Chemical variation in Al 2 O 3 CaONa 2 OK 2 O space: controls on the peraluminosity of the South Mountain Batholith

The South Mountain Batholith (SMB) of southwestern Nova Scotia is a large, highly differentiated, peraluminous, granitic batholith in which the average A/CNK (mol Al 2 O 3 /mol(CaO + Na 2 O + K 2 O)) increases from 1.16 to 1.23 with chemical evolution. We use vector analysis of variations solely in Al 2 O 3 –(CaO + Na 2 O + K 2 O) space to assess the fractionation, assimilation, fluid, and source controls on the peraluminosity of the SMB. With increasing chemical evolution, Al 2 O 3 decreases, CaO decreases sharply, Na 2 O is approximately constant, and K 2 O increases in the early and middle stages but decreases in the most evolved stage. Initial 87 Sr/ 86 Sr and 143 Nd/ 144 Nd 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 Al 2 O 3 –(CaO + Na 2 O + K 2 O) 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.