MESOPROTEROZOIC FERROAN MAGMATISM IN THE SOUTHWESTERN USA

Mesoproterozoic ferroan granites make up ~25% of the exposed Precambrian crust in the southwestern USA. Most of the plutons were emplaced between 1.44 – 1.40 Ga, but the presence of 1.36 Ga and 1.08 Ga granitoids indicate that the magmatism continued episodically for over 300 m.y. There is no correlation between magma composition and age. For example, highly ferroan, alkalic to alkali – calcic granites were emplaced at 1.43 Ga (Sherman) to 1.08 Ga (Pikes Peak). Anorthosite and ferroan alkali monzonite complexes also were emplaced over the same time period.

The Sherman batholith serves as a type example of this Mesoproterozoic magmatism. The Sherman batholith consists of Sherman granite with lesser amounts of leucogranite and porphyritic granite. On Harker diagrams the porphyritic and Sherman granites define linear trends with coeval mafic rocks. This chemical evidence for mixing is evident in the field. Sherman granite commonly contains mafic enclaves that show varying degrees of disaggregation. On its northern margin, where the Sherman batholith intrudes Archean gneiss, Nd and Sr isotopes suggest that no sample can contain more than 35% Archean crust. These data indicate that the Sherman granite originated from fractional crystallization or partial melting of highly evolved tholeiitic sources. The porphyritic granite formed by mixing of Sherman and mafic magmas. All magmas assimilated some felsic crust, with the leucogranite recording the greatest amount.

We suggest that mantle-derived basaltic melt is fundamental to the origin of the Mesoproterozoic ferroan granites. Assimilation of partial melts of metasedimentary rocks produced much of the chemical variation in these rocks across the southwest USA: decreasing Fe-index and increasing peraluminosity correlates with increasing d¹⁸O (Anderson & Morrison, Lithos, 2005). Chemically the Mesozoic ferroan granites resemble granites and rhyolites produced in modern rifts. However, although extensional structures are associated with some intrusions, syn-rift sedimentary rocks are not present. This supports a model in which insulation by the Rodinia supercontinent caused periodic instabilities in the underlying mantle, upwelling and partial melting, underplating of mafic magmas , and ultimately producing anorthositic and ferroan granite magmatism.