UNRAVELING THE COMPLEX MESOPROTEROZOIC METAMORPHIC HISTORY OF NORTHWEST LAURENTIA WITH LU-HF GARNET GEOCHRONOLOGY

We have determined Lu-Hf dates for several metamorphic tectonites in northern Idaho to constrain the metamorphic history of northwest Laurentia. Many of these samples, which appear to have simple systematics based on lower uncertainties and MSWDs, have dates that cluster near 1.4-1.3 Ga and 1.1-1.0 Ga. Garnet pelites from the Snow Peak and Clarkia areas yield relatively precise dates in both intervals: 1085±2.4 Ma; 1078±17 Ma; 1314±2.3 Ma; 1379±8 Ma [1, 2]. In between are several intermediate dates: 1151±41; 1198±79, 1206±8.4, and 1255 ± 28 [1, 2]. All analyses were performed on clean garnet fragments picked from whole garnets; no attempt was made to separate core and rim material.

These results indicate either a prolonged series of garnet-producing events or complications in the Lu-Hf systematics. To address this issue, Nesheim et al. [1] separated garnet cores and rims from a Clarkia pelite (08HL-04) by microdrilling thin slabs. Cores from 08HL-04 gave a date of 1347±10 Ma with a younger date of 1102±43 Ma for the rims, clearly indicating two episodes of garnet growth. A mixed garnet fraction gave an intermediate date of 1306±18 Ma. In many garnets, cores have higher Lu contents and Lu/Hf ratios than rims due to incorporation of HREEs during garnet growth. These higher Lu/Hf ratios exert strong control on the slope of the regression for mixed garnet fractions and therefore bias toward older ages. This is clearly the case for 08HL-04.

Because of the small size of many garnets and the rather large sample requirements for Lu-Hf and Sm-Nd garnet geochronology, it is not always practical or possible to separate core and rim material. In these cases, analyzing multiple garnet fractions can often reveal multiple growth zones by scatter in the regressions. Absence of evidence for multiple growth zones using this approach, however, is not evidence of absence of multiple events; care must be taken in interpreting garnet ages. In spite of these caveats, Lu-Hf and Sm-Nd garnet geochronology remains a powerful tool in unraveling metamorphic histories. A clear example of this is the garnet geochronology of the northern U.S. Cordillera, where this technique has revealed a rich Mesoproterozoic metamorphic history that was previously largely unknown.

[1] Nesheim et al., 2012, Lithos, 134, 91-107. [2] Zirakparvar et al., 2010, CJES, 47, 161-179.