A Paleozoic fore-arc complex in the eastern Central Asian Orogenic Belt: Petrology, geochemistry and zircon U-Pb-Hf isotopic composition of paragneisses from the Xilingol Complex in Inner Mongolia, China

Abstract

The Central Asian Orogenic Belt (CAOB) is one of the largest and most complex accretionary collages and responsible for considerable Phanerozoic juvenile crustal growth. The Xilingol Complex is a key tectonic unit within the eastern CAOB and consists of strongly deformed quartz-feldspathic gneisses with lenticular or quasi-lamellar amphibolites. Whether the complex in the belt represents Precambrian basement is controversial and here a combined analysis of petrology, geochemistry and geochronology of quartz-feldspathic gneisses from the complex is reported. The rocks belong to upper amphibolite facies and three stages of metamorphism are recognized based on mineral assemblages: prograde metamorphism (Bt-I+Ms-I+Pl+Qz+Mag), peak metamorphism (Alm+Sil+Kfs), and retrograde metamorphism (appearance of Bt-II+Ms-II). Both Precambrian and Paleozoic age populations are identified from each sample. The Precambrian zircons are sourced from magmatic rocks and the crystals display a wide range of 207Pb/206Pb ages from 3141±17Ma to 837±24Ma, and significant variation in εHf(t) values from −17.84 to +10.57 with TDM2 model ages from 3209 to 1389Ma. The Paleozoic zircons have 206Pb/238U ages from 494±14Ma to 402±10Ma, variation in εHf(t) values from −12.82 to +7.72 and TDM2 model ages from 2252 to 960Ma. The biotite 40Ar-39Ar dating yields a plateau age of 301.2±1.9Ma with a low temperature step age 288.1±1.7Ma. The samples have a wide range of whole-rock SiO2 (63.7–89.3%) and Al2O3 contents (5.66–16.3%), with Fe2O3T varying from 0.60% to 6.06%. All of them have trace element compositions consistent with those of the upper continental crust. The protoliths of paragneisses in the Xilingol Complex are wackes and litharenites, representing a part of fore-arc sedimentary sequence (>319Ma) with a major phase of diagenesis age between ca. 387 and 382 Ma in an active continental margin of the South Mongolian microcontinent. The Precambrian zircons from the complex are proposed to have been originally derived from the South Mongolian or those similar microcontinents in the CAOB. These microcontinents have been originally derived from the Tarim craton, instead of the Siberia or North China cratons or Gondwanaland. The prograde biotites from the paragneisses with ages of ca. 312–301Ma recorded the beginning of the accretionary wedge-continent collage during the northward subduction of the Paleo-Asian Ocean, which may last to ca. 282Ma recorded by the retrograde biotites. Detrital materials from the South Mongolian microcontinent and the Baolidao arc instead of the North China craton were deposited in the fore-arc basin indicating the presence of the Solonker zone separating the northern subduction zone from the southern subduction zone in the CAOB. The CAOB is formed as the model of multiple subduction zones with different subduction polarities.