山东胶莱盆地下白垩统莱阳组物质组分特征及其对源区的制约

Characteristics of material components from the Lower Cretaceous Laiyang Formation in Jiaolai basin,Shangdong province,eastern China and constraints to the provenance.

位于中国东部苏鲁造山带北侧的胶莱盆地发育早白垩世莱阳组,厚约2000m,为一套陆相碎屑沉积,包括砾岩、砂岩和页岩,以及少量泥灰岩、白云岩和凝灰岩。它可以分为4个岩性段,每个段大致形成一个向上变细的旋回。砾岩砾石成分主要为花岗片麻岩、黑云斜长片麻岩、混合花岗岩和大理岩,其次是黑云片岩、灰岩和沉积碎屑岩、火山岩和花岗岩等。莱阳组砂岩主要由岩屑长石砂岩以及岩屑砂岩组成,具有低的成分成熟度,Q/(F+L)平均为0.42(0.19～0.84)。石英主要由单晶石英组成,长石包括斜长石、微斜长石等,岩屑主要为火山岩和变质岩,其次为沉积岩。6个砂岩地球化学样品分析表明,砂岩的SiO_2、Al_2 O_3、TiO_2、Fe_(2)O_3~T、MnO、MgO、CaO、Na_2 O、K_2O的平均含量分别为70.53%、13.10%、0.41%、3.09%、0.06%、1.23%、1.69%、3.38%和3.38%,类似于晚元古代造山带砂岩成分。莱阳组砂岩稀土元素分析表明,La含量为38.4×10'(28.0×10^(-6)～50.5×10^(-6)),Ce含量为69.3×10^(-6)(58.1×10^(-6)～81.3×10^(-6)),稀土总量为134×10^(-6)～197×10^(-6),轻稀土明显富集,La//Yb为20.18(11.91～25.98),(La/Yb)_N为13.60(8.03～17.52),有中等到弱的Eu负异常(δEu=0.58～0.88),Ce为弱的负异常(δCe=0.78～0.97)。Ce含量、稀土总量和Eu异常揭示源区岩石形成时的构造背景主要为大陆岛弧。砾石统计揭示,莱阳组物源区的剥蚀序列按时间顺序应该是花岗片麻岩、黑云斜长片麻岩→大理岩→脉石英、灰岩和碎屑岩→正长岩。砂岩点统计、F1-F2判别图、La-Th-Sc和Th-Sc-Zr/10判别图揭示,莱阳组砂岩主要源区属于大陆弧的长英质火成岩区,少许来自再循环造山带。砂岩Zr/Y比值揭示莱阳组砂岩与分布在山东荣成地区具有大陆弧构造背景的片麻岩具有亲缘性。这表明山东胶莱盆地莱阳组砂岩源区主要来自苏鲁造山带,其次有少许来自扬子陆块。造山带周缘盆地的陆源沉积物中,砂岩的碎屑组分以及地球化学特征具有源区构造背景的继承性,反映的是源岩形成的构造环境。

The Lower Cretaceous Laiyang Formation, about 2000m thick, develops in the Jiaolai Basin located in the north side of the Sulu orogen, eastern China. It consisted of terrigenous sediments such as conglomerates, sandstones and shales, and a little marl, dolomite and tuff. The Formation can be divided into four members, and each member composes an upward-fining cycle. According to the counting of eight gravel points of the Formation, the gravel compositions of the conglomerates comprise most granitic gneiss, migmatitic granite and marble, biotite-plagioclase gneiss, biotite schist, limestone and detrital rocks, volcanic rocks and vein quartzes etc. Based on the point counting analyses of 10 thin sections of sandstone samples, the Laiyang Formation consists of most lithic arkose and little litharenite, which have low compositional maturity （average 0. 42 （0. 19 - 0. 84） of Q/（ F ＋ L） ratio）. In the sandstones, the monocrystalline quartzes are most, the feldspars are mainly plagioclase and microcline, and the lithics consist mostly of volcanic and metamorphic rocks and little sedimentary rocks. The geochemistry analysis of 6 sandstone samples shows that average concentrations of SiO2 , Al2O3 , TiO2 , total Fe, MnO, MgO, CaO, Na2O, K2O are 70. 53%, 13.10% , 0.41% , 3.09% ,0.06% , 1.23%, 1.69%, 3.38% and 3.38% respectively, and are similar to the compositions of the sandstone from Late Proterozoic orogen. The Laiyang Formation sandstones have REE gross of 134. 6 × 10^-6 - 197.0 × 10^-6, La concentration of average 38. 4× 10^-6（28.0× 10^-6 -50. 5 × 10^-6）, and Ce concentration of average 69. 3 × 10^-6（58. 1× 10^-6 - 81.3× 10^-6 ）. In the sandstones, LREE are relatively rich, La/Yb ratio is 20. 18 （11.91 -25.98）, （La/Yb） Nis 13.60 （8.03 - 17. 52） ,SEu is 0. 58 -0. 88, and δCe is 0.78 - 0.97. It shows markedly negative Eu and slight negative Ce anomaly. The Ce contents, the total rare elements and the Eu anomaly imply that the source rocks formed under a continental island-arc tectonic setting. According to the gravel compositions, a denudation sequence of provenance for the Laiyang Formation has been set, in ascending order, as granitic gneiss, migmatitie granite, biotiteplagioclase gneiss, biotite sehist→marble→vein quartz, quartzite, limestone, detrital rocks→orthosite. Based on the point count data of the sandstones, the F1-F2 discrimination diagram, and La-Th-Sc and Th-Sc-Zr/10 discrimination diagrams, the source rocks of the Laiyang Formation mostly belong to magmatic arc provenance, and little do to recycled orogen provenance. The Zr/Y ratios of the sandstones indicate that they are related with gneisses forming under a continental island are tectonic setting, which develop in Rongeheng area belonging to the Sulu orogen. These indicate that most source rocks of the Laiyang Formation are gneisses which came from the Sulu orogen which went through a tectonic evolution stage of continental arc, and a little source rocks are derived from Yangtze block. This result brings to light the tectonic setting inheritance of the detrital compositions and geochemical characteristics of detrital rocks formed in peripheral basins of an orogen belt, that is, they do not reflect tectonic setting of their source rocks, the gneisses, but protoliths of metamorphic rocks.