Emplacement depth and cooling evolution of gabbroic stocks on Greenwich Island, Antarctica: an integrated approach using crystal size distribution (CSD) and two-pyroxene geothermobarometry

Field studies were conducted on Greenwich Island (South Shetland Islands, West Antarctica) as part of the 8th Turkish Antarctic Expedition (TAE-IV) and the Türkiye–Ecuador Bilateral Cooperation program. This study combines field observations, two-pyroxene geothermobarometry, and crystal size distribution (CSD) analysis to constrain the emplacement depth and cooling history of gabbroic stocks around Fort Williams Point, Greenwich Island.

Field investigations documented that Fort Williams Point hosts basaltic volcanic rocks and gabbroic to microgabbroic intrusions exposed across the area. The gabbroic bodies crop out as isolated stocks and dikes forming independent ridges and hills. Contacts between these intrusive bodies and volcanic rocks is not observable in the field. Petrographically, the gabbroic rocks are dominated by plagioclase, clinopyroxene, and orthopyroxene and minor amounts of olivine. They display holocrystalline porphyritic texture in which larger crystals are embedded in a relatively fine-grained groundmass. Two-pyroxene geothermobarometric calculations based on the mineral chemistry data of the intrusions indicate crystallization temperatures of approximately 1020–1035 °C and pressures in the range of 1.8–2.2 kbar which point to emplacement at shallow crustal levels.

Crystal Size Distribution (CSD) analysis of plagioclase was performed on approximately 300 crystals from two samples and displays a multi-segmented pattern, characterized by high population densities at small crystal sizes and a distinct break in slope toward larger sizes. The fine-size segment is interpreted to reflect an early stage dominated by high nucleation rates, likely triggered by rapid cooling following the emplacement. The change in CSD slope indicates a shift towards growth-dominated crystallization as cooling rates decreased and the system approached thermal stabilization. This evolution in crystallization regime suggests that the intrusions experienced initially rapid cooling consistent with shallow-level emplacement followed by progressively slower cooling within small intrusive bodies.

Overall, the combined geothermobarometric results and CSD patterns suggest that the gabbroic stocks of Fort Williams Point, Greenwich Island were emplaced within small and shallow magma storage zones and experienced rapidly changing thermal conditions during crystallization.