The Yukon-Tanana terrane (YTT) is a composite arc built on the rifted margin of western North America. The YTT is present in the southeastern part of the St. Cyr area in south-central Yukon, but in the northwest section the structural and lithologic relationships are poorly known. This study describes the structural geometry of the northwest section of the St. Cyr area, west of the Canol Road. The lithological, geochemical, and geochronological characteristics of the Tower Peak unit are investigated in an effort to determine its origin and terrane affinity. In the study area, the Tower Peak unit sits tectonically above a low-grade mafic-ultramafic unit. In turn, this mafic-ultramafic unit is thrust over phyllite and marble creating a composite klippe. Detrital zircons from the marble indicate a maximum depositional age of 368 Ma. This result is consistent with the footwall being part of the Devonian to Mississippian Finlayson assemblage, the oldest arc of the composite YTT. The Tower Peak unit is a massive but fractured, sub-greenschist facies metabasalt. Whole-rock and trace element geochemistry of four Tower Peak samples suggest an oceanic or island arc setting for deposition of the Tower Peak unit. Zircons separated from three samples of the Tower Peak unit give a range of U-Pb ion probe ages. Three distinct age populations exist: 600-2582 Ma, 433-303 Ma, and 140-190 Ma. The preferred interpretation is that the Tower Peak unit is Mesozoic in age and may derive from Stikinia or Quesnellia.