Granitoid magmas preserved as melt inclusions in high-grade metamorphic rock

Abstract

This review presents a compositional database of primary anatectic granitoid magmas, entirely based on melt inclusions (MI) in high-grade metamorphic rocks. Although MI are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks, MI in crustal rocks that have undergone anatexis (migmatites and granulites) are a novel subject of research. They are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions. Primary MI in high-grade metamorphic rocks are small, commonly 5-10 μm in diameter, and their most common mineral host is peritectic garnet. In most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage (nanogranitoid inclusions) with quartz, K-feldspar, plagioclase, and one or two mica depending on the particular circumstances. After their experimental remelting under high-confining pressure, nanogranitoid MI can be analyzed combining several techniques (EMP, LA-ICP-MS, NanoSIMS, Raman). The trapped melt is granitic and metaluminous to peraluminous, and sometimes granodioritic, tonalitic, and trondhjemitic in composition, in agreement with the different P-T-a H2O conditions of melting and protolith composition, and overlap the composition of experimental glasses produced at similar conditions. Being trapped along the up-temperature trajectory-as opposed to classic MI in igneous rocks formed during down-temperature magma crystallization-fundamental information provided by nanogranitoid MI is the pristine composition of the natural primary anatectic melt for the specific rock under investigation. So far ~600 nanogranitoid MI, coming from several occurrences from different geologic and geodynamic settings and ages, have been characterized. Although the compiled MI database should be expanded to other potential sources of crustal magmas, MI data collected so far can be already used as natural "starting-point" compositions to track the processes involved in formation and evolution of granitoid magmas.

Phase equilibria modelling of residual migmatites and granulites: An evaluation of the melt‐reintegration approach

“…Silicate glass is present as melt inclusions (MI) within rock‐forming minerals (Figure ) and in the matrix of the enclaves (Figure ). Melt inclusions are often distributed in zonal arrangements (Figure ) which reflect the progressive growth of the host mineral (Acosta‐Vigil, Cesare, London, & Morgan, ) and which represent the most compelling evidence for primary trapping (Acosta‐Vigil et al., ; Bartoli, Acosta‐Vigil, Ferrero, & Cesare, ; Bartoli, Cesare, Remusat, Acosta‐Vigil, & Poli, ; Cesare, Acosta‐Vigil, Bartoli, & Ferrero, ; Cesare, Acosta‐Vigil, Ferrero, & Bartoli, ; Ferrero et al., ).…”

Radiogenic Ca isotopes confirm post-formation K depletion of lower crust

“…Typical reactions involve the transformation of amphibole and mica into feldspar, pyroxene, sillimanite/kyanite, and (at higher pressures) garnet. These transformations are typically accompanied by the generation of granitic liquids (rich in incompatible elements and containing dissolved H 2 O) that are generally lost from the system, but found preserved as microscopic melt inclusions (e.g., Bartoli et al, 2016;Stepanov et al, 2016;Ferrero et al, 2018). There is also evidence that fluid loss in the absence of melt generation could contribute to the geochemical changes, as suggested by the common observation of high Th/U in granulite facies rocks (Rudnick and Gao, 2014).…”

Evaluating the Changes from Endogranitic Magmatic to Magmatic-Hydrothermal Mineralization: The Zaaiplaats Tin Granites, Bushveld Igneous Complex, South Africa

“…Normative Qz-Ab-Or plot of the granites from the Zaaiplaats tin field (Triangles). (A) White squares and solid lines represent haplogranite, hydrous eutectic points and cotectic lines respectively, which vary due to pressure [47][48][49][50][51]. The white diamond represents the anhydrous minimum [52].…”