Multi-stage crustal magma reservoirs of ultrapotassic rocks recorded by zoned clinopyroxene

Abstract

(a) Main crustal processes within magmatic plumbing system underneath the ultrapotassic Zhuopan pluton . (b) Petrogenetic model for the 37–30 Ma mafic potassic–ultrapotassic volcanic and plutonic rocks in western Yunnan, highlighting their distinct magmatic ascent paths and processes in the crust. • Clinopyroxene zoning and textures record a complex magma history. • Clinopyroxenes crystallized at two crustal levels with distinct depths. • Crustal contamination and magma recharge took place in the magma system. • The Zhuopan pluton originated from a metasomatized lithospheric mantle. • Potassic–ultrapotassic volcanic and plutonic rocks differ in element systematics. Ultrapotassic rocks that are rare and volumetrically minor have attracted much attention due to their genetic link with the terrestrial mantle evolution and specific tectonic settings. However, crustal processes that may be involved in the generation of these rocks are poorly known despite extensive studies on their mantle sources. The ultrapotassic Zhuopan pluton in western Yunnan, southeastern Tibetan Plateau, consists chiefly of clinopyroxene-rich mafic–intermediate rocks and well preserves clinopyroxene textures and zoning, implying its potential record of valuable information on crustal processes. This pluton was emplaced at ∼ 36 Ma, suggesting its formation in the post-collisional setting associated with the India–Asia collision. Clinopyroxenes have three distinct types and form complex zoning patterns, with colorless clinopyroxene at the core, pale green at the mantle, and dark green at the rim. The clinopyroxene–liquid thermobarometer indicated that the clinopyroxenes crystallized in two magma reservoirs at depths of ∼ 18 and ∼ 10 km. The in situ clinopyroxene Sr isotopes revealed that wall-rock contamination had taken place and then was punctuated by recharge with a relatively evolved magma, leading to the resorption of pre-existing clinopyroxenes. The Zhuopan plutonic rocks were formed through evolution within multi-stage crustal reservoirs from the primary mafic magma derived from a subduction-metasomatized lithospheric mantle. Together with the coeval (37–30 Ma) potassic–ultrapotassic volcanic counterparts, we determined that the two differ in elemental systematics. This may have resulted from distinct magmatic ascent paths and processes in the crust. We also concluded that the generation of these coeval potassic–ultrapotassic rocks was triggered by massive lithospheric delamination.