A NEW INTERPRETATION FOR THE AGES OF ADIRONDACK PLUTONISM: IMPLICATIONS FOR THE NATURE AND TIMING OF MESOPROTEROZOIC DEFORMATION AND METAMORPHISM

On the basis of field relations and previous zircon U-Pb results, the established chronology of three major suites of Adirondack plutonic rocks includes: (1) anorthosite-mangerite-charnockite-granite (AMCG)--~1.17-1.15 Ga (SHRIMP data on cores), (2) Hawkeye granite gneiss (HG)--~1.10 Ga (multi-grain TIMS data), and (3) Lyon Mountain granite gneiss (LMG)--~1.05 Ga (SIMS data on zoned rims; zoned cores were interpreted as inherited from AMCG sources). Relative ages constrain that HG intruded AMCG, and that LMG intruded HG and AMCG. In addition, HG usually is strongly foliated whereas LMG is weakly foliated to massive. The existing ages and structural evidence supported LMG emplacement after (or very late during) the 1.09-1.05 Ga Ottawan phase of the Grenvillian orogeny.

New high resolution CL imagery shows that zircon from HG and LMG at their type localities are composed of fine, concentric, oscillatory-zoned cores, and at least two growth episodes of rims. In places, outermost medium-gray rims invade older dark-gray inner rims, giving the misleading appearance of broad oscillatory zoning. Age of HG cores is 1160 ± 10 Ma; ages of HG inner and outer rims are 1039 ± 16 and 1006 ± 15 Ma, respectively. Age of LMG cores is 1142 ± 10 Ma; ages of LMG inner and outer rims are 1055 ± 7 and 1028 ± 28 Ma, respectively. Oscillatory-zoned cores have average Th/U of 0.32 (HG) and 0.43 (LMG); average values of Th/U for inner and outer rims are 0.18 and 0.11 (HG) and 0.11 and 0.18 (LMG). In addition, cores have REE patterns that are typical of igneous origin (i.e., increasing towards HREE), whereas REE in rims show decreasing values towards HREE, suggesting that rims formed cogenetically with garnet. Contrary to previous interpretations, we conclude that the cores formed during igneous emplacement, whereas the rims formed during subsequent metamorphism.

Our new age data indicate that HG and LMG plutons at their type localities should be considered to be late Shawinigan and post-Shawinigan, respectively. This contentious conclusion implies that, although the well-dated high P-T Ottawan event was responsible for formation of ubiquitous zircon rims, it likely was only a minor, or localized, fabric-forming event. Most of the strong deformation occurred between emplacement of HG and LMG.