PETROGENESIS OF MESOPROTEROZOIC OAK CREEK AND WEST MCCOY GULCH PLUTONS, COLORADO - AN EXAMPLE OF CUMULATE UNMIXING OF A MIDCRUSTAL, 2-MICA GRANITE OF ANOROGENIC AFFINITY

The 1.44 Ga old Oak Creek batholith is a composite suite of foliated g ranitoid intrusions consisting of two main facies: (1) a porphyritic f acies ranging from quartz monzonite to monzogranite and (2) a medium-g rained leucogranite. The porphyritic facies comprises most of the bath olith and is distinctly rich in feldspar. The medium-grained facies is more silica rich, and it occurs as small, irregular bodies within the porphyritic facies. Occurring 20 km to the west, the West McCoy Gulch stock is a 1.46 Ga, unfoliated pluton that is similar to the medium-g rained facies at Oak Creek. Based on several independent thermobaromet ers, the batholith is estimated to have been emplaced at 11 to 14 km ( 3 to 4 kbar). Near liquidus temperatures were at 880 +/- 60-degrees-C and, for more evolved members, solidus temperatures were 650 +/- 20-de grees-C. The estimated oxygen fugacity (at 2 log units above QFM) is t ypical of other magnetite series granites. Compared to most granites, these granites contain high Fe/Mg ratios, high K and REE concentration s, and low Mg and Ca concentrations. The least differentiated portions of the medium-grained facies at Oak Creek and the West McCoy Gulch pl uton could have formed by 20-30% aggregate melting of calc-alkaline to nalite to granodiorite. Other models, including derivation from melt-d epleted granulite are precluded. The porphyritic facies at Oak Creek h as large variations in elements that concentrate both in mafic mineral s (Fe, Mg, Ti, Cr, and Sc) and feldspars (Ba, Sr), and these elements form fairly linear plots in Harker diagrams. Element modeling combined with the field evidence suggests that this unit formed by variable un mixing of magmatic cumulate crystals and evolving liquid. The medium-g rained facies is interpreted as being a more evolved melt derived from the crystal-rich porphyritic facies. The West McCoy Gulch pluton coul d have formed in a similar manner to the medium-grained facies by crys tal fractionation from a non-exposed parent. Cumulate-evolved melt unm ixing may be a predominately mid-crustal phenomenon. Most A-type grani tes are homogeneous, shallow intrusions but may represent fractionated liquids derived from crystal-rich fractions such as that exposed in t he Oak Creek batholith.