This thesis investigates the metamorphic processes occurring in the contact aureole of the Torres del Paine Intrusive Complex (TPIC) in southern Patagonia. The TPIC, emplaced in the upper crust ( 750 bars) between 12.59 and 12.43 Ma, consists of a bimodal intrusion: (1) a felsic granitic laccolith underplated by (2) a series of mafic sills. These intruded into the metasediments of the Cerro Toro and Punta Barrosa formations, which previously experienced anchizonal to greenschist-facies regional metamorphism. The metasediments consist of turbiditic sequences composed of pelites, sandstones, conglomerates, and minor carbonate layers.
The TPIC emplacement generated a narrow contact aureole, whose thickness varies between 400 m at the base and 150–200 m at the top, which is marked by the appearance of cordierite and biotite produced by the breakdown of chlorite. Further towards the intrusion, K-feldspar forms along with additional cordierite as muscovite disappears. Phase petrology calculations, based on bulk rock compo- sitions of the metapelites, predict chlorite and muscovite breakdown at 490◦C and 545◦C, respectively, at 750 bars. The appearance of these mineral reactions with respect to the contact varies spatially around the aureole, influenced by the thickness of the granitic intrusion at different locations.
This thesis uses the well-characterized mineral isograds of the contact aureole, along with the known thermal conditions of the intrusion, to study three key metamorphic processes: (1) Ti incorporation into biotite; (2) carbonaceous material maturation; and (3) the kinetics of cordierite growth. Samples collected along transects perpendicular to the intrusion were analyzed to assess the impact of varying thermal histories on these processes.
Temperature estimates based on phase petrology calculations, particularly the forming reactions of cordierite and K-feldspar, were used to infer the minimum temperature at various distances from the intrusion by interpolation of the isograds. Significant intra-sample variation of the biotite Ti content was observed, suggesting that equilibrium was only locally achieved. The variation in Ti correlated with Al content, which also varied systematically with distance from the intrusion. The Ti content covaries with the Al content and increases towards the intrusion, showing a direct correlation with rising temperature, as it has been previously observed, and decreasing pressure, as biotite closer to the top of the intrusion formed under pressures approximately 200 bars lower than biotite at the bottom. The pressure dependence of Ti solubility, typically observed at higher pressures (>6 kbar), was evident at pressures below 1 kbar in this study. The Ti substitution mechanism shifted from Ti-spinel exchange at low pressures to Ti-oxy and vacancy exchanges at higher pressures. A model incorporating temperature, pressure, and total Al content successfully reproduced the observed Ti trends in biotite, highlighting the complex pressure-temperature behavior of Ti solubility under low-pressure conditions.
To investigate the influence of thermal history on carbonaceous material maturation, a 2D thermal model of the intrusion was constructed. The model incorporated detailed thermal histories for dif- ferent areas, based on mineral isograds and phase petrology estimates. The results showed that the temperatures predicted by the model closely match those expected from observed mineralogical changes. Raman spectroscopy of carbonaceous material (RSCM) was used as a geothermometer, given its correlation with temperature and its irreversible nature. Raman spectra were measured for samples from different distances around the intrusion, and the R2 = D1/D1 + G band ratio was used to estimate temperatures. However, RSCM temperatures were consistently lower than those predicted by phase petrology and thermal modeling. This discrepancy arises from the kinetic control of the carbonaceous material maturation, which is enhanced in rapidly cooling plutons where cooling times are <105 years. In such settings, RSCM temperatures do not reflect peak metamorphic conditions. The thermal model was used to relate the observed discrepancies to thermal histories, showing that in plutons with faster cooling rates, maturation lags behind temperature increases. A model adapted from vitrinite reflectance, which describes early maturation stages, was applied to the data, successfully reproducing the observed D1/G band ratios. This model emphasizes the importance of accounting for pre-existing maturation states in applying RSCM thermometry, although further refinement is needed. The thermal model was also applied to examine the size distributions of cordierite porphyroblasts (RCSDs) across the aureole. Comparison of RCSDs revealed that rapid heating rates promoted diffusion- controlled growth, while slower rates favored nucleation-controlled growth. These findings emphasize the importance of localized thermal conditions in shaping cordierite crystal growth dynamics. A model integrating nucleation- and diffusion-controlled growth was developed to understand these mechanisms more thoroughly. Preliminary results suggest that equilibrium during metamorphism was restricted to small domains surrounding individual crystals, with accelerated nucleation and high intergranular diffusion coefficients further limiting equilibrium to localized scales.
The three processes studied in this thesis — Ti incorporation in biotite, carbonaceous material matura- tion, and cordierite growth — demonstrate that disequilibrium conditions are central to the metamor- phism observed in the contact aureole of the TPIC. Ti solubility in biotite shows locally equilibrated behavior, while carbonaceous material maturation is kinetically controlled, and cordierite growth exhibits localized equilibrium due to rapid nucleation and diffusion. These findings underscore the significance of disequilibrium processes in contact metamorphism and highlight the need to account for local thermal histories and reaction kinetics when interpreting metamorphic systems.