Testing the Use of Zircon Chemistry to Infer Crustal Thickness in the Gangdese Arc, Southern Tibet

The Gangdese Arc spans the ~2000 km-long southern margin of the Tibetan Plateau and was formed between ~50 and 150 million years ago (Ma), when the Indian plate subducted beneath the Asian plate. Recent work (Tang et al., 2021; Geology) near Lhasa, Tibet, in the East-central part of the arc has used rare-earth element chemistry of zircon crystals in modern sediments to infer crustal thickness changes over time. They infer major pulses of magmatism at ~45-50 and ~65-90 Ma, and increases in crustal thickness between ~75-100 Ma and ~30-55 Ma. We tested Tang et al.’s interpretations by measuring zircon age and chemistry in sands collected near Lhasa and in an area ~1000 km to the west. All data show a major pulse of magmatism at ~45-50 Ma, but the older 75-100 Ma pulse is absent in the west. We find no consistent trends in zircon rare-earth element chemistry over time; rather (a) different magmatic centers have different zircon compositions, and (b) some zircon compositions correlate with Ti content, which is temperature-sensitive. Evidently, zircon chemistry depends on factors other than crustal thickness, including temperature.