ISOTOPIC INFRASTRUCTURE OF ZIRCON AGE PEAKS; IMPLICATION FOR INCREASED CRUSTAL PRESERVATION

The episodic temporal distribution of zircon crystallization ages is thought to represent one of two geologic phenomena, either variable magmatic activity or a bias introduced because the magmatic rocks generated in late stage convergence and continental collision are more likely to be preserved in the geologic record. The peaks of ages correspond with the assembly of the supercontinents and their respective orogenies through time. We use U-Pb, O, and Hf isotopes in zircon to examine the internal structure of the dominant ~1 Ga age peak associated with the Grenville orogeny and the assembly of Rodinia. O, in combination with Hf isotopes in zircon are especially useful in differentiating zircons from mantle and crustal derived magmas. To dissect the Grenville age peak samples have been analyzed from throughout the Grenville orogen that span the supercontinent cycle from subduction (1.5 – 1.25 Ga), collision (1.25 – 0.95 Ga), collapse and eventually break-up (0.95 – 0.5 Ga). Preliminary O isotope ratios in zircon show the proportion of those from mantle-derived magmas varies significantly during the three main stages of orogeny. The proportions of zircons from mantle-derived magmas are high prior to 1.25 Ga and then drop steeply and remain low until after 1.0 Ga where the proportion of mantle-derived increases significantly. This is consistent with a classical model of collisional orogenesis where collision is preceded by subduction zone magmatism with significant input from the mantle, followed by collision related magmatism involving greater crustal contributions. Following collision, the over-thickened crust delaminates and/or collapses allowing for an influx of mantle-derived magmas. We contend this is further evidence that zircon age peaks are the product of increased preservation of continental crust due to the assembly of supercontinents.