Abstract Day 4 of the Field Forum is dedicated entirely to the magmatic evolution of the Tuolumne Intrusive Complex. The Tuolumne Intrusive Complex is mostly located in Yosemite National Park and is superbly exposed due to a series of glaciations that started 2–3 million years ago and ended ca. 10,000 yr B.P. one of the many reasons why it has become one of the most studied intrusions in the world. We will be driving from Mammoth Lakes to Yosemite National Park taking Hwy 395 north and then Hwy 120 west crossing the entire Tuolumne Intrusive Complex before we start off at its western margin. On this side, the Tuolumne Intrusive Complex intruded into the rocks of the Yosemite Valley Intrusive Suite, Sentinel, and Yosemite Creek granitoids (Day 3). Today we will make stops along Hwy 120 while traversing the Tuolumne Intrusive Complex from west to east ( Fig. 4-1) looking at exposures near the road and discussing field data, structural geology, geochronology, and geochemistry from the Tuolumne Intrusive Complex…

Abstract Our goals today are several-fold. We have now spent five days examining different parts of the Mesozoic Sierran arc, and hopefully discussions are already under way attempting to integrate both the shared and distinct characteristics of these individual magma plumbing systems and synchronous tectonics. We will briefly continue these discussions below. Our main focus will be to consider the arc as a whole and introduce a number of new regional data sets related to the tectonic and magmatic components of this arc. By the end of the day, we hope that our discussions have evolved to a consideration of the overall petrologic evolution of the arc, the tectonic and magmatic arc tempos, and their potential links. Without an airplane, or satellite, or Hollywood earth coring machine, it is difficult to take you to field locations where we can observe large sections of the arc. Instead, as we travel west back across the arc, we have selected a number of scenic overview stops, where we will introduce and discuss these new data sets while looking at gorgeous views of the arc…

The Lakeview Mountains pluton is a concordant teardrop-shaped pluton located at a marked deflection of the structural grain of the prepluton rocks within the northern Peninsular Ranges batholith. This dynamically emplaced 100 Ma pluton lies within the western transition zone and consists of biotite-hornblende tonalite that lacks K-feldspar. The pluton is characterized by ubiquitous schlieren that range from black hornblende-biotite rock to near-white quartz-plagioclase rock, imparting an extreme outcrop-scale mineral and chemical heterogeneity to the pluton. Geometrically, the schlieren define three structural sets; one is concordant, and the other two constitute a northeast- and northwest-oriented conjugate set. The orientation of the concordant schlieren resulted from the outward expansion of the pluton, and the orientation of the conjugate set is in response to regional stresses at the time of emplacement. Based on chemical analysis of systematically collected samples, the pluton consists of two chemically distinct parts. Initially emplaced magma formed an ellipsoidal body concordant with the regional northwest structural grain. This early-emplaced magma formed a zoned body having a relatively potassic core and a mafic outer part. Later-emplaced magma expanded the pluton to the north-northeast, deflecting the regional structural grain of the batholith, and forming the teardrop-shaped outline of the composite pluton. The later-emplaced magma was more mafic than the initial magma, producing a more mafic core and a relatively higher-potassium outer part. Variations in major and trace elements, specific gravity, magnetic susceptibility, and magnetite content, in addition to aeromagnetic and pseudogravity anomalies, all show similar patterns within the pluton. Bodies of hypersthene gabbro, large masses of melanocratic and leucocratic tonalite, and numerous potassic granitic pegmatite dikes are concentrated in the more mafic part of the pluton, and interpreted as the last to crystallize. The leucocratic and melanocratic tonalite bodies are interpreted to be late-emplaced giant schlieren. Initial 87 Sr/ 86 Sr ratios (Sr i) have only subtle, limited systematic variation, reflecting a relatively uniform magma source. Rb/Sr ratios also are relatively constant from the early- to late-emplaced magma, indicating the absence of, or only slight, fractional crystallization from the early- to late-emplaced magma. Sr i values of the hypersthene gabbro, mafic enclaves, and granitic pegmatites are the same as the tonalite of the pluton. The deflection of the regional structural grain by outward expansion of the pluton is interpreted to be a result of dynamic emplacement of the magma. Highly attenuated mafic enclaves in a prepluton mixed granitic rock unit that partly surrounds the pluton are also interpreted to have developed in response to outward expansion during the dynamic emplacement. A comb-layered gabbro, located along the contact of the earlier-formed part of the pluton, is interpreted as an early water-rich magma emplaced in a border area of the pluton protected from primary flow and dynamic strain. The high-potassium pegmatite bodies are interpreted to have formed from residual, immiscible, water-charged fluids derived from the low-potassium tonalite magma, which are concentrated in the mafic last part of the pluton to crystallize.