The Paleomagnetic And Detrital Zircon U-Pb Geochronology Hf Isotope Composition Studies Of Paleozoic In The Hexi Corridor Belt And Its Tectonic Significance

Detailed paleomagnetic studies on the upper Devonian, lower Carboniferous and upper Permian strata,and detrital zircon U-Pb geochronology and Hf isotope composition on the Cambrian and Devonian strata have been carried out in the eastern part of the Hexi Corridor belt, Northwest China. These results are e used to discuss the paleogeography and tectonic evolution history of the Qilian orogenic belt during the Paleozoic.Systematic thermal demagnetization and detailed rock magnetic studies have been conducted on sediments of upper Devonian (a total of 826 orientated drill-core samples from 6 sampling profiles), lower Carboniferous (80 orientated drill-core samples from 7 sites) and upper Permian (113 orientated drill-core samples from 12 sites distributed in 3 sampling profiles). The characteristic high-temperature remanence components obtained in the Upper Devonian and Upper Permian have passed fold and reversal tests. The corresponding paleomagnetic poles are at?=34.00°S, ?=46.50E, dp=4.2°, dm=8.3° and ?=26.2°N, (?=18.0°E, dp=7.5°, dm=13.3°,respectively. The middle-temperature components obtained from the samples of lower Carboniferous have passed fold and reversal tests, which however, were probably remagnetized through the "orogenic fluids" during the early Permian in the Qilian Orogenic Belt. The middle-temperature remanence component yields a pole position at?=12.1°N, ?=10.7°E, dp=4.2°, dm=8.3°.Combining with the early Paleozoic reliable paleoinagnetic data obtained in the studied areas, the apparent polar wander path (APWP) and the changes of paleolatitudes of the Hexi Corridor belt in the Paleozoic are established. Comparing with palaeolatitudal changes and APWPs of the Hexi Corridor belt,the South China Block (SCB) and the Australian Block during the Cambrian and Late Devonian, and also evidences on paleontology, detrital zircon geochronology and Nd isotopic characteristics of magmatic rocks,tectonic implications are drawn as follow. First,the Hexi Corridor belt and SCB were probably close each other before the Late Carboniferous. The Hexi Corridor belt, SCB and the Australian Block were adjacent and belonged to East Gondwana before the Late Devonian. The paleo-latitude of Hexi Corridor agreed with that of NCB during the Late Permian. However, there was relative separation between the eastern Hexi Corridor belt and the North China. By comparing the Permian and early-middle Triassic apparent polar wander path of the eastern Hexi Corridor with that of the North China block, there are obvious similarities and systematic bias. If the apparent polar wander path of the eastern part of Hexi Corridor suffers from a 32° counterclockwise rotation around an Euler pole at 44°N and 84°E, then two APWPs will be consistent. The results show that the Hexi Corridor belt converged to the NCB after the Late Triassic.U-Pb geochronology and Hf isotopes studies on detrital zircons separated from middle Cambrian and late Devonian sediments in the Eastern Hexi Corridor were carried out. There were three major U-Pb age groups in the Middle Cambrain samples:0.6-0.8Ga (peak at 669Ma), 1.0-1.3Ga (peak at 1173Ma),1.6-1.8Ga (peak at 1710Ma),which were signifcantly different from the detrital zircons U-Pb age spectra from the upper strata of the Hexi Corridor. The characters of the detrital zircons U-Pb age spectra and Hf isotopes indicated that the detrital zircons aged in 1.0-1.3Ga may originate from Southwestern Australia, which was consistent with previous paleomagnetic results. By combination of paleomagnetic results, we suggest that the Hexi Corridor belt was located in northwest Australia in the Middle Cambrian. U-Pb geochronology and Lu-Hf isotopes studies on detrital zircons from Late Devonian samples showed that the detrital zircons aged in 0.4-0.5Ga may originate from the Ordovician volcanic rocks in the North Qilian Orogenic Belt, the detrital zircons aged in 0.8-1.0Ga may originate from the Central Qilian Block, and the detrital zircons aged in 1.8-2.1Ga may come from Paleoproterozoic magmatic rocks in the eastern part of Alashan Block. The characteristics of Hf isotopic of the detrital zircons age in?2.5Ga are significantly different from the contemporaries detrital zircons in North China. The detrital zircons aged in 1.8-2.1Ga, which originated from Alashan Block,are firstly appeared in the Paleozoic strata. This result implies that the North Qilian Ocean had already been closed in the Late Devonian.