Fe-Al-rich tridymite-hercynite xenoliths with positive cerium anomalies: preserved lateritic paleosols and implications for Miocene climate

We report isotopic and chemical compositions of unusual tridymite-hercynite xenoliths in middle Miocene Niutoushan tholeiites from the southeast coastal area of China. These xenoliths are characterized by positive cerium (Ce) anomalies and extremely high Al2O3 (32-34 wt.%) and total iron oxide (20-22%). They have Sr-87/Sr-86 of 0.7050-0.7058, epsilon(Nd)(0) values of + 3.2 to +4.2. Pb-206/Ph-204 ratios of 18.8-19.1, and delta(18)O values of +5.2parts per thousand to +6.1parts per thousand. Their chemical and isotopic compositions suggest that these xenoliths represent preserved aluminous lateritic paleosols that are not genetically related to host tholeiites. These lateritic palcosols with strongly desilicated minerals were formed by intense chemical weathering under warm and humid tropical conditions (with annual average temperature of > 19 degreesC and the annual rainfall of >165 cm) in SE China during the interval from 17 to 15 Ma. The formation age of the paleosols corresponds to a period characterized by slow uplift of the Himalayan-Tibetan Plateau region (and thus less consumption Of CO2) after 17 Ma, and eruptions of 17-15 Ma Columbia River flood basalts, the Vogelsberg basalts, and eastern China basalts (and thus more input Of CO2 into the atmosphere). The tridymite-hercynite xenoliths in the Niutoushan basalts thus preserve evidence of extraordinary climatic greenhouse conditions in the middle Miocene that would otherwise have been lost by the erosion of paleosols. (C) 2004 Elsevier B.V. All rights reserved.