Chemická zonálnosť Ti-andraditového granátu v Ca-skarne z oblasti Magnetový vrch pri Tisovci (Slovenská republika)

Analyzed garnets from the Magnet hill area near Tisovec (Slovak Republic) are part of a Ca-skarn mineral association consisting of diopside, clinochlore and calcite. Compositionally they correspond to Ti-rich andradite (Adr50.9-73.7) with minor grossular (Grs3.8-44.5) and schorlomite (Sch0.5-41.5) components. Garnets contain up to 13 wt. % TiO2 and in all of them YFe3+>YTi ratio prevails. Both sector and oscillatory chemical zoning were observed, which is primarily caused by variable distribution of Ti contents within individual garnet crystals. The three principal zones were distinguished in BSE imaging. The brightest are Ti-enriched zones with Ti content ranging from 0.85 to 0.50 apfu. Titanium gradually decreases in transitional zone (0.17 - 0.40 apfu) and reaches the minimum values in the dark zones (0.01 - 0.13 apfu). In the Ti-enriched zones the content of Si4+ and Al3+ is decreased due to substitution of Ti4+ and Fe3+ and assumed hydrogarnet substitution (SiO4)4- ↔ (O4H4)4-.

Skarn formation between metachalk and agglomerate in the Central Ring Complex, Isle of Arran, Scotland

A skarn mineral assemblage occurs at the junction between vent pyroclastics and a xenolithic Cretaceous chalk block which subsided into the collapsed caldera of the Central Ring Complex, Isle of Arran, Scotland. Adjacent to the metachalk marble an andradite garnet exoskarn zone has developed at the expense of the carbonate. An andradite grossular/diopsidic clinopyroxene endoskarn zone has formed in the surrounding agglomerate, and a magnetite exoskarn is present in places between the andradite and garnet/pyroxene zones. The andraditic exoskarn garnets have fluor-hydrogarnet components, indicating that fluorine was present in the metasomatic fluid. From petrographic evidence, three distinct episodes of exoskarn garnet crystallization can be recognized, in which the fluor-hydrogarnet component steadily increased as a function of time, which probably reflects falling temperature. The REE compositions of the exoskarn minerals are regarded as having been largely inherited from the carbonate, and the exoskarn garnets increasingly fractionated HREE with time. The endoskarn and agglomerate have also been epidotized. The REE signatures of epidotes appear to be inherited partially from precursor clinopyroxenes or feldspars, which have been replaced by epidote. Late-stage vein minerals include prehnite, laumontite and K-rich laumontite, and their REE compositions appear to have been derived from the marble, probably via REE fluoro-complexes in the fluid.