Abstract
The Maloin Ranch pluton is a layered composite intrusion at the southeast margin of the main body of the Laramie Anorthosite. The layering defines a body shaped like a half bowl, which contains ferrodiorite at its base, overlain progressively by fine-grained mon-zonite and porphyritic monzonite, monzosyenite, and porphyritic granite. The trace element data presented here, together with isotopic data for these rocks (Kolker, 1989) show that each of the members of this composite intrusion has a separate origin. Field, geochemical, and isotopic evidence are consistent with a comagmatic relation between ferrodiorite and anorthositic rocks, either by fractionation or immiscibility. The fine-grained monzonite is not the result of fractionation of ferrodiorite, or mixing of ferrodiorite with a more evolved magma, such as monzosyenite or granite. Trace-element data for the finegrained monzonite suggest an independent deep-crustal source that was depleted in some incompatible elements.
At intermediate levels in the intrusion, porphyritic monzonite formed by mixing of finegrained monzonite (or biotite gabbro) and monzosyenite magmas, not by fractionation of the fine-grained monzonite (Kolker and Lindsley, 1989). Textures, REE patterns, and other trace-element data show that the monzosyenite is a feldspar cumulate, with a highly variable proportion of liquid. Trace-element and isotopic characteristics of the Maloin monzosyenite are consistent with derivation from a more evolved deep crustal source or more extensive fractionation, compared to the fine-grained monzonite. REE data suggest that some of the overlying porphyritic granite is a late-stage segregation related to the monzosyenite and not to the overlying Sherman Granite. Associated granitoid dikes and small fine-grained intrusive bodies have a diverse history reflecting late-stage mobility of granitic melts with varying fluid contents.
