Pearcy, L.G., 1991, Island-arc petrogenesis and crustal growth: Examples from Oregon and Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 184 p., illust., maps.
The bulk compositions determined by mass balance of two exposed sections of intra-oceanic island-arc crust, the Talkeetna volcanics and Border Ranges ultramafic-mafic complex in SE Alaska, and the Canyon Mountain Complex in NE Oregon, are basalt (MgO = 11%) and probably basaltic andesite (MgO = 8%), respectively, with unfractionated REE abundances approximately 10 times chondrite. Simple accretion of arcs such as these cannot generate an LREE-enriched, andesitic, post-Archean continental crust. To do so requires modification of their bulk compositions by a combination of processes, including (a) delamination of basal cumulates to make the crust less mafic, (b) addition of alkalic magmas and accreted rocks to enrich the average crust in LREE and other lithophile elements, and (c) partial melting of the lower crust combined with delamination of the residue to do both of the above. The 2- to 3-km-thick ultramafic-to-mafic 'transition zone' (TZ) in the Early Permian Canyon Mountain Complex represents the products of mantle melts that crystallized in a nascent island-arc setting. This zone consists of complexly interlayered pyroxene-rich cumulate rocks, with igneous textures and mineralogy pointing to formation by in situ crystallization and fractionation at 5-10.5 kb (15-30 km depth) under water-poor conditions. Major- and trace-element compositions of clinopyroxene, and the order and relative proportions of phases crystallized indicate that parental magmas were primitive island-arc tholeiitic basalts with some similarities to boninites. Variations in clinopyroxene chemistry reveal trace-element heterogeneity in the parental melts, which may be due to either variable source compositions or disequilibrium mantle melting. Magmas entered the TZ in at least three batches, forming crude, large-scale, cyclic units in some locations. During slow cooling, the TZ rocks were apparently deformed and uplifted to near-surface levels. The entire Canyon Mountain Complex most likely represents incipient arc magmatism in the Olds Ferry terrane. The complex was transferred to the dismembered forearc region (that is, Baker terrane) by tectonic erosion of the leading edge of the arc crust. It was then uplifted and emplaced within serpentinite-matrix melange along faults lubricated by serpentinite diapirs.
Theses and Dissertations
