Doctoral Dissertations

Qu Qi

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Lawrence A. Taylor

Committee Members

Harry Y. McSween, Theodore C. Labotka, A. J. Pedraza

Abstract

Cenozoic basaltic rocks from SE China can be grouped into three different series: tholeiites, alkali basalts, and picritic-nephelinitic basalts. Each basaltic series has distinctive geochemical features and is not derived from a common source rock by different degrees of partial melting or from a common parental magma by fractional crystallization. The petrography and major- and trace-element geochemistry of the tholeiites from SE China are similar to Hawaiian tholeiites, implying that the mantle source for these Chinese continental tholeiites is similar to that of the oceanic island basalts - an asthenospheric mantle. The alkali basalts and picritic-nephelinitic basalts are enriched in incompatible trace elements and are the products of partial melting of an enriched lithospheric mantle, or are the mixing products of an asthenospheric magma with a magma derived from an enriched lithospheric mantle. Radiogenic isotopic ratios of these rocks indicate that this enrichment of the lithospheric mantle source (mantle metasomatism) was a relatively young event, which is in agreement with the observation from mantle peridotite xenoliths from this region. Geochemical data on mantle peridotite xenoliths from SE China demonstrate that the lithospheric mantle under this region is heterogeneous. The depleted signatures of Sr and Nd isotopic compositions and major-element contents (low CaO and AI2O3) require an old depletion event, probably mid-Proterozoic, and the enrichment of LREE in the depleted peridotites implies a young metasomatic event shortly before Cenozoic magmatism. Major-element compositions of the peridotite xenoliths are controlled largely by the degree of partial melting, and the compositions of mineral phases are controlled by both P-T conditions and degree of partial melting which the host rocks have experienced. The equilibrium P-T-ƒO2 conditions, determined from coexisting mineral phases, indicate that SE China had a high heat flow similar to current oceanic basins or rifting regions during the Cenozoic. The later metasomatism did not change the redox state in the upper mantle. A rare variety of Fe-Al-rich xenolith, tridymite-hercynite rock, was entrained in the Cenozoic tholeiite from Niutoushan, southeastern China. The tridymite-hercynite rock consists of hercynite (40-50%), tridymite (35-45%), ilmenite (-2%), and glass (10-20%). Mineral grains are smaller than 0.05 mm with a granular texture. Compared to normal igneous rocks, this tridymite-hercynite rock has higher FeO, AI2O3, and lower Si02, CaO, MgO, and alkalis (Na2O + K2O). Based upon the available mineralogical and geochemical data, three possible origins have been evaluated: (1) The product of an extreme tholeiitic fractionation under low oxidation conditions; but the major element geochemistry appears to negate this model. (2) The Fe-rich and Si-poor portion of silicate-immiscible liquids from an extremely fractionated residual tholeiite magma; however, the corresponding Si-rich end-member fraction is missing, and the composition of the tridymite-hercynite rock is different from that of any Si-poor melt phase of known immiscible silicate liquids. (3) The product of metamorphism, including partial to complete melting, of a sedimentary protolith. An Fe-Al-rich shale (quartz 5%, kaolinite 75%, hematite 20%) has the approximate composition of the tridymite-hercynite rock. The trace-element abundances of the xenoliths suggest that the source rock for the Fe-Al-rich sedimentary protolith was an undifferentiated volcanic rock at the active continental margin.

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