Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas

Abstract

We obtained 453.2 ± 1.8 Ma and 449.4 ± 1.8 Ma (2{sigma}) laser step-heating 40Ar/39Ar plateau ages for phengite from quartzite mylonites from the blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia (northern China). These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess argon incorporation because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the quartzite mylonite unit occured about 200 Ma before final closure of the Paleo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone. We argue that the Early Paleozoic evolution of the Ondor Sum complex occurred along the northeastern Cimmerian margin of Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. The later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the East Gondwanan margin and northward drift of the detached North China craton. An extensive review shows that this type of two-stage scenario probably also applies to the geodynamic evolution of other micro-continents like, South China, Tarim, a number of Kazakh terranes, Alashan, Qaidam and Kunlun, as well as South Kitakami and correlatives in Japan, and probably Indochina. Like the North China craton, these were bordered by Early Paleozoic subduction-accretion complexes, island arcs or contained calc-alkaline volcanic margins, like for example, the central Tienshan, North Qinling, North Qaidam-Altun, North Qilian and Kunlun belts in China, as well as the Oeyama and Miyamori ophiolites and Matsugadaira-Motai blueschist belt of Japan and the dismembered Sergeevka ophiolite of the southern part of the Russian Far East. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in Early Paleozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred

Tectonic evolution of the North China Block: from orogen to craton to orogen

“…They yield sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 1973 + 4 Ma and 1834 + 5 Ma, with a core residual age of 2.0-2.1 Ga (Peng et al 2005(Peng et al , 2007. Zhao et al (2001aZhao et al ( , b, 2005Zhao et al ( , 2006, Wilde et al (2003), and Kröner et al (2005a, b, 2006 have suggested that the c. 1.9-1.8 Ga granulite event in the NCC is related to the continent-continent collision between the Eastern and Western Blocks of the craton. This model is supported by the interpretation of clockwise metamorphic P-T -t paths that show crustal thickening related metamorphism at 1.85 Ga, in support of a collision at this time.…”

Triassic 40Ar/39Ar ages from the Sakaigawa unit, Kii Peninsula, Japan: implications for possible merger of the Central Asian Orogenic Belt with large-scale tectonic systems of the East Asian margin

“…These observations imply that the Solonker suture zone continued to the coastal area of the Sea of Japan (Figs. 1, 2;de Jong et al 2006). The South Kitakami terrane of northeast Japan (Fig.…”

“…A number of authors explored the possibility that the Central Asian Orogenic Belt continues southeastward from mainland Asia into the 250-235 Ma-old, mainly metasedimentary Hida-Oki terrane in Japan ( Fig. 1; Arakawa et al 2000;Jahn et al 2000;de Jong et al 2006). Indeed, metamorphic rocks and synorogenic granites from the easternmost Central Asian Orogenic Belt in northeastern China yielded ca.…”

The Late Paleozoic Evolution of the Central Inner Mongolia, China: A Perspective From Metamorphic Study

“…Based on the previous study, four periods of metamorphism can be identified in the central Inner Mongolia since the Paleozoic (Figure): the first period is the high P/T metamorphism occurred in the Late Silurian (426 Ma), represented by the Early Paleozoic Ondor Sum Group which is attributed to the subduction of the Paleo-Asian ocean [De et al, 2006]; the second period is the medium P/T metamorphism in the early Devonian (400 Ma), represented by the Baoyintu Group in the western Inner Mongolia, which is related to the orogenic progress as a result of collision, following the closure of the Paleo-Asian ocean [Chen et al, 2015]; the third period is the low P/T metamorphism in the Middle-Late Carboniferous (309-345 Ma), represented by Xilin Gol complex, which is probably related to the regional extension after collision process; the fourth period is the low-medium P/T metamorphism in the Early Triassic (240 Ma), represented by the regional greenschist to epidote-amphibolite facies metamorphism in the central Inner Mongolia, which is attributed to the thickening process due to the closure of limited ocean basins. It is suggested that the Solonker suture zone may not represent a typical suture after closure of the Paleo-Asian ocean but a tectonic belt that records the closure of the limited basins in the early Mesozoic.…”