Textural and Chemical Evolution of Unidirectional Solidification Textures in Highly Differentiated Granitic Rocks at Kharaatyagaan,Central Mongolia

Unidirectional solidification texture (UST) in an aplite body is recognized in the Neoproterozoic highly differentiated granitic rocks at Kharaatyagaan, central Mongolia. On the basis of crystal morphology, two main types of UST were identified in the aplite body: (i) thin crenulate UST layers; and (ii) thick intergrowth UST layers. Bulk geochemistry indicates that the Kharaatyagaan UST‐bearing aplite and aplite dike are alkaline, and are enriched in light rare‐earth elements. Scanning electron microscopy and cathadoluminescence imaging of UST quartz from Kharaatyagaan show four types of quartz: euhedral quartz phenocrysts with well‐developed concentric growth zoning (Qa1) in the aplite; euhedral quartz with weak growth zoning in the aplite (Qa2); UST quartz exhibiting distinct growth zones (Qu1); and UST quartz showing mosaic texture (Qu2). Crystallization temperatures determined by the Ti‐in‐quartz geothermometer of Qa1 and Qu1 quartz range between around 500°C and 780°C and Qa2 and Qu2 range between about 490° and 630°C. The cathodoluminescence textures of quartz are predominantly caused by variations in the trace elements contents of quartz. The Qa1 and Qu1 quartz crystals are characterized by high Ti and Al concentrations in the quartz lattice, and are observed in the bottom of the Kharaatyagaan hill, which formed in the early, less evolved magmatic stage. The Qa2 and Qu2 UST quartz characterized by low Ti and variable Al concentrations are found at the top. The UST layers crystallized along the upper part of the magma chamber in the presence of fluid phases exsolved from felsic magma.