One of the microstructural features of deformed minerals is the development of lattice preferred orientation (LPO) by intracrystalline deformation mechanisms. The latter make active several slip systems of different orientation depending on both the metamorphic grade and deformation conditions. Active slip systems reflect the temperature, strain rate conditions and kinematics of deformation (PARK et alii, 2001). Among fabric studies, quartz c-axis preferred orientation is probably the best known and most often interpreted in geological materials (SCHMID & CASEY, 1986; WENK, 1994). The interpretations are usually made with the assumption that the LPO in rocks are obtained during deformation or by the deformation processes. Situations which contradict the usual assumption are commonly found in circumstances such as post-kinematic contact metamorphism of deformed rocks that had an LPO developed before the thermal metamorphism.In this case, care needs to be exercised when interpreting the LPO, because there is a possibility of modification of LPO during static recrystallization. On the other hand, REE & PARK (1997), suggested that the LPO in annealed artificial material (octa-chloropropane) has a good memory of former deformation occurred prior than annealing, in agreement whit experimental results obtained by deformed quartzite samples and subsequently annealed from HEILBRONNER & TULLIS (2002).The present study deals with mylonitic rocks outcropping in the southern sector of the Serre Massif (southern Calabria) where an extensional shear zone developed under greenschist to amphibolite facies conditions (P=0.03 GPa; T=470 °C) has been recognised (ANGÌ et alii, 2010). The shear zone affected the crystalline basement of the so called Paragneissic Mammola Complex (PMC) (COLONNA et alii, 1973), intruded by the Late Variscan granitoids (ROTTURA et alii, 1990).GEOLOGICAL SETTING The Serre Massif is a crustal sector of the northern Calabrian Peloritani Orogen bordered by the Catanzaro trough to the north and the Aspromonte Massif to the south, consisting of intermediate-lower to upper crustal metamorphic rocks intruded by large granitoid bodies. The whole sector has been interpreted by SCHENK (1990) as a complete piece of continental crust tilted in Paleogene time by means of Alpine tectonics coupled with the more recent (Pliocene) spreading of Thyrrenian sea. Nevertheless several authors (CAGGIANELLI et alii, 2000 and references therein) have distinguished different tectonic units within this massif recognising different tectono-thermal evolution of crystalline rocks as well as different depth of plutonic bodies emplacement.According to SCHENK (1980), the extensive calc-alkaline magmatism which intruded the upper crust, whose products range from diorite to granite, coincides with the peak metamorphic conditions in the lower crust portion, dated 295 Ma. During this migration the eastern part of this crustal section experienced a sort of shortening; differently the western margin underwent extension and uplift. Despite post-Hercynian tectonic events, the original Hercynian structural setting is quite well preserved in the Sila and Serre massifs (CAGGIANELLI & PROSSER, 2001).In the study area, two metamorphic Hercynian Complexes characterized by different evolution (ATZORI et alii, 1977) that came into contact before the intrusion of the late-Hercynian granitoids have been recognized: a) the lowermost Mammola-Paragneiss Complex (MPC), constituted by paragneisses, leucocratic gneisses and amphibolites and b) the uppermost Stilo-Pazzano Complex (SPC), which includes low grade greenschist facies metapelites, metalimestones, quarzites and metavolcanics. At the top of the SPC, a composite sedimentary cover lies unconformably over the crystalline basement.The tectono-polymetamorphic evolution of the Mammola-Paragneiss Complex, consisting of polyphase Hercynian regional metamorphism, locally affected by pervasive contact metamorphism due to the late-Hercynian intrusive bodies has been investigated by ANGÌ et alii (2010). Undeformed dykes cut the regional mylonitic foliation of the MPC basement rocks and limit its metamorphic history to Hercynian time (ROMANO et alii, 2011 and references therein).PETROGRAPHYA main foliation characterise host rocks collected near the intrusive contact, consisting of alternated fine-grained quartz and mica domains. The main assemblage is given by quartz, feldspar, biotite, and Fe-oxides. Thermal effects due to the contact metamorphism have substantially modified the pre-existing texture producing widespread recrystallisation making recognisable different generations of minerals (i.e. quartz and biotite) constituting polygonal microstructure composed by equant, polygonal quartz grain, exhibiting different grain size (10 to 150 microns). Quartz boundaries often meet at right angles the basal plane of mica flakes aligned parallel to the foliation.Two quartz typologies were recognized on the basis of their different size: the first is represented from small (ca 50 microns) equant strain-free grains of quartz with minor undulose extinction and with straight grain boundaries meeting at 120° triple junctions defining a foam texture, whereas the other one is constituted by quartz grains (ca. 400 microns) with lobate boundaries and with extensive bulging phenomena well preserved in relic mylonitic domains. A lot of plagioclase grains also contribute to