Lead isotopic study of the Xigaze ophiolite (Tibet): the problem of the relationship between magmatites (gabbros, dolerites, lavas) and tectonites (harzburgites)

Abstract

The Pb isotopic study of the Xigaze ophiolite (Tibet) shows that the tectonic (harzburgites) and magmatic (gabbros, dolerites, lavas) units bear different isotopic signatures, the harzburgitic rocks show much higher 207Pb/ 204Pb values than the samples from the magmatic unit. Analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes. U Pb analysis allowed determination of the crystallization age of the magmatites at 120 ± 10 m.y. This age is compatible with paleontological age determinations and thus demonstrates that U Pb behaved more or less as a closed system since formation of the ophiolite. If an exchange with seawater has occurred, it must necessarily have happened shortly after crystallization of the Xigaze oceanic crust. Dolerite dykes crossing the harzburgites show similar Pb isotopic composition as the magmatites, indicating that they were emplaced when the ultramafic rocks were already brittle; however, the time difference of the emplacement was probably not larger than 280 m.y. This makes it evident that two sources with different isotopic signatures, one similar to normal MORB, the other with much higher 207Pb/ 204Pb ratios, were active in the immediate neighborhood. One possible environment which can account for this association is a propagating ridge.