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Abstract:
We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia. south-west of Lake Baikal (50.5degreesN, 95degreesE). The majority of samples are high- alumina basalts and basaltic andesites having island-are affinities. They were derived from an evolved parental magma (Mg # greater than or equal to 0.60 Crsimilar to180 ppm, Nisimilar to95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-are rocks have high abundances of incompatible trace elements fight rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)(n) = 14.6-5.1) and negative Nb anomalies (Nb/La = 0.37-0.62). but low Zr/Nb ratios (7-14). Initial epsilon(Nd) values are around +5.5, initial Pb isotopic compositions are Pb-206/Pb-204 = 17.39-18.45, Pb-207/Pb-204 = 15.49-15.61. Pb-208/Pb-204 = 37.06-38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La= 11-130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8-15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare- earth element patterns [(La/Yb)(n) = 0.6-2.4] and higher epsilon(Nd) values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La 0.81-1.30), but Zr/Nb is high (21-48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island- like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (similar to6.5. a low epsilon(Nd) value (around 0), and radiogenic Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204; (2) a back-arc basin component similar to N-MORB with a flat rare- earth element pattern and a high epsilon(Nd) value (around + 8.5), and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-are system and an associated back-arc basin.
