PHYSICAL UNMIXING DURING FORMATION OF MAGMATIC STRUCTURES AND IMPLICATIONS FOR WHOLE ROCK ELEMENT AND ISOTOPE CHEMISTRY AT POHOLE DOME, SIERRA NEVADA, CA

Magmatic structures are found in plutons worldwide, yet the origin of structures (such as migrating tubes/ladder dikes) is debated, with explanations including flow segregation of crystals, shearing of buoyant plumes of magma, and liquid immiscibility. Here we present both whole rock element and isotope chemistry on a suite of magmatic structures.

Pothole Dome is an area centrally located within the Cathedral Peak (CP) phase of the Tuolumne intrusive complex, Sierra Nevada, CA. Pothole Dome is unusually rich in magmatic structures such as: tubes, pipes, troughs, mafic ellipsoids, two magmatic fabrics, small scale plumes and diapirs, and crystal accumulation and melt escape structures. These magmatic structures are often distinguished from the main CP as either felsic concentrations or mafic schlieren of different geometries, depending on structure type. Structures in Pothole Dome can be split into an early group with trends toward the NW and N/S, and a late group with E/W orientations.

Whole rock major, trace, and rare earth elements and Nd, Sr, and Pb isotopes were measured for mafic and felsic parts of magmatic structures and compared to host magmas adjacent to these structures. Whole rock element and isotope measurements of magmatic structures are distinct from the main CP, with the main CP plotting at intermediate values between the more mafic and more felsic parts of these structures regardless of isotope system or chemical, with few exceptions. Our study suggests that the main phase of the CP consists of a mixture of different mineral populations that were subsequently physically separated to form the magmatic structures. Whole rock element and isotope results of the main CP phase are an average of the mixed populations, while schlieren and felsic phases in magmatic structures represent physically “unmixed” phases that have retained their original pre-mixing chemical signatures. Thus, unmixing in Pothole Dome is likely hydrodynamically controlled by flow processes. Isotopes of magmatic structures are likely averages dominated by the relative abundances of minerals within the structure, as opposed to a chemical process related to the formation of the structure. These findings highlight the importance of single mineral geochemistry in determining the true meaning of a rock or structure’s chemical signature.