鲁西下常庄中三叠世磁铁角闪岩的岩石地球化学和Sr--Nd同位素：对岩石成因和构造背景的制约

摘要

The systematically petrographical,geochemical and Sr--Nd isotopic research of the Middle Triassic Xiachangzhuang magnetite amphibolites (242 Ma)show that they are mainly composed of hornblende (40% ~45%),magnetite (35% ~45%),plagioclase (1 0% ~1 5%)and clinopyroxene (1% ~3%).The magnetite amphibolites have low contents of SiO2 (24. 25% ~38. 40%),K2 O (0. 02% ~1 . 09%),Na2 O (0. 1 6% ~1 . 68%),and high concentrations of MgO (6. 86%~1 1 . 81%),Fe2 OT3 (21 . 69%~36. 62%),TiO2 (7. 37%~1 3. 46%).They are enriched in light rare earth elements (LREEs)and depleted in high field strength elements (HREEs)(∑LREE/∑HREE=7. 09 ~8. 97)with high (La/Yb)Nvalues (10. 67 ~16. 62)and weak negative Eu anomaly (δEu=0. 83~1 . 01 ).Additonally,they are enrichment of LILEs (Rb,Ba and Sr),nondepletion of HFSEs (Nb,Ta,Zr and Hf),and show normal Pb and Ti anomalies.The whole-rock 87 Sr/86 Sr initial ratios range from 0. 702 645 to 0. 706 534,corresponding to εNd (t)and model ages (TDM1 )ranging from -7. 51 to -1 2. 59 and 237 4 to 2 41 3 Ma,respectively.The above results suggest that the protolith of the Xiachangzhuang magnetite amphibolites may be hornblende gabbros derived dominantly from partial melting of enriched lithospheric mantle of North China Block.The forming age of the Middle Triassic Xiachangzhuang magnetite amphibolites is similar to the peak period age of the Dabie --Sulu UHP metamorphism,which suggests that they might form in a transformation tectonic setting changing from ocean-continent subduction to the continent-continent collision in Early Mesozoic.%下常庄中三叠世（242 Ma）磁铁角闪岩的岩相学、岩石地球化学和Sr-Nd同位素的研究结果表明，下常庄磁铁角闪岩主要由角闪石（40％～45％）、磁铁矿（35％～45％）和斜长石（10％～15％）及少量的单斜辉石（1％～3％）组成；具低的 SiO2（24．25％～38．40％）、K2 O （0．02％～1．09％）和 Na2 O （0．16％～1．68％）含量，较高的MgO（6．86％～1 1．81％）、Fe2 OT3（21．69％～36．62％）和TiO2（7．37％～13．46％）含量；轻稀土元素（LREE）相对富集、重稀土元素（HREE）相对亏损（∑LREE／∑HREE ＝7．09～8．97、（La／Yb）N ＝10．67～16．62），无明显的Eu异常（δEu＝0．83～1．01）；富集大离子亲石元素（Rb、Ba、Sr），高场强元素（Nb、Ta、Zr、Hf）不亏损，具Pb和Ti的正异常；全岩87 Sr／86 Sr初始比值介于0．702645～0．706534之间，εNd （t）值变化于－7．51～－12．59之间，TDM1＝2374～2413 Ma。上述结果暗示，下常庄磁铁角闪岩的原岩可能为角闪辉长岩，起源于受下地壳物质混染的华北地块富集岩石圈地幔的部分熔融。下常庄中三叠世磁铁角闪岩原岩的形成时代与大别—苏鲁高压-超高压变质的峰期时间一致，暗示它们可能形成于中生代早期洋壳俯冲-消减向陆-陆碰撞造山转换的构造背景。