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ARCO Resources Technology, Exploration and Production Research Center, 2300 W. Plano Pkwy, Plano, Tx 75075, U.S.A.
In this paper the geological occurrence, geochemistry and mineralogy of ultrapotassic (K2O/Na2O > 3 (molar ratio)) and perpotassic (K2O/Al2O3 > 1 (molar ratio)) igneous rocks, especially lamproites, are reviewed and discussed in the context of compositionally-similar mantle-derived melts.
Lamproites are K- and Mg-rich igneous rocks (typically K2O > 5 wt.%, MgO > 5 wt.%) which possess an exotic and diagnostic mineralogy and geochemistry. Known lamproites occur in 21 major suites or localities in continental regions with a variety of geological and tectonic environments; they range in age from the early Proterozoic dykes at Holsteinsborg, W Greenland, and Chelima, India, and Precambrian pipe at Argyle, W Australia, to the Middle Pleistocene flows and the Recent volcanics of the Leucite Hills, Wyoming, and Gaussberg, Antarctica, respectively. Intrusive and extrusive forms of lamproites include flows, a variety of pyroclastics (welded tuffs, piperno, air-fall tuffs, volcanic breccias etc.), cinder cones, dykes, sills and diatremes. Whereas kimberlite diatremes tend to be carrot shaped, the shape of olivine lamproite diatremes approximates a sherbet-glass. The recent discovery of diamondiferous lamproties of large volumetric proportion in the E and W Kimberleys, NW Australia, and the reclassification of the diamondiferous micaceous peridotite at Prairie Creek, Arkansas, as a lamproite substantiate their economic importance. The 21 lamproite suites considered here tend to be localized marginal to continental craton cores in areas that overlie fossil Benioff zones, in contrast with the general occurrence of kimberlites more interior to continental cratons.
The petrographic diversity of lamproites has historically hindered the development of a concise and universal classification and nomenclature. Lamproites are distinguished from kimberlites and alkali basalts (and lamprophyres) in terms of mineralogy, mineral chemistry, geochemistry and volcanic extrusive character. Relative to kimberlites, lamproites are enriched in K, Si, Ti, Al, Rb, Sr, Zr and Ba and depleted in CO2, Ca, Mg, Fe, Ni, Co and Cr. Lamproites are characterized by the general presence of phlogopite, diopside, leucite and K-richterite, occasional glass, olivine, sanidine, priderite, perovskite, wadeite, apatite and chrome spinel, and very rare ilmenite. Lamproite amphiboles, diopsides and phlogopites are distinctly depleted in Al2O3 relative to those of nearly all other igneous rocks. Lamproite magmas are produced by the partial melting of old refractory mantle peridotite (approaching a dunite or harzburgite in mineralogy) that was enriched in K-bearing and other incompatible-element-enriched phases, such as phlogopite and apatite, most probably as a result of some metasomatic event which occurred prior to melting. In contrast with alkali basalt and kimberlite melts which are apparently produced from the partial melting of a CO2-enriched mantle periodtite (i.e. a source with a relatively high CO2/H2O ratio), water is the key volatile species involved with lamproite petrogenesis (source with a low CO2/H2O ratio).
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