Crystallisation History of Granitic Magma: Evidence from Trace Element Zoning

The evolution of the major and trace clement chemistry of a magma is dependent on a number of factors, especially the order of erystallisation and the degree to which early formed crystals reequilibrate with the magma as it cools. The chemical zoning which is observed in many minerals, even for coarse-grained rocks formed by slow cooling, demonstrates that equilibrium between minerals and melt is rarely maintained (Paterson and Stephens 1992). The zoning can be used to understand the progressive evolution of the magma during crystallisation. Indeed since the early formed crystals are effectively removed from equilibrium, the progressive changes within minerals should give a strong indication of how the magma might evolve during fractional crystallisation. The petrography and mineral chemistry of two granodiorites from a single plutonic complex have been studied in detail. Analyses have been made of the chemical zonation of both major and accessory minerals by ion and electron microprobes. The two granodiori tes differ in major element chemistry with BPSRT1 (T1) being lower silica (SiO2 = 60%) than BPSRG1 (G1) (SiO2 = 73%); both are dominated by the major mineral phases plagioclase, quartz and K-feldspar and contain the accessories apatite and zircon. While biotite is present in both rocks, it is subordinate to hornblende in T1. Importantly the two rocks differ in the dominant REE-bearing accessory mineral, G1 containing monazite and T1 titanite. Although the Ba, Sr, Nb and the Rare-EarthElements (REE) contents are higher by over a factor of 2 in T1, compared to G1, the overall chondrite normalised REE pattern for both is similar: the REE concentrations fall rapidly through the light REE (LREE) but with an essentially fiat pattern through the heavy REE