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
- 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.
- 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 (
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.