Monte Grighini Complex is a NW-SE elongated dome-shaped metamorphic complex which outcrops in central Sardinia in the eastern side of Campidano graben. This Complex consists of a pile of variscan Nappe. From top to bottom they are: Gerrei Unit, Castello Medusa Unit, M. Grighini Unit. Gerrei Unit (Upper Unit) crops out along the Flumendosa antiform and consists of Porfiroidi auct. (Middle Ordovician), metasandstone, metavolcanics, Upper Ordovician-Silurian sedimentary succession and Devonian limestones (Carmignani et al., 2001). In the Monte Grighini area, the Gerrei Unit is made up of porphiroids, phyllites and mylonitic granitoids outcropping in the south-west of the complex (Musumeci 1991). This Unit has been affected by polyphasic Variscan tectonic events (D1,D2) followed by a late D3 shear deformation. D1 gives rise to NW-SE isoclinals folds with sub-horizontal axial plane schistosity (S1). D2 event causes prevailing simple shear deformation which results in structure and microstructure related to NE-SW compressional event (Carosi et al., 1990). The Castello Medusa Unit crops out in tectonic windows along the Flumendosa antiform. It consists of a sedimentary succession made up of metasandstones to shales and limestones of Cambrian-Carboniferous age. The Monte Grighini Unit (Lower Unit), located in the north-east side of the complex, shows the higher metamorphic grade represented by mostly mylonitic micaschist and gneiss of amphibolite facies. In this Unit, a D2 deformational event gave rise to a pervasive S2 schistosity deforming previous D1 structures and transposing the S1 schistosity. Monte Grighini Unit is also intruded by a suite of granitoids, mostly monzogranite and leucogranite (Musumeci 1992). Rb/Sr and Ar/Ar radiometric data give an age of about 305-300 Ma (Carmignani et al., 1987; Laurenzi et al., 1991). Along the southern side, the granitoids are overlained by Upper Unit with the contact marked by a cataclastic fault zone. The leucogranite can be found mostly close to the cataclastic fault zone. Granitoids follow the regional trend (NW-SE) and show a pervasive subvertical schistosity and mineralogical lineation related to the late D3 deformation event. Several outcrops show a rapid change in the intensity of mylonitic deformation. One of these has been investigated in some detail. The studied outcrop in 1.5 metres shows a transition from slightly to strongly deformed leucogranite. On the basis of granulometric, chromatic and schistosity morphology, five layers have been identified from each of which were taken some samples. The layer subdivision follow the pervasive foliation that affects the entire outcrop with N162°-N170° direction and 60°-65° W dipping. The firs layer (L1) consists of about 40 cm of massive, coarse-grained, leucocratic rock with just a mild foliation and a moderate alteration grade. At the outcrop scale, it is possible to recognize plagioclase and K-feldspar phenocrysts and dark biotite layers parallel to the main foliation. Moving toward NW there is a sharp transition to a thin (4 cm) strongly foliated, very fine-grained rock layer (L2). This layer consists of very thin dark- and light-colored alternating levels. Layer L3, 10 cm in thickness, is similar to the previous one but shows a more spaced foliation and a coarser grain size. The color becomes darker than L2, due to millimetric trails and lenses of mafic minerals (biotite). In this layer the foliation envelopes the millimetric crystals aggregates. Layer L4 (70 cm thick) is light-colored as compared to layer L3 and the alternating levels appear to be thicker. In this Layer grain size increases again, several minerals can be observed at the naked eye and foliation appears less pervasive than the previous layer L3. In Layer 5, (30 cm thick) millimetric-thick very fine grained levels alternate with prevailing medium grained ones. Coarse-grained portion appear as a microcrystalline matrix with lens and levels of mafic minerals. Several thin sections has been realized for each level for microstructural, mineralogical, and petrographic analyses. Mylonitic granitoid consists of the following paragenesis: quartz, k-felspar, plagioclase, muscovite, biotite, iron oxides and ± tourmaline, ± apatite, ± zircon, ± epidote. Almost all samples are characterized by k-feldspar phenocrysts within a quartz + phyllosilicate-rich matrix. k-felspar porphyroclasts sometimes show antithetic microfaults. Generally quartz has