FLUCTUATIONS OF THE ARAL SEA LEVEL

The geochemistry and environment of formation of the Precambrian Fuchuan ophiolite, situated in the eastern part of the southern margin of the Yangtze block in southern Anhui province, China, are discussed. The serpentinized peridotites have high Mg# (94–95) and low CaO and Al₂O₃ contents, high Cr# (100?Cr/(Cr + Al)) spinels, and LREE-enriched patterns. The pyroxenites and gabbros exhibit flat to LREE-depleted patterns and positive ε_Nd(T) (+2.1 to +2.6). The spilites have LREE-enriched patterns (La/Yb)_N = 1.5-3.7], positive ε_Nd(T) (+3.2), negative Eu anomalies (Eu/Eu?)_N = 0.71-0.87], and low ratios of Nb/La(<0.8), Ti/Y (<350), and Ti/V (<30). Both diorites and keratophyres have LREE-enriched patterns and negative Eu anomalies. Seawater alteration is suggested by: (1) negative Ce anomalies exhibited by all samples, (2) large variations in δ¹⁸O, and (3) a horizontal trajectory to the higher ε_Sr values in the ε_Nd(T)-ε_Sr (T) diagram. The geochemical data for the Fuchuan ophiolite are consistent with formation in an arc-related setting, where fluids derived from a subducting slab induced further melting in an overlying lherzolite source.