Combined Sm-Nd and Lu-Hf dating of garnets from the Putomayo foreland basin in south-central Colombia and implications

Garnet-whole rock (Grt-WR) ages of metapelites determined by the Lu-Hf decay system are almost always older than those determined by the Sm-Nd system. Unambiguous interpretation of the observed age differences has been hindered by a lack of adequate information about grain size, diffusion data for Hf in garnet, and in many cases about peak metamorphic conditions and cooling rates, all of which affect the closure temperatures of these decay systems. As part of a broader study on basement rocks from the Andean Putomayo foreland basin in south-central Colombia, we have determined the Lu-Hf and Sm-Nd Grt-WR ages of these rocks using painstakingly handpicked garnets of ~50 μm radius, and obtained ages of 1070 × 5.6 and 1007 × 2.9 Ma, respectively. By modeling the retrograde Fe-Mg zoning in garnet adjacent to biotite according to an asymptotic cooling model (1/T = 1/To + ηt) with the diffusion data from [1], an initial cooling rate of ~2-5 °C/Ma is obtained independently of the geochronological data; peak P-T conditions of ~8 kb, 675 °C are imposed by garnet-orthopyroxene thermobarometry. Using the above data in conjunction with the Nd diffusion data from [2] and Hf diffusion data from our recent study, we obtain closure temperatures for the Lu-Hf and Sm-Nd decay systems in garnet of ~545-565 °C and 415-430 °C, respectively. Results from analytical solutions [3, 4] and a more flexible numerical method are found to be in good agreement with one another. The calculated difference of closure temperatures predicts a difference of ~105-40 Ma between the ages determined by the two decay systems, as compared to the observed age difference of 63 × 6 Ma. The predicted peak metamorphic age derived from the measured and calculated resetting ages of the two decay systems is between ~1030 and 1185 Ma, as compared to the Lu-Hf age of 1070 ×1.9 Ma; we are currently working to obtain U-Pb zircon ages to better constrain this peak metamorphic age. In calculating these results, we have made the simplifying assumptions that there was no retention of prograde radiogenic 176Hf in garnet and no rotation of the Lu-Hf Grt-WR isochron during cooling as a result of the somewhat lower closure temperature of Lu compared to that of Hf. Further work is in progress to refine the model taking these effects into account and explore the consequence of using diffusion data from other groups [5, 6]. A review of available experimental REE in garnet diffusion data suggests that low-REE garnets produce diffusion coefficients which are in agreement with [2], while higher-REE garnets produce diffusion coefficients which fall on the Arrhenius trend of [5] and [6]. Because there is no obvious reason why the experimental data presented in [2] or [6] should be erroneous, it is concluded that the variation in these datasets represents a change in diffusion mechanism as a consequence of garnet REE concentration. We hope that the results of this study will help to elucidate the circumstances under which each of these datasets is appropriate. [1] Chakraborty and Ganguly (1992) CMP 111, 74-86. [2] Tirone et al. (2005) Geochim. Cosmochim. Acta 69, 2385-2398. [3] Dodson (1973) CMP 40, 259-274. [4] Ganguly & Tirone, EPSL 170, 131-140. [5] Carlson (2012) Amer. Mineral. 97, 1598-1618. [6] Van Orman et al. (2002) Cont. Min. Petr. 142, 416-424.