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Título

METAMORPHIC EVOLUTION OF A GARNETITE FROM THE SOUTHERN CARPATHIANS, ROMANIA

Texto do resumo

Garnetites are unusual rocks constituted mostly of garnet, and other phases often occur in minor proportions. Their formation is not well understood and can occur in igneous, metamorphic and/or metasomatic environments. These rocks are occasionally found in high-pressure (HP) metamorphic terrains. In these cases, garnetites tend to present eclogite mineral assemblages also containing omphacite. The Southern Carpathians in Romania stand out as a host of garnetites with well-preserved metamorphic paragenesis. This orogen was formed due to the accretion of Peri-Gondwanan microcontinents onto major European cratonic terrains during the Variscan orogeny. It hosts several occurrences of mafic and ultramafic rocks, which underwent high-P metamorphism and are candidates of suture zone markers. In this context, occurrences of garnetites are found in the central region of the South Carpathians (the Voineasa Unit), where detailed studies on metamorphism are still scarce. This work aims to understand the evolution of the mineral assemblage and the peak temperature metamorphic conditions of the formation of a garnetite using petrography, mineral chemical analysis coupled with compositional maps, and conventional thermometry. The studied garnetite is heterogeneous, presenting garnet-rich portions cut by minor clinopyroxene-rich venules and other portions where clinopyroxene is a more significant constituent. Amphibole, rutile, ilmenite, apatite and epidote are widespread and occur as accessory phases. We inferred five stages of metamorphic evolution for this rock: the stage 1 is recorded by garnet cores and high-jadeite (Jd) clinopyroxene (Cpx1). Garnet rims mark the stage 2. Garnet exhibits abrupt concentric zoning for XGrossular and XPyrope, with grossular-rich cores and pyrope-rich rims. Zoning for XAlmandine and XSpessatine is also concentric but irregular and gradual, with both decreasing toward the rim. Omphacite exhibits irregular zoning for XJd, which ranges from 0.20 to 0.28. Rutile is frequently observed as inclusions in both phases. In the stages 3 and 4, symplectites around Cpx1 were formed, where amphibole (Amp1), low-Jd clinopyroxene (Cpx2) and plagioclase are found. Amphibole (Amp1) also exhibits irregular zoning for Na and Al, with both elements decreasing rimward (NaM4 = 0.6-0.2; AIIV = 1-2.4). The stage 5 is related to the formation of later venules, filled with Ca-rich amphibole (Amp2), chlorite and plagioclase. Ilmenite, apatite and epidote are ambiguous but are likely related to stages 2 and 3. We conclude that the garnetite underwent at least two stages of mineral growth during high-P metamorphism. The garnet zoning suggests episodic growth, marked by the formation of garnet cores likely at stage 1, followed by the formation of garnet rims at stage 2. The rock underwent later retrograde metamorphism, during which symplectites were formed. Garnet-clinopyroxene thermometry was used to estimate the temperatures of stages 1 and 2, considering pressures of 1, 1.5 and 2 GPa, based on the literature data from other rocks in nearby areas. For stage 1, results are of ~600-850 °C, while stage 2 yielded temperatures of ~640-890 °C. The next steps of the study involve calculating phase diagrams to determine the P-T path of metamorphism and correlating the studied rock with other high-P metamorphic rocks in the orogen. This work is supported by Instituto Serrapilheira (SERRA-1912-31510).

Palavras Chave

High-pressure metamorphism; symplectite; XMapTools; Variscan orogeny; Alpine orogeny