ATTRIBUTING DEFORMATION PHASES TO SPECIFIC OROGENIC EVENTS IN THE EAST-CENTRAL ADIRONDACK HIGHLANDS

The Shawinigan orogeny (c. 1200-1150 Ma), intrusion of anorthosite-mangerite-charnockite-granite magmas (AMCG, c. 1160-1140 Ma), and the Ottawan orogeny (c. 1090-1050 Ma) are the major tectonic events that produced the geology of the Adirondack Highlands terrane – a southern extension of the Canadian Grenville province. Deformation models for the terrane typically include at least three phases. The first and third phases are often attributed to the Shawinigan and Ottawan orogenies, respectively, while the second phase is the least well constrained and may be attributed to deformation during the last stages of the Shawinigan orogeny or during the early Ottawan orogeny. However, given the extent of the terrane, the robustness of such models across the Adirondack Highlands needs testing. To this end, an extensive outcrop immediately west of Ticonderoga, New York, in the east-central Adirondack Highlands, is being studied to connect its preserved deformation regionally and to known tectonic events.

The outcrop is predominately foliated garnet-biotite-quartzofeldspathic migmatitic paragneiss, foliated garnet amphibolite, and undeformed granitic pegmatite. Paragneiss protolith was likely a clastic sedimentary rock deposited in the Trans-Adirondack backarc basin, while the geochemistry of the amphibolite indicates it was a gabbroic member of the c. 1155 Ma AMCG suite. Deformational features in the paragneiss and amphibolite are parallel and regional mapping demonstrate that the features were produced during D₂ that transposed D₁ features associated with migmatization. Zircon U-Pb SHRIMP-RG analysis of the migmatite and pegmatite constrain migmatization to 1186±25 Ma and the end of penetrative deformation to prior to 1051±25 Ma.

Taken together the results demonstrate that D₁ and migmatization in the east-central Adirondack Highlands can be attributed to the Shawingan orogeny, while D₂ can either be attributed to the latest Shawinigan orogeny shortly after AMCG intrusion or to the Ottawan orogeny prior to intrusion of the pegmatite. Given the evidence for significant deformation and metamorphism during the Ottawan orogeny, it is not unreasonable to attribute D₂ to the Ottawan orogeny, but work continues to assess the effects of D₃ and to better constrain D₂ via U-Th-Pb_(total) monazite geochronology.