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

Abstract

Detailed 3‐D analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand‐sample scale, tight, upright fold has revealed a complex deformation history. The fold, dominated by interlayered quartz–mica schist and quartz‐rich veins, preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica‐rich layers. Garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities. Garnet porphyroblast cores and rims pre‐date the macroscopic fold and preserve successive foliation inflection/intersection axes (FIAs), which have the same trend but opposing plunges on each limb of the fold, and trend NNE–SSW and NE–SW, respectively. The FIAs are oblique to the main fold, which plunges gently to the WSW. Inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica‐rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32°, whereas equivalent surfaces in tabular garnet adjacent to quartz‐rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17°. This potentially could be explained by garnet porphyroblasts that grew over a pre‐existing gentle fold and did not rotate during fold formation, but is more easily explained by rotation of the porphyroblasts during folding. Tabular porphyroblasts adjacent to quartz‐rich layers rotated more relative to the fold axis than those within mica‐rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz‐rich layers. This work highlights the importance of 3‐D geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix.

Extensional v. contractional origin for the southern Menderes shear zone, SW Turkey: tectonic and metamorphic implications

“…SSITs appear so obviously a product of rotation of the porphyroblast within a foliation as progressive shearing occurred that no one considered the possibility that this might not be the case until the 1980s. However, a technique for quantitatively measuring the axes of SSITs was developed (Hayward, 1990;Bell, Forde & Wang, 1995) and a large amount of data has now been published on the orientation of spiral axes around folds (Bell & Hickey, 1997;Hickey & Bell, 2001;Bell & Chen, 2002;Timms, 2004), around oroclines (Bell & Mares, 1999;Yeh & Bell, 2004) and along and across orogens (Bell & Bruce, 2006;Bell, Hickey & Upton, 1998;Bell, Ham & Hickey, 2003, Bell, Ham & Kim, 2004Bell et al 2005;Cihan, 2004;Cihan & Parsons, 2005). He also generated a non-coaxial strain field diagram that mimicked asymmetric crenulation cleavages and straight to slightly sigmoidal inclusion trails that are common in so many porphyroblasts.…”

Tectono-metamorphic history recorded in garnet porphyroblasts: insights from thermodynamic modelling and electron backscatter diffraction analysis of inclusion trails

“…Today, both rotational (Prior, 1987;Chan & Crespi, 1999;Ikeda et al, 2002) and non-rotational (Fyson, 1980;Johnson, 1990;Aerden, 1995) behaviours have been documented in nature. The observed geometries of inclusion trails have been explained, for instance, by flexural flow folding (Visser & Mancktelow, 1992), coaxial deformation associated with passive folding (Ramsay, 1962;Stallard & Hickey, 2001;Timms, 2003), or deformation partitioning of bulk non-coaxial deformation into pure and simple shear components (Bell, 1981;Lister & Williams, 1983).…”

Porphyroblast rotation during crenulation cleavage development: an example from the aureole of the Mooselookmeguntic pluton, Maine, USA

“…Porphyroblasts are therefore of central importance because they are generally the only features in multiple‐deformed metapelites that preserve information about the deformation history beyond what can be extracted from the matrix microstructure (Zwart, 1960, 1962; Vernon, 1978, 1989; Bell et al. , 1986; Williams, 1994; Johnson & Vernon, 1995; Karlstrom & Williams, 1995; Chan & Crespi, 1999; Johnson, 1999; Hickey & Bell, 2001; Holcombe & Little, 2001; Timms, 2003). Of equal importance, porphyroblast microstructures show characteristics that should make them useful for: (1) determining deformation kinematics (Rosenfeld, 1968; Simpson & Schmid, 1983; Bell, 1985; Busa & Gray, 1992; Passchier et al.…”

“…, 1991; Davis, 1993; Morgan et al. , 1998), and (7) investigating folding mechanisms (Ramsay, 1962; Visser & Mancktelow, 1992; Solar & Brown, 1999; Williams & Jiang, 1999; Hickey & Bell, 2001; Stallard & Hickey, 2001; Timms, 2003; Evins, 2005).…”