Trace element compositions and petrogenetic implications of zircons in high-pressure granulites

Trace element compositions of zircons play a pivotal role in unveiling their origins, the geological significance of zircon U-Pb ages, and the tectonic settings in which their host rocks formed. However, the trace element composition and petrogenesis of zircons in high-grade metamorphic and anatectic rocks, and trace element differences between metamorphic and magmatic zircons, remain unclear. Here, we present an integrated data set of new cathodoluminescence images, U-Pb ages, and trace element compositions of zircons from pelitic, felsic, and mafic high-pressure granulites in the eastern Himalayan orogen, and compare them to a compilation of previously reported metamorphic zircon trace element data from various metamorphic rocks. Zircons in the granulites studied are predominantly subhedral, have prismatic and ovoid shapes, and mostly have an inherited core (magmatic and detrital, respectively) and a metamorphic rim. Metamorphic domains of zircons show planar, patchy, and fir-tree zoning, and yielded metamorphic, anatectic, and melt crystallization ages of 44−9 Ma, while the inherited magmatic cores of the zircons show oscillatory zoning, and yielded early Paleozoic protolith ages of 510−480 Ma. The metamorphic zircon domains in the granulites generally have high Hf, U, and Sc contents, and low heavy rare earth element, Y, Th, Ti, Nb, and Ta contents. These compositions indicate that the metamorphic zircons formed during the growth of garnet, rutile, and Th-rich minerals (monazite, allanite, and titanite), and the host rocks underwent prolonged high-pressure granulite-facies metamorphism and partial melting. This study shows that the metamorphic zircons from garnet-bearing metamorphic rocks have lower Yb, Y, Nb, and Ta contents than those of zircons from magmatic rocks in different tectono-magmatic settings. We determined that the metamorphic zircons in garnet-bearing metamorphic rocks have higher U/Yb and Sc/Yb ratios than zircons in various magmatic rocks, and that the metamorphic zircons in ultrahigh-temperature rocks have high Th/U ratios (mostly >0.1), similar to those of magmatic zircons, whereas metamorphic zircons from other metamorphic rocks have low Th/U ratios (mostly <0.1). In general, this study deciphered the characteristics of trace elements of zircons in various garnet-bearing metamorphic rocks.