PLAGIOCLASE COMPOSITIONS OF POISON LAKE CHAIN PRIMITIVE BASALTS, LASSEN REGION, SOUTHERN CASCADES

The Poison Lake chain (PLC) is located 30 km east of Lassen Peak in the southern Cascades. Eight compositionally distinct groups of calc-alkaline basalts erupted in a small area (<30 km²) over a short period of time (~100 ±10ka; 1). Eruption of PLC basalts are likely the result of proximity to the Basin and Range where extension allowed small batches of magma to reach the surface with minimal modification. Major and trace element compositions and isotope ratios of PLC primitive basalts (Mg# = 59-76) indicate they are not related to one another by crustal processes, but are consistent with heterogeneous mantle source components. Investigation of mantle heterogeneities have used three of the most primitive basalt units and a fourth unit from a unit with a compositionally distinctive whole rock composition (basalts of Pittville Road, bt) to represent four unique mantle domains. Among the four units, plagioclase compositions in the three most primitive units have the highest An core compositions (>0.70 An), only minor normal zoning (<5% An), lower FeO (<1.2%), BaO (<0.08%) and SrO (<0.43%) than the bt unit. The three most primitive units have plagioclase that are distinguished from one another by combined variations of FeO, MgO, SrO and BaO. Elsewhere in the PLC, less primitive basalts have plagioclase crystals with distinctive textural variations including >10% An normal zoning, reverse and oscillatory zoning and in some cases sieve textures as well as more evolved An compositions and lower FeO and SrO. Such variations are associated with crustal processing, which has also been modeled from geochemical compositions of these samples, but are not observed in the mineral or whole rock compositions of the most primitive samples. Thus, plagioclase trace element compositions in the most primitive samples are distinct from plagioclase in units that have undergone crustal processing, and are consistent with geochemical interpretations that the most primitive PLC basalts are good representations of heterogeneous mantle- compositions. (1) Muffler et al., 2011