MAGMATIC STRUCTURES IN THE TUOLUMNE INTRUSIVE COMPLEX RECORD PHYSICAL DIFFERENTIATION OF MAGMAS AT THE EMPLACEMENT LEVEL

Regional to mineral scale analysis of schlieren-defined magmatic structures in the Tuolumne Intrusive Complex (TIC) supports a flow-sorting mechanism to explain their formation. Across four mapped areas of the Cathedral Peak (CP) unit, truncations in 309 troughs define an outward-younging pattern towards the contact with the older Half Dome (HD) intrusive units; average younging orientations are ~100°, ~140°, ~240° and ~310°, matching the migration directions of tubes. Mapping of complex schlieren zones at 1:10 scale highlights the sharp truncation of schlieren and mineral sorting by grain size and density.

Whole rock compositions of schlieren across all TIC units are also consistent with crystal sorting and accumulation of dense minerals from their nearby host magma. Schlieren compositions form linear arrays at high angles to the main TIC compositional trend in major and trace elements, with felsic components of magmatic structures typically showing some overlap with the main TIC array.

Hornblende (Hbl), concentrated in schlieren layers from the Kuna Crest Lobe (KCL), the Porphyritic Half Dome (pHD) and the CP, is olive green-brown, euhedral, shows patchy zoning and is dominantly magnesio-hornblende, whereas Hbl in the adjacent host has similar cores but relatively more green and blue-green rims of actinolitic composition. Schlieren Hbl major and trace element compositions overlap considerably with Hbl from neighboring host samples, indicating that schlieren Hbl are locally sourced from surrounding magma. However, schlieren Hbl records higher temperature magmatic history (>800-775 °C; Putirka, 2016 thermometer) that is absent in host pluton Hbl, precluding a liquid immiscibility or diffusion model. By 775 °C, host pluton Hbl grew concurrently with schlieren Hbl until ~750 °C. Actinolitic rims in the host pluton record the lowest near-solidus temperatures <680 °C.

Schlieren-bound magmatic structures provide evidence that the TIC, and other schlieren-bearing upper-crustal magma bodies are capable of local convection, with exchange of melt and crystals, while map-scale younging patterns indicate a regional control on the formation of schlieren-bound structures. This supports the existence of at least moderate chemically and physically evolving magma mush systems in upper crustal intrusive suites.