Abstract

The Laramie batholith, which is exposed over an area of about 2000 km2 in eastern Wyoming, has gradational contacts on the north and southeast with metamorphic terranes (the northern and central metamorphic complexes, respectively). The northern metamorphic complex is composed dominantly of gneiss, and the central metamorphic complex is composed dominantly of gneiss and migmatite. Anorthosite and related syenites intrude the central metamorphic complex on the south, and mafic dikes cross-cut all rocks in the northern and central parts of the range.

Foliation in the northern metamorphic complex strikes west-southwest to east-southeast. In the central metamorphic complex the strike varies from northeast (on the north) to northwest (on the south). Relict foliation (striking dominantly northeast) and occasional amphibolite roof pendants occur in the batholith.

Elements in the batholith appear to have lognormal-type distributions with Fe, Na, Sr, Zr, Mn, Ca, Ti, Al, and Mg showing positive skewness and Si, K, Ni, and Rb negative skewness. Element dispersion increases as concentration decreases. The batholith, on the whole, is extremely uniform in composition and appears to have followed a typical calc-alkaline differentiation trend. Gneisses of the northern metamorphic complex are enriched in Fe, Mg, Ca, Ti, Mn, and Ni, and are depleted in Al, Na, and Rb compared to batholithic granitic rocks. Gneisses and migmatites of the central metamorphic complex are similar in composition to the batholith, differing only in relative enrichment of Ca and Sr and depletion of Si, K, and Rb.

K/Rb and Rb/Sr ratios are interpreted in terms of a crystal-melt equilibria origin both for the batholith and the light, granitic migmatite bands. These ratios indicate that the light migmatite bands were probably not produced by partial melting of the enclosing gneisses or by the introduction of material derived from the batholith.

The normative composition of the batholith, interpreted in light of experimental data in the system Ab-An-Or-Q-H2O, is also most easily explained in terms of a crystal-melt equilibria origin. Such compositional data, however, are not sufficient to support or negate any of the popular theories regarding the origin of the migmatites in the central metamorphic complex.

Considered collectively, the field, petrographic, and major and trace element data presented in this paper are perhaps most easily interpreted in terms of an origin for the Laramie batholith, involving the fractional crystallization of an intermediate or mafic magma derived by the partial melting of the lower crust or upper mantle.

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