Differentiating Shawinigan and Sttawan orogenesis in the Central Adirondacks

Abstract

The rocks at Chimney Mountain provide a rare glimpse into primary intrusive relations and exceptionally well-preserved pre-Shawinigan metasedimentary rocks in the Adirondack Highlands despite a strong Ottawan thermal overprint. A near vertical contact between granite (ca. 1172 Ma) and a shallowly dipping and structurally intact sequence of quartzose to calc-silicate metasedimentary rocks is exposed on the southern fl ank of Chimney Mountain in the Central Adirondacks. The contact is marked by foliation truncation and a metasomatic aureole with randomly orientated porphyro blasts of enstatite rimmed by anthophyllite (max. 5 cm) and phlogopite (max. 2 cm), and a zone of granular, quartzrich rock. The granite is non- to weakly foliated and has a shallow, north-plunging, mineral lineation as do the metasedimentary rocks. Zircons separated from a diopsidebearing quartzite (82% SiO2; 0.75 m thick) are of variable size (up to 400 μm), equant, and contain, on average, >1000 ppm uranium. Scanning electron microscope investigation indicates that there is little variation in a uniformly dark cathodoluminescence response, no discernible cores or rims, few inclusions, and partially faceted to round morphologies. Zircon U-Pb sensitive high-resolution ion microprobe (SHRIMP II) ages of 1042 ± 4 Ma and 1073 ± 15 Ma are coincident with Ottawan metamorphic ages from the Adirondack Highlands. Zircons from the intrusive granite yield large cores with typical anorthosite-mangerite-charnockitegranite (AMCG) ages (ca. 1171.6 ± 6.3 Ma) and sparse, thin younger rims (ca. 1060-1090 Ma) readily distinguishable by cathodoluminescence. Despite the younger zircon ages, the metasedimentary rocks and their fabric must predate the crosscutting granite. The thermal effect of the Ottawan event was likely enhanced by volatile fl uxing and resulted in recrystallization and resetting of zircons in the metasedimentary rocks. However, it had limited effects on zircons in the granite and produced only thin metamorphic rims emphasizing the importance of local geochemical conditions to the response of zircon to metamorphism. Elzevirian or Shawinigan fabrics are preserved as the dominant foliation; the lineation and folding is likely late (post-1170 Ma) Shawinigan or Ottawan (ca. 1050 Ma). Titanites from the same metasedimentary sequence yield a range of 238U /206Pb ages from 969 to 1077 Ma, with a maximum probability age of 1035 Ma, similar to other titanites in the Adirondack Highlands. Ottawan paleotemperatures, estimated by zirconium in titanite thermometry, range from 787 to 818 °C. © 2011 Geological Society of America.