Quantitative petrological evidence for the origin of plagioclase megacrysts: Constraints from the Wulong gabbro, North China Craton

The Paleoproterozoic Wulong porphyry gabbro dykes on the southern margin of North China Craton host uncommon plagioclase megacrysts that offer novel perspectives into the origin of megacrystic textures and the dynamics of their magmatic settings. U–Pb isotopic dating of zircon and titanite reveal that the Wulong gabbro was emplaced at ca. 1831 Ma. The plagioclase megacrysts (1–6 cm long) present petal-shaped aggregates, isolated islands, and flowing shapes, with oscillatory growth zonings and mineral inclusion (e.g., epidote and garnet) zonings. The Sr and Ba distributions within the plagioclase megacrysts exhibit a sawtooth-like pattern, which is interpreted as being caused by the thermal cycling resulting from the multi-batch replenishment of hot magma. Also as a result of thermal cycling, magmatic convection is induced, leading to the aggregation of the plagioclase megacrysts. Phase equilibrium modeling of these mineral assemblages suggests that the pressure and temperature during plagioclase crystallization were approximately 1,000 MPa and 800°C, respectively. The multicomponent diffusion model for garnet estimates the residual time of the magma chamber at ~2,000 years. According to our thermodynamic model, the temperature of the mafic magma involved in the multi-batch underplating process is estimated to be 940℃. The quantitative calculation model of the thermal cycling indicates that near-liquidus storage is not stable, and its rapid eruption or emplacement was easily triggered by underplating magmas.

华北克拉通（North China Craton）南缘的古元古代武隆斑状辉长岩脉，产出罕见的斜长石巨晶（plagioclase megacrysts），为探究巨晶结构的成因及其岩浆环境动力学机制提供了全新视角。对其中的锆石（zircon）与榍石（titanite）开展U-Pb同位素定年结果显示，武隆辉长岩的侵位时代约为1831 Ma。该斜长石巨晶（单颗长度1~6 cm）呈现花瓣状集合体、孤立岛状与流状形态，发育振荡生长环带及绿帘石、石榴子石等矿物包裹体环带。斜长石巨晶内的锶（Sr）与钡（Ba）分布呈现锯齿状模式，这被认为系热岩浆多批次补给引发的热循环所致。同样由热循环诱发的岩浆对流，进一步促使斜长石巨晶发生聚集。针对该矿物组合开展的相平衡模拟表明，斜长石结晶阶段的压力与温度分别约为1000 MPa与800℃。基于石榴子石的多组分扩散模型估算，岩浆房的滞留时长约为2000年。通过热力学模型估算，参与多批次底侵过程的镁铁质岩浆温度为940℃。热循环定量计算模型显示，近液相线储留状态并不稳定，底侵岩浆极易触发其快速喷发或侵位。