The evolution of the Čanište epidote-bearing pegmatite, Republic of Macedonia: evidence from mineralogical and geochemical features

Abstract

The epidote-bearing Čanište pegmatite and adjacent Upper Carboniferous granodiorites cut Precambrian gneises, at the western slopes of the Selečka Mts., the Eastern Pelagonian zone, FYRO Macedonia. The pegmatite exhibits zonal internal structure with the following sub-units: the wall zone (amazonite microcline ± biotite, quartz), the first intermediate zone (epidote + hematite + grossular + muscovite + quartz + almandine ± zircon, beryl, microcline, quartz), the second intermediate zone (albite + quartz ± microcline) and the core (massive quartz). According to the microprobe data epidote belongs to clinozoisite subgroup with formula (Ca_1.96-1.99Mn_0.02-0.03Fe²⁺_0.00-0.02)(Al_2.17-2.46Fe³⁺_0.51-0.82Ti_0.00-0.01)(Si₂O₇)(Si_0.99-1.00Al_0.00-0.01O₄)O(OH). The occurrences of almandine and zircon with low U, Th and REE content, are indicative to weakly evolved granitic/granodioritic rocks. The absence of aplites suggests steady pressure condition during the course of pegmatite crystallization. Microthermometric data combined by the two-feldspar geothermometer gained pressure from 4.8 to 5.6 kbar for the second intermediate zone. The wall zone, composed of amazonite microcline, crystallized at temperature between 650 and 760. Dropping of melt temperature below 550°C, under the oxygen fugacity between 10^-22 and 10^-19.5 bar, was the principal trigger for deposition of minerals in the first intermediate zone. The residual fluid, depleted in Ca, Fe and K, and enriched in water, Na and Si, caused deposition of the second intermediate zone (albite + quartz) at temperature between 445 and 465°C. The massive quartz core crystallized in the very last stage of the pegmatite evolution (T ≈ 400-480°C) from melt residue enriched in silica, water and CO₂ content.