Mineralogical and Geochemical Constraints on the Petrogenesis of Post-collisional Potassic and Ultrapotassic Rocks from Western Yunnan, SW China

Abstract

Paleogene mafic potassic and ultrapotassic volcanic rocks in western Yunnan, China, show a compositional spectrum from potassic trachybasalt to latite (MgO 624^218 wt %; SiO 2 445^591wt %). These rocks have high K 2 O (307^528 wt %), relatively low Na 2 O (099^418 wt %) and high K 2 O/Na 2 O ratios (091^389). They share geochemical features such as depletion of Ta, Nb and Ti relative to other similarly incompatible elements and enriched SrN d isotopic compositions (initial 87 Sr/ 86 Sr of 07056^07101; e Nd (t) of ^097 to ^436). The rocks contain abundant olivine and clinopyroxene phenocrysts and xenocrysts. Clinopyroxene phenocrysts show complex zoning patterns (e.g. normal, reverse and oscillatory) and are all characterized by high Mg-number (077^090), low TiO 2 (013^029 wt %), Al 2 O 3 (073^168 wt %) and Na 2 O (022^042 wt %) with similar Ti/Al (006^016) and convex-upward rare earth element (REE) patterns, which are apparently in equilibrium with the host melts. Green cores in some clinopyroxene phenocrysts are characterized by low Mg-number (050^074) and Ti/Al (5005), high Al 2 O 3 (166^363 wt %), Na 2 O (087^217 wt %) and Al VI /Al IV (038^076), and have chondrite-normalized REE patterns convex-upward from La to Dy and convex-downward from Dy to Lu.