Zircon U-Pb geochronology and major, trace elemental and Sr-Nd-Pb isotopic geochemistry of mafic dykes in western Shandong Province, east China: Constrains on their petrogenesis and geodynamic significance

Mesozoic mafic dykes in western Shandong Province (Luxi), SE, the North China Craton (NCC) provide an opportunity to examine the nature of their mantle source and the secular evolution of the Mesozoic lithospheric mantle beneath SE NCC. Chronological, geochemical and Sr-Nd-Pb isotopic analyses were carried out on two selected Mesozoic mafic dykes from Mengyin and Zichuan in western Shandong, respectively. Detailed SHRIMP zircon U-Pb dating yields emplacement ages of 144 +/- 2 Ma for the Mengyin mafic dykes and of 143 +/- 2 Ma for the Zichuan mafic dykes. Both are enriched in LILE (Rb, Ba, Sr, Pb) and LREE without Eu anomalies, but are depleted in HFSE (Nb, Ta and Ti). The dykes have relatively radiogenic Sr (Sr-87/Sr-86)(i) (0.706 similar to 0.707) and negative epsilon(Nd)(t) (-6.5 to -4.4), but remarkably unradiogenic Pb (Pb-206/Pb-204 = 16.99-18.36). Both dykes may have experienced crystal fractionation of olivine and clinopyroxene. The Mengyin mafic dykes are characterised by high MgO (9.66-17.97 wt.%), Mg-# (66-74), Cr (809-1208 ppm) and Ni (171-390 ppm), indicative of derivation from mantle-derived melts with minor fractionation. In contrast the Zichuan mafic, dykes have relatively low MgO (3.38-4.14 wt.%), Cr (24-56 ppm) and Ni (14-24 ppm), suggesting that they may have originated from an extremely evolved magma. There is evidence in support of multiple enrichment events induced by hybridism of foundering lower crust at mantle depths. For example, phlogopite, amphibole and rutile metasomatism may have affected the lithospheric mantle beneath Mengyin, whereas phlogopite and carbonatite metasomatism modified the lithospheric mantle below Zichuan. This study sheds light also on the geodynamic significance of the 144-143 Ma mafic dykes, for example, we propose that lithospheric thinning underneath southeastern NCC of eastern China was caused by the removal of the lower lithosphere (mantle and lower crust). (C) 2008 Elsevier B.V. All rights reserved.