Two-mica F-Li-B-rich monzogranite apophysis of the Larderello batholith cored from 3.5km depth
Cavarretta, G.; Puxeddu, M.
published: Dec 14, 2001
The well Monteverdi 7 (MV7), at Larderello, crossed granite apophyses in the depth range 3418-3581m below ground level. They consist of a fine-grained hypidiomorphic two-mica S-monzogranite, with a K-Ar age of 3.8 ± 0.1Ma. Early magmatic muscovite with F≤1.09 wt % crystallized between 600° and 540 °C; and the late subsolidus one between 540 ° and 300 °C. Biotite is a siderophyllite-rich variety with F≤1.2 wt %. Two generations of plagioclase were distinguished: the first (Ab19-87) shows progressive P2O5 enrichments, up to 0.16wt %; the second (Ab88-95) is almost devoid of P2O5. K-feldspar occurs as crypto- to microperthitic hypersolvus albite-rich microphenocrysts (T>650 °C) and late fine-grained albite-poor crystals. Titanite and apatite contain F≤4.66 and F≤4.34 wt % respectively. Tourmaline has features intermediate between those of late-magmatic and early hydrothermal types. Euhedral cordierite yielded Na contents indicating temperatures mostly in the range 735 °-650 °C; the low oxide sum indicates PH2O = 200-240MPa, suggesting a crystallization during magma ascent. Anhedral andalusite and rare sillimanite with hercynite are the aluminarich minerals.The chemistry of the MV7 rocks is similar to that of the F-Li-B-rich peraluminous two-mica rhyolites and granites of Alaska, Peru, South-West England, Portugal, Czech Republic, Mongolia and Himalaya. The MV7 magma had early temperatures of 800 °-850 °C. The water content increased from 4.2-4.4 wt % at 800 °-850 °C up to more than 10wt % at 650 °C. Water saturation, LiCl content up to 3 wt %, F and B enrichments of the last residual melts, presumably had a synergistic effect lowering the solidus temperatures to about 540 °C. Primary muscovite was stable at these temperatures for PH2O = 115MPa. This value was calculated on the basis of known rock thicknesses and densities from geothermal drilling data. Early magmatic muscovites have values of the Fe3+/(Fe3+ + Fe2+) ratio mostly ranging from 0 to 0.05 and few values in the range 0.05-0.10; late magmatic and subsolidus muscovites show a continuous transition from 0.10 to 0.50: the fO2 increase may be attributed to hydrogen leakage due to tectonic activity, connecting the oxidizing supergene environment with the deep reducing one. From the early to the final stage of crystallization the MV7 residual melts were gradually enriched in F and Na and followed a path parallel to the QzAb side of the QzAbOr diagram. During the very last stage of crystallization, such enrichment caused a massive albitization of the MV7 granite.