LITHOGEOCHEMISTRY OF BUFFALO HUMP FORMATION, PROTEROZOIC DEER TRAIL GROUP, WASHINGTON: IMPLICATIONS FOR PROVENANCE, TECTONIC SETTING AND WEATHERING

Petrographic and geochemical analyses of selected samples of Buffalo Hump Formation (BHF) of Proterozoic Dear Trail Group, Washington, were carried out to determine the detrital mineralogy and geochemical signatures for the purpose of deducing provenance composition, tectonic setting and weathering conditions. These aspects of BHF are important to establish its, much debated, correlation with Beltian rocks of Western Montana. Buffalo Hump Formation (BHF) is composed of two lithologic units; a lower fine-grained argillite unit and an upper quartz arenite unit which is medium- to very coarse-grained and moderate to poorly sorted. The upper arenite unit is dominantly siliciclastic having 57% Qm, 38% Qp, and 5% rock fragments (including MRFs). Removal of feldspar by diagenetic processes has prohibited the interpretation of original composition of a large number of framework grains and matrix. However, the whole rock geochemistry has provided insights to characterize the protolith and provenance conditions. The systematics and distribution patterns of few immobile and rare earth elements (REEs) suggest the derivation of detritus from a hetrogenetic source having variations in composition with time.

The geochemical analyses revealed that the BHF is composed of 22% Al₂O₃ which makes it a highly aluminous composite of metasediments. On the other hand it has 3.6% FeO_(t), 0.5% Na₂O and 2.7% K₂O that are considered low concentrations. Average CIA value; ≈80, is indicative of intense chemical weathering. The concentrations of LILE and transition elements; Co, V, Ni and Zn, are significantly lower than PAAS whereas variations and distribution patters of REEs indicate that some samples are depleted in HREE, but most of the samples have reduced values of LREE, and lower Eu/Eu* and Gd_N/Yb_N ratios. Concentrations of Cr and Ga are much higher suggesting the derivation of sediments from Archean crust. The geochemical signatures suggest that protolith of BHF was composed of highly differentiated granite rocks and juvenile mantle material. Metamorphic rocks associated with the granite intrusions also contributed to the BHF in its upper part. Discrimination diagrams suggest the deposition of Buffalo Hump sediments in a shallow water basin on passive continental margin with local rifting and mafic magmatism.