Geochronology and geochemical characteristics of Precambrian granite in the Main Islands, Republic of Seychelles

Abstract

The Seychelles microcontinent located in western Indian Ocean consists of a large number of granitic islands. The oldest known rocks of the Seychelles microcontinent are Neoproterozoic and comprise the Main Islands (Mahé and Praslin group). Previous studies demonstrated that the emplacement age of the Neoproterozoic granites is ~750 Ma although the age of Île aux Récifs is ~50 million years older (808.8±1.9 Ma). New weighted-mean zircon 238U/206Pb ages for rocks collected from northern Mahé (750.6±4.1 Ma, 756.9±4.5 Ma, 756.4±5.1 Ma) and western Praslin (761±12 Ma, 754.2±6.9 Ma, 753.8±4.9 Ma) are reported in this study. The new radioisotopic ages overlap with previous results but a significant number of inherited zircons were identified that fall within two major groups: ~790 Ma to ~820 Ma and ~850 Ma to 880 Ma. Geochemically, the Seychelles granites are characterized by high SiO2 and are metaluminous to peraluminous, alkali-calcic to calc-alkalic, and ferroan. The negative correlation of SiO2 with Al2O3, Fe2O3, CaO, TiO2, MgO and P2O5 indicates that the rock experienced crystal fractionation either during or after emplacement. They also show the enrichment in light rare earth elements compared to heavy rare earth elements with distinct negative Eu anomalies (0.1-0.84). Mahé samples illustrate positive εNd(t) = +1.85 to +2.83, and ISr values from 0.7022 to 0.7031 while those of Praslin shows negative εNd(t) = -1.52 to -1.29 and extremely low ISr values. Consequently, it appears that the Mahé and Praslin granites are derived from different sources. Tectonomagmatic discrimination of the granites is compatible with within-plate or post-orogenic environment. The ~50 million year gap between the oldest and youngest rocks, and their compositional (within-plate vs. volcanic-arc) dichotomy are difficult to reconcile. It is likely that Neoproterozoic granites of the Seychelles microcontinent were not associated with a volcanic-arc but rather related to post-collision extension or rifting. Furthermore, it is possible that the age of Île aux Récifs may be indicative of inherited zircon and not the emplacement age.

The Seychelles microcontinent located in western Indian Ocean consists of a large number of granitic islands. The oldest known rocks of the Seychelles microcontinent are Neoproterozoic and comprise the Main Islands (Mahé and Praslin group). Previous studies demonstrated that the emplacement age of the Neoproterozoic granites is ~750 Ma although the age of Île aux Récifs is ~50 million years older (808.8±1.9 Ma). New weighted-mean zircon 238U/206Pb ages for rocks collected from northern Mahé (750.6±4.1 Ma, 756.9±4.5 Ma, 756.4±5.1 Ma) and western Praslin (761±12 Ma, 754.2±6.9 Ma, 753.8±4.9 Ma) are reported in this study. The new radioisotopic ages overlap with previous results but a significant number of inherited zircons were identified that fall within two major groups: ~790 Ma to ~820 Ma and ~850 Ma to 880 Ma. Geochemically, the Seychelles granites are characterized by high SiO2 and are metaluminous to peraluminous, alkali-calcic to calc-alkalic, and ferroan. The negative correlation of SiO2 with Al2O3, Fe2O3, CaO, TiO2, MgO and P2O5 indicates that the rock experienced crystal fractionation either during or after emplacement. They also show the enrichment in light rare earth elements compared to heavy rare earth elements with distinct negative Eu anomalies (0.1-0.84). Mahé samples illustrate positive εNd(t) = +1.85 to +2.83, and ISr values from 0.7022 to 0.7031 while those of Praslin shows negative εNd(t) = -1.52 to -1.29 and extremely low ISr values. Consequently, it appears that the Mahé and Praslin granites are derived from different sources. Tectonomagmatic discrimination of the granites is compatible with within-plate or post-orogenic environment. The ~50 million year gap between the oldest and youngest rocks, and their compositional (within-plate vs. volcanic-arc) dichotomy are difficult to reconcile. It is likely that Neoproterozoic granites of the Seychelles microcontinent were not associated with a volcanic-arc but rather related to post-collision extension or rifting. Furthermore, it is possible that the age of Île aux Récifs may be indicative of inherited zircon and not the emplacement age.