Petrogenesis of Triassic crystal-rich and crystal-poor volcanic rocks in the East Kunlun orogenic belt: Evidence from in-situ zircon-apatite-quartz and whole-rock geochemical compositions

Abstract

Coeval crystal-rich and crystal-poor volcanic rocks are widely regarded as natural archives for exploring the disaggregation of mush and the relevant crustal crystal-melt-volatile interactions. However, the formation mechanisms and genetic links of both types of volcanic rocks remain elusive. This study integrated zircon UPb geochronology and Hf isotopic compositions, in-situ zircon-apatite-quartz compositions, and whole-rock geochemistry for Triassic crystal-rich and crystal-poor volcanic rocks in the Dulan area of the East Kunlun Orogen Belt (EKOB). The results show that these volcanic rocks are dominated by rhyolitic ignimbrites with contrasting crystal proportions. Compared with crystal-poor volcanic rocks, crystal-rich volcanic rocks display relatively lower SiO₂ and total rare earth elements, but higher TiO₂ and MgO. However, both types of volcanic rocks exhibit similar zircon UPb ages, REE and trace element patterns, and zircon ε_Hf(t) values, indicating derivation from a common magmatic source, most plausibly linked to partial melting of mafic lower-crustal components. Zircon textures and in-situ compositions also indicate that they have a broadly overlapping evolutionary trend, but those crystal-rich volcanic rocks had experienced a longer duration of magmatic evolution. The presence of zircon antecrysts and feldspar glomerocrysts supports a long-lived (∼2 Myr) crystal-rich mush reservoir. Compositional variations of whole-rock, zircon, and apatite further indicate the mush reservoir underwent complex crystal-melt segregations, primarily governed by the fractionation of plagioclase and recorded by co-crystallizing zircon and apatite. Compared with the crystal-poor volcanic rocks, the crystal-rich volcanic rocks contain abundant quartz and apatite that commonly shows reverse zoning, and record relatively high-temperature signatures. Combined with new regional studies, we propose two-stage crystal-melt segregations to generate Triassic crystal-rich and crystal-poor volcanic rocks in the EKOB. Early crystal-melt segregation favored extraction of relatively H₂O-rich, melt-dominated magma to form the crystal-poor volcanic rocks, whereas subsequent injections of hydrous mafic magma recharge rejuvenated the mush and triggered the later segregation, ultimately generating the crystal-rich volcanic rocks.