Temporal and structural evolution of the Early Palæogene rocks of the Seychelles microcontinent

The Early Palæogene Silhouette/North Island volcano-plutonic complex was emplaced during the rifting of the Seychelles microcontinent from western India. The complex is thought to have been emplaced during magnetochron C28n. However, the magnetic polarities of the rocks are almost entirely reversed and inconsistent with a normal polarity. In this study we present new in situ zircon U/Pb geochronology of the different intrusive facies of the Silhouette/North Island complex in order to address the timing of emplacement and the apparent magnetic polarity dichotomy. The rocks from Silhouette yielded weighted mean 206Pb/238U ages from 62.4 ± 0.9 Ma to 63.1 ± 0.9 Ma whereas the rocks from North Island yielded slightly younger mean ages between 60.6 ± 0.7 Ma to 61.0 ± 0.8 Ma. The secular latitudinal variation from Silhouette to North Island is consistent with the anticlockwise rotation of the Seychelles microcontinent and the measured polarities. The rocks from Silhouette were emplaced across a polarity cycle (C26r-C27n-C27r) and the rocks from North Island were emplaced entirely within a magnetic reversal (C26r). Moreover, the rocks from North Island and those from the conjugate margin of India are contemporaneous and together mark the culmination of rift-related magmatism.


Geological Background
The Mascarene Plateau is comprised of submerged fragments of ancient continental crust in the western Indian Ocean that covers an area of ~115 000 km 2 and extends for ~2000 km from north to south 1,[8][9][10][11] . The plateau is exposed above the water surface at a few localities (Seychelles, Mauritius, Réunion, Rodrigues and Cargados Carajos Shoals). The Seychelles microcontinent is an elliptical-shaped block located at the northern end of the Mascarene Plateau (Fig. 1a) 18 . The exposed islands and islets of the Seychelles are composed mostly of granitic rocks with subordinate volcanic and mafic intrusive rocks and are ringed by coral reefs 15,19,20 .
Geologically there are three groups of islands that correspond to geography, composition and age (Fig. 1b). The granitic rocks of the Mahé Group are grey in colour and range in age from 748 ± 1.2 Ma to 764 ± 1.9 Ma whereas the Praslin Group of granitic rocks are red to pink in colour and are 750 ± 1.8 Ma to 759 ± 2.0 Ma with île aux Recifs ~50 Ma older at 809 ± 1.9 Ma 9, 21 . The Mahé and Praslin groups likely formed at an Andean-type setting on the margin of Rodinia 9,10 . The youngest islands, Silhouette and North Island, are located at the western edge of the Main Islands and are mainly composed of syenite with a minor amount of mafic to intermediate rocks (Fig. 1b).
Silhouette is located ~20 km NW of Mahé and covers an area of 20.1 km 2 and is considered to be a ring complex that has a syenitic rim and a granitic core (Fig. 1c). There are two outcroppings of volcanic rocks along the easternmost portion of the island at Pointe Vareur-Pointe Ramasse and Pointe Zeng Zeng (Fig. 1c). The volcanic rocks are trachytic tuffs that contain fragments of other texturally distinct silicic and mafic rocks and were intruded by microgranites along contours suggesting a sill-like emplacement relationship 14 . A total of five samples targeted for radio-isotopic dating were collected from Silhouette. Three fayalite-bearing syenites were collected from the northern end of Baie Cipailles (near Grebau) whereas an additional two fayalite-absent granitic rocks (1 syenite, 1 microgranite) were collected near Pointe Vareur (Fig. 1c). North Island (île du Nord) is ~5 km north of Silhouette and has an area of 2.01 km 2 . The principal rock-type found on North Island is a buff grey syenite however there is a gabbro 'float' near the centre of the island and there is a small exposure of darker olivine-biotite-bearing gabbro/diorite to the southwest which forms a portion of the Congoment promontory ( Fig. 1d) 15,19 . In the north, porphyritic microsyenite dykes and dark-coloured veins are also observed trending to the northwest. A total of five samples were collected for radio-isotopic dating. Three samples were collected from Congoment (2 syenites, 1 diorite) whereas the remaining two were collected from the main north-south trending syenite body (Fig. 1d).

