A NEW FIND OF CALC-ALKALINE LAMPROPHYRES IN WOOD COUNTY, WISCONSIN, USA: A BRIEF SUMMARY OF THE EVIDENCE

In this research effort, a new find of calc-alkaline lamprophyres in the Pembine-Wassau Terrane is outlined and characterized. As the samples contain primarily biotite phenocrysts in a groundmass of orthoclase, they are classified as minettes, which are members of the calc-alkaline suite of lamprophyres. This represents the first characterization of these lamprophyres, and serves as a brief summary of the evidence so that future inquiries may proceed with preliminary knowledge of the minettes’ petrology, geochemistry, and genesis. The Wood County minettes are highly alkaline (potassic) in character and are composed of phlogopitic biotite, diopsidic augite, and lesser amphibole in a groundmass of potassium feldspar, FeTi-oxides, apatite, accessory sodic plagioclase, and sphene. The Wood County minettes display subduction-related affinities in their major and trace element systematics, such as enrichment in LILE and a relative depletion in HFSE. Additionally, the trace element values for the studied minettes plot within the arc volcanism field on tectonic discrimination diagrams. Based on the comparison of trace element abundances with melting model calculations, the empirical values obtained here can be generated by extremely minute degrees of partial melting (0.2%) of a spinel peridotite followed by a sequence of fractionation. Geochemically, the lamprophyres studied here display pronounced Ba enrichments coupled with negative Nb-Ta and Zr-Hf anomalies on primitive mantle-normalized trace element plots, which indicate a relative depletion in the HFSEs, and are typical of low-volume, arc-related magmas. Sr contents vary between the samples, possibly indicating varying amounts of plagioclase or titanite in the samples. Ba/Rb ratios are much lower (4.14-5.4) than the samples’ Ba/Sr ratios (14.4-30.9), indicating the predominance of phlogopite over pargasite in the source. The samples display high Th (~100 times primitive mantle), U (~100 times primitive mantle), Cs (~300-600 times primitive mantle), Rb (~200-500 times primitive mantle), and Ba (~1000 times primitive mantle).