Pallaturated forms in metamorphosed (possibly ultramafic or dolomit-rich) protocunnamite–garnet–biotite assemblages at high PC-r compressed conditions, typically pressures > 1.5 GPa and temperatures 500–700°C, in subduction setups or steep geothermal gradient terrains. - Midis
Pallaturated Forms in Metamorphosed Protocunnamite–Garnet–Biotite Assemblages Under High-PC Compression: Insights from Ultra-Mafic to Dolomit-Rich Protococunnamite–Garnet–Biotite Facies
Pallaturated Forms in Metamorphosed Protocunnamite–Garnet–Biotite Assemblages Under High-PC Compression: Insights from Ultra-Mafic to Dolomit-Rich Protococunnamite–Garnet–Biotite Facies
Abstract
Pallaturated mineral textures in metamorphosed protocunnamite–garnet–biotite assemblages represent critical indicators of high-pressure (HP) metamorphic environments, particularly within subduction zones and steep geothermal gradient terranes. These distinctive pallaturated forms—characterized by intergrown, isotopic exchange zones between paleocunnamite, garnet, and biotite—manifest predominantly under high PC (pressure-composition) conditions exceeding 1.5 GPa and temperatures between 500–700°C. This article explores the petrogenetic significance of pallunted occult textures in ultramafic-derived to dolomit-rich metamorphosed assemblages, shedding light on fluid flecks, retrograde exchange, and subduction dynamics.
Understanding the Context
Introduction
Metamorphosed rocks preserved in modern and exhumed subduction complexes reveal complex facies histories and mineral reaction pathways under extreme pressures and moderate temperatures. Among the most diagnostic mineral assemblages are protocunnamite–garnet–biotite mixtures, often associated with HP-grade conditions (>1.5 GPa, 500–700°C). A defining feature of these assemblages is the development of pallaturated forms—texturally subdued, intergranular, or partially planar exchanges concentrated in biotite–quartz–or related phyllosilicate domains—revealing dynamic metamorphic equilibration and fluid-assisted reactions.
Understanding pallaturated forms in dolomit-rich ultramafic to metasedimentary terranes enhances interpretation of subduction fluid migration, retrograde metamorphism, and the P–T–t evolution of deep crustal and mantle wedges.
Key Insights
Mineral Assemblage Context and High-PC Conditions
Protocunnamite–garnet–biotite assemblages form under medium- to high-grade metamorphism typical of deep subduction zones and buoyant cratonic margins with steep geothermal gradients. These assemblages often develop in rocks transitioning from dolomitic carbonate or ultramafic parent rocks (e.g., olivine-rich probes), where metamorphic reactions generate coarse-grained, equation-rich mineral textures.
Under high PC (pressure-composition) regimes (>1.5 GPa) and moderate temperatures (500–700°C), the stability field shifts toward ultramafic (polydinellite, pentlandite-rich), metadolomitic, and low-actinote biotite facies. Here, garnet nucleates and grows preferentially due to its stability at high pressures, incorporating significant Mn, Fe, and often Mg-rich cations. Biotite coexists as a structurally dominant phyllosilicate capable of fluid-mediated element exchange.
Pallaturated Textures: Formation Mechanisms and Significance
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Pallaturated textures denote subtle to pronounced relic intergrowths where mineral phases, such as residual protocunnamite veils or cryptic garnet-biotite interlayers, remain within a matrix of coexisting minerals but are no longer discrete coeval. In the context of high-P/H_CT (high pressure–channel temperature) metamorphism, palluration develops through:
- Diffusion-controlled ionic exchange across grain boundaries driven by steep chemical gradients and sustained fluid activity.
- Retrograde reaction fronts where mineral breakdown (e.g., pristine protocunnamite) is cross-cut by pallitated intergrowths reflecting partial re-equilibration under stagnant or slowly fluid-present conditions.
- Preferred orientation of biotite–garnet–protocunnamite domains aligned by differential deformation, creating textural “pallatures” visible under petrographic and electron probe analysis.
These textures are diagnostic markers of PNT (protocol, nacenta, and texturally over-grown) re-equilibration under non-equilibrium fluid influence, especially where retrograde fluid infiltration introduces alkali and volatile components penetrating through relic primordial fabrics.
Implications for Subduction Zone Dynamics
The presence of pallaturated forms in ultramafic to dolomit-rich settings strongly supports two key subduction-related scenarios:
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Deep Subduction of Carbonate-Carrying Oceanic Crust and Mafic Probes
Ultramafic protococunnamite–garnet–biotite assemblages frequently originate from sub-continental lithospheric mantle or oceanic mantle subjected to high P–T conditions. Pallaturated textures record fluid-assisted exchange during descent, indicating dynamic interaction between slab-derived fluids and host rocks under sustained >1.5 GPa pressures. Fluid constituents rich in Mg, Si, and endemic elements (e.g., Ni, Cr) migrate across grain boundaries, fostering pallitated intergrowths that preserve a transient snapshot of metamorphic equilibration. -
Steep Geothermal Gradients and Retrograde Metamorphic Mixing Zones
In terrains with steep geothermal gradients, subduction-related metamorphism advances rapidly along the pressure–temperature path. Pallaturated forms thus reflect late-stage retrogression, where partial breakdown of unreacted protocunnamite by diffusion-limited fluid influx produces textural mosaics integrating primordial and recently modified domains. These relics offer insight into the timing, duration, and composition of fluid pulses that influenced arc magmatism and deep crustal rheology.
