Geochemical Journal, Vol. 52, 2018
Huifei Tao1*, Zhen Qiu2, Hongjie Ji1, 3 and Junli Qiu1
1Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, PR China
2China PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
3University of Chinese Academy of Sciences, Beijing 100049
(Received July 3, 2017; Accepted December 13, 2017)
In this study, a combined methods of geochemistry, petrology and mineralogy are used to analyze the Lower Carboniferous clastic rocks from the northeastern Junggar, China, and then the weathering intensity, recycling, provenance and tectonic setting are discussed.
The chemical index of alteration (CIA) values and the A-CN-K diagram show that the detritus of the mudrocks experienced a weak to moderate weathering intensity. The low Th/U and Zr/Sc ratios show that the mudrocks experienced a minimal recycling process. The low compositional and textural maturities of the interlayer sandstone reflect a proximal provenance. The low contents of Cr and Ni of the samples exclude the southern adjacent Armantai ophiolites as the potential provenance. Therefore, the nearby northern Dulate arc was the likely source area for the mudrocks. The XRD analysis reveals that the mudrocks contain abundant of quartz, plagioclase (albite) and clay minerals, and some calcite and apatite. The samples have high contents of P2O5, meanwhile, show statistically significant positive correlation between P2O5 and CaO, Y, Sr, and light rare earth elements, also indicating abundant phosphates minerals (e.g. apatite) in the mudrocks. The discriminate plots of the geochemistry (major, trace and rare earth elements) suggest that the source rock compositions are mainly felsic rocks and mixed with a few basic rocks. The tectonic discrimination diagrams of major and trace elements indicate that the mudrocks were originated from oceanic island arcs, which formed in an extensional tectonic environment. Combined with previous studies, the geochemistry data imply there are two evolution stages occurred in the northern Junggar area during the early late Paleozoic time. The northern Junggar was amalgamated to the southern margin of the Chinese Altai before Devonian, and then experienced extensional movements during the early Devonian to early Carboniferous periods. The Dulate arc and arc-related basins in this area were formed during this extensional course.
Key words: Major and trace elements; Lower Carboniferous; Mudrock; Tectonic setting; Junggar