(OH)4, with antigorite

being the blue-green to colorless variety, lizardite being the

green to white variety, and chrysotile being the fibrous white,

yellow, or gray variety commonly used as asbestos.

Many serpentinites are associated with tectonically

emplaced fragments of oceanic crust and arc rocks known as

ophiolites, where the ultramafic rocks have been hydrated

and converted to serpentinites during seafloor metamorphism

and during regional tectonics associated with emplacement of

the ophiolites. Because of their very low strength many serpentinites

are strongly deformed and may form serpentinite

mélanges containing blocks of ophiolitic and other rock

types. These are typically incorporated into the mélange during

thrusting and emplacement of the ultramafic body in a

convergent margin setting. Some serpentinites have formed

diapirs and intruded overlying rocks, most in accretionary

wedge settings, where hydration of ultramafic rocks lowered

the density of the serpentinized ultramafic rocks. This caused

them to become negatively buoyant and to be forced upward

as diapirs into the overlying accretionary wedge, typically

intruding along faults and other preexisting structures.

See also ASBESTOS; CONVERGENT PLATE MARGIN PROCESSES;

MÉLANGE; OPHIOLITES.

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