Transcrustal magmatic processes of the Tianshuigou dolerite sill from the ca. 1.32 Ga Yanliao large igneous province: Insights from zoned plagioclase macrocrysts

Abstract

Transcrustal magmatic processes within the magmatic plumbing systems of large igneous provinces (LIPs) remain poorly constrained. Here we present petrographic textures, whole-rock and mineral geochemistry, and in situ Sr isotopes of several types of zoned plagioclase macrocrysts identified as antecrysts in the shallowly emplaced Tianshuigou dolerite sill from the Yanliao LIP. The goal is to reveal the transcrustal magmatic processes from middle to shallow crust. Our results characterize three zones in the plagioclase macrocrysts: core, mantle, and rim, and we determine that these zones crystallized in different parts of the LIP plumbing system under distinct P-T conditions. The cores of plagioclase macrocrysts originated from a mid-crustal (ca. 17.4 km deep) magma reservoir consisting of crystal mush. The mantle zone of plagioclase macrocrysts records the magma ascent and/or convection processes in a shallower chamber at a depth of ca. 6 km. The rim and groundmass plagioclase crystallized initially in a shallower chamber at a depth of ca. 6 km, and further crystallization of the rim and groundmass occurred during final ascent of the crystal-bearing magma and intrusion into the Xiamaling Formation as dolerite sills. Geothermometers show that the core crystallized at ∼1214 ± 36 °C, the mantle crystallized at ∼1169 ± 36 °C, and the rim and groundmass plagioclase crystallized over a range from ∼1154 ± 36 to 1108 ± 14 °C. The hygrometer data suggest the magma plumbing system is relatively dry (H2O contents <1 wt%). The plagioclase macrocrysts generally show normal zonation from core to rim reflecting overall evolution of magma composition in the plumbing system. Geochemical data suggest that magmas were contaminated by the mafic lower crust prior to crystallization of the plagioclase macrocryst cores in the middle crustal magma reservoir, but that little or no contamination occurred during the magma storage and the ascent in the middle-upper crust. Our results suggest that the zoned plagioclase macrocrysts in mafic sill swarms may record the magmatic processes from the middle to the shallow crust in LIP plumbing systems. The processes that occurred in magma plumbing systems in middle crust play a vital role in the compositions of shallowly emplaced intrusions in LIPs.