THE CONTROL OF DEFORMATION PARTITIONING ON THE LOCATION AND TIMING OF METAMORPHIC REACTIONS

The matrix preserves little memory of the extended deformation and metamorphic history that most rocks within orogens have undergone because bedding is generally reactivated during younger deformations, and this destroys (via decrenulation) or rotates younger foliations into parallelism with S0. Porphyroblasts preserve evidence of this history, but have been regarded as showing the remains of a foliation that was formed only 1 or 2 deformations prior to that seen in the matrix. We have found that the memory of successive deformations and metamorphic reactions recorded by porphyroblasts is an order of magnitude more extensive than this.

Foliation intersection axes preserved in porphyroblasts (FIAs) reveal a consistent succession of 5 FIAs with constant trends across a large tract of Acadian Vermont. FIA sets represent periods of time over which the direction of bulk shortening remained constant during orogenesis. This extensive history has been confirmed by absolute dating of monazite grains within the foliations defining the FIAs. They reveal a progression in foliation ages from 431±2 to 349±3 Ma within porphyroblasts and from 366±3 to 327±5 Ma in pervasive matrix foliations. The last 4 of the 5 FIA sets began forming prior to 424±3, 405±6, 386±6 and 366±4 Ma, respectively.

Significantly, these ages also reveal greater than 75 million years of successive metamorphic reactions from which the porphyroblasts have formed. What controls whether a reaction takes place at a particular location, once the T and P and bulk composition are appropriate, is whether the deformation partitions through that location at the scale of a porphyroblast. This control is so intimate that porphyroblasts stop and start growing episodically at different times in the same compositional horizon on opposite limbs of regional folds and even locally on mesoscopic folds. Furthermore, large-scale competency contrasts can affect regional partitioning of deformation and thus the localization of reaction. We demonstate this using pseudosections on which we correlate reactions using the succession of FIAs.