THE LONGEVITY OF THE CARTHAGE-COLTON SHEAR ZONE: INSIGHTS FROM MULTIPHASE U-PB GEOCHRONOLOGY, TI THERMOCHRONOMETRY, AND TRACE ELEMENT GEOCHEMISTRY

The Carthage-Colton shear zone (CCSZ) separating the Adirondack Highlands and Lowlands has been interpreted as a long-lived crustal weakness most recently reactivated during exhumation of the Adirondack segment of the Grenville Orogen. We present zircon, apatite, and titanite U-Pb geochronology, Ti in zircon thermometry, and trace element geochemistry to better constrain the timing, conditions, and rates of deformation along this exhumed mid-crustal shear zone displaying both a ductile and brittle history. Near Colton, NY recent incision of the Raquette River exposes a near continuous transect across the Carthage-Colton Shear Zone from the Highlands to Lowlands. Mylonitic gneisses from this transect yield U-Pb zircon ages from ca. 1180-1150 Ma representing Shawinigan plutonism. Interstitial pegmatite, between meter-scale brecciated blocks of ultramylonite, yields a complex inherited zircon population consisting of ca. 1330, 1260, and 1160 Ma populations and was intruded at ca. 1050 Ma. Trace element analyses indicate different of zircons have unique trace element patterns related to their source: one group of zircons showing HREE-enriched chondrite-normalized patterns with typical positive Ce and strongly negative Eu anomalies, and the second group of zircons revealing profound enrichment in L-REE and lack Eu anomaly. Zircon, titanite, and apatite U-Pb results from a single diopsidite boudin track the later part of metamorphic history ranging from zircon (ca. 1047±10 Ma), titanite (ca. 1068±6 Ma and 1025±5 Ma) and apatite (ca. 924±17 Ma). The similarity of zircon and titanite U-Pb ages indicate rapid cooling of mylonites from zircon crystallization temperatures (ca. 760 °C based on Ti-in-zircon thermometry) to titanite closure temperatures (ca. 650 °C) in Ottawan orogenic phase. Other apatite samples yield ages as old as ca. 972 Ma and these ages may be interpreted either as a regional-scale cooling of the crust after the end of orogeny, or long-lived hydrothermal alteration along fault systems similar to the eastern Adirondack Highlands. Overall, our results expand our understanding of the nature of basement rocks, assess different zircon formation mechanisms, and document rapid cooling rates that indicate crustal exhumation facilitated by extensional detachment faulting.