Abstract:

 Clinopyroxene phenocrysts with reverse zoning have been observed in Wudalianchi potassic lavas. The cores are salites characterized by lower Mg# (68~77), TiO2 (0.23~0.50 wt.%), Cr2O3 (<0.06 wt.%) and higher Al2O3 (3.4~5.0 wt.%), Na2O (0.43~0.78 wt.%), FeO (8.8~11.0 wt.%) and MnO contents than those of the exteriors. Normalized REE patterns of the cores are relatively flat, and they also exhibit prominently negative anomaly of Eu and Sr (Eu/Eu* =0.35~0.63, Sr/Sr* =0.03~0.17), suggesting an origin from metamorphic rocks in equilibrium with plagioclase. In the plot of Mg#-TiO2, the overall composition of the cores falls within the compositional range of clinopyroxenes in lower crustal low-Mg granulites from North China Craton. As a result, the cores are inferred to be xenocrysts from granulites in the lower continental crust. The exteriors and normal pyroxene phenocrysts are similar in their compositions, which are higher in Mg# (81~85), TiO2 (0.40~1.65 wt.%), Cr2O3 (0.03~0.25 wt.%), and lower in Al2O3 (2.1~3.4 wt.%), Na2O (0.34~0.63 wt.%), FeO (4.6~6.6 wt.%), MnO. Their LREE and HREE are highly fractionated (La/Yb)N=3.23~7.89, which are consistent with the basaltic whole rocks. The exteriors were in equilibrium with the host magmas, as is confirmed by both major and trace element modeling based on their partition coefficients. Thus the exteriors are suggested to be magmatic pyroxenes formed by overgrowth on the eroded cores. The Mg# of Wudalianchi normal pyroxene phenocrysts are negatively correlated with TiO2 contents, pointing to a certain extent of magmatic evolution in crustal depth, consistent with the gradual increasement of Al2O3 and Na2O contents from the inside out of the exteriors. The granulite facies nature of the cores further constrains the evolution to have occurred in the lower continental crust. Thermobarometer estimations also suggest that the crystallization of normal pyroxenes and exteriors of reversely zoned pyroxenes were in serial magma chambers in the lower crust. Pyroxene phenocrysts crystallized in a later stage are more enriched in incompatible elements than those of earlier pyroxenes, indicating that fractional crystallization have enhanced the enrichment of incompatible elements of Wudalianchi basalts. Calculated melts in equilibrium with pyroxene phenocrysts are extremely similar with the ultimately erupted lavas in their trace element patterns. Considering the remarkably uniform whole-rock trace element patterns, we suggest that the evolution of Wudalianchi basalts in the crust were mainly controlled by the fractional crystallization in magma chambers, and crustal contamination (also mixing with crust-derived magmas) has had negligible effects on whole rock compositions.

Key words: clinopyroxene, reverse zoning, Wudalianchi, magmatic evolution