Earth Science Frontiers ›› 2021, Vol. 28 ›› Issue (1): 353-362.DOI: 10.13745/j.esf.sf.2020.11.29

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Discussion on the features and mechanism of cryptoexplosive breccia in Wu’an, Hebei Province

LI Ruipeng1,2(), CUI Xiaoliang1, SU Shangguo1,*(), ZHANG Yanan1, LIANG Cuntao1, CHEN Xuegen1   

  1. 1. School of Earth Sciences and Resources,China University of Geosciences(Beijing), Beijing 100083, China
    2. YunchuangRuiCheng Data Technology Ltd. Company, Shenzhen 518101, China
  • Received:2020-01-10 Revised:2020-12-04 Online:2021-01-25 Published:2021-01-28
  • Contact: SU Shangguo

Abstract:

Cryptoexplosive breccia is widely distributed in Wu’an, Hebei Province. The study of its forming process of is important to the understandings of the interaction between magma and fluid and the formation of Fe deposit. We carried out in situ mineralogical analysis by electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in combination with detailed field and microscope investigation. The results show that the Wu’an breccia is mainly distributed between the Hongshan intrusive complex (mainly diorite) and its host rock (limestone). The gravels are mostly angular or subangular-shaped and could be easily pieced together. Secondary enlargement of calcite is often seen on the edge of the angular gravels of different colors due to different Fe contents. The matrix of breccia is mainly composed of calcite and hematite. A composition zone can be observed in hematite, which has higher FeO content in the core and higher MgO and Al2O3 contents in the edge. The total iron is negatively correlated with SiO2 content. The trace elemental characteristics of hematite show enrichments of Ba, K and U and depletions of high field-strength elements, such as Th, Ta, Nb, Zr and Hf, which may indicate that the magma was formed in a magma-fluid environment. The core hematite zone is enriched in Ti and Pb. The enrichment of Ti indicates the hematite’s magmatic origin, while the enrichment of Pb indicates a later involvement of hydrofluid superposition. According to this study, the genetic model of the cryptoexplosive breccia in the Wu’an area is as follows: The trapping of rising deep magmatic fluid by overlying host rock resulted in boiling and overpressure of the fluid, which led to cryptoexplosion as the overlying rock strata suddenly decompressed and broke up and the magmatic fluid cemented the upper rock mass into rocks.

Key words: Wu’an City in Hebei Province;, cryptoexplosive breccia, magmatic hydrothermal fluid, hematite

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