Origin and chemical evolution of the 1360 Ma San Isabel batholith, Wet Mountains, Colorado: A mid-crustal granite of anorogenic affinities

Abstract

The 1360 Ma San Isabel batholith is a weakly foliated hornblende-biotite-sphene monzogranite consisting predominantly of two variants: a coarse-grained facies and a medium-grained equigranular facies. It is a deeper-seated portion (mid-crustal; 5-7 kbar) of the extensive rhyolite and upper-crustal granite terrane that occurs in parts of Missouri, Oklahoma, Kansas, and Texas. Mafic minerals are abundant, variable in amount (to as high as 35%) and segregated into mafic-enriched zones. The San Isabel magma probably formed in an anorogenic setting, as suggested by (1) high total alkali and high FeO/(FeO + MgO) ratios, (2) relatively dry nature, (3) trace-element plots in tectonic discrimination diagrams, (4) similar composition of its leucogranite differentiates to shallower A-type granites, and (5) contemporaneity with other A-type granites. Compared to many granitic plutons, the batholith contains small, negative Eu anomalies, low SiO 2 (56.1-72.0 wt%), moderate Sr, and high Fe, Ti, K, Mn, Ba, Sc, REE9s, and FeO/(FeO + MgO). There are large and fairly linear ranges in the abundances of elements concentrated in mafic minerals (for example, Fe, Ti, Mg, and Ca) and smaller ranges (more scatter) in elements concentrated in the felsic minerals (for example, Eu anomaly size and Ba). The San Isabel magma is thus distinctly mafic in comparison with shallow A-type magmas. The San Isabel magma could have formed by >20% aggregate melting of a tonalite/andesite or by