Reaction progress of clay minerals and carbonaceous matter in a contact metamorphic aureole (Torres del Paine intrusion, Chile)

Abstract

This study reports on reaction processes in a transition zone from contact to regional metamorphism by using Raman spectroscopy on carbonaceous matter (RSCM), illite “crystallinity” (Kübler index, KI), chlorite geothermometry, and thermal modeling. The thermal effect due to the emplacement of the Torres del Paine intrusion (TPI, assembly time of ca. 150 kyr) had different consequences for inorganic and organic compounds of the host rock. The thermal alteration of the pre-intrusive regional metamorphosed host rock is documented by elevated RSCM temperatures, high-temperature chlorite generations, and the appearance of epidote and retrograde Fe-rich chlorite. Microprobe analysis on chlorite indicates incomplete re-equilibration as evidenced by various chlorite populations of individual contact metamorphic samples. This study indicates that the maturity of organic matter is the most reliable and unequivocal indicator on timescales of several thousand years to determine the lateral extension of the TPI contact aureole. Raman geothermometry reveals that the lateral extension of the contact-influenced zone expands up to a distance of 1.5 km and, thus, expands to ca. 1.1 km further out than the macroscopically mappable hornfels contact aureole. The best match between measured (Raman geothermometry) and calculated (thermal modeling) ΔTmax values (ΔT=54 ∘C) is achieved with a total intrusion assembly time of 150 kyr, a magmatic temperature of 800 ∘C, a two-batch model (batch repose time of 10 kyr) with five pulses per batch, short heating durations (3 kyr), and long pulse repose times (15 kyr).