Geochemical and petrological diversity of a transcrustal magmatic system driven by mushy magma mixing: Insights from the Triassic dike swarms in East Kunlun orogen, northern Tibetan Plateau

Abstract

Geochemical and petrological diversity within transcrustal magmatic systems usually reflects the magma properties and magmatic processes and thus is critical to understanding the origin of magmatic complexes and the evolution of continental crust. Herein, we present an integrated study on the petrology, mineralogy, geochronology, geochemistry, and Sr-Nd-Hf isotopes of Triassic mafic-felsic dikes in the East Kunlun orogenic belt, northern Tibetan Plateau, to elucidate the nature and evolution of the transcrustal magmatic system. The studied dikes intruding into the granodiorite pluton (ca. 235−233 Ma) comprise coeval ca. 220−218 Ma gabbroic diorite porphyry, diorite porphyry, granodiorite porphyry, and alkali-feldspar granite, resembling composite dike swarms. The macrocrysts in these dikes show various zoning patterns, indicating episodic magma recharge and crystal resorption. The compositional gap between the intermediate-mafic dikes (SiO2 = 52.9−67.8 wt%) and the granitic dikes (SiO2 >75 wt%), as well as their homogeneous whole-rock Sr-Nd isotopes, with (87Sr/86Sr)i = 0.708387−0.710995 and εNd(t) = −5.83 to −4.34, but variable zircon Lu-Hf isotopes, i.e., εHf(t) = −7.67 to −0.36, demonstrates that magma mixing rather than cogenetic fractional crystallization accounts for their origin. In combination with thermobarometric insights, these results suggest that the mafic and felsic parental magmas originating from an enriched lithospheric mantle and ancient continental crust, respectively, were ultimately emplaced and stagnated at varying crustal depths (∼22−30 km and 8−17 km). Subsequently, the felsic magma mush was replenished and rejuvenated by the underplated mafic magma, leading to varying degrees of crystal-melt and/or melt-melt mixing. This mush-facilitated crust-mantle magma mixing is an important mechanism accounting for the compositional diversity of the transcrustal magmatic system.