Petrophysical properties of Variscan mylonitic leucogranite of Monte Grighine, west-central Sardinia

Abstract: Monte Grighini Complex consist of a pile of variscan Nappes and of a suite of granitoid rocks, mostly monzogranite and leucogranite (Musumeci, 1992), that intrudes the tectonic unit of Monte Grighini. Rb/Sr and Ar/Ar radiometric data give, for granitoid rocks, an age of about 305-300 Ma (Carmignani et al., 1987; Laurenzi et al., 1991). A late variscan shear zone affects the SW side of the complex and produces a wide cataclastic and mylonitic zone, along which leucogranite can be mostly found. In this zone an outcrop, that shows a rapid change in the intensity of mylonitic deformation, has been investigated in detail. The outcrop is affected by a pervasive N162-170° foliation dipping 60-65° W. Petrographic and XRD analysis show the following mineralogical assemblage: quartz, k-felspar, plagioclase (albite), muscovite, biotite, iron oxides and ± tourmaline ± apatite ± zircon ± epidote. Almost all samples are characterized by k-feldspar porphyroclasts in a quartz-phyllosilicate matrix. On the basis of porphyroclast/matrix ratio protomylonite, mylonite and ultramylonite layers have been identified. In protomylonitic layers k-felspar phenocrysts are fractured and sometimes perthitic. Quartz occurs in microcrystalline lens and bands following the foliation. Phyllosilicate matrix consists of biotite, subordinate muscovite and chlorite. Mylonitic layers are similar to the previous ones but show a decrease in grain-size. Ultramylonitic layer consists of rare k-feldspar relics in a very fine-grained phyllosilicate matrix. The following physical properties have been determined in 12 series of samples with varying degree of deformation: the open porosity, the total porosity, the real and apparent density, the water imbibition coefficient, and the water saturation index. To calculate the closed and total porosity the real density of ground samples were determined. The ultrasonic velocity of the studied samples has been determined in three directions: 1) perpendicular to the foliation, 2) parallel to the foliation and to the direction of mineral stretching lineation, 3) parallel to the foliation and perpendicular to the direction of mineral stretching lineation. The overall results indicate that the samples of protomylonite, mylonite and ultramylonite have different physical properties values. The average values of the real density of samples vary from a minimum of 2.66±0.02 g/cm3 in the protomylonitic samples, to a maximum of 2.72±0.01 g/cm3, in the ultramylonitic samples. The mylonitic samples show intermediate values with an average of 2.68±0.01 g/cm3. The open porosity shows a similar trend; it ranges from 4.1±0.2% in the protomylonitic samples, to 6.3±0.9% in the mylonitic samples, to 9.2±0.5% in the ultramylonitic samples. Therefore, there is a positive correlation between the degree of mylonitization and these physical parameters. The ultrasonic velocity values show on one side of the greater porosity of ultramylonites than proto-mylonites and mylonites, on the other side confirm a higher anisotropy degree in the ultramylonitic samples releaved perpendicular to the foliation. Preliminary results seem to suggest a relationship between the petrographic and petrophysical characteristics of the layers. References Carmignani L., Cherchi G.P., Del Modo A., Franceschelli M., Ghezzo C., Musumeci G. & Pertusati P.C. (1987). IGCP project n°5. Newsletter. Higgins M. W. (1991). USGS Prof. Paper 687. Laurenzi M.A., Del Moro A., Musumeci G. & Pardini G. (1991). Terra abstract, 3, 1, p. 501-502. Musumeci G. (1992). Geodinamica Acta: 5, 119-133, Paris.