Paleomagnetic results from Lower Devonian sandstones of the Niqiuhe Formation in the Duobaoshan area and its constraints on paleoposition of the Xing’an block

Abstract

Two common but contradictory views toward the timing of the final closure of the eastern Paleo-Asian Ocean (PAO) are either the Pre-Late Devonian or the Late Permian to Early Triassic. Previous paleomagnetic studies support a consolidation of the unified North China and Mongolia by the Late Permian. However, Pre-Late Permian paleogeography of the eastern Central Asian Orogenic Belt (CAOB) is not well understood due to the scarcity of reliable paleomagnetic records. The Xing’an block plays a key role in the evolution of the CAOB, however, no paleomagnetic results are available so far to provide constraints on its Late Paleozoic paleo-position. We performed a systematic paleomagnetic study on a purple siltstone section of the Lower Devonian Niqiuhe Formation near the Duobaoshan County, Heilongjiang Province. Representative fresh end samples were subjected to rock magnetic analysis to identify their dominant magnetic carriers and particle size distribution. Rock magnetic experiments include measurements of magnetic susceptibility variation dependence of temperature ( κ - T ) curves, hysteresis loops, acquisition of isothermal remanent magnetization (IRM) and back-field demagnetization of saturated IRM (SIRM) curves. The results indicate that the magnetic remanence carriers are dominant by hematite with minor contributions of magnetite. 112 cylindrical specimens were thermally demagnetized using an ASC TD-48S furnace and measured in a 2G-755-4K SQUID system in the Magnetotectonics Lab of Peking University. A total of 11 sites revealed identical bi-components character. The low temperature component resembles the present geomagnetic field in geographic coordinates, while the high temperature characteristic remanence (ChRM) component was reversed polarity both before and after tilt correction. Optimal concentration is achieved at 35.7% unfolding percentage, which suggests the magnetization was acquired during the late stage of a folding process. A previous structural study indicated that stress field of the research area switched from NW-SE to N-S orientation during the Late Permian, which is supported by the predominant NE-SW oriented Ordovician to Devonian strata as well as the angular unconformity between the Devonian and Upper Permian strata. The folding time of the Niqiuhe Formation is likely Middle Devonian to Middle Permian. Supported by evidence from a suite of granites near our sampling section (less than 3 km) dating at 309−299 Ma along with the universal reversed polarity of all 11 sites, we argue that the ChRM component was acquired during the Permo-Carboniferous Reversed Superchron (~318−262 Ma), when the study area was affected by a granitic intrusion thermal event (309−299 Ma). The ChRM component ( D / I =28.6°/−33.2°, k =18.6, α 95=10.9°) corresponds to a paleomagnetic pole of λ p/ φ p=17.3°S/97.1°E ( dp / dm =7.0°/12.4°). For a common reference site (43°N/114°E), our results suggest the Xing’an block was situated at 27.7°±12.4°N during the Late Carboniferous. Coeval results obtained from rocks of different lithology suggest 28.5°±9.7°N and 19.9°±14.8°N for the Uliastai continental margin and the Central Mongolia-Euruna block. After applying a blanket flattening factor f =0.6 for results obtained from clastic sedimentary rocks, the paleolatitudinal difference between the Xing’an block, Uliastai continental margin, and the Central Mongolia-Erguna block is less than 3.2°±12.8°. Late Carboniferous paleomagnetic poles from those three blocks are distributed along a small circle. Significant post-Permian local rotations are likely attributed to extensive Mesozoic strike-slip faults among the eastern CAOB. Those three blocks were probably consolidated into a unified block, situated at ~28°−30°N during the Late Carboniferous and dominant by the cold water Borel realm fauna. By contrast, the North China and Songliao-Xilinhot blocks was situated at an equatorial region (4.3°S−12°N) and dominant by the warm water Tethyan realm fauna, while Siberia was located at ~45°−50°N. The Paleo-Asian Ocean therefore remained open and the main basin was the Hegenshan ocean rather than the Solonker ocean.