The development of spiral-shaped inclusion trails during multiple metamorphism and folding T. H. BELL AND A. CHEN School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia (tim.bell@jcu.edu.au) ABSTRACT Three periods of mineral growth and three generations of spiral-shaped inclusion trails have been distinguished within folded rocks of the Qinling-Dabie Orogen, China, using the development of three successive and differently trending sets of foliation intersection axes preserved in porphyroblasts (FIAs). This progression is revealed by the consistent relative sequence of changes in FIA trends from the core to rim of garnet porphyroblasts in samples with multiple FIAs. The first and second formed sets of FIAs trend oblique to the axial planes of macroscopic folds that dominate the outcrop pattern in this region. The porphyroblasts containing these FIAs grew prior to the development of the macroscopic folds, yet the FIAs do not change orientation across the fold hinges. The youngest formed FIAs (set 3) lie subparallel to the axial planes of these folds and the porphyroblasts containing these FIAs formed in part as the folds developed. The deformation associated with all three generations of spiral-shaped inclusion trails in garnet porphyroblasts involved the formation of subhorizontal and subvertical foliations against porphyroblast rims accompanied by periods of garnet growth; pervasive structures have not necessarily formed in the matrix away from the porphyroblasts. The macroscopic folds are heterogeneously strained from limb to limb, doubly plunging and have moderately dipping axial planes. The consistent orien- tation of Set 1 FIAs indicates that the development of spiral-shaped inclusion trails in porphyro- blasts with FIAs belonging to Set 2 did not involve rotation of the previously formed porphyroblasts. The consistent orientation of Sets 1 and 2 FIAs indicate that the development of spiral-shaped inclusion trails in porphyroblasts with FIAs belonging to Set 3 did not involve rotation of the pre- viously formed porphyroblasts during folding. This requires a fold mechanism of progressive bulk inhomogeneous shortening and demonstrates that spiral-shaped inclusion trails can form outside of shear zones. Key words: FIAs; fold mechanisms; foliation development; heterogeneous strain. INTRODUCTION Quantitative measurement of the orientations of the axes of sigmoidal or spiral-shaped inclusion trails in porphyroblasts around folds only began in earnest in the last decade of the 20th Century. It has been sug- gested that spiral-shaped inclusion trails form during large-scale shearing, such as that which might accom- pany nappe development (e.g. Rosenfeld, 1968), even though they are found in porphyroblasts in multiply deformed rocks. Further, Williams & Jiang (1999) argued that spiral inclusion trails may only form within shear zones. However, inclusion trail geometries from a large-scale shear zone with a simple history that lacks evidence for multiple deformation events, show that garnet porphyroblasts, which grew during the development of the shear zone, contain inclusion trails that are predominantly straight and constantly oriented throughout the zone (Jung et al., 1999). Since the development of techniques for routine measurement of the 3-D orientation of the axes of sigmoid, staircase and spiral-shaped inclusion trails (Hayward, 1990; Bell et al., 1995) there has been a decade of measurement of foliation intersection axes preserved in porphyroblasts (FIAs). These data allow examination of how porphyroblasts behave around folds. Such inclusion trail geometries may be explained by variation in the timing of porphyroblast growth relative to folding (Bell & Forde, 1995), or by fold models that involve rotation or nonrotation of the porphyroblasts (e.g. Stallard & Hickey, 2001a). Deformation is inherently heterogeneous (Bell, 1981) making structural and metamorphic correlation dif- ficult over any distance along or across an orogen. FIAs have been measured around doubly plunging and heterogeneously strained folds (Bell & Hickey, 1997), across large scale shear zones, and along and across orogens (Bell & Mares, 1999). These data allow periods of porphyroblast growth and foliation development to be recognized that cannot be distinguished in other ways, and these features have been correlated with directions of tectonic shortening (e.g. Bell et al., 1998). Thus FIA data provide a quantitative way of integrat- ing the metamorphic, structural and tectonic history of rocks within orogens and testing whether long held concepts of deformation and metamorphism are J. metamorphic Geol., 2002, 20, 397–412 # Blackwell Science Inc., 0263-4929/02/$15.00 397 Journal of Metamorphic Geology, Volume 20, Number 4, 2002