Mineralogical constraints on the petrogenesis of trachytic inclusions, Carpenter Ridge Tuff, Central San Juan volcanic field, Colorado

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Although bulk-rock normative analyses of the trachytic inclusions from the Carpenter Ridge Tuff yield abundant quartz and minor corundum, a portion of the phenocryst assemblage is indicative of an alkaline parentage. Sanidine and biotite contain up to 8 and 5 wt% BaO respectively. In addition, both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica-undersaturated magma. Amphibole is magnesiohastingsite with 3 wt% TiO2 and less than 0.3 mole fraction vacancies in the A site. Clinopyroxene compositions straddle the calcic augite-salite boundary. Chrondite-normalized REE patterns are similar for both inclusions and rhyolites. The inclusions are slightly poorer in REE and have a positive Eu anomaly versus the negative anomaly of the rhyolites. The similarity in REE patterns would seem to indicate that the two rock types are genetically related with the positive Eu anomalies resulting from feldspar accumulation. However, this possibility is denied by the antithetic alkaline and subalkaline phenocryst assemblages of the two rock types. We suggest that the best explanation for these discrepancies is that a mildly silica-undersaturated magma was the parent for the phenocrysts. This magma intruded the Carpenter Ridge chamber, and because the crystallization temperatures of both magma overlapped, the alkaline magma mixed with the ambient rhyolite to form a hybrid. This hybrid consisted of a portion of the phenocryst assemblage from the alkaline magma but the bulk-rock chemistry depended upon the proportions of the endmember liquids. The abundance of normative quartz, the minor normative corundum, and the similarity of REE patterns indicates that the inclusions are mixtures dominated by the rhyolitic component. Additional processes such as liquid-state diffusion, crystal accumulation, and alkali loss may have contributed to obscure the compositions of the initial liquids involved in the inferred mixing process.