The Ironside Mountain batholith consists of the 170-Ma Ironside Mountain pluton, the quartz diorite of Happy Camp Mountain, the 167-Ma Denny intrusive complex, and the West China Peak intrusive complex. The contact aureole of the Ironside Mountain pluton is present in both the western and eastern Hayfork terranes, which indicates that emplacement followed regional thrusting along the Wilson Point thrust. The batholith marks the beginning of a 15-m.y.-long episode of arc magmatism. Al-in-hornblende barometry suggests emplacement at ∼4 kb. The Denny and West China Peak complexes are predominantly gabbro to olivine pyroxenite characterized by calcic plagioclase. Along with the quartz diorite of Happy Camp Mountain, these plutons have calcic bulk compositions, abundant amphibole, and low FeO/(FeO + MgO). Their chemical and petrographic features suggest oxidized, H 2 O-rich parental magmas. In contrast, the Ironside Mountain pluton has higher FeO/(FeO + MgO), shows K 2 O enrichment with increasing SiO 2 to ultrahigh-K compositions, and is typified by 2- and 3-pyroxene assemblages in which hydrous mafic silicates are sparse or absent, except in the most evolved rocks. The elements Rb, Zr, and Ba all behave incompatibly. Initial Nd and Sr isotope ratios (170 Ma) are uniform at 87 Sr/ 86 Sr ϵ Nd of ∼5.2 and of ∼0.7037. These data suggest that the pluton lacks significant crustal input, unlike younger Middle Jurassic plutons, and that it evolved primarily by fractional crystallization of a reduced, H 2 O-poor arc tholeiite. Enrichment of K 2 O is interpreted to result from differentiation at relatively high pressure (≥8 kb), under conditions in which augite fractionation predominated over olivine fractionation. High-P fractionation without appreciable crustal input is probably related to the thermal structure of the crust. Emplacement immediately after regional thrusting placed parental magmas into cool, nonreactive host rocks. Succeeding Middle Jurassic magmatism encountered progressively hotter crust, with consequent crustal melting and assimilation.