NEW SHRIMP II AGES FOR ADIRONDACK CHARNOCKITE AND FERRODIORITE RELATED TO MASSIF ANORTHOSITE: IMPLICATIONS FOR GEOLOGIC HISTORY AND AMCG AGE

SHRIMP II analyses of charnockites from Sabael, Minerva and Gore Mt. yield bimodal ages for cores and rims: 1158 ± 22 Ma and 1041 ± 32 Ma (Sabael); 1155 ± 25 Ma and 1041 ± 7 Ma (Gore); 1153 ± 18 Ma and 1050 ± 7 Ma (Minerva). All three charnockites are low in silica (58-61%), rich in iron (7-8%), and contain ~8% alkalis. Each is adjacent to massif anorthosite (Sabael- Snowy Mt., Minerva-Marcy; Gore-Oregon Dome) and contains abundant anorthosite-derived, blue-gray andesine xenocrysts with perthitic reaction rims as well as xenolithic andesine clusters, both from adjacent anorthosite. Zircon rims are narrow and unzoned, whereas cores are large and commonly zoned. Rims are interpreted as metamorphic overgrowths formed during the Ottawan Orogeny, and cores as primary igneous zircons. Since the charnockites contain xenocrystic/xenolithic material from adjacent massif anorthosite, the latter can be no younger than the ca 1155 Ma charnockites. This is supported by SHRIMP II dating of an anorthosite-derived ferrodiorite dike cutting Oregon Dome anorthosite. Zircon rims yield an age of 1061 ± 10 Ma while igneous cores give ages of 1155 ± 12 Ma interpreted as the emplacement age of the ferrodiorite. It follows that the Oregon Dome anorthosite can be no younger than ca. 1155 Ma and is probably coeval with adjacent ca 1155 Ma Gore Mt. charnockite. Similarity between these ages, and ubiquitous presence of andesine xenocrysts in charnockites, indicates that all of these rocks, including the massif anorthosites, were emplaced together as an AMCG suite at ca 1155 Ma. The ca 1050 Ma ages associated with zircon rims and small, equant, multifaceted grains are clearly metamorphic and correspond to regional Ottawan metamorphism ~100 Ma after AMCG magmatism. These SHRIMP II results corroborate similar multigrain ages from 11 other Adirondack AMCG occurrences (McLelland et al,1988)none of which contain significant pre-1160 Ma zircon cores or show inheritance problems. Such properties are consistent with relatively low zircon solubility in AMCG melts. Supporting this is a new SHRIMP II age of 1158 ± 11 Ma for the Diana syenite (a similar age has been found by Bickford and Hill, pers comm). These results support a ca 1140 -1160 Ma age for the Adirondack AMCG suite and lend no credence to arguments purporting a much younger age for the suite.