From Western to Eastern Alps, widespread occurrence of whiteschists and other Mg-metasomatic rocks (e.g. leucophyllites) derived from post-Variscan granitoids has been reported. Firstly considered as formed by isochemical metamorphism of a sedimentary protolith, these rocks are now interpreted as originated by metasomatism of orthogneisses, thought both the timing of metasomatism and origin of metasomatic fluids are still a matter of debate. The common occurrence of these metasomatic rocks testifies for the widespread presence of local processes of Mg-metasomatism in the Alps. Field, petrological, geochemical (bulk-rock chemical composition and stable isotope) and fluid inclusion data previously reported are integrated to compare the genetic processes and the tectonic scenarios responsible for the origin of these rocks. Despite the heterogeneous data, the large range of peak metamorphic conditions (from the lowermost greenschists-facies to the ultrahigh-pressure medium-temperature eclogite facies) and the distinct time of metasomatism (from rifting to exhumation), some common features can be recognized: i) they occur along shear zones within the metagranitoids; ii) they display a simple MgO–Al2O3–SiO2–H2O±K2O-silicate mineralogy; iii) based on major- and trace-element compositions, they can be grouped in four homogeneous Classes, representative for a progressive increase in Mg-metasomatism from the host rock to the centre of the shear zone; iv) from Class 0 (protolith) to Class 3 (Mg-rich rocks) the lithologies show an increase in Mg, Ni, H2O, Fe, Cr, and a decrease in Na, Ca, Sr, P, K, Rb, Si; v) fluid inclusion data, coupled with bulk-rock chemical compositions indicates that the metasomatic fluid was a Ni-Mg-rich brine, maybe containing also Fe and Cr, and possibly undersaturated in Si; vi) δ18O and δD data are indicative for seawater, locally mixed with meteoric water, as source of the metasomatizing fluid. All the data indicate that Mg-metasomatic rocks from the Alps are originated by the same process, that assumes highly channelized fluids (derived from ultramafics that have previously interacted with seawater) that infiltrated the continental crust along high strain zones and produced chromatographic fractionation of major and trace elements. However, distinct peak metamorphic conditions and timing of metasomatism from each locality suggest at least three tectonic scenarios in which this genetic process can occur: rift-related ocean-continent transition, oceanic-continental subduction, and continent-continent collision. In these scenarios, three kinds of ultramafic rocks could have originated the metasomatic fluid: sub-continental ultramafics, oceanic serpentinites, and mantle-wedge ultramafics.
Common genetic processes but different tectonic scenarios for the origin of whiteschists and other continental Mg-metasomatic rocks from the Alps
FERRANDO, Simona;COMPAGNONI, Roberto;CASTELLI, Daniele Carlo Cesare
2011-01-01
Abstract
From Western to Eastern Alps, widespread occurrence of whiteschists and other Mg-metasomatic rocks (e.g. leucophyllites) derived from post-Variscan granitoids has been reported. Firstly considered as formed by isochemical metamorphism of a sedimentary protolith, these rocks are now interpreted as originated by metasomatism of orthogneisses, thought both the timing of metasomatism and origin of metasomatic fluids are still a matter of debate. The common occurrence of these metasomatic rocks testifies for the widespread presence of local processes of Mg-metasomatism in the Alps. Field, petrological, geochemical (bulk-rock chemical composition and stable isotope) and fluid inclusion data previously reported are integrated to compare the genetic processes and the tectonic scenarios responsible for the origin of these rocks. Despite the heterogeneous data, the large range of peak metamorphic conditions (from the lowermost greenschists-facies to the ultrahigh-pressure medium-temperature eclogite facies) and the distinct time of metasomatism (from rifting to exhumation), some common features can be recognized: i) they occur along shear zones within the metagranitoids; ii) they display a simple MgO–Al2O3–SiO2–H2O±K2O-silicate mineralogy; iii) based on major- and trace-element compositions, they can be grouped in four homogeneous Classes, representative for a progressive increase in Mg-metasomatism from the host rock to the centre of the shear zone; iv) from Class 0 (protolith) to Class 3 (Mg-rich rocks) the lithologies show an increase in Mg, Ni, H2O, Fe, Cr, and a decrease in Na, Ca, Sr, P, K, Rb, Si; v) fluid inclusion data, coupled with bulk-rock chemical compositions indicates that the metasomatic fluid was a Ni-Mg-rich brine, maybe containing also Fe and Cr, and possibly undersaturated in Si; vi) δ18O and δD data are indicative for seawater, locally mixed with meteoric water, as source of the metasomatizing fluid. All the data indicate that Mg-metasomatic rocks from the Alps are originated by the same process, that assumes highly channelized fluids (derived from ultramafics that have previously interacted with seawater) that infiltrated the continental crust along high strain zones and produced chromatographic fractionation of major and trace elements. However, distinct peak metamorphic conditions and timing of metasomatism from each locality suggest at least three tectonic scenarios in which this genetic process can occur: rift-related ocean-continent transition, oceanic-continental subduction, and continent-continent collision. In these scenarios, three kinds of ultramafic rocks could have originated the metasomatic fluid: sub-continental ultramafics, oceanic serpentinites, and mantle-wedge ultramafics.
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