EPISODIC GRANITIC MAGMATISM IN SPACE AND TIME: CONSTRAINTS FROM IGNEOUS AND DETRITAL ZIRCON AGE SPECTRA

Igneous zircon age spectra from granitic rocks are complex with many age peaks in contrast to detrital spectra, in which age peaks are broad and hump-shaped. Some age peaks do not occur in both igneous and detrital age spectra. Possible contributing factors to this mismatch are, 1) broad detrital age peaks mask other peaks, 2) removal of crustal sources by erosion, such that the only record of some granitic events resides in detrital zircons, and 3) inadequate sampling of granitoids because they are covered by younger rocks.

Ten major granitic episodes are recognized in the Archean of which only the 2700-Ma event occurs on ? 7 cratons. Granitoid age peaks at 3300, 2800, 2680, 2650, 2630, 2600, and 2550 Ma occur on 5-6 cratons each, and events at 2550 and 2500 Ma are found chiefly in the North China, Central Africa and India cratons. Events older than 3700 Ma are limited to the Yilgarn, Slave, Nain and North China cratons. Large peaks in Archean detrital ages at 2680, 2600 and 2480 Ma occur in sediments from 3-4 continents each. Major granitoid events during the Proterozoic occur at 2100, 1900, 1860, 1750, 1700, 1650, 1550, 1375, 1200 and 1100 Ma, of which only the 2100 and 1700-Ma events occur in ? 5 orogens. A large detrital peak from ? 3 continents is recognized only at 1900 Ma. There are no well-defined granitoid events between 2450 and 2200 Ma.

These age distributions clearly indicate that single, short-lived mantle events at 2.7 and 1.9 Ga (or any other times) cannot explain the prolonged crustal magmatic episodes in the late Archean or in the Proterozoic. The large volume of granitoids produced in the late Archean may reflect the first widespread subduction on Earth. This is followed by 200 My of stagnant-lid mantle cooling, with modern plate tectonics stabilizing after 2500 Ma.