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, lo w solubility and low elemental diffusivities combine to preserve in it a record of most of the important events that have affected it, its h ost rocks, and the crust of which it is a part. Zonation in zircon gra ins delineates the boundaries of discrete geochemical packages formed at different times, each effectively a closed system. The elemental an d isotopic compositions of these packages reflect the timing and condi tions of growth events, and the morphology df the zonation indicates q ualitatively the nature of both growth and intervening degradation eve nts. Cathodoluminescence (CL) and backscattered electron (BSE) imaging reveals detailed zonation patterns that are commonly invisible or bar ely visible with conventional transmitted and reflected light microsco py. Characteristic patterns are visible in almost all zircons that ser ve to distinguish igneous from metamorphic growth, to distinguish trun cation surfaces of different types (e.g., sedimentary fracturing vs. r esorption), and possibly to identify ancient metamictization. Zircons from many rocks record multistage histories that reflect two or more e vents; those from rocks such as peraluminous granites and high-grade p aragneisses are especially likely to reveal long and complex histories . Studies of zonation patterns in zircons provide a clear, though qual itative, history of a rock and its heritage. Furthermore, they provide the basis for a quantification of that history. Elemental and isotopi c compositions can reveal the environment in which a zone grew. U-Pb a nalysis 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 fa lsely suggest Pb loss, and important growth zones may be missed entire ly. Thus, the combined use of CL, BSE, electron microprobe and ion pro be methods can elucidate complex crustal histories.