Cretaceous (~100 Ma) high-silica granites in the Gajin area, Central Tibet: Petrogenesis and implications for collision between the Lhasa and Qiangtang Terranes

Abstract

High-silica granites that occur as isolated plutons are widely distributed in collisional orogenic belts and are considered a hallmark of continental collision processes. The late Mesozoic collision between the Lhasa and Qiangtang Terranes along the Bangong–Nujiang Suture Zone was key event in the evolution of the Tibetan Plateau; however, the timing of this collision remains debated. Here, we report the ages and geochemical characteristics of granites from the Gajin area in the north-central Lhasa Terrane, central Tibet. Zircon UPb dating reveals that the Gajin granites formed at ~100 Ma. These granites exhibit uniformly high SiO2 contents (74.9–78.4 wt%), are weakly peraluminous compositions, and have high K2O, moderate Na2O and Al2O3, and low Fe2O3T, MgO, and CaO contents. They are characterized by enrichments in Rb, Cs, and U, and by depletions in Zr, Hf, Nb, and Ti. These characteristics, in combination with high Rb/Sr ratios and pronounced negative Eu anomalies, indicate that the Gajin granites are highly fractionated rocks. The granites have enriched whole-rock Nd (εNd[t] = −4.3 to −2.9) and relatively depleted and slightly variable zircon Hf (εHf[t] = −1.0 to +5.5) isotope compositions. The Gajin granites were probably generated by high degrees of crystal fractionations of magma produced by partial melting of mature crustal sources. The Cretaceous magmatic rocks in the north-central Lhasa Terrane record a magmatic lull between ~100 and 85 Ma, which corresponds to a transition in both geochemical composition (e.g., HfNd isotopes) and rock association at ~100 Ma. Combining the above characteristics with the development of a regional angular unconformity around the time of the formation of the granites, we suggest that the initial continental collision between the Lhasa and Qiangtang Terranes probably occurred between 110 and 90 Ma. Therefore, the ~100 Ma high-silica granites in the Gajin area were formed during the continuing continental convergence that followed the initial collision between the Lhasa and Qiangtang Terranes. The generation of discrete and monotonous high silica granites may constitute evidence for a significant tectonothermal transition in evolution of the continental crust.