A Petrological and Mineralogical approach to the Saint Martin Volcanics, Caribbean

In the island of Saint Martin, selected lava samples reveal a diverse volcanic suite that comprises of tholeiitic and calc-alkaline basalts, basaltic andesites, andesites and dacites. Basalts and basaltic andesites exhibit variable MgO (3.6-7.9 wt.%), CaO (6.9-11.8 wt.%) and TiO₂ (0.5-1.0 wt.%) contents. Andesites and dacites generally show lower MgO (1.2-5.5 wt.%) and Al₂O₃ contents (11.7-18.0 wt.%). Interestingly, a dacite sample exhibits relatively enhanced MgO (5.5 wt.%), comparable to the recently reported melatonalites in St. Martin^[1]. Alteration was rather moderate, as revealed from their LOI (0.5-2.6 wt.%) and the presence of prehnite, chlorite and sericite. The rock forming minerals in the basalts and basaltic andesites include compositionally broad plagioclase (Ab_5-74An_26-94Or_0-4), diopside and augite (Wo_42-52En_29-45Fs_9-21), magnesiohornblende  (MgO=13.0-19.8 wt.%) and phlogopite (FeO_t/MgO≈0.57, Al₂O₃=14.4-14.7 wt.%). Andesites and dacites contain plagioclase (Ab_10-76An_1-90Or_0-86), diopside and augite (Wo_45-48En_40-42Fs_10-15), enstatite (Wo_1-4En_55-71Fs_29-43), magnesiohornblende (MgO=13.2-17.5 wt.%), biotite (FeO_t/MgO≈1.5, Al₂O₃=12.1-12.9 wt.%) and phlogopite (FeO_t/MgO≈0.5, Al₂O₃=13.4-14.0 wt.%). LREE in basalts and basaltic andesites are either slightly depleted or variably enriched [(La/Yb)_CN=0.6-4.7], with enhanced HREE (10.2-21.2xCN) and negative Eu anomalies (Eu_CN/Eu*=0.7-0.9). Andesites and dacites display comparable LREE patterns [(La/Yb)_CN=0.9-3.8], followed by differentiated HREE (11.7-24.8xCN) and pronounced negative Eu anomalies (Eu_CN/Eu*=0.6-0.9). Petrogenetic modelling calculations reveal that the primary hydrous basaltic magma was generated at pressure and temperature ~1.6 GPa and ~1280 ^oC respectively, after partial melting ~14% of a depleted DMM source. The least differentiated basalts evolved after ~50% fractional crystallization of olivine, plagioclase, clinopyroxene and amphibole. Subsequent additional removal of plagioclase and Fe-Ti oxides further differentiated the residual melts toward andesitic and dacitic compositions.

Acknowledgments: Georgia Kolovadi was financially supported by the «Andreas Mentzelopoulos Foundation».

Reference: [1] Koutsovitis, P. et al., 2025. Granitoids from St. Martin/Maarten Island, Caribbean: Insights on the role of mantle processes in the Lesser Antilles arc. Lithos, 494-495, 107926.