Petrogenesis of post-collisional felsic rocks and implications for continental growth: Insights from a Paleogene monzonitic intrusion, southeastern Tibetan Plateau

Abstract

It is unclear if substantial continental growth occurs after continent–continent collision. Paleogene magmatism immediately after the India–Asia collision produced widespread felsic intrusions in the southeastern Tibetan Plateau (TP) and these intrusions provide an excellent opportunity to study continental growth in a post-collisional setting. In this paper, we report new petrological and geochemical data for a representative intrusion, the Paleogene Weishan monzonitic intrusion in the Simao Block of the southeastern Tibetan Plateau. The quartz monzonite porphyries in the Weishan intrusion are geochemically similar to adakitic rocks. They have enriched Sr–Nd isotopic compositions, with (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7071–0.7072 and Ɛ_Nd(t) values from −4.0 to −3.5. These features are indicative of an origin of these rocks from melts that were produced by the interaction between melts derived from delaminated lower crust and overlying mantle peridotite. The geochemical similarities between the Weishan intrusion and coeval felsic rocks in the southeastern TP suggest a common origin. Detailed quantitative modeling demonstrates that only a small amount of mantle component (less than 20 %) was involved in the formation of these rocks, and at the same time, a large volume of the lower crust (with a thickness of roughly 10 km) was recycled into mantle by delamination. Therefore, we suggest that substantial continental growth might not have occurred in the southeastern TP region during the Paleogene after continent–continent collision.