Initial subduction-related magmatism in southern Alaska identified by geochemistry and zircon Hf-O isotopes

Abundant arc-type magmatic and metamorphic rocks exist on Earth today, which provide insights into the equilibrium state of the subduction process. However, magmatic samples generated during the initial stage of subduction is largely unknown. This hinders our understanding of the subduction initiation process and by inference, the onset of plate tectonics as well as the history of crustal formation. To address this issue, we carried out a comprehensive geochemical-geochronological study of a suite of Late Triassic to mid-Jurassic plutonic rocks from southern Alaska that potentially represent magmas from the initial to mature stages of arc formation. While all studied samples show typical arc-type geochemical signatures, i.e., enrichment of large ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs) relative to the heavy rare earth elements (HREEs), the Late Triassic trondhjemites show unique geochemical features such as strongly positive ε_Hf(t) and ε_Nd(t) coupled with lower δ¹⁸O (average 4.77‰ ± 0.09‰). These signatures, along with its higher zircon saturation temperatures compared with younger plutonic rocks, are best explained by shallow partial melting of subducting high-temperature hydrothermally altered lower oceanic crust (i.e., gabbro). If true, these surprising findings would open up new ways to study subduction initiation which would have important bearing on future research on the onset of global plate tectonics and the formation of the continental crust.