Petrographic features of foliated leucocratic body in the Migmatite Complex from northeastern Sardinia

COLUMBU, STEFANO;COSTAMAGNA, LUCA GIACOMO;CRUCIANI, GABRIELE;Elter F. M;FANCELLO, DARIO;FRANCESCHELLI, MARCELLO;Scodina M.

2014-01-01

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Abstract

At Punta Bados, a few kilometers north of Olbia city, an elongated foliated leucocratic body striking N160°-170°, characterized by coarse-grained garnets up to 4-5 cm in diameter is hosted in a sequence of igneous and sedimentary derived migmatite and fibrolite-rich metapelites with minor metabasite and metaultramafics (Cruciani et al., 2002, 2008). The leucocratic body shows an alternation between poorly and moderately foliated domains, as well as a great variability in garnet size and abundance. Rock grain size is strongly variable reaching up, locally, a pegmatitic texture. The contact between the leucocratic body and the metapelites is marked by a 30 cm-thick cataclastic band with S-C structures and kinematic indicators suggesting a dextral shear component. It mainly consists of quartz, plagioclase, microcline, muscovite, biotite and garnet, with zircon, apatite and tourmaline as accessory minerals. Plagioclase is mainly unzoned albite (An~5). Garnet is unzoned to poorly zoned with composition Alm75, Pyr10, Sps15, Grs<1mol.%. It is free from inclusions and/or from other microstructural relics. Muscovite occurs as centimetre-sized crystals with Si content near to 6.15 a.p.f.u. and XNa = 0.10. The whole-rock composition of a few representative samples shows following major element content ranges: SiO2: 72.8-74.7; Al2O3: 15.0- 15.4; Fe2O3tot: 0.5-0.7; MnO: 0.05-0.08; MgO: 0.1-0.7; CaO: 0.4-2.0; Na2O: 4-5; K2O: 3.2-4.8 wt.%. The rocks are peraluminous with normative composition matching those of granite/trondhjemite. Preliminary data suggest that barium ranges from 71 to 634 ppm, whereas Rb and Sr vary between 36-252 and 54-327 ppm, respectively. Rare earth element (REE) content of the leucocratic body is ~50 ppm with moderately fractionated pattern for light and medium REE, and flat for heavy REE (CeN/YbN ~1.5, LaN/SmN ~1.5, GdN/YbN ~1.0) with no Eu anomaly. Field geological data and petrographic features indicate that the leucocratic felsic body of Punta Bados resembles the foliated leucogranites described in Golfo Aranci and Arzachena areas (northern Sardinia) interpreted as anatectic bodies emplaced during the late deformative event related to the exhumation of the metamorphic basament.