ABSTRACT The Late Cretaceous Tuolumne Intrusive Suite in the Sierra Nevada batholith (California, USA) has been well studied for magmatic processes. The superb alpine exposures also allow one to investigate the development of magmatic fractures in these systems. Utilizing a cognitive framework of mental models, this field trip highlights and explores three fracture types that are rarely described in the literature and appear limited to magmatic systems. The first type is tabular fracture clusters (TFCs), which are a series of closely (sub-cm) spaced, sub-parallel fractures. Most TFCs in the Tuolumne Intrusive Suite strike ~020, but in some locations there is an orthogonal set that strikes ~110. Both sets have a sub-vertical dip. The second fracture type is magma-healed fractures. These fractures offset igneous features (e.g., layering), but there is no extant fault with grain-size reduction or evidence of cataclasis. Our interpretation is that these features formed while the system contained melt, and that the melt healed any brittle deformation features that may have formed. The third fracture type is primary en echelon P-fractures, recognized by the formation of epidote-filled, pull-apart zones. There are both right-lateral and left-lateral P-fracture arrays, which are spatially associated with the right-lateral Cascade Lake shear zone (part of the Sierra Crest shear zone system). These three types of fracture systems do not fit into any preexisting fault/fracture categories. These observations suggest that disciplinary categories (e.g., fault systems) may need to be expanded to form more categories (splitting). This category reorganization may be facilitated by taking a perspective that addresses both the Earth processes and mental processes that reason about them. During this two-day field trip, we will visit the tabular fracture clusters and healed magmatic fractures in Yosemite National Park, and the P-fracture arrays in the Saddlebag Lake area.

Abstract Much of today will be spent in the plutons that are the principal host rocks of the western margin of the Tuolumne Intrusive Complex ( Fig. 3-1 ). These include the Sentinel and Yosemite Creek granodiorites (Kistler, 1973; Bateman, 1992) and the Yosemite Valley Intrusive Suite ( Bateman, 1992 ). The Sentinel and Yosemite Creek Granodiorites were tentatively included as part of the Tuolumne by Kistler and Fleck (1994) and Bateman (1992), but assignment of these plutons to the Tuolumne has been hampered by lack of detailed study. We and our students and colleagues have been mapping and conducting focused studies on the petrology, geochemistry, and structure of both plutons recently, but the results are presently only in theses and abstracts (Petsche, 2008; Fulmer and Kruijer, 2009; Bliekendaal, 2012; van der Linde, 2012; Johnson, 2013). Reasonably accessible exposures of these units can be seen along Highway 120 within Yosemite National Park

Abstract This study investigates the internal anatomy and petrogenesis of the Tuolumne Intrusive Suite (TIS), which comprises metaluminous, high-potassium, calc-alkaline granitoids typical of the Sierra Nevada batholith. Although the TIS has often been cited as an example of a large magma chamber that cooled and fractionated from the margins inward, its geochemistry is inconsistent with closed-system fractionation. Most major elements are highly correlated with SiO 2 , but the scattered nature of trace elements and variations of initial Sr and Nd isotopic ratios indicate that fractional crystallization is not the predominant process responsible for its chemical evolution. Isotopic data suggest mixing between melts of mantle-like rocks and a granitic melt similar in composition to the highest-silica TIS unit. Monte Carlo models of magma mixing confirm that such processes can reproduce the observed variations in major elements, trace elements and isotopic ratios. Thermobarometry suggests emplacement at depths near 6 km and crystallization temperatures ranging from 660 to 750 °C. Feldspars, hornblende, biotite and magnetite exhibit evidence of extensive low-temperature subsolidus exsolution. The TIS as a whole trends toward more evolved isotopic compositions and younger U–Pb zircon ages passing inward. This pattern indicates a general increase in the proportion of felsic, crustally derived melt in the mixing process, which may have resulted from net accumulation of heat added to the lower crust by intrusion of mantle-derived mafic magma. However, the bulk geochemical and isotopic compositions of the equigranular Half Dome Granodiorite, the porphyritic Half Dome Granodiorite and the Cathedral Peak Granodiorite overlap one another and the contacts between them are commonly gradational. We interpret these map units to represent a single petrological continuum rather than distinct intrusive phases. The textural differences that define the units probably reflect thermal evolution of the system rather than distinct intrusive events.

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…