Journal of Petrology
Antonio Garcia-Casco, Rafael Luis Torres-Roldan
ABSTRACT
Medium-grade metapelites from the Torrox unit (Western Alpujarrides, Betic Cordilleras)
provide an example of the phase relationships to be expected from extensional
collapse of thickened crust. The rocks contain assemblages formed by combinations
of St-Grt plus or minus Ky plus or minus Sil-And, and are characterized by a
marked state of textural and compositional disequilibrium that originated during
post-peak, near-isothermal decompression at moderately high temperatures (from
>10 to 2-3 kbar, at 550-650 degrees C). Major disequilibrium features include
abundant reaction textures, metastable coexistence of the three A1s polymorphs,
wide heterogeneities in the composition of phases within single samples and
unsystematic/abnormal Mg-Fe partitioning among biotite, garnet and staurolite.
Reaction relations and compositional trends of phases can be shown to be consistent
with expectations from model systems, however, indicating that disequilibrium
was a consequence of reaction overstepping along the isothermal decompression
path. Ensuing fast, near-isobaric cooling abruptly terminated metamorphism,
and allowed for the preservation of disequilibrium features after decompression.
Given that the rocks decompressed at relatively high temperature, the occurence
of reaction overstepping suggests that decompression proceeded at a very fast
rate, consistent with a bulk exhumation velocity in the range of 5-10 km/Ma,
as estimated from available radiometric data for this stage.
Keywords
Decompression;disequilibrium;extensional collapse;medium-grade metapelites;reaction
overstepping
Return to header
Current Contents
................
J. Petrology Editor's Page
This page maintained by P Stuart and
J Rosenbaum
©1996 last updated 15 May 1996