THE MIDDLE JURASSIC IRONSIDE MOUNTAIN BATHOLITH: POTASSIC MAGMAS IN A PRIMITIVE ARC SETTING

The Ironside Mountain batholith consists of the volumetrically dominant 170 Ma Ironside Mountain pluton (IMp), quartz diorite of Happy Camp Mountain, and the ~169 Ma Denny and West China Peak intrusive complexes. The contact aureole of the IMp is present in both the western and eastern Hayfork terranes, which indicates emplacement followed regional thrusting. Al-in-hornblende barometry suggests emplacement at ~4 kbar. The Denny and West China Peak complexes are predominantly gabbro to olivine pyroxenite and are 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, water-rich parental magmas. In contrast, the IMp has intermediate plag compositions that decrease with differentiation, and higher FeO/(FeO+MgO) than other plutons in the batholith. It shows K₂O enrichment with increasing SiO₂ to ultrahigh-K compositions, and is typified by 2- and 3-pyroxene assemblages in which primary hydrous mafic silicates are sparse or absent, except in the most evolved rocks. Rb, Zr, and Ba all behave as incompatible elements. Nd and Sr isotope ratios are uniform at epsilon Nd of ~5.2 and initial ⁸⁷Sr/⁸⁶Sr 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, water-poor arc tholeiite. Enrichment of K₂O is interpreted to result from differentiation at relatively high pressure (≥ 8 kbar), 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. The batholith marks the beginning of a 15 m.y.-long episode of arc magmatism during which time crustal contamination of mafic magmas and generation of crustal melts became commonplace. Emplacement of the batholith immediately after regional thrusting placed parental magmas into cool, non-reactive host rocks, resulting in predominantly closed-system fractionation. Succeeding Middle Jurassic magmatism encountered progressively hotter crust, with consequent crustal melting and assimilation.