摘要
内蒙古四子王旗地区发现有一套早白垩世钾玄岩。主量元素、微量元素、稀土元素以及 Sr-Nd 同位素测试结果表明,岩石以低硅(SiO_2含量为48.11%～52.77%)、高钛(TiO_2含量为1.94%～2.77%,平均2.48%)、高钾高碱(K_2O+Na_2O 为5.32%～7.02%、K_2O/Na_2O 比值为0.48～1.10)为特征,属于钾玄岩系列。它们富集大离子亲石元素、轻稀土元素,亏损高场强元素,具有微弱的 Eu 负异常(δEu=0.81～0.98),其地球化学特征总体与洋岛玄武岩(OIB)相似。Rb/Nb、Ba/Nb、La/Nb、K/Nb 和 Zr/Nb 比值主要落在 EM I OIB 范围内。^(87)Sr/^(86)Sr 和^(143)Nd/^(144)Nd 比值分别变化在0.7063～0.7077和0.51196～0.51243之间,岩石的 Nd、Sr 同位素组成接近 EM I 地幔端元。随着ε_(Nd)的减小,P_2O_5、Rh、Sr、Ba 和 Zr 逐渐富集,表明四子王旗钾玄岩没有经历明显的地壳物质混染。利用微量元素地球化学原理,推断四子王旗钾玄岩母岩浆源于含金云母地幔橄榄岩减压低度部分熔融。根据其地球化学特征可以认为,四子王旗钾玄岩是华北板块早白垩世岩石圈减薄事件的产物。
There is a suit of shoshonite discovered recently in Siziwangqi area, Inner Mongolia. The shoshonitic basalts are characterized by low-Si （ SiO2 48.11% - 52.77% ） , high-Ti （ TiO2 1.94% - 2.77% , averaged by 2.48% ） , high-K （ K2O ＋ Na2O 5.32% -7.02% , K2O/Na2O 0.48 - 1.10）; The rocks are also enriched in large ion lithophile element （LILE）, light rare earth elements （LREE）, and depleted in high field strength elements （HFSE） , slightly negative Eu anomaly （δEu = 0.81 - 0.98 ）. Their chemical characteristics are similar to those of oceanic island basalt （OIB）. Trace element ratios such as Rb/Nb, Ba/Nb, La/Nb, K/Nb and Zr/Nb are consistent with the EM Ⅰ OIB geochemical signatures. ^87Sr/^86 Stand ^143Nd/^144 Nd ratios are in the ranges of 0. 7063 to 0.7077 and of 0. 51196 to 0.51243, respectively, showing the EM I geochemical signatures. The progressive enrichment of P2O5, Rb, Sr, Ba and Zr with decreasing εNa values, exclude the possibility that the magmas were contaminated by crustal materials. Trace-element compositions and isotopic characteristics suggest that the shoshonitic parental magma was derived from the small degree decompression melting of phlogopite-bearing mantle peridotites. Based on the geochemistry, Siziwangqi shoshonites are the products of early Cretaceous lithospheric thinning of the North China craton.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第11期2791-2800,共10页
Acta Petrologica Sinica
基金
国家自然科学基金项目(批准号:40072074)资助。
