Subsolidus crystallisation in the A-type Pikes Peak batholith

Abstract

<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>