HR: 0830h
AN: S21F-0387 [PDF]
TI: Seismic Evidence for a Mantle Source for Mid-Proterozoic Anorthosites and Implications for Models of
Crustal Growth
AU: * Musacchio, G
EM: gemma@mi.ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Milano
via Bassini, 15, Milano, 20133
Italy
AU: * Musacchio, G
EM: gemma@mi.ingv.it
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Mooney, W D
EM: mooney@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB:
Voluminous anorthosite intrusions are common in mid-Proterozoic crust. Historically, two end-member models have been proposed
for the origin of these anorthosites. In the first model anorthosites derive from fractionation of a mantle source leaving a
residue of meta-gabbro in the lower crust; in the second model anorthosites are the product of partial melting of the lower
crust with residual pyroxene and high-grade minerals (i.e., a pyroxenitic and/or metapelitic lower crust). Although a general
consensus has developed that the first model provides the best fit to petrologic and geochemical constraints, the sparse
evidence for mafic and ultramafic counterparts to the anorthosites leave the issue still unresolved.
We use the absolute P-wave velocity and the ratio between P- and S-wave velocities (Vp/Vs) to infer the composition of the
lower crust beneath the Marcy anorthosite (New York State, USA). Seismic refraction data reveal a lower crust 20 km thick,
where Vp and Vp/Vs ranges from top to bottom between 7.0 and 7.2 +/- 0.1 km/s and between 1.84 and 1.81 +/- 0.02,
respectively. Laboratory measurements on rock samples indicate that these seismic properties are typical of plagioclase-rich
rocks. Magmatic underplating of basaltic melts is a mechanism to form plagioclase-rich bulk composition for the Grenville
crust. At the bottom of the lower crust, increase of P-wave velocity, slight decrease of Vp/Vs ratios and the presence of a
low reflective seismic Moho are additional observations supporting crust-mantle interactions related to magmatic
underplating. High P-wave velocity (8.6 km/s) in the upper mantle may indicate the ultramafic portion (e.g., pyroxenites) of
the underplated magma has become eclogite.
High average P-wave velocity (6.7 km/s) and VP/VS (1.81) and the exceptional abundance of anorthosites-norites-troctolites
among the rocks exposed at the surface, indicate that the Grenville Proterozoic crust may have a unique plagioclase-rich bulk
composition. We suggest magmatic underplating, occurring either over a wide time span or with separate syn- and
post-collisional magmatic pulses, as being a major crust-forming mechanism operating in the Proterozoic.
DE: 7205 Continental crust (1242)
DE: 7230 Seismicity and seismotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 8125 Evolution of the Earth
SC: Seismology [S]
MN: 2003 Fall Meeting