Sources of Cenozoic intra-plate magmatism in Central Asia: implication from geochemistry and Sr-Nd-Pb isotope data
V.M.Savatenkov*, E.A.Kudryashova**, A.M.Kozlovsky**, V.V.Yarmolyuk**
* Institute of Precambrian Geology and Geochronology RAS, St.-Petersburg; ** Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow, Russia.
During Later Cenozoic within Central Asia the series of independently evolving volcanic areas (so-called Central Asian Volcanic Province (CAVP)) has formed . They are Udokan (UVA), Vitim (VVA), Southern Baycalian (SBVA) and Khangai (KVA) volcanic areas. They are situated far from lithospheric plate margins and are characterized by predominance of sub-alkaline and alkaline basalts. So, they relate to intra-plate processes. Geophysical studies of CAVP  revealed presence of large-scale ledge of an asthenospheric mantle (or hot field of mantle) with roof elevated on 100 km to surface. Moreover, directly beneath some volcanic areas the local elevation attending the 50 km to surface was discovered. These elevations were interpreted as mantle diapires produced by thermal field of the mantle. Because the feeding area was common the structurally separated volcanic areas were developed simultaneously . This fact also indicates the relation of CAVP with mantle plumes. Volcanic areas are characterized by the duration of development exceeding a few 10 Ma, as well as by relations to grabens systems, the stable character of magmatism, the regular migration corresponding to lithosphere displacements over hot spot, and by the link to dome-like elevation at modern relief.
The geochemical characteristics of the basalts from the different areas of CAVP demonstrate no significant differences. The basalts show smooth normalized REE spectrum with relative LREE enrichment. The trace element enrichment implies that the basalts of CAVP correspond to OIB. However, CAPV basalts show enrichment in Ba and K and depletion in U, Th, Zr, Hf and heavy REE compared with OIB. Because geochemical characteristics of basalts from different CAVP area are similar the source of investigated basalts is common and melting environment is similar.
In spite of these similarities, some differences between the basalts from different areas as well as basalts from single volcanic area occur. For example, the basalts from SBVA and KVA contain lower MgO compared with the basalts from UVA and VVA; that mean higher fractionating degree. In addition, within KVA the basalts vary from high-Mg to low-Mg types. These variations of chemical composition occur from centre to edge of KVA and they apparently indicate different levels of magma generation. These levels appear to link with mantle dome structure.
ÁIn εNd versus εSr space, the basalts from CAVP form superposition confined by two trends from PREMA-like mantle (FOZO ), one of which extend to EMI source and other extend to EMII source. In according to the εNd versus εSr diagram, the basalts from SBVA and KVA derived from the source contained mixture of EMI and EMII components. The basalts from VVA form trend from FOZO to EMII . End-member of the basalts from UVA accords with a weak depleted mantle isotope signatures.
In 206Pb/204Pb versus 207Pb/204Pb space, basalts from KVA and UVA also form some trend from FOZO. First group of the samples forms array shifted to the field of ancient (Archean) recycled material. The latter was primarily enriched in U and then partly loosed it during Proterozoic period. Other group of the samples forms trend toward the field of low-U/Pb ancient recycled material. This conclusion is in agreement with low Th and U concentrations in the studied basalts. End-member component of the basalts from UVA is a weak depleted mantle. This suggests shorter history of enriched source evolution compared with source of the basalts from KVA.
Thus geochemical and isotopic data demonstrate that source of the studied basalts appear to be involved material of recycled lithosphere. The basalts from KVA and SBVA generally exhibit low εNd value (up to -12), reflecting recycled lithosphere was ancient. In addition, their Pb isotope signature results from the long-term evolution of enriched source. In contrast, isotope data on the basalts from UVA and VVA indicate the shorter history of enriched source evolution. Sr-Nd-Pb isotope differences between end-member sources of the basalts from southern (KVA, SBVA) and northern (UVA, VVA) reflect sublithospheric mantle heterogeneity. So, sublithospheric mantle apparently contain buried lithosphere fragments of different age. Based on SrãNdãPb isotopic results, we suggest that common component for all of the studied basalts was asthenospheric mantle with εSr ~ -15, εNd ~ +5, 206Pb/204Pb ~ 18.5, at least in Central Asia.
This work is supported by the Russian Academy Foundation (grants 07-05-00876, 05-05-64000, 07-05-90100).
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