福建早侏罗世火山岩岩石地球化学特征及岩石成因研究

The petrogeochemical characteristics and petrogenesis of early Mesozoic volcanic rocks,Fujian Province

选择较有代表性的永定、平和、长泰—同安、闽清—永泰等地早侏罗世火山岩岩石主量、微量和Sr、Nd同位素分析测试成果,开展岩石地球化学特征对比研究,探讨岩浆起源、演化和不同类别的岩石成因。研究认为,早侏罗世火山岩主要属亚碱性钾质-普通系列,玄武岩属亚碱性高铁拉斑玄武岩系列。玄武岩稀土含量较高,配分曲线与典型OIB玄武岩变化趋势一致;高场强元素(HFSE)除Nb略亏损外,Ti、Zr、Nd具有明显正异常,Ta具有弱正异常;原始地幔标准化曲线位于典型大陆弧玄武岩之上。流纹岩稀土含量高,轻稀土富集,Eu负异常明显,高场强元素Nb、Zr等具有清晰正异常,Ti、P负异常明显。岩石地球化学特征研究表明早侏罗世火山岩岩石成因与地幔岩浆作用有关:基性单元——玄武质岩石岩浆主要来自于软流圈,亏损的地幔源区可能有早期富集岩石圈物质加入或是由于软流圈地幔上涌萃取了岩石圈地幔富集组分。酸性单元——流纹质岩石主要形成于上地壳,但不排除有幔源物质的混合。安山岩-英安岩是底侵的镁铁质岩浆与下地壳部分熔融岩浆混合均一化形成典型的MASH岩浆。早中生代处于印支运动后造山大陆裂解的地球动力学背景,后造山岩石圈伸展拉张致使软流圈减压上涌和部分熔融,所产生的岩浆沿北东向展布的裂解区域喷出地表形成火山岩带。

According to the test results of major elements, trace elements and Sr, Nd isotopes in volcanic rocks which were chosen from Fuiian typical regions like Yongding, Pinghe, Chang- tai-Tong＇an, and Minqing-- Yongtai area, the petrogeochemical characteristics of early Juras- sic volcanic rocks have been comparative researched the origin and evolution of magma and the petrogenesis of different rocks have been discussed. Based on the research, early Jurssic volcanic rocks mainly belong to subalkaline potassic-ordinary series and basaltic rocks belong to subalkaline ferrian tholeiite system. The basalt contains relative high rare-earth. The curve of partition map slightly changes around the OIB basalt curve and the general change trend of two curves are totally the same. The HFSE show that.. Nb is slight negative anoma- ly Ti, Zr and Nd are obvious normal anomaly and Ta is weak normal anomaly. The LILE show that： Th is obvious normal anomaly, while Sr is negative anomaly~ the standard curve of primitive mantle is above typical continental basalt. The rhyolite contains high REE, es- pecially LREE enrichment. Europium has evident negative anomaly. The HFSE show that： Nb and Zr has clear normal anomaly, while Ti and P has evident negative anomaly. The IdLEs show that： Rb and Th have normal anomaly, while Ba and Sr have negative anomaly. Petrogeochemical charact：ristics show that the genesis of early Jurassic volcanic rocks was related to the mantle magmatism. Basic unit： basaltic rocks have the characteristics of high Zr/Ba, Zr/Y and Ti/Y, low La/Nb, La/Ta, Ba/Nb, K/Ti, Rb/Zr, and similar chondrite Nd isotope, which indicate magma was mainly from rheosphere and depleted mantle source was possible added by early enriched lithospheric material or the upwelling of asthenospheric mantle extracted the enrichment component of lithospheric mantle. Acid unit： rhyolitic rocks were mainly from supracrust, but also possible from the mixture of mantle substances. Transitional unit： andesite and dacite were typical MASH magma which homogenized be- tween mafic magma and lower crust. Early Mesozoic was under the geodynamics background of orogenic continental break-up after Indosinian movement and the extension of postorogenic lithosphere resulted in reducing pressure, upwelling and partly melting of asthenosphere, which the magma erupted along the NE-trend cracking areas and finally volcanic belts were built.