AGES AND GEOCHEMICAL SIGNATURES OF SERPENTINITES FROM THE QILIAN FOLD BELT OF NORTHWEST CHINA

Late Proterozoic and Early Paleozoic serpentinites in the Qilian fold belt along the southwestern margin of the North China block range from lherzolite to harzburgite in composition with MgO contents from 39 to 46 wt.%. Trace element profiles are characterised by a progressive enrichment in incompatible elements, LREE enrichment and negative Ce-, positive Eu- anomalies. Two groups may be recognised within these patterns: samples from the Qing-Shui Creek and Baijing Temple localities, along with the Lha-Jyi harzburgitic samples have U-shaped REE profiles with moderate LREE enrichment (La/Smn up to 6.6) and prominent Eu anomalies (Eu/Eu* up to +2.0). Samples from the Ta-Den Creek and Ayou Creek localities, along with a lherzolitic sample from the Lha-Jyi terrane show only minor LREE enrichment (La/Smn<2) and have only weak positive Eu anomalies (Eu/Eu*<+1.5). The concentrations of incompatible trace elements are greater than in peridotites residual from MORB generation. Likewise, the range in eNd(500Ma) of +5.5 to -15 shown by the serpentinites is less than for Caledonian oceanic crust. Most samples have therefore undergone an enrichment event. The increase in immobile trace elements such as Th, manifest in ratios such as Th/Nb (0.1 to 0.9) relative to values of <0.07 in MORB and peridotite residues suggests the serpentinites were metasomatised in a collisional tectonic setting. The evidence therefore supports conclusions drawn from the associated volcanic rocks of a supra-subduction zone setting for the Lha-Jyi, Baijing Temple and Ta-Den Creek assemblages, and a collisional margin setting for the Qing-Shui Creek samples.