New Mexico Geological Society Annual Spring Meeting — Abstracts

The Aspen Basin mtgmatHes are part of a large (≥ 8km²) pelite roof pendant contained within the Proterozoic plutonic rocks of the central Santa Fe Range. Three lithologic types dominate the plutonic rocks: quartz diorite, granodiorite, and two mica granite. The quartz diorites and granodiorites are both porphyritic and exhibit weak to moderate foliations. The two mica granite is rarely foliated and, along with associated pegmatite and aplite dikes, intrudes the foliated plutonic rocks.

The migmatites consist largely of concordant layers of paleosome and neosome. The paleosomes are thought to represent layers of unmelted original pelite. These contain assemblages of plagioclase + quartz + biotite + muscovite + oxides and exhibit textural evidence of metamorphic equilibrium. The neosomes (layers once melted) consist of a leucosome (solidified melt) and a melanosome (mafic rich residual solids). Leucosomes are trondhjemites with plagioclase + quartz ± muscovite ± biotite assemblages and igneous textures. Melanosome assemblages are dominated by foliated biotite with minor muscovite 2: garnet + sillimanite and are depleted in quartz and plagioclase. Also present within migmatites are discordant dikes of trondhjemite.

It is suggested that migmatite leucosomes formed by grain boundary melting of plagioclase + quartz (+ water). Thi s implies the production of a trondhjemite (K-poor) magma by partial melting of a K-rich pelite. Utilizing the above melting reaction, the presence of sillimanite, and the stabiltiy of quartz + muscovite, the metamorphic P-T conditions at Aspen Basin are constrained to 680-770°C and 5-9kb.

Although the evidence is not conclusive, migmatite formation may be contemporaneous with intrusion of the two mica granite.