Linking the thermal evolution and emplacement history of an upper-crustal pluton to its lower-crustal roots using zircon geochronology and geochemistry (southern Adamello batholith, N. Italy)

Abstract

The Val Fredda igneous complex in the southern Adamello batholith (N. Italy) consists of dioritic to gabbroic sills and dykes that were injected at 6–10 km depth into partly crystallized tonalites and granodiorites. High-precision U–Pb age determinations by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA–ID–TIMS) show very similar dispersion of zircon U–Pb dates over 90–200 ka and identical age distributions with a dominant mode at 42.5 Ma for six samples ranging in composition from gabbro to granodiorite. The co-variation of the probability density curves of zircon dates from mafic and felsic units suggests that they shared a common thermal history with periods of enhanced and reduced zircon growth, reflecting lowered and increased magma temperatures, respectively. However, trace element compositions, Ti-in-zircon temperatures and Hf isotopic compositions of zircon from mafic lithologies are distinctly different from those in felsic zircon and suggest their crystallization occurred in isotopically and chemically diverse magma batches. These magma batches formed in the lower crust from mingling and mixing of residual melts (derived from fractional crystallization of mainly amphibole from basaltic melt) with crustal partial melts at high temperatures above zircon saturation. Zircons crystallized during incipient cooling of these magmas and were entrained into the ascending melts, which were emplaced and rapidly solidified in the upper crust. The reported age dispersions imply that fractional crystallization and hybridization in the lower-to-middle crust, ascent into the upper crust and solidification did not last for more than 200 ka. The small magma volumes and flux also preclude significant zircon crystallization at the upper crustal emplacement level.