Abstract

More than a dozen composite mafic-granitic dikes crop out along the northern margin of the San Isabel batholith, Wet Mountains, Colorado. The dikes crosscut and contain xenoliths of 1441 Ma granitic dikes and are in turn intruded by, but probably are coeval with, late-stage granitic dikes of the 1360 Ma San Isabel batholith. Only two other mafic representatives of the mid-Proterozoic anorogenic suite are known from Colorado. In the field, the dikes contain up to one-third granitic material by volume. Textures range from a swirling, lacy, mortarlike granite within basalt to subovoid pillows of basalt in granite. Many of the pillows contain small ovoids of the granite. The mutual inclusions and rounded to cuspate liquidlike contacts with minor mechanical mixing strongly indicate that these dikes formed through commingling of mafic and granitic liquids. The mafic dikes have the mineralogy and chemistry of lamprophyres, containing andesine, amphibole, biotite, sphene, and magnetite, with minor quartz and epidote and rare K-feldspar. Chemically, they average average 52% SiO2, and they are enriched in incompatible elements including the light rare earth elements. They are tholeiitic, compositionally similar to within-plate basalts with rift affinity. They partially mixed with the granite to produce intermediate-composition rock with 56% SiO2. The granite is a monzogranite with biotite, hornblende, and sphene. Its mineralogy and chemistry are characteristic of the San Isabel batholith. The lamprophyres may be related to mantle-derived mafic liquids that were hypothesized to have melted lower crust to produce the San Isabel batholith. The lamprophyres may have migrated up around the margin of the batholith and mixed with late-stage granite to form these composite dikes.

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