L

◙ L-tectonites, L-S tectonites ◙ lamprophyres ◙ leucosome ◙ lithostatic stress ◙ ◊ ◊

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lamprophyres

Lamprophyres are relatively uncommon rocks named from the Greek for bright porphyry, for their light colored phenocrysts set within a darker groundmass. Lamprophyres are commonly gray to greenish, but may be dark gray, or a dark red color in the few centimeters close to contacts (such as this with a microgranite). Lamprophyres are ultrapotassic, alkaline silica-undersaturated, ultramafic hypabassal igneous rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium. Their unusual mineralogy renders them impossible to classify by QAPF or TAS schemes. Non-specific usage of 'lamprophyres' includes ultrapotassic mafic igneous rocks with a primary mineralogy comprising amphibole or biotite, with a groundmass containing feldspar. A more specific classification defines the "lamprophyre facies" as characterized by phenocrysts of mica and/or amphibole together with lesser clinopyroxene and/or melilite set in a groundmass which may comprise plagioclase, alkali feldspar, feldspathoids, carbonate, monticellite, melilite, mica, amphibole, pyroxene, perovskite, Fe-Ti oxides and glass. Rock (1987) subdivided lamprophyres into four branches: calc-alkaline lamprophyres, alkaline lamprophyres, ultramafic lamprophyres and lamproites. Lamprophyres are found in small volumes in dikes, lopoliths, laccoliths, stocks, and small (hypabassal) intrusions. Lamprophyre genesis involves deep, volatile-driven, partial melting in a subduction zone, which promotes: --- ◙ lithophile element (K, Ba, Cs, Rb) enrichment, high Ni and Cr --- ◙ high potassium and sodium concentrations (commonly with silica undersaturation) --- ◙ volatile enrichment, providing biotite (phlogopite) and amphibole (pargasite) mineralogy --- ◙ general lack of fractional crystallisation --- ◙ high Mg# (MgO//FeO + Fe2O3) Many lamprophyres have a decidely primitive chemistry, and some lamproites are associated with diamonds, lamprophyre dikes are also associated with mesothermal lode gold deposits worldwide (particularly Archaean lamprophyres). "Greenstone-hosted quartz-carbonate vein deposits (GQCV) are more abundant and significant, in terms of total gold content, in Archean terranes. However a significant number of world-class deposits are also found in Proterozoic and Paleozoic terranes. In Canada, they represent the main source of gold and are mainly located in the Archean greenstone belts of the Superior and Slave provinces. They also occurred in the Paleozoic greenstone terrane of the Appalachian orogen and in the oceanic terranes of the Cordillera. "[s] Lamprophyre melt is not appreciably gold-enriched relative to the continental crust, so that the spatial association of lamprophyres with gold deposits is probably related to a shared tectonic setting of crustal weakness and flagging terranes susceptible to later mineralization. "Greenstone-hosted quartz-carbonate vein deposits (GQCV) typically occur in deformed greenstone belts of all ages, especially those with variolitic tholeiitic basalts and ultramafic komatiitic flows intruded by intermediate to felsic porphyry intrusions, and sometimes with swarms of albitite or lamprophyre dyke. They are distributed along major compressional to transtensional crustal-scale fault zones in deformed greenstone terranes commonly marking the convergent margins between major lithological boundaries such as volcano-plutonic and sedimentary domains."[s] Cenozoic examples include magnesian rocks (Mexico, South America) and young ultramafic lamprophyres (Gympie, Australia). (image above right - click to enlarge - Lamprophyre dyke cross-cutting ankerite vein, Campbell Mine, Red Lake. Courtesy of Natural Resources Canada.) Rock, N.M.S. (1987) The nature and origin of lamprophyres: an overview. In `Alkaline Igneous Rocks' (Fitton, JG and Upton, BGJ editors), Geol.Soc.Spec.Publ. 30, Blackwell Scientific Publications, 568pp., 191-226.

links: images: formations: lamprophyre dikes in granite, VT; lamprophyre dyke, 2, Parsons Beach, Helford Estuary, Cornwall; lamprophyre dike cutting through the core of an anticline in the contact-metamorphosed Stanley Fm., Arkansas; two lamprophyre dikes (cut contact between the Arkansas Novaculite and the overlying Stanley Formation); hypabyssal plug is cut by two dikes, one of monzonite (cu) and the other lamprophyre dike, an alkali-lamprophyre (camptonite ... a lamprophyre in which plagioclase is more abundant than alkali feldspar, and total feldspar exceeds feldspathoids), white material is caliche, Huerfano Butte, Walsenburg, CO; lamprophyre dike cutting Stanley Shale (Mississippian), 2; lamprophyre dikes intruded along fractures in metagraywacke about 360 million years ago; eroded lamprophyre dike, near the summit of Mt. Jo., Adirondacks, NY; eroded lamprophyre dike between two apparent "dikes", which are the indurated remains of the Cuchara Formation metamorphosed to quartzite, NE of the Devil's Stairway, CO; lamprophyre dikes crosscutting limestone, Mt. Royal, Que; lamprophyre dike, Ayer's cliff, Que; Asgard Range and Mt Odin: Peneplain sill caps the mountains, then a band of Lower Paleozoic granites, then the Basement sill, another band of granites with lamprophyre dikes, then a band of doleritic scree, then granite and dikes again to the superficial outwash on the Wright valley floor, Antarctica, second strata from top on Mt. Jason is granite with lamprophyre dikes; lamprophyre dike, 2; mafic dykes (hornblende lamprophyre) intruding Vredenburg Granite, cutting magmatic layering, en echelon dike geometry, Groot Paternoster Point; alkaline lamprophyre sheet cutting thinly bedded metaquartzites, 2, 3, contact between brown lamprophyre dyke and quartzites, alkaline lamprophyre dyke cutting metacarbonates, Brand-se-baai; Middelplaat potassic lamprophyre dyke swarm, 2, reworking of dike-gneiss contact; Steinkopf, Spektakel intrusive suite, Western Namaqualand; hand-specimens: lamprophyre (diorite porphyry or andesite porphyry), Rossland gold camp, seBC, Canada; lamprophyre; close-up: lamprophyre; lamprophyre dyke; cut section of lamprophyre, Parsons Beach, Helford Estuary, Cornwall; lamprophyre, Que; lamprophyre ('F' indicates potassic feldspars); camptonite; thin-section: lamprophyre; lamprophyre hornfels; lamprophyre with coarse-grained titanaugite phenocryst; diagrams: schematic diagram illustrating geometric relationships between structural element of veins and shear zones and deposit scale strain axes; webpages: Lamprophyres; Dyke Swarms gallery;