STUDY OF THE SYSTEM KAlSi206-Ca2MgSi207- NaAISi308—Si02 UNDER ATMOSPHERIC PRESSURE AND ITS SIGNIFICANCE

Abstract

Bulk compositions of many simplified ultrapotassic rocks lie in the system leucite-akermanite-albite-SiOp. The basal plane le^cite-akermanite-Si02 and two other joins containing 25 and 40 wt £ albite were therefore, studied to learn the extent of solid solutions within various phases and to establish different reactions inhibiting the coexistence of melilite and plagioclase as well as leucite and sodium-rich plagioclase. The study was also made to ascertain the course of crystallization of liquid within the system to elucidate the cause of coexistence of both silica-rich and silica-poor ultrapotassic rocks in the same volcanic field. Study of the join leucite-akermanite-SiCU under one atmospheric pressure shows that it cuts through the primary phase volumes of leucite, melilite, diopside, alkali feldspar and tridymite/cristobalite. This join, therefore, is pseudoternary with three piercing points (except for tridymite and cristobalite all phases are solid solutions): (1) Lc32Ak5R^10 at 1284° + 5°c (leucite + akermanite + diopside + liquid), (2) Lc55Aic2^43 at 1005° ± 5°c (leucite + diopside + K-feldspar + liquid), and (3) Lc46Ak2Q52 at 970° + 10°C (diopside + K-feldspar + ti^<}ym-i-te- + .liiquA<0 . (iii) Just below the piercing temperatures wollastonite also appears. Final assemblages within silica-poor but akermaniterich part of the system are leucite + akermanite + diopside + wollastonite + liquid. Akermanite was found to be absent in the silica-poor as well as akermanite-poor region. Final assemblage in this portion is leucite + diopside + wollastonite + K-feldspar + liquid. In the silica-rich section leucite reacts with Si02 to form K-feldspar. Appearance of diopside and wollastonite is attributed to the reaction between akermanite and Si02. Leucite and akermanite completely breaks to the respective products in the silica-rich region and final assemblage in this portion is tridymite + diopside + wollastonite + K-feldspar. This study shows that melilite leucitite can be produced from a melilite italite, a melilitite or a leucitite liquid. The liquid approaching the composition of melilite leucitite magma can further differentiate to crystallize wollastonite-melilite leucitite. Wollastonite-pyroxenebearing phonolite can be produced from a wollastonite leucitite, a pyroxene-bearing phonolite, a wollastonite phonolite or a wollastonite-pyroxene trachyte. The liquid of the later composition may also differentiate to wollastonite- pyroxene-bearing rhyolite in suitable conditions.

Beginning of melting is variable in different parts of the system. While compositions in silica-poor but akermaniterich region melts at 1166 + 10°C, mixtures in the silica-rich portion of the system begin to melt at 1057°+ 10°C. Study of the joins 25 and 40 wt '/<, leucite-akermanite- Si02 with albite reveals that the increase in albite content restricts the appearance of leucite and melilite with alkali feldspar. Composition of alkali feldspar coexisting with leucite and melilite at 800°C lies between K-Fels62 = Ab^- - and K-Fels,o i Ab,,. g. Because of reaction with leucite and akermanite, albite disappears, and nepheline appears along with other phases below the liquidus. The composition of nepheline appearing in these joins varies from Ne6g JKs27 7 Si02 2.6 to Ne75 3KS2Q 3Si°o • ** is "e^e^ "that as the albite content increases, feldspar field expands at the expense of leucite field. Nearly isotropic nature of melilite shows the incorporation of sodamelilite in akermanite to form solid solution. This sodamelilite may form by addition reaction of wollastonite and nepheline. Because of the complex nature of the phases, the join leucite~akermanite-albite-Si02 is not a true quaternary. If presence of small amounts of possible alumina incorporated by diopside (as Ca-Tschermak's molecule) and melilite (as gehlenite) is ignored, the system may be treated as a quaternary join of the quinary system KAlSi04- NaAlSi04-Ca0-Mg0-Si02» A flowsheet diagram drawn on the basis

of this assumption, shows the presence of two invariant points in silica-poor region of the system. Final invariant assemblages at these points are diopside + wollastonite + leucite + melilite + nepheline + liquid (1030 + 20°C), and diopside + wollastonite + leucite + melilite + alkali feldspar + liquid (1055°+ 20°C), respectively. Final invariant assemblage in silica-rich part of the system is same as observed in the join leucite-akermanite- Si02, but composition of alkali feldspar depends upon the leucite and albite contents. This study shows that a wollastonite-melilitenepheline leucitite magma can be derived from liquids of either melilite-nepheline leucitite, or wollastonitemelilite- nepheline italite, or wollastonite-nepheline leucitite, or wollastonite-melilite nephelinite or wollastonite- melilite leucitite. On the other hand a wollastonitemelilite- bearing phonolitic leucitite can be obtained from one of the following melts : a wollastonite-melilite-bearing phonolite, a wollastonite-alkali feldspar-bearing melilitite, a wollastonite-melilite leucitite, a wollastonite-bearing phonolitic leucitite or a phonolitic melilite leucitite.