229-13 Partial melting of a metabasalt with eclogite relics in a hot subduction zone due to breakdown of phengite and amphibole, Sierra Nevada, California

Session: Crustal Petrology (Posters)

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Abstract:

Metabasalts from the metamorphic sole of the Tr-J Tuolumne Ophiolite in the Sierra Nevada Foothills Metamorphic Belt, California, consist of garnet-epidote amphibolite with eclogite relics. Eclogite, evidenced by omphacite inclusions in garnet, is overprinted by a partial melt event visible in outcrop as leucocratic veins and zones. Here, I describe microscopic mineral textures in garnet inclusions and the surrounding matrix of epidote and pargasitic amphibole that formed during the melt event.

Garnets have both inclusions and mineral-filled fractures indicating a partial melt event. Polysilicate inclusions are common. Some have K-feldspar, quartz, and amphibole with thin mineral films/moats and cuspate contacts with surrounding garnet. Others have euhedral clinozoisite and amphibole surrounded by plagioclase, K-feldspar, and phengite. Both types of inclusions often have negative crystal habits and decrepitation cracks that extend from the inclusion. Large fractures filled with quartz, K-feldspar, and euhedral amphibole crosscut the garnet and are sealed by pargasite growth at the rim of the garnet.

Polysilicate leucosomes are always present surrounding garnet and locally present elsewhere in therock. The leucosomes are composed of plagioclase, some K-feldspar, phengite, euhedral pargasite (high Al cores, lower Al rims), and graphic intergrowths of clinozoisite and plagioclase. Rutile indicates that melt was present at relatively high pressures. Garnets are embayed or have a negative crystal habit when in contact with the surrounding leucosome. The embayments are sometimes filled with euhedral pargasite, surrounded by a thin film of K-feldspar or plagioclase, indicating back reaction between the garnet and melt.

The lack of prograde phengite and the presence of K-feldspar and phengite in recrystallized melt indicates breakdown of phengite or biotite. Both pargasite and epidote are heavily corroded and surrounded by reaction products (still being analyzed) indicating that they likely participated in the melt-producing reaction. It is not clear at this point whether melting was driven by heating or by decompression during exhumation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11038

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