Petrogenesis of Shangyu gabbro-diorites in western Shandong: Geochronological and geochemical evidence

• 来源期刊: 中国科学D辑（英文版）
• 年，卷（期）: 2008,（4）
• 页码范围: 481-492
• 页数: 12页
• 语种: 英文
• DOI: 10.1007/s11430-008-0029-0

摘要

Chronology and geochemistry of the Shangyu gabbro-diorite in western Shandong were studied to understand their petrogenesis and the nature of the Mesozoic lithospheric mantle. The Shangyu intrusion is mainly composed of a suite of gabbro-diorite. Zircons from the intrusion display euhedral-subhedral in shape and have high Th/U ratios (1.23-2.87), implying their magmatic origin.LA-ICP-MS zircon U-Pb dating results for two samples indicate that they were formed in the Early Cretaceous, yielding weighted mean 206Pb/238U ages of 129±1 Ma and 134±2Ma, respectively. Except for early cumulate such as sample QT-19, their SiO2 and MgO contents range from 50.12% to 56.37% and from 3.52% to 6.37%, respectively. Moreover, the gabbro-diorites are characterized by high Mg# (0.54-0.63),enrichment in Na (Na2O/K2O ratios more than 1 ), Cr (73×104-217×10-6) and Ni (34×10-6-241 ×10-6), and intensive enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Their initial 87Sr/66Sr ratios and εNd(t) values range from 0.70962 to 0.71081 and from -16.60 to -13.04, respectively. Taken together with the Early Cretaceous high-Mg diorites and the mantle xenoliths from the Tietonggou and Jinling as well as basalta from the Fangcheng and Feixian, it is suggested that the primary magma for the Shangyu gabbro-diorites should be derived from the enriched lithospheric mantle intensively modified by continental crust. The Sr-Nd-Pb isotopic compositions for the Early Cretaceous high-Mg diorites in western Shandong display a trend of spatial variations, i.e., initial 87Sr/86Sr, 207Pb/204Pb and 208Pb/204Pb ratios decreasing and εNd(t) values increasing from southeast to northwest in western Shandong, which is consistent with the tectonic model that the Yangtze Craton subducted beneath the North China Craton oriented in north-west direction in the Early Mesozoic.