Petrogenesis of Late Cretaceous andesitic porphyrites in the Zhongcang area: Implications for early uplift and tectonic evolution of the central Tibetan Plateau

Abstract

The central Tibetan Plateau is widely recognized to have undergone significant surface uplift during the Late Cretaceous; however, the deep-seated geodynamic processes driving this uplift remain debated. Magmatic rocks, as direct records of lithospheric processes, provide critical constraints on the nature and evolution of the deep crust and mantle. This study presents a comprehensive petrographic, zircon U-Pb geochronological, geochemical, and Sr-Nd-Hf isotopic investigation of Late Cretaceous andesitic porphyrites from the Zhongcang area in the northern Lhasa Terrane, central Tibetan Plateau. Geochronological results indicate that the Zhongcang andesitic porphyrites crystallized at ∼90 Ma. These rocks are characterized by relatively high SiO₂ (62.00–62.96 wt%) and moderate K₂O (0.96–1.14 wt%) contents, elevated Mg# values (51.6–53.7), high Sr (369–399 ppm) and low Y (10.2–10.9 ppm) concentrations, and high Sr/Y ratios (34.49–38.25), geochemically resembling high-Mg# adakitic rocks. They exhibit enrichment in large-ion lithophile elements and depletion in high-field-strength elements, with Th/U (3.82–4.42) and Nb/Ta (12.5–14.9) ratios comparable to those of continental crust. The heterogeneous zircon Hf isotopic compositions (ε_Hf(t) = −7.7 to +10.7) indicates involvement of both juvenile and ancient crustal components, whereas whole-rock Sr-Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.704716–0.705051, ε_Nd(t) = +0.23 to +0.59) suggest a mainly juvenial crustal source. Pressure estimates from whole-rock and zircon trace element barometry indicate that the crustal thickness of the northern Lhasa Terrane reached ∼70 km during the early Late Cretaceous. Given that this period corresponds to the post-collisional setting of the Qiangtang-Lhasa terrane convergence, we propose that the Zhongcang andesitic porphyrites were derived from magmas generated by the delamination of the thickened juvenile lower crust with locally ancient components, which subsequently interacted with mantle materials during ascent. This lower crustal delamination not only contributed to surface uplift in the central Tibetan Plateau but may have also facilitated lithospheric thinning following crustal thickening.