Between about 1.8–1.6 Ga, an accretionary belt ∼1300 km wide was added to the southern margin of the Archean Wyoming craton. In the prevailing arc-accretion model (Condie, 1982; Karlstrom and Bowring, 1988; 1993), most of this material is considered to be mantle-derived juvenile crust, and therefore represents newly formed crust (at 1.8–1.6 Ga). The interesting and highly provocative paper by Bickford and Hill in this issue of Geology (p. 167–170) challenges the view that crustal growth in southern Laurentia primarily involved accretion of juvenile arc rocks to an Archean core. Although the Bickford and Hill paper focuses specifically on Paleoproterozoic crustal evolution of southern Laurentia, it has relevance to mechanisms and rates of crustal formation and growth throughout geologic time.
Bickford and Hill present an alternative interpretation for Paleoproterozoic crustal evolution in which they suggest that significant volumes of older (ca. 1.85 Ga Trans-Hudson/Penokean) crust may be present in southern Laurentia, and that rifting of this crust produced the widespread bimodal volcanic sequences present in the southwestern United States (see also Hill and Bickford, 2001). The principal lines of evidence cited by Bickford and Hill to support their interpretations are the presence of pre-1800 Ma inherited zircons and Nd and Pb isotopic signatures suggestive of older crust, and the abundant rhyolite-dominated bimodal volcanic sequences present in southern Laurentia. In their model, rifting of older crust resulted in basaltic magmatism that provided a heat source for the partial melting of Trans-Hudson/Penokean crust to form the rhyolites. The basaltic component of these bimodal sequences would represent juvenile additions to the crust.
The rifting and arc-accretion models are not mutually exclusive. The arc-accretion model allows for some involvement of older crust (e.g., Jessup et al., 2005); in fact, its proponents identified the only known (to this date) Trans Hudson/Penokean age crust in the southwestern United States, the 1.84 Ga Elves Chasm Gneiss (Hawkins et al., 1996). The Bickford and Hill rifting model explicitly states that arc accretion probably did play a role in Paleoproterozoic crustal evolution of southern Laurentia. The principal unanswered question centers on the relative proportions of mantle-derived juvenile crust versus older continental crust in the accretionary belt. If the crust is largely juvenile, the Paleoproterozoic accretionary orogen represents formation of voluminous new crust at 1.8–1.6 Ga. If large volumes of older continental crust are present, the orogen may largely represent redistribution of existing crust (via melting of older crust and resultant rhyolite magmatism), with minor mantle-derived additions in rifts, rather than a protracted crust-forming event.
Thus, a major implication of the Bickford and Hill model is that much of what has conventionally been considered a period of major crustal formation in southern Laurentia may actually represent a time of large-scale reworking of older crust. Stated another way, the volume of new crust added to southern Laurentia between 1.8 and 1.6 Ga may have been substantially less than implied by the arc accretion model. This discussion of the relative volumes of juvenile versus older crust in the Paleoproterozoic orogen of southern Laurentia parallels the debate over crustal accretion models for the Neoproterozoic Arabian-Nubian shield nearly two decades ago (Pallister et al., 1990).
As indicated by Bickford and Hill, resolution of this debate will require, at the least, acquisition of a large amount of geochemical and isotopic data to (1) establish the regional extent of older crust, and (2) determine whether the bimodal sequences are rift related or arc related. Techniques to be applied include U-Pb zircon dating to establish the extent of inheritance and recycling; Nd, Pb, and Hf studies to test the distribution and age of older crust; and trace element analysis to determine the tectonic setting in which igneous rocks were formed. Unfortunately, data from such studies may not necessarily lead to unequivocal resolution of the controversy. For example, inherited zircons in plutonic and volcanic rocks simply indicate that there was “communication” between older continental crust and the locus of magma production. The presence of inherited zircons does not require that older crust exists either in the area of magma generation or in the column of rock through which the magma ascends. One can envision a situation such as the Aleutian arc where detrital zircons from the continental part of the arc could be dispersed along the trench adjacent to the oceanic part of the arc. These zircons could then be incorporated into arc magmas through subduction. Isotopic studies may hold more promise in delineating areas of older crust; however, estimates of volumes of older crust may have large uncertainties. Trace-element chemistry of mafic rocks may be useful in distinguishing between oceanic arc and continental rift settings; however, high-grade metamorphism of much of the Paleoproterozoic in the southwestern United States, and the complex factors that can influence the final trace-element composition of deformed and metamorphosed rocks, may yield equivocal results at best. A combination of the aforementioned methods may hold the most promise for distinguishing between the arc-accretion and rifting models. The results of collaborative geochronologic and geochemical studies will ultimately be required to place improved quantitative limits on the amount of juvenile crustal material that underpins southern Laurentia.
