Correlación entre parámetros químicos, cristalográficos y espectroscópicos en la termometría de grafito aplicada a una aureola de contacto del monzogranito de La Soledad (Andes venezolanos)

Abstract

Graphite samples from a metamorphic contact aureole between phyllites of the Cerro Azul Association (Palaeozoic) and La Soledad Monzogranite, in the Venezuelan Andes, were studied by chemical (% inorganic carbon and isotopic distribution), crystallographic (DRX) and spectroscopic (Raman) techniques in order to assess changes in the graphite in the vicinity of the contact, the correlation between the different parameters, and the determination of the higher temperature reached by the host rock during igneous intrusion. The δ13C reached less negative values near the monzogranite, caused by devolatilization; the graphite present just in contact with the pluton experienced retrograde recrystallization, which causes a shift towards more negative values. The calculated degree of graphitization intervals (GD = 53–80) corresponds to a well-structured mineral with ordered packaging. The peak metamorphic temperature at the contact was calculated from crystallographic (XRD) and spectroscopic (Raman) parameters with great agreement in both techniques, registering the 528 ± 16 and 526 ± 20 oC respectively. The metapelitic rocks reached the Cordierite Zone (cordierite + biotite + muscovite) in the contact aureole where the graphite is well ordered and in hexagonal microtexture. Factors such as fluid activity and the subsequent retrograde recrystallization have an effect on isotopic redistributions after the intrusive event, as well as on the crystallinity change rate with the temperature, avoiding a clear correlation between the isotopic variations of 13C in graphite and the temperature.