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

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 SiO₂ (56.1-72.0 wt%), moderate Sr, and high Fe, Ti, K, Mn, Ba, Sc, REE's, 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 <10% melting of a calc-alkaline basalt. Mixing of the melts derived from the two sources might best explain the relatively low silica and high mafic-mineral content of the monzogranite. Accumulation or crystallization of the observed minerals from the magma could have produced the compositional variations in the batholith. The most silica-rich portions of the San Isabel are similar in composition to classic, shallower A-type granites. Thus, this may be one way in which the shallower and silica-rich A-type granites have formed.