GARNET SM-ND AND LU-HF GEOCHRONOLOGY OF ECLOGITES FROM WEIHAI, SULU UHP TERRANE, EASTERN CHINA

The Sulu ultrahigh pressure (UHP) terrane is the eastern extension of the Dabie orogenic belt, offset ~500 km to the northeast by the Tanlu fault, formed by the collision between North China and South China cratons during the early Mesozoic. The Weihai area is in the northernmost part of the Sulu UHP terrane, and consists mainly of gneiss, eclogite, amphibolite, marble, and ultramafic rocks. In the last decade, the timing of metamorphism has been determined mainly by laser ablation zircon U-Pb dating on metamorphic overgrowths (240 – 200Ma) [1], and by a few younger garnet-whole rock Sm-Nd isochrons (159 – 156 Ma) [2]. These Sm-Nd analyses have also yielded very radiogenic present-day whole-rock Nd isotope compositions, with epsilon Nd values ranging from +260 to +174, the result of extreme Sm/Nd fractionation (f = +6.1 – +4.6) in these rocks. The garnet Sm-Nd dates of the Weihai eclogites are significantly younger than those of other eclogites from the Sulu and Dabie UHP terranes [3] and may not represent prograde metamorphism [2] but, rather, cooling ages. Therefore, we have used the Lu-Hf system in conjunction with Sm-Nd to investigate the coupled behavior of both isotope systmes in the eclogites from Weihai UHP terrane.

In this study, we collected eclogites throughout the Weihai UHP belt (Weihai city – Wendeng – Rongcheng, from north to south). On the field scale, the margins of some of the eclogite enclaves have been transformed to amphibolite facies. The garnets are of ~1 – 5 mm in size, and pink to brown in color. These samples are coarse grained, consisting of ~20 – 50% garnet, 10 – 30% omphacite, 10 – 20% quartz and other retrograded granulite-facies minerals. The mineral assemblages of these eclogites indicate retrograde processes. In the thin section, exsolution of pyroxene + rutile ± apatite ± hornblende has been found within garnet. Garnets in these eclogites are mantled by plagioclase + diopside + hornblende. In addition to the petrography, we will explore the Lu-Hf and Sm-Nd isotope behavior in the garnet, clinopyroxene, and bulk-rock.

[1] Zhang et al., Gondwana Res., 16 (2009) 1-26. [2] Jahn et al., Chem Geol., 127 (1996) 61- 79. [3] Cheng et al., J. Met. Geol., 26 (2008), 741–758