Rock Magnetic Results from the Early Ordovician Limestone Rocks in the Northern Qaidam Block, Tibetan Plateau

Abstract

The early Ordovician location of the Qaidam Block remains unclear, mainly due to the sparse paleomagnetic data to quantitatively determine the coeval position. The early Ordovician limestone is widespread in the northern Qaidam Block providing an ideal window to obtain a paleomagnetic result. The results derived from the limestone are, however, frequently constrained by the hydrothermal fluid activities after the strata formed and complex magnetic minerals in the samples. Therefore, systematic petrological and rock magnetic studies were conducted to identify the content and composition of magnetic minerals in the limestone samples from the early Ordovician Duoquanshan Formation in the northern Qaidam Block. The rock magnetic and petrologic results show that the magnetic carriers in the limestone samples are dominated by magnetite with a small amount of pyrrhotite, goethite, and hematite. These results are consistent with the characteristics of stepwise demagnetization. Combined with the results published previously, the secondary remanence component carried by pyrrhotite might be produced by thermal fluid or magma active by the multiple Tethys evolution.

Keywords

• early Ordovician
• Duoquanshan Formation
• rock magnetism
• limestone
• pyrrhotite
• northern Qaidam Block