A preliminary study of the Chinese Continental Scientific Drilling (CCSD) PP3 hole on the Gangshang garnet peridotite body in the Sulu UHPM belt.

A 705-m-deep hole (CCSD-PP3) was drilled on the Gangshang garnet peridotite body in the Sulu UHPM belt, of northern Jiangsu Province. The body, which is about 1370 m×700 m in size, lies under 10-20 m of Quaternary sediment. Four lithologic units are recognized in the drillcore, which are from top to bottom peridotite ( 370 m) , gneiss and eclogite (50 m) , peridotite (60 m) and gneiss (225 m). The eclogites, which are intercalated with the gneissic rocks, contain pseudomorphs of coesite preserved in omphacite, indicating that the rocks have undergone ultrahigh pressure metamorphism ( UHPM). The gneissic rocks include both orthogneiss and paragneiss. The peridotite consists of intercalated dunite and garnet-bearing dunite with transitional contacts. The garnet grains contain numerous inclusions of pre-metamorphic minerals, such as olivine, clinopyroxene and spinel, and are themselves partly replaced by secondary minerals such as amphibole, chlorite and spinel. The dunite and garnet-bearing dunite show a consistent composition; e.g. , SiO2=42. 17-44.79 wt% (average 43.78) , MgO = 46.35-48.98 wt% (average 47.42) and, Mg#(100×Mg/(Mg+Fe)=91.89-92.75, (average 92.4) , indicating the most depleted peridotite known in the Dabie-Sulu UHPM belt. These rocks also have low A12O3 (0.66-0. 27 wt% ) , CaO (0. 01-0. 35 wt% ) and total REE abundances but LREE are relatively enriched to HREE. SHRIMP U-Pb zircon dating gives two groups of age for the garnet-bearing dunite: 726±56 Ma for the cores of grains and 240 ±2.7 Ma for the new grown zircons. The older group is similar in age to granitic intrusions elsewhere within the Dabie-Sulu belt, whereas the younger ones coincide with previously reported UHP metamorphic ages. Thus, the Gangshang garnet peridotite body records at least two tectonic events; one related to the emplacement of the body into the crust and the other to UHPM during subduction and collision.