Actinolite xenolith in sandorite lamprophyre in the Precambrian of Ontario, Canada.

This is part of an igneous dike composed of a rare igneous rock called lamprophyre. Such rocks are ultrapotassic, mafic to ultramafic, intrusive igneous rocks. This one is a diamondiferous spessartite lamprophyre called sandorite, with 47% silica and 5% black amphibole macrocrysts, up to 3 millimeters in size. The sandorite here has xenoliths ("xeno" = foreign; "lith" = rock), which are pieces of rock that have fallen from the walls or roof of a cooling magma chamber. At this site, they include mantle and crustal xenoliths.

Spessartite lamprophyres are defined in the literature as lamprophyres having phenocrysts of hornblende amphibole or clinopyroxene in a groundmass of sodic plagioclase feldspar, with accessary olivine, biotite mica, apatite, and opaque oxide minerals.

Sandorite dikes here represent the hypabyssal facies of a sandorite volcanic complex. The surrounding host rocks are 2.7 billion year old gabbros and volcanic tuffs of the Catfish Assemblage.

Seen here is a moderately large, coarsely-crystalline xenolith composed of long blades of greenish actinolite amphibole. This is not the primary mineralogy of the xenolith - it has been interpreted as a metapyroxenite mantle xenolith. Upper mantle rocks are dominated by ultramafic igneous rocks such as peridotite (= olivine-bearing) and pyroxenite (= almost exclusively pyroxene).

Geologic unit and age: Sandor Dike, Neoarchean, 2.703 Ga (a zircon from a gneissic xenolith has been dated to 2.684 Ga)

Locality: roadcut on the eastern side of Route 17, northeast of Wasp Lake & east of McCormick Lake, north of the town of Wawa, Ontario, southeastern Canada (48° 12’ 42.13" North latitude, 84° 50' 52.24" West longitude)

See info. at: <a href="https://en.wikipedia.org/wiki/Lamprophyre" rel="noreferrer nofollow">en.wikipedia.org/wiki/Lamprophyre</a> and