INVESTIGATING THE EXTENT OF CRYSTAL MIXING IN THE NORTHERN TUOLUMNE INTRUSIVE COMPLEX WITH MINERAL INCLUSION PATTERNS IN K-FELDSPAR

Recycling of older magma mushes into younger during emplacement is sometimes inferred to have made ‘space’ for the emplacement of subsequently intruding younger magma batches. This has been suggested for the southern portion of the Tuolumne Intrusive Complex (TIC) in the Sierra Nevada batholith, where cathodoluminescence (CL) and major oxide and trace element analyses of K-feldspars and plagioclase suggest that the major TIC units are characterized by multiple feldspar populations (Oppenheim et al., this meeting; Memeti et al., 2014). The goal of this study is to determine if crystal transfer from the marginal units into the youngest, most voluminous Cathedral Peak unit also occurred in the northern TIC.

The TIC is composed of granodiorite units containing euhedral K-feldspar phenocrysts ranging from 3 mm grains in the equigranular Half Dome (eHD) to 12 cm megacrysts in the Cathedral Peak (CP) unit. Copious mineral inclusions in the K-feldspars are used as a tracer to determine individual populations of K-feldspars in each unit to investigate if inter-and intra-unit magma interaction occurred. This is completed with petrographic analysis and CL on thin sections from the eHD and porphyritic Half Dome (pHD), and CP granodiorites. Observations include the determination of shape and grain size of the K-feldspars, identification of specific types and size of mineral inclusions, and the zoning patterns of the inclusions.

Inclusions in the TIC K-feldspars include plagioclase, biotite, hornblende, sphene, apatite, and zircon which is consistent with the observations made on feldspars in the southern TIC. The 92-88 Ma eHD and pHD feldspar megacrysts are 1-4 cm and enriched in inclusions. The 88-84.5 Ma CP contains megacrystic K-feldspars that lack inclusions, although some have inclusion-rich cores, suggesting that feldspar recycling could have occurred from the pHD into the CP units, where the K-feldspars likely continued growth. Our preliminary results and the data obtained from the southern TIC support that crystal transfer from older into younger units, and within units, and therefore magma mixing, has occurred in both the northern and southern TIC, indicating that extensive magma mush bodies were present during the later stage of TIC evolution and late Cretaceous regional magma focusing.