PRECAMBRIAN GEOLOGY OF YELLOWSTONE NATIONAL PARK (YNP) AND SURROUNDING AREAS: PETROGENESIS OF LEUCOGRANITES

Precambrian leucogranites in Yellowstone National Park (YNP) are relatively unusual, but significant. An earlier related study found a single leucogranitic pluton that is a peraluminous, S-type granite representing a dry, high-temperature crustal melt (821-863°C and 7.0-7.3 kbar). The current investigation extends the range of leucogranitic rocks across the Slough Creek area of YNP, allows for further constraints on their petrogenesis, and details the relations with proximal igneous rocks.

Meter-scale leucogranite outcrops are geographically isolated and occur proximal to a major shear zone, intruding a suite of mesozonal/catozonal (magmatic epidote-bearing) felsic plutons. The leucogranites cross-cut metaluminous igneous and meta-igneous units present, implying that the leucogranite suite is younger and is derived from a different source area. These rocks consist primarily of quartz, K-feldspar, and plagioclase of varying compositions, as well as biotite, muscovite, apatite, zircon, and secondary chlorite. Although the leucogranite previously studied in this area contains garnet, none has been found so far in this investigation. Subsolidus igneous textures encountered include myrmekite, perthite and antiperthite. Locally, the leucogranites display varying degrees of mylonitization with blastomylonitic and recrystallization textures. Deformation twinning in the feldspars is common. The amount of recrystallization and the occurrence of grain boundary migration in the quartz grains imply late-stage, high-temperature deformation. Point counting of representative samples of the leucogranites classifies the rocks as granites and granodiorites.

Whole rock geochemical analyses (XRF and ICP-MS) are in progress. Backscattered electron imaging reveals that zircon grains are commonly metamict, with some crystalline zircon remaining. Therefore, mineral separation will isolate zircon grains to be used for LA-ICPMS analysis for U-Pb dating. The working interpretation of the leucogranites is that they may have been generated by partial melting of a pelitic or felsic gneiss in response to late decompression melting along shear zones, similar to the leucogranites (e.g. Manaslu) of the present day Himalaya Mountains.