Detecting Peritectic Garnet in the Peraluminous Cardigan Pluton, New Hampshire

Abstract

The peraluminous Cardigan Pluton of New Hampshire contains restitic garnetites with abundant peritectic garnet that serves as a basis for comparison with garnet in the host-rock. The peritectic garnet is inclusion-rich and contains high concentrations of heavy rare earth elements (HREE), Sc, and Zr and low concentrations of Zn, P, and U. The high HREE, Zr, and Sc result from biotite and zircon melting, buffering liquid compositions. Because the HREE and Sc are compatible in garnet, the peritectic garnet is rich in these elements. Some garnet crystals in the host-rocks have cores with identical inclusions and trace element compositions to the peritectic garnet in the garnetites. We infer that these are also peritectic garnet grains dispersed in the host magma. These cores are mantled by inclusion-poor garnet. These mantles, along with other garnet crystals that are inclusion-poor throughout, show rimward trends of decreasing HREE, Sc, and Zr, and increasing Zn, P, U, and Eu/Eu*. We interpret the trace element variations as records of fractionation as the melt decreased in HREE, Zr, and Sc and was enriched Zn, P, and U. The rimward increase in Eu/Eu* records the effect of feldspar fractionation. Thus the peritectic garnet preserves evidence of crystallization from buffered melts whereas the phenocrystic garnet shows variable trace element abundances indicative of crystallizing from fractionating melts. These differences provide criteria for the recognition of peritectic garnet in the host pluton.