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Detrital zircon record of Paleozoic orogeny in eastern North America

Michael Palin, Ian Campbell, Kenneth Eriksson , and Charlotte Allen

Zircon grows mainly from melts of intermediate to felsic composition derived from the crust. In general, the more heat supplied to the crust during an orogenic event, the greater the amount of melting and the more zircon produced. Although crustal melting behavior can be influenced by previous thermal history, zircon productivity is a first-order measure of the thermal perturbation associated with an orogeny. Detrital zircons provide a robust record of past orogenic events, in some cases preserving samples of crystalline rock no longer present or exposed.

In the central and southern Appalachian Mountains of eastern North America, the vast majority of detrital zircons from modern river sands and Neoproterozoic through late Paleozoic sandstones yield Grenville ages (1250-950 Ma). Although detrital zircons with Paleozoic ages are rare (<1%) in the sandstones, they comprise about 17% of those in the river sands. These grains display age peaks at 420-500 Ma and 350-380 Ma which correspond well with the Taconic and Acadian orogenies (Fig. 5). The low Th/U ratios and narrow age distribution of the Acadian zircons may reflect a metamorphic origin in contrast to the magmatic character of Taconic zircons. The scarcity of detrital zircons with Alleghenian ages is unexpected in light of the widespread occurrence of late Paleozoic plutons in the southern Appalachians and indicates a possible sampling bias related to erosional base level. Nonetheless, the Alleghenian orogeny must have involved much less new zircon production than the Grenville suggesting that the two collisional events were fundamentally different with respect to the extent of crustal melting. The lack of Paleoproterozoic zircons of South American or African heritage is equally surprising considering that the accreted Carolina terrane originated from margin of the Amazonian Craton and that Alleghenian collision took place with Africa.

Figure 5: Th/U ratios (A) and age spectra (B) of modern detrital zircons with Neoproterozoic/Paleozoic ages collected from rivers draining the central and southern Appalachian Mountains. Also shown are age ranges of the 3 Paleozic orogenies in the Appalachains: Taconic (480-420 Ma), Acadian (400-350 Ma), and Alleghenian (330-250 Ma).