A BACK-ARC SETTING IS PROBLEMATIC FOR GRENVILLIAN ANORTHOSITE-FERROAN GRANITE SUITES

A recent model proposes that the ca. 1.4-1.5 Ga magmatic belt that runs across North America from the southwestern US to Labrador and Fennoscandia represents an active margin of Laurentia and that the anorthosite-ferroan granite (AFG) suites therein were emplaced in a back-arc environment in the hinterland of this arc. Although this model is enticing, it is problematic because it fails to take into account the petrologic constraints imposed by this distinctive rock suite.

AFG magmatism forms by differentiation of relatively dry, reduced tholeiitic melts at the base of the crust. Anorthosites commonly contain high-Al pyroxenes that record crystallization at high pressures deep in the crust. High pressures also favor the crystallization of clinopyroxene over plagioclase which, coupled with the reducing nature of tholeiitic melts, causes Na₂O + K₂O to increase relative to CaO during differentiation, producing the ferroan and alkalic to alkali-calcic compositions characteristic of the granitic magmas. The tectonic environment producing this suite must allow tholeiitic melts to pond at the base of the crust. The only Phanerozoic massif anorthosite complex is the Åir massif, which is spatially and temporally associated with the Nigerian granites, which formed in a failed rift during the opening of the Atlantic in the Mesozoic.

Rifting does occur in the back-arc environment, but the major problem with this model is that AFG suites are not found in the back-arc portions of any Phanerozoic arcs. Cordilleran granites are overwhelmingly magnesian-calc-alkalic and metaluminous. The most siliceous Cordilleran granites may be ferroan, but the peraluminous nature of these ferroan granites indicate that they formed primarily from crustal melting.

For the model explaining Grenvillian AFG magmatism as back-arc continental to be valid it must explain the following:

1) why the back-arc environment in the Grenville, unlike those in Phanerozoic environments, had the proper conditions for the formation of ferroan granites and anorthosites.

2) why such conditions were consistent over nearly 5000 km of length of the magmatic belt at ca. 1.4 Ga, and

3) why such conditions occurred ONLY in the Mesoproterozoic margin of Laurentia and did not occur afterward.