- UW Department of Geology and Geophysics
New field investigations in the Wet Mountains of Colorado reveal informative structural-plutonic relationships surrounding Mesoproterozoic intrusions. Gneisses and schists of the Wet Mountains host syntectonic ∼1.7-Ga and ∼1.4-Ga plutons plus two to three generations of sills and stocks. Comparison of two study areas reveals a variation in metamorphic grade, crustal position, and structural rigidity of gneisses hosting the 1.4-Ga intrusions, with implications for the interpretation of dynamic versus anorogenic intrusive settings. An episode of post-1.4-Ga mineral growth was recorded in the Wet Mountains by overprinting mineral textures and 40Ar/39Ar hornblende ages of 1369 ± 4 to 1342 ± 6 Ma.
Mineral textures and rock fabrics provide evidence of three significant Proterozoic deformational events in the Wet Mountains. Predominant northwest- to west-striking foliation is a second-phase fabric, S2, developed during regional plutonism at 1.66–1.7 Ga. S0, relict sedimentary layering, and S1, an earlier penetrative foliation, are preserved within cordierite.
Fabric development and metamorphism during 1.4-Ga magmatism varied across the range. Middle amphibolite-grade gneisses of the Arkansas River Canyon in the north give way to stromatic migmatites in the central Wet Mountains. S2 was completely transposed in two discrete shear zones. The Five Points Gulch shear zone strikes approximately north-south and records sinistral-oblique displacement along a sillimanite mineral lineation. The Newlin Creek shear zone strikes northwest, with top-southwest transport.
Fabric within 1.4-Ga intrusions varies from locally developed foliation on discordant margins (northern Wet Mountains) to strong concordant foliation in extensive sills (central Wet Mountains). Later sills are less well-foliated and slightly discordant, indicating syntectonic granitic emplacement. Blocks of host gneiss were assimilated along some sill margins, attesting to a similarity in temperature between intruded material and country rock.
The variation in degree of metamorphic recrystallization, degree of transposition, and style of intrusion from north to south in the Wet Mountains is attributed to southward increase in temperature and structural depth. This study suggests that pluton emplacement depth influenced structural development and thus bears on the interpretation of dynamic versus anorogenic context for 1.4-Ga magmatism.