typical fabric of dynamic recrystallisation: relics of large old quartz grain pass laterally into domains of recrystallised small grains (50- 100 μm). L1 is characterized by k-feldspar, quartz and plagioclase phenocrysts, floating in a matrix of microcrystalline quartz and phillosilicate-rich levels (protomylonite). k-feldspar phenocrysts are generally fractured and sometimes show albite exsolutions (perthite). Quartz occurs mostly as microcrystalline lenses and bands which follow the weak foliation. Plagioclase range in composition between albite and oligoclase. Phyllosilicates (largely biotite and muscovite) occur in thin trails and sometimes submillmetric crystals arranged along the foliation. Towards the ultramylonitic zone, muscovite content decreases up to its almost complete disappearance. Based on its average porphyroclast/matrix ratio of about 60/40, the rock can be classified as protomylonite (Higgins, 1971). Layer 2 is characterized by a strong shift to ultramylonitic conditions with a porphyroclast/matrix ratio of 2/98. In this layer it is not possible to distinguish any mineral except some strongly fractured k-feldspar relics. In Layer 3 porphyroclast/matrix ratio decreases up to 30/70 whereas the grain size increase. This sample can be classified as mylonite. In this layer clusters of euhedral tourmaline (?) crystals not following the pervasive rock foliation have been found. Muscovite has been rarely found. In layer L4 the texture is again protomylonitic, characterized by increasing grain size and phenocrists abundance (60/40 porphyroclast/matrix ratio); micas are well visible in these samples with a local intergrowth of biotite and muscovite. Layer L5 again shows that porphyroclast/matrix ratio varying from 40/60 to 25/75 therefore it can be classified as mylonite. Fractures are filled by euhedral and microcrystalline quartz. In this levels locally occurs millimetric ultramylonitic trails. Field observations and petrographic analyses suggest that protomylonite, mylonite and ultramylonite repeteadly alternate at the centrimetric to decimetric scale.
TRANSITION FROM PROTOMYLONITE TO ULTRAMYLONITE IN THE VARISCAN LEUCOGRANITE FROM MONTE GRIGHINI COMPLEX, WEST-CENTRAL SARDINIA: PRELIMINARY PETROGRAPHIC AND PETROPHYSICAL DATA
Fancello D;COLUMBU, STEFANO
2011-01-01
Abstract
Monte Grighini Complex is a NW-SE elongated dome-shaped metamorphic complex which outcrops in central Sardinia in the eastern side of Campidano graben. This Complex consists of a pile of variscan Nappe. From top to bottom they are: Gerrei Unit, Castello Medusa Unit, M. Grighini Unit. Gerrei Unit (Upper Unit) crops out along the Flumendosa antiform and consists of Porfiroidi auct. (Middle Ordovician), metasandstone, metavolcanics, Upper Ordovician-Silurian sedimentary succession and Devonian limestones (Carmignani et al., 2001). In the Monte Grighini area, the Gerrei Unit is made up of porphiroids, phyllites and mylonitic granitoids outcropping in the south-west of the complex (Musumeci 1991). This Unit has been affected by polyphasic Variscan tectonic events (D1,D2) followed by a late D3 shear deformation. D1 gives rise to NW-SE isoclinals folds with sub-horizontal axial plane schistosity (S1). D2 event causes prevailing simple shear deformation which results in structure and microstructure related to NE-SW compressional event (Carosi et al., 1990). The Castello Medusa Unit crops out in tectonic windows along the Flumendosa antiform. It consists of a sedimentary succession made up of metasandstones to shales and limestones of Cambrian-Carboniferous age. The Monte Grighini Unit (Lower Unit), located in the north-east side of the complex, shows the higher metamorphic grade represented by mostly mylonitic micaschist and gneiss of amphibolite facies. In this Unit, a D2 deformational event gave rise to a pervasive S2 schistosity deforming previous D1 structures and transposing the S1 schistosity. Monte Grighini Unit is also intruded by a suite of granitoids, mostly monzogranite and leucogranite (Musumeci 1992). Rb/Sr and Ar/Ar radiometric data give an age of about 305-300 Ma (Carmignani et al., 1987; Laurenzi et al., 1991). Along the southern side, the granitoids are overlained by Upper Unit with the contact marked by a cataclastic fault zone. The leucogranite can be found mostly close to the cataclastic fault zone. Granitoids follow the regional trend (NW-SE) and show a pervasive subvertical schistosity and mineralogical