THE CONTAMINATION OF GRANITIC MAGMA BY METASEDIMENTARY COUNTRY-ROCK MATERIAL: AN EXPERIMENTAL STUDY

Abstract

Melting experiments involving metasedimentary and granitic rocks provide an opportunity to study reactions and products of granite contamination by assimilation of metasedimentary material from the Meguma Group, host rocks to the South Mountain Batholith (SMB), in Nova Scotia. The main products of melting metapelitic and metapsammitic rocks for 4–26 days, at 700 or 800°C, 200 MPa, at H2O-undersaturated or H2O-saturated conditions, are new magmatic solids such as cordierite and magnetite, and a highly peraluminous partial melt. Quartz, biotite, and plagioclase are the dominant relict phases. Melting of the metasedimentary rocks from the Meguma Group under similar conditions, but juxtaposed against a granodiorite from the South Mountain Batholith (SMB), or a synthetic haplogranitic glass, have run products similar to those from the melting the metasedimentary rocks or the SMB granodiorite only, except for a zone along the contact of the two materials, where melt fractions are higher, and magmatic cordierite or K-feldspar may be more abundant than in the rest of the charge. The experiments at 800°C produce melt fractions of >50 vol.%, with the highest fractions of melt occurring along the contact between metasedimentary and granitic material, whereas in the 700°C experiments, only the metapelitic rocks of the Meguma Group in contact with the haplogranite generate a significant fraction of partial melt (~50 vol.%). These results suggest that at temperatures of ≥800°C, the assimilation of Meguma Group country-rock material through partial melting may have been an important process in the SMB. At temperatures of ≤700°C, minor partial melting may have caused disintegration of Meguma Group country-rocks rather than their assimilation. At both temperatures, new magmatic phases, partly composed of country-rock material and partly composed of magmatic material, may form along the contact between metasedimentary rocks and SMB host magmas.