Publication details for Professor Yaoling NiuHuang, X.L., Niu, Yaoling, Xu, Y.G., Chen, L.L. & Yang, Q.J. (2010). Mineralogical and Geochemical Constraints on the Petrogenesis of Post-collisional Potassic and Ultrapotassic Rocks from Western Yunnan, SW China. Journal of Petrology 51(8): 1617-1654.
- Publication type: Journal Article
- ISSN/ISBN: 0022-3530
- DOI: 10.1093/petrology/egq032
- Keywords: Green-core clinopyroxene, High-Mg olivine, Mantle metasomatism, Potassic and ultrapotassic rocks, Tibetan Plateau.
- Further publication details on publisher web site
Author(s) from Durham
Paleogene mafic potassic and ultrapotassic volcanic rocks in western Yunnan, China, show a compositional spectrum from potassic trachybasalt to latite (MgO = 6 center dot 24-21 center dot 8 wt %; SiO2 = 44 center dot 5-59 center dot 1 wt %). These rocks have high K2O (3 center dot 07-5 center dot 28 wt %), relatively low Na2O (0 center dot 99-4 center dot 18 wt %) and high K2O/Na2O ratios (0 center dot 91-3 center dot 89). They share geochemical features such as depletion of Ta, Nb and Ti relative to other similarly incompatible elements and enriched Sr-Nd isotopic compositions (initial Sr-87/Sr-86 of 0 center dot 7056-0 center dot 7101; epsilon(Nd)(t) of -0 center dot 97 to -4 center dot 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 center dot 77-0 center dot 90), low TiO2 (0 center dot 13-0 center dot 29 wt %), Al2O3 (0 center dot 73-1 center dot 68 wt %) and Na2O (0 center dot 22-0 center dot 42 wt %) with similar Ti/Al (0 center dot 06-0 center dot 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 center dot 50-0 center dot 74) and Ti/Al (< 0 center dot 05), high Al2O3 (1 center dot 66-3 center dot 63 wt %), Na2O (0 center dot 87-2 center dot 17 wt %) and Al-VI/Al-IV (0 center dot 38-0 center dot 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 center dot 1 wt %), 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 fO(2) values calculated from the spinels indicate that the parental melts were oxidized, with high Fe3+/Fe-Tot or low Fe2+/Fe-Tot, which explains the high Mg-number [= Mg/(Mg + Fe2+)] of the melts and the high-Mg olivines. The high fO(2) of the ultrapotassic magmas is probably inherited from their high fO(2) 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.