Remagnetization Under Hydrothermal Alteration of South Tibetan Paleocene Lavas: Maghemitization, Hematization, and Grain Size Reduction of (Titano)magnetite.

The Paleocene lavas from Dianzhong Formation (E1d) in Linzhou basin of southern Lhasa terrane are a key target for paleomagnetic investigations into the timing and paleolatitude of the initial India‐Asia collision. Controversy exists, however, on whether these rocks preserve a primary remanent magnetization. Here we reanalyze previously published thermal demagnetization data and report detailed rock magnetic results and petrographic observations of these rocks. We find that the original magnetic carrier, a magmatic multidomain Ti‐poor titanomagnetite, underwent significant grain size reduction and was variably reacted to single‐domain maghemite and nano‐hematite. Such strong alteration may have resulted from successive hydrothermal events: a first event related to the ∼52 Ma dike intrusions into the E1d that accompanied a massive ignimbrite eruption deposited above the E1d producing heating up to 300°C; and a secondary event related to the 42–27 Ma southward overthrusting of the basin, heating the E1d up to 130–145°C. Unblocking/inversion temperature spectra of the authigenic maghemite and nano‐hematite overlap with those of the titanomagnetite, implying that the primary remanence of the E1d lavas has been contaminated or replaced by thermoviscous and chemical remanent magnetizations. Thus the isolated characteristic remanent magnetization from these rocks, whether slightly or completely altered, cannot be considered primary and should not be used for paleolatitudinal determination. Our study confirms that hydrothermal alteration can seriously jeopardize the remanence carried by titanomagnetite and thus should be tested for paleomagnetic investigations of rock units from tectonically active areas. Plain Language Summary: The study of past magnetic directions preserved in rocks (paleomagnetism) enables to constrain the paleolatitudes of the southern Asian margin where the rocks formed and is thus key to understand the timing and latitude of the initial India‐Asia collision. However, paleomagnetic studies of Paleogene rocks of southern Tibet have returned a surprisingly wide spread of paleolatitude data. One possible explanation is unrecognized remagnetization, a process that may relate to subsequent hydrothermal alteration. Such hydrothermal alteration has recently been documented for Paleocene lavas in the Linzhou basin, paleomagnetic signals from which have previously been interpreted either as primary or secondary (re)magnetizations. Here we analyze previously published and newly acquired paleomagnetic, rock magnetic, and petrographic data of these rocks to identify whether hydrothermal alteration caused remagnetization, and how such events may be fingerprinted. We find that the original magnetic carrier is Ti‐poor titanomagnetite. Hydrothermal alteration, however, led to significant changes in these titanomagnetite crystals and mineral transformations to maghemite and hematite, associated with acquisition of secondary magnetizations with the magnetic behaviors similar to the primary magnetization. This can be misleading for the interpretation of the magnetic directions obtained from these rocks and imply that they are not suitable for paleolatitudinal determination. Key Points: Hydrothermal alteration intensively destroyed the original multi‐domain titanomagnetite and recorded magnetization in the Dianzhong lavasTitanomagnetite experienced serious grain size reduction and has variably converted to authigenic single‐domain maghemite and nano‐hematitePrimary remanent magnetization has been replaced or contaminated, these lavas should not be used to determine Lhasa terrane paleolatitude [ABSTRACT FROM AUTHOR]