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 = 6.24-21.8wt %; SiO2 = 44.5-59.1wt %).These rocks have high K2O(3.07-5.28 wt %), relatively low Na2O(0.99-4.18 wt %) and high K2O/Na2O ratios (0.91-3.89). They share geochemical features such as depletion of Ta, Nb and Ti relative to other similarly incompatible elements and enriched SrNd isotopic compositions (initial 87Sr/86Sr of 0.7056-0.7101; Nd(t)of-0.97 to -4.36).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 (0.77-0.90), low TiO2 (0.13-0.29 wt %), Al2O3 (0.73-1.68 wt %) and Na2O (0.22-0.42 wt %) with similar Ti/Al (0.06-0.16) 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 (0.50-0.74) and Ti/Al (>0.05), high Al2O3 (1.66-3.63 wt %), Na2O(0.87-2.17 wt %) and AlVI/AlIV (0.38-0.76), and have chondrite-normalized REE patterns convex-upward from La to Dy and convex-downward from Dy to Lu.We interpret these green cores as xenocrysts from the wall-rocks. All these observations indicate complex magma chamber processes including extensive fractional crystallization, phenocryst accumulation and multiple melt replenishment of similar parental melts with discernible, but limited crustal contamination. All the olivine crystals (Fo = 94-75%) have high CaO contents (>0.1wt %), indicative of a magmatic origin. The NiO decrease with decreasing Fo is consistent with the effect of fractional crystallization. High-Mg olivines (i.e. those with Fo > 90) are best interpreted as having crystallized from the ultrapotassic magma system (vs mantle olivines) because their spinel inclusions have high Cr-number and low Al2O3 and TiO2, consistent with a magmatic (vs mantle) origin. High fO2 values calculated from the spinels indicate that the parental melts were oxidized, with high Fe3+/FeTot or low Fe2+/FeTot, which explains the high Mg-number [= Mg/(Mg + Fe2+)] of the melts and the high-Mg olivines. The high fO2 of the ultrapotassic magmas is probably inherited from their high fO2 metasomatically enriched lithospheric mantle source. Very low-degree partial melting of metasomatized mantle lithosphere best explains the petrogenesis of these ultrapotassic rocks. The metasomatism may have been relatively recent, probably since the Emeishan flood basalt magmatism in the late Paleozoic in the region. The metasomatic agent may be dominated by a carbonatitic melt, which has imprinted the enriched Sr-Nd-Pb isotopic signature and incompatible element enrichments with conspicuous negative Ta-Nb-Ti anomalies seen in the resulting potassic and ultrapotassic volcanic rocks. Fractionation of Ti-rich amphiboles during melt ascent may have also magnified the geochemical signatures of these rocks. © The Author 2010. Published by Oxford University Press. All rights reserved.