Fluid exsolution in magmas is a process that, in many silicic upper crustal reservoirs, starts at relatively low crystallinities (near liquidus), and precedes the precipitation of many ore bodies, including pegmatites. As any magmatic system approaches its solidus, the amount of the exsolved fluid phase increases and becomes progressively dominant over melt, allowing local over pressurization and the generation of pegmatitic pods/dykes. Such pegmatitic bodies show several features that point to both magmatic and hydrothermal environments, linking those realms and providing a unique opportunity to document and understand the magmatic-hydrothermal transition within silicic magmatic systems. We studied the 1.1 Ga classic A-type Pikes Peak granite (Colorado, United States) and one of its many internally-hosted pegmatites, the Wellington Lake pegmatite, to investigate the changes that occur within a granitic system as it crosses its theoretical water-saturated solidus and continues crystallizing beyond it. Textural and geochemical analyses of quartz, plagioclase, and K-feldspar minerals, as well as fluid inclusion studies, demonstrate this magmatic to hydrothermal transition in the granite and the pegmatite. Different thermometers (Ti-in-quartz, 2-feldspars, fluid inclusions) document the temperature evolution of the granitic system, from >850°C for the hottest magmatic minerals to <400°C for the pegmatite core. The magmatic-hydrothermal transition is recorded by plagioclase and quartz rims that yield temperatures well below the traditionally inferred haplogranite solidus. In the pegmatite, the magmatic-hydrothermal transition is observed between the graphic granite wall zone, which shows homogeneous quartz geochemical signatures at near-solidus conditions (700–670°C), and the intermediate zone, which crystallized at much colder temperatures (470–420°C). Although a significant process, our calculations suggest that subsolidus precipitation from exsolved, solute-rich magmatic fluids represent less than 20% of the total volume of the granite.

<p>Understanding magmatic activity on the Early Earth remains a challenge for geoscientists, as most of its rock record has been destroyed or altered. The oldest exposed rocks belong to the Tonalite-Trondhjemite-Granodiorite (TTG) plutonic suite, only rarely associated with volcanic units of the same age. For this reason, TTGs are often interpreted as magmas that have not erupted, and their compositions thought to represent <em>melts</em>. However, if TTGs are the left-overs from shallow magma reservoirs that have lost some melt to the now-eroded volcanic record, their bulk composition would be at least partly biased towards<em> crystal cumulates</em>. As post-emplacement metamorphism typically overprints many of the chemical characteristics of the initial magmatic minerals, the more resistant magmatic minerals (quartz and zircons) within sedimentary successions derived from these systems provide the best chance of identifying volcanic lithologies that have been completely eroded. Here we use a novel approach to show that Ti-in-quartz and Ti-in-zircon thermometers can be used to recognise different magmatic sources in sedimentary rocks. In quartz, Ti thermometry calibrated against blue cathodoluminescence obtained from scanning electron microscopy allows for fast and statistically meaningful Ti quantification in hundreds of sedimentary quartz grains. This imaging-derived Ti distribution matches well with the distribution of Ti concentrations obtained by LA-ICP-MS spot measurements of individual crystals. We compare this quartz record to Ti distributions in zircons, which have the benefit of also providing a crystallisation age. We applied these techniques to the Pikes Peak Batholith (CO, USA), a 1.1 Ga A-type granite hosting several pegmatites, and the Tava sandstone, a series of Cryogenian intra-granite sedimentary dikes that represents the oldest terrestrial sediments in the Front Ranges of Colorado. Our data successfully separates plutonic from pegmatitic crystals and shows that quartz and zircon crystals in the Tava Sandstone crystallised at statistically higher temperatures than the ones observed in the Pikes Peak Batholith, implying potential contribution from a volcanic source that is no longer available on the surface. The proposed techniques can therefore be used to identify eroded magmatic lithologies and to estimate proportions of different magmatic components (volcanic, plutonic, pegmatitic) in sediments.</p>

