Detecting the thermal aureole of a magmatic intrusion in immature to mature sediments: a case study in the East Greenland Basin (73°N)

Geophysical Journal International doi: 10.1093/gji/ggt396 Detecting the thermal aureole of a magmatic intrusion in immature to mature sediments: a case study in the East Greenland Basin (73◦ N) Charles Aubourg,1 Isabelle Techer,2 Laurent Geoffroy,3 Norbert Clauer4 and François Baudin5 1 Laboratoire des Fluides Complexes et leurs Réservoirs, CNRS (UMR 5150), Université de Pau, France. E-mail: GIS/CEREGE, CNRS (UMR 6635), Université de Nı̂mes et d’Aix-Marseille, CEREGE, 150 rue Georges Besse, F-30035 Nı̂mes Cedex 1, France 3 Université de Brest, CNRS, UMR 6538 Domaines Océaniques, IUEM Place N. Copernic, F-29280 Plouzané, France 4 Laboratoire d’Hydrologie et de Géochimie de Strasbourg, CNRS-UdS (UMR 7517), Université de Strasbourg, 1 rue Blessig, F-67084 Strasbourg, France 5 Institut des Sciences de la Terre – Paris (ISTeP), CNRS (UMR 7193), UPMC – Université Paris 06, 4 place Jussieu, F-75252 Paris Cedex 05, France 2 Laboratoire Accepted 2013 September 27. Received 2013 September 18; in original form 2013 February 07 SUMMARY The Cretaceous and Triassic argillaceous rocks from the passive margin of Greenland have been investigated in order to detect the thermal aureole of magmatic intrusions, ranging from metric dyke to kilometric syenite pluton. Rock-Eval data (Tmax generally <468 ◦ C), vitrinite reflectance data (R0 < 0.9 per cent) and illite cristallinity data (ICI > 0.3), all indicate a maximum of 5 km burial for the argillaceous rocks whatever the distance to an intrusion. The K–Ar dating of the clays <2 µm fraction suggests that illites are mostly detrital, except near magmatic intrusions where younger ages are recorded. To get more information about the extent of the thermal aureole, rock magnetism data were determined. At distance away from the thermal aureole of the syenite intrusion, Triassic argillaceous rocks reveal a standard magnetic assemblage compatible with their burial (R0 ∼ 0.4 per cent). It is constituted essentially by neoformed stoichiometric magnetite (Fe3 O4 ). In contrast, within the thermal aureole of the magmatic intrusions, the Cretaceous argillaceous rocks contain micron-sized pyrrhotite (Fe7 S8 ), firmly identified through the recognition of Besnus transition at 35 K. The thermal demagnetization of natural remanence carried by this pyrrhotite shows a diagnostic ‘square shouldered’ pattern, indicating a narrow grain size distribution of pyrrhotite. The extension of this diagnostic pyrrhotite maps a ∼10-km-thick aureole around the syenitic pluton. Away from this aureole, the magnetic assemblage is diagnostic of those found in argillaceous rocks where organic matter is mature. Key words: Magnetic mineralogy and petrology; Continental margins: divergent; Pluton emplacement; Arctic region. 1 I N T RO D U C T I O N Sediments intruded by a magmatic body potentially develop an aureole with minerals associated with the intrusion’s thermal field. These neoformed minerals could serve as proxies for a progressive burial of these host rocks (Bishop & Abbott 1995). Alternatively, identification and characterization of these minerals can help describe hidden thermal aureoles that might have some impact on the oil generation in source rocks. Focusing strictly on magnetic minerals of rocks that consist mostly of clay-type minerals (mudstones, claystones and siltstones), varied studies have connected the occurrence of iron oxides (magnetite and hematite) or iron sulphide (pyrrhotite) with the description of thermal aureoles (Katz et al. 1998; Gillett 2003). The conditions at which these mineral formed depend on the fugacity of oxygen and sulphide that prevailed when the thermal aureole developed (Gillett 2003). 160  C It is also known that iron sulphides and iron oxides form during burial (Brothers et al. 1996; Cairanne et al. 2004; Moreau et al. 2005; Kars et al. 2012). For instance, Aubourg et al. (2012) have defined the contours of a magnetic diagenesis consisting of three successive magnetic windows in which greigite, magnetite and pyrrhotite formed. Greigite forms in the anoxic subsurface up to several tens of metres (Roberts et al. 2011), being even the predominant magnetic mineral, and issues from the detrital iron oxides that were altered and dissolved due to bacterial activity. Crystallizing at depths of about 2 km (Kars et al. 2012), magnetite is nanometric in size with concentration of about tens of parts per million volume (ppmv). Micron-size pyrrhotite (Fe7 S8 ) forms to depths of about 8 km (Rochette 1987; Crouzet et al. 1999; Schill et al. 2002). It might also be added that these neoformed minerals coexist in ranges of depths that are still under debate. Greigite becomes unstable at temperatures above 200 ◦ C, which corresponds to a burial >8 km The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society. GJI Geomagnetism, rock magnetism and palaeomagnetism Geophys. J. Int. (2014) 196, 160–174 Advance Access publication 2013 November 7 Thermal aureole of a magmatic intrusion of the halo intrusion, the maturation of the organic matter and mineral composition of the clay fraction were also evaluated. The fine fractions of clays (<2 µm) were also dated by the K–Ar method. Overall, the evidence of organic matter and clay indicate that the burial of argillaceous series was less than 5 km, without demonstrating clearly a thermal perturbation or chemical intrusion in the halo. In contrast, analysis of magnetic data shows that micron-sized pyrrhotite develops in argillaceous rocks, which are presumably in the heat halo due to pluton intrusion. 2 GEOLOGICAL FRAMEWORK 2.1 Geodynamic evolution of the NE Atlantic province The NE Atlantic area (Fig. 1) is known to have had a complex structure and evolution. Following the Caledonian orogeny and throughout the Palaeozoic and Mesozoic times, the weak lithosphere located between the Greenland and European cratons suffered episodic periods of extension and thinning followed by long-term thermal subsidence and sedimentation. The maximum of stretching/thinning occurred at the transition between Jurassic and Cretaceous (ScheckWenderoth et al. 2007). This complex evolution contributed to build a complex rift system in the NE Atlantic with, locally, extreme crust thinning. The KT boundary was characterized by a transient regional uplift (Dam et al. 1998). During the Palaeocene epoch (i.e. ∼C27– C26 magnetochrons), large volumes of mafic magma intruded the NE Atlantic and continental crusts, and extruded as flood basalts. In both the Baffin Bay and NE Atlantic, the breakup and earliest oceanic accretion occurred during the Eocene time (Chron 24B, age Figure 1. Simplified tectonic map of the Greenland and Norwegian seas. The box localizes the Fig. 2. Three main stages of magnetic anomalies are indicated. RR, Reykjanes Ridge; KR, Kolbeinsey Ridge; MR, Mohn Ridge; WJMFZ, West Jan Mayen Fracture Zone; VB, Voring Basin. (Roberts et al. 2011). Magnetit (...truncated)