1 A Method of Determining Early Deformation and Metamorphism History in Orogenic Belts Porphyroblast inclusion trails from orogenic belts represent early-formed foliations composed of oriented metamorphic minerals preserved in rocks in the processes of porphyroblast growth (i.e. progressive and retrogressive metamorphism). Since they were studied in the 1920s, such inclusion trails have been recognized as products of syntectonic rotation due to progressive ductile deformation, which may be used as a known
Spiral garnet porphyroblasts are known to record lengthy periods of deformation and metamorphism by preserving single or multiple FIAs(Foliation Intersection Axis)formed normal to tectonic shortening directions.Thanks to technological advances in X-ray computed micro-tomography(XCMT),FIAs can now be readily determined in relatively large samples in contrast to previous methods that require the preparation of a set of radial vertical and horizontal thin sections of samples.XCMT scanning not only alleviates tedious thin section based procedures but also illuminates the complete internal architecture of a rock sample allowing three-dimensional(3D)quantitative shape analysis of an individual porphyroblast as well as precise measurement of FIAs.We applied the technique to a sample from the Hunza Valley in the Karakoram metamorphic complex(KMC),NW Himalayas,containing numerous garnet porphyroblasts with spiral-shaped inclusion trails.The XCMT imaging reveals an E-W trending FIA within the sample,which is consistent with orthogonal N-S collision of the India-Kohistan Island Arc with Asia.Garnet long axes(X_(GT))have variable plunges that define a broad sub-vertical maximum and a small sub-horizontal maximum.The X_(GT) principle maxima lie at N-090 and N-120.Smaller maxima lie at N-020 and N-340.Geometric relationships between X_(GT) axes and FIA orientation in the sample suggest that porphyroblast shapes are controlled by the geometry of the lens-shaped microlithons in which they tend to nucleate and grow.The orientation of inclusion trails and matrix foliations in the sample are correlated with three discrete tectono-metamorphic events that respectively produced andalusite,sillimanite and kyanite in the KMC.Late staurolite growth in the sample reveals how the rocks extruded to the surface via a significant role of roll-on tectonics,which can be correlated with the Central Himalayas.
A succession of 5 FIA trends（foliation intersection or inflection axes in porphyroblasts） preserved in high temperature-low pressure regime PreCambrian rocks in the Texas Creek, Arkansas River region reflected by the fold axial plane traces and schistosity data in this region. Similar fold axial plane trace data measured in Palaeozoic rocks in Chester Dome, Vermont, which is high temperature to medium pressure regime, only preserve the effects of the youngest FIAs of the all 5 FIA sets that obtained in this region. The other three FIA sets have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks has tended to flatten all but the very largest early-formed structures, preserving only those folds that were more recently developed.
