2. 上海大学力学与工程科学学院，上海200444

为44. 43% ～ 46. 72%，Ni 值主要为1 100 × 10 － 6 ～ 2 381 × 10 － 6，计算得出的Fo 值为85. 60 ～ 87. 71。在岩石化学成分上，SiO2含量为38. 51% ～ 42. 86%，MgO 含量为26. 81% ～ 31. 28%，TFeO 含量为12. 17% ～ 14. 52%，TiO2

含量为1. 00% ～ 1. 45%，Na2O + K2O 含量为0. 17% ～ 0. 67%，在岩石化学分类上属麦美奇岩。因玻基辉橄岩中存在较多辉石斑晶而与传统麦美奇岩存在明显区别，因此暂将研究区超基性熔岩称为“麦美奇质岩石”。选用橄榄石最高Fo 值和其寄主岩石TFeO 含量估算出上述麦美奇质岩石原生岩浆中的MgO 含量≥18. 18%，据此限定地幔熔融温度和压力下限分别为1 580℃和3. 9 GPa。结合岔路口地区麦美奇质岩石具有洋岛玄武岩微量元素特征以及前人研究成果，认为该区麦美奇质岩石形成于晚二叠世—晚三叠世的伸展环境，其地球动力学背景与地幔柱有关。

2. School of Mechanics and Engineering Science，Shanghai University，Shanghai 200444，China

38. 51% ～ 42. 86%，MgO content is 26. 81% ～ 31. 28%，TFeO content is 12. 17% ～ 14. 52%，TiO2 content is 1. 00% ～ 1. 45%，and Na2O + K2O content is 0. 17% ～ 0. 67%． They belong to meimechite by petrochemical classification． In consideration of the presence of pyroxene phenocrysts in the vitric pyroxene peridotites，it is obviously different from the traditional definition of meimechite． Therefore，the authors refer to the newly discovered ultramafic lavas in the study area as“meimechitic rocks”． Using the highest Fo value of olivine and the TFeO content

of its host rock，the MgO content in the parent magma of the meimechitic rocks is estimated to be ≥18. 18%．Based on this evidence，the mantle melting temperature and pressure are limited to be higher than 1 580℃ and 3. 9

GPa，respectively． The meimechitic rocks in the Chalukou area show OIB-like trace element characteristics． This，along with the results of previous studies，suggests that the meimechitic rocks in the area were formed in the Late

Permian to Late Triassic extensional setting related to the mantle plume．