Preliminary report on the excess argon bearing K–feldspar from metagranite in the Brossasco–Isasca UHP Unit of Dora–Maira Massif, Italy

Abstract

The metagranite, that still preserves the igneous structure, is composed mainly of K-feldspar, plagioclase, quartz, biotite, white mica and accessory ilmenite. Quartz, K-feldspar, plagioclase and biotite preserve the original igneous shape, but are either re-equilibrated or replaced by new phases. Quartz occurs as fine-grained granoblastic aggregate, statically derived from inversion of coesite. Plagioclase is now composed of a finegrained mineral aggregate including albite, zoisite, phengite, titanite and apatite. Biotite is surrounded and partly replaced by fine-grained white mica.K-Ar analyses of K-feldspar from the metagranite were carried out, giving 42.6 ± 0.9 Ma. This age, significantly older than the SHRIMP zircon U-Pb age (35.4 ± 1.0 Ma) previously estimated for the metamorphic peak of the Brossasco-Isasca UHP unit, is apparent due to inherited excess argon (~3.4%) from the host lithologies. This inherited excess argon is interpreted as related to the fact that K-feldspar has trapped the excess argon wave generated by the argon release from micas (that have large amount of radiogenic argon) during exhumation and cooling of the host lithologies.

Timing of exhumation of meta-ophiolite units in the Western Alps: New tectonic implications from 40Ar/39Ar white mica ages from Piedmont Zone (Susa Valley)

“…This implies that, at least in the northern DM, the D1-related P-T peak age should predate the Early Oliogocene age calculated in the southern DM (i.e. in the Brossasco-Isasca Unit; Moniè and Chopin, 1991;Gebauer et al, 1997;Itaya et al, 2017), wherein a complex tectonic stack of continental crust and ophiolitic units occurs (Henry et al, 1993;Groppo et al, 2019;Balestro et al, 2020).…”

Laser step–heating ⁴⁰Ar/³⁹Ar analyses of biotites from meta–granites in the UHP Brossasco–Isasca Unit of Dora–Maira Massif, Italy

“…The high argon pressure environment created by the interaction between metasediments, peridotite and fluids would have allowed the trapping of a large amount of excess argon within phengitic micas. Recently, Itaya et al (2017) reported excess argon bearing K-feldspar from metagranite in the UHP Brossasco-Isasca Unit of DoraMaira Massif, Italy, and pointed out the possibility that the K-feldspar has trapped EAW generated by the argon release from micas during exhumation and cooling of the host lithologies. Thus, instead of total or partial resetting of the original age, the phenomenon of the trapping of the excess-argon by minerals through argon diffusion may be quite common in polymetamorphic MP-HP-UHP metamorphic rocks, depending on the closure temperature of minerals and on the presence of transient argon pressure.…”

“…Itaya et al (2009) and Itaya and Tsujimori (2015) later named the same phenomenon as 'Excess-Argon Wave'. Since then, the phenomenon of the trapping of 'Excess-Argon Wave' by minerals has been increasingly observed in biotite, kyanite, white mica, and K-feldspar from many types of lithologies (Itaya et al, 2005(Itaya et al, , 2009Itaya and Tsujimori, 2015;Itaya et al, 2017).…”

“…The perthitic exsolutions were likely formed during cooling of the Permian granite, while it intruded into the Variscan metamorphic basement. Biotite is surrounded and partly replaced by fine-grained white mica (Compagnoni et al, 2012;Itaya et al, 2017;Figs. 2B, 2C, 2E, and 2F Ar analyses of individual biotite crystals were carried out with the step-heating method in which the temperature ranges are from~500 to~1300°C.…”

“…The insets show the areas of photomicrograph of (E) and ( A d v a n c e P u b l i c a t i o n A r t i c l e igneous shape but are either re-equilibrated or replaced by new phases. Quartz occurs as a fine-grained granoblastic aggregate, statically derived from inversion of coesite (Compagnoni et al, 2012;Itaya et al, 2017;Figs. 2B and 2C).…”