lineation related to the late D3 deformation event. Several outcrops show a rapid change in the intensity of mylonitic deformation. One of these has been investigated in some detail. The studied outcrop in 1.5 metres shows a transition from slightly to strongly deformed leucogranite. On the basis of granulometric, chromatic and schistosity morphology, five layers have been identified from each of which were taken some samples. The layer subdivision follow the pervasive foliation that affects the entire outcrop with N162°-N170° direction and 60°-65° W dipping. The firs layer (L1) consists of about 40 cm of massive, coarse-grained, leucocratic rock with just a mild foliation and a moderate alteration grade. At the outcrop scale, it is possible to recognize plagioclase and K-feldspar phenocrysts and dark biotite layers parallel to the main foliation. Moving toward NW there is a sharp transition to a thin (4 cm) strongly foliated, very fine-grained rock layer (L2). This layer consists of very thin dark- and light-colored alternating levels. Layer L3, 10 cm in thickness, is similar to the previous one but shows a more spaced foliation and a coarser grain size. The color becomes darker than L2, due to millimetric trails and lenses of mafic minerals (biotite). In this layer the foliation envelopes the millimetric crystals aggregates. Layer L4 (70 cm thick) is light-colored as compared to layer L3 and the alternating levels appear to be thicker. In this Layer grain size increases again, several minerals can be observed at the naked eye and foliation appears less pervasive than the previous layer L3. In Layer 5, (30 cm thick) millimetric-thick very fine grained levels alternate with prevailing medium grained ones. Coarse-grained portion appear as a microcrystalline matrix with lens and levels of mafic minerals. Several thin sections has been realized for each level for microstructural, mineralogical, and petrographic analyses. Mylonitic granitoid consists of the following paragenesis: quartz, k-felspar, plagioclase, muscovite, biotite, iron oxides and ± tourmaline, ± apatite, ± zircon, ± epidote. Almost all samples are characterized by k-feldspar phenocrysts within a quartz + phyllosilicate-rich matrix. k-felspar porphyroclasts sometimes show antithetic microfaults. Generally quartz has typical fabric of dynamic recrystallisation: relics of large old quartz grain pass laterally into domains of recrystallised small grains (50- 100 μm). L1 is characterized by k-feldspar, quartz and plagioclase phenocrysts, floating in a matrix of microcrystalline quartz and phillosilicate-rich levels (protomylonite). k-feldspar phenocrysts are generally fractured and sometimes show albite exsolutions (perthite). Quartz occurs mostly as microcrystalline lenses and bands which follow the weak foliation. Plagioclase range in composition between albite and oligoclase. Phyllosilicates (largely biotite and muscovite) occur in thin trails and sometimes submillmetric crystals arranged along the foliation. Towards the ultramylonitic zone, muscovite content decreases up to its almost complete disappearance. Based on its average porphyroclast/matrix ratio of about 60/40, the rock can be classified as protomylonite (Higgins, 1971). Layer 2 is characterized by a strong shift to ultramylonitic conditions with a porphyroclast/matrix ratio of 2/98. In this layer it is not possible to distinguish any mineral except some strongly fractured k-feldspar relics. In Layer 3 porphyroclast/matrix ratio decreases up to 30/70 whereas the grain size increase. This sample can be classified as mylonite. In this layer clusters of euhedral tourmaline (?) crystals not following the pervasive rock foliation have been found. Muscovite has been rarely found. In layer L4 the texture is again protomylonitic, characterized by increasing grain size and phenocrists abundance (60/40 porphyroclast/matrix ratio); micas are well visible in these samples with a local intergrowth of biotite and muscovite. Layer L5 again shows that porphyroclast/matrix ratio varying from 40/60 to 25/75 therefore it can be classified as mylonite. Fractures are filled by euhedral and microcrystalline quartz. In this levels locally occurs millimetric ultramylonitic trails. Field observations and petrographic analyses suggest that protomylonite, mylonite and ultramylonite repeteadly alternate at the centrimetric to decimetric scale.
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