辽东菜园子古近纪玄武岩和深源捕虏晶:对新生代岩石圈地幔性质及深部作用过程的制约

CAIYUANZI PALEOGENE BASALTS AND DEEP-DERIVED XENOCRYSTS IN EASTERN LIAONING,CHINA:CONSTRAINTS ON NATURE AND DEEP PROCESS OF THE CENOZOIC LITHOSPHERIC MANTLE

通过对辽东菜园子玄武岩中的橄榄石和单斜辉石捕虏晶进行K-Ar定年及岩石地球化学测试,研究华北克拉通东部新生代岩石圈地幔的性质及深部作用过程,结果表明,辽东菜园子玄武岩形成于古近纪,K-Ar年龄为36.33 Ma±0.97 Ma和39.34 Ma±1.27 Ma;属于碱性玄武岩系列;玄武岩中Sr,Ba相对富集,Zr,Hf相对亏损;ISr和εNd(t)值分别介于。0.704 73～0.705 16和+0.22～+2.27之间。橄榄石捕虏晶发育环状裂隙和扭折带,具有明显的成分环带,自核至边Mg#值逐渐降低(核部Mg#值介于88.7～94.3之间,边部Mg#值介于76.6～77.8之间),核部与新生代玄武岩地幔橄榄岩包体中橄榄石的成分类似,边部与玄武岩中橄榄石斑晶成分相当。单斜辉石捕虏晶为透辉石,具有反环带结构,核部具有较高的FeO,MnO,Na2O,Al2O3质量分数,反映了壳源成因;过渡部分MgO,Cr2O3质量分数明显升高,反映了壳源物质与软流圈物质反应的结果;向边部MgO,Cr2O3质量分数逐渐降低;FeO,MnO,Na2O,Al2O3质量分数逐渐升高,暗示与寄主岩浆反应的效应。上述结果表明,华北克拉通东部新生代岩石圈地幔具有亏损性质,但岩浆形成过程中有陆壳物质加入。环带状橄榄石捕虏晶反映了新生代岩石圈地幔中橄榄岩与熔体之间相互作用的存在。

In order to reveal the nature and deep process of the Cenozoic lithospheric mantle in eastern North China Craton （NCC）, the K-Ar dating, petro-geochemistry, and mineral chemistry of xenocrysts from the Caiyuanzi basalt in eastern Liaoning are studied in this paper. The results indicate that the Caiyuanzi basalts formed in the Paleogene with the ages of 36.33 Ma±0.97 Ma and 39.34 Ma±1.27 Ma, belong to alkaline basalt series, and are characterized by relative enrichment in Sr,Ba and depletion in Zr,Hf. Their Isr and ε Nd values range from 0. 704 73 to 0. 705 16 and ＋0.22 to ＋ 0.27, respectively. The olivine xenocrysts display circle cracks, kink band, and obvious compositional zoning. Their Mg values gradually decrease from core to rim,being 88.7-94.3 and 76.6-77.8,respectively. The former is similar to those of olivines in the mantle-derived peridotite xenoliths from the Cenozoic basalt,and the latter is chemically consistent with the olivine phenocrysts in the basalt. The clinopyroxene （Cpx） xenocrysts of diopsides with reversal zoned structure are rich in FeO, MnO, Na2O, and Al2O3 in the cores, implying that they could come from the crust. However,the cpx xenocrysts are obviously rich in MgO,Cr2O3 in the transition,suggesting that the reaction between the crust-derived Cpx and melting from the astheno- sphere could happen. The increases of FeO, MnO, Na2 O, and Al2O3 contents from transition to rim for Cpx xenocrysts should be attributed to the reaction of the host magma with the xenocrysts. The above-mentioned lines of evidence suggest that the Cenozoic lithospheric mantle in the eastern NNC is characterized by depletion,and that the delaminated continental crust material could be involved in the formation of primary magma. The occurrence of the zoning olivine xenocrysts implies that the reaction between the mantle peridotite and the melting could have taken place in the Cenozoic lithospheric mantle.