Mineralogy of nepheline syenites from the Poços de Caldas alkaline massif, SE Brazil: chemistry, x-ray data and microtextures of feldspars

Abstract

Nepheline syenites (NeS) of the Poços de Caldas Alkaline Massif constitute mostly small bodies of miaskitic-intermediate or agpaitic rocks. The alkali feldspars of most NeS are highly potassic (OR > 80-85) and are either orthoclase or highly ordered microcline. The microcline shows a multiple twinning that resembles that of "chess-board" albite. This twinning may appear in isolated turbid spots in otherwise clean monoclinic crystals or occupy the whole grain (incipient, poor, good twinning); microperthites, when present, occur in the twinned areas. In some NeS the K-feldspars are clean, wholly twinned with larger twin lamellae (very good twinning). Secondary albite (tabular albite) is seen in some facies either included in, or bordering, the K-feldspars. In the singlemost important NeS intrusion, the Pedreira NeS, the alkali feldspars are richer in Na, Ca and Sr, very turbid and show a "diffuse twinning" resembling a cross-hatched pattern. The X- ray diffractograms show a mixture of orthoclase and intermediate microcline. Estimated geothermometric temperatures indicate subsolidus reequilibration between feldspar and nepheline, particularly when the K- feldspars show good or very good twinning. The microtextures of the K-feldspars are the result of a continuous process of recrystallization of non-twinned earlier crystals, controlled by late magmatic fluids. The turbid patches probably represent regions of abundant micropores facilitating fluid circulation. The twinning styles of the K-feldspars may be due to the composition of the fluid phase and of the feldspars. Sodium redistribution by deuteric fluids may have produced the secondary and perthitic albite.