Download Research Focus: Crust formation in the western United

Research Focus: Crust formation in the western United States
Ernest M. Duebendorfer
Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011, USA
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.
REFERENCES CITED
Condie, K.C., 1982, Plate tectonics models for Proterozoic continental accretion
in the southwestern United States: Geology, v. 10, p. 37–42.
Hawkins, D.P., Bowring, S.A., Ilg, B.R., Karlstrom, K.E., and Williams, M.L.,
1996, U-Pb geochronologic constraints on the Paleoproterozoic crustal
evolution of the Upper Granite Gorge, Grand Canyon, Arizona: Geological
Society of America Bulletin, v. 108, p. 1167–1181.
Hill, B.M., and Bickford, M.E., 2001, Paleoproterozoic rocks of central Colorado:
Accreted arcs or extended older crust?: Geology, v. 29, p. 1015–1018.
Jessup, M.J., Karlstrom, K.E., Connelly, J., Williams, M., Livaccari, R., Tyson,
A., and Rogers, S.A., 2005, Complex Proterozoic crustal assembly of southwestern North America in an arcuate subduction system; the Black Canyon
of the Gunnison, southwestern Colorado, in Karlstrom, K.E., and Keller,
G.R., eds., The Rocky Mountain region—An evolving lithosphere: Tectonics,
geochemistry, and geophysics: Geophysical Monograph, v.154, p. 21-38.
Karlstrom, K.E., and Bowring, S.A., 1988, Early Proterozoic assembly of tectonostratigraphic terranes in southwestern North America: The Journal of
Geology, v. 96, p. 561–576.
Karlstrom, K.E., and Bowring, S.A., 1993, Proterozoic orogenic history of Arizona,
in Van Schmus, W.R., Bickford, M.E., Anderson, J.L., Bender, E.E., Anderson, R.R., Bauer, P., Robertson, J.M., Bowring, S.A., Condie, K.C., Denison,
R.E., Gilbert, M.C., Grambling, J.A., Mawer, C.K., Shearer, C.K., Hinze,
W.J., Karlstrom, K.E., Kisvarsanyi, E.B., Lidiak, E.G., Reed, J.C., Jr., Sims,
P.K., Tweto, O., Silver, L.T., Treves, S.B., Williams, M.L., and Wooden, J.L.,
1993, Transcontinental Proterozoic provinces, in Reed, J.C., Jr., Bickford,
M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus,
W.R., eds., Precambrian: Conterminous U.S.: Boulder, Colorado, Geological
Society of America, The Geology of North America, v. C-2, p. 188-211.
Pallister, J.S., Cole, J.C., Stoeser, D.B., and Quick, J.E., 1990, Use and abuse of
crustal accretion calculations: Geology, v. 18, p. 35–39.
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