Remagnetization of carbonate rocks in southern Tibet: Perspectives from rock magnetic and petrographic investigations

Abstract

The latitudinal motion of the Tibetan Himalaya—the northernmost continental unit of the Indian plate—is a key component in testing paleogeographic reconstructions of the Indian plate before the India‐Asia collision. Paleomagnetic studies of sedimentary rocks (mostly carbonate rocks) from the Tibetan Himalaya are complicated by potentially pervasive yet cryptic remagnetization. Although traditional paleomagnetic field tests reveal some of this remagnetization, secondary remanence acquired prior to folding or tilting easily escapes detection. Here we describe comprehensive rock magnetic and petrographic investigations of Jurassic to Paleocene carbonate and volcaniclastic rocks from Tibetan Himalayan strata (Tingri and Gamba areas). These units have been the focus of several key paleomagnetic studies for Greater Indian paleogeography. Our results reveal that while the dominant magnetic carrier in both carbonate and volcaniclastic rocks is magnetite, their magnetic and petrographic characteristics are distinctly different. Carbonate rocks have “wasp‐waisted” hysteresis loops, suppressed Verwey transitions, extremely fine grain sizes (superparamagnetic), and strong frequency‐dependent magnetic susceptibility. Volcaniclastic rocks exhibit “pot‐bellied” hysteresis loops and distinct Verwey transitions. Electron microscopy reveals that magnetite grains in carbonate rocks are pseudomorphs of early diagenetic pyrite, whereas detrital magnetite is abundant and pyrite is rarely oxidized in the volcaniclastic rocks. We suggest that the volcaniclastic rocks retain a primary remanence, but oxidation of early diagenetic iron sulfide to fine‐grained magnetite has likely caused widespread chemical remagnetization of the carbonate units. We recommend that thorough rock magnetic and petrographic investigations are prerequisites for paleomagnetic studies throughout southern Tibet and everywhere in general.

Paleomagnetic and Geochronologic Results of Latest Cretaceous Lava Flows From the Lhasa Terrane and Their Tectonic Implications

“…Chen et al () reported Cretaceous paleomagnetic results of three limestone sites (22 specimens) from the LT. The results have no fold test and no rock magnetic studies (e.g., Huang, Lippert, Zhang, et al, ) and yield a paleolatitude of 11.3° ± 13.1°N for the reference point. In addition, similar ChRM directions of 11 sandstone sites from the western part of the Qiangtang terrane have also been reported (Chen et al, ).…”

“…Based on the position of the India‐Asia collision, the evolution of the TH, which is traditionally regarded as the northernmost continental unit of India, can be used to date the collision. There have been many paleomagnetic studies on Paleozoic‐Paleogene rocks from the TH, but many of these rocks have been remagnetized (e.g., Appel et al, ; Besse et al, ; Huang, van Hinsbergen, Dekkers, et al, ; Huang, Lippert, Dekkers, et al, ; Huang, Lippert, Zhang, et al, ; Klootwijk & Bingham, ; Liebke et al, ; Schill et al, ; Tong et al, ). Importantly, the paleomagnetic results from the late Cretaceous and Paleocene limestones reported by Patzelt et al () and Yi et al () have previously been regarded as primary remanences.…”

“…Importantly, the paleomagnetic results from the late Cretaceous and Paleocene limestones reported by Patzelt et al () and Yi et al () have previously been regarded as primary remanences. However, these results have been reevaluated for their primary origin and found to be unequivocally remagnetized during or after the limestones were folded (Huang, Lippert, Dekkers, et al, ; Huang, Lippert, Zhang, et al, ). Recently, Yi et al () argued that the associated secondary remanence was most likely acquired before the folding.…”

“…However, only paleomagnetism can quantitatively constrain the paleogeographic evolution of the precollisional leading edges of Asia and India (e.g., Lippert et al, ). Because India‐Asia convergence during the late Cretaceous and Palaeocene was initially north‐south directed, the overlapped paleolatitudes of the TH with the LT defines the collision in a paleomagnetic reference frame (e.g., Chen et al, ; Dupont‐Nivet et al, ; Huang, Van Hinsbergen, Lippert, et al, ; Lippert et al, ), if all the rocks preserve a primary remanence (Huang, Lippert, Dekkers, et al, ; Huang, Lippert, Zhang, et al, ).…”

Comment on “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision” by Huang et al.

“…Their result of best grouping at ~25% of unfolding yields a paleolatitude of ~20°N, i.e., compatible with the two‐stage collision model of van Hinsbergen et al [], which they obviously regard as a valid scenario. Huang et al [, ] also present a large set of new rock magnetic results which they interpret as evidence for remagnetization. In this comment we discuss the validity of the fold tests and the possibility of remagnetization.…”