CRYSTAL SIZE DISTRIBUTIONS OF CORDIERITE IN THE TORRES DEL PAINE CONTACT METAMORPHIC AUREOLE, PATAGONIA

Quantitative analysis of cordierite crystal size distributions (CSDs) in contact metamorphic host rocks (Cerro Toro Formation) of the Torres del Paine laccolith (southern Chile) is used to assess the mechanism of crystal growth, and infer the qualitative amount of overstepping responsible for the observed mineral textures. The thermal history is complex, because the laccolith was formed by at least 3 successive pulses of granites (12.59 to 12.50 Ma). The oldest granite forms the top of the intrusion, the youngest the bottom.

The cordierite forming reactions are the chlorite break-down reaction (zone I, ca. 490°C/750bar) and the phengite break-down, which is accompanied by a modal decrease in biotite and the appearance of k-feldspar (zone II, 530-550°C/750bar). Crystal sizes of the nearly spherical, poikiloblastic cordierite crystals were determined from thin section images using image analysis techniques and subsequently corrected for intersection probability. The range of cordierite diameters in all samples are similar at smaller crystal sizes. However, samples from below the laccolith have the largest crystals (max. diameter = 0.17mm), samples from zone I in the roof have crystals with a maximum diameter of 0.13 mm, and cordierite crystals from zone II in the roof have a maximum diameter of 0.095mm. Rocks from zone II have a larger number of small cordierite crystals than rocks from zone I.

Since each intrusive pulse intruded quickly as a thin sheet at very shallow depths, and isotope data indicate hardly any fluid flow, heating is dominated by conduction. In this case, samples at comparable distances below and above the intrusion should have similar heating rates. However, multiple heating at the base of the growing laccolith, as well as inherently slower cooling below the intrusion resulted in distinctly different cooling paths above and below the intrusion. Observed CSDs are significantly different from samples below and above the intrusion suggesting that reaction kinetics were slow enough to allow an important part of the pro-grade reaction history to take place during cooling, after temperatures began to decline from the thermal maximum. Furthermore, none of the profiles suggest significant Oswald ripening.