285-2 Evidence of Magma Mingling and Mixing in a Mafic Magmatic Enclave in the Tuolumne Intrusive Complex, California, USA

Abstract:

Magmatic enclaves are interpreted to reflect processes such as magma mingling and mixing, and as such can provide insight into pluton construction and open-system processes that influence evolution of magmatic systems. We are studying in detail a single large (~0.5 m), heterogenous, relatively mafic magmatic enclave (MME) from the equigranular Half Dome unit (eHD) of the Tuolumne Intrusive Complex (TIC), California. This enclave comprises three distinct lithologies: (1) dark grey enclave interior; (2) medium grey discontinuous rind on (1); (3) white discontinuous rind on (2). The enclave is enclosed within typical eHD granodiorite. Contact between (1) and host eHD is sharp; contact between (1) and (2) is diffuse; contact between (2) and (3) is sharp; and contact between (3) and host eHD is diffuse.

We characterize rock textures and plagioclase and amphibole major and trace-element compositions in this MME using SEM (BSE imaging and EDS quantitative analysis) and LA-ICP-MS. (1) and (2) contain both macrocrysts (~ 1 - 5 mm) and microcrysts (~ 0.05 – 0.60 mm) and (3) contains macrocrysts of plagioclase, amphibole, biotite, and alkali feldspar. Plagioclase macrocrysts display three textural patterns: 1) concentric zoning with some inclusions; 2) no visible (in BSE) zoning with few inclusions; 3) an anorthite-rich, ring-like zone and many inclusions. All plagioclase macrocrysts, regardless of overall pattern, have ~An₃₅ cores and ~An₂₅ rims. The anorthite-rich, ring-like zone and cores of plagioclase microcrysts in (1) are distinctly rich in Ca (~An₅₅). Rims of microcrysts in (1) are ~An₂₅.  These zoning patterns and compositional changes suggest that macrocrysts with the An-rich ring were entrained first, followed by the macrocrysts with concentric zoning, and finally the macrocrysts with no visible zoning. This suggests at least 3 stages of mingling during the formation of this enclave.

We estimate crystallization pressures using an amphibole geobarometer from Médard and Le Pennec (2022) using a different amphibole formula calculation from Gualda and Vlach (2005). Resulting pressures are: (1): 300 – 400 MPa; (2): 300 – 450 MPa; (3): 350 – 450 MPa; host eHD: 200 – 300 MPa. This variation in estimated crystallization pressures imply a multi-stage history of the enclave over a depth range on the order of 5 km, possibly involving several magmas and mingling events prior to emplacement of the eHD.

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