Tertiary high-pressure metamorphism recorded in andalusite-bearing mica-schist, southern Pirin Mts., SW Bulgaria

摘要

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 mm size commonly display an inclusion-rich core, which is relatively poor in Ca (Alm(0.67)Gr(0.03)PY(0.10)SP(0.20)). Following resorption of the core, a garnet mantle with significant zonation developed (inner mantle: Alm(0.65)Gr(0.25)PY(0.05)SP(0.05); outer mantle: Alm(0.76)Gr(0.13-)PY(0.09)SP(0.02)). Following another resorption event, the garnet rim (Alm(0.715)Gr(0.09)Py(0.125)SP(0.07)) 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 degrees 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.% Y2O3), intermediate-, and low-Y (<0.4 wt.% Y2O3) populations. The high-Y population, which probably formed prior to high-pressure (HP) garnet, yielded an age of 45.8 +/- 5.8 (2 sigma) 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.