Geochronologic evidence for a cold arc–continent collision: The Taiwan orogeny

Abstract

In order to constrain the tectonic consequences of the collision of the Luzon arc with the SE Chinese continental margin during the Neogene Taiwan orogeny, we assess the thermal history of the poly-metamorphic para-autochtthonous rocks of the continental margin in Taiwan. To accomplish this we report a new SIMS U–Pb age of prograde metamorphic zircons and interpret this age in the context of existing geochronology and thermochronology. Zircons extracted from the Fenniaolin amphibolite near Nanao, Taiwan, in the contact aureole of a granodiorite, are interpreted to have crystallized during the first lower amphibolite facies prograde metamorphism based on [1] their equant shape rather than the prismatic shape typical of magmatic zircon, [2] their microcrystalline or “micro-saccharoidal” aggregate structure and patchy and sector zoning rather than oscillatory zoning visible in cathodoluminescent imaging, [3] their low concentrations of U (most<40ppm), [4] their chlorite inclusions, and [5] their inclusion within prograde metamorphic amphiboles. The apparent absence of magmatic zircon is also consistent with the low Zr concentrations of the N-MORB metabasaltic protolith. SIMS analyses of 18 zircon grains yield a concordant 206Pb/238U age of 86±1Ma, analytically indistinguishable from the age of the adjacent pluton that caused the contact metamorphism. We thus conclude that the contact metamorphism of this intrusion caused the first lower amphibolite facies metamorphism of these rocks in the mid-Cretaceous.Existing 40Ar/39Ar and Rb–Sr cooling ages of both amphibole and muscovite at ~80Ma (e.g. Lan, 1990; Lo and Onstott, 1995) show that the granodiorite and its contact aureole cooled rapidly from ~700° to <400°C in~5m.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 ~5kb, equivalent to a depth of 15–20km. Cooling ages of biotite and K-feldspar in the literature reflect slow cooling at <2°C/m.y. for more than 50m.y. to at least 20Ma, 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 15–20km depth in a forearc setting in the Early Cretaceous. This intrusive event caused ‘regional contact metamorphism’ at about 85Ma, after which the passive margin setting preserved these rocks at mid-crustal depths until the beginning of the Taiwan orogeny (~5Ma). These depths (10–12km) 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.