Amphibole-bearing migmatites from the Variscan Belt of NE Sardinia, Italy: Partial melting of mid-Ordovician igneous sources

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• 作者：Gabriele Cruciani ; Marcello Franceschelli ; Stefan Jung ; Mariano Puxeddu ; Daniela Utzeri
• 关键词：Amphibole-bearing migmatite ; Petrography ; Geochemistry ; P– ; T conditions ; NE Sardinia ; Variscan orogeny
• 刊名：Lithos
• 出版年：2008
• 出版时间：October 2008
• 年：2008
• 卷：105
• 期：3-4
• 页码：208-224
• 全文大小：2114 K

文摘

Amphibole-bearing migmatites from north-eastern Sardinia, Italy, are characterized by tonalitic to granodioritic leucosomes made up of quartz, plagioclase, K-feldspar, biotite, ± amphibole, and garnet. The leucosomes show higher SiO₂, Na₂O, Sr and lower TiO₂, Fe₂O_3tot, MgO, MnO, P₂O₅, K₂O and Rb content as compared to the mesosomes. The granodioritic leucosomes differ from the tonalitic leucosomes for significantly higher Ba and K₂O content. The mesosomes and the leucosomes show slightly fractionated REE patterns with moderate negative and positive Eu anomalies, respectively. The leucosomes show lower ⁸⁷Rb/⁸⁶Sr ratios (0.279–0.581) than the mesosomes (0.634–1.121), whereas the ¹⁴⁷Sm/¹⁴⁴Nd ratios are similar in leucosomes (0.12–0.14) and mesosomes (0.11–0.14). Mineralogical, geochemical and isotopic data suggest that the migmatites formed by in situ partial melting of a biotite + plagioclase + quartz-bearing protolith with 2–4 wt. % added water. Variable degrees of melt loss were responsible for the observed compositional variability in the leucosomes, whereas solid-state re-equilibration of the migmatites leads to a general re-equilibration of mineral compositions after partial melting. The amphibole-bearing migmatites record maximum P–T conditions of 700–750 °C and 1.0–1.2 GPa, probably lower than, but near to, the P–T conditions of peak metamorphism. Zircon morphology suggests an igneous origin for the migmatite protolith. Pb–Pb zircon dating yielded a mean value of 452 ± 3 Ma and an isochron age of 461 ± 12 Ma which is interpreted as the emplacement age of the migmatite protolith.