THE ALKALI OLIVINE GABBRO AND ALKALI SYENITE ON US HIGHWAY 50, WESTERNMOST GUNNISON COUNTY, COLORADO

Gabbro in a roadcut on US 50, 1.7 mi east of the Gunnison-Montrose county line, intruded Mancos Fm. Outcrops extend 1 to 1.5 mi to the north and south. The rock plots in the chemical field of trachybasalts (wt %; 50.8 SiO2; 3.7 K2O; 3.8 Na2O) and is an alkali olivine gabbro. Other major oxides, REEs [56 ppm La; 23 (La/Lu)n; no Eu anomaly], and other trace element abundances support the alkali gabbro designation. Phenocrysts include plagioclase (An 53 Ab 42 Or 5 to An 61 Ab 36 Or 3), augite (Wo 45 Fs 15 En 40), serpentinized olivine (Fo 68; grain preserved in a cpx grain), magnetite, and Mg-biotite (0.74 Mg*). Small plagioclase laths (An 54 Ab 43 Or 3) and enclosed cpx (Wo 45 Fs 13 En 41), biotite, and iron-rich aluminous chlorite (0.38 Mg*; 13 wt % Al2O3) comprise an interstitial matrix. Angular analcime patches are bordered by tabular, sanidine crystals (Or 53 Ab 44 An 3), and sparse, small amygdules are filled with analcime and natrolite. In addition to features described for the main body, outcrops in the roadcut close to the base of the intrusion contain 5 % subhedral brown hornblende phenocrysts and a few opx (Wo 2 Fs 32 En 66) grains that show strong evidence for resorption in the magma. Concentrically banded chalcedony filled in some of the original open space.

Sparse alkali syenite seams and dikes (1 mm to a few m thick) comprised of tabular orthoclase crystals (Or 97 Ab 3), minor biotite (Mg* 0.48), augite (Wo 45 Fs 27 En 28), and sodic plagioclase cut the gabbro. These plot in the chemical field of phonolites [57.3 SiO2; 6.8 K2O; 5.0 Na2O; 92 ppm La; 18 (La/Lu)n; strong Eu anomaly]. Albite (Ab 97 An 3) has exsolved to form coarse film and patch perthites. Angular spaces between feldspar crystals are filled with analcime. Biotite from the alkali syenite gave a conventional K-Ar age of 77 Ma. The alkali gabbro is also probably early Laramide in age; both rocks are distinctly different from other Laramide intrusive rocks of the Colorado Mineral Belt.