大别山沙村和椒子岩基性-超基性岩锆石U-Pb定年、元素和碳氧同位素地球化学研究

Zircon U-Pb Age, Element and Isotope Geochemistry of Mesozoic Mafic-Ultramafic Rocks at Shacun and Jiaoziyan in North Dabie

对大别山北部沙村和椒子岩基性 -超基性岩进行了锆石U Pb定年、全岩主量和微量元素分析 ,全岩和单矿物氧同位素分析以及全岩碳含量及其同位素组成的测定。结果表明 ,这些基性 -超基性岩表现出强烈的轻稀土富集 ,高场强元素 (Nb ,Zr和Ti)负异常以及Pb和Ba正异常。锆石U Pb年龄指示了早白垩世 (12 2～ 12 8Ma)的岩浆结晶年龄 ,但是在 10 5～ 116Ma期间受到了热液蚀变。全岩和单矿物氧同位素比值变化较大 (全岩 :1.1‰～ 6 .6‰ ,单斜辉石 :3.85‰～5 .7‰ ,斜长石 :2 .8‰～ 7.3‰ ,锆石 :3.85‰～ 6 .0 4‰ ) ,大多数锆石具有与正常地幔锆石 ((5 .3±0 .3)‰ )不同的δ18O值 ,部分样品矿物对之间保存了氧同位素平衡分馏 ,而部分样品则表现出明显的氧同位素不平衡分馏 ,指示它们受到了岩浆期后亚固相水 -岩相互作用的扰动。全岩碳含量和同位素组成具有较大的变化范围 ,分别为 0 .0 3%～ 0 .18% ,δ13 C值为 - 2 7.0‰～ - 5 .8‰。将这些中生代基性 -超基性岩的元素和同位素数据与大别 -苏鲁榴辉岩比较后发现 ,它们之间具有较为一致的地球化学特征 ,因此可能具有一定的成因联系。这些榴辉岩的原岩为基性火成岩 ,对应于扬子板块北缘与新元古代裂谷构造有关的岩浆活动。古老富集岩石圈地幔及其上覆地壳部分?

Zircon U Pb dating, major and trace elements of whole rocks, oxygen isotope compositions of mineral separates, and carbon concentrations and isotope compositions of apatite and whole rocks were determined for Mesozoic mafic ultramafic intrusions at Shacun and Jiaoziyan in the Dabie Mountains. The results show that these mafic ultramafic rocks are characterized by strong LREE enrichment, negative HFSE anomalies (Nb, Zr and Ti), and positive anomalies of Pb and Ba. Zircon U Pb dating yields the Early Cretaceous (122 to 128Ma) magma crystallization ages and also the younger post magmatic hydrothertmal overgrowth ages (105 to 116 Ma). There is a large variation in oxygen isotope ratios of minerals and their host rocks (whole rock: 1.1 ‰～6.6 ‰, clinopyroxenite: 3.85 ‰～5.7 ‰,plagioclase: 2.8 ‰～7.3 ‰, zircon: 3.85 ‰～ 6.04 ‰ . Most zircons have δ 18 O values different from normal mantle zircon. Equilibrium fractionations of oxygen isotopes among minerals have been preserved for some samples, but for the others obvious disequilibriaum of oxygen isotopes is detected, indicating alteration by post magmatic subsolidus water rock interactions. Carbon concentrations and isotope compositions vary widely from 0.03 % to 0.18 % and from -27.0 ‰ to -5.8 ‰, respectively. Comparison of element and isotope data for Mesozoic mafic ultramafic rocks with eclogites from the Dabie Mountains shows that they share many common geochemical features, and thus there should be genetic links between them. Protholiths of these ecologites are considered mafic rocks, corresponding to rift magmatism in the northern margin of the Yangtze plate related to Neoproterozoic rifting tectonics. Partial melting of old lithosphere mantle and overlying crust is considered to produce these early Cretaceous mafic ultramafic rocks.