SM-ND ISOTOPIC, AND GEOCHEMICAL CHARACTERIZATION OF MAFIC MAGMATISM IN THE BELT-PURCELL AND WYOMING BASINS, COMPARING THE SINGLE PULSE CA. 780 MA GUNBARREL AND THE MULTIPLE PULSES OF THE MOYIE-PURCELL CA. 1460 MA INTRAPLATE MAGMATIC EVENTS

Meso- and Neoproterozoic intraplate magmatism exposed along the western side of the Laurentian margin between southern British Columbia, Canada, and the Montana-Wyoming border (MWB), USA can be divided into at least 12 geochemical groups, 9 of which group into two Large Igneous Provinces (LIPs): 8 in the ca. 1460 Ma Moyie-Purcell LIP (MPL) which consists of i) the Purcell lavas and other dykes and sills in the USA, ii)the gabbroic Moyie sills and dykes in Canada. This event is composed of tholeiitic basalts and basaltic andesites. The ca. 780 Ma Gunbarrel LIP (GL) is 1 geochemical group, extends from Yukon Territories in Canada to the MWB in the USA, although herein we focus on tholeiitic basalt to basaltic andesite dykes, sills and volcanic rocks from MWB to Idaho. We integrate new geochemical and isotopic data with previous studies to model the MPL in terms of mantle source, interactions with lithosphere (mantle and crust) and fractionation, and contrast these results with the GL.

The MPL and GL have distinct geochemical fingerprints. Some units of both LIPs exhibit a negative Nb-Ta anomaly interpreted to reflect interaction with subduction-modified lithospheric mantle. The MPL consistently plot with lower (Gd/Yb)_PM (1.5-1.0) than the GL which has a higher ratio (1.8-1.4), indicating different depths of melting above and below, but adjacent to the garnet-spinel transition. (La/Sm)_PM for the MPL (3.1-1.2) have much more variable crustal and lithospheric mantle contribution compared to the more tightly constrained slope (2.3-1.4) of the GL. Sm-Nd isotopic analyses show a single GL pulse but at least 4 distinct isotopic signatures within the 8 geochemical MPL pulses, indicating mantle melting at or near the garnet-spinel transition with contributions from crustal and lithospheric mantle sources. Samples from the northern Belt-Purcell basin have tightly constrained eNd_(i) ~(-1) compared to highly positive eNd_(i) (0 to +7 ) values from those in the southern Belt-Purcell and Wyoming basins. GL dykes have a positive eNd_(i) of ~(+1). There are 3 geochemical groups of dykes of unknown age within the Wyoming basin that have low eNd_(1460Ma?) of (-8 to 0), indicating greater crustal contribution or a different magmatic source.