Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion

Abstract

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Factors Affecting the Thickness of Thermal Aureoles

“…In the field, the limit of the thermal aureole is determined by microcline becoming disordered sanidine and by quartz recrystallization, corresponding to an isograd of ∼500 • C. The edge of the aureole at the floor and lower wall of the intrusions is at 390 m from the contact. Bufe et al (2014) note that the aureoles temperatures at this level are lower (about 100 • C less) than the temperature at the intrusion roof and upper wall (Manning et al, 1993). The difference is attributed to heterogeneous release of latent heat of crystallization.…”

“…Asymmetric aureoles can be explained by incremental emplacement (Figure 10) or by the presence of a fault on one side of the pluton along which the intrusion has been exhumed (Alvarez-Valero et al, 2014). Interestingly, higher temperatures at the roof than at the floor are reported for Skaeregaard mafic intrusion (Bufe et al, 2014) whereas the opposite is true for Manaslu leucogranite intrusion (Guillot et al, 1995). It suggests a magma emplacement by over-accretion for the mafic case and by under-accretion for the felsic case in good agreement with geochronological data that indicate ages that become younger toward the floor for incrementally-emplaced granite intrusions and toward the roof for mafic intrusions (Leuthold et al, 2012).…”

“…Several asymmetric aureoles have been reported (Guillot et al, 1995;Alvarez-Valero et al, 2014;Bufe et al, 2014). Asymmetric aureoles can be explained by incremental emplacement (Figure 10) or by the presence of a fault on one side of the pluton along which the intrusion has been exhumed (Alvarez-Valero et al, 2014).…”

“…Only exceptionally do outcropping conditions allow for the simultaneous observation of the different sides of a pluton. Bufe et al (2014) fitted the thermal aureoles of the basaltic layered intrusion of Skaergaard (Greenland) with an instantaneously emplaced magma sphere 4 km in radius. In the field, the limit of the thermal aureole is determined by microcline becoming disordered sanidine and by quartz recrystallization, corresponding to an isograd of ∼500 • C. The edge of the aureole at the floor and lower wall of the intrusions is at 390 m from the contact.…”

“…Black diamonds are data from Reverdatto et al (1970) for meso-or hypabyssal metamorphic facies that are amphibolite hornfels and muscovite hornfels. Red dots are for (1) the Manaslu (Guillot et al, 1995), (2) the Alta stock (Cook and Bowman, 1994), (3) the Western Adamello tonalite (Floess and Baumgartner, 2015), (4) the Cupsuctic (Bowers et al, 1990), (5) the Skaergaard (Bufe et al, 2014).…”