Geochemistry of garnets from a tonalite and granitic aplite-pegmatite veins fro...

• Author(s): Lima, S. S. M. ; Neiva, Ana Margarida R. ; Ramos, João Farinha
• Date: 2009
• Persistent ID: http://hdl.handle.net/10400.9/754
• Origin: Repositório do LNEG
• Subject(s): Geoquímica; Garnet; Tonalite; Aplitos; Pegmatito granítico; Ciborro-Aldeia da Serra (Concelho de Montemor-o-Novo, Portugal); Zona de Ossa-Morena (Portugal)
• Document Type: Article
• Language: English

Description

Electron-microprobe analysis of garnet from a tonalite and aplite-pegmatite and pegmatite veins cutting different lithologies from an area located in the Ossa-Morena Zone, southern Portugal, show distinct compositions, mainly andradite with a subordinate grossular component in tonalite, and predominantly spessartine with a subordinate almandine component [and rarely, of mainly almandine with a subordinate spessartine component] in the veins. In general, single crystals of garnet are unzoned in tonalite, but commonly show a Mn-enriched core in veins, as the ratio Mn/(Mn + Fe) in the melt is controlled by garnet. However, reverse zoning also occurs owing to resorption of garnet at lower temperatures. A fractionation trend involving a decrease of Mn/(Mn + Fe) occurs in garnet from a complex aplite-pegmatite vein with 22 alternating layers cutting schists; values decrease toward the hanging wall because garnet is the sole host of Mn, which is depleted in the granitic magma.