Microthermometric Study of Silicate Melt Inclusions in Variscan Granites from SE Germany: Volatile Contents and Entrapment Conditions

Abstract

Melt inclusions in granitic rocks are generally crystalline and must be homogenized to glass before analysis. We employ a simple kinetic technique with microthermometry to estimate total volatile contents and P–T conditions of trapping of the melts from their homogenization behaviour. This information complements chemical data from microprobe or ion probe analysis, and can be obtained from inclusions too small for direct analysis. We present here values for volatile contents (as equivalent wt % H2O), trapping temperatures and trapping pressures derived from studies of melt inclusions in some Variscan granites of the Erzebirge, Fichtelgebirge and Oberpfalz, Germany. The volatile concentrations range from 3 to 10 equiv. wt % H2O. Microprobe analysis shows that the most volatile-rich inclusions contain up to 8 wt % F. Estimates of trapping pressures ad temperatures suggest that crystallization began at an emplacement level of 3–6.5 km near 700°C with ‘normal’ biotite and two-mica granites, and ended with topaz–albite granite (∼650°C) and pegmatitic phases down to 520°C. This wide range of trapping temperatures and volatile contents is a general feature of melt inclusions in granites. Many of the granites studied are highly evolved geochemically, but the extreme volatile enrichment typical for these also occurs locally in the ‘unspecialized’ granites.