Trace element and isotopic constraints on magmatic evolution at Lassen Volcanic Center

Abstract

Magmatic evolution at the Lassen volcanic center (LVC) is characterized by a transition from predominantly andesitic to predominantly silicic volcanism with time. Magmas of the andesitic, or “Brokeoff phase” of volcanism range in composition from basaltic andesite io dacite, whereas those of silicic, or “Lassen phase” range in composition from basaltic andesite to rhyolite. The compositions of magmas from each phase define well organized but distinct variation trends. Compared with Brokeoff‐phase magmas of similar SiO2 content, most Lassen‐phase magmas contain lower concentrations of most incompatible minor and trace elements. Based on the behavior of both incompatible and compatible trace elements, the geochemical trends defined by the Brokeoff‐phase magmas cannot be ascribed merely to fractional crystallization from a single or multiple mafic parental magmas, Moreover, the Lassen‐phase magmas cannot be derived from the Brokeoff‐phase magmas by fractional crystallization. Rather, the geochemical trends that characterize each volcanic phase define arrays that primarily indicate mixing between well‐homogenized silicic and heterogeneous mafic magmas. The distinctive mixing‐dominated arrays for each volcanic phase manifest the generation and evolution of two physically distinct, but genetically related magma systems. The LVC magmas have Sr, Nd, and Pb isotopic characteristics that approximate two‐component mixing arrays. One isotopic component is similar in composition to that of NE Pacific Ocean ridge and seamount basalts (“MORB” component), the other to mafic Mesozoic granitoids sampled from the neighboring KSamath and Sierra Nevada provinces (“KSN” component). The isotopic compositions of the most silicic LVC magmas lie within the ranges defined by the more mafic LVC magmas, which in turn lie within broad ranges defined by primitive mafic lavas sampled from the Lassen region. The lack of a correlation between the major element and isotopic compositions of LVC magmas seriously limits any model for magmatic evolution that relies on assimilation of old middle to upper crust by isotopically homogeneous mafic magmas during their ascent through the crust. Alternatively, the isotopic and geochemical uniformity of the most silicic magmas of the Brokeoff and Lassen phases suggests that they are well‐homogenized partial melts. The likely source region for these silicic melts is the lower crust, which we envision to consist primarily of mafic igneous rocks that are similar in geochemical and isotopic diversity to the regional mafic lavas. Magmatic evolution at LVC can be viewed in terms of a series of mantle melting events that subsequently stimulated meiting in a progressively increasing volume of the lower crust. In general, the LVC magmas represent slightly fractionated mixtures of the mantle‐derived mafic magmas and silicic partial melts of the lower crust, the latter melts increasing in relative proportion over the history of the volcanic center. The voluminous rhyolitic lavas and pyroclastic materials erupted during the early Lassen phase represent lower crustal melts thai pooled into sufficient volumes to avoid significant blending and dilution with intruding mafic magmas. The geochemical and isotopic heterogeneity of both Brokeoff‐phase andésites and mafic magmatic inclusions in the silicic Lassen‐phase magmas must in part record the corresponding diversity of the mantle‐derived contributions to arc magmagenesis in this region.