Myrmekite and antiperthite formation in deformed granite: Chemical and crystallographic constraints on formation mechanisms

The chemical and crystallographic characteristics of myrmekite and antiperthite in deformed Tiantang Zhai granite of the Central China Orogen are studied to reveal their origin. We infer two paths of silica transfer during formation of myrmekites: (a) silica released by magmatic quartz during formation of early coarse-grained myrmekites, and (b) silica released from the myrmekitic vermicular quartz during formation of a second generation of fine-grained myrmekites. Formation of myrmekites occurred during syn-kinematic deformation of the granite. The silica transfer was promoted by the high density of crystallographic defects in the proto-quartz and by the low interfacial energy, as indicated by crystallographic orientation relationship(s) (CORs), of defect-free myrmekitic quartz.

Systematic lattice correlation of {110} Feldspar // {11-21} Quartz is observed in nearly all myrmekites, attributed to the close lattice spacing match between feldspar and quartz within these lattice planes. The myrmekites formed under high silica supersaturation developed multiple subdomains with different orientations of quartz. Subdomain lacking CORs between quartz and plagioclase tapered out, indicating that the interfacial energy differences control the orientation evolution in each subdomain. The variations in CORs among different subdomains reveal the processes of quartz nucleation, propagation and possible dissolution in myrmekite. The study introduces an approach for investigating the growth features and evolution of intergrowth through the occurrence of CORs within intergrowths, wherein the constituent minerals do not share comparable crystallographic structures.

Antiperthite comprising faceted inclusions of alkali feldspar developed in the vicinity of myrmekite. Precipitation of alkali feldspar in plagioclase was mediated by fluid present along cracks in the plagioclase host. Compositional evidence indicates that myrmekite and antiperthite may have formed simultaneously, serving as mutual sources and sinks for Na, K, and Ca exchanged between their respective domains during formation. This study reveals ubiquitous mass transfer among intergrowths formed during late magmatic hydrothermal stage, accompanied by ongoing adjustments in the crystallographic orientations of their constituent minerals, aim at reducing strain energy and achieving minimal interfacial energy.