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Shearing of K-feldspar Megacryst: from Magmatic to Brittle Deformation

 

Roberto Weinberg
School of Geosciences, Monash University, Australia

 

 

 

 

 

This page compares the deformation of K-feldspar megacrysts in granite in different conditions, from syn-magmatic deformation in the Guojialing pluton, in the Shandong Province in China, to ductile deformation in the Copacabana Range in Catamarca Province in Argentina, to brittle deformation in the Quebrada de Belen shear zone also in Catamarca.

 

K-feldspar Megacryst Deformed During Melting: Zanskar Shear Zone, Himalaya

sense of shear sense of shear
a) K-feldspar porphyroclast with asymmetric tails of leucosomes indicative of top to the left. Interpretation: normal shearing during melting (water-fluxed melting, no peritectic minerals). Note truncation of layering across a diagonal plane (from lower right to upper left) indicative of movement along that plane. Photo parallel to lineation and perpendicular to foliation. b) Example of K-feldspar porphyroclast with asymmetric leucosome tails indicative of top to the left shearing in the presence of melt. Photo parallel to lineation and perpendicular to foliation.

Deformation During Granite Crystallization, Guojialing Pluton, Shandong Province, China

Megacrysts of K-feldspar Megacrysts of K-feldspar
a) Sigma-shaped K-feldspar megacryst with tails of felsic granite. Three K-feldspar crystals in the central part are linked by a felsic matrix band indicative of extension between crystals during solidification and seggregation of magma into the necks. Asymmetry of individual crystals and of the combined object indicates top-to-right movement sense. b) Tiled and deformed K-feldspar sigma clasts (with length parallel to S-plane), indicative of top-to-right movement sense. Note darker and lighter bands throughout the rock. Central megacryst has been broken and fracture was filled with magma indicating syn-magmatic fracturing. Note the bands richer in mafic minerals deflecting above and below the central, fractured crystal, and bands of more felsic granite in the shadows of the megacrysts also indicative of syn-magmatic deformation.
Megacrysts of K-feldspar
c) Typical general appearance of flow banding: felsic bands are associated with aligned K-feldspar megacrysts, mafic bands develop in between.

K-feldspar Deformation During Ductile Shearing, Sierra de Copacabana, Catamarca Province, Argentina

antithetic fracturing synthetic fracturing
d) Sheared K-feldspar megacrysts, top to the left shear sense. . e) Sheared K-feldspar megacrysts, top to the left shear sense.
antithetic fracturing Tensile cracks
f) Constrasting behaviour in Copacabana. K-feldspar megacrysts in sheared pegmatitic band. The one in the centre-left is not visibly deformed while the matrix is strongly foliated, whereas the megacryst on the right is fractured. Plane of outcrop is at high angle to the stretching lineation. g) Rounded megacryst looking down plunge of the stretching lineation.

Brittle Deformation of K-feldspar Megacrysts in Belen Shear Zone, Catamarca Province, Argentina

antithetic fracturing synthetic fracturing
h) Sheared megacryst within the Belen Shear Zone. . i) W-side-up (left up) on 260/sv. Sheared Kfs megacryst, synthetic microfaults.
antithetic fracturing Tensile cracks
j) E-side up, antithetic and synthetic microfaults in K-feldspar megacrysts. . . k) Tensile cracks in K-feldspar megacryst.
Hourglass megacryst Hourglass megacryst
l) Hourglass phenocrysts of K-feldspar with no evident internal fractures. m) Hourglass megacrysts of K-feldspar. E-side-up (right-side-up) shear sense and fractures in Kfs are synthetic.