T143 Plate Tectonics Paradigm and Future Directions of Research—Discovery in Light of Its 50-Year History II

Monday, 5 November 2018
1:30 PM - 5:30 PM
Room 243

LATE CARBONIFEROUS-PERMIAN SUBDUCTION-ACCRETION TECTONICS OF THE ALXA TECTONIC BELT (NW CHINA), SOUTHERN CENTRAL ASIAN OROGENIC BELT

Paper number: T143-7
Program: Technical Sessions
Time: 3:05 PM – 3:20 PM

Authors

  • Dongfang Song Presenting Author
    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
  • Wenjiao Xiao
    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing
  • Chunming Han
    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing

Abstract

The Alxa Tectonic Belt (ATB) is a key area for understanding the late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt (CAOB). Three different perspectives exist on the Late Carboniferous – Permian (C2-P) tectonic setting for the ATB: (1) subduction-accretion, (2) continental rifting, and (3) mantle plume. To solve this issue, we carry out field and geochronological studies on the C2-P sedimentary rocks, accretionary complexes and magmatic rocks.

The C2-P sedimentary rocks mainly distribute in the northern and southwestern ATB. The C2-P strata in the southwestern ATB consists of volcano-sedimentary rocks. An angular unconformity exists between the tightly folded Middle Permian and gently folded Upper Permian strata. Large amounts of intermediate-felsic porphyry such as andesite, dacite and rhyolite are present in the sedimentary rocks. On the Q-F-L diagram, the Permian sandstones are plotted in the field of dissected to undissected arc. Detrital zircon U-Pb-Hf studies show that the Permian sediments were sourced from Permian juvenile magmatic processes mixed with limited recycled Precambrian basement.

The Engger Us accretionary complex includes mélange and coherent units. The mélange consists of sandstone, pillow basalt, chert and limestone. The chert and limestone display as block in the matrix of sheared basalt. The cherts contain Late Permian radiolarian fossils, which indicates oceanic crust existed until Late Permian. The coherent unit is composed of slate, phyllite, metasandstone and limestone, which are highly deformed into tight to open folds with axial plane and crenulation cleavages.

C2-P magmatic rocks consist mainly of granitoids with minor volcanic rocks. LA-ICP-MS zircon U-Pb dating show the timing of magmatism is from ~317 to ~287 Ma. Field, geochemical and Hf-in-zircon isotopic data show this magmatism can be best interpreted as formed in a continental arc setting due to the southward subduction of the PAO. The presence of A2-type granites indicates a retreating accretionary system existed during C2-P in the ATB.

The C2-P sediments are interpreted as arc-related basin deposition, the Ennger Us mélange and coherent unit forms the accretionary complex, and the magmatic rocks were formed in a continental arc setting. The ATB was under a subduction-accretion setting during the C2-P.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-317543

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