Petrology and geochemistry of the Middle Jurassic Ironside MountainBatholith; evolution of potassic magmas in a primitive arc setting

Abstract

The Ironside Mountain batholith consists of the 170-Ma IronsideMountain pluton, the quartz diorite of Happy Camp Mountain, the 167-MaDenny intrusive complex, and the West China Peak intrusive complex.The contact aureole of the Ironside Mountain pluton is present in boththe western and eastern Hayfork terranes, which indicates thatemplacement followed regional thrusting along the Wilson Point thrust.The batholith marks the beginning of a 15-m.y.-long episode of arcmagmatism. Al-in-hornblende barometry suggests emplacement atapproximately 4 kb. The Denny and West China Peak complexes arepredominantly gabbro to olivine pyroxenite characterized by calcicplagioclase. Along with the quartz diorite of Happy Camp Mountain,these plutons have calcic bulk compositions, abundant amphibole, andlow FeO/(FeO+MgO). Their chemical and petrographic features suggestoxidized, H (sub 2) O-rich parental magmas. In contrast, the IronsideMountain pluton has higher FeO/(FeO+MgO), shows K (sub 2) O enrichmentwith increasing SiO (sub 2) to ultrahigh-K compositions, and istypified by 2- and 3-pyroxene assemblages in which hydrous maficsilicates are sparse or absent, except in the most evolved rocks. Theelements Rb, Zr, and Ba all behave incompatibly. Initial Nd and Srisotope ratios (170 Ma) are uniform at epsilon (sub Nd) ofapproximately 5.2 and (super 87) Sr/ (super 86) Sr of approximately0.7037. These data suggest that the pluton lacks significant crustalinput, unlike younger Middle Jurassic plutons, and that it evolvedprimarily by fractional crystallization of a reduced, H (sub 2) O-poorarc tholeiite. Enrichment of K (sub 2) O is interpreted to result fromdifferentiation at relatively high pressure (> or =8 kb), underconditions in which augite fractionation predominated over olivinefractionation. High-P fractionation without appreciable crustal inputis probably related to the thermal structure of the crust. Emplacementimmediately after regional thrusting placed parental magmas into cool,nonreactive host rocks. Succeeding Middle Jurassic magmatismencountered progressively hotter crust, with consequent crustalmelting and assimilation.

65 samples from this reference