Genesis of zircon and its constraints on interpretation of U-Pb age

Abstract

Zircon V-Pb dating is the most commonly used method for isotopic geochronology. However, it has been a difficult issue when relating zircon V-Pb ages to metamorphic conditions in complex metamorphic rocks. Much progress has been made in the past decade with respect to the genesis of zircon and its constraints on interpretation of V-Pb age. Three methods have been proposed to link zircon V-Pb age to metamorphic conditions: ( i ) internal structure; (ii) trace element feature; (iii) mineral inclusion composition. Magmatic zircon shows typical oscillatory zoning and/or sector zoning, whereas metamorphic zircon has internal structures such as no zoned, weakly zoned, cloudy zoned, sector zoned, planar zoned, and patched zoned ones. Zircons formed in different geological environments generally have characteristic internal structures. Magmatic zircons from different rock types have variable trace element abundances, with a general trend of increasing trace element abundances in zircons from ultramafic through mafic to granitic rocks. Zircons formed under different metamorphic conditions have different trace element characteristics that can be used to relate their formation to metamorphic conditions. It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet. Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions. Therefore, interpretation of zircon V-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on.

2.24 Ga mafic dykes from Taihua Complex, southern Trans-North China Orogen, and their tectonic implications

“…The prismatic crystal shape, high U, Y, P and Ca contents and unimodal age indicate that the zircon grains from the Zhaiwa mafic dykes are not detritus grains while detrital zircon grains are generally rounded/subrounded with eroded margins, low U, Y, P and Ca contents and bi-or multimodal ages (Köppel and Sommerauer, 1974). It is noteworthy that the zircon grains from the Zhaiwa mafic dykes have wide zoning, indicating their formation in high temperature (because the temperatures affect diffusion rates of trace elements), which is consistent with typical zircon grains derived from mafic rocks (Corfu et al, 2003;Wu and Zheng, 2004). Twenty analyzed zircon grains from the Zhaiwa mafic dyke (ZW51) yielded a concordia age of 2235 ± 43 Ma (MSWD = 2.4) and a weighted mean 207 Pb/ 206 Pb age of 2242 ± 25 Ma (MSWD = 2.4).…”

“…The elevated Th/U values, together with the oscillatory magmatic zoning, indicate a magmatic origin for the analyzed zircon grains (Corfu et al, 2003;Wu and Zheng, 2004). The prismatic crystal shape, high U, Y, P and Ca contents and unimodal age indicate that the zircon grains from the Zhaiwa mafic dykes are not detritus grains while detrital zircon grains are generally rounded/subrounded with eroded margins, low U, Y, P and Ca contents and bi-or multimodal ages (Köppel and Sommerauer, 1974).…”

Late Miocene provenance change on the eastern margin of the Yinggehai-Song Hong Basin, South China Sea: Evidence from U–Pb dating and Hf isotope analyses of detrital zircons

“…5) and have high Th/U values (>0.4). All of these features indicate that the majority of the analyzed zircons are of a magmatic origin (Wu and Zheng, 2004).…”

“…Most of the grains show oscillatory growth zoning in CL images (Fig. 5), and have high Th/U ratios (>0.4), indicating that the majority of the analyzed zircons are of a magmatic origin (Wu and Zheng, 2004).…”

Zircon U-Pb geochronology and heavy mineral composition constraints on the provenance of the middle Miocene deep-water reservoir sedimentary rocks in the Yinggehai-Song Hong Basin, South China Sea

“…Most grains are oscillatory growth zoning under CL (Fig. 6) and have high Th/U values (>0.4) (Appendix A), indicates that the majority of the analyzed zircons are of magmatic origin (Wu and Zheng, 2004). Some grains also show typical metamorphic Th/U values (<0.1).…”