Late Variscan trondhjemites and granites make up small plutonic complexes intruded in migmatitic paragneisses in the northeastern Peloritani Mountains (southern Italy). In particular, two different varieties of trondhjemites occur: s.s., with typical CaO contents (>1.5 wt%), and low-Ca, with lower CaO contents comparable with those of the granites. Furthermore, trondhjemite sub-varieties with anomalous Na2O and K2O contents also occur. The s.s. trondhjemites and the granites have petrographic and geochemical features consistent with generation by water-fluxed and dehydration melting of metapelitic rocks, respectively. Low- Ca trondhjemites show significant similarities with the granites, but are characterized by massive subsolidus crystallization of turbid plagioclase at expense of magmatic K-feldspar. Secondary plagioclase is present in different amounts in all rock types, but it appears as an important rock-forming component only in the low-Ca trondhjemites, indicating these rocks as the product of infiltration metasomatism of the associated granites. All the granitoids were affected at varying extents by shear-related deformation that, in the most strained samples, resulted in significant tectonic grain size reduction, therefore producing an increase of the rock permeability as well as a parallel increase of the surface-controlled rock reactivity. Shear-zone activity played a significant role in driving the metasomatic modifications, as documented by nearly ubiquitous examples of secondary plagioclase growing along micro-shear zones filled with new grains of quartz and feldspars. Furthermore, occurrence in many granitoid samples of chessboard patterns in quartz indicates high-temperature deformation of more than 650 °C. Such high shearing temperatures suggest shear-zone activity at the same time or soon after the emplacement of the late Paleozoic plutons. Late Variscan shearing deformation starting at submagmatic conditions, during the cooling of the c. 310-300 Ma Aspromonte-Peloritani granitoid bodies, is consistent with the activity of the crustal-scale shear zone that controlled the emplacement of the 13-km thick Serre Batholith (central Calabria), between c. 300 and 290 Ma. This context provides viable conditions for pervasive fluid migration across the sheared crust, as well as a possible source for the metasomatizing fluids, that could have been released during crystallization of the huge volumes of granitic magmas of the Serre Batholith. Since metasomatic replacement of feldspar is largely pseudomorphic, its original magmatic habit is often preserved and, at the same time, most derived bulk rock geochemical features can be interpreted as reflecting igneous processes. This leads to the final consideration that shear zones can drive large-scale metamorphic reactions able to produce deceptive metamorphic rocks that pretend to be magmatic.

A mini-atlas of shear-related microstructures in late Variscan granitoid rock from the northeastern Peloritani Mountains (southern Italy)

Fiannacca P.
Investigation
;
Fazio E.
Investigation
;
Cirrincione R.
Supervision
;
Pezzino A.
Supervision
2018

Abstract

Late Variscan trondhjemites and granites make up small plutonic complexes intruded in migmatitic paragneisses in the northeastern Peloritani Mountains (southern Italy). In particular, two different varieties of trondhjemites occur: s.s., with typical CaO contents (>1.5 wt%), and low-Ca, with lower CaO contents comparable with those of the granites. Furthermore, trondhjemite sub-varieties with anomalous Na2O and K2O contents also occur. The s.s. trondhjemites and the granites have petrographic and geochemical features consistent with generation by water-fluxed and dehydration melting of metapelitic rocks, respectively. Low- Ca trondhjemites show significant similarities with the granites, but are characterized by massive subsolidus crystallization of turbid plagioclase at expense of magmatic K-feldspar. Secondary plagioclase is present in different amounts in all rock types, but it appears as an important rock-forming component only in the low-Ca trondhjemites, indicating these rocks as the product of infiltration metasomatism of the associated granites. All the granitoids were affected at varying extents by shear-related deformation that, in the most strained samples, resulted in significant tectonic grain size reduction, therefore producing an increase of the rock permeability as well as a parallel increase of the surface-controlled rock reactivity. Shear-zone activity played a significant role in driving the metasomatic modifications, as documented by nearly ubiquitous examples of secondary plagioclase growing along micro-shear zones filled with new grains of quartz and feldspars. Furthermore, occurrence in many granitoid samples of chessboard patterns in quartz indicates high-temperature deformation of more than 650 °C. Such high shearing temperatures suggest shear-zone activity at the same time or soon after the emplacement of the late Paleozoic plutons. Late Variscan shearing deformation starting at submagmatic conditions, during the cooling of the c. 310-300 Ma Aspromonte-Peloritani granitoid bodies, is consistent with the activity of the crustal-scale shear zone that controlled the emplacement of the 13-km thick Serre Batholith (central Calabria), between c. 300 and 290 Ma. This context provides viable conditions for pervasive fluid migration across the sheared crust, as well as a possible source for the metasomatizing fluids, that could have been released during crystallization of the huge volumes of granitic magmas of the Serre Batholith. Since metasomatic replacement of feldspar is largely pseudomorphic, its original magmatic habit is often preserved and, at the same time, most derived bulk rock geochemical features can be interpreted as reflecting igneous processes. This leads to the final consideration that shear zones can drive large-scale metamorphic reactions able to produce deceptive metamorphic rocks that pretend to be magmatic.
Variscan shear zones, fluid-rock interaction, NE Sicily
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11769/347696
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