The evolution of continental magmatic arcs and relevance to continental crustal formation
Understanding the origin of continents is a
fundamental issue in geology. The average continental crust is intermediate in
composition. Cordilleran-type batholiths represent aerially tremendous tracts
of petrogenetically young sialic basement extending along the entire western
margin of the
Major batholithic belts of North and South America have been studied in great detail. What commonly escapes observation and prevents us from unique interpretations are the roots of arcs, the deep crustal counterparts of the well-studied shallow batholithic exposures. In contrast to the extensive study of deeply exhumed rocks from collisional environments and of high pressure-low temperature oceanic terranes, knowledge of deep continental arc crustal sections is in its infancy. Rare exposures of deeper crustal levels in arc terranes provide important insights into the nature of the lower crust of the North-American Cordillera and a testing ground for most hypotheses on the rates and mechanisms of continental growth in arcs.
Read a summary of our new data and interpretations on the California arc in a recent article in the UA Geology Newsletter. Click here for a pdf file or for a more complete set of results in a paper published in GSA Today click here.
currently working in two arc-related lower crustal exposures: the western Santa
Lucia Mountains in central California and the Xolapa terrane in southern
Mexico. A summary of results from ongoing research in each area is given below.
A. Santa Lucia Mountains
Our work in the central coastal Californian Salinian block deals with the evolution of the middle crust of the Cretaceous magmatic arc. Our current work concerns a narrow slice of the Salinian block, the Coast Ridge belt, which exposes rocks that were at depths of 25-30 km during arc activity. We have made a detailed map of a transect across the Coast Ridge belt, described three previously unknown Cretaceous intrusions, and in collaboration with Dr. George Gehrels have constrained ages of deformation, intrusion, metamorphism and uplift using U/Pb ages of zircons. Additionally we have carried out Sr and Nd isotopic analysis on samples of the most common Coast Ridge belt rocks.
This work has improved our understanding of the Salinian arc in a number of ways. For instance we show in our study area, previously mapped as entirely metamorphosed sedimentary rocks, that Cretaceous igneous rocks make up roughly a third of the area. Additionally, our work suggests that the framework rocks are dominantly Cretaceous orthogneiss. It is therefore likely that the Coast Ridge belt more than doubled in thickness during arc activity. Our Sr and Nd isotopic work shows that the magmas, which span a compositional range from gabbro to tonalite, assimilated significant amounts of framework material, and field evidence suggests that most assimilation occurred at depths greater than those exposed. This work is helping to constrain questions of mass balance, magma evolution, and crustal growth at continental arcs.
B. The Xolapa terrane
The Xolapa terrane is an extensive (600 x 100 km) mid- to lower-crustal exposure of a late Mesozoic - Cenozoic Cordilleran arc that resulted from the subduction of the Cocos plate beneath Mexico. It is arguably the largest North-Central American exposure of deep, arc-related rocks. Rocks of upper amphibolite to granulite facies are commonly exposed throughout the arc. No modern, quantitative studies have been carried out in the Xolapa. We are currently trying to map the exposure depths within the Xolapa in order to focus on the deepest crustal exposure. Preliminary data indicate that the arc crust is exposed to a maximum depth of about 45 km.
Early Cenozoic granulite facies tonalite of the Xolapa complex
Magma mingling products in the Huatulco batholith on the Fort Huatulco beach
Personnel: Ducea, Mark Barton,
grad students Steve Kidder and Li Chao.
Collaborators: Jason Saleeby (Caltech), Scott Patterson (USC), Chris Andronicos (UTEP), Sue De Bari (
· Ducea, M., 2002, Constraints on the bulk composition and root foundering rates of continental arcs; A California arc perspective. Journal of Geophysical Research, Vol 107, No. B11, 2304, doi:10.1029/2001JB000643. Click Here to download article (pdf).
· Ducea, M.N., 2001, The California arc: thick granitic batholiths, eclogitic residues, lithospheric-scale thrusting, and magmatic flare-ups, Geol. Soc. Am. Today, 11, 4-10.
· Ducea, M.N., and Saleeby, J.B., The age and origin of a thick mafic ultramafic root from beneath the Sierra Nevada batholith, Contributions to Mineralogy and Petrology, 133: 169-185, 1998.
· Ducea, M.N., and Saleeby, J.B., Silica-rich glass inclusions in ultramafic xenoliths from the Sierra Nevada, California, Earth and Planetary Science Letters 156: 101-116, 1998.
· Ducea, M.N., and Saleeby, J.B., Buoyancy sources for a large unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry, Journal of Geophysical Research, 101: 8229-8241, 1996.Page last updated