Uncommon high-temperature Na-metasomatized rocks close to sodic fenites are hosted in coeval granodiorites from the late-Variscan (286 ± 2 Ma) Sàrrabus pluton (SE Sardinia). They show sharp contacts with the host rocks and consist of small, irregular lenses of “spotted” rocks and hectometric layered, albititic bodies, scattered along a 15 km-long WNW trending extensional shear zone. Alkali metasomatized rocks locally preserve the original textural and mineralogical features of granodiorites and display pervasive coarse recrystallization textures, with a wide range of mineral assemblages. Spotted rocks display variably albitized plagioclase and anhedral K-feldspar, with interstitial aggregates of low-Ti diopside, partially or totally replaced by hastingsitic amphibole, producing the “spotted” macroscopic aspect. As the metasomatism becomes more pervasive, xenomorphic or granoblastic textures with low Tiandraditic garnet coexisting with a clinopyroxene close to aegirine-augite occur in the rock. Several stages of metasomatism are revealed by dark mica + albite occurring as micro granoblastic aggregates along intergranular boundaries, and as chessboard albite replacing K-feldspar. Magnetite, titanite and apatite are ubiquitous accessory phases. Rare quartz with mortar textures occur in veinlets and pegmatitic pockets. Albitites show εNd286 in the range of -6.29 ÷ -6.78, overlapping the values observed for host granodiorites (-5.53 ÷ -6.40). 87Sr/86Sr286 is in the range of 0.70922 ÷ 0.71140, slightly higher than 0.70865 ÷ 0.70969 of granodiorites. This confirm alkali metasomatism of a granodioritic protholith, caused by rock-interaction with Na-enriched, silica undersaturated fluids. Phase relationships for metasomatic systems and several calibrations constrain the P-T-fO2 conditions in the range of 2.0 ÷ 2.2 kbars, 450 ÷ 710°C, and -18.5 ÷ -16.2, respectively. Several stages of metasomatism occurred inthe granodioritic intrusion while the latter was still cooling. These stages were controlled by fault development and by continuous variations in physico-chemical parameters of the fluids, with T decreasing from near-solidus to sub-solidus and hydrothermal conditions. Schematically, metasomatic stages may be summarized into: (1) HT quartz removal (desilication) + albitization + primary biotite destabilization + Ca-silicate (Cpx, Grt) formation (2) fluid-controlled metasomatic reactions leading to partial consumption of Cpx and Grt and to Amph, Tit and Mt production, and (3) final widespread albitization with hydrothermal hastingsite, Bt and local quartz precipitation. Overall, high-temperature processes resulted in a singular “sodic fenite” stage, with coarse recrystallization textures under near-solidus temperatures that overlaps those of magmatic stage. High temperature textures are then overlapped by lower temperature stages with a general increase in Na, fO2 and aH2O in the fluids, with a final and widespread albitization which may be interpreted as a true episyenitization stage.
High-temperature albitites from late-Variscan Sàrrabus pluton (SE Sardinia, Italy)
Naitza S.;Ferrero S.;
2022-01-01
Abstract
Uncommon high-temperature Na-metasomatized rocks close to sodic fenites are hosted in coeval granodiorites from the late-Variscan (286 ± 2 Ma) Sàrrabus pluton (SE Sardinia). They show sharp contacts with the host rocks and consist of small, irregular lenses of “spotted” rocks and hectometric layered, albititic bodies, scattered along a 15 km-long WNW trending extensional shear zone. Alkali metasomatized rocks locally preserve the original textural and mineralogical features of granodiorites and display pervasive coarse recrystallization textures, with a wide range of mineral assemblages. Spotted rocks display variably albitized plagioclase and anhedral K-feldspar, with interstitial aggregates of low-Ti diopside, partially or totally replaced by hastingsitic amphibole, producing the “spotted” macroscopic aspect. As the metasomatism becomes more pervasive, xenomorphic or granoblastic textures with low Tiandraditic garnet coexisting with a clinopyroxene close to aegirine-augite occur in the rock. Several stages of metasomatism are revealed by dark mica + albite occurring as micro granoblastic aggregates along intergranular boundaries, and as chessboard albite replacing K-feldspar. Magnetite, titanite and apatite are ubiquitous accessory phases. Rare quartz with mortar textures occur in veinlets and pegmatitic pockets. Albitites show εNd286 in the range of -6.29 ÷ -6.78, overlapping the values observed for host granodiorites (-5.53 ÷ -6.40). 87Sr/86Sr286 is in the range of 0.70922 ÷ 0.71140, slightly higher than 0.70865 ÷ 0.70969 of granodiorites. This confirm alkali metasomatism of a granodioritic protholith, caused by rock-interaction with Na-enriched, silica undersaturated fluids. Phase relationships for metasomatic systems and several calibrations constrain the P-T-fO2 conditions in the range of 2.0 ÷ 2.2 kbars, 450 ÷ 710°C, and -18.5 ÷ -16.2, respectively. Several stages of metasomatism occurred inthe granodioritic intrusion while the latter was still cooling. These stages were controlled by fault development and by continuous variations in physico-chemical parameters of the fluids, with T decreasing from near-solidus to sub-solidus and hydrothermal conditions. Schematically, metasomatic stages may be summarized into: (1) HT quartz removal (desilication) + albitization + primary biotite destabilization + Ca-silicate (Cpx, Grt) formation (2) fluid-controlled metasomatic reactions leading to partial consumption of Cpx and Grt and to Amph, Tit and Mt production, and (3) final widespread albitization with hydrothermal hastingsite, Bt and local quartz precipitation. Overall, high-temperature processes resulted in a singular “sodic fenite” stage, with coarse recrystallization textures under near-solidus temperatures that overlaps those of magmatic stage. High temperature textures are then overlapped by lower temperature stages with a general increase in Na, fO2 and aH2O in the fluids, with a final and widespread albitization which may be interpreted as a true episyenitization stage.
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