A COMPARISON OF MIDDLE CRUST FORMED IN ISLAND ARC AND CONTINENTAL MARGIN SETTINGS FROM THE ALASKA RANGE

The Alaska-Aleutian Range batholith (ARB) records long-lived and spatially overlapping plutonism from ca. 210 to 25 Ma, with at least three distinct stages of pluton emplacement separated by magmatic lulls. The oldest stage includes rocks from the Jurassic Talkeetna arc, considered a type example of an intra-oceanic arc (IO). Arc volcanic rocks and coeval plutons from this stage (210-150 Ma) make up the eastern portion of the ARB and are key components of the Peninsular terrane of southern Alaska. The Talkeetna arc was accreted to the southern Alaska margin during the second ARB stage, which lasted from ca. 110 Ma to 55 Ma. Arc accretion and associated deformation are thought to have been completed by ca. 80 Ma, but intrusive magmatism during this continental margin (CM) stage persisted for ca. 25 My after “docking” of the Talkeetna arc. Volcanic rocks from these two older stages are absent or only locally preserved in the study area. The youngest ARB stage, from ca. 44 to 25 Ma, is represented by volcanic deposits locally intruded by cogenetic plutons. Here we focus on middle crustal plutons associated with the two older stages.

Similarities and differences in the chemistry of ARB magmatic stages have important implications for mechanisms and timing of middle crust formation in different tectonic settings, and suggest constraints on the production of new continental crust. Zircon U-Pb ages from both IO and CM stages show that both are represented by ca. 50 My of continuous crystallization intervals, and both stages feature the onset of high Dy/Yb, Sr/Y, and Nb/Yb compositions after ca. 25 My. However, CM rocks are always higher in Nb/Y, and are more alkaline (less calcic) at similar degrees of differentiation relative to IO rocks. Average CM trace element composition reflects global estimates of average continental middle crust, whereas IO rocks do not. While both stages indicate an increasing role for partial melting of lower crust over time, the initially thicker continental margin crust (relative to a juvenile oceanic arc) at the incipient stage of collision may yield a greater fraction of lower crustal melting, which, under modern geothermal conditions, may be required to form new middle crust with a “continental” trace element endowment.