Results
The zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) weighted mean 206 Pb/ 238 U results for this study are listed in Table 1. The complete dataset is available in the online supplementary information (Datasets S1 and S2). The zircons from Silhouette and North Island vary in size (50 μm to 450 μm long) and display oscillatory zonation (Figs S1 and S2). Most crystals are euhedral to subhedral with equant to elongate shapes but some crystals have fragmented shapes. The fragmented shapes are probably caused by the mineral separation process. The measured Th/U ratios for all zircons are ≥0.4 and within the range of typical igneous zircons 22 .
Temporal evolution of the Silhouette/North Island complex. The results from this study are within the range of previously reported ages of the Silhouette/North Island complex 1, 2, 14, 23 . However, we are able to identify a distinct spatial-temporal progression from Silhouette to North Island. Figure 2a shows the weighted mean 206 Pb/ 238 U ages with respect to latitude and the 2012 geomagnetic polarity timescale 24 . There is a clear latitudinal secular variation as the ages systematically decrease northward. The rocks from North Island are ~2 million years younger than the rocks from Silhouette. Although there is overlap, it is clear that there is a statistically meaningful age difference between the rocks of the two islands.
The identification of secular variation within the Silhouette/North Island complex has a number of important magmatic and tectonic implications for the Seychelles microcontinent. The age gap between Silhouette and North Island suggests there are at least two periods of magmatism within the complex. The trachytic tuffs and the fayalite-absent syenitic rocks from eastern Silhouette (Pte. Vareur-Pte. Ramasse) likely formed the initial volcanic-hypabyssal complex during the early stages (~63 Ma). The only rocks from Silhouette that were found to contain inherited zircons are located in the eastern part of the island where the microgranite and fayalite-absent syenite were collected 23 . Based on the abundance of xenoliths and micro-xenoliths within the trachytic rocks from eastern Silhouette, it is likely that they erupted violently thereby brecciating the country rocks and allowing them to be incorporated into the tuff. The emplacement of the fayalite-bearing rocks could represent a second period of magmatism as they are texturally distinct from the fayalite-absent syenites. However, the fayalite-bearing and fayalite-absent rocks have similar whole rock and Sr-Nd isotopic compositions and ages thus it is possible that the rocks formed by the same process but that the principal difference between their magmas was volatile (H 2 O, CO 2 , Cl, F) content as fayalite crystallizes under relatively dry conditions 2, 15, 25 . The last period of silicic plutonism (~60 Ma) is represented by North Island. The syenitic rocks from North Island are texturally and mineralogically different from the syenites of Silhouette 15, 19 . Moreover the whole rock Sr-Nd-Hf isotopes are subtly different. Although there is overlap, the rocks from the Silhouette have slightly higher 87 Sr/ 86 Sr i (0.7035 to 0.7061) ratios, and lower ε Nd (t) (+0.5 to +2.9) and ε Hf (t) (+3. 8 (Fig. 2b) 14,24 . It is clear that the ages from North Island cluster within magnetochron C26r and are thus compatible with the magnetic polarity results (Fig. 2a). The ages from Silhouette, however, straddle the C26r-C27n-C27r magnetochrons. The precision of the data from this study is insufficient to resolve the short duration of magnetochron C27n (0.296 Ma) but a microgranite collected from eastern Silhouette has a normal polarity and could have been emplaced during that short normal polarity timeframe 14 . Therefore it is possible that the rocks of Silhouette were emplaced across magnetochrons C26r-C27n-C27r (62.221 Ma to 63.494 Ma) and may have either a normal or reverse polarity 14,24 .

