Abstract:

        There are three petrogenetically different pyroxenites, namely cumulate pyroxenite, metamorphosed oceanic crust, and
metasomatic pyroxenite. Type I pyroxenites formed as crystallization segregates from mantle-derived melts under pressure
of 1.5~2.5 GPa. They commonly show cumulated or igneous textures and do not form perfect linear correlations in a CaO/MgOSiO2/ MgO plot. Type I pyroxenits show no Eu anomaly and their Sr-Nd-O isotopes are similar to those of mantle rocks. Type II pyroxenites are metamorphic products of recycled oceanic crust. They commonly show a deformed texture, forming a linear correlation in a CaO/MgO-SiO2/MgO plot and show a positive Eu anomaly. While theirεNd values are identical to those of the MORB, their Sr-O isotopes show distinct features. Metasomatic pyroxenites result from interactions between melts and peridotite. They are commonly surrounded by harzburgite or dunite. Minerals in metasomatic pyroxenites are less variable than other two types of pyroxenites. Dispersed in the CaO/MgO-SiO2/MgO plot, theirεNd values are lower than those of the Type II pyroxenites,and their Sr isotopic ratios show a restricted range. The δ18O values of metasomatic pyroxenites are variable, either lower than,higher than, or similar to the mantle value. To explain the co-existence of pyroxenites of different origins within a single ultramafic massif, the following model is conceived: the oceanic upper crust, including basalts and sediments, produces considerable elementally variable fluids and magmas, which react with surrounding peridotites and convert it to pyroxenites and also form type I pyroxenites by high-pressure crystal segregation along magma conduits in the peridotites. The oceanic lower crust consisting of troctolite and gabbroic rocks retained their original compositions and translated to type Ⅱ pyroxenites without partial melting or other significant metasomatic reaction during subduction and exhumation at subsolidus conditions.

Key words: pyroxenites, high-pressure crystallization, recycling oceanic crust, metasomatism, discrimination