Composition of Crustal Melts at the Source Area: Information from Glassy Melt Inclusions in Anatectic Enclaves
Tuesday, 16 December 2014: 4:30 PM
Crustal anatexis, together with melt extraction and ascent to upper crustal levels, generate plutons and volcanic edifices of granitoid composition. This process constitutes the main mechanism for the differentiation of the continental crust. A recent breakthrough in the study of crustal anatexis is the discovery of former melt inclusions in peritectic minerals of anatectic rocks. These melt droplets show now as glassy inclusions in rapidly cooled anatectic enclaves within volcanic rocks, or as polycrystalline aggregates (nanogranites) in migmatites. Analysis of glassy inclusions and of rehomogenized nanogranites provide direct information on the composition of crustal melts at the source of crustal magmas, on the extent of equilibration between melt and residue, and on the fluid regime during anatexis. A comprehensive geochemical study (≈350 EMP, 100 LA-ICPMS and 80 nanoSIMS analyses) of matrix glasses and glassy melt inclusions in Pl and Grt of anatectic enclaves within El Hoyazo dacite (Betic Cordillera, S Spain), recording melt compositions during regional anatexis at ≈700-850 ºC and 0.5-0.7 GPa, shows that melts are leucogranitic (FeOt+MgO+TiO2=1.0-2.0 wt%), moderately to strongly peraluminous (ASI=1.10-1.25), with H2O concentrations well below saturation (3-5 wt%). They are heterogeneous and spread around the 0.5-0.7 GPa haplogranite H2O-undersaturated eutectics. Glassy inclusions in Pl are more heterogeneous, richer in normative Qtz and H2O, and poorer in FeOt and CaO compared to glassy inclusions in Grt and matrix glass. All glasses have moderate to high concentrations of LILE and low to very low concentrations in FRTE, HFSE and REE. Glass inclusions in Pl and Grt have higher concentrations of LILE, lower concentrations of Y, Zr, REE, and lower values of Th/U compared to matrix glasses. Surprisingly, and in spite of the compositional heterogeneity, glasses are at or close to equilibrium with their residue regarding most of the trace elements, except for some degree of undersaturation in accessory minerals Zrn and Mnz. The compositions of glassy inclusions in Pl and Grt and matrix glasses are interpreted to record the compositional evolution of melt during the prograde melting of the enclaves from ≈700 to 800-850 ºC.