Search

This work presents an integrated structural, kinematic, and petrochronological study of the Monte Grighini dome within the Variscan hinterland–foreland transition zone of Sardinia (Italy). The area is characterised by dextral transpressive deformation partitioned into low- and high-strain zones (Monte Grighini shear zone, MGSZ). Geothermobarometry of one sample of sillimanite-bearing mylonitic metapelite indicates that the Monte Grighini shear zone developed under high-temperature (~ 625 °C) and low-pressure (~ 0.4–0.6 GPa) conditions. In situ U–(Th)–Pb monazite geochronology reveals that the deformation in the shear zone initiated at ca. 315 Ma. Although previous studies have interpreted the Monte Grighini shear zone to have formed in a transtensional regime, our structural and kinematic results integrated with constraints on the relative timing of deformation indicate that it shows similarities with other dextral ductile transpressive shear zones in the Southern European Variscan belt (i.e., the East Variscan Shear Zone, EVSZ). However, dextral transpression in the Monte Grighini shear zone started later than in other portions of the EVSZ within the framework of the Southern European Variscan Belt due to the progressive migration and rejuvenation of deformation from the core to the external sectors of the belt. [ABSTRACT FROM AUTHOR]

Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Garnet presents an internal foliation identified by the isoorientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldspathic and micaceous layers. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of garnet core, garnet mantle, and rim. The compositional isopleths that match the composition of the garnet core intersect with isopleths for the highest Si contents in K-white mica at about T 430–490 °C, P 1.3–1.4 GPa. Garnet rim isopleths and isopleths for the lowest Si contents in K-white mica indicate P-T conditions close to 560–630 °C/0.6–1.1 GPa. The resulting P-T path is clockwise, subdivided into two separate stages. The first stage is a prograde segment suggested by the garnet core to mantle compositional variation whereas the second reflects garnet rim growth during exhumation. • Garnet crystals up to 1 cm are characterized by core, mantle, rim, and outer rim • Clockwise P-T path shows a prograde segment up to pressure peak (1.7GPa) • High – pressure conditions pre-date Barrovian metamorphism in Sardinia [ABSTRACT FROM AUTHOR]