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Título

NEW MONAZITE AGES AND CHEMISTRY BY LASS-ICP-MS OF THE HIGH-PRESSURE FELSIC GRANULITES OF THE CARVALHOS KLIPPE, SOUTHERN BRASÍLIA OROGEN, SE BRAZIL

Texto do resumo

During the amalgamation of West Gondwana in the Neoproterozoic, the collision between an active margin (Paranapanema paleocontinent) and a passive margin (São Francisco paleocontinent) generated the Southern Brasília Orogen (SBO) in southeast Brazil. The Andrelândia Nappe System represents a metasedimentary domain subducted to mantle depths, resulting in the formation of high-pressure granulites at metamorphic conditions of ~14 kbar and 850 °C (Carvalhos Klippe, SBO). This work focused on monazite petrochronology via laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) to constrain the timing of the high-pressure metamorphism of the rutile-kyanite-garnet-orthoclase granulites from the Carvalhos Klippe. Monazite compositional X-ray maps reveal complex compositional zoning which suggests different episodes of growth and consumption during the high-pressure granulite facies metamorphism. In-situ LASS-ICP-MS analyses were conducted to obtain isotopic U-Pb ratios and trace element composition at the mineral volume to correlate the age associated with specific compositional domains. The results obtained encompass distinct patterns of heavy rare earth elements (HREE), Y content, and Eu/Eu* ratios which may reflect monazite growth in different stages along the metamorphic pathway. The compositional zoning of HREE, Y, and Eu/Eu* may indicate distinct episodes of monazite crystallization during the metamorphism between ca. 620-600 Ma. Prograde ages were interpreted in monazite enriched in HREE and Y with older ages. In addition, the timing of the metamorphic peak was determined in HREE-Y-depleted monazite inclusions in garnet porphyroblasts, and the retrograde stage is marked by monazite HREE-Y-enriched domains interpreted to reflect the partial replacement of garnet by biotite + quartz. The high-pressure felsic granulites of the Carvalhos Klippe likely record at least ca. 20 Ma of monazite growth.

Palavras Chave

monazite petrochronology; high-pressure granulites; Carvalhos Klippe; LASS-ICP-MS