Three-dimensional geometric analysis of a fold in a garnet-bearing schist: significance for fold mechanisms and porphyroblast timing and behaviour during fold evolution
N. E. Timms
Department of Earth Sciences, University of Liverpool, Liverpool, L69 3GP, UK
n.e.timms@liverpool.ac.uk
Detailed 3-D analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand-sample scale, Ramsay type 2 (‘similar’), 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 on each limb of the fold. Garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural and chemical discontinuities. Garnet porphyroblasts hosted completely in quartz-poor domains are idioblastic, and those hosted in narrow quartz-poor domains between quartz-rich ribbons are tabular.
Inclusion trail surfaces in cores of idioblastic garnet define a pre-existing but very open fold with an oblique axis to that of the main fold, whereas the equivalent inclusion trail surfaces in the tabular garnet porphyroblasts define a tighter fold with an axis oblique to that of the main fold. Foliation inflection/intersection axes preserved within porphyroblasts (FIAs) trend NNE-SSW in the garnet cores, and NE-SW in the garnet rims on each limb, and are oblique to the main fold axis. The main fold formed after garnet porphyroblast growth.
The consistent preservation of linear inclusion microstructures in the idioblastic garnet porphyroblasts across the fold is strong evidence that porphyroblasts hosted in quartz-poor zones have not rotated significantly relative to each other, or the fold axis during subsequent folding. The data is incompatible with current models of tangential longitudinal strain, flexural flow, axial slip, or pure homogeneous shear acting together or alone in these zones. However, the tighter fold defined by inclusion trails in the tabular garnet porphyroblasts indicates that these porphyroblasts have rotated approximately relative to the main fold axis. Layer-parallel shearing was concentrated at the interfaces between quartz-rich and quartz poor layers in the matrix. In these domains, some of the tabular garnet porphyroblasts spun with the main fold limbs.
The fold formed by progressive bulk inhomogeneous shortening and development of a crenulation cleavage, especially in the quartz-poor layers, followed by progressive reactivation parallel to the folded layering, especially adjacent to the 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.