BADDELEYITE (ZRO2) AND ZIRCON (ZRSIO4) FROM ANORTHOSITIC ROCKS OF THELARAMIE ANORTHOSITE COMPLEX, WYOMING - PETROLOGIC CONSEQUENCES AND U-PB AGES

The Zr-bearing minerals baddeleyite (ZrO2) and zircon (ZrSiO4) occur w ithin plagioclase-rich (61-95% plagioclase) cumulates of the Laramie a northosite complex (LAG), southeastern Wyoming. In each of the examine d samples, zircon is present as relatively coarse (1-2 mm) interstitia l grains, and baddeleyite occurs as small (0.05 mm) inclusions within cumulus plagioclase. Zircon crystallized between cumulus plagioclase c rystals near solidus temperatures from highly fractionated, Zr-saturat ed liquids. The resultant shape of zircon was controlled by the form o f he remaining pore space. The origin of baddeleyite in the anorthosit ic rocks of the LAC is less well constrained. It may have crystallized early from the anorthositic parental magmas at relatively low silica activities; however, this would require baddeleyite saturation at extr emely low Zr concentrations in the parental magmas (much less than 100 ppm). Baddeleyite and zircon U-Pb ages reveal that several petrologic ally distinct intrusions were emplaced and crystallized in the LAC ove r a relatively restricted 1-3 m.y. interval at ca. 1434 Ma. The Pb-207 /Pb-206 ages obtained for the baddeleyite and zircon in each sample ar e identical within error (+/-1-3 m.y.), and U concentrations are unifo rmly low (<240 ppm), supporting a genetically related origin for the m inerals. Two anorthositic layered cumulates and a crosscutting, oxide- rich troctolite from the Poe Mountain anorthosite have crystallization ages that are identical within error: 1434.4 +/- 0.6, 1434.5 +/- 0.6, and 1434.1 +/- 0.7 Ma, respectively. The only earlier period of anort hositic magmatism that can be identified from the Poe Mountain anortho site is represented by a leucogabbroic xenolith (1436.2 +/- 0.6 Ma), w hich settled onto the floor of the magma chamber that produced the lay ered cumulates. A sample composed almost entirely of zoned, iridescent , plagioclase megacrysts from the Chugwater anorthosite yields a badde leyite age of 1435.4 +/- 0.5 Ma, intermediate between the ages of the xenolith and the layered anorthositic rocks. All the ca. 1.4-1.5 Ga an orthosites in North America, including the LAG, are located near or on Paleoproterozoic boundaries between Archean cratons and accreted Prot erozoic island are terranes. These preexisting crustal structures appe ar to play a major role in the origin and ascent of anorthositic magma s. The evidence for anorthositic magmatism at 1.43 Ga in the LAC sugge sts that there may be a strong genetic link between these high-tempera ture mafic magmas and the regional production of anorogenic granites i n the western and southwestern U.S., many of which have similar crysta llization ages in the interval of 1.43-1.44 Ga.