Dual polarity directions in basaltic-andesitic dykes-reversal record or self-reversed magnetization?

Abstract

S U M M A R YRock magnetic analyses were performed on basaltic-andesitic dyke samples from the Lhasa Block, which were previously used for palaeomagnetic investigations of the India-Asia collision. The aim of the study is to prove whether antipodal directions separated in these samples represent different polarities of the Earth's magnetic field or a self-reversal magnetization. Reflected light microscopy, domain pattern observations and energy dispersive X-ray analyses revealed two generations of titanomagnetite (TM) grains, one consisting of largesized (∼20 µm to more than 100 µm) and inhomogeneous Ti-poor TM, and another with small (smaller than ∼10 µm) and rather homogeneous Ti-rich TM grains. Partial thermoremant magnetization experiments and reflected light microscopy show that magnetostatic or superexchange interaction between these two phases is unlikely. Temperature dependence of saturation magnetization and low-temperature curves of isothermal remanent magnetization disprove a possible N-type behaviour of the samples. A self-reversal due to ionic reordering can be ruled out as it requires a high degree of oxidation of the Ti-rich TMs, which was not observed. In summary, a self-reversal magnetization is very unlikely in the studied dyke samples, and therefore the observed antipodal directions most probably represent a record of different polarity epochs of the Earth's magnetic field. This conclusion implies that the time of remanence acquisition in the studied dykes was sufficiently long to average out paleosecular variation, supporting the significance and reliability of the palaeomagnetic results.Key words: Paleomagnetism applied to tectonics; Reversals: process, timescale, magnetostratigraphy; Rock and mineral magnetism. I N T RO D U C T I O NThe evolution of the Himalaya-Tibetan orogen is of particular interest for understanding continental collision processes causing crustal uplift and associated climate change. Palaeomagnetism can provide direct constraints for the age of the India-Asia collision by comparing palaeolatitudes of the pre-collisional continental margins of India and Eurasia. The extent and latitudinal position of the passive northern Indian margin was determined by Patzelt et al. (1996) and recently confirmed by Yi et al. (2011). Some earlier results from the Lhasa Block (Westphal et al. 1983; Achache et al. 1984;Chen et al. 1993), which formed the southern rim of Eurasia before collision, were published in the 1980s and 1990s. A recent paper of Aitchison et al. (2007), postulating a 34 Ma age for the India-Asia continental collision, revived an intensive discussion on the evolution of the collision and stimulated several new palaeomagnetic studies on the Lhasa Block (Chen et al. 2010;Dupont-Nivet et al. 2010;Liebke et al. 2010;Sun et al. 2010;Tan et al. 2010). The new results largely improved the database and allowed a more significant determination of the position of Eurasia's southern margin. A compilation of palaeolatitudes supports a collision age between about 60 and 48 Ma. Alm...

Paleomagnetic and 40Ar/39Ar geochronological results from the Linzizong Group, Linzhou Basin, Lhasa Terrane, Tibet: Implications to Paleogene paleolatitude and onset of the India–Asia collision

“…The paleomagnetic investigations of this study and Chen et al (2010), Liebke et al (2010Liebke et al ( , 2012 and Sun et al (2010) are all consistent in confirming that the Lhasa Terrane was located around 10 ± 5°N during formation of the Dianzhong and Nianbo formations ($64-50 Ma). This estimate is found to be compatible with other Cretaceous observations (Section 6.1).…”

“…The collective results from the Dianzhong and Nianbo formations of the Linzizong Group ($64-50 Ma) imply a low paleolatitude of 10 ± 5°N for the southernmost leading edge of the Asian continent (Chen et al, 2010;Liebke et al, 2010Liebke et al, , 2012Sun et al, 2010, and this study). Conservative assessment of deformation resulting from the India-Asia collision presupposes no paleomagnetically-detectable crustal shortening or thickening within the stable Indian Craton.…”

“…More recently, a comprehensive review on the 110-50 Ma paleomagnetic data of the Lhasa Terrane proposed a quasi-stationary paleolatitude of $20°N (van Hinsbergen et al, 2012;Lippert et al, 2014). This estimate is, however, $10-15°N higher than the one resolved from the lower part of the Linzizong Group (Najman et al, 2010), in which paleolatitudes for the southern margin of the Lhasa Terrane yielded $6.7°N for the Dianzhong Formation (Chen et al, 2010), $11.6°N for the Nianbo Formation (Chen et al, 2010;Sun et al, 2010), and $13.2°N for dykes intruded into the Nianbo Formation (Liebke et al, , 2012. This discrepancy in paleolatitudes might result from (i) complexities of paleomagnetic data from the Pana Formation, (ii) inadequate sampling of the Dianzhong and Nianbo formations from the Linzhou Basin, http://dx.doi.org/10.1016/j.jseaes.2014.09.007 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved.…”

What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

“…The acquisition of the new TVRM likely occurred after E 2p had been tilted ~30° toward the north. This new TVRM might also be carried by the dykes intruded in E 1d , because the in situ magnetization directions of the dykes are similar to that of the in situ LTC of the E 1d lavas; the magnetic carrier is Ti‐rich titanomagnetite in both units [ Liebke et al ., ; Liebke et al ., ].…”

Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India‐Asia collision?

“…Paleomagnetic results to address this problem in the 1980s and early 1990s [ Zhu et al ., ; Pozzi et al ., ; Westphal and Pozzi , ; Achache et al ., ; Lin and Watts , ; Chen et al ., ] were partly inconclusive. Recently, this research question has been taken up again by several groups [ Chen et al ., ; Dupont‐Nivet et al ., ; Liebke et al ., ; Sun et al ., ; Tan et al ., ; Chen et al ., ; Liebke et al ., ; Sun et al ., ; van Hinsbergen et al ., ; Huang et al ., ; Lippert et al ., ; Yang et al ., ]. Surprisingly, and rather paradoxically, the paleolatitude estimates in these recent studies vary from 5°N to 30°N despite the fact that the underlying data are all from the same rock sequences—the regionally extensive Linzizong Group of the Lhasa terrane, which ranges in age from ~69 to ~48 Ma.…”