The Melilite-Bearing High-Temperature Skarns of the Apuseni Mountains, Carpathians, Romania

Abstract

The melilite-bearing skarns of Cornet Hill (CH) and Upper Cerboaia Valley (UCV), in the Apuseni Mountains of Romania, occur at the contact between monzodiorite bodies of Ypresian age (Paleocene) and Neojurassic calcitic marbles. Typical wollastonite -grossular -diopside endoskarns are separated from exoskarns (tilleyite and spurrite or wollastonite at CH, wollastonite only at UCV), at most places, by a melilite-rich rock, in which veins and vein-like zones of recrystallization are composed only of idiomorphic melilite crystals reaching 15 cm across. Titanian garnet and wollastonite are the principal minerals associated with melilite (and also monticellite, perovskite, vesuvianite, cuspidine, spurrite, tilleyite, calcite, hydroxylellestadite, hydrogrossular and other minor alteration-induced minerals). A different association that includes aluminian diopside and grossular occurs (1) as veinlets in the marble close to the skarns and (2) as relict inclusions in endoskarns. From the geometrical relationships of the zone sequences and the veins, the textural features of the mineral associations and the inferred conditions of fluid-mineral equilibrium, these mineralogical peculiarities are interpreted as resulting from the superposition of two main stages. Firstly, there was circulation of a comparatively CO 2 -rich fluid formed the early aluminian diopside -grossular endoskarns, with depletion in Si (and Fe, Na, K) and inert behavior of Mg, Al, Ti. Then, a high-temperature (750°C) fluid circulated on both sides of the contact between marble and endoskarns, and developed the melilite-rich, titanian-garnet-bearing rocks partly at the expense of previously formed endoskarns, and spurrite or wollastonite (CH) or wollastonite (UCV) exoskarns at the expense of marble. The pressure of CO 2 was very low, less than 26 bars at UCV and 16 bars at CH, with a H 2 O pressure less than 750 bars. Not only Si and Ca were mobilized, but also Mg, Al and Ti, leached from the endoskarns and deposited in the veins and the nearby part of exoskarns. This stage, which occurred in the temperature range corresponding to the end of the crystallization of plagioclase in the monzodiorite, has pegmatitic chemical and textural features. The main flow of fluid ended with the development of tilleyite partly at the expense of spurrite and wollastonite at CH, and local high-temperature (about 710°C) recrystallization of the zonation, mostly in veins, especially in the endoskarn-exoskarn boundary, but also within the endoskarns. A monticellite -gehlenite association appeared in the melilite-rich rocks, later followed by vesuvianite, whereas in the endoskarn, vesuvianite developed together with coarse-grained wollastonite and grossular.

Mineralogy and petrology of the angrite Northwest Africa 1296

“…The presence of silica in apatite seems to be real as the analysis of apatite included in pyrrhotite reveals 8% SiO 2 and only 0.3% Al 2 O 3 . The presence of silica in apatite is a typical feature of carbonate-rich magmatic rocks (Rønsbo 1989) and skarns (Pascal et al 2001). …”

The crystal chemistry and the compressibility of silicate-carbonate minerals: Spurrite, galuskinite and tilleyite

“…Spurrite occurs characteristically in skarns formed during the shallow intrusion of igneous rock into limestone in USA (Tilley, 1929;Joesten, 1976), Mexico (Wright, 1908), Russia Galuskina et al, 2009) and Canada (Rosa and Martin, 2010); and it was also found in the pyrometamorphic rocks of Hatrurim Formation in Israel where the combustion of organic fossil fuel provided a heat source (Sokol et al, 2008). Tilleyite was initially discovered in thermally metamorphosed limestones from a contact zone in California, USA (Larsen and Dunham, 1933), and later confirmed to abound in the high-temperature skarn in Cornet Hill in Romania Pascal et al, 2001;Grice, 2005), where spurrite was occasionally found in association. Galuskinite is a newly discovered mineral found in altered silicatecarbonate xenoliths a few meters in diameter in the Birkhin gabbro massif of Russia.…”

The role of aqueous silica concentration in controlling the mineralogy during high temperature contact metamorphism: A case study from Fuka contact aureole, Okayama, Japan

“…The petrogenetic grids used to deduce the observed assemblages were explained using reactions involving quartz, calcite and the minerals in the sequence. However, at Fuka (and elsewhere in Apuseni mountains, Pascal et al, 2001;Marincea et al, 2001) mineralogy and textural features of the skarn does not suggest a progressive sequence of reactions involving quartz. The formation of spurrite and tilleyite is suggestive of its formation with out early wollastonite.…”

“…High temperature conditions for spurrite and gehlenite skarn formation have been reported elsewhere as well. In a recent study on the formation of spurrite and gehlenite bearing skarn in Apuseni Mountains, Romania, Pascal et al (2001) reported temperature conditions of around 750°C at 750 bars and a maximum concentration of silica of 2.9 ×10 −4 m. Their study also indicated that the variation of CO 2 in the fluid is controlled by the activity of silica, since CO 2 is produced during the formation of spurrite, which in turn needs silica in the fluid to react with the calcite. Our results are also in agreement with this, however the skarn formation at Fuka occurred at a higher temperature condition that those in Apuseni.…”