<p>Pegmatites are texturally, mineralogically, and geochemically zoned rocks that show distinctive features such as graphic granite in the wall zones, coarse-crystalline material in the centre, and unusual mineralisation sometimes of economic significance. They are usually considered to be derived from silicate melts, but a significant fluid supply is also required to reproduce their unique characteristics. These fluids are commonly enriched in flux anions such as F<sup>-</sup>, Cl<sup>-</sup>, CO<sub>3</sub><sup>2-</sup>, and BO<sub>3</sub><sup>3- </sup>. Many studies have investigated the petrogenetic processes that led to pegmatite crystallisation, yet not all aspects of pegmatite formation have been fully understood. Notably, the nature of the precipitating medium remains uncertain for the different zones of the pegmatite. In order to better understand the transition from a silicate-melt-dominated crystallisation to fluid-dominated precipitation, we aimed to produce a temperature profile across the pegmatite and its host granite. We analysed quartz crystals from the different zones of the Wellington Lake Pegmatite and the host rock, a syenogranite of the Pikes Peak Batholith, in Colorado (USA). This NYF-type pegmatite consists of a fine-grained graphic granite wall zone, a coarse-grained quartz and albite intermediate zone, and pure blocky quartz core zone with REE-dominated mineralisation including fluocerite, bastnäsite, thorite, columbite, zircon, and cassiterite. Quartz trace element data (Al, Ti, Ge) suggest that the granite crystallized over a range of conditions, with Ti-in-quartz temperatures varying from 800 to 550°C. The wall zone of the pegmatite crystallised over a more constricted range, with temperatures on the order of ~660 to 630°C, just below the experimentally determined H<sub>2</sub>O-saturated haplogranite solidus. Finally, the intermediate and core zones of the pegmatite show much colder conditions, with fluid inclusion homogenisation temperatures calibrated for typical pegmatite pressures ranging from 450°C (for 300 MPa) or 380°C (for 200 MPa) for the intermediate zone to 380°C (for 300 MPa) or 325°C (for 200 MPa) for the core. These results suggest crystallisation from a range of conditions transitioning from hydrous silicate melt-based mineral precipitation at the high temperature end (in the core of quartz crystals in the granite) to sub-solidus Al-Si-Na-enriched fluid precipitation in the interstitial quartz of the granite and in the pegmatite. Textural and geochemical zoning in the pegmatite records the transition from near-magmatic conditions in the borders to colder and more hydrothermal conditions in the core.</p><p> </p>

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<p>The granite solidus curve is generally placed around 660-700°C, depending on the pressure conditions and water content of the melt. However, recent studies have documented evidence of <em>subsolidus</em> crystallisation in granitic rocks, with minerals recording temperatures <660 °C, posing a debate on the state of the melt or fluid from which precipitation occurred. We utilise the Ti-in-quartz thermometer in quartz crystals of the Pikes Peak batholith, a 1.1 Ga A-type granitic pluton in Colorado (USA) and one of its many pegmatites, the Wellington Lake Pegmatite, to investigate the range of crystallisation temperatures for this system. In the granite, quartz crystals start to crystallise at typical magmatic temperatures, above 800°C, and progress to very cold conditions, below 500°C, overlapping with temperatures from the monomineralic quartz core of the pegmatite. This very low-temperature crystallisation is observed in cathodoluminescence (CL) images as (1) dark rims in the crystals and (2) fluid inclusion-filled fractures. By linking the quartz growth zones observed in CL images with the Ti content of the crystals, we estimate that a maximum of ~1/3 of the quartz volume, and likely less, corresponds to subsolidus crystallisation in the granite. Further geochemical evidence and comparison with the pegmatite indicates that the chemistry of the dominant precipitating medium undergoes pronounced changes throughout cooling and crystallisation, likely transitioning from a silicate melt at high temperatures to a solute-rich, hydrous supercritical fluid near and below the granite solidus. Further supporting the presence of a hydrous supercritical fluid, the assemblage of the different zones of the pegmatite (a fine-grained graphic granite wall zone, a coarser grained quartz-albite intermediate zone and a pure blocky quartz core) do not record typical eutectic compositions, implying that the pegmatite would not have precipitated from an evolved “granitic” melt. Thus, we suggest that subsolidus precipitation from a solute-rich supercritical fluid is a late, but significant process in the Pikes Peak granite. This event is associated with the progressive enrichment in water and other volatile elements during “second boiling”, ultimately leading to the transition from magmatic to hydrothermal conditions, and sourcing the numerous pegmatite dykes and pods in and around the pluton.</p>