Origins of ultramafic rocks in the Sulu Ultrahigh-pressure Terrane, Eastern China

Abstract

Ultramafic rocks in the Sulu belt are associated with high-pressure (HP) and ultra-high-pressure (UHP) rocks, a metamorphic product of the northern margin of the Yangtze Craton (YZC). These ultramafic rocks are important because they provide information relevant to the nature and evolution of the Mesozoic collisional belt. We selected ultramafic rocks from Yangkou Bay (YKB), Suoluoshu (SLS) and Hujialin (HJL) in the central region of Sulu belt. The ultramafic rocks in YKB and SLS are hydrated to form serpentinites and they contain low concentrations of moderately incompatible elements (Al, Ti, and V), high contents of Ir-group platinum-group elements (IPGE; Ir, Os, and Ru; 12.8–21.7ppb in total), and high ratios (1.2–5.5) of IPGE to Pd-group PGE (PPGE) in bulk rocks. Spinel contains high Cr (Cr#=atomic ratio of Cr/[Cr+Al], 0.57–0.79). The data suggest that they likely represent hydrated forearc mantle peridotites underlying the margin of the North China Craton (NCC). Dunite samples from HJL are not fully hydrated with loss on ignition (LOI) values ranging from 6.6 to 13.2wt.%, and contain olivine grains with high forsterite components (Fo=100*Mg/[Mg+Fe], 91.7–92.4) and NiO contents (0.36–0.41wt.%). Spinel grains show high Cr# (0.68–0.76). The bulk rock contains high IPGE (5.0–22.7ppb total) and show high ratios (up to 8.8) of IPGE to PPGE. The data suggest that they are also residual mantle peridotites after high degrees of influx partial melting in the subduction setting. The geochemical features of our serpentinite and dunite samples are different from peridotites of young Cenozoic subcontinental lithospheric mantle (SCLM) that have been brought to the surface as xenoliths of young volcanic rocks. Instead, our samples are similar but even more refractory than peridotites of relict Archean SCLM below the NCC that were enclosed as xenoliths in Paleozoic–Mesozoic igneous rocks. Thus, the studied ultramafic rocks likely represent the relic of old refractory lithospheric mantle peridotites that underwent partial melting in the margin of the NCC during the subduction of oceanic lithosphere before the collision with the YZC. The peridotites in the forearc mantle were dragged into the subduction channel during the Mesozoic subduction of the YZC below the NCC, and exhumed together with the buoyant granitic metamorphic rocks in the Sulu belt.