Primary Devonian paleomagnetic results from the Qinling orogenic belt and its implication for the evolution of the Proto-Tethys Ocean

The Qinling Orogenic Belt (QOB) is one of the most important orogens in Eastern Asia formed by the collision between the North China Block (NCB) and the South China Block (SCB). The evolution history of the QOB is essential to the assembly processes of the major blocks in China and the evolution history of the Proto-Tethys Ocean (Shangdan Ocean). Paleomagnetism can quantitatively restore the paleo-position of blocks, which is key to studying the related tectonic evolution. Hindered by the complex tectonic process, few paleomagnetic results have been reported from the QOB. Here we reported a primary paleomagnetic study from the northern QOB by conducting both rock magnetic and paleomagnetic experiments on the early Devonian Lajimiao pluton (~413Ma) in the North Qinling belt (NQB), to constrain its paleo-position and the evolution of the QOB during the early Paleozoic period.

253 cores from 28 sites were drilled by portable gasoline drills, and oriented by a magnetic compass and also a sun compass if possible. Rock magnetic experiments indicate that the main magnetic mineral in most of the samples is mainly magnetite in a pseudo-single domain or multi-domain state. Both thermal demagnetization and alternating-field demagnetization were applied to obtain the characteristic remanent magnetization. The Fisher-mean direction of the low-temperature/coercivity component is roughly consistent with the present geomagnetic field (PGF), suggesting that it is probably a viscous remanent magnetization caused by the PGF. The high-temperature/coercivity component yielded a Fisher-mean direction Ds/ Is = 355.8°/19.1° in stratigraphic coordinates, corresponding to a paleomagnetic pole of 65.8°N/299.9°E (A95=2.4°). It is the first Devonian paleomagnetic pole among the scarce paleomagnetic results from the QOB. This pole indicates that the NQB may have been located at a low latitude at the early Devonian, probably in proximity to both the North China and South China blocks. However, the difference between the coeval paleomagnetic poles from the three blocks (NQB, NCB, SCB) may hint the assembly process of the several major blocks is not simple and direct. Anyway, the newly obtained paleomagnetic pole from the NQB would be able to refine our understanding of the tectonic evolution of the QOB and the Proto-Tethys Ocean.