Existing 40Ar/39Ar and Rb–Sr cooling ages of both amphibole and muscovite at ~ 80 Ma (e.g. Lan, 1990; Lo and Onstott, 1995) show that the granodiorite and its contact aureole cooled rapidly from ~ 700° to < 400 °C in ~ 5 m.y., indicating that the ambient temperature of the country rocks at the time of intrusion was less than the closure temperature of Ar diffusion in muscovite. Existing thermobarometry on rocks in the contact aureole yield pressures of ~ 5 kb, equivalent to a depth of 15–20 km. Cooling ages of biotite and K-feldspar in the literature reflect slow cooling at < 2 °C/m.y. for more than 50 m.y. to at least 20 Ma, suggesting atectonic cooling.
The crustal history reconstructed from these data shows that the regional ‘gneissic’ terrane of the northern Tananao complex may have been caused by coalescing contact aureoles of multiple mid-Cretaceous intrusions (the ‘Nanao intrusive event’), and not by the Neogene Taiwan orogeny. On the contrary, the Tananao rocks probably achieved middle greenschist conditions and 15–20 km depth in a forearc setting in the Early Cretaceous. This intrusive event caused ‘regional contact metamorphism’ at about 85 Ma, after which the passive margin setting preserved these rocks at mid-crustal depths until the beginning of the Taiwan orogeny (~ 5 Ma). These depths (10–12 km) and temperatures (~ 200–250 °C) were high enough to account for the late Miocene lower greenschist facies cleavages and the Pliocene–Pleistocene fission track cooling ages without additional loading and heating from the Taiwan orogeny.