广西大瑶山地区下泥盆统莲花山组碎屑岩地球化学特征及其对物源区和构造环境的制约

Geochemistry of Clastic Rocks from Lower Devonian Lianhuashan Formation in Dayaoshan Area of Guangxi,China:Implications for Provenance and Tectonic Setting

受加里东期郁南运动和广西运动影响,华南板块西南部大瑶山地区下泥盆统莲花山组不整合于寒武系之上。对下泥盆统莲花山组砂岩开展岩石学和地球化学研究,探讨其物质来源和构造环境。砂岩碎屑组分以石英和长英质沉积岩岩屑为主,分选和磨圆中等,指示近源沉积。SiO_(2)含量(质量分数,下同)(74.19%~95.35%,平均值为85.72%)较高,Al_(2)O_(3)含量(1.23%~14.73%,平均值为6.48%)和TiO_(2)含量(0.09%~1.05%,平均值为0.38%)较低,Al_(2)O_(3)/SiO_(2)值(0.01~0.20)极低。砂岩亏损大离子亲石元素,富集高场强元素,稀土元素总含量为(12.48~281.10)×10^(-6),轻、重稀土元素分异明显((La/Yb)N值为3.70~29.40),呈现弱Eu负异常(0.66~1.14,平均值为0.88)。SiO_(2)/Al_(2)O_(3)值(5.04~77.28,平均值为22.30)较高,表明成分成熟度高。成分变异指数(0.61~2.13,平均值为1.21)变化较大,指示源岩包含再循环的古老沉积物与第一次循环物质;化学蚀变指数为82.13~91.68,反映了强烈化学风化作用。综合地球化学、碎屑锆石U-Pb年代学、岩相古地理和大地构造演化研究成果认为,大瑶山地区下泥盆统莲花山组沉积于被动大陆边缘环境,物源为北侧大瑶山—大明山隆起带和南侧云开地区的隆升剥蚀物质,源岩以寒武系黄洞口组为主,包含少量云开地区新元古代沉积岩和广西运动形成的花岗岩类(470~435 Ma)。

The Lower Devonian Lianhuashan Formation is unconformly unlained by Cambrian strata due to Early Paleozoic Yunan orogeny and Kwangsian orogeny in Dayaoshan area of Guangxi.The new petrology and whole-rock geochemical data of Lower Devonian Lianhuashan Formation in Dayaoshan area were presented in order to constrain their provenance and sedimentary tectonic setting.The sandstones are composed of the predominated moderately sorted and rounded quartz and sedimentary lithic fragments,indicating a close detrital provenance.These samples have obviously high SiO_(2) contents(74.19%-95.35%with an average of 85.72%),low Al_(2)O_(3) contents(1.23%-14.73%with an average of 6.48%),TiO_(2) contents(0.09%-1.05%with an average of 0.38%)and Al_(2)O_(3)/SiO_(2)(0.01-0.20).The sandstones have low REE contents((12.48-281.10)×10^(-6))and are characterized by depletion in LILE,enrichments in HFSE and LREE((La/Yb)_(N) is 3.70-29.40)coupled with slightly negnative Eu anomaly(0.66-1.14 with an average of 0.88).The high SiO_(2)/Al_(2)O_(3)(5.04-77.28 with an average of 22.30)implies the high grade of maturity.The wide ranges of ICV(0.61-2.13 with an average of 1.21)and high CIA(82.13-91.68)indicate that the source rocks contain recycled ancient sediments and first-cycle materials,which have experienced high degree of chemical weathering and alteration in the heat and humid climate environment of tropical or subtropical zone.Integrated with geochemistry,clastic zircon U-Pb geochronology,paleogeography and tectonic evolution,it is suggested that Lower Devonian Lianhuashan Formation is deposited in passive margin,and its provenance may be Dayaoshan-Damingshan area in the north,and Yunkai area in the south.The source rocks of Lower Devonian Lianhuashan Formation mainly comprise Cambrian Huangdongkou Formation,and minor Neoproterozoic sedimentary rocks of Cathaysia block and the granitoids formed in Kwangsian orogeny(470-435 Ma).