A Study On Geochemical Characteristics Of Datong Basalts

The Quaternary Datong basalt is located to the west of the Daxing'anling-Taihangshan Gravity Lineament?DTGL?.The research of geochemical characteristics of the basalt will place important constraint on the origin as well as the interaction between lithosphere mantle and asthenosphere in the western North China Craton.According to the different landform and lithology,Datong basalt can be roughly divided into east and west regions along the NE-oriented Chenzhuang-Xubao fault.The western region is characterized by numbers of volcanic cones with dorminant alkaline basalt,whereas the eastern region is characterized by overflowing tholeiitic basalt with few cones.Many olive and clinopyroxene phenocrysts are observed under the microscope.As the Ni and Cr abundance decrease with that of Mg O,we conclude that these phenocrysts should be formed by crystallization.The Si O₂ contents of these rocks vary from 45.02%to 53.3%and?K₂O+Na₂O?from 3.59%to 6.53%.The trace element normalized curves show relatively rich light rare earth elements??La/Yb?_N=5.8-31.6?,and the primitive mantle-normalized spider patterns show both positive Rb,Ba,and Sr anomalies?LILE enrichment?and Nb,Ta,and Zr anomalies?HFSE enrichment?,similar to those of Ocean Island-like basalt?OIB?.According to the simulation calculations of La/Yb vs.Sm/Yb,it is suggested that the Datong basalts derive from low degree partial melting of garnet-bearing lherzolite,with the melting degree of the alkaline basalt about 1.5-3%and the tholeiitic basalt about 4-8%.The Datong basalts have relatively lower ⁸⁷Sr/⁸⁶Sr?0.703302-0.705102?ratios and higher¹⁴³Nd/¹⁴⁴Nd?0.512561-0.512963?and¹⁷⁶Hf/¹⁷⁷Hf?0.282922-0.283072?ratios,falling into the OIB range.The ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd-²⁰⁶Pb/²⁰⁴Pb diagrams show that the Datong basalts are a binary mixture of PREMA and EMI.The differences of the geochemical characteristics can be explained by the two reasons:?1?The different main and trace element features are formed by various degrees of partial melting;?2?The isotope characteristics indicate that the Datong basalts are mainly produced by the partial melting of the depleted asthenosphere,with varied addition of the few amount of enriched lithospheric components.It is speculated that the formation of Datong basalt has been due to the extension of the lithosphere since the Cenozoic.The convective degree of the asthenosphere in the Datong area has increased.The melt with the PREMA geochemical characteristics assimilated the lithosphere with EMI characteristics to form a‘pudding'.In the western part of Datong volcanic field,the large thickness and weak extension of the lithosphere assimilated less‘pudding'and formed the depleted alkaline basalts with lower degree of partial melting.Whereas in the eastern part,due to the thin lithosphere,the asthenosphere assimilated more‘pudding'and formed the enriched tholeiitic basalts with a higher degree of partial melting.