Palaeoarchaean materials in the Tibetan Plateau indicated by zircon

The early tectonic evolution of the Lhasa Terrane remains poorly understood, although evidence for a substantial prehistory has been reported recently. We have carried out <i>in situ</i> zircon U–Pb dating and Hf–O isotopes of late Early Cretaceous monzogranites and get a surprising package of inherited zircons, not only because of their age profile, but also because the oldest Palaeoarchaean zircons are euhedral. The discovery of Palaeoarchaean euhedral zircons in the region suggests the presence of extremely old rocks in Tibet. Zircons from the Nagqu monzogranite yield five age peaks at ~3.45 Ga, ~2.56 Ga, ~1.76 Ga, ~900 Ma, and ~111 Ma. They have large variations in <i>ε</i>_Hf(<i>t</i>) values (−45.1–9.2) and old Hf model ages (924–3935 Ma), with variable <i>δ</i>¹⁸O values of −5.80–9.64. Palaeoarchaean zircons (~3.20–3.45 Ga) are euhedral with magmatic zircon characteristics. One of the grains has negative <i>ε</i>_Hf(<i>t</i>) value (−4.8), old Hf model age (3935 Ma), and high positive <i>δ</i>¹⁸O value (7.27), which suggests an ancient crustal origin. The source of Palaeoarchaean euhedral zircons should be proximal to prevent long-distance transport and abrasion, whereas the late Early Cretaceous monzogranites are I-type. Thus, Palaeoarchaean euhedral zircons are most likely captured from the country rocks by assimilation at depth or may be relics of previous magmatic zircons. Notwithstanding their exact history, Palaeoarchaean euhedral zircons indicate Palaeoarchaean materials near Nagqu in the Tibetan Plateau. The inherited zircons also experienced a Late Palaeoproterozoic event (~1.76 Ga) likely related to the evolution of the India block. The peak at ~900 Ma suggests affinity to the Qiangtang and Himalaya blocks.

拉萨地体（Lhasa Terrane）的早期构造演化迄今仍不甚明晰，尽管近期已有大量关于其早前演化史的研究报道。本研究对早白垩世晚期二长花岗岩开展了原位（in situ）锆石U-Pb定年与Hf-O同位素分析，获得了一组令人意外的继承锆石：不仅其年龄谱特征反常，最古老的古太古代（Palaeoarchaean）锆石还呈自形晶产出。该区域古太古代自形锆石的发现，指示青藏高原腹地存在极古老的岩石。那曲二长花岗岩中的锆石呈现5个年龄峰值，分别约为3.45 Ga、2.56 Ga、1.76 Ga、900 Ma及111 Ma；其εHf(t)值变化范围宽（-45.1~9.2），Hf模式年龄跨度为924~3935 Ma，δ¹⁸O值介于-5.80~9.64之间，存在显著分异。年龄约3.20~3.45 Ga的古太古代锆石均为自形晶，具备岩浆锆石特征。其中一颗锆石具有负的εHf(t)值（-4.8）、极老的Hf模式年龄（3935 Ma）以及高正值的δ¹⁸O值（7.27），指示其源自古老地壳。古太古代自形锆石的物源应属近源，以避免长距离搬运与磨蚀；而该早白垩世晚期二长花岗岩属于I型花岗岩。因此，这些古太古代自形锆石最可能是在深部同化作用过程中从围岩中捕获的，或是早期岩浆锆石的残留体。尽管其具体成因仍待厘清，这些古太古代自形锆石仍指示青藏高原那曲地区附近存在古太古代物质。继承锆石还记录了一次晚古元古代事件（约1.76 Ga），该事件可能与印度地块的演化相关。约900 Ma的年龄峰值则指示其与羌塘地块及喜马拉雅地块具有亲缘性。