Trace element geochemistry of the Louis Lake batholith of early Precambrian age, Wyoming

The behavior of Rb, Sr, Eu, Zr and Ni in the Louis Lake batholith as a function of increasing fractionation from the granodiorite to the aplite stage is similar to the predicted and often observed behavior for these elements in calc-alkaline suites and can be explained in terms of the removal of feldspar (Rb, Sr, Eu), zircon (Hf, Zr), and magnetite (Ni) during fractional crystallization. Cs decreases slightly and Ba and REE remain approximately constant over this interval and require special conditions to explain their behavior. Increasing relative fractionation of the REE are not observed over this range. An increasing Eu anomaly is observed from the granodiorite to the granite stage and can be related to plagioclase removal. The absence of a Eu anomaly in the aplites may be caused by partial reequilibration of the aplitic magmas with the voluminous granodiorite. Striking depletions in Rb and Cs relative to K are observed in the aplites and are interpreted in terms of escaping late-stage magmatic fluids enriched in these elements. Rb is contained in biotite during the early and middle stages of fractionation in K-feldspar during the latter stages. The D_Rb ^{Biot KF} does not vary significantly with fractionation while the D_Rb ^{Biot Plg}decreases by more than a factor of three. La, Ce, and Sm are contained chiefly in minor phases (dominantly apatite and sphene) in all but the very late stages of crystallization where they are contained chiefly in feldspars. D ^{Biot KF} and D ^{Biot Plg} for La and Sm do not change appreciably with fractionation. D_Ce ^{Biot KF}and D_Ce ^{Biot Plg} are less than the La and Sm distribution coefficients and increase significantly at the quartz monzonite fractionation stage. This increase is tentatively interpreted in terms of a decrease in F_O2 causing a reduction of some Ce⁴⁺ to Ce³⁺. Element distributions in the Louis Lake batholith when compared with those in the surrounding metagraywacke terrane indicate that the batholith could not have been readily produced by complete or partial melting of this terrane. The lack of a significant depletion in Ni and the low K Na ratio in the batholith as a whole do not favor an origin by fractional crystallization of high-alumina basalt. Also the absence of a Eu anomaly and the low K Na ratio do not support an origin by partial melting of amphibolite. An origin for the parent granodiorite magma involving hydrous partial melting of quartz eclogite together with minor amounts of more highly differentiated rocks in the upper mantle is consistent with the geochemical data.