High-¹⁸O igneous rocks from the Tuscan Magmatic Province, Italy

Abstract

The ¹⁸O/¹⁶O ratios were measured for 60 rocks and coexisting minerals from the Plio-Pleistocene, calc-alkaline, Tuscan Magmatic Province, Italy. The δ¹⁸O values of these magmas were as follows: Elba, Giglio, and Montecristo granodiorites (11.4 to 12.1); M. Cimini rhyolites and trachytes (11.2 to 11.7); Roccastrada, S. Vincenzo, and M. Amiata rhyolites (12.3 to 13.4); and the Tolfa rhyolites and quartz latites (15.3 to 16.4). The latter are by far the highest δ¹⁸O values yet reported for primary volcanic rocks. The extremely high δ¹⁸O values in the Tuscan Province indicate that these magmas all formed by melting or large-scale assimilation of high-¹⁸O argillaceous sedimentary rocks, in agreement with previous evidence based on the high ⁸⁷Sr/⁸⁶Sr ratios and ubiquitous presence of cordierite. Low δ¹⁸O values (+5 to +6) were found in the feldspar, but not the quartz, of the small Porto Azzuro stock on Elba, as well as in the marginal facies of the M. Capanne stock, indicating that these bodies interacted with low-¹⁸O meteoric-hydrothermal fluids subsequent to solidification. The M. Cimini lavas apparently formed about 1 m.y. ago by mixing of high-¹⁸O Tuscan magmas containing large K feldspar crystals with potassic, undersaturated, low-¹⁸O magmas of the adjacent Roman Co-Magmatic Province. These large sanidine crystals did not completely equilibrate ¹⁸O/¹⁶O with the coexisting magma.