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

The Paleocene lavas from Dianzhong Formation (E(1)d) 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 & SIM;52 Ma dike intrusions into the E(1)d that accompanied a massive ignimbrite eruption deposited above the E(1)d producing heating up to 300 & DEG;C; and a secondary event related to the 42-27 Ma southward overthrusting of the basin, heating the E(1)d up to 130-145 & DEG;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 E(1)d 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.