东太平洋洋隆Siqueiros转换断层带的地球化学特征

东太平洋洋隆Siqueiros转换断层带(西经103°~104°、北纬8°20′~8°30′),位于中美洲之西可可斯板块与太平洋板块交接处(图1).该转换断层的玄武岩,成分从原始的到富化的,变化很大.最原始的玄武岩产于A-B走滑断层内,其MgO含量高达14.91%;最富化的玄武岩产在西部洋脊与转换断层交叉部位,其MgO含量较低,成分变化大.转换断层内的洋中脊玄武岩可分三类:富化的洋脊玄武岩(E-MORB)、原始的正常洋脊玄武岩(Primitive N-MORB)和正常洋脊玄武岩(N-MORB).Siqueiros转换断层区已确定有两个地幔源:一个位于转换断层西部下面的富集地幔,其特点是稀土总量高,富轻稀土,低MgO、K2O和TiO2,而高87Sr/86Sr;另一个是位于转换断层中-东部下面的亏损地幔,来自这个地幔源的玄武岩具有低87Sr/86Sr、低稀土元素和微量元素含量,但MgO含量较高.

Geochemical Characteristics of Siqueiros Transform,East Pacific Rise

Basalts recovered from the Siqueiros transform exhibit a wide compositional range from primitive to evolved compositions.The most primitive samples collected from strike-slip fault A-B,have the highest MgO contents up to 14.91 wt%.The most evolved samples with lower MgO content were collected from the western ridge-transform intersection.Three classes of MORBs are distinguished in the transform domain: E-MORBs,Primitive N-MORBs and N-MORBs.Two mantle sources are identified in the Siqueiros region.One is the enriched mantle with highest REEs,enriched LREEs,low MgO,K2O and TiO2 and high 87Sr/86Sr isotopic concentrations,which lies underneath the western part of the transform domain.The second source is the depleted mantle,which lies underneath the mid-eastern transform domain.Basalts from this source have low 87Sr/86Sr isotopic compositions,lower REEs and trace elements and higher contents.