RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 2, NO. 2, PAGES 153–183, doi:10.2205/2000ES000038, 2000

Mongolia Cenozoic volcanism

UIPE, Moscow, Russia

Abstract

[1]  Described in this paper is the Cenozoic volcanic activity of Mongolia, including the description of the basalts and their deep crustal inclusions. Seventeen individual areals of basalt volcanism were identified in Mongolia on the basis of the geologic and geomorphologic data available. The wide areal development of Cenozoic volcanic activity began at the Cretaceous-Paleocene boundary with its peak in the Miocene. Our isotope dating allowed us to identify several time periods of volcanic activity. The study of the lateral changes in the behavior of petrogenic elements allowed us to identify the Hangai Upland as a specific structural element of Mongolia, or as its "symmetry center", relative to which the SiO₂, FeO, MgO, and K₂O contents vary in the submeridional and latitudinal directions. The single finding of young acid lavas in Central Mongolia suggests the possibility of, at least, the local manifestation of bimodal magmatism, characteristic of rift-type structural features. Most of the crustal xenoliths found in the basalts, studied in this region, show the distinct indications of at least superposed metasomatic alterations, which resulted in the appearance of mica, amphibole, ilmenite, apatite, and other minerals. Our comparative analysis of the geochemical properties of the basaltoids in the Transbaikalia region, Mongolia, and partially in Northeast China showed that their REE concentrations prove the intensive fractionation of the melt, for which the basalts had been generated. Our Spider diagrams based on ultrabasic inclusions from the Khamar-Daban and Bartoi basalts showed that the xenoliths found in the latter had been impoverished in most of the minor elements, compared to the Khamar-Daban basalts. All inclusions from the Transbaikalia, Mongolia, and China basalts showed the uniform distribution of minor elements in spite of their enrichment (compared to chondrite) in Cs, Ce, Ti, and Yb. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios showed that the basalt and glass compositions of the peridotite inclusions follow the mantle sequence of the DM, N-MORB, and EM reservoirs. The analysis of the crustal structure of the study region and the geophysical data available (including tomography) proved the presence of "hot" low-velocity upper mantle blocks (diapirs) under most of the areas of the Cenozoic volcanism. Our mineral geothermobarometry allowed us to plot paleogeotherms which proved that the subcrustal layer of the upper mantle had been heated to 100-200^oC during the time of about 20 million years. We agree with the view of the rift-type endogenic conditions of the Cenozoic volcanism in Mongolia. Most of its volcanic areas experienced the pre-rifting conditions of the rifting evolution.

Received 15 August 2000; published 15 September 2000.

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