Geochemical fingerprints and pebbles zircon geochronology: Implications for the provenance and tectonic setting of Lower Cretaceous sediments in the Zhucheng Basin (Jiaodong peninsula, North China)

Abstract

This paper conducts a petrogeochemical analysis of the Lower Cretaceous Laiyang Group’s sandstones, compares the results with the Neoproterozoic and Mesozoic intrusive rocks in the southern Sulu Orogen (also called the Jiaonan Orogen), and performs an LA-ICP-MS zircon geochronology analysis of the granitic gneisses in the conglomerates of the Laiyang Group and the intrusive rocks in the Jiaonan Orogen. The results show that the major element proportions of the Longwangzhuang Formation (LWZ Fm) and Qugezhuang Formation (QGZ Fm) of the Laiyang Group in the Zhucheng Basin are similar. The values of various indices for the LWZ Fm are similar to the average sandstone content of active continental margins, whereas, the values for the QGZ Fm are similar to those of continental island arcs. The comparison shows that the REE characteristics of the LWZ Fm and QGZ Fm of Laiyang Group are similar to those of the Neoproterozoic granitic gneisses in the Jiaonan Orogen but obviously different from those of the Early Cretaceous intrusive rocks. A tectonic setting discrimination diagram reveals that the provenance of the Laiyang Group includes features of active continental margins and continental island arcs. A number of indicators, e.g., the sandstone type, the Chemical Index of Alteration, the Chemical Index of Weathering, the Plagioclase Index of Alteration and the Index of Chemical Constituent Variation, indicated that the sandstones did not undergo intense weathering and were deposited near the source area. The zircon ages of the granitic gneiss material in the conglomerates at the base of the Laiyang Group are 790 ± 8.4 Ma, close to the ages of the Neoproterozoic granitic gneiss in the Jiaonan Orogen (739–819 Ma), and very different from the ages of the Early Cretaceous intrusive rocks. Combining with paleocurrent directions, geochemical character, the Neoproterozoic granitic gneisses in the Jiaonan Orogen may represent the primary provenance of the Laiyang Group in the Zhucheng Basin. During the depositional period of the Laiyang Group, the source rocks did not experience intense weathering, which indicated the Jiaonan Orogen experienced rapid uplift during this time, and coincided with the high exhumed rate of 2.0 km Ma −1 from before ca. 128 to 123 Ma in Jiaonan Orogen. The rapid subsidence during the formation of the Laiyang Group in the Zhucheng Basin and the rapid uplift of the Jiaonan Orogen are the result of a single regional extensional event associated with the lithospheric thinning and destruction of North China and peripheral cratons.