SUBDUCTION AND EXHUMATION OF ULTRAHIGH-PRESSURE ROCKS: FIELD AND DRILLING STUDIES IN EASTERN CHINA

A US-China project was established through the support of the US NSF Continental Dynamics program to investigate the size, protoliths and structures of the subducted continental slab, and the formation and exhumation of UHP rocks, particularly garnet peridotites. The Chinese Continental Scientific Drilling project drilled 3 pre-pilot holes to depths of up to 2 km and one 5-km main hole in east central China and successfully recovered many core samples of felsic gneiss, eclogite and garnet peridotite. Kinematic microstructures of continuous oriented cores together with surface mapping and seismic reflection profiles indicate eroded, asymmetric, antiformal UHP + HP slabs intruded by numerous Mesozoic granitic plutons. Ubiquitous occurrences of coesite inclusions in zircons of felsic gneisses from the main hole indicate that the supracrustal rocks (>90 % felsic gneisses + <10% eclogites) were subducted to depths >100 km. Drill core samples thus show the Sulu UHP slab to be >5 km thick. SHRIMP dating of zoned zircons identified 3 discrete age groups: latest Proterozoic protolith ages (>680 Ma) in inherited cores, a UHP event in coesite-bearing mantles at 220-240 Ma, and a late amphibolite-facies overprint in quartz-bearing rims at 210 ± 10 Ma; these data constrain the rate of exhumation at >5 km/Ma. The presence of anomalously low δ¹⁸O values in UHP minerals of eclogitic and felsic rocks suggests extensive meteoric water-rock interactions under Snowball Earth conditions during the Neoproterozoic and limited fluid infiltrations during continental subduction and exhumation. Both crustal-hosted and mantle-derived garnet peridotites occur; the former represent igneous layered mafic-ultramafic cumulates of various compositions, whereas the later were depleted, metasomatized ultramafic fragments from mantle wedge, some of which contain minor eclogite pods. Both types of ultramafics were metamorphosed under calculated “forbidden zone” P-T conditions (<5°C/km), involving P up to ≈6.0 GPa. These mantle rocks contain trace amounts of zircon, sulfide and micron-sized inclusions, and exhibit polymorphic transformations and mineral exsolution. SHRIMP ages of 230 ± 10 Ma for zircons from garnet peridotite and enclosing eclogite suggest mantle fragments were involved in coeval Triassic subduction and exhumation.