川西南美姑地区下三叠统飞仙关组地球化学特征及其对物源和构造环境的指示意义

针对川西南美姑地区飞仙关组碎屑岩进行岩石学和地球化学方面的研究,以探究其物质来源以及构造背景。川西南美姑地区飞仙关组为一套碎屑岩,三角洲相沉积。碎屑岩主量元素中w(TFe2O3)为3.46%~11.23%,w(MgO)为2%~7.98%,w(TiO2)为1.51%~2.62%。微量元素中亲铁镁矿物元素w(Cr)为66×10^-6~426×10^-6,w(Co)为18.8×10^-6~30.6×10^-6,w(Sc)为16.9×10^-6~23.7×10^-6。REE分布模式和Al2O3/TiO2比值与峨眉山高钛玄武岩较为相似,指示着飞仙关组的物质来源于晚二叠世的峨眉山高钛玄武岩。样品的成分异变指数(ICV)值为0.95~3.49,普遍大于1,暗示着飞仙关组的物质来源大部分未经历过风化-沉积成岩的过程,为第一次循环碎屑。飞仙关组碎屑岩CIA值为60.49~72.44,反映了飞仙关组时期该地区经历了温暖、湿润环境下中等程度的化学风化作用。同时碎屑岩中的TFe2O3+MgO,TiO2,Al2O3/SiO2地球化学指标最接近大洋岛弧特征值,Th-Co-Zr/10,La-Th-Sc,Th-Sc-Zr/10图解亦指示着飞仙关组的源岩形成于大陆岛弧-大洋岛弧的构造环境中。结合飞仙关组的岩石学和地球化学特征,初步认为晚二叠世时期在大洋岛弧的构造背景下峨眉山玄武岩喷发溢流而出,而后隆升,为飞仙关组的沉积提供了充足的物质来源。同时,随着飞仙关组的不断沉积,亦逐渐有其他物源混入。

GEOCHEMICAL CHARACTERISTICS OF THE LOWER TRIASSIC FEIXIANGUAN FORMATION IN MEIGU AREA,SOUTHWESTERN SICHUAN AND ITS SIGNIFICANCE FOR THE PROVENANCE AND TECTONIC SETTING

Characteristics of petrology and geochemistry of the Triassic Feixianguan Formation clastic rocks in Meigu County of southwestern Sichuan are studied.It shows that the Feixianguan Formation clastic rocks were deposited in a deltaic environment.Geochemical analysis of main elements reveals that the w(TFe2O3)ranges from 3.46%to 11.23%,w(MgO)from 2%to 7.98% and w(TiO2)from 1.51%to 2.62%.Trace element analysis shows that w(Cr)ranges from 66×10^-6 to 426×10^-6,w(Co)from 18.8×10^-6 to 30.6×10-6,and w(Sc)from 16.9×10^-6 to 23.7×10^-6.Its REE pattern and Al2O3/TiO2ratio of the clastic rocks are similar to the high-titanium Emeishan Basalt,therefore,the material of Feixianguan Formation came from the high-titanium Emeishan Basalt.The fact that the ICV of the samples from Feixianguan Formation is in the range of 0.95~3.49(most of them >1)indicates that the material of Feixianguan Formation did not go through the process of weathering-sedimentary diagenesis and the deposition materials belonged to the first cyclic clastic.CIA value of the sample from Feixianguan Formation ranges from 60.49to 72.44,reflecting that the Feixianguan Formation experienced medium chemical weathering in a warm and humid environment.The values of TFe2O3+MgO,TiO2,Al2O3/SiO2obtained from the Feixianguan Formation are similar to the characteristic values of oceanic island arc,and the plots of Th-Co-Zr/10,La-Th-Sc,Th-Sc-Zr/10indicate that the source rock of the Feixianguan Formation formed in the environment of continental island arc or oceanic island arc.According to the characters of petrology and geochemistry of the Feixianguan Formation,it is considered that in Late Permian at the oceanic island arc environment,the Emeishan Basalt erupted,overflowed and subsequently uplifted,providing enough material for deposition of Feixianguan Formation.Meanwhile, other material sources were mixed in the deposition of Feixianguan Formation.