form this polygonal texture. Bulging and grain boundary migration recrystallization microstructures are frequently observed. Formation of quartz subgrains is incipient. The coesistence of different quartz fabrics as well as the textural re-equilibration of the other minerals is the result of annealing after dynamic recrystallization by thermal effects associated with the intrusion of granitoids. Two generations of biotite occur too. The former is represented by laths parallel to the main foliation, which have often toothed edges indented with quartz subgrains growing at their margin (bulging texture). The latter constitutes the decussate plates overgrowing the first mica generation and, at place, mimetically replacing sigmoid shaped crystals (early mica-fish). Both of them are partially to totally retrogressed to chlorite.Epidote overgrows clusters of static biotite. Patchy relics of cordierite are replaced by white mica, quartz and chlorite. Sporadic sillimanite relics are surrounded by biotite laths. Porphyroclastic plagioclase crystals have amoeboid shape. Apatite occurs as porphyroclast. It is characterized by a lot of very tiny microinclusions, especially in the outer edge, forming a corona-type structure.Samples collected about 2 km far from the granitoids are less affected by thermal metamorphism and have preserved the mylonitic features such as fine grain size and ribbon quartz grains. The quartz c-axis pattern, obtained from these sample using an image-assisted analysis computer program, show an asymmetric cross girdle quartz c-axes fabric indicating that mylonitic deformation phase occurred at upper greenschist facies conditions. These samples show sometimes an increased grain size (up to 300 microns) compared to those close to the intrusive contact with wavy grain boundaries, bulging and formation of sub-grain boudaries indicative of grain boundary migration recrystallization (GBMR), exhibits a weakened c-axis quartz pattern probably partly affected by thermal metamorphism.QUARTZ MICROSTRUCTURESA variety of quartz microstructures is present in quartz-feldspar mylonites from the Mammola village area (Serre Massif), which show varying degrees of recrystallization: bulging and GBMR are frequently observed. In addition, adjacent to quartz layers elongated parallel to the main foliation, mica domains occur, consisting of aligned mica flakes, whose basal plane meet at right angles the quartz boundaries. Two generations of quartz occur: the first is represented by equant strain-free grains of quartz with minor undulose extinction and with straight grain boundaries defining a foam texture; the other one is constituted by quartz grains with lobate boundaries well preserved in relic mylonitic domains. The coesistence of different quartz fabrics as well as the textural re-equilibration of the other minerals is the result of annealing after dynamic recrystallization by thermal effects associated with the intrusion of granitoids.The annealed samples all have equant straight-sided, polygonal grains; annealing appears to reduce grain boundary lobateness The static annealing following deformation does not greatly affect the pattern or strength of the CPO, even though it completely changes the grain size and shape.The AVA technique was applied on a well preserved oblique foliation microstructure of samples, due to the effects of combined sub-grain rotation and grain boundary migration re-crystallisation regime. It allowed to obtain an asymmetric single girdle quartz c-axes fabric, consistent with a top-to-the ENE/NE sense of shear. These c-axes quartz patterns, suggesting that basal <a> slip was more active than prism <a> slip during the shearing phase, are indicative of a shearing temperature ranging from 400 °C to 500 °C (HIRTH & TULLIS, 1992). CONCLUSIONSApproaching the contact with plutonic complex, the metapelites display a variation in the minero-textural re-equilibration (i.e. metamorphic annealing) and an increase in the static mineral-growth. Several samples exhibit textural features clearly related to the static re-crystallisation, which are represented by: a) strain-free quartz level aggregates with slightly undulose extinction and straight grain boundaries, b) a network of triple junctions among grains of re-crystallised oligoclase-andesine plagioclase; c) biotite, garnet and feldspar static mineral-growth, overprinting the earlier regional metamorphic assemblage; d) randomly oriented tabular plates porphyroblasts of biotite with irregular rims occur as well as static garnet, represented by subhedral to euhedral inclusions-free almandine-rich rim on previous regional garnet.The microstructural investigation of a suite of samples collected at different distance from the intrusive contact of the granitoid outcropping in the southern termination of the Serre Massif (near the Mammola village) allowed us to confirm that, although the previous mylonitic features are clearly annealed by thermal effect re-crystallisation in rich-quartz-level domains, a memory of the relic strong quartz c-axes pattern is recorded, consistent with a top-to-the ENE/NE sense of shear in the present-day geographic coordinates, as suggested also by microstructural observations and structural field data.
|Titolo:||Quartz annealing microstructures in sheared rocks from the Serre Massif (Calabria, Italy)|
|Data di pubblicazione:||2012|
|Appare nelle tipologie:||1.1 Articolo in rivista|