Lamprophyre

Article By:

Bachinski, Sharon W. Department of Geology, University of New Brunswick, Fredericton, Canada.

Last reviewed:November 2019

DOI:https://doi.org/10.1036/1097-8542.369400

Any of a heterogeneous group of gray to black, mafic igneous rocks characterized by a distinctive panidiomorphic and porphyritic texture in which abundant euhedral, dark-colored ferromagnesian (femic) minerals (dark mica, amphibole, pyroxene, olivine) occur in two generations—both early as phenocrysts and later in the matrix or groundmass—while felsic minerals (potassium feldspar, plagioclase, analcime, melilite) are restricted to the groundmass (see illustration). Compared with the common igneous rocks, lamprophyres are chemically peculiar. They have low silicon, moderate aluminum, and variable calcium contents, but are rich in alkalies (potassium or sodium or both), magnesium, iron, and volatile constituents (H₂O and CO₂), and contain a wide variety of such minor and trace elements as titanium, phosphorus, barium, strontium, rubidium, zirconium, lanthanum, uranium, thorium, chromium, nickel, and cobalt. Many lamprophyres contain ocelli, small spheroidal bodies consisting of alkali-rich feldspar, analcime, or calcite with minor femic minerals.

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