Remagnetization of Upper Triassic Sandstones From the Central Tibetan Plateau: Mechanism and Age

Abstract

Sandstones, common in continuous stratigraphic successions, can provide valuable paleogeographic and paleoenvironmental information. However, the original depositional remanent magnetization (DRM) preserved in sandstone is often overprinted by secondary chemical remanent magnetization (CRM), complicating geologic interpretations. To evaluate the depositional and remanence acquisition history of the Upper Triassic Duoburi Formation sandstones from the central Tibetan Plateau, we radiometrically dated (U‐Pb) detrital zircon grains and calcite veins, reanalyzed the remanence directions, and conducted integrated rock magnetic, spectroscopic, and petrographic experiments. Detrital zircon U‐Pb dates are >400 Ma, significantly older than the Late Permian ‐ Early Jurassic biostratigraphic age. Lithology and comparison of detrital zircon age distributions suggest a sediment provenance of proximal Paleozoic strata and a maximum depositional age similar to that of the nearby Mailonggang Formation at 222.9 ± 1.3 Ma, older than the calcite vein U‐Pb ages of 200.1 ± 2.4 Ma. The dominant magnetic carrier shifts from magnetite to hematite/goethite with sandstone color changing from gray to red. All magnetic minerals are predominantly fine‐grained, smaller than the typical detrital magnetite/hematite grains carrying the DRM. Their authigenic origin was further confirmed by petrographic observations. Trace amount of detrital magnetite is identified, but the remanence residing in it is dwarfed by the authigenic magnetite/hematite signal. Combined with underrepresentation of secular variation, we hypothesize that the remanence retained in the Duoburi sandstones is a CRM. Formation of authigenic magnetic minerals and CRM acquisition might have occurred during orogenic action at ∼200 Ma when the North and South Lhasa terranes sutured.