Inter-relationships between deformation partitioning, metamorphism and tectonism

摘要

Thrusting from the east loaded the thick Pomfret dome stratigraphic sequence in Vermont to such an extent that by the time the first schistosity had formed it was 20. km deep. This occurred without garnet growth even though rock compositions were ideal for this phase to grow before they reached this depth. The rocks remained at this depth until garnet growth ceased ~. 50. million. years later after 5 periods of FIA development (foliation intersection/inflection axes preserved within porphyroblasts). The first phase of the garnet growth in each sample from the Pomfret dome was overstepped in pressure, nucleating well above the incoming phase boundary for this phase at ~7 kbar for whatever FIA set was the first to develop. This was not the case 45. km S in the Chester dome where a thin stratigraphic sequence overlay a basement high of gneiss. Lateral ramping against this basement thinned the thrust sheet preventing overstepping. Frontal ramping to the WNW had the same effect. The pressure did not increase in both regions to ~. 7. kbars until FIA 2.Approximately 50% of the rocks sampled around the Pomfret dome did not grow garnet during FIA 0. PT pseudosections and overstepped garnet phase boundaries indicate that all would have grown garnet if the bulk composition and PT were the only controlling factors. If metastability alone was a factor the other 50% should have grown garnet during the development of FIA 1. They did not, and this pattern was repeated for FIAs 2 and 3. Why, where and when garnet first grew in this PT overstepped environment was recorded by the inclusion trail geometries in each sample; all grew at the start of crenulation-producing events. The variable partitioning of a succession of differently oriented crenulation deformations through the region from FIA to FIA controlled where garnet growth first occurred. Successive FIAs shifted the bulk shortening direction relative to competent rocks, deforming sites previously protected and protecting others.The NW-SE trend of FIA 0 resulted in most deformation pervasively partitioning NE of the competent gneiss beneath the Chester dome and Green Mountains. Consequently, the bulk of porphyroblast growth within the Pomfret dome region occurred at this time. The effects of NW-SE bulk shortening partitioned through similar amounts of gneiss during FIA 1 generating the same percentage of new sites for garnet growth in both regions. As a result, ~. 60% of all garnet growth within the Pomfret region had occurred before the N-S directed bulk horizontal shortening during FIA 2 began, resulting in an increase in bulk competency. This caused deformation to preferentially partition pervasively through the previously more competent Chester region generating ~35% of all garnet cores plus loading through crustal thickening to a depth of ~. 7. kbar. This increase in pressure prior to FIAs 3 and 4 enabled other rocks with different bulk compositions to grow garnet for the first time during these later periods of orogeny. It also resulted in considerable growth on the rims of garnet porphyroblasts that had grown much earlier during the deformation history.