METAMORPHISM OF METAPELITES IN THE LITTLE COTTONWOOD CONTACT AUREOLE, UTAH, USA

The geometry and excellent exposure of the Little Cottonwood contact aureole allow a detailed study of systematic changes of mineral assemblages in metapelites as a function of distance from the contact, as well as along the contact. The Little Cottonwood granodiorite stock (30 Ma [1]) is located in the Wasatch Mountains, southeast of Salt Lake City. The Wasatch block started uplifting about 10 Ma ago, along a normal fault in the western part, resulting in 10-30° antithetic rotation [2, 3, 4, 5] and a minimum throw of 4 km [5, 6].

Here we present a metamorphic map of the aureole, based on approx. 500 samples. The metamorphic assemblages of metapelites are divided into: (i) biotite (bt) zone (bt + muscovite (mu) + quartz (qtz) + chlorite (chl) + plagioclase (plg)); (ii) cordierite (crd) + andalusite (and) zone (crd + and + bt + mu + qtz ± chl ± plg); (iii) K-feldspar (kfs) + and or kfs + crd zone (kfs + and/crd + bt + mu + qtz ± plg); (iv) kfs + sillimanite (sil) zone (sil + kfs + and/crd + bt + mu + qtz ± plg); and (v) a migmatite zone (kfs + sil + and/crd + bt + mu + qtz + plg). The vertical relief of 2000m and the rotation of the block are nicely reflected in the mapped isogrades. Together with phase petrology calculations they suggest significant rotation of the Wasatch block. Assemblages at the intrusive contact change from and + kfs in the east to sil + kfs in the west at current equivalent altitudes. Andalusite is metastable throughout zones (iv) and (v). Sil never forms as a reaction product of and, but rather replaces mu and bt, so that the appearance of sil reflects either mu or bt breakdown, depending on rock chemistry. The Wasatch front faults cut through the western contact aureole, complicating detailed phase petrology analysis in that part of the aureole.

[1] Crittenden et al. (1973) J. Res. US Geol. Survey 1, 173 - 178; [2] John (1987) PhD thesis, Stanford University, pp. 236; [3] Pak (1998) PhD thesis, Madison University, pp. 252; [4] Wohlers & Baumgartner (2013) J. metamorphic Geol. 31, 301-312; [5] Naeser et al. (1983) Geol. Soc. Am. Memoir 157, 29 - 36; [6] Parry & Bruhn (1987) Geology 15, 730 - 744