ELEGANT SIMPLICITY IN PROGRADE-RETROGRADE METAMORPHISM OF EARLY PROTEROZOIC METASEDIMENTS, FRONT RANGE, COLORADO

The northern Front Range preserves a remarkably detailed record of mineral-zone boundaries formed in dominantly pelitic sediments metamorphosed about 1750 Ma during regional deformation and widespread intrusion of calc-alkaline batholiths (Boulder Creek Granodiorite). The lowest-grade rocks, chlorite-sericite phyllites exposed near the eastern mountain front, retain primary sedimentary structures that define the axis of a regional-scale NE-SW syncline overturned toward the SE. Progressive mineral-zone boundaries, also synformal but with a WNW axial trend, are superimposed across the primary stratigraphy and define a pattern of increasing metamorphic grade toward the N, W and S. Garnet, staurolite, and cordierite appear sequentially in the up-grade direction and are joined by andalusite in middle-grade rocks. Where sillimanite appears, biotite increases due to reactions that consume garnet, staurolite, and cordierite. Andalusite disappears nearby up-grade and the reaction muscovite + quartz=Kspar + sillimanite occurs within the sillimanite stability field. Partial melting begins in rocks with biotite as the sole Fe-Mg phase; increased melting (undersaturated) leads to incongruent melting of biotite and formation of Mg-garnet and Mg-cordierite in the residue. Retrograde assemblages follow the prograde migmatite front: volatiles expelled from anatectic melts produce muscovite-tourmaline-topaz in less-melted or unmelted prograde rocks, whereas extensively melted (dehydrated) terranes have retrograde patchy andalusite and Fe-spinel. The prograde-retrograde patterns were not disturbed by widespread younger bodies of Silver Plume Granite (1400 Ma), although mineral isotopic systems were largely reset at this time. Speculation about relict, very high P-T metamorphism (7-10 kb) based on limited microprobe data is incompatible with the simple regional mineral-zone pattern and ignores the distinctly different parageneses of garnet, cordierite, and andalusite in middle-grade rocks vs migmatitic rocks. Timing relationships for structural fabric also support a simple, one-event history. Peak mineral growth was coincident with peak deformation in migmatite terranes, whereas porphyroblastic minerals overprint schistosity and cleavage fabrics in lower-grade terranes.