Open-system controls on the non-linear evolution of K-feldspar megacryst populations in a zoned felsic crystal mush

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ID: 323035
2026
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Abstract
Abstract The origin and timing of K-feldspar megacryst formation in granitoids remain debated, particularly whether megacrysts represent late-stage coarsening products or early formed crystals subsequently modified during open-system reservoir evolution. Here, we integrate field relationships, petrography, crystal size distribution (CSD) analysis, and mineral-scale geochemistry to constrain the life cycle of K-feldspar megacrysts in the Early Miocene Çataldağ Pluton (Western Anatolia), a zoned felsic system comprising granodiorite, quartz monzonite, and granite. Coeval granitic units lacking euhedral megacrysts occur alongside megacryst-bearing lithologies, providing a key constraint on the compositional and thermal conditions required for megacryst development. K-feldspar megacrysts locally approaching 100 mm occur throughout the principal lithologies and within mafic microgranular enclaves (MMEs). Lithology-scale pooled CSDs reveal non-linear, lithology-dependent evolution of crystal populations. Granite has the largest characteristic length and the shallowest coarse-segment CSD slope, whereas granodiorite and quartz monzonite have shorter characteristic lengths and broadly similar CSD slopes. Small-size downturns in both granodiorite and quartz monzonite are consistent with depletion of smaller crystals by recharge-related thermal cycling and coarsening, although sampling truncation may also contribute at the smallest sizes. These distributions indicate that megacryst populations did not evolve through a simple monotonic granodiorite–quartz monzonite–granite sequence. Plagioclase chadacryst compositions overlap matrix fields but locally extend to higher anorthite contents, particularly in quartz monzonitic and MME-related populations. Their alignment parallel to K-feldspar growth faces indicates heterogeneous nucleation and partial growth within chemically modified boundary layers around advancing megacrysts. These textures and compositions record crystallization and recycling within heterogeneous but physically connected mush domains. Non-monotonic Ba zoning and Ba variability further indicate episodic dissolution–regrowth and enlargement during transient K–Ba-enriched growth windows generated by recharge, incomplete mixing, and local melt-domain enrichment. K-feldspar megacrysts enclosed within MMEs record mechanical transfer and recycling between compositionally distinct magma domains. We propose that K-feldspar megacrysts began growing early upon entry into the K-feldspar stability field and were subsequently modified by recharge-related thermal cycling, coarsening, crystal recycling, and continued growth within evolving crystal-rich mushes. They do not record a single continuous growth history common to all lithologies, preserve lithology-specific combinations of growth, dissolution, recycling, and selective survival within a long-lived open-system reservoir. Megacryst formation thus emerges as a time-dependent, open-system population process controlled by the interplay between thermodynamic saturation, kinetic growth, recharge-driven thermal cycling, crystal recycling, equilibration-driven coarsening, and transient chemical perturbations, rather than as a product of simple static late-stage textural coarsening. This framework links crystal population dynamics to reservoir-scale magmatic processes and provides a testable model for megacryst formation in felsic plutonic systems.
Reference Key
openalex_W7171849814 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Ömer Kamacı, Şafak Altunkaynak
Journal journal of petrology
Year 2026
DOI
10.1093/petrology/egag063
URL
https://doi.org/10.1093/petrology/egag063
Keywords Keywords not found

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