Mineralogical study and the origin discussion of Purang ophiolite peridotites, western part of Yarlung-Zangbo Suture Zone (YZSZ), Southern Tibet

The Purang ophiolite is characterized by containing a large mantle peridotite massif ca. 600km(2) in area. In order to discuss the lithofacies variation and origin, a transect (AB) about 10km across the ophiolite which located in the east of the Purang mantle peridotite massif is sampled. The mantle peridotite is composed dominantly of harzburgite which mainly distribute within the peridotite massif, and the banded lherzolites mainly distribute in the slide of the peridotite massif, and the ratio of the exposure area of harzburgites and lherzolites is about 4 : 1. The transect also shows that the lherzolites change to the harzburgites gradually, the contents of O1 and Sp increase, the contents of Opx and Cpx decrease, and the Fo and NiO contents of olivine in the peridotites increase gradually from lherzolite to harzburgite. The orthopyroxene porphyroblasts are enstatite (En = 85. 1 similar to 90. 1), with Mg-# of 88 to 92. 0 and Al2O3, contents of 0. 89% to 5. 16%, whereas the clinopyroxenes are endiopside and diopside with low Al2O3, (1. 16% similar to 6. 02% and high Me (90 similar to 94). Spinels in the lherzolites have much lower Cr-# (19 similar to 32) than those in the harzburgites (25 similar to 72). On the other hand, the modal mineralogy, mineral compositions and the degree of partial melting change both gradually and abruptly within the harzburgites across the AB transect, indicating that a part of the harzburgites are not only the simple residues of partial melting but maybe experienced later fluids/melts metasomatism. On the basis of the spinel-olivine-clinopyroxene mineral chemistry, we calculate that the lherzolites are the residues of 5% similar to 12% partial melting of the mantle source, whereas the harzburgites are formed by 12% similar to 32% partial melting. The research results demonstrate that the evolution from lherzolites to the harzburgites owing to the partial melting differences, there is little evidence that the evolution is controlled by the variation of the tectonic setting. Thus, the method distinguishing the tectonic setting of peridotites by the variation of spinel and other mineral chemistry simply deserves to be discussed.