Abstract: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 empl… Show more
“…As in many other rifted-margin CFB provinces like the Karoo (Melluso et al 2008), silicic magmatism in the Deccan generally postdates thick flood basalt sequences (e.g. Sheth & Melluso, 2008;Owen-Smith et al 2013;Sheth & Pande, 2014;Shellnutt et al 2017).…”
Most continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with 40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.
“…As in many other rifted-margin CFB provinces like the Karoo (Melluso et al 2008), silicic magmatism in the Deccan generally postdates thick flood basalt sequences (e.g. Sheth & Melluso, 2008;Owen-Smith et al 2013;Sheth & Pande, 2014;Shellnutt et al 2017).…”
Most continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with 40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.
“…(2017) for discussion of isotopic evidence). The present‐day subaerial volume of the DT lava flows is about 600,000 km 3 (Jay & Widdowson, 2008; Richards et al., 2015) along with a significant volume offshore in the Arabian Sea (G. Bhattacharya & Yatheesh, 2015; Calvès et al., 2011; Fainstein et al., 2019; Gombos Jr et al., 1995; P. Kumar & Chaubey, 2019; D. Pandey et al., 2011), and some small‐volume Deccan‐related intrusions in Seychelles (Devey & Stephens, 1991; Ganerød et al., 2011; T. M. Owen‐Smith et al., 2013; Shellnutt et al., 2017). Estimates of the total pre‐erosional DT lava flow volume range from 1 to 2 × 10 6 km 3 (Colleps et al., 2021; Jay & Widdowson, 2008; G. Sen, 2001; Sukheswala, 1981).…”
Section: Observational Constraints On Eruptive Tempomentioning
confidence: 99%
“…Pandey et al, 2011), and some small-volume Deccan-related intrusions in Seychelles (Devey & Stephens, 1991;Ganerød et al, 2011;T. M. Owen-Smith et al, 2013;Shellnutt et al, 2017). Estimates of the total pre-erosional DT lava flow volume range from 1 to 2 × 10 6 km 3 (Colleps et al, 2021;G.…”
“…This sequence is topped by the thick lower Grudja claystone and Grudja shaly marl. The Early Eocene unconformity corresponds to a stop of accretion in the Mascarene basin at C27, and a ridge jump to initiate seafloor spreading between Seychelles and India at C27 with the arrival of the Reunion hotspot initiating the Deccan Traps of India (66 Ma) and the basalts on the Seychelles micro‐continent (62.4 ± 0.9 Ma, Shellnutt et al., 2017).…”
We have analyzed the MZ6 onshore‐offshore wide‐angle seismic profile of the MOZ3‐5 survey to investigate the crustal structure of the South Mozambique passive margin. The NNW‐SSE, 625 km‐long profile runs across the Mozambique coastal plain (MCP), the Continental Shelf and Slope, the Almirante Leite Ridge (ALR), the North Natal Valley (NNV), the Ariel Graben and the Dana Plateau of the Mozambique Ridge. Forward modeling through combined interpretation of the multichannel seismic, the main reflected and refracted phases of the wide‐angle, drill hole data and bathymetric data reveal: (a) a sedimentary cover poorly compacted up to 3 km‐thick, intruded by magmatic dykes that reach the seafloor at the ALR forming 0.5 to 5 km‐wide corrugated mounts, (b) between 2 and 7 km, thick magmatic or volcano‐clastic deposits are observed both at the MCP and NNV, forming a 40 km‐wide terrace at the center of MZ6 and southward‐dipping reflectors in the southern part interpreted as the Pre‐Neocomian Formation contemporary of the Karroo and/or Bombeni‐Movene magmatic events reached in several wells, (c) onshore, the 3‐layered crust reaches 39 km thickness, gradually thinning to ∼27 km at the southern end of MZ6. In the deepest layer, velocities exceed 7.15 km/s, reaching at its base 7.55 km/s at the vertical of the ALR. (d) the seismic Moho is marked by a strong reflection on the wide‐angle data. These results suggests that the basement is composed of slightly thinned and altered continental crust, most likely intruded by several phases of intense magmatism.
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