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Occurrence of foliated leucocratic bodies within Migmatite Complex of NE Sardinia: preliminary data
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Geochemical constraints on petrogenesis of Ca-rich leucosomes from Porto Ottiolu migmatites (NE Sardinia)
No full textAnticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: Geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block
Full text linkIn the Migmatite Complex from NE Sardinia, a large lensoid body of coarse-grained, dark-green amphibolite with a schistose to weakly massive aspect crops out. Within this amphibolite centimetre-sized layers locally occur which contain millimetric porphyroblastic garnet. We investigated the amphibolite and the layers applying microstructural analyses and thermodynamic modelling in the NCKFMASH+Ti+Mn system in order to reconstruct the pressure-temperature (P-T) metamorphic evolution. The amphibolite underwent a burial path, recorded by the compositional zoning of garnet, that started at pressures of 0.8 GPa and showed only a slight increase in temperature leading to peak P-T conditions. The garnet rim records peak P-T conditions of 1.3-1.4 GPa at 690-740 °C. As the early exhumation of the amphibolites occurred already at lower temperatures than the burial, an anticlockwise P-T path results which is in contrast to the typical clockwise P-T paths reported for several high-pressure metamorphic rocks from NE Sardinia. We interpret the anti-clockwise path by the location of the studied rocks in the lowermost part of the upper plate and their burial to depths of around 45 km during the Variscan continental collision between Laurussia and Gondwana. This process could have affected some rock slices of the upper plate only owing to tectonic erosion by the downgoing plate. The subsequent uplift occurred in an exhumation channel where these slices were continuously cooled by the upper portion of the lower continental plate
P–T conditions in mylonitic gneiss from Posada Shear Zone, NE Sardinia
No full textThe Posada Valley shear zone is part of a regional-scale mylonitic belt that runs from Posada (NE Sardinia) to Asinara Island (NW Sardinia) separating the Variscan Medium Grade Metamorphic Complex to the south from the Migmatite Complex to the north. In the Posada Valley, a dextral top–to–the SE shear belt developing ductile and brittle–ductile D2 mylonites has been recognized. In southern Gallura the dextral shear movement follows a sinistral top–to–the NW shear belt, coeval to the initial D2 post–collisional phase. At Punta Orvili, a few kilometers north from Posada village, a sequence of mylonitic gneiss with subordinate calc–silicate nodules and metabasite lenses crop out. The main foliation at the mesoscale is the S2 schistosity striking N 60–80°and dipping 30°–50° SE. Locally, the S2 schistosity is affected by centimetre size, sinistral strike–slip greenschist shear zones, striking N 30° and dipping 60° SE. Three mineral lineations have been recognized on the S2 schistosity: a feldspar+quartz lineation trending N30°–50° and plunging 20°–30° SW; a quartz lineation trending N 40°, plunging 30° SW; a biotite±chlorite lineation trending N 20° and plunging 15°–30° SW. The mylonitic gneiss consists of quartz, plagioclase, K-feldspar, biotite, white mica, fibrolite, ± garnet. Centimetric garnet porphyroblasts, locally observed, are Alm58–68, Pyr8–12, Grs2–3, Sps16–31. The mylonitic gneiss is characterized by the widespread occurrence of millimetre– to centimetre–sized nodules enveloped by the S2 foliation. The nodules occur in four textural types: (i) quartz–rich, (ii) K-feldspar–rich, (iii) plagioclase–rich, and (iv) sillimanite–rich nodules. The K-feldspar–rich nodules are made up of polygonal aggregates of submillimetric K-feldspar crystals or by elongated, anhedral K-feldspars up to 5mm in length associated with smaller quartz–feldspathic grains. Perthite exsolutions and very thin albite rims are often observed in K-feldspar. The plagioclase–rich nodules consist of plagioclase (An20) aggregates with subordinate quartz and K-feldspar. The feldspar rich–nodules probably document an incipient melting affecting the rock prior of the mylonitic deformation. Application of the garnet–biotite geothermometer and GASP geobarometer to some selected mylonitic gneiss yielded P–T conditions of P ~ 6 kbar, T = 540–620°C. These P–T conditions most likely refers to the development of the D2 deformation, i.e. to the development of the sinistral top–to–the NW shear deformation. In conclusion, the Posada shear zone is first characterized by a medium– to high–grade sinistral shear movement followed by dextral shear movement as shown by Carosi et al. (2012) in southern Gallura
Geothermobarometry of Al-silicate-bearing migmatites from the Variscan chain of NE Sardinia, Italy: a P-T pseudosection approach
No full textThis paper investigates Al-silicate-bearing migmatite from NE Sardinia by using the P-T pseudosection approach with the aim to determine the P-T conditions of partial melting and those of melt crystallization. P-T pseudosections were calculated in the NCKFMASH system within the P-T range 500-800°C, 0.1-1.5 GPa by using the average compositions of metapelitic greywacke, average mesosome and average trondhjemitic leucosome, respectively. The P-T pseudosections calculated for the average metapelitic greywacke and for the average mesosome, contoured for melt volume %, Si/Al and Na/K molar ratios in melt point to P–T conditions 700-740°C, 1.1-1.3 GPa which are indicative of partial melting. The P-T pseudosection calculated for the average composition of trondhjemitic leucosomes, contoured for kyanite and biotite modal content and for XMg ratio in biotite indicates P-T conditions of 660-730°C, 0.75-0.90 GPa for the crystallization of the melt. The comparison between the Na/K and Si/Al ratios in leucosomes, and the same ratios modeled for the anatectic melt by an haplogranitic melt model is thus a powerful tool for the reconstruction of P-T conditions of partial melting also in pelitic rocks, provided that leucosomes represent pure melts and are not contaminated by restitic phases or feldspar cumulates
PETROGRAPHIC FEATURES OF FOLIATED LEUCOCRATIC BODY IN THE MIGMATITE COMPLEX FROM NORHEASTERN SARDINIA
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Microstructures in granulitized kyanite-eclogite from N-E Sardinia, Italy: insights into the recontructions of metamorphic evolution
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