INSIGHT INTO CONTROLS ON THE CRYSTALLIZATION HISTORY OF GABBROIC DIKES IN THE BAKERS BRIDGE GRANITE, SOUTHWESTERN COLORADO

The ~1.7 Ga Bakers Bridge Granite in the Needle Mountains complex is cut by ~1.4 Ga gabbro dikes. Some dikes preserve a mineral and textural zonation exemplified by megacrystic centers that grade into fine-grained margins. Field studies combined with petrography, petrochemical studies, and electron microprobe data reveal possible mechanism(s) for the creation of the zonation.

Field and petrographic evidence indicates that the dike formed from crystallization of a single batch of mafic magma. The interior of the dike is porphyritic with plagioclase phenocrysts up to 5 cm in maximum dimension that locally define a crude trachytic texture. Towards the margins the proportions of phenocrysts decrease and there is a progressive fining of grain size, an increase in the abundance of hydrous minerals (chlorite, hornblende, apatite), quartz, and a decrease in plagioclase phenocrysts. Although some dikes in the area contain partially digested granite xenoliths on the margin, none were observed at the study site.

Bulk-rock chemical analyses reveal an increase in incompatible elements (CaO, NaO, Al₂O₃, K₂O, Rb, Ba, Sr) and a decrease in SiO₂ from dike margins to interior. Except for SiO₂, most of the element trends are consistent with fractional crystallization and concentration of more felsic constituents in the interior of the dike (i.e. Na, K, Al, etc.). Electron microprobe analyses of plagioclase show an increase in SrO and NaO from the interior to the margin, and a decrease of F in apatite.

On the basis of field and petrologic data we argue that multiple mechanisms were involved to create the textural and mineralogical changes in the gabbro dike. The dike underwent differential cooling to produce the distinct textural changes in different zones. Rapid cooling of the melt on the margins accompanied by the release of hydrous minerals whereas the interior crystallized later and involved the growth of plagioclase megacrysts. The trachytic texture in the interior of the dike indicates flow differentiation and concentration of megacrysts. Though no field evidence was observed for assimilation and partial melting of the host granite, the higher SiO₂ concentrations and modal quartz on the margin could reflect this process. These results provides insight into the ~1.4 Ga mafic magmas in the Needle Mountains complex.