Rejuvenation of a granitic magma reservoir by mafic injections: Evidence from zircon UPb geochronology, Hf isotopes and trace element compositions (Porto complex, Western Corsica)

Abstract

The Porto complex (western Corsica, France) is a post-Variscan, shallow-level intrusive system where mafic and felsic rocks are closely associated. To assess the timeframe and impact of successive injections of mantle-derived melts on a granite magma reservoir and gain a better understanding on the evolution of the complex, we performed high precision UPb dating combined with trace element and Hf isotope compositions of zircon separated from both mafic intrusives and associated granites.

Zircons from the granites have similar ²⁰⁶Pb/²³⁸U ages and initial Hf isotopic compositions. The saturation of zircon in the felsic magma occurred before emplacement and mingling with the mafic magma, between 282.55 ± 0.28 Ma and 281.65 ± 0.42 Ma, and at ∼800 °C. The initial ɛ_Hf values of zircons (3.7 ± 2.0 to 5.3 ± 1.0) indicates that the felsic magma source experienced addition of crustal material to a mantle component. Interaction with late-hydrothermal fluids, most likely related to an event affecting the Porto complex after its emplacement, was responsible for the modification of LREE, Ti, Fe and Mn compositions in zircons affected by intense metamictization.

Zircons from the mafic rocks have slightly enriched initial ɛ_Hf values (5.7 ± 1.6 to 6.9 ± 1.6) relative to depleted mantle composition. UPb zircon dating identified two chronologically distinct mafic melts injections, at 280.93 ± 0.21 Ma and 279.63 ± 0.15 Ma, which are representative of the time of mafic magma emplacement and mingling with the felsic magma.

To explain the long-time interval occurring between the saturation of zircon in the felsic magma and the time the granite emplaced, mingled and crossed its solidus, we propose that during cooling, the zircon-bearing granite magma reservoir was intruded, most likely at the base of the felsic mush, by injections of hotter, mafic magmas that caused at least two successive events of rejuvenation and remobilization, which promoted the shallow intrusion of variably differentiated felsic magmas.