REE characteristics and genesis of alkaline-rich porphyry, Yunnan Province

Abstract

The Yunnan Himalayan alkaline-rich porphyry occurs as a compound rockbelt and consists of calca-alkalic, alkaline and peralkaline intrusions. Its origin is in debate. The paper deals with its origin by studying rock’s REE and Sr isotope. Although the rocks are different in their REE contents varying from 77.53 µg/g to 1 798.3 µg/g, they have very similar features in REE parameters. On the triangalar diagram of REEs, the sample dots are concentrated on the end area of light REEs, representing a product of low-degree melting of upper mantle or lower crust materials. The initial 87Sr/86Sr values of rocks vary between 0.706 4 and 0.709 8, showing a feature of mantle-crust mixed source. Moreover, REEs show a logarithmic linear positive correlation between them. This type of correlation strongly supports the fractional-partial melting model. The result of geochemistry inversion shows that the source rock of alkaline-rich porphyry is plagioclase-bearing harzburgite and of mantle-crust mixed type. At the early state of melting, some crust components of the source rock were partially melted into intermediate-acidic magma; with the crust components consumed, the magma evolved to basic.