拉脊山混杂带东段峡门早奥陶世埃达克岩的确认及地质意义

拉脊山混杂带东段峡门地区侵入岩可进一步解体为英云闪长岩和花岗闪长岩。通过LA ICP MS锆石U Pb同位素测年,获得英云闪长岩锆石U Pb年龄为(480.7±1.9)Ma(MSWD=0.80),代表岩体侵位时代为早奥陶世。主量元素特征表明岩石具高硅(SiO 2=59.10%~63.45%)、高铝(Al 2 O 3=15.40%~16.19%)、富钠贫钾(Na 2 O/K 2 O=1.41~2.03)的特征,属于钙碱性高钾钙碱性系列岩石。稀土元素球粒陨石标准化配分模式图上呈轻稀土富集(ΣLREE=148.22×10^-6~173.46×10^-6)、重稀土亏损(ΣHREE=10.04×10^-6~14.40×10^-6)的右倾型曲线,显示轻微Eu负异常(δEu=0.92~0.96)。岩石明显富集Rb、Ba、Th等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,同时形成Ta、P、Ti均亏损的“TNT”谷,显示出弧花岗岩特征。岩石具高Sr(591.9×10^-6~922.9×10^-6)、低Y(12.44×10^-6~18.79×10^-6)和低Yb(1.24×10^-6~1.90×10^-6)特点,具有较高的Sr/Y(34.85~55.76)值,与典型的埃达克岩地球化学特征一致。结合区域资料综合研究认为,峡门早奥陶世埃达克岩源于俯冲板片流体交代地幔楔诱发下地壳富含角闪岩相的基性玄武质岩石部分熔融,形成于俯冲、消减作用下的活动大陆边缘火山弧环境,说明该时期拉脊山洋盆在峡门地区已处于板块碰撞前期阶段。

Confirmation and Geological Significance of Early Ordovician Adakite in Embouchure Area of the Eastern Segment of the Lajishan Mixed Belt

The intrusive rocks in embouchure area of the eastern segment of the Lajishan mixed belt can be further disintegrated into Yingyun diorite and granodiorite.The zircon U-Pb isotopic dating is carried out by LA-ICP-MS.The zircon U-Pb age of Yingyun diorite is(480.7±1.9)Ma(MSWD=0.80),which indicates that the emplacement age of rock mass is early Ordovician.The characteristics of principal elements show that the rocks have high silicon(SiO 2=59.10%—63.45%),high aluminum(Al 2 O 3=15.40%—16.19%),rich sodium and little potassium(Na 2 O/K 2 O=1.41—2.03).The rocks belong to calc-alkaline-high-potassium calc-alkaline series rocks.The standardized partition pattern diagram of rare earth element meteorite shows the right dip curve of light rare earth enrichment(ΣLREE=148.22×10^-6—173.46×10^-6)and heavy rare earth loss(ΣHREE=10.04×10^-6—14.40×10^-6),with the negative anomaly of slight Eu(δEu=0.92—0.96).The rocks are obviously enriched in large ion lithophile elements such as Rb,Ba,Th and deficient in high field strength elements such as Nb,Ta,Ti,P.At the same time,the‘TNT valley with loss of Ta,P,Ti is formed,which shows the characteristics of arc granite.The rock has the characteristics of high Sr(591.9×10^-6—922.9×10^-6),low Y(12.44×10^-6—18.79×10^-6)and low Yb(1.24×10^-6—1.90×10^-6),and a high ratio of Sr/Y(34.85×55.76).The geochemical characteristics are consistent with those of the typical adakite.Combined with the comprehensive study of the regional data,it is considered that the early Ordovician Adakite in embouchure area was originated from the partial melting of basement basaltic rocks which was induced by subduction plate fluid metasomatic mantle wedges and rich in hornblende facies,and was formed under the action of subduction and consumption of the volcanic arc environment of the active continental margin.This indicates that the oceanic basin of Laji Mountain was already in the early stage of plate collision in embouchure area during this period.