Anticlockwise rotation of the Seychelles microcontinent and the structural influence on the
Silhouette/North Island complex. The locations of Silhouette and North Island appear to be structurally controlled. There are distinct NW-SE and N-S structural lineaments that are evident from the Main Islands morphology and are complementary to regional Bouguer gravity maps (Fig. 1b) 27 . The structural lineaments likely exerted a significant control on the morphology of the Main Islands and it is probably not a coincidence that both Silhouette and North Island are located in close proximity to the junction between two lineaments 27 . Interestingly, the long axis of Silhouette is aligned with a NW-SE lineament whereas the long axis of North Island is aligned with a N-S lineament. The different orientations of the islands, and possibly their locations, suggests that they were emplaced under different stress regimes that reflect the secular evolution of the regional stress field as the Indian Ocean developed 6,7,28 .
Paleomagnetic data indicate the Seychelles microcontinent rotated 29° ± 12.9° in an anticlockwise direction after it rifted from India (~64 Ma) to ~60 Ma or possibly later 14 . If the Seychelles microcontinent is rotated back to its original orientation before rifting (66 Ma) and the formation of the first magnetochron in the East Somali basin (C28), the lineaments are oriented NNW-SSE and NNE-SSW. The original position of the Seychelles microcontinent prior 66 Ma has a tight fit with the western Indian margin. By fitting the triangular geometry of the Seychelles into the western Indian margin, a much smoother continental margin with a general N-S alignment can be reconstructed (Fig. 3a). The orientation of the Seychelles microcontinent also agrees with the general N-S trending Late Cretaceous India-Madagascar rift as the Laxmi Basin stopped developing at ~62.5 Ma 28 . There are a number of dyke swarms emplaced along the western margin of India that are related to the Deccan Traps and include the ENE-WSW striking Narmada-Tapi swarm, the NNE-SSW striking Nasik-Pune swarm and the N-S striking Coastal swarm (Fig. 3b) 29,30 . The Nasik-Pune and Coastal swarms were emplaced during E-W tensional stress and generally match the orientation of the reoriented Seychelles lineaments 29,31,32 . Silhouette was emplaced concurrently (~63 to ~62 Ma) with the dyke swarms during the E-W directed tensional stress as the Seychelles microcontinent rifted from India. It is possible that the initial orientation of the lineaments favoured the emplacement of magmas along the N-S axis when the microcontinent initially rifted and then, after rotation, became the NW-SE lineaments (Fig. 3c). Approximately two million years after Silhouette, North Island (~60 Ma) was emplaced along a N-S lineament after the rotation ceased. The principal plate stress changed from E-W to N-S due to the northward movement of India and the initiation of N-S sea-floor spreading along the Carlsberg Ridge (Fig. 3d). Therefore we suggest the geographic position and orientation of North Island relative to Silhouette is likely due to the structural controls (lineaments, rotation) within the Seychelles microcontinent as it rifted from India and subsequently rotated.

Conclusions
In situ zircon U/Pb age dates of the Early Palaeogene rocks from the Seychelles identify a secular variation between Silhouette Island and North Island. The northward decreasing age progression is structurally consistent with the anticlockwise rotation of the Seychelles microcontinent after rifting from western India. Moreover the new ages are in agreement with the magnetic polarities of the rocks suggesting the Silhouette/North Island complex was emplaced over a ~3 million year period beginning during magnetochron C27r and ending during magnetochron 26r. Furthermore, syenites from North Island and trachytes from western India are contemporaneous (~60 Ma) and together mark the culmination of rift-related magmatism.

Methods
Geochronology. Zircons were mechanically separated at the Yu-Neng Rock and Mineral Separation Company (Lanfang, Hubei) using a steel jaw crusher, magnetic separation and heavy-liquids. The crystals were linearly mounted on a glass slide covering a diameter of one inch. A mold was placed over the zircons and epoxy was poured over the zircons to ensure transfer of the minerals to the epoxy. Cathodoluminescence (CL) images were taken for selecting suitable positions for spot U/Pb analyses at Institute of Earth Sciences, Academia Sinica. Zircon U/Pb isotopic analyses were performed by the LA-ICPMS technique using an Agilent 7500s ICP-MS and a New Wave 193-nm laser ablation system set up at the Department of Geosciences, National Taiwan University 33 . A spot size of 40 μm with laser repetition rate of 5 Hz was used. Calibration was performed by using the zircon standards GJ-1, 91500 and Plešovice for data quality control [34][35][36] . Measured U-Th-Pb isotopic ratios were calculated using the GLITTER 4.4.4. software 37 . Common lead was directly corrected using the common lead correction method 38,39 , and the weighted mean U/Pb ages and concordia plots were carried out using Isoplot v. 4.1 40 . The full table of zircon U/Pb results can be found in Dataset 1 and Dataset 2.
Plate Reconstruction. GPlates 1.5 software and the sample source data provided: http://www.earthbyte. org/Resources/earthbyte_gplates_1.5_data_sources.html was used to reconstruct paleogeography evolution of Seychelles during the Early Palaeogene. A global rotation model was used to reconstruct the paleogeography of India, Africa and Madagascar using Madagascar as the anchor plate 41 . The age of oceanic crust is reconstructed following the Global Present Day Age grid 42 . The evolution of Seychelles microcontinent is modified according to our new age results and previously models 3,4,14 .