Formation of shallow-water glaucony in weakly oxygenated Precambrian ocean: An example from the Mesoproterozoic Tieling Formation in North China Dongjie Tang a,b,⇑ , Xiaoying Shi a,c,⇑ , Jianbai Ma c , Ganqing Jiang d , Xiqiang Zhou e , Qing Shi a,b a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Beijing), Beijing 100083, China b Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing 100083, China c School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China d Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA e Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info Article history: Received 20 December 2016 Revised 18 March 2017 Accepted 21 March 2017 Available online 22 March 2017 Keywords: Glaucony Seawater redox conditions Mesoproterozoic Tieling Formation North China platform abstract Authigenic glaucony precipitation in Phanerozoic oceans takes place mostly in middle shelf to upper slope environments with low depositional rate or sediment starvation. Precambrian glaucony, however, is more common in stratigraphic successions deposited from shallower-water environments with high and variable depositional rates. This phenomenon has long been noticed in literature, but the controlling factors of shallow-water glaucony precipitation in Precambrian oceans have not been adequately inves- tigated. To better understand the glauconitization processes in Precambrian oceans, we have conducted an integrated study of the glaucony in stromatolitic carbonates of the Mesoproterozoic Tieling Formation (ca. 1437 Ma) in North China, using sedimentological, mineralogical, and geochemical data obtained from field observations, petrography, XRD, SEM, quantitative EDS and ICP-MS analyses. Macro- and micro- scopic observations show that the Tieling glaucony fills voids of varying sizes and shapes, and records dif- ferent maturation stages of glauconitization. Geochemical analyses show that the Tieling glaucony has high K 2 O (avg. > 8%) but low and variable total Fe 2 O 3 (TFe 2 O 3 ) contents (1.92–13.65 wt%). The TFe 2 O 3 contents increase with maturation of glaucony. Titration results show that the Tieling glaucony contains both Fe 3+ and Fe 2+ ions, but has Fe 2+ /Fe 3+ ratios much higher than that of the Phanerozoic glaucony. REE results of glaucony-hosting carbonates show weak negative and positive Ce anomalies with average Ce/ Ce / ratio close to 1.0, suggesting carbonate precipitation near the redoxcline of Fe-Mn oxides. All these features suggest that the Tieling glaucony was precipitated in seawater around the Fe-redoxcline, where both Fe 2+ and Fe 3+ were available throughout the glaucony maturation stages. High Fe 2+ /Fe 3+ ratios in the depositional environments led to Fe 2+ occupation at octahedral sites of glaucony and negative charges on octahedrons, which resulted in high K content (to balance the negative charges on octahedrons) and low TFe 2 O 3 (limited by dioctahedral structure). The formation of the Tieling glaucony and other similar Precambrian glauconies is likely controlled by low oxygen concentration in seawater and a shallow redoxcline that controls the availability of Fe and K cations during initial precipitation and maturation of glaucony. The shift of authigenic glaucony precipitation from shallow water in the Precambrian to deep water in the Phanerozoic may record the deepening of ocean chemocline in response to increased ocean oxygenation. Ó 2017 Elsevier B.V. All rights reserved. 1. Introduction Glauconite [(K,Na)(Fe,Al,Mg) 2 (Si,Al) 4 O 10 (OH) 2 ] refers to a phyl- losilicate mineral of dioctahedral mica group with 2:1 + interlayer ion structures, while the term glaucony is used to represent a ser- ies of green clay minerals with a wide range of chemical/miner- alogical compositions including glauconitic mica, glauconitic smectite, and ferric illite (Banerjee et al., 2015, 2016; Odin and Létolle, 1980). Glaucony typically occurs as 60–1000 lm green clay aggregates formed through marine authigenesis in sedimentary rocks (e.g., Banerjee et al., 2015, 2016). Although detrital and reworked glaucony is sporadically present in both modern and http://dx.doi.org/10.1016/j.precamres.2017.03.026 0301-9268/Ó 2017 Elsevier B.V. All rights reserved. ⇑ Corresponding authors at: State Key Laboratory of Biogeology and Environ- mental Geology, China University of Geosciences (Beijing), Beijing 100083, China. E-mail addresses: dongjtang@126.com (D. Tang), shixyb@cugb.edu.cn (X. Shi). Precambrian Research 294 (2017) 214–229 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres