北淮阳东段徐家湾岩体地质和地球化学特征及LA-ICP-MS锆石U-Pb年龄

徐家湾二长花岗岩体位于北淮阳构造带内桐柏-桐城与郯城-庐江两大断裂的交汇处,岩体呈岩株状侵入新元古界老变质岩层中。本文利用原子吸收光谱和ICP-MS法测定了岩体主量和微量元素的含量,表明岩体具SiO₂和Al₂O₃较高、富碱、过铝质、Mg^#小等特征。大离子亲石元素(LILE)Rb、Ba富集,Sr亏损;高场强元素(HFSE)Y、Th、Nb、Hf、U富集,Ta、P、Ti亏损;岩体整体亏损HFSE,富集LILE;La_N/Yb_N与LREEs/HREEs值均较大,具较弱的δCe负异常,显示该岩体为过铝质A型花岗。LA-ICP-MS锆石U-Pb定年获得徐家湾二长花岗岩侵位年龄在128.0±0.9~129.6±1.4 Ma之间,是早白垩世岩浆活动的产物。研究认为徐家湾二长花岗岩体形成于造山后的伸展环境,形成岩体的岩浆源于岩石圈地幔,并受到地壳物质的混染。

Geochemical Characteristics and LA-ICP-MS Zircon U-Pb Geochronology of Xujiawan Monzogranite in the Eastern Part of North Huaiyang and Their Geological Significance

The Xujiawan monzogranitic intrusion is located at the junction of the Tongbai-Tongcheng fault and the Tancheng-Lujiang fault in North Huaiyang tectonic belt. The rocks intruded into the Neoproterozoic metamorphic rocks as stocks. In this study, major and trace element contents of the rocks are determined by Atomic Absorption Spectrometer and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Results show that these rocks have high contents of SiO₂ and Al₂O₃ and are alkali-rich and peraluminous with low Mg^# values. They are rich in large ion lithophile elements such as Rb and Ba, but are poor in Sr. They are rich in high field strength elements such as Y, Th, Nb, Hf, U, but are poor in Ta, P, Ti. In general, these rocks are deplete in HFSE but enriched in LILE. They have high La_N/Yb_N and LREE/HREE ratios and have weak Ce anomalies. Major and trace elements indicate that these rocks belong to peraluminous A-type granite. LA-ICP-MS Zircon U-Pb dating of Xujiawan monzogranite yield ages of 128.0 0.9 Ma to 129.6 1.4 Ma, indicating that these intrusions were produced by the early Cretaceous magmatism. Xujiawan monzogranite may have formed in a post-collisional extensional setting. The magma may be derived from the mantle and contaminated by the continental crust.