DISTINGUISHING AND CONSTRAINING THE TIMING OF SHAWINIGAN AND OTTAWAN OROGENY DEFORMATION IN THE EASTERN ADIRONDACK HIGHLANDS NEAR TICONDEROGA, NEW YORK STATE

The Adirondack Highlands in northern New York State is part of the southern extension of the Mesoproterozoic Grenville Province. Rocks of this terrane originated in a 1400-1250 Ma arc to backarc setting but were modified by the 1200-1160 Ma Shawinigan orogeny, the c. 1155 Ma anorthosite-mangerite-charnockite-granite (AMCG) magmatic suite, and the 1090-1050 Ma Ottawan orogeny. Distinguishing between Shawinigan and Ottawan orogenic features is challenging but was largely accomplished in an outcrop near Ticonderoga, New York, through regional mapping, detailed structural analysis, and geochronology. Such work is important to build a better understanding of how poly-orogenic events constructed the terrane and of the broader Grenville Province.

The outcrop of focus consists of garnet-biotite-quartzofeldspathic migmatitic paragneiss that is cross-cut by unstrained ameboid bodies of granitic pegmatite. At the east end of the outcrop is a garnet poikiloblastic amphibolite with geochemistry suggesting the rock originated as an AMCG suite gabbro. Both the paragneiss and amphibolite are penetratively deformed by an on average 082, 44 SE foliation and shallow eastward plunging mineral lineation, indicating that the foliation postdates the AMCG suite. Regional mapping demonstrate that this foliation (S₂) is the dominant fabric in both paragneisses and AMCG rocks, but in paragneisses was formed by transposition of an earlier migmatitic fabric (S₁) absent in AMCG rocks.

U-Pb zircon dating by ion probe (SHRIMP-RG) reveal that paragneiss migmatization occurred at 1186±25 Ma, while the pegmatite was emplaced at 1051±25 Ma. Combing this outcrop data with regional observations indicate that S₁ and migmatization can be attributed to the Shawinigan orogeny. With S₂ developing between c. 1155 Ma (AMCG intrusion) and c. 1051 Ma (pegmatite emplacement), it could be attributed to either the last stages of the Shawinigan orogeny and AMCG intrusion, or during the Ottawan orogeny prior to c. 1051 Ma. Work continues to better constrain the timing of S₂. These results indicate that the Adirondack Highlands, and likely many poly-orogenic terranes, have composite fabrics and emphasize the need for detailed research throughout such terranes.