Felsic plutonic rocks formed in arcs are buoyant with respect to mantle peridotite over the entire range of relevant pressures and temperatures. They tend to remain at the Earth?s surface, to form the fundamental building blocks of continental crust. In the Aleutians, most felsic plutonic rocks have compositions that overlap estimates for the bulk composition of the continental crust, and that are distinctly different from spatially associated lavas. Understanding the genesis of Aleutian felsic plutonic rocks is a key to understanding continental genesis and evolution via arc magmatism, a key science goal for the MARGINS and GeoPRISMS Initiatives. The PIs will address the following questions: (1) Do Aleutian plutonic rocks have an isotopically distinct source composition, compared to nearby lavas? If they do, this is vitally important since it is commonly assumed that erupted basalts are representative of the magmatic flux from the mantle into arc crust. If not, we will evaluate how they can be explained as the result of different differentiation processes operating on the same parental melt. (2) Has there been compositional variation in the Aleutian arc over time? Do differences between plutonic and volcanic rocks represent temporal evolution of the arc, or different modes of magma transport and emplacement for different magma compositions? And (3) are high viscosity felsic magmas preferentially emplaced in plutons, while low viscosity, mafic magmas preferentially form lavas? What biases does this introduce, when lavas are presumed to be representative of arc magmatic processes and compositions? Broader Impacts: The Aleutian arc poses numerous hazards to society. Understanding subduction processes helps to predict, avoid, and/or mitigate the hazardous consequences of volcanic eruptions, landslides and earthquakes. During this project, a graduate student will be trained in research, and will likely be able to help design and participate in a larger field research program based on this pilot project.
The goal of this project was to determine the origin of compositional difference between the continental crust and average island arc lavas. The formation and accretion of island arcs is considered one of the most important processes in the formation of continental crust, both today and likely throughout Earth history. Understanding compositional differences between arc volcanic rocks and average continental crust is therefore a key to understanding continental genesis and evolution via arc magmatism, an important science goal of the GeoPRISMS Initiatives. The classic textbook view of magmatism dictates that the eruptive magmas (the lavas) represent fractionated melts from an evolving magma body while the intrusive magmas (the plutons) represent the counterpart - the crystal cumulates from the same magma. However, prior studies in the Aleutian volcanic arc, Alaska, had suggested that the volcanic and plutonic rocks may have distinct compositions, and that the composition of the Aleutian plutonic rocks may be similar to the composition of average continental crust. Investigating the origin of the differences between volcanic versus plutonic magmas, and better constraining the compositions of arc plutons, could therefore provide insight into the origin of continental crust through arc magmatism. In this pilot study we investigated the relationship between arc plutonic and volcanic rocks using existing samples from the intra-oceanic Aleutian arc. We examined major and trace element contents and isotopic compositions of 22 mafic to intermediate plutonic rocks across the Aleutians arc and compared them with spatially associated volcanic rocks. Our new U-Pb zircon geochronology indicates that the studied plutonic rocks are Eocene to Miocene (9-40 Ma) in age. We found that the plutons show stronger calc-alkaline signatures (i.e., higher SiO2 at a given Mg#) compared to Holocene volcanic rocks from the same region. The plutonic rocks also have higher εNd- and εHf-values, and lower Pb isotope ratios than the volcanic rocks. The plutons therefore resemble volcanic compositions from the western Aleutian arc isotopically, and have compositions similar to the bulk continental crust. The isotopic signatures of our plutonic samples cannot be generated by shallow level crustal assimilation or magma fractionation in the Aleutians. Instead, the data require distinct parental magma sources for the plutonic and volcanic rocks. A first-order implication of our findings is that erupted magmas alone might not be an accurate guide to the composition of the bulk arc crust. Our new data may reflect different processes of transport and emplacement for the magmas that form plutons and lavas. The sources for the calc-alkaline magmas likely have higher SiO2, H2O and fO2 than the sources for tholeiitic magmas. As a result, calc-alkaline magmas may undergo a rapid viscosity increase during mid-crustal degassing due to their initially high water contents and preferentially form plutons. With currently available data, we cannot rule out the possibility of temporal variations of Aleutian magma source compositions, from Paleogene-Neogene "isotopically depleted" and predominantly calc-alkaline compositions to Holocene "isotopically enriched" and tholeiitic compositions. Our study highlights the need for more detailed studies of Aleutian plutons and pre-Holocene, central Aleutian lavas.
