Are carbonates from the India-Asia collision remagnetized ?
Résumé
Widespread carbonate rocks from the Tibetan plateau have been extensively used to constrainterrane paleolatitudes involved in the India-Asia collision. However, their reliability in preserving aprimary magnetization has been recently put into question.A transformation of pyrite tomagnetite has been recently proposed as a cause for late re-magnetizations in Paleocene TethyanHimalaya carbonates (1) and late Triassic carbonates from the Qiantang (2), thus discarding suchCharacteristic Remanent Magnetizations (ChRM) for tectonic purposes. We have re-examined thepaleomagnetic data obtained on late Triassic carbonate rocks from the Qiantang. Our SEMobservations indicate pristine pyrite in non-weathered carbonate rocks. Optical microscopeobservations in reflected light demonstrate that pyrite, when it is weathered, is transformed toiron hydroxides minerals but not to magnetite. This is at odds with previously proposed pyrite tomagnetite transformation hypothesis mainly based on interpretations of Scanning ElectronMicroscope data (SEM/EDS). We thus interpret the ChRM more likely related to an early diageneticmagnetization of Late Triassic age. Knowing that the arguments put forward for a remagnetizationof Triassic carbonates are the same as those proposed for the remagnetization of Paleocenecarbonates, the ChRM in some Paleocene carbonates could also be of early diagenetic origin.However, there is also a growing number of studies where remagnetization is obvious in theTethyan Himalaya and undetected remagnetizations (3) are likely the cause of the largedifferences in the estimation of the size of Greater India. These examples show the urgent need topublish the complete demagnetization dataset in an open database like MAGIC or the FAIR datainitiative from (4) in order to reassess previous interpretations if we want to solve problems likethe size of Greater India and hypothesis like the Greater India basin
