PETROGRAPHIC EVIDENCE FOR POLYMETAMORPHISM IN THE CLASSIC BARROVIAN METAMORPHIC SEQUENCE OF SOUTHEAST NEW YORK

Preliminary field and petrographic analysis of rocks across the classic Taconic metamorphic gradient in Dutchess County from chlorite to sillimanite grade allow us to establish the relationship between fabric development and porphyroblast growth. The dominant fabric Sn, dips to the SE and is axial planar to folds in Sn-1 (S1/S0). In the chlorite zone it forms a discrete (stylolitic) cleavage, but at higher grades it is defined by muscovite and lesser biotite truncating S1. Sn in turn is truncated by randomly oriented porphyroblasts of biotite, garnet + biotite +/- chloritoid, and staurolite +/- kyanite +/- sillimanite at appropriate metamorphic grades. Sn and its late porphyroblasts together apparently constitute M1. Overprinting this is Sn+1 defined by muscovite + biotite that deforms these porphyroblasts and crenulates Sn. Overprinting these fabrics and minerals is a second generation of randomly oriented biotite, garnet, and staurolite porphyroblasts (apparently defining a second thermal pulse or metamorphism, M2). In higher-grade rocks these textures and structures are overprinted by a low-grade fabric Sn+2 that dips gently to the south-southeast. This last schistosity is defined by muscovite + chlorite that truncates Sn and Sn+1 and wraps around all earlier porphyroblasts. Finally, randomly oriented chlorite porphyroblasts can be found cutting all fabrics within the biotite to higher-grade rocks. Clearly this classic Barrovian sequence is more complicated than previously recognized, with either (1) alternating pulses of deformation and heating and cooling in the Taconic orogeny, or (2) by the poly-metamorphic effects of the Taconic, (Salinic?), Acadian, and possibly even the Alleghanian orogenies.