Petrogeochemical and geochronological framework of element mobilities during magmatic-metasomatic processes (Campiglia Marittima, Tuscany)

Processes of post-magmatic element mobilisation have been reported for a number of magmatic-hydrothermal systems occurring in diverse geodynamic contexts. To constrain the processes that control these types of systems is a challenging task because the effects of hydrothermal/metasomatic processes on element distribution are poorly known, also the relationships of alteration types to mineralisation stages have not been well documented. This study presents the results of a detailed petrogeochemical and geochronological investigation, involving mineral and bulk-rock analysis as well as a precise CA-ID-TIMS U-Pb zircon dating. Based on these data, a model has been developed to explain the role of metasomatic processes in mobilising elements in a granitic system and to define the timing of igneous events controlling the mobility of fluids observed at Campiglia Marittima magmatic-hydrothermal system. At Campiglia the occurrence of multiple magmatic events over about 1 Ma generated an intense metasomatic fluid circulation. These magmatic processes began with the emplacement of the Botro ai Marmi monzogranitic pluton (~5.4 Ma), that was followed by mafic and later felsic porphyritic bodies (from 4.9 to 4.5 Ma) crosscutting the contact aureole generated by the monzogranite intrusion in the carbonate host rock. The closing event is represented by the early Pliocene (~4.4 Ma) rhyolitic extrusive complex of San Vincenzo. The hydrothermal metasomatic activity related to the whole igneous cycle generated proximal endo- and exoskarn as well as distal mineralised skarn bodies, exploited for Cu-Pb-Zn-Ag for over twenty-seven centuries and which are considered as a classic example of contact exoskarn generated by the interaction between a magmatic body and a marble host rock. This study focuses on (i) the characterisation of the different metasomatic lithofacies occurring at the pluton-host carbonate contact, resulting from prolonged fluids-rock interaction, (ii) the identification of the metasomatic processes that generated chemical transformation and a consequent replacement of the original granite and host carbonate, (iii) the identification of the geochemical processes responsible for the significant element mobilisation, regarding also the local mobilisation of usually poorly mobile elements, such as HFSE and REE. Thus, this study carried out detailed textural and geochemical investigations of the monzogranite, the host carbonate, and the products of their hydrothermal- metasomatic alteration, in the field, under the optical and electron microscope, and by QEMSCAN, EPMA and (LA)- ICP-MS. The obtained results allowed to discriminate between metasomatic processes occurred at variable temperature, fluid composition and pH, as well as to recognise preferential pathways for fluid circulation. Moreover, the reconstruction of the mineral paragenetic sequence coupled with chemical analysis allows to reconstruct the sequence of these metasomatic events. To define the chronological framework of the multiple igneous episodes occurring in the Campiglia Marittima area and to constrain the multiple hydrothermal episodes, a precise CA-ID-TIMS U-Pb dating has been carried out at the University of Geneva, on carefully selected zircon grains from the Botro ai Marmi monzogranite, the San Vincenzo rhyolite and the Temperino mafic porphyry crosscutting the metasomatic aureole. The Campiglia system offers exposures of the full range of emplacement types for magmas and related fluids, thus represents a prime case study to investigate the timescales of mechanism of magma deep storage, extraction, transfer, and shallow emplacement/eruption. New U-Pb CA-ID-TIMS geochronology from the Campiglia igneous system allows to reconstruct the evolution of crustal-derived and mantle-derived magmas that fed plutonic, subvolcanic and volcanic units over 1000 ka. Distribution of zircon ages is at odds with what can be expected for the crystallization interval of an igneous body. Indeed, that interval is short for the Botro ai Marmi pluton (100 ka), intermediate for the subvolcanic mafic Temperino porphyry (450 ka), and long for the volcanic San Vincenzo rhyolite (700 ka). The youngest zircons from the Botro ai Marmi granite have ages identical to 40Ar-39Ar ages of biotite from the granite and metasomatic phlogopite from skarn crosscutting the granite. The Temperino mafic porphyry and the San Vincenzo rhyolite show younger sanidine ages (emplacement/eruption age). The youngest zircon age from the pluton is therefore assumed to approximate the age of emplacement and final crystallization of the melt, whereas the zircons from the Temperino mafic porphyry and the San Vincenzo rhyolite are considered antecrystic, derived from re-mobilised earlier magma extracted from a deeper reservoir at the emplacement age. The new documentation of an extended period of crystallization for the Campiglia igneous system (about 1000 ka) matches with observations for the long-lived magmatic systems of Larderello and Elba Island. These data support the existence of multiple crustal reservoirs far larger than the outcropping igneous products. The observed mantle signature of metasomatic fluids suggests the presence of hidden mantle-derived reservoir able to activate episodically crustal melting and magma transfer to shallow levels. This sequential magmatic activity (both from mantle and crust) could be controlled by multiple, small batches of mafic magma, that did not lead to the formation of a single, homogeneous, hybrid pluton at the emplacement level.