The Laramie Anorthosite Complex and the Sherman Batholith, Southeastern Wyoming

ABSTRACT

The Laramie anorthosite complex and Sherman batholith in southeastern Wyoming represent one of the most accessible and best-studied anorthosite-mangerite-charnockite-granite (AMCG) suites in the world. The Laramie anorthosite complex and the Sherman batholith were emplaced between 1440 Ma and 1430 Ma. The Laramie anorthosite complex consists of three anorthosite plutons. The Chugwater anorthosite, which is the oldest at 1435.6 ± 0.3 Ma, forms a partial dome on the southern margin of the complex, whereas the Poe Mountain anorthosite, which was emplaced at 1434.4 ± 0.4 Ma, forms a partial dome on the northern margin of the complex. These two plutons do not touch; both are intruded by the Snow Creek anorthosite. In the center of the complex, the anorthositic plutons are intruded by the Strong Creek complex, a suite of mafic to granitic rocks. On the margins of the complex, anorthositic plutons are intruded by the monzonitic Sybille, Maloin Ranch, and Red Mountain plutons. The Sybille and Maloin Ranch plutons only slightly postdate the Chugwater and Poe Mountain anorthosites, whereas the Red Mountain pluton is the youngest intrusion in the Laramie anorthosite complex at 1431.3 ± 1.2 Ma.

The Sherman batholith occurs in several localities across southeastern Wyoming. The Mule Creek lobe lies on the northeastern margin of the Laramie anorthosite complex, where it has been intruded by the Laramie anorthosite complex. The main Sherman batholith lies to the south of the Laramie anorthosite complex, and some portions of the Sherman batholith are transitional with portions of the Maloin Ranch pluton. Other exposures of the Sherman batholith lie in the Medicine Bow Mountains to the west of the Laramie Mountains. Contact relations and U-Pb ages show that the Sherman batholith straddles the age of the Laramie anorthosite complex. The Mule Creek lobe was emplaced at 1437.7 ± 2.4 Ma, whereas the main granite in the southern lobe of the Sherman batholith is 1433.0 ± 1.5 Ma, and the late granitic dikes in the batholith date from 1430.6 ± 2.6 Ma.

The Laramie anorthosite complex is mostly anhydrous, and anhydrous portions are present in the Sherman batholith. The anhydrous rocks are dominated by olivine, pyroxenes, and, to a lesser degree, Fe-Ti oxides. The rocks are unmetamorphosed, except for localized hydration. Fe/(Fe + Mg) values in olivine and pyroxene from the anorthositic rocks range from 0.5 to 0.8, and in the monzonitic rocks, these values range from 0.8 to >0.95. Similar to the monzonitic rocks of the Laramie anorthosite complex, the Sherman batholith contains very iron-enriched ferromagnesian minerals. In places, these include fayalite, ferroaugite, and inverted pigeonite, but the ferromagnesian minerals in most of the Sherman batholith are generally Fe-rich hornblende and biotite.

Both the Laramie anorthosite complex and the Sherman batholith have been intruded across the Cheyenne belt, a major tectonic boundary that separates the Archean Wyoming Province from the Proterozoic rocks that lie to the south; as such, it, along with the Nain plutonic suite, is one of the two AMCG suites in North America that has intruded Archean rocks. The fact that portions of the Laramie anorthosite complex and Sherman batholith intrude Archean rocks allows petrologists to use Nd, Sr, and Pb isotopes to argue with certainty that the AMCG suite in southeastern Wyoming had mantle sources, although the plutons of the Laramie anorthosite complex and Sherman batholith record various degrees of assimilation of continental crust.

Our model posits that the Laramie anorthosite complex and Sherman batholith were both derived from a high-Al basaltic magma, evidence of which is found as high-Al gabbros that occur as dikes and small intrusions throughout the Laramie anorthosite complex. This magma was emplaced at the base of the crust, where it underwent differentiation at high pressures. The rocks rich in olivine, pyroxene, and Fe-Ti oxides produced by this differentiation remained at the crust-mantle boundary, whereas the plagioclase produced by this differentiation, which was around An50 (50% anorthite), rose diapirically into the overlying continental crust, along with the evolved melt, to produce the Laramie anorthosite complex. To this point, our model mostly agrees with those of many current workers. However, we infer that C-O fluids are likely to have played a major role in the evolution of the Laramie anorthosite complex. We also differ from many researchers by arguing that the alkalic to alkali-calcic nature of the Sherman batholith also reflects high-pressure differentiation, and its ferroan composition suggests differentiation from tholeiitic basalt. It is evident the Laramie anorthosite complex and Sherman batholith both evolved from high-pressure differentiation of mantle-derived melts. Along with having similar conditions of formation, the two igneous suites are coeval, cospatial, and potentially comagmatic.