Acta Petrologica et Mineralogica

The Proterozoic Eon is a critical period of the Earth’s evolution, and the study of its rock records in this period is important for understanding the Earth’s evolution in the Precambrian. The correct interpretation of rock genesis is the basis of the study of rock geotectonics and the premise of revealing the change of earth environment based on rock records. A set of silicon-rich layered rocks generally developed at the bottom of the Changcheng System in the North China Craton, which is considered to be a set of marine quartz sandstones and siliceous rocks formed by shallow metamorphism in rift valley deposition. This paper focuses on the silica-rich layered rocks of Changzhougou Formation at the bottom of the Great Wall System exposed in Huyugou, Changping, Beijing. Through detailed field geological observations, petrological and mineral analysis of typical samples, SEM/EDS (Scanning Electron Microscopy/X-ray Spectroscopy), CL (Cathodoluminescence) imaging, in-situ LA-ICP-MS (Laser Ablation-inductively Coupled Plasma Mass Spectrometry) analysis of quartz, and petrological, micro-thermometry, muffle furnace heating-quenching experiments, and SEM/EDS, EPMA (Electron Probe Microanalysis), LRM (Laser Raman Microscopy) analysis of melt inclusions in quartz, a new understanding of the volcanic origin of the silicon-rich stratified rocks at the bottom of the Great Wall System has been proposed. The results indicate that the silicon-rich stratified rocks at the bottom of the Great Wall System include two types: clastic structures (previously defined as quartz "sandstone") and mosaic structures (previously defined as "quartzite"). The main mineral compositions are quartz and K-feldspar, with minor amounts of zircon, rutile, and other minerals. In the clastic structure rocks, quartz and K-feldspar clasts exhibit angular to subangular shapes, and K-feldspar shows no signs of clay alteration at the edges. The CL of quartz in the two types of rocks mostly shows blue-purple luminous characteristics. Additionally, the formation temperature estimated by the quartz titanium saturation thermometer is concentrated between 550~650℃, indicating characteristics of high-temperature quartz. Both types of rocks contain iron-titanium-rich two-phase melt inclusions in quartz and K-feldspar. Micro-thermometry results of the melt inclusions show that during the heating process to 1 000℃, the composition of the two-phase immiscible melt inclusions changes continuously, when the temperature reaches 1 000℃, with a color reversal occurring at about 1 050℃, and critical homogenization around 1 350℃. Muffle furnace heating-quenching experiments also confirmed that the two-phase melt inclusions in the samples quenched after heating to 1 350℃ became homogeneous. SEM/EDS and EPMA results indicate that the main components of the melt inclusions are iron and titanium oxides, with the dark part being relatively rich in iron and the light part being relatively rich in titanium, and the Fe/Ti ratio of the two phases of the melt inclusions varies greatly. LRM analysis results show that both phases of the melt inclusions are amorphous. Integrating the structural characteristics of the rocks, the CL luminescence characteristics of quartz, the formation temperature estimation of quartz, and the micro-thermometry results of the melt inclusions in quartz, it is believed that the siliceous strata at bottom of the Changcheng System should be siliceous volcanic rocks and volcaniclastic rocks. This indicates that there was a large-scale siliceous volcanic activity in the North China Craton during the Paleoproterozoic, which is related to the rifting of the Columbia Supercontinent. It provides new evidence for the geological evolution of the North China Craton in the Proterozoic. The discovery of iron-titanium-rich melt inclusions in quartz also offers new perspectives on the formation of Precambrian magmatic iron and titanium deposits, particularly the enrichment mechanism of iron and titanium during the magmatic process.

North China craton; Great Wall system; silicon-rich volcanic rocks; melt inclusions; melt immiscibility