Rotated staurolite porphyroblasts in the Littleton Schist at Bolton, Connecticut, USA

Abstract

Staurolite porphyroblasts, 1.5-8cm in length and 0.3-2cm in width, in the Littleton Schist at Bolton, Connecticut, contain curved quartz inclusion trails which document synkinematic rotations of at least 135". The orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation. Serial sections of four differently orientated crystals and U-stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical 'symmetry axis' characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are, in all cases, parallel to the long axis of the porphyroblast. The cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c-axis section through the initial core of a crystal. The amount of rotation about the c-axis decreases linearly along the length of the crystal away from the nucleation site.The sense and amount of rotation recorded by a porphyroblast is related to its orientation. A tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c-axes. The total rotation reflected by the inclusion trails increases as a function of the angle between the c-axes of the staurolite crystals and the slip direction.Initially random staurolite porphyroblasts rotated during growth, as a consequence of laminar shear in the surrounding viscous matrix. This interpretation is quantitatively consistent with: the staurolite preferred orientation; its coincidence with the apparent slip direction; the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast; and the twisted conical shape of the family of surfaces defined by the inclusion trails.

Polyphase deformation and garnet growth in pelitic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast-matrix relationship

“…One group of workers (Bell 1985;Bell and Johnson 1989;Bell et al 1992aBell et al , 1992bHickey and Bell 1999) have strongly argued that porphyroblasts do not rotate with respect to an external frame of reference and that even 'snow-ball' type spiral inclusion trails can be reinterpreted as a product of repeated overprinting deformation and transposition of the matrix foliation around a growing and non-rotating porphyroblast. The other group supports the existing and 'traditional' view of rotation of porphyroblasts in non-coaxial flow to explain strongly folded and rotated inclusion trails in porphyroblasts (Schoneveld 1979;Vernon 1988;Busa and Grey 1992;Visser and Mancktelow 1992;Passchier et al 1992;Williams and Jiang 1999;Jiang and Williams 2004). However, there is an inherent ambiguity in the interpretation of spiral inclusions by either rotation and non-rotation models (Johnson 1993a, b).…”

“…Tectonic interpretation of the observed inclusion patterns in porphyroblasts, as discussed above, greatly depends on the orientation of the section planes with respect to the porphyroblast rotation axis and the orientation of the matrix foliation (Powell and Treagus 1970;Busa and Grey 1992). In the case of non-coaxial deformation producing strong monoclinic shape symmetry in the rock, sections normal to the vorticity vector (rotation axis of porphyroblasts, if any) will give the maximum amount of information.…”

“…However, it should be kept in mind that the inclusion trail geometry observed in thin sections is essentially two-dimensional and depends on the position and orientation of the plane of section with respect to the porphyroblast. Great care should be taken to orient the sections at a high angle to the porphyroblast rotation axis and should be cut ideally through the porphyroblast near its centre (Powell and Treagus 1970;Busa and Grey 1992). In areas of repeated deformation and metamorphism, the angular relationship between the inclusion trails preserved in the porphyroblasts (internal schistosity -S i ) and the matrix foliation (external schistosity -S e ) provides key evidences for understanding the nature of the superposed deformational and metamorphic events.…”

Fan‐shaped staurolite in the Bolton syncline, eastern Connecticut: evidence for porphyroblast rotation during growth

“…Centimetre‐sized prismatic staurolite porphyroblasts are common in the pelitic schists of the Littleton Formation in the Bolton syncline of eastern Connecticut. Some of the prismatic staurolite porphyroblasts contain sigmoidal‐shaped inclusion trails, which have been interpreted as evidence for shear‐induced rotation during growth (Rosenfield & Eaton 1985; Busa & Gray, 1992a). On the contrary, Bell et al.…”

Garnet porphyroblast timing and behaviour during fold evolution: implications from a 3‐D geometric analysis of a hand‐sample scale fold in a schist

“…Growth ceases when zones of progressive shear impinge on their margins as the crenulation cleavage develops (Bell, 1985;Bell et al, 1986;. Busa & Gray, 1992;Hickey & Bell, 1999, 2001Stallard & Hickey, 2001;Bell & Chen, 2002). Porphyroblast rotation is inhibited due to the lack of shear coupling with the matrix.…”