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Contact aureoles as constraints on regional P‐T trajectories: an example from the Northern Alabama Piedmont, USA
Author(s) -
GIBSON RICHARD G.,
SPEER J. A.
Publication year - 1986
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1986.tb00352.x
Subject(s) - pluton , metamorphism , geology , metamorphic rock , geochemistry , mineral , mantle (geology) , mineralogy , paleontology , tectonics , chemistry , organic chemistry
Contact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphic P‐T trajectories, thereby allowing reconstruction of more complete P‐T loops than would be otherwise possible. In the Northern Alabama Piedmont of the southern Appalachians, Wedowee Group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the Blakes Ferry Trondhjemite. Textural evidence indicates that an early bt+st+grt assemblage was replaced by bt+chl+grt during the latter stages of regional metamorphism. Changes in AKFM topology, complex Fe‐Mg‐Ca garnet zoning, and the latestage appearance of epidote indicate that a sequence of continuous reactions (bt+st = grt+ ms followed by chl+ms+Ca‐grt+Ca‐pl=bt+ (Fe+Mg)‐grt+ep) occurred in response to increasing pressure and resulted in the observed changes in mineral assemblage. Pl‐ms‐bt‐grt thermobarometry indicates conditions of 580° 65°C, 8.5±0.8 kbar for equilibration of grt+ bt+chl. Pluton emplacement, subsequent to penetrative deformation, caused textural annealing and mineral re‐equilibration by the continuous reaction bt+(Fe+Mg)‐grt+ep = chl+ms+Ca‐grt+Ca‐pl within 50 m of the pluton. Conditions of 510±65°C, 5‐7 kbar are inferred. A reconstructed P‐T trajectory for this area is characterized by (1) early moderate‐ T , moderate‐ P metamorphism; (2) an increase in P to approximately 8.5 kbar; and (3) decompression and slight cooling prior to pluton emplacement. The compressional phase of this path is interpreted to result from underthrusting of the Wedowee metasediments to mid‐crustal levels during Palaeozoic crustal thickening. Late‐stage decompression prior to intrusion records uplift of these rocks in response to movement on structurally lower thrusts.