Stratigraphy and structural summary of the Cooma Metamorphic Complex

The Cooma Metmorphic Complex, in southern N.S.W. is an Abukuma‐type facies sequence comprising a series of zones successively denned by chlorite, biotite and knotted schists (andalusite or retrogressed cordierite?) of the Greenschist Facies, and continues via a Transition Zone to sillimanite K‐feldspar gneisses of the Amphi‐bolite Facies. The sequence is developed over some 12 km, about a central anatectic granite with a migmatite halo. The high‐grade area has been intruded by syntectonic phases of the Murrumbidgee Batholith. The alternating psammitic and pelitic meta‐sediments of the sequence have been deformed by two groups of folds. Group 1 consists of subhorizontal F 1 and subvertical F 2, sets of coplanar folds which both have a schistosity (S 2), as their axial surface. Group 2 consists of F 3 folds with a metamorphically differentiated crenulation cleavage (S 3) as axial surface, and F 3’ (related but slightly later folds) having a kink‐like crenulate axial surface structure (S3'). Three successive structural‐metamorphic events are defined by the microfabrics S 2, S 3 and S 3'. The sequence of development of porphyroblasts is related to this sequence of structural events. Syntectonic growth of porphyroblasts is relatively uncommon at Cooma. Instead, each deformation stage appears to have been followed by a stage of post‐tectonic porphyroblast growth. Porphyroblasts of earlier growth‐stages have been partly retrogressed by subsequent deformations. There is evidence that the metamorphic zones of the Cooma Metamorphic Sequence have become progressively narrower with respect to successive deformation events.