Microstructural evidences for mineralogical inheritance in partially molten rocks: example from the Vosges Mts

Abstract

During orogenic processes continental crust experiences significant partial melting. Repeated thermal pulses or fluctuation in fluid content can even cause multiple anatectic events that result in complex intrusion suits. In the Vosges mountains, France, two main generations of magmatic rocks are recorded. The first magmatic event occurred at ca. 340 Ma, and is represented by extensive K-Mg granitoids magmatism. The second magmatic event occurred at ca. 325 Ma and produced large quantity of felsic anatectic melts which further pervasively intruded and compositionally and texturally reworked previously formed granitoids. Detailed field and microstructural observations revealed continuous transitions from porphyritic granite with large euhedral Kfs and Pl phenocrysts (Type I granite) via intermediate granite (Type II) to fine-grained apparently isotropic granite (Type III) dominated by the neo-crystallized melt. The Type I granite preserves the original magmatic assemblage and has only incipient amount of the newly crystallized melt. The new melt-crystallized material forms narrow, fine-grained pathways along grain boundaries or cuts across pre-existing magmatic grains and forms an interlinked network. With increasing amount of the newly crystallized material the original magmatic grains are resorbed and show highly corroded shapes. The early formed feldspars grains have strong compositional zoning, with oscillatory zoned cores reflecting range of original magmatic compositions and rims showing later melt overgrowths. Original magmatic feldspars have different composition from the new phases crystallizing in the partially molten granite. We interpret the fine-grained microscopic corridors as melt pathways that were exploited by the new magma. We suggest that this melt pervasively migrated through the older granitoids resulting in mixture of inherited “xenocrysts” and of new melt-derived crystals. The interaction between the new melt and previously crystallized granitoids results in variety of granite textures and fabrics. These reflect different degrees of equilibration between the bulk rock and the passing melt. Finally, Type III granite carries mixed isotopic signature intermediate between the type I granite and the surrounding metasediments and granulites, suggesting mixing of the original granite with new later magma with source in these rocks.