Archean trondhjemitic crust at depth in Yangtze Craton: Evidence from TTG xenolith in mafic dyke and apatite inclusion pressure in zircon

Abstract

Granitic rocks are the most abundant plutonic rocks in the continental crust, and their Archean counterparts include tonalite, trondhjemite and granodiorite (TTG), which dominate the early Earth’s rock archive. Understanding Archean crustal structure have been impeded not only by the rarely exposed deep Archean crustal sections but also by the lack of universally applicable barometers for granitic rocks. Here, we focus on ~2.9 Ga trondhjemite xenoliths brought from depth by mafic dykes that intrude shallower country rocks with the same age and chemistry as the xenolith, in the Kongling Complex, South China. The mafic dykes were emplaced at or after 2.0 Ga and were hybridized by the xenoliths showing signs of partial melting. Exploring the apatite-in-zircon system, for the first time, we establish maximum range of remnant pressure for the inclusions inside zircon hosts. The results yield systematically higher inclusion pressures for the apatites fully encapsulated in xenolith zircons than in the country rock. Based on this data, we envisage a widespread and at least locally thick trondhjemitic crust in the Yangtze Craton by the Mesoarchean.