Apatite and Zircon as key indicators for effusive-explosive transition in the Leningcou Volcano: Middle Triassic Volcanic activity in the East Kunlun Orogenic Belt

Volcanic eruptions play a critical role in the Earth's system by linking the lithosphere, atmosphere, hydrosphere, and biosphere. Determining the mode of volcanic eruptions is essential for assessing the potential hazards posed by volcanic activity to humans. Although significant progress has been made in monitoring active volcanoes and analyzing them with modern techniques, our understanding of the effusive-explosive transition in ancient volcanic eruptions remains limited. This study focuses on the Leningcou ancient volcano in the Triassic Chachaxiangka area of the East Kunlun Orogenic Belt on the northern Tibet Plateau. It aims to investigate the products around the ancient volcanic caldera through the study of accessory minerals and to identify potential factors influencing the effusive-explosive transition in volcanic eruptions.

Our research reveals that the Leningcou ancient volcano is composed of intermediate to acidic pyroclastic rocks and lava, with lava mainly exposed in the center of the ancient crater. Zircon U-Pb dating results indicate that this volcano was active during the Middle Triassic, approximately 242 Ma. Combined with whole-rock Sr-Nd isotopes and apatite Nd isotopic analysis, we suggest that the volcanic products originate from a similar crust-mantle mix source region and experienced varying degrees of crustal contamination. Using apatite hygrometers, we calculated the water content of the melts from the volcanic crater products, finding similar values with an average ranging between 3.3 and 3.7 wt% for both effusive and explosive volcanic rocks. The water content in the rhyolite melt of effusive eruptions reaches the highest value of 3.7 wt%. By analyzing redox-sensitive elements in apatite and zircon, we found that rhyolitic melts from explosive eruptions exhibit lower oxygen fugacity (ΔFMQ <0), whereas the andesitic and rhyolitic melts of effusive eruptions show higher oxygen fugacity (ΔFMQ >1).

Our research proposes a new model suggesting that a recharge event of mafic magma in the shallow crustal magma reservoir reactivated the crystal mush and triggered explosive eruptions. Higher melt water content introduced by new magma, along with a more open magma reservoir, are often key to factors in the transition from effusive to explosive eruptions.