Paleomagnetic Results of Permo-Carboniferous Volcanic-sedimentary Strata in Mid-eastern Inner Mongolia, China: Implications for Tectonic Evolution of the Eastern CAOB

Abstract:

There has been hotly debating over the closure time of the eastern Paleo-Asian Ocean and the tectonic evolution of the eastern CAOB (Central Asian Orogenic Belt) for decades. To better puzzle out this controversy, we carried out a detailed paleomagnetic study on the Permo-Carboniferous volcanic-sedimentary strata in mid-eastern Inner Mongolia, northeast of China. More than 820 samples were collected from 81 sites and titanium-poor magnetite and hematite are proved as the principal magnetic carriers. (1)In Kingan Block, 9 sites of volcanic rocks from Dashizhai Formation (P₁) were calculated to get a mean magnetic direction Dg/Ig = 285.5°/77.4°, kg = 68.2, α₉₅ = 6.8° before and Ds/Is = 206.5°/48.2°, ks = 100.8, α₉₅ = 5.5°, N = 9 after bedding correction, which suggests a paleolatitude of 34.5°±5°N. Both the positive fold test and reversal test suggest a pre-folding magnetization and thus may indicate a primary remanence. (2)Three volcanic sections of Baoligaomiao Formation (C₃-P₁) from Uliastai Passive Margin were sampled and a mean magnetic direction derived from 16 sites is Dg/Ig = 30.1°/31.8°, kg = 16.3, α₉₅ = 9.8° before and Ds/Is = 65.6°/58.1°, ks = 39.8, α₉₅ = 6.1°, N = 16 after bedding correction. The corresponding paleomagnetic pole Plat. /Plong = 43.1° N/186.7°E, A₉₅=8° suggests a paleolatitude of 38.7°±6.3°N. A primary remanence is confirmed by positive fold test. (3) In the northern margin of NCB (North China Block), a ChRM is successfully isolated from 6 sites of basaltic rocks in Elitu Formation (P₂) as Dg/Ig = 351.1°/67.2°, kg = 2.1, α95 = 71.8° before and Ds/Is = 351.1°/29.1°, ks = 32.7, α95 = 71.8°, N = 16 after bedding correction, and thus yielded a paleomagnetic pole as Plat. /Plong = 63.1° N/313.5°E, A₉₅=9.5°, which suggests a paleolatitude of 17.2°±7.2°N. A positive fold test and reversal test indicate that the remanence should be primary. The paleomagetic pole of Kingan Block and Uliastai Passive Margin are located at a common small circle and thus indicates they may share a similar paleolatitude, the huge longitudinal difference should be ascribed to relative rotation afterwards . To the contrary, the paleolatitude of the northern margin of NCB and southern Mongolia is much lower than that of Kingan and Uliastai, which may suggest a remain-opening paleogeography of the Paleo-Asian Ocean by middle Permian.