Partial to Complete Decoupling of Th-U-Pb Ages, Chemical Domains, and Monazite-Xenotime Immiscibility Gap Thermometry in Orthogneiss Monazite (Khanom Core Complex, Southern Thailand)

Abstract

We investigate the relationships between monazite Th-U-Pb ages, chemical domains, and monazite-xenotime immiscibility gap thermometry from two Indosinian orthogneisses from the Khanom Core Complex (Southern Thailand), with the aim of better understanding the relationships of these systems and their possible implications for petrochronology. Our data reveal that direct dating of chemical domains in monazite and temperature estimates can be ambiguous. On one side, U, Si, and Ca contents do not influence the Th-U-Pb ages or monazite-xenotime immiscibility gap temperatures, reinforcing the geological reliability of these age and thermometric data. However, the variability in REE enrichment and depletion and the underlying mechanisms remain poorly understood, with post-magmatic processes likely playing a major role in these geochemical modifications. Principal component analysis shows that monazite chemistry is the primary driver of data variation, with age and temperature estimates largely decoupled from chemical signatures, further underscoring the complexity of monazite petrochronology. Pseudosection modeling then suggests a protracted history of monazite growth and recrystallization, spanning approximately 40 million years. These findings challenge previous petrochronological postulations about coupled substitution mechanisms and highlight the necessity of using integrated analytical approaches to decipher complex metamorphic histories in monazite-bearing rocks. The study emphasizes the importance of considering the multifaceted nature of monazite systems when interpreting their petrochronological data.