The Role of Manganese in the Paragenesis of Magmatic Garnet: An Example from the Old Woman-Piute Range, California

Abstract

Strongly peraluminous plutons of the Old Woman-Piute Range, southeastern California, display the differentiation sequence: muscovite-biotite granite $$\rightarrow$$ muscovite-garnet-biotite granite $$\rightarrow$$ garnet-muscovite granite $$\rightarrow$$ garnet-muscovite aplite and pegmatite. The garnets show no evidence of reaction, but biotites in garnet-bearing rocks are rimmed by muscovite. Metamorphic assemblages in the country rocks suggest pressures of emplacement less than 4-5 kb. The plutons become relatively enriched in manganese with differentiation. Whole rock ratios of molar Mn/(Fe + Mg) increase from 0.01 in least felsic, garnet-free rocks to 0.3 in most felsic, garnet-rich rocks. MnO concentrations show a corresponding rise in biotites (0.4 to 1.8 wt %) and, less clearly, in garnets (9 to 25%). The differentiation sequence, textural relations, and mineral and whole rock compositions suggest that garnet paragenesis was controlled by the reaction: $$biotite + Mn-rich liquid \rightarrow garnet + muscovite$$. A great majority of all reported plutonic garnets are Mn-rich, with greater than ten percent spessartine component, and occur in very felsic rocks, indicating that manganese enrichment in differentiated magmas may be the controlling factor in paragenesis of most granitoid garnets. Experimental work (e.g., Green 1978a) showing that such Mn-rich garnets are stable in magmas down to relatively low pressures ($$\leq 3 kb$$) is consistent with this conclusion.