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

Abstract

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 percent) and illite cristallinity data (ICI > 0.3), all indicate a maximum of 5km burial for the argillaceous rocks whatever the distance to an intrusion. The K-Ar dating of the clays <2µm 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 percent). It is constituted essentially by neoformed stoichiometric magnetite (Fe3O4). In contrast, within the thermal aureole of the magmatic intrusions, the Cretaceous argillaceous rocks contain micron-sized pyrrhotite (Fe7S8), firmly identified through the recognition of Besnus transition at 35K. 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.

The thermal maturity of sedimentary basins as revealed by magnetic mineralogy

“…In addition, pyrrhotite may also be present closer to the intrusion contact where palaeotemperatures were above 220°C, (Katz et al, 1998). In the East Greenland volcanic margin, Aubourg et al (2014) reported the occurrence of >1 μm pyrrhotite, characterized by a well‐developed Besnus transition at 35K, in sediments sampled in the thermal aureole of a magmatic intrusion. Therefore, the pyrrhotite detected in our samples is most likely related to contact metamorphism where palaeotemperature exceeded 250°C.…”

“…Our rationale is based on the principle that the formation of new minerals carrying a remanent chemical magnetization reflects the thermal history of a rock (e.g. Aubourg, Techer, Geoffroy, Clauer, & Baudin, 2014; Kars, Aubourg, Pozzi, & Janots, 2012). In particular, changes in magnetic assemblages are often the result of precipitation of iron that is released during the alteration of pyrite (Brothers, Engel, & Elmore, 1996) or clays (Katz, Elmore, & Engel, 1998) during early diagenesis (Roberts, 2015; Roberts & Weaver, 2005; Rowan & Roberts, 2006; Rowan, Roberts, & Broadbent, 2009), thermal maturation of sedimentary rocks (Banerjee, Elmore, & Engel, 1997) and increasing burial (Aubourg, Pozzi, & Kars, 2012).…”

“…In the case of sedimentary basins affected by volcanism, several studies have connected the occurrences of magnetite, hematite and pyrrhotite with specific degrees of thermal and hydrothermal alteration in the contact metamorphic aureoles surrounding individual sill and dyke intrusions (e.g. Aubourg et al, 2014; Gillett, 2003; Katz et al, 1998) or around larger intrusions (Just, Kontny, Wall, Hirt, & Martín‐Hernández, 2004; Petronis, O'Driscoll, & Lindline, 2011). Indeed, the formation of these magnetic minerals is controlled by the fugacity of oxygen and sulphur that are specific to thermal aureoles (Gillett, 2003).…”

Contact metamorphic reactions and fluid–rock interactions related to magmatic sill intrusion in the Guaymas Basin

“…In sedimentary basins, the occurrence of pyrrhotite may be an indication of burial temperatures exceeding 200 • C (Crouzet et al, 2001). Several studies reported the formation of pyrrhotite in thermal aureoles of magmatic intrusions (Gillett, 2003;Wehland et al, 2005;Ledevin et al, 2012;Aubourg et al, 2014). At Site U1546, pyrite is present down to 350 m b.s.f.…”