Remagnetization of the Jurassic limestones in the Zaduo area, Eastern Qiangtang Terrane (Tibetan Plateau, China): implications for the India–Eurasia collision

Abstract

Summary A series of terranes were accreted to Eurasia in the region of what is now the Tibetan Plateau, including the Qaidam-Qilian, the Songpan-Ganzi, the Qiangtang, the Lhasa and the Tethyan Himalaya terranes. The drift history of the Qiangtang Terrane and the timing of the Lhasa-Qiangtang collision is controversial. To contribute to this topic, here, we paleomagnetically investigate the Middle-Upper Jurassic limestones of the Yanshiping group in the Zaduo area (32.5° N, 95.2° E), in the eastern Qiangtang Terrane. Twelve sites (133 samples) were processed. A major challenge in paleomagnetism is the possibility of remagnetization that interferes with paleogeographic reconstructions. Both thermal and alternating field demagnetizations were carried out to isolate the characteristic remanent magnetization (ChRM). Despite the positive reversals test, rock magnetic information points to a remagnetized ChRM. The ChRM is residing in stable single-domain (SSD) magnetite grains with cogenetic superparamagnetic (SP) particles. The co-occurreance of SSD and SP magnetites generate distinct rock-magnetic properties often referred to as the ‘remagnetized fingerprint’ in limestones. This remagnetization process is also manifested by the widespread occurrence of gypsum veinlets in the limestones. The site-mean direction of the 12 sites after tilt-correction is Ds = 30.6°, Is = 35.6°, κs = 182.9, α95 = 3.2°, corresponding to a paleolatitude of ∼19.7°± 2.8° N for the study area. The corresponding paleopole (59.8° N, 202.7° E with A95 = 2.8°) points to an NRM acquired after the India-Eurasia collision. The original sediments were likely anoxic because of the high organic carbon fluxes that prevailed during their deposition. After the India-Eurasia collision, it is envisaged that conditions became more oxic, giving rise to oxidation of iron sulphides to authigenic magnetite and the CRM acquisition. The Zaduo area in the Eastern Qiangtang Terrane has experienced ∼15.7° ± 3.2° (∼1740 ± 350 km) of latitudinal crustal shortening since the Eocene. In addition, the clockwise rotation responding to the India-Eurasia collision is also detected in the Zaduo area.

Remagnetization of Carboniferous Limestone in the Zaduo Area, Eastern Qiangtang Terrane, and Its Tectonic Implications

“…The widely distributed limestone on the EQT has been one of the main targets for paleomagnetic studies to constrain its drift history. Many paleomagnetic studies have been carried out on limestone of the EQT, including rocks of the late Paleozoic (Cheng et al, 2012, Cheng et al, 2013Yang et al, 2017) and the Mesozoic (Lin and Watts, 1988;Cheng et al, 2012;Ren et al, 2013;Yan et al, 2016;Ran et al, 2017;Cao et al, 2019;Zhou et al, 2019;Fu et al, 2021Fu et al, , 2022. Nevertheless, due to the complexity of limestone, some discordant results existed.…”

The Early Cretaceous Zaduo Granite, Eastern Qiangtang Terrane (China)—An Attempt to Constrain its Paleolatitude and Tectonic Implications

“…Component 1 has low coercivity and contributes ~15% to the SIRM. It is interpreted to be the result of thermally activated component 2 (Egli, 2004; Heslop Huang et al, 2015;Zhang et al, 2016;Fu et al, 2021), which results in a left-skewed distribution that must be fitted with an extra component in software that only considers symmetric log-Gaussian functions. Component 2 is the dominant magnetic carrier in the granite and contributes >80% to the SIRM; it is typically interpreted to be magnetite (e.g., .…”

“…The nearby Middle-Upper Jurassic limestones of the Yanshiping Group were reported to be remagnetized during the India-Eurasia collision process (Fu et al, 2021). However, igneous rocks have a considerably lower porosity that grossly diminished the circulation of fluids.…”

“…Paleomagnetism is an effective approach for quantifying terrane drift history. Many studies have been carried out on the Mesozoic paleographic positions of the Qiangtang Terrane (Lin and Watts, 1988;Dong et al, 1990;Otofuji et al, 1990;Dong et al, 1991;Huang et al, 1992;Chen et al, 1993;Cheng et al, 2012;Song et al, 2012;Huang et al, 2013;Ren et al, 2013;Song et al, 2015Song et al, , 2020Tong et al, 2015;Yan et al, 2016;Chen W. et al, 2017;Ran et al, 2017;Meng et al, 2018;Cao et al, 2019Cao et al, , 2020Zhou et al, 2019;Fu et al, 2021;Guan et al, 2021). These studies have provided extensive knowledge on the tectonic evolution of the Qiangtang Terrane.…”

“…Therefore, the tectonic evolution of the Zaduo area during the Late Jurassic to Early Cretaceous likely provides key information to address questions as to when the Lhasa-Qiangtang collision occurred and how the region deformed in response to the India-Asia collision. Investigations on the Middle-Upper Jurassic limestones of the Yanshiping Group in this area indicate that primary natural remanent magnetization (NRM) was overprinted by a chemical remanent magnetization (CRM) during the India-Eurasia collision (Fu et al, 2021). Igneous rocks are less prone to remagnetization than limestones.…”