Petrogenesis and geodynamic mechanisms of the Late Cretaceous magmatic ‘flare-up’ in the southern Lhasa Terrane, Tibet

Abstract

Exploring magmatic ‘flare-up’ events can be used to unravel dynamic Earth processes including plate subduction and crustal growth. However, the cause and mechanism of magmatic ‘flare-up’ remain unresolved. The abundant magmatic records that are preserved in the Lhasa Terrane on the southern Tibetan Plateau make it an important region to study the episodic nature of the secular magmatism. This paper presents geochronological, mineralogical, and geochemical data that describes the Dazhuka diorites in the central-western and Sangri granites in the eastern parts of the southern Lhasa Terrane. Zircon LA–ICP–MS U Pb data indicate that these rocks were generated around ~95–90 Ma. Both are enriched in large ion lithophile elements and depleted in high field strength elements, indicating geochemical affinity with subduction-related magmatic rocks. The Dazhuka diorites have low SiO 2 contents (54.49–56.72 wt%), with moderate Mg# (41.7–42.7) and rare earth element fractionation ((La/Yb) N = 6.9–8.6), and homogeneous whole-rock 87 Sr/ 86 Sr i (0.704204–0.704480), ε Nd (t) (+3.5 − +4.1), and zircon ε Hf (t) (+10.2 − +13.3). These features suggest that the Dazhuka diorites originated in the spinel-garnet transition zone within the lithospheric mantle of a mantle wedge environment. The Sangri granites have high SiO 2 (64.36–69.54 wt%), Al 2 O 3 (14.27–15.61 wt%), and (La/Yb) N (4.6–49.6) and Sr/Y (34.7–119.3) ratios, indicating geochemical affinity with adakites. Low MgO (1.3–2.22 wt%) and depleted Sr-Nd-Hf isotopes (whole-rock 87 Sr/ 86 Sr i = 0.704099–0.705453, ε Nd (t) = +2.9 − +3.5, zircon ε Hf (t) = +7.1 − +9.3) suggest that the granites were derived from oceanic crust. The types of magmatic rocks that are distributed over the central-western and the eastern parts of the southern Lhasa Terrane differ, with higher magmatic temperatures in the east as compared to the central-west. These results indicate that different areas in the southern Lhasa Terrane may have experienced different geodynamic processes, with oceanic ridge subduction in the eastern part of the southern Lhasa Terrane and normal oceanic plate subduction in the central-western part. These different types of subduction processes are likely to have caused the magmatic ‘flare-up’ and variation in the magmatism of southern Tibet. • Both Dazhuka diorites and Sangri granites formed in early Late Cretaceous. • Dazhuka diorites were derived from the lithospheric mantle. • Sangri granites with adakitic affinity were sourced from the oceanic crust. • Multiply Neo-Tethys Ocean plate subduction processes result in magmatic ‘flare-up’.