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

Abstract

The Eastern Qiangtang Terrane is an orogenic-like belt around the Eastern Himalayan syntaxis (EHS). The deformation history of this terrane must be known to understand how the EHS region responded to the Lhasa-Qiangtang collision and the closure of the Bangong-Nujiang Ocean (BNO). Here, we present a new paleomagnetic investigation on an Early Cretaceous granite (∼126 Ma) in the Zaduo area, Eastern Qiangtang Terrane. Petrographic observations reflect crystallization from primary melts with only limited subsequent alteration (chloritization of biotite). Magnetite appears to be the dominant carrier of the characteristic remanent magnetization (ChRM) based on stepwise demagnetization of the natural remanent magnetization, supplemented by detailed rock magnetic measurements, including magnetization versus temperature, and acquisition curves of the isothermal and anhysteretic remanent magnetization. End-member modeling of those acquisition curves helped to constrain the paleomagnetic analysis. The inconsistent demagnetization behavior between alternating field (AF) demagnetization at high levels and thermal demagnetization was attributed to the development of gyroremanent magnetization in the AF demagnetization generated by fine-grained single domain magnetite. The ChRM directions from 92 granite samples in geographic coordinates yield an average of declination (Dg) of 2.6° and inclination (Ig) of 38.6° (precision parameter k = 51.4, and 95% confidence cone α95 = 2.1°). The amount of tilting of the granite is poorly constrained which makes proper correction rather tedious. We compared the expected bedding attitudes (Strikeexp = 43.1°, Dipexp = 46.1°) derived from published data (Huang et al., 1992;Tong et al., 2015) with the average observed bedding attitudes (Strikeobs = 54°, Dipobs = 32°) of the Middle-Upper Jurassic sandstones of the Yanshiping Group that was intruded by the Early Cretaceous granite. The discrepancy between the expected and measured bedding attitudes implies that the strata of the Yanshiping Group in the Zaduo area were already tilted prior to the intrusion of the ∼126 Ma Zaduo granite, which was attributed to the Lhasa-Qiangtang collision and the closure of the BNO. The collision led to a series of geological events, such as the tilting of the strata, the ophiolite emplacement, the development of a peripheral foreland basin, and the magmatic activity gap. The tilting/folding of the strata was generally delayed by the layer parallel shortening processes during the early stages of the deformation, thus suggesting an older Lhasa-Qiangtang collision (i.e., >126 Ma).

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

“…The most common mechanisms to explain remagnetization include the thermoviscous resetting of existing magnetic minerals (Kent, 1985) and chemical remanent magnetization through magnetic mineral growth associated with orogenic fluids (e.g., Jackson, 1990;Elmore et al, 2012;Huang et al, 2015). Given that only the Late Triassic and Early Cretaceous intrusive rocks are found near the section (Figure 1B), they are deemed a less likely mechanism for the remagnetization as 1) the obtained pole is most likely of the Early Cenozoic one; 2) The nearby Permo-Triassic volcanic rocks and Early Cretaceous intrusive rocks both contain primary magnetizations (Guan et al, 2021;Fu et al, 2022). In general, limestone perhaps has not been heated for a sufficient amount of time at such a temperature to make the thermal resetting feasible by the intrusive rocks (Dekkers, 2012); and 3) The nearby Middle Jurassic limestone was remagnetized during the Eocene (Fu et al, 2021).…”

“…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.…”

Paleomagnetic studies of Jurassic and Cretaceous rocks in the Eastern Qiangtang Terrane, Tibetan Plateau, China

“…Rock magnetic analyses suggest that SD to pseudo-single domain (PSD) magnetite is the main magnetic carrier, and the positive fold and reversals tests support a primary origin of the NRM . A second target is the ~126Ma Cretaceous Zaduo granite where microscopic and rock magnetic results support a primary remanent magnetization acquired during its cooling albeit without support of paleomagnetic field tests (Fu et al, 2022b (chapter 1)). In contrast, paleomagnetic results of the limestones of the Jurassic Buqu Formation show that their primary NRM was overprinted by a CRM during the Paleogene, although the samples pass the reversals test (Fu et al, 2022a (chapter 2)).…”

“…5.8, Table S5.3). The granite is classified as S-type with crustal affinity (QGSI, 2005;QGSI, 2014;Fu et al, 2022b), while the volcanic samples were collected from the rhyolite dacite flows and tuff layers ; magnetic mineral amounts vary and result in correspondingly variable paramagnetic contributions.…”

“…In the Zaduo area of the Eastern Qiangtang Terrane, the Permo-Triassic tuff and rhyolite and the Cretaceous granites are reported to retain a primary NRM residing in magnetite Fu et al, 2022b (chapter 1)), while the Carboniferous and the Jurassic limestones have a secondary remanence also residing in magnetite (Fu et al, 2022a (chapter 2); Yu et al, 2022b). The remagnetized characteristic remanent magnetization (ChRM) is residing in stable single domain (SD) and cogenetic superparamagnetic (SP) grains of authigenic magnetite, which is absent in the samples with a primary NRM.…”