Isolation of Primary Remanent Magnetization From Himalayan Rocks: Insights From Partially Remagnetized Upper Cretaceous Oceanic Red Beds in Southern Tibet, China

Abstract

Oceanic red beds, preserving primary depositional remanent magnetization, play a key role in reconstructing the Tethyan paleogeography. However, partial remagnetization caused by chemical processes could be pervasive in these rocks, leading to flawed reconstructions, and thus, differentiating secondary and primary remanences is important. In this paper, we conduct multiple X-ray diffraction, petrographic, diffuse reflectance spectroscopy, and rock magnetic analyses on the Upper Cretaceous oceanic red beds (CORBs) from the Cailangba section in the Gyangze region of the Tethyan Himalaya. Our results reveal that the CORBs contain coarse-grained detrital hematite with a narrow coercivity distribution, as well as fine-grained authigenic hematite with a broad coercivity distribution. The coarse-grained population is mainly composed of >400 nm hematite grains and unblocks close to the Néel temperature (675°C), consistent with a detrital origin. In contrast, the fine-grained population is mainly composed of <30−400 nm hematite grains and progressively unblocks below 650°C, consistent with a chemical (authigenic) origin. In addition to these two populations of hematite, a small amount of goethite, unblocking below 120°C, is detected. Due to the distinct unblocking temperature spectra of these two populations of hematite, isolating the primary detrital remanence of the coarse-grained hematite from the chemical remanence of the fine-grained hematite in the CORBs through high-resolution thermal demagnetization treatment is feasible. This study lends confidence to the paleomagnetic studies of these oceanic red beds in Tethyan paleogeographic reconstructions.