Emplacement age of leucogranite in the Kampa Dome, southern Tibet

Abstract

Himalayan leucogranite is an important rock to decipher the orogenic evolution of the Himalayan orogen. It has been recognized that the leucogranite occurred as two separate, but parallel belts within Himalaya. Leucogranite in the north belt is mainly exposed an intrusion component within the gneissic dome. Although most domes have been intensively studied, the Kampa Dome, located in the east of Southern Tibet, is less investigated due to its inaccessibility. In this study, we conducted a comprehensive set of U(–Th)–Pb zircon, monazite and xenotime dating and trace element composition analyses using laser ablation technique, in order to constrain the emplacement age and lithological nature of the Kampa leucogranite. Zircons from gneiss are mainly magmatic and define a protolith age of ca. 498Ma, but no Cenozoic age is documented. Zircons from leucogranite display inherited magmatic core overgrown by thin dark rim, which occasionally gives a much younger age of 27.7±1.0Ma. However, monazites from leucogranite yield a Th–Pb age of 25.7±0.5Ma to 26.8±0.4Ma, which is comparable to the U–Pb age of 24.8±0.4Ma obtained from xenotimes. For the granitic gneiss into which the leucogranite intruded, monazites and xenotimes give ages of 25.8±0.6Ma to 26.6±0.8Ma and 25.0±0.4Ma to 26.0±1.0Ma, respectively. These ages are comparable to those obtained from the leucogranite, and suggest that the metamorphic monazite and xenotime from gneiss formed during the leucogranite emplacement. Trace element analyses indicated that zircons from leucogranite have high P (up to 3721ppm) and REE concentrations, but have low in Th/U ratios (0.04–0.16). Similarly, monazites from leucogranite generally show extreme HREE depletion and significant negative Eu anomaly with some displaying significantly tetrad effect. These geochemical features indicated that the Kampa leucogranite was highly fractionated during its evolution.