Composition and Evolution of Topaz Rhyolite Magmas of the Spor Mountain Formation, USA: A Study of Melt Inclusions in Minerals

Abstract

Using various methods of melt inclusion investigation, including electron and ion microprobe analysis, we estimated the composition and evolution of topaz rhyolite magmas of the Spor Mountain Formation (USA). Primary melt inclusions were found in quartz from the topaz rhyolite. Based on their phase composition, two types of melt inclusions were distinguished: (1) glassy and (2) partly or completely crystallized. The daughter minerals of the crystallized inclusions are albite, sanidine, fluorite, and fluorine-rich (up to 7 wt %) Li-annite. Heating experiments with second-type melt inclusions revealed high temperatures of their partial homogenization (to glass + gas) ranging from 1040 to 1100°C. The analysis of glasses of the homogenized melt inclusions in quartz from the topaz rhyolites distinguished two groups of melts. Melts of the first group are rhyolitic in composition and have high SiO₂ and Al₂O₃ contents: 73–75 and 12–13 wt % respectively; their Na₂O content is 4.8–5.8 wt %. The H₂O content varies from 0.7 to 1.9 wt %. The melts of the second group correspond to the trachydacite composition. Compared with the melts of the first group, they are depleted in SiO₂ (67–69 wt %) and enriched in Al₂O₃ (16.5–18.8 wt %) and Na₂O (6.8–7.6 wt %). Their H₂O content does not exceed 0.1–0.3 wt %. The F and Cl contents of the melts of both groups are 0.7–1.8 and 0.20–0.37 wt %, respectively. The analysis of trace elements in the glasses of homogenized melt inclusions of both groups showed high contents of Be, B, Li, Nb, Ta, Th, U, Rb, Y, and LREE. In addition to silicate glass, an unusual fluoride phase was found within the glassy (unheated) melt inclusions of the first type. This phase shows high contents (wt %) of F (34.93), Al₂O₃ (34.43), and Na₂O (35.20) at 14.87 wt % SiO₂ and 3.44 wt % K₂O, which allowed us to suppose that a specific sodium aluminofluoride melt separated from magma enriched in alumina, alkalis, and fluorine. The data obtained from the study of melt inclusions in quartz from the Spor Mountain topaz rhyolite provided insight into the evolution of the rhyolite melt during rock formation. It was shown that quartz crystallized from an eutectic melt enriched in Be, B, Li, Nb, Ta, Th, U, Rb, and Y with elevated F (up to 1.8 wt %) and H₂O contents (0.7–1.9 wt %). The injection of basaltic melt into a silicic magma initiated intense degassing and led to the melting of the mineral association that had crystallized by the time of the injection. During this process, fluorine-containing phases, such as fluorite, topaz, and F-annite, were melted. The residual melt became significantly enriched in fluorine and trace elements, leading to the separation of a specific aluminofluoride liquid. The further evolution path of the silicate melt included the fractional crystallization of feldspars.