TIMESCALES OF COLLISIONAL METAMORPHISM FROM COMBINED SM-ND, LU-HF AND U-PB THERMOCHRONOLOGY

Resolving the timing and rates of orogenic metamorphism is fundamental to our understanding of the evolution of the continental crust, and plays a critical role for deciphering the timescales over which orogenic –and by extension tectonic– processes operated in the geologic past. However, placing robust geochronometric constraints and estimating the absolute time durations of different segments of a metamorphic path can prove difficult, especially in high-grade (e.g., granulitic) terranes where the prograde mineral compositional zoning has been significantly modified or obliterated.

The common observation that garnet-whole rock isochron ages obtained using the ¹⁷⁶Lu-¹⁷⁶Hf decay scheme are older than the ¹⁴⁷Sm-¹⁴³Nd isochrons from the same rock has led to the widespread view that the former system records higher mean closure temperatures. Nevertheless, recent experimental Lu and Hf diffusion data from garnet have shown that these elements can undergo significant diffusive decoupling even at moderate metamorphic temperature conditions, potentially leading to spurious age results. Despite this complexity, if sufficient independent constraints on the thermal history of a rock can be placed, the degree of partial retention of prograde radiogenic Hf in garnet can be estimated and the extent of perturbation of the isochron addressed.

We present a detailed geochronologic study of a granulite-grade metasediment from the collisional Putumayo Orogen of Amazonia, where Sm-Nd, Lu-Hf and U-Pb isotopic dates from garnets and apatites of controlled radius are interpreted using a quantitative numerical diffusion-modeling scheme. Because the length-scale dependency of diffusion imparts contrasting closure-age profiles in crystals of variable dimensions as these cool through their partial retention temperature, this approach allows us to place robust constraints on the timing and duration of different segments of the prograde and initial retrograde path experienced by the metasediments. It is shown here that if grain-size controlled garnet crystals can be dated by Sm-Nd and Lu-Hf methods, and independent constrains on the high-T cooling path be placed, the degree of diffusive modification of the Lu-Hf system can be used to place first-order constraints on the duration of the prograde metamorphic path.