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

The paleolatitudinal motion of the Tibetan Himalaya - the northernmost continental unit of the Indian plate - is a key component in the reconstruction of India-Asia collision history. Paleomagnetic studies of sedimentary rocks from Tibetan Himalaya, mostly carbonates, are complicated by pervasive remagnetization. It is sometimes uncovered by classical paleomagnetic field tests, but can also remain masked: when remagnetization would have occurred before the folding. Here we present a systematic rock magnetic and petrographic investigations of 117 samples from Jurassic to Paleocene strata of the Tibetan Himalaya exposed in the Tingri area. The four sampled sections, located at 28.48°N/87.04°E, 28.71°N/86.72°E, 28.69°N/86.71°E, and 28.68°N/86.73°E with increasing stratigraphic level, are composed mainly of carbonate rocks, except some clastic units of Oxfordian-Albian or Danian age. Our rock magnetic tests (thermomagnetic runs, hysteresis measurements, low-temperature magnetic remanence and susceptibility experiments) reveal that the dominant magnetic carrier of these carbonate rocks is magnetite, which has the characteristic rock magnetic properties of remagnetized carbonates, including `wasp-waisted' hysteresis loops, suppressed Verwey transitions, high frequency-dependent magnetic susceptibility, and extremely fine grain sizes with >70% of ferrimagnetic material being superparamagnetic at room temperature. SEM observations and EDS analysis show that magnetite grains are pseudomorphs of early diagenetic pyrite. We suggest that oxidation of early diagenetic pyrite to fine-grained magnetite is responsible for widespread chemical remagnetization of the Tibetan Himalayan carbonate rocks. If oxidation occurred pre-folding, the remagnetization could be easily mistaken for a primary magnetization in case only fold tests are used to constrain the timing of the natural remanent magnetization (NRM). Therefore, thorough rock magnetic and petrographic studies are prerequisites for paleomagnetic studies of carbonate rocks throughout southern Tibet. Although most carbonate rocks in southern Tibet are not suitable for paleogeographic reconstruction, we note that the Early Cretaceous volcanoclastic sandstones within the sampled sections appear to retain a primary NRM.