The Monte Filau Orthogneiss represents the southernmost metaigneous body cropping out in the External Zone of the Variscan chain of Sardinia. It derives from an Ordovician granitoid intruded in the Monte Settiballas Micaschist at about 460 Ma. Three different petrographic facies can be distinguished in the orthogneiss: (i) a dark, biotite-rich facies, (ii) a leucocratic coarse-grained facies and (iii) a leucocratic, fine-grained facies (quartz + plagioclase + K-feldspar + muscovite + biotite ± garnet ± sillimanite ± andalusite). The Monte Filau Orthogneiss shows a polyphasic deformation. The first schistosity is recognizable in the field only in the eastern part of the dome and is evidenced by the orientation of mm-thick quartz-feldspar-bearing porphyroblasts or biotite crystals. The main structure in the field is a N-S striking gneissic pervasive foliation dipping toward West and East in the western and eastern part of the dome, respectively. The foliation is marked by a mm-thick quartz-feldspathic layering that changes to a biotite-bearing gneissic structure in the biotite-rich facies and near the contact with the andalusite-bearing micaschists. In the leucocratic, fine-grained facies the foliation is poorly recognizable except for andalusite-bearing facies in which it is marked by the orientation of andalusite crystals. The NNW-SSE striking mineral lineation gently plunging toward north are highlighted by orientation of biotite on the foliation planes. Late deformational structures include asymmetric folds, shear bands, transposition of mineral lineation and NS striking upright folds. Sillimanite, garnet and andalusite were observed mainly in the leucocratic fine-grained facies. Sillimanite in the fibrolite variety as well as andalusite were observed enclosed in the coarse-grained muscovite. Garnet has been observed as isolated single crystals and in clusters of small crystals oriented along the main schistosity. Worthy of note is the occurrence of dark spindles (term after [1]) up to half a centimeter mainly composed of quartz, feldspar, coarse-grained muscovite and andalusite. In the Western side the gneissic foliation is overprinted by low - temperature (greenschist facies) mylonitic fabric characterized by new growth of quartz, albite, muscovite, and chlorite. In conclusion, the history of Monte Filau Orthogneiss consists of different stages of metamorphic evolution and deformative events recorded in different portion of the orthogneiss. A high-grade metamorphic stage, with P-T conditions P= 3-5 kbar and T near 700 °C, and a later stage under low-temperature conditions (T ca. 400°C) were recognized. Our data support a continuous evolution from amphibolite to greenschist facies with local preservation of high metamorphic grade structures, possibly being consistent with an almost completely Variscan history. References: [1] Mazzoli C and Visonà D (1992) In: Carmignani L and Sassi FP (Eds). IGCP No. 276, Newsletter 5, 175-182.
Metamorphic and structural data of the Monte Filau Orthogneiss, SW Sardinia (Italy)
Cruciani G.Primo
;Dulcetta L.
Secondo
;Franceschelli M.Penultimo
;
2021-01-01
Abstract
The Monte Filau Orthogneiss represents the southernmost metaigneous body cropping out in the External Zone of the Variscan chain of Sardinia. It derives from an Ordovician granitoid intruded in the Monte Settiballas Micaschist at about 460 Ma. Three different petrographic facies can be distinguished in the orthogneiss: (i) a dark, biotite-rich facies, (ii) a leucocratic coarse-grained facies and (iii) a leucocratic, fine-grained facies (quartz + plagioclase + K-feldspar + muscovite + biotite ± garnet ± sillimanite ± andalusite). The Monte Filau Orthogneiss shows a polyphasic deformation. The first schistosity is recognizable in the field only in the eastern part of the dome and is evidenced by the orientation of mm-thick quartz-feldspar-bearing porphyroblasts or biotite crystals. The main structure in the field is a N-S striking gneissic pervasive foliation dipping toward West and East in the western and eastern part of the dome, respectively. The foliation is marked by a mm-thick quartz-feldspathic layering that changes to a biotite-bearing gneissic structure in the biotite-rich facies and near the contact with the andalusite-bearing micaschists. In the leucocratic, fine-grained facies the foliation is poorly recognizable except for andalusite-bearing facies in which it is marked by the orientation of andalusite crystals. The NNW-SSE striking mineral lineation gently plunging toward north are highlighted by orientation of biotite on the foliation planes. Late deformational structures include asymmetric folds, shear bands, transposition of mineral lineation and NS striking upright folds. Sillimanite, garnet and andalusite were observed mainly in the leucocratic fine-grained facies. Sillimanite in the fibrolite variety as well as andalusite were observed enclosed in the coarse-grained muscovite. Garnet has been observed as isolated single crystals and in clusters of small crystals oriented along the main schistosity. Worthy of note is the occurrence of dark spindles (term after [1]) up to half a centimeter mainly composed of quartz, feldspar, coarse-grained muscovite and andalusite. In the Western side the gneissic foliation is overprinted by low - temperature (greenschist facies) mylonitic fabric characterized by new growth of quartz, albite, muscovite, and chlorite. In conclusion, the history of Monte Filau Orthogneiss consists of different stages of metamorphic evolution and deformative events recorded in different portion of the orthogneiss. A high-grade metamorphic stage, with P-T conditions P= 3-5 kbar and T near 700 °C, and a later stage under low-temperature conditions (T ca. 400°C) were recognized. Our data support a continuous evolution from amphibolite to greenschist facies with local preservation of high metamorphic grade structures, possibly being consistent with an almost completely Variscan history. References: [1] Mazzoli C and Visonà D (1992) In: Carmignani L and Sassi FP (Eds). IGCP No. 276, Newsletter 5, 175-182.
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