ND, SR AND PB ISOTOPIC GEOCHEMISTRY OF PALEO-MESOPROTEROZOIC ALKALINE-RICH INTRUSIONS FROM THE NORTHERN PART OF THE NORTH CHINA CRATON: EVIDENCE OF THE LITHOSPHERIC MANTLE ENRICHMENT

Abstract

The Paleo-Mesoproterozoic alkaline-rich intrusive rocks, mostly distributed in or near Yan-Liao triple-junction aulacogen system and Liao-Ji aulacogen along the north part of the North China Craton (NCC), are composed of basic to intermediate rocks dominated with syenite suites with elevated contents of total alkali and potassium. In this paper a systematic geochemical study on Nd, Sr and Pb isotopic characteristics is presented. All the εNd (t) values of these alkali-rich rocks are negative and the range (-3.4 ~ -7.5) is in a small scale with an average of -4.8. These are consistent with the Nd isotopic characteristics of the Paleo-Mesoproterozoic basic-ultrabasic rocks in this area (εNd (t)= - 4 ~ - 8 ), and also similar to the subalkaline extrusive in Yan-Liao aulacogen (ε Nd(t) = - 4.3 ~ - 8.9), which indicate that these alkaline-rich intusives are of enriched mantle origin. Their initial 87Sr/86 Sr values have a relatively restricted range from 0.7028 to 0.7053, with an average of 0.7041. The 206Pb/204 Pb (14.500 ~ 15.701, 14.968 average), 207Pb/204Pb (14.887 ~ 15.150, 14.984 average) and 208 Pb/204 Pb (34.178 ~ 36.537, 35.057 average) ratios for K-feldspar show that most of these alkali-rich rocks have very low ratios except for those from the Liangtun-Kuangdonggou complex. On the εNd(t) - εSr (t) diagram, all the spots are near the mantle array line and close to the EMI endmember, which indicates that these rocks have a close connection to the materials from the EMI enriched mantle. Their lead-isotope evolution also illustrates that they have relation with mantle and lower crust. Furthermore, a simulative calculation of the two endmembers reveals that the average ratio of the lower crust contamination is about 14%. According to this study, the depletion degree of lithospheric mantle of NCC has become weak and weak since ca. 2.5Ga, and the mantle enrichment appeared partly at about 2.2Ga, all the lithospheric mantle was enriched at 1.85Ga and then became more and more enriched. It's also suggested that the lithospheric mantle enrichment of NNC is related to the crust materials carried by subduction at early time and the subsequent mantle fluid metasomatism.