No abstract

No abstract

The Ondor Sum Group in the central Inner Mongolia is mainly composed of meta-basic volcanics intercalated with ferruginous quartzite and quartz schist, and has been interpreted as slices of oceanic crust or an ophiolite suite of the Early Paleozoic or much older ages. This paper presents new LA-ICP-MS zircon U-Pb dating and geochemical data for the meta-basic volcanics. The results show that zircons in the meta-basic volcanics were derived from complicated sources, most of which may be captured by basic magma from the country rocks or other sources. They yield a large age range from the Late Archean to Early Mesozoic with the youngest age group between 246 and 261 Ma, constraining the protolith of the meta-basic volcanics formed in the Late Permian to Early Triassic. The meta-basic volcanics have an affinity to E-MORB in geochemistry, and also a similarity toward OIB, representing a tectonic setting of limited intra-continental ocean basin. This limited basin might have been related to the continuous extension of the area since the Early Permian and finally closed in the Early Mesozoic. meta-basic volcanics, zircon U-Pb dating, Ondor Sum Group, Central Asian orogen Citation:Chu H, Zhang J R, Wei C J, et al. A new interpretation of the tectonic setting and age of meta-basic volcanics in the Ondor Sum Group, Inner Mongolia.

No abstract

No abstract

No abstract

No abstract

The Xilingol Complex comprises biotite gneisses and amphibolite interlayers with extensive migmatization. Four representative samples were documented and found to record either two or three metamorphic stages. Phase modelling using THERMO-CALC suggests that the observed assemblages represent the final stages that underwent cooling from temperature peaks, and are consistent with a fluid-absent solidus in P-T pseudosections. Their P-T conditions are further constrained to be 5-6 kbar/680-725°C and 4-5 kbar/650-680°C for two garnet-bearing gneiss samples, 4-5 kbar/660-730°C for a cordierite-bearing gneiss sample, and 4-5 kbar/ 680-710°C for an amphibolite sample based on mineral composition isopleths, involving measured Mg content in biotite, anorthite in plagioclase, grossular and pyrope in garnet and Ti content in amphibole. The peak temperature conditions recovered are 760-790°C or >760°C at 5-6 kbar based on the composition isopleths of plagioclase, biotite, garnet and especially the comparison of melt contents between the calculated and observed. A pre-peak heating process with slight decompression can be suggested for some samples on the basis of the core-rim increase in the plagioclase anorthite, and the stability of ilmenite. Zircon U-Pb dating using the LA-ICP-MS method provides systemic constraints on the metamorphic ages of the Xilingol Complex to be 348-305 Ma, interpreted to represent the post-peak cooling stages. Moreover, metagabbroic dykes that intruded into the Xilingol Complex yield 317 AE 3 Ma from magmatic zircon, and are considered to have played a significant role for heat advection triggering the high-T and low-P metamorphism. Thus, the clockwise P-T paths involving pre-peak heating, peak and post-peak cooling recovered for the Xilingol Complex are consistent with an extensional setting in the Carboniferous that developed on a previous orogen in response to addition of mantle-derived materials probably together with upwelling of the asthenospheric mantle. K E Y W O R D S Central Asian Orogenic Belt, extensional setting, HT-LP metamorphism, Xilingol Complex, zircon age dating