Rotation of irregular staurolite porphyroblasts in a simple shear dominated shear zone controlled by initial growth orientation and aspect ratio

In a mica schist unit of the Bossòst dome (Central Pyrenees, Spain) idioblastic interkinematic staurolite porphyroblasts with straight inclusion trails (Si) have rotated with respect to the main schistosity in the same sense by different amounts. Orientation data of 600 porphyroblasts were obtained from planes parallel to stretching lineation (xz), perpendicular to lineation (yz) and parallel to schistosity (xy). Interpenetrating growth twinning resulted in highly irregular shapes with aspect ratios (RA) ranging from 1 to 5. Initial random porphyroblast growth is inferred from variable length axis (LA)–Si angles. Maximum rotation of 120° is observed in blasts with Si oriented orthogonal to the mineral long axis, implying a shear strain of γ = 4.3 in simple shear flow. Absence of back rotation is indicative of dominant simple shear flow (WK ≥ 0.8–0.9). Elongated porphyroblasts (RA > 3) with Si parallel to LA have rotated very little, because they were already in a stable position when shearing commenced. Rotation of staurolite porphyroblasts was possible because decoupling between blasts and matrix was weak and no significant strain partitioning had developed. The wide variety of shapes with different aspect ratios formed by simple blasts and interpenetrating growth twins of staurolite are potential shear strain and vorticity gauges.

Research highlights
► In simple shear dominated flow porphyroblasts can rotate relative to one another.
► Porphyroblast rotation is dependent on aspect ratio and initial orientation.
► Rotated irregularly shaped staurolite can serve as shear strain and vorticity gauges.