关键词: 海绿石, 海水氧化还原状态, 中元古代, 铁岭组, 华北地台

Abstract:

The origin of glaucony in the Phanerozoic sediments has been fairly understood, which generally results from marine authigenesis with slow sedimentary rate, especially in outer shelf environments. In contrast, the origin of glaucony in the Precambrian sediments is still poorly constrained, particularly that why it was commonly deposited in much shallower environments with relatively high sedimentary rate is not fully understood. To address this issue, we carried out an integrated study, including field, petrography, XRD, SEM, EDS, ICPMS investigations on the glaucony and on its host carbonate in stromatolitic reef of the Mesoproterozoic Tieling Formation. Three types of autochthonous glaucony were recognized based on their distribution and substrate porosity, including type 1 (in the most porous belt between stromatolitic column and micritic matrix), type 2 (in less porous matrix) and type 3 (in dense stromatolitic column) glaucony. They were all characterized by constant and high K2O contents (av.>8%), indicating initial high K+ concentration in pore water, and an evolution from type 3 to type 1 glaucony by addition of total Fe2O3 and simultaneous release of Al2O3. These features likely suggest that substrate permeability has controlled the glaucony evolution, and glauconization requires persistent supplement of Fe. Since the stromatolitic carbonates are basically devoid of detrital inputs, Fe may have been largely sourced from seawater. In this case extremely shallow Feredox chemocline was required, which is also supported independently by Ce anomalies in the carbonates. Thus, we proposed that porous substrate, high K+ concentration and suboxic conditions in the Mesoproterozoic shallow seawaters have favored the precipitation and evolution of glaucony.

Key words: glaucony, seawater redox conditions, Mesoproterozoic, Tieling Formation, North China Platform