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

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.