Using Zircon Chemistry as a Discriminating Tool for Geodynamical Context of Magma Formation

Abstract:

Zircon, a widespread accessory mineral in rocks of felsic composition, is frequently used in various disciplines of geosciences as a petrogenetic, geochronologic and geothermometric tool. It can survive metamorphism and partial melting, and it is chemically and physically very resistant to erosion. As a consequence, zircon is very common in sedimentary rocks of terrigenous origin. Over the last decades, several studies have focused on detrital zircon grains to track the eroded source of sedimentary rocks. However, despite the fact that zircon chemistry allows its source rock to be dated and characterized in terms of crystallization temperature and composition, the detailed geodynamical context of the source rock is still very difficult to reconstruct.

The goal of this study is to infer the geodynamical crystallization environment of zircon from its geochemical signature. Differentiated igneous rocks from several geodynamic environments, including intraplate, active margin, ocean rift, and pre-, syn-, and post-collisional settings, were sampled, and zircon grains were separated, mounted in epoxy, and imaged by cathodoluminescence. The trace element signature of zircon was determined using (in-situ) laser ablation-high-resolution-inductively coupled plasma-mass spectrometry (LA-HR-ICP-MS). Zircons non representative of original magmatic values (LaN(ormalised) > 1, PrN > 10 and SmN/LaN < 10) had been disregarded. Y and Yb seem to behave similarly if we compare zircons from all our samples. But preliminary observation of trace elements such as Nb, Ta, and Th plotted against rare earth elements allows us to characterise and discriminate several geodynamic environments. More particularly, Y/Th ratio and Eu anomaly seem to be significant to distinguish ocean rift and intraplate settings, thus confirming the geochemical control of the zircon crystallisation.