ZIRCON ZONATION PATTERNS AS REVEALED BY CATHODOLUMINESCENCE AND BACKSCATTERED ELECTRON IMAGES - IMPLICATIONS FOR INTERPRETATION OF COMPLEX CRUSTAL HISTORIES

Zircon exhibits an extraordinary memory. Its stability, durability, low solubility and low elemental diffusivities combine to preserve in it a record of most of the important events that have affected it, its host rocks, and the crust of which it is a part. Zonation in zircon grains delineates the boundaries of discrete geochemical packages formed at different times, each effectively a closed system. The elemental and isotopic compositions of these packages reflect the timing and conditions of growth events, and the morphology df the zonation indicates qualitatively the nature of both growth and intervening degradation events. Cathodoluminescence (CL) and backscattered electron (BSE) imaging reveals detailed zonation patterns that are commonly invisible or barely visible with conventional transmitted and reflected light microscopy. Characteristic patterns are visible in almost all zircons that serve to distinguish igneous from metamorphic growth, to distinguish truncation surfaces of different types (e.g., sedimentary fracturing vs. resorption), and possibly to identify ancient metamictization. Zircons from many rocks record multistage histories that reflect two or more events; those from rocks such as peraluminous granites and high-grade paragneisses are especially likely to reveal long and complex histories. Studies of zonation patterns in zircons provide a clear, though qualitative, history of a rock and its heritage. Furthermore, they provide the basis for a quantification of that history. Elemental and isotopic compositions can reveal the environment in which a zone grew. U-Pb analysis of a zone provides an age for its growth. SHRIMP analyses that are not guided by detailed knowledge of zonation can straddle two (or more) zones and a discordant U-Pb result from such an analysis may falsely suggest Pb loss, and important growth zones may be missed entirely. Thus, the combined use of CL, BSE, electron microprobe and ion probe methods can elucidate complex crustal histories.