Synkinematic intrusion of peraluminous and associated metaluminous granitic magmas, Whipple Mountains, California

Abstract

ABsrRAcr The autochthon of the Whipple Mountains detachment terrain (California) contains two generations of peraluminous intrusive bodies. each one coeval with a distinct metamorphic event. Garnetbiotite granodiorite and two-mica adamellite plutons were generated during an early ductile metamorphism. Subsequent Late Cretaceous mylonitization was synchronous with the intrusion and formation of a sheeted sill complex. Peraluminous sills, which contain two-mica tonalite and garnet-two-mica granodiorite, form a maior central portion of the compositionally zoned sill complex. The structurally uppermost and lowermost sills, composed of biotite granodiorite and hornblende-biotite guartz diorite. respectively, are metaluminous. The primary white mica in the peraluminous rocks contains 19.5-32.5% of a titaniferous ferriceladonite end-member. K( Mgo Ti,,.: ) Fe3 + (Si3.oAlo.4) Oro( OH ) u. Chemographic analysis shows that these'celadonitic muscovites are consistent with shallow levels of intrusion: a depth of 9.6-11.5 km is calculated. Shallow depth is also suggested by the Mn-rich composition of garnet, vhich contains 42.647.4 mole Vo almandine. the remainder being comprised primarily of subequal proportions of spessartine and grossular. Biotite compositions indicate crystallization under fairly oxidizing conditions (Ni-NiO to MnG-MnsOr buffer curves), an inference consistent with the abundance of magnetite and the near-absence of ilmenite. These peraluminous intrusive rocks are thus set apart from S-type granitoid rocks. The composition of the magmas that generated the older and younger peraluminous suites is only weakly aluminasaturated when compared with $type granitoid rocks of other orogenic belts; this is due to a high content of Na and Ca. not to low Al. The marked similarity of melts coeval with the metamorphic events is indicative of crustal anatexis involving exceedingly similar source materials. Although fractional crystallization was dominated by plagioclase. the removal of hornblende or allanite (or both) performed a major rote in increasing the peraluminous nature of one intrusive bodv.