Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana-derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the NRM.
|Original language||English (US)|
|Journal||Earth and Planetary Science Letters|
|State||Published - Oct 1 2019|
Bibliographical noteFunding Information:
Data to support this article are available in supplementary material Tables S1–S3. This project was funded by the Netherlands Organization for Scientific Research ( NWO ) with a Rubicon grant ( 825.15.016 ) to W.H. The first author was also supported by a visiting research fellowship from the Institute for Rock Magnetism (IRM) at the University of Minnesota, which is funded by the Instruments and Facilities program of NSF . G.D.-N. acknowledges support from ERC consolidator grant MAGIC 649081 and the project of “Probing Deep and Surface Processes in Central Tibet” from Agence Nationale de la Recherche , ANR, France. We thank Peter C. Lippert at Utah Pmag Center and Peat A. Solheid at the IRM for laboratory assistance. Pierrick Roperch is especially appreciated for collecting the samples, applying thermal demagnetization, and sharing his ideas about the remanence acquisition in carbonates. Comments from Prof. John W. Geissman have greatly improved this manuscript.
© 2019 Elsevier B.V.
- Tibetan Plateau
- Triassic limestone