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Abstract: High-precision in-situ ion microprobe (SIMS) oxygen isotope analysis of zircons from two diorite intrusions associated with the late Caledonian Lochnagar pluton in Scotland has revealed large differences in the degree of heterogeneity in zircon δ 18O between the diorites. Zircon crystals from the Cul nan Gad diorite (CnG) show a unimodal distribution of oxygen isotope values (δ 18O=6.0±0.6‰ (2σ)) and no or only minor grain-scale variation. Those from the Allt Darrarie diorite (AD1) show a large range in δ 18O and an apparent bimodal distribution with modes of 6.6±0.4‰ and 7.3±0.4‰. Variations of up to 1.2‰ occur between and within grains; both an increase and decrease in δ 18O with zircon growth has been observed. The δ 18O composition of growing zircon can only change if open-system processes affect the magma composition, i.e. if material of contrasting δ 18O composition is added to the magma. The variability in AD1 is interpreted to represent a cryptic record of magma mixing. A ‘deep crustal hot zone’ is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks. The overall small number of zircons with mantle-like δ 18O values (5.3±0.6‰ (2σ)) in the Lochnagar diorites is largely the product of crustal differentiation rather than crustal growth. The δ 18O of quartz from the CnG and AD1 diorites shows only minor variation (CnG: 10.9±0.5‰ (2σ), AD1: 11.7±0.6‰ (2σ)) within single populations, with no evidence of mixing. Quartz–zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids, and in case of the AD1 diorite after the inferred magma mixing from a homogenised, higher δ 18O melt. High-precision SIMS oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early-stage magma mixing and constraining the compositions and origins of the component magmas. A combination of zircon, quartz and whole-rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration. [Copyright &y& Elsevier]