Nd、Sr and Pb isotopic geochemistry of late-Mesozoic alkaline-rich intrusions from the Tanlu Fault zone:evidence of the magma source.
Tanlu Fault Zone,a large deep fault zone in eastern China,cuts four tectonic units (the North China Craton,the Yangtz Craton,the Hinggan-Inner Mongolia-Jilin-Heilongjiang orogenic belt and the Sulu-Dabie orogenic belt) straight.It is well known that there is a nearly 2000 kilometer-long alkali-rich intrusive rock belt composed of many late-Mesozoic alkaline-rich intrusions in this zone.Based on the pioneers' research,a systematic and thorough geochemical study on their Nd,Sr and Pb isotope is presented in this paper for the first time.The distinct different Nd,Sr and Pb isotopic characteristics of the alkali-rich intrusive rocks in the four tectonic units indicates their different source regions:the late-Mesozoic alkaline intrusions from the Hinggan-lnner Mongolia-Jilin- Heilongjiang orogenic belt are derived from the young crust related to depleted mantle,but the others from the North China Craton,the Yangtz Craton and the Sulu-Dabie orogenic belt are mainly originated from the EMI-type enriched mantle.Farther more,the enrichment degree of the lithospheric mantle from the latter three units is obviously different from each other:the lithospheric mantle from the Sulu- Oabie orogenic belt has the highest enrichment (averageee_(Nd) (t) value is-17.06),that from the North China Craton (average e_(Nd) (t) value is-10.03) takes second place,and the enrichment of the mantle in the Yangtz Craton (averageeNd (t) value is-6.41) is the lowest.It is suggested in this paper that the most enriched lithospheric mantle from the Sulu-Dabie orogenic beh results from the releasing of pressure in the subduetion-exhumation in early Mesozoic era in the Sulu-Dabie orogenic,which conduces to the formation, precipitation and metasomatism of melt/fluids,and then reconstructs the lithospherie mantle from the North China Craton.