The Lower Cretaceous Laiyang Formation, about 2000m thick, develops ill 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 oil 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 similar to 0. 84) of Q/(F + L) ratio). In the sandstones, the monocrystalline quartzes are most, the feldspars arc 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 X 10(-6) similar to 197. 0 x 10(-6), La concentration of average 38. 4 X 10(-6) (28.0 x 10(-6) similar to 50.5 X 10(-6)), and Ce concentration of average 69. 3 X 10(-6) (58. 1 x 10(-6) similar to 81. 3 x 10(-6)). In the sandstones, LREE are relatively rich, La/Yb ratio is 20.18 (11. 91 similar to 25. 98), (La/Yb)(N) is 13. 60 (8.03 similar to 17.52), delta Eu is 0. 58 similar to 0.88, and delta Ce is 0. 78 similar to 0.97. It shows markedly negative Eu and slight negative Cc anomaly. The Cc contents, the total rare elements and the Eu anomaly imply that the source rocks formed under a continental island-are 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, migmatitic granite, biotiteplagioclase gneiss, biotite schist-marble-vein quartz, quartzite, limestone, detrital rocks -> orthosite. Based oil 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 are 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 arc tectonic setting, which develop in Rongcheng 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 all orogen belt, that is, they do not reflect tectonic setting of their source rocks, the gneisses, but protoliths of metamorphic rocks.
