EVOLUTION OF THE PRECAMBRIAN ROCKS OF YELLOWSTONE NATIONAL PARK (YNP): MAFIC TO INTERMEDIATE PLUTONS

Mafic to intermediate igneous bodies occur in the Precambrian rocks exposed in the Black Canyon and in the Garnet Hill areas of northern YNP. Many of these mafic bodies occur as sheets that cut regional foliation or as tens of meters-scale lenticular bodies that are generally conformable with foliation. On Garnet Hill, the mafic bodies occur in an injected migmatitic complex that cuts across quartz monzonitic and tonalitic stocks and regionally metamorphosed pelitic schists. These rocks range in composition from mafic (amphibolite) to intermediate (hornblende quartz diorite). These rocks locally contain relict igneous clinopyroxene, and were subsequently metamorphosed in the epidote amphibolite facies resulting in the assemblage: hornblende, plagioclase, epidote, chlorite, biotite, and quartz. The depth of emplacement was ~12-15 Km, based on thermobarometry determined from metamorphic assemblages in pelitic schists in the country rock. The amphiboles are zoned to actinolite rims indicating retrograde metamorphism in the greenschist facies. Elemental abundances show a range of SiO₂ contents from 49 to 60%, Mg# from 0.25-0.40, with low LREE abundances (La_n 30-100x) and La/Nb between 4 and 10. Incompatible trace elements (e.g., Zr) show an irregular, but negative, correlation with Mg#. The La/Nb ratios show Nb depletion and suggest derivation in a subduction environment or from altered sub-continental lithospheric mantle. A U/Pb zircon age acquired by LA-ICP-MS for a hornblende quartz diorite body located at Garnet Hill is 2.799 +/-0.003 Ga (2σ). Geochemical data suggest similarities to the petrogenesis of rocks of intermediate composition in the Long Lake magmatic complex in the adjacent Beartooth Mountains, where intermediate to felsic magmatic rocks were generated at the same time (~2.8 Ga over a ~40 Ma time span), but from different geochemical sources and depths of melting (Mueller et al., 2010). Intermediate compositions in northern YNP may, therefore, be shallower crustal equivalents to the Long Lake Magmatic complex. Alternatively, these rocks could be related to the post-orogenic (2.8 Ga) mafic dikes of the adjoining Beartooth Mountains.