华北陆块东部晚中生代基性岩浆作用的地球化学研究

GEOCHEMISTRY OF THE LATE MESOZOIC MA FIC MAGMATISM FROM THE EASTERN NORTH CHINA BLOCK:A COMPARATIVE STUDY

华北陆块晚中生代(早白垩世)基性火成岩沿着郯庐断裂带存在显著的元素-同位素地球化学分区。其西侧(鲁西区)火成岩在主量元素上以高Mg、Fe和低Al、Ti为特征,在微量元素上富集LILE、LREE和强烈亏损HFSE(Nb、Zr、Hf、Ti和P),同时显示出EM1型Sr-Nd同位素组成;其东侧(鲁东区)火成岩则相对低Mg、Fe和高Al、Ti,在微量元素上也表现出LILE、LREE富集和Nb、P亏损的特征,Ti、Zr、Hf异常不明显,并表现出EM1+EM2的Sr-Nd同位素组成。鲁西区基性火成岩源于陆内岩石圈伸展-减薄作用下古老而富集的岩石圈地幔减压部分熔融作用的结果,鲁东区则为被俯冲陆壳物质改造的富集岩石圈地幔减压部分熔融作用产物。中生代华北陆下岩石圈地幔性质仍为古老富集型地幔,因此至少在早白垩世前不可能存在地幔柱作用和大规模的下地壳拆沉作用。

Geochemical investigations on late Mesozoic(early Cretaceous)mafic igneous rocks from both areas separated by Tan -Lu Fault show clear geochemical and isotopic zonation.Generally,samples from the western part of the Tan -Lu Fault(termed the west Shandong Zone(WSZ))occupy relatively higher MgO,FeO * but lower Al 2 O 3 and TiO 2 ,exhibiting LILE,LREE enrichment while significantly nagetive HFSE an d Ti anomalies as well as EM1-like Sr -Nd isotopic compositions.These rock s were interpreted as decom-pression melting products of an aged and enriched lithospheric mantle ma inly composed of chemically re-fractory garnet harzburgites with d isseminated phlogopites.Compared with those from WSZ,the counterparts from the eastern part(termed the east Shandong Zone(ESZ)),usually display relatively lower MgO,FeO * but higher Al 2 O 3 and TiO 2 ,exhibiting LILE,LREE enrichment while significant Nb and P negative an omalies as well as EM1+EM2Sr -Nd isotopic data.They were derived from an enriched mantle source,which had been modified by Triassic subducted continental crust prior to magma gen eration.All these observations ten d to support that the lithospheric man tle beneath the NCB was mainly composed of aged and enriched mantle reservoirs of relatively lower density during the early Cretaceous,thu s it is unlikely for significant cont inental lower crust delamination to have occ urred neithor an abnormally upwelli ng mantle plume underneath the NCB during nor shortly before the meltin g event.