Zn-rich hercynite-magnetite assemblage in gneiss and metasedimentary rocks from Lavadores (Northern Portugal)

• Type: Article in International Scientific Journal
• Date: 2010

Authors

Anjos Ribeiro (Author) FCUP; Helena Sant'Ovaia (Author) FCUP; Maria Armanda Dória (Author) FCUP

Journal

Title: Geochimica et Cosmochimica Acta

Vol. 74 No. 12, Supplement

ISSN: 0016-7037

Publisher: Elsevier

Indexing

ISI Web of Science

Scientific classification

FOS: Natural sciences > Earth and related Environmental sciences

Other information

Abstract (EN): This study describes the occurrence of hercynite in metapelitic and gneiss-migmatitic rocks outcropping in Praia de Lavadores (Northern Portugal) as metamorphic host rocks of the post-tectonic granite, ‘Granito de Lavadores’. The spinel hercynite occurs mainly in pelitic and quartz-pelitic metasedimentary lithologies (micaschists and quartzmicaschists) in paragenetic association with quartz, sillimanite (fibrolite) and magnetite- ulvospinel series. In the gneissmigmatitic rocks, the occurrence of hercynite is restricted to small restitic nodules with granoblastic texture, and is associated with magnetite-ulvospinel series, andalusite, quartz, cordierite and biotite. The semi-quantitative mineralogical study of the hercynite has pointed out a relatively constant composition, with values of Al2O3, very close to 56%, FeO total between 20 and 21% and a component of ZnO between 11 and 12%. The Zn-rich hercynite-magnetite-fibrolite assemblage in the metapelitic rocks points out one of two possibilities: (i) that the biotite was consumed in a dehydration reaction, which generated hercynite, Fe-oxides Ti (magnetite-series ulvospinel) and ‘melt’[1]; or (ii) that the spinel is as a result of dehydration of Zn-rich staurolite [2]. The magnetite-rich pelitic rocks, as it has already been evidenced by the high values of magnetic susceptibility and isothermal remanent magnetization curve [3], and its association with a Zn-rich spinel shows the correlation of these rocks with staurolite-rich metapelitic, occurring to the east of this coastal zone [4].The geological context, the paragenetic assemblage, either in metaspelitic or in gneiss-migmatitic rocks and the Zn-rich hercynite, indicates HT/L to MP metamorphic conditions in anomalous high thermal gradient in a late-orogenic setting. These conditions can be intensified by magmatic and/or metamorphic fluids advection over a late-orogenic period of crustal thinning, accompanied by processes of migmatization.

Language: English

Type (Professor's evaluation): Scientific

Notes: Comunicação apresentada na Conferência "Goldschmidt 2010: Earth, Energy, and the Environment