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The unusual association of cordierite and cummingtonite (� gedrite + chlorite + biotite + ilmenite + plagioclase + quartz) defines a metamorphic facies within aluminous, low-Ca amphibolites from the Proterozoic rocks of the Gold Brick District, east of Gunnison, Colorado. More Fe-rich bulk chemistries in the same facies are characterized by assemblages consisting of cordierite+-gedrite + garnet + chlorite + biotite + ilmenite + plagioclase + quartz, whereas more Mg-rich compositions are characterized by cordierite + anthophyllite + chlorite + biotite + ilmenite � plagioclase + quartz. The assemblage gedrite 4- cummingtonite + chlorite + biotite + ilmenite + plagioclase + quartz was also observed. Coexisting cordierite+...
Locally, voluminous andesitic volcanism both preceded and followed large eruptions of silicic ash-flow tuff from many calderas in the San Juan volcanic field. The most voluminous post-collapse lava suite of the central San Juan caldera cluster is the 28 Ma Huerto Andesite, a diverse assemblage erupted from at least 5-6 volcanic centres that were active around the southern margins of the La Garita caldera shortly after eruption of the Fish Canyon Tuff. These andesitic centres are inferred, in part, to represent eruptions of magma that ponded and differentiated within the crust below the La Garita caldera, thereby providing the thermal energy necessary for rejuvenation and remobilization of the Fish Canyon magma body....