Generation of ferroan (“A-type”) granite in a subduction zone setting: Petrology of the Golden Horn batholith, North Cascades, Washington, USA

The Eocene Golden Horn batholith (North Cascades, northwestern United States) is an unusual example of coeval ferroan (A-type) and I-type granitoids that formed in a subduction zone setting. Associated mafic to intermediate dikes and stocks (adakites and calc-alkaline basalts to andesites) attest to the presence of mafic magmas in the shallow crust during batholith assembly and imply anomalously hot conditions in the mantle that are attributed to rupture or rollback of the subducting slab. Field, geochemical, and Sr-Nd-Pb isotopic data support generation of the ferroan magmas at midcrustal depth by melting of calc-alkaline plutonic rocks or by protracted differentiation of a parental basalt, and production of the I-type magmas by partial melting of gabbroic rocks at greater depth. Metasomatism by fluids bearing Na, F, and high field strength elements enriched some ferroan rocks to produce peralkaline granite. Mixing between the I-type and ferroan magmas also produced a hornblende-biotite granite with rapakivi feldspars, which is the most voluminous phase of the batholith. Results of this study suggest that ferroan granitoids rarely form in continental arcs because that tectonic regime limits voluminous basalt intrusion into the upper crust, and not because of a lack of suitable source rocks. Their presence in an arc setting likely indicates rollback or tearing of the slab or the presence of a slab window.