Three‐dimensional morphologies of metasedimentary and mafic enclaves From Ascutney Mountain, Vermont

Abstract

Two types of enclaves are found in the main syenite unit of the Cretaceous igneous complex at Ascutney Mountain, Vermont. Metasedimentary xenoliths consist of fine‐grained hornfels with bands and lenses of coarser‐grained quartz. Mafic enclaves of igneous origin contain phenocrysts in an aphanitic groundmass. We have reconstructed the three‐dimensional shapes of a metasedimentary xenolith and 15 mafic enclaves from serial sections created by sawing blocks of syenite containing enclaves into parallel slabs. Enclave outlines on each side of the slabs were digitized and computer software was used to reconstruct three‐dimensional images. Results are presented for three typical mafic enclaves and for both the hornfels and quartz portions of the metasedimentary enclave. Stereographic drawings show that the metasedimentary and the mafic enclaves have complex shapes in three dimensions, with branches, lobes, and concavities. Syenite from the host magma penetrated the xenolith along hornfels‐quartz boundaries, producing complementary concave‐convex surfaces. Adjacent mafic enclaves commonly have complementary surfaces and are not observed to impinge on each other. Syenite also penetrated both the metasedimentary and mafic enclaves along finger‐like projections. Syenite infiltration into the metasedimentary xenolith apparently occurred by a combination of mechanical and chemical processes, controlled by the presence of compositionally favorable boundaries. A similar model is suggested for the mafic enclaves, in which syenite infiltration occurred preferentially along quartz‐feldspar grain boundaries or involved larger‐scale compositional heterogeneities within the mafic enclaves. Three‐dimensional views of enclaves provide information complementary to chemical and petrologic studies of assimilation.