Daniela Gasperini

Geochemical and Sr–Nd isotope disequilibria during multi-stage anatexis in a metasedimentary Hercynian crust

The processes responsible for the compositional evolution of metamorphic terranes that underwent disequilibrium melting reactions control the petrology, the geochemistry and the isotopic composition of the resulting anatectic magmas. The high-grade metamorphic Tarra Padedda Complex (northern Sardinia, Italy) provides a suitable opportunity to study the isotopic (Sr–Nd) and geochemical evolution of both the anatectic magmatic products and their source. The metamorphic complex consists of migmatites (diatexites, metatexites and hornblende-bearing migmatites) and sillimanite-bearing metasediments, which were intruded by syntectonic peraluminous granitoids, aplites and pegmatites. The diatexites consist of stromatic migmatites and nebulites. Neosomes in stromatic migmatites show either granitic or trondhjemitic leucosomes, and hornblende-bearing or hornblende-free melanosomes. Petrographic characteristics of the melanosomes and hornblende-bearing migmatites suggest two distinct biotite-dehydration melting reactions: (a) biotite + sillimanite + plagioclase + quartz → garnet ±K-feldspar + melt; and (b) biotite + plagioclase (An~40) + quartz → hornblende + plagioclase (An > 60) + titanite + melt. In contrast, textural features in metatexites favour dehydration melting of muscovite: muscovite + quartz + plagioclase → sillimanite + K-feldspar + melt. A progressive variation of chemical composition coupled with a decrease of Sr and Nd crustal isotopic signatures is observed both within the metamorphic rocks and the granitoids. Field relationships, petrological, geochemical and isotope evidence of the Tarra Padedda Complex are consistent with a multi-stage anatexis of a heterogeneous metasedimentary source within the same Hercynian orogenic cycle. The variation of Sr and Nd isotope compositions within both the granitoids and the migmatites is interpreted in terms of anatexis of the various metasedimentary components, via different melting reactions involving micas plus variable amounts of accessory phases (apatite, monazite, zircon and/or xenotime).

Coexistence of calc-alkaline and ultrapotassic alkaline magmas at Mounts Cimini: evidence for transition from the Tuscan to the Roman Magmatic Provinces (central Italy)

The volcanic complex of Mts. Cimini (~0.90-1.30Ma) represents the geographical and chronological transition between the Tuscan Magmatic Province (TMP) and the Roman Magmatic Province (RMP), in central Italy. Major and trace elements, and Sr, Nd and Pb isotopes of whole-rock, as well as mineral chemistry analyses, were carried out on samples representative of the different petrographic and chronological units of Mts. Cimini. In particular, we focused on the olivine-bearing latites of Mts. Cimini that are the most mafic magmas, belong to the last phase of this volcanic activity, and are heterogeneous in highly incompatible element ratios and Sr-isotope compositions. We suggest that such heterogeneity reflects the occurrence of a heterogeneous upper mantle beneath central Italy, in which different portions, e.g., the sources of both the TMP and RMP, are characterized by distinct geochemical and petrographic features. In this scenario, about 900ka ago, the olivine-bearing latites mark the progressive decline of the TMP magma production in favour of partial melting of the RMP mantle region, thus recording the coexistence of both ultrapotassic alkaline and calc-alkaline magmas in the same volcanic region. 1

Evidence for transition from the Roman to the Tuscan Magmatic Provinces in Mts. Cimini olivinlatites

Mo isotope proxy for ancient anoxia: Insights from a modern anoxic basin COREY ARCHER*, DEREK VANCE, TANYA GOLDBERG, BO THAMDRUP, GENARO ACUNA AND SIMON POULTON Dept. of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK (c.archer@bristol.ac.uk) School of Civil Engineering and Geosciences, Newcastle University, UK Institute of Biology, University of Southern Denmark CIMAR, Universidad de Costa Rica

Geochemical study of the Garrotxa volcanic field (NE Spain Volcanic Province)

1 pagina.-- Resumen del trabajo presentado en la 19th Annual V.M. Goldschmidt Conference, V.M. Goldschmidt Conference.