Initial Closure of the Longmu Co‐Shuanghu Paleo‐Tethyan Ocean: Constraints Based on New Paleomagnetic Data From the Middle Triassic Volcanic‐Sedimentary Sequence, Southern Qiangtang Terrane.

Understanding the geodynamics of the Tethys requires solid constraints on the evolution of the Tethyan Ocean. However, the exact closure time of the Longmu Co‐Shuanghu Paleo‐Tethyan Ocean remains poorly defined. To address this issue, we report a combined paleomagnetic, rock magnetic, and petrographic study of the Middle Triassic (∼240 Ma) volcanic‐sedimentary sequence of the southern Qiangtang Terrane. Magnetite, along with minor amounts of pyrrhotite and hematite, are the magnetic carriers in the limestone, whereas magnetite and titanomagnetite constitute the principal magnetic carriers in the volcanic rocks. After tilt correction, the analysis of characteristic remanent magnetizations (ChRMs) from 18 sites yielded a mean direction of Ds = 55.4°, Is = 36.2°, α95s = 6.4°, and k = 29.9. The volcanic rock sites effectively averaged out the paleomagnetic secular variation, and the overall ChRMs passed the fold test. The determined paleomagnetic pole at 39.4°N, 175.6°E, with A95 = 5.7°, indicates a paleolatitude of ∼20.1 ± 5.7°N. A comparison of paleolatitudes between the northern and southern Qiangtang terranes reveals that the Longmu Co–Shuanghu Paleo‐Tethyan Ocean was initially closed before or during the Middle Triassic. Available geological observations, including magmatic, metamorphic, and sedimentological records from the northern and southern Qiangtang terranes and the suture zone indicate a closure time ranging from late Middle Triassic to early Late Triassic. Thus, the combined geological records and paleomagnetic results suggest that the LongmuCo–Shuanghu Paleo‐Tethyan Ocean was initially closed in the late Middle Triassic. Key Points: A primary magnetic remanence in the Middle Triassic Zuoqing Co Formation volcanic‐sedimentary sequence was acquiredThe southern Qiangtang terrane was located at 20.1 ± 5.7°N during the Middle TriassicThe initial closure of the Longmu Co‐Shuanghu Paleo‐Tethyan Ocean was in the late Middle Triassic [ABSTRACT FROM AUTHOR]