ANATEXIS AT THE MICROSCALE: FUSED GRANITOID XENOLITHS IN HOLOCENE BASALT, BLACK ROCK DESERT, UTAH

Quenched anatectic xenoliths in basalt provide the opportunity to examine high temperature partial fusion of granitoids to determine microscale melting mechanisms. Holocene olivine tholeiite basalt from the Ice Springs volcanic center, Black Rock Desert, west-central Utah, contains silicic inclusions that have undergone up to 90% partial melting. The presence of excess ⁴⁰Ar in basalt glass yields K/Ar “ages” of 4-16 Ma, which is consistent with the degassing or assimilation of these xenoliths. The xenoliths occur in basalt flows, agglutinate and as distinct individual clasts in cinder cones. They are typically rounded and range in diameter from <1 to 25 cm. Textural and chemical analyses reveal disequilibrium melting caused by rapid super-liquidus heating under H₂O-saturated conditions. Xenoliths consist of adhered host basaltic glass, anhedral plagioclase (An_39-19), alkali feldspar (Or_38-66) and quartz, plus vesicular colorless and brown glasses. Basalt glass contains perfectly euhedral olivine microphenocrysts, and olivine-liquid thermometry yields host rock temperatures of 1170–1190 °C. EPMA and LA-ICPMS analyses show that anatectic glass is heterogeneous in composition (62-77% SiO₂), and is broadly distributed in the haplogranite system plotting between the Ab-Or base and the ternary minimum, whereas some glass associated with quartz is exceptionally high in SiO₂ (80-95%). Colorless glass is formed by the melting of feldspar with or without quartz. Local concentrations of metastable Fe-bearing corundum and an aluminous pseudobrookite-like phase in colorless glass are attributed to muscovite breakdown. Regions of brown glass are inferred to form from the breakdown of biotite or hornblende. The presence of hydrous phases in the protolith is evident from pumice-like vesicularity measured by computed tomography (CT) scanning from 51 to 74 volume percent (average 62%), locally approaching the theoretical fragmentation threshold for magmas. A viscosity contrast of 10³ Pa s between the host basalt melt and anatectic glass allows for limited mixing. Although the colorless and brown glasses are separated by optically distinct boundaries, 2-D CT scanning and chemical profiling reveal gradational transitions resulting from proximal diffusion or mixing.