Isotopic Disequilibrium in Migmatitic Hornfels of the Gennargentu Igneous Complex (Sardinia, Italy) Records the Formation of Low 87Sr/86Sr Melts from a Mica-Rich Source

• Published in: Journal of petrology 2018-07, Vol.59 (7), p.1309-1328
• Main Authors: Gaeta, Mario, Giuliani, Andrea, Di Rocco, Tommaso, Tecchiato, Vanni, Perinelli, Cristina, Kamenetsky, Vadim S
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egy062

Abstract Isotopic disequilibrium is increasingly recognized as a common feature of magmatic systems, but the details of the mechanism(s) underpinning the development of isotopic disequilibrium during partial melting processes are not fully understood. Partial melting of mica-rich lithologies may be predicted to generate melts enriched in radiogenic Sr compared to the bulk protolith compositions due to the typically high Rb/Sr ratio coupled with low melting temperature of mica in crustal rocks. Here we report a puzzling case study where the Sr-isotope composition of the melt fraction (leucosome) of partially molten metapelites (migmatites) is less radiogenic than the restitic component (melanosome). The examined migmatites fringe (∼50 m wide zone) a low-pressure (≤200 MPa), high-temperature (∼1050°C) quartz-dioritic intrusion, which was emplaced in the Gennargentu Igneous Complex (Sardinia, Italy) at 306 ± 26 Ma (bulk-rock Rb/Sr dating). The migmatites derive from anatexis of the muscovite-rich metapelitic wall-rocks. They include a quartzo-feldspathic leucosome and a melanosome containing cordierite, K-feldspar, plagioclase, biotite, Fe–Ti oxide minerals and both corundum and hercynite. The leucosome has a less radiogenic Sr and more radiogenic Nd isotope composition than the melanosome (87Sr/86Sr (306 Ma) = 0·71068 and 0·71536; εNd(306 Ma) = -6·4 and -9·2, respectively), with bulk migmatite samples having intermediate compositions. The significantly lower content of mica in the migmatites compared to the protolith indicates that muscovite and, to a lesser extent, biotite largely contributed to melt formation. However, the leucosome volume (∼50%) estimated through mass balance calculations is considerably higher than the amount of melt (≤10 vol. %) generally produced by mica-dehydration melting in the crust, suggesting that partial melting was enhanced by an external hydrous fluid. The O-isotope composition of the migmatites is lower than the typical metapelite values (>10‰) but overlaps with the δ18 O range of the quartz-diorites (8·8–9·9‰), suggesting that such a hydrous fluid was released from the quartz-dioritic intrusion. We put forward a model whereby the anatexis temperature conditions (T