P-T-time characterization of the Trans-Amazonian Orogeny in the Imataca Complex, Venezuela

Metaigneous and metasedimentary rocks from the Imataca Complex of Venezuela record high temperature decompression at granulite facies peak metamorphism. Preservation of convincing reaction assemblages in many samples allows determination of relative timing of development of observed assemblages. Sillimanite pseudomorphs after andalusite and absence of kyanite requires a high temperature /pressure (T/P) prograde geothermal gradient. Two-pyroxene thermometry and garnet-orthopyroxene-plagioclase-quartz assemblages yield peak T and P in the range of 750-800°C and 8-8.5 kbar. One sample of garnet-bearing meta-norite contains orthopyroxene with approximately 6% Al₂O₃, requiring either higher peak pressures or disequilibrium growth of orthopyroxene from garnet. Breakdown of garnet + sillimanite to hercynite + quartz and of garnet + sillimanite + quartz to cordierite require decompression at high temperature; mineral compositions in these assemblages indicate P near 6 kbar for T in the range of 500-700°C. Coexisting orthopyroxene + clinopyroxene + plagioclase + quartz apparently react to form abundant hornblende and strongly zoned plagioclase with An₇₀ cores and An₃₅ margins. Garnet is also compositionally zoned in some samples. Finally, alkali feldspar and plagioclase coexist in some samples without perthitic intergrowths, and these retrograded feldspars yield temperatures near 550°C. Rhombohedral oxides are extensively and complexly exsolved in some samples; exsolution products include ilmenite, hercynite and corundum. In other samples, exsolution of oxides has apparently been complete, and only homogeneous, discrete grains of rhombohedral and cubic oxides exist. These completely exsolved oxides yield apparent retrograde temperatures below 600°C. These observations indicate that the Imataca Complex reached granulite facies metamorphic conditions along a high T/P gradient, followed closely by tectonic loading. Subsequent decompression and slow cooling allowed significant progress along several retrograde reactions, as well as equilibration of feldspar compositions and extensive exsolution in oxide phases. The observed compositional zoning in garnet and plagioclase developed during retrograde reactions. A one-dimensional heat flow model constrained by Rb-Sr and ⁴⁰Ar/³⁹Ar cooling ages suggests the Imataca Complex was uplifted from 32 to 16 km at a rate of 0.5-1 km Ma¹ after which isobaric cooling ensued. The P-T path given by this model is consistent with the retrograde assemblages observed.