Chemical variation in Al2O3–CaO–Na2O–K2O space: controls on the peraluminosity of the South Mountain Batholith

Abstract

The South Mountain Batholith (SMB) of southwestern Nova Scotia is a large, highly differentiated, peraluminous, granitic batholith in which the average A/CNK (mol Al2O3/mol(CaO + Na2O + K2O)) increases from 1.16 to 1.23 with chemical evolution. We use vector analysis of variations solely in Al2O3–(CaO + Na2O + K2O) space to assess the fractionation, assimilation, fluid, and source controls on the peraluminosity of the SMB. With increasing chemical evolution, Al2O3 decreases, CaO decreases sharply, Na2O is approximately constant, and K2O increases in the early and middle stages but decreases in the most evolved stage. Initial 87Sr/86Sr and 143Nd/144Nd isotopic ratios for granites and average Meguma Supergroup country rocks suggest an upper limit of ~33% of wall-rock contamination for the most evolved rocks, if the most primitive rocks are uncontaminated. The trend of chemical evolution of the SMB through Al2O3–(CaO + Na2O + K2O) space is the resultant of all input vectors (processes). In the early stages, those processes are fractional crystallization of plagioclase ± K-feldspar ± cordierite ± biotite and contamination by country rocks. In the later stages, those processes are fractionation of plagioclase ± K-feldspar ± andalusite ± muscovite, further contamination by country rocks, and selective partitioning of Ca–Na–K into aqueous fluid phases. Clear geochemical evidence for variation in the source composition is lacking.