Anorthosites Proterozoic

Seismic evidence for a mantle source for mid-Proterozoic anorthosites and implications for models of crustal growth... 

For many years, a debate on the origin of the mid-Proterozoic meta-anorthosites has revolved... 

Anorthosite massifi Proterozoic shield Archaean O-NYNEX refraction profile... 

Fig. 1. The Proterozoic Grenville Province of the easternmost part of the North American craton. The Marcy Anorthosite is crossed by the O-NYNEX high-resolution seismic refraction profile. Dashed lines indicate seismic refraction profiles.

The seismic properties of the Grenville lower crust are inconsistent with pyroxene-rich or high-grade metapelites (Fig. 2). This is crucial in determining which source, mantle or crust, is responsible for generating the mid-Proterozoic meta-anorthosites, as we will discuss below. 

Longhi et al. 1999) and amphibolite (Christensen Mooney 1995 Rudnick Fountain 1995). For any of these compositions, partial melting and extraction of the anorthosite suite had to result in a general increase of restitic minerals (pyroxene and garnet). The occurrence of this process at a large scale, as the volume of mid-Proterozoic meta-anorthosites would require, would have produced a restitic bulk composition of the lower crust and a decrease of VpjVs ratios to values inconsistent with our observations. 

Dempster, T. J., Preston, R. J. Bell, B. R. 1999. The origin of Proterozoic massif-type anorthosites evidence from interactions between crustal xenoliths and basaltic magma. Journal of the Geological Society, London, 156, 41-46. 

Longhi, J., Auwera, J. V., Fram, J. Duchesne, J. C. 1999. Some phase equilibrium constraints on the origin of Proterozoic (massif) anorthosites and related rocks. Journal of Petrology, 40, 339-362. 

McLelland, j. 1994. Multi-stage polybaric genesis of Proterozoic massif anorthosite example from the Adirondack mountains, New York. Abstracts with Programs, Geological Society of America, 26, 40. 

ScoATES, J. S. Frost, C. D. 1996. A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites. Geo-chimica et Cosmochimica Acta, 60, 95-107. 

Thomas, M. D. 1990. Deep structure of the Middle Proterozoic anorthositic intrusions in the eastern Canadian Shield insights from gravity modeling.-... 

Proterozoic anorthosite, Labrador, Canada (Simmons and Hanson 1978). 

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