Unique coesite-bearing zircon from allanite-bearing gneisses: U-Pb, REE and Lu-Hf properties and implications for the evolution of the Sulu UHP terrane, China

Abstract

Zircon from allanite-bearing para- and ortho-gneisses provides a unique insight into the geologic evolution of the Sulu ultrahigh-pressure (UH P ) terrane in eastern China. Laser Raman, cathodoluminescence (CL) imaging combined with trace-element, U-Pb, and Lu-Hf isotope data indicate that zircon grains consist of three distinct zones. Inherited magmatic core records a Neoproterozoic protolith age, the coesite-bearing mantle reveals a UH P metamorphic age of 230 ±7 Ma, and the low- P mineral-bearing rim retains an age of 210 ±3 Ma for the amphibolite-facies retrogression. Positive eHf(t) values for the inherited cores reflect involvement of juvenile materials in the protolith. Low 176 Lu/ 177 Hf ratios of coesite-bearing mantles indicate their formation during /after garnet crystallization. Their 176 Hf/ 177 Hf ratios are homogenous and more radiogenic than those of the inherited cores, implying fractionation of the Lu-Hf system during isotope equilibration within the metamorphic mineral assemblage. Moreover, coesite-bearing mantles show geochemical characteristics that are distinctly different from those of previous studies: (1) The chondrite-normalized pattern shows a steep slope from HREE (heavy rare earth elements) to LREE (light rare earth elements) with a moderately negative Eu anomaly (mean Eu/Eu* = 0.53). Such a pattern requires that phases enriched in LREE ( e.g. , allanite) and europium ( e.g. , K-feldspar) were stable during UH P conditions. (2) The positive Ce anomaly is very pronounced (Ce/Ce* of 612–657), e.g. , about seven to eight times higher than that of the inherited core; (3) The uranium content is very low (6–25 ppm) and the Th/U ratio elevated (0.52–2.83). These data for the coesite-bearing mantle are distinctly different from those of previous studies. We interpret the data to suggest high O fugacity during the UH P and retrograde metamorphic evolution, which also played a critical role in restraining diamond growth in Sulu-Dabie UH P rocks.