Complex magmatic processes recorded by clinopyroxene phenocrysts in a magmatic plumbing system: A case study of mafic volcanic rocks from the Laiyang Basin, southeastern North China Craton

Abstract

Understanding the evolution of magmatic plumbing systems requires an understanding of the mechanisms of magma ascent from the magma source to the surface. We undertook textural and in situ geochemical studies of clinopyroxene phenocrysts in Early Cretaceous mafic volcanic rocks from the Laiyang Basin, China. Four distinct clinopyroxene populations were recognized based on their textures and zoning patterns in the mafic volcanic rocks (types 1, 2, 3, and 4). Type 1 is reversely zoned and contains rounded, irregularly resorbed, low-Mg cores (Mg# = 60–75) surrounded by clear, high-Mg mantles (Mg# = 75–90). Type 2 phenocrysts are typically normally zoned and characterized by relatively high-Mg cores (Mg# > 85) and thin lower Mg# rims (Mg# = 70–85). Type 3 comprises strong oscillatory zoned crystals (Mg# = 72–85) that are generally normally zoned and, in some cases, exhibit substantial sector zoning. Type 4 crystals are euhedral and unzoned phenocrysts. The textural and compositional features of the clinopyroxenes record complex magmatic processes including fractional crystallization, recycling of early formed crystals, magma convection, and magma recharge and subsequent mixing. Clinopyroxene–melt equilibrium pairs in the volcanic rocks yielded a relatively wide range of temperatures (984–1264°C) and pressures (0.1–10.1 kbar). The P–T array defines two main magma storage zones: (1) a deep, high-T, magma reservoir (~22–28 km), and (2) a shallow, low-T, magma reservoir (~8–19 km). At pressures of >3 kbar, the amount of water in melts in equilibrium with the clinopyroxene is in the range 1.5–3.0 wt.%, but at pressures of <3 kbar, the melt water contents decrease to 0.1 wt.%, indicative of extensive magma degassing. The clinopyroxene phenocrysts have a wide range of 87Sr/86Sr ratios from 0.70683 to 0.71366. The broad range of 87Sr/86Sr ratios (0.70796–0.71171) for the most primitive clinopyroxene (Mg# > 89) might have been inherited from a heterogeneous mantle source. However, some type 1 crystals with more radiogenic Sr isotopic compositions may record assimilation of crustal material during evolution of the mantle-derived magma. The textural and compositional features of the studied clinopyroxene phenocrysts suggest that Laiyang volcano is fed by a vertically extensive magmatic plumbing system that extends from shallow to deep levels, in which geochemically distinct magmas crystallized and mixed under variable physicochemical conditions.