鲁西隆起侏罗系碎屑主物源来自华北北缘:锆石U-Pb和Hf同位素年代学证据

鲁西隆起保存并出露比较完整的晚中生代沉积记录,是研究华北东部晚中生代构造演化的重要窗口。本文采用碎屑锆石LA-MC-ICP-MS测年方法分析鲁西隆起北缘淄川地区坊子组和三台组中的两个砂岩样品,以此探讨华北东部侏罗纪的沉积物源并约束构造古地理格架。两个砂岩样品具有近似的U-Pb年龄谱。古元古代和新太古代两组U-Pb年龄及其Hf同位素组成与广泛出露于北部的华北克拉通基底一致,根据缺乏新元古代中期(850～700 Ma)岩浆年龄和三叠纪变质年龄的事实,以此可以排除扬子板块以及现今临近于鲁西隆起东南部的苏鲁造山带物源。坊子组和三台组碎屑锆石显生宙年龄(393～256 Ma)记录的晚古生代岩浆活动均未发现于鲁西隆起及其邻近地区,而可以与大量出露于华北北缘及其北部的兴-蒙造山带的岩浆活动进行对比。此外,相当部分的显生宙碎屑锆石(183 Ma、462 Ma和324～154 Ma)具有正的ε_Hf(t)值(0.9～12.7)同位素特征,也与兴-蒙造山带特征相似,且三台组砂岩中显生宙碎屑锆石及其中ε_Hf(t)值为正值的比例较坊子组均增多。研究认为,华北北缘及其北部的兴-蒙造山带在侏罗纪时从早到晚不断抬升、剥蚀,形成相对华北克拉通内部的高地势特点,由此大量的剥蚀产物向南输运而成为鲁西隆起侏罗系的主要碎屑物源。

Major provenance of Jurassic sediments in Luxi uplift,eastern North China,derived from the northern North China Block: Evidences from detrital zircon U-Pb and Hf isotopic geochronology

Relatively successive sequences of Late Mesozoic are preserved and exposed in Luxi uplift,eastern North China Block(NCB),which is an important region to study the Late Mesozoic tectonic evolution of the eastern NCB.In this study,detrital zircon U-Pb-Hf isotopic compositions of two Jurassic arkosic sandstones,respectively sampled from the Fangzi and Santai Formations,are measured through LA-MC-ICP-MS method,with aim to trace the Jurassic sediment provenances and tectonic-paleogeographic configuration.The two sample have very similar U-Pb age spectrums which can be divided into four major groups:Phanerozoic(Ⅰ),Late Mesoproterozoic to Early Neoproterozoic(Ⅱ),Paleoproterozoic(Ⅲ)and Neoarchean(Ⅳ).Detrital zircons of Group Ⅲ and Group Ⅳ broadly match the age spectra of the basement of NCB which exposed widely in the northern part.No Middle Neoproterozoic(850～700 Ma)magmatic zircon regarded as the provenance diagnostic for the South China Block(SCB)and Triassic metamorphic zircon are found in this study,ruling out the possibility of clastic provenance transportation from the Sulu orogen(belongs to SCB),adjacent to the Luxi area at present,to the Luxi area.The detrital zircons age of Phanerozoic age(393～256 Ma)from the two samples recording the Late Paleozoic magmatic activities which are not found both in Luxi and its peripheral areas,but can be well compared with that of the north NCB and the northern Xing-Meng orogen belt(XMOB).Furthermore,considerable proportion of Phanerozoic zircons(183 Ma, 462 Ma and 324～154 Ma)from the two samples with positive ε_Hf(t)values(+0.9～+12.7)have similar Hf isotopes characteristic to the XMOB as well.In addition,the proportions of Phanerozoic zircons and the Phanerozoic zircons with positive ε_Hf(t)value in the sandstone of Fangzi Formation are both higher than that of Santai Formation.This research proposes that an evident mountain or provenance region once increasingly developed along northern NCB during Early to Late Jurassic,easily providing mass clastic materials southward and becoming the major provenance of the Jurassic sediments in Luxi.