Projects


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 Americas. Cordilleran-type batholiths also represent the best Phanerozoic analogues to the average upper-crustal composition of continental masses. Therefore it has been proposed that continental convergent margins represent the main sites of crustal differentiation and growth. Observations of magmatism in continental arcs and batholiths and derivative petrogenetic models come principally from upper- to mid-crustal levels exposed at the Earth's surface. These observations are complemented by experimental studies of the plausible sources and liquid line of descent of arc magmas. Many of the first order questions regarding the origin of large granitic batholiths and their relevance to crustal growth have not been uniquely answered by these constraints, in particular, the relative importance of contributions from the mantle wedge underlying arcs, pre-existing continental crust, or the subducting oceanic crust. This uncertainty leaves unresolved the extent to which the continental crust is internally differentiated or largely extracted from the mantle during arc magmatism. Whether continents grow at convergent sites, and by what mechanism and with what rates, remain open questions.

 

 

The key unanswered questions in arc tectonics are:

  • Determining the duration of magmatism and magmatic fluxes in continental arcs;
  • Explaining the non-steady state nature of most continental magmatic arcs;
  • Testing the relationships between arc flare-ups and intracrustal deformation;
  • Estimating the bulk chemistry of continental arcs at lower crustal depths;
  • Identifying crustal and sub-crustal reservoirs involved in continental arc magmatism;
  • Constraining the depths and mechanisms of generation of large batholiths;
  • Testing the hypothesis of lower crustal convective removal in arcs.

 

 

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.

 

The major approaches to answering the questions outlined above within our research group are:

  • Field mapping of arc terranes;
  • Compilation of extensive databases containing all map, petrologic, geochemical and geochronological information on various Cordilleran arc terranes;
  • Detailed geochronological and geochemical studies of deep crustal exposures of arc-related rocks;
  • Geochronological and geochemical studies of deep crustal and upper mantle xenoliths from arc environments;
  • Thermodynamic modeling of melting processes in arc environments;

 

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.

We are 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 (
Western Washington).
Relevant publications:

·         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.



HOME

RETURN TO PROJECTS

Page last updated January 4, 2005. Questions or Comments Mail to mducea@geo.arizona.edu