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

Abstract

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 °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.% Y 2 O 3 ), intermediate-, and low-Y ( 2 O 3 ) 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.