Tertiary high-pressure metamorphism recorded in andalusite-bearing mica-schist, southern Pirin Mts., SW Bulgaria
European Journal of Mineralogy Volume 28 Number 6 (2016), p. 1187 - 1202
published: Dec 1, 2016
ArtNo. ESP147052806010, Price: 29.00 €
A garnet-bearing schist, rich in white mica, was sampled from the southern Pirin Mountains, western margin of the Rhodope Metamorphic Complex, near the Popovi-Livadi pass and investigated with the electron microprobe. Garnet grains of mmsize commonly display an inclusion-rich core, which is relatively poor in Ca (Alm0.67 Gr0.03 Py0.10 Sp0.20). Following resorption of the core, a garnet mantle with significant zonation developed (inner mantle: Alm0.65 Gr0.25 Py0.05 Sp0.05; outer mantle: Alm0.76 Gr0.13- Py0.09 Sp0.02). Following another resorption event, the garnet rim (Alm0.715 Gr0.09 Py0.125 Sp0.07) formed. At the same time and later, plagioclase, biotite, clusters of unoriented muscovite (Si around 6.2 per double formula unit = pdfu), and porphyroblasts of andalusite crystallized. Earlier potassic white mica, occurring as a core of oriented flakes, preserves Si contents up to 6.6 pdfu. On the basis of contoured P – T pseudosections, the P– T evolution of the studied rock was reconstructed: (1) the garnet core represents a low-P granulite or high- T amphibolite stage; (2) the mantle domain and the Si contents in phengite reflect peak- P conditions of 16 kbar at 500°C and a subsequent exhumation to conditions of 10 kbar and 565°C; (3) slight heating occurred at 6.5 kbar to produce the garnet rim. Analysed monazite can be subdivided into high-Y (> 1.3 wt.% Y2 O3), intermediate-, and low-Y (< 0.4 wt.% Y2 O3) populations. The high-Y population, which probably formed prior to high-pressure (HP) garnet, yielded an age of 45.8 ± 5.8 (2σ) Ma. Monazite of the low-Y population, which is also enclosed in the outer garnet mantle, gave an age of 42.1 ± 5.2 Ma. It is concluded that the deduced HP metamorphism is an Early Eocene event related to the deep burial of rocks by continent-continent collision and their exhumation in the exhumation channel. The late low-P heating event included formation of andalusite during retrogression, which is interpreted to have been caused by emplacement of the nearby Teshovo granite at 31 Ma.