Types and genesis of the Neoproterozoic glauconites, Longshan area, Changping district, Beijing

Abstract

Multiple sets of glauconites occur in the Neoproterozoic outcrops of the Longshan area in Beijing. This study examines the glauconite-bearing clastic rocks of the Qingbaikou System, whose mode of occurrence and genesis remain unclear. The glauconites occur in granular, colloidal, detrital pseudomorphic, pigment-infested and rimmed forms. Considering their varied color, morphology and chemical composition, these glauconites are classified into five types. Type I are granular glauconites with high K2O and low TFeO content. Type II are yellow-green granular glauconites with low K2O content. Type III are yellow-brown granular glauconites with strong oxidation. Type IV and V are not considered to be glauconites, in principle. Type IV is a yellow colloidal form whose chemical formula is difficult to determine, while Type V is dark brown detrital glauconite whose chemical composition was altered to form iron oxides. The formation of glauconite is controlled by material sources and redox conditions. Here, the iron components, glauconite and intermediate-basic volcanic rock in the underlying Xiamaling Formation and the felsic conglomerate & coarse feldspar sandstones in the lower Changlongshan Formation are the source for the glauconites in the Changlongshan Formation. The covariant relationship between oxides and maturity levels reveals that Type I granular glauconites have the origin of pseudomorphic replacement of feldspar or quartz. Type II glauconites have a similar trend with the origin of layer lattice theory, and Type III granular glauconites were formed either by early pseudomorphic replacement or layer lattice genesis and later, by alteration. Type IV colloidal glauconites formed under the condition of authigenic cementation in the early stage, most of which were oxidized and altered in the later stage. Type V detrital granular glauconites formed by later re-transportation and allochthonous re-deposition of autochthonous authigenic glauconite or mica pseudomorphic glauconite by the origin of layered lattices. In North China, the presence of Neoproterozoic glauconite makes it difficult to identify the precise systems tract and differentiate the “condensed segments” due to its wide distribution. Different glauconites origins underwent various mechanical and chemical processes, or stages of diagenesis, at various times.