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Abstract
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).
Highlights
Magmatic intrusions emplaced in sedimentary basins produce local heat anomalies which affect both organic and inorganic compounds of the host rock
Temperatures > 260 ◦C are considered thermally altered by the Torres del Paine intrusion (TPI)
The impact of the contact thermal alteration of the pre-intrusive regional metamorphosed host rock is documented by elevated Raman temperatures, high-temperature chlorite generations, and the appearance of epidote and retrograde Fe-rich chlorite
Summary
Magmatic intrusions emplaced in sedimentary basins produce local heat anomalies which affect both organic and inorganic compounds of the host rock. Contact metamorphic processes of inorganic material typically include dehydration, decarbonation, and host-rock melting reactions (e.g., Jamtveit et al, 1992) During these processes, clay minerals become less expandable due to a decrease in the occurrence of smectite interlayers (Pytte and Reynolds, 1989; Esposito and Whitney, 1995; Nadeau and Reynolds, 1981; Abad et al, 2014). Pytte and Reynolds (1989) suggested that the reaction is controlled by kinetic rather than equilibrium factors, meaning that changing physical boundary conditions (i.e., temperature, time, and K+ activity) has the potential to disproportionately affect clay mineral compositions They documented a zone of decreased expandability extending into the country rock up to a distance of approximately twice the intrusion thickness
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