Abstract
Petrographic and geochemical data for mylonites from a metric-scale shear zone in mantle peridotites from the Finero massif (Southern Alps) record large mineralogical and geochemical modifications compared to surrounding coarse-grained ultramafic rocks, which were pervasively deformed in presence of hydrous melts. The mylonites are composed by olivine and orthopyroxene and, less frequently, clinopyroxene, phlogopite, and pargasite porphyroclasts enclosed in a fine-grained matrix of phlogopite and olivine, with subordinate amounts of orthopyroxene, clinopyroxene, pargasite, and chromite. P-T estimates indicate that deformation occurred under granulite- to upper-amphibolite facies conditions. Field relationships and U-Pb dating indicate that the shear zone was active during Lower Jurassic and/or later, in an extensional setting at the western margin of the Adria plate, which led to the opening of the Alpine Tethys. The major and trace element composition of the porphyroclasts in the mylonites significantly differ from those in the hosting coarse-grained ultramafics. Porphyroclasts were chemically active during deformation acting as source (diffusion-out) or sink (diffusion-in) for some trace elements. The chemical modifications were promoted by the interaction with aqueous fluids and the composition varied from mantle- (enriched in Ni, Co, Li, Na, REE, Y, and Sr) to crustal-derived (enriched in Zn, K, Al, Ti, and Fe).
Highlights
Shear zone evolution is commonly associated to bulk and mineral chemistry changes due to fluid–rock interaction [1,2,3], since shear zones provide important conduits for fluid movement through the crust [4] and lithospheric mantle [5,6]
Phlogopite Peridotite unit (Ivrea-Verbano Zone). This shear zone has been the subject of previous microtextural and geochemical studies [1,2,14,15,16], which demonstrated that bulk compositions and mineral chemistry underwent significant changes during shear deformation
This work provides a complete on major, minor, of and element mineral chemistry coarse-grained to sheared peridotites is discussed, taking into account thedata information covering all theharzburgites major lithologies of the Finero
Summary
Shear zone evolution is commonly associated to bulk and mineral chemistry changes due to fluid–rock interaction [1,2,3], since shear zones provide important conduits for fluid movement through the crust [4] and lithospheric mantle [5,6]. The element behavior during deformation has mostly been the subject of research based on bulk chemistry and major-element mineral compositions [1,3,4,5,6,10,11,12,13,14,15]. This shear zone has been the subject of previous microtextural and geochemical studies [1,2,14,15,16], which demonstrated that bulk compositions (major and trace elements) and mineral chemistry (major elements) underwent significant changes during shear deformation.
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