Multiple crustal and mantle inputs in post-collisional magmatism: Evidence from late-Variscan Sàrrabus pluton (SE Sardinia, Italy)

The Sarrabus pluton is formed by multiple short-lived intrusions emplaced at about 286 Ma at shallow crustal levels within the external part of the South Variscan Orogenic Belt. A chemical and Sr and Nd isotopic study on the Variscan post-collisional magmatism from the Sarrabus pluton reveals the repeated bimodal character of the intrusions, in which heterogeneous crustal sources and mantle-derived calcalkaline magmas are involved. Products of this magmatic activity occur as intrusive units and mafic/felsic dykes intruded in post-collisional regime along extensional faults during tectonic exhumation. Pluton growth started with an early stage of emplacement of broadly granodioritic magma with subordinate mafic magma batches (stage 1) followed by large intrusions of metaluminous to subaluminous and subordinately peraluminous granites (stage 2). In stage 1, the occurrence of remnants of stratified olivine-bearing gabbroic rocks indicates the intrusion of mafic magmas which experienced low-pressure crystal/liquid fractionation. Mafic magmas may represent an external heat supply for melting of different crustal materials belonging to an inferred Precambrian crystalline basement underlying the Paleozoic rocks of the Variscan nappe pile. Strong evidence for heterogeneous crustal sources is constrained by isotope data. Peraluminous granites and felsic dikes display initial Sr-87/Sr-86 in the range of 0.7140 divided by 0.7215 and a roughly constant epsilon Nd286 (-7.4 to -7.5). Conversely, a peculiar less radiogenic character, in the range of 0.7030 divided by 0.7067/-5.5 divided by -6.2, is observed for metaluminous to subaluminous varieties. Calculated Neodymium Crustal Index (NCI) confirmed a progressive increase in crustal magmas generation during the pluton growth, from stage 1, recording minor mixing processes between mantle-and crustal-derived peraluminous melts, to stage 2, where extensive crustal melting occurred, originating metaluminous to subaluminous granites. Possible crustal sources for metaluminous/subaluminous and peraluminous granites are (Pan-African) amphibolites and metasedimentary rocks, respectively. Two-stage depleted-mantle Nd model ages cluster at 1.4 and 1.6 Ga for metaluminous/subaluminous and peraluminous granites, respectively, well comparable with other segments of the European Variscan belt. Remarkably, last magmatic pulses resulted in widespread subalkaline bimodal mafic/felsic dykes that overlapped the Sr-Nd signature recorded by major intrusions. This similar geochemical affinity between late dykes and the first intrusives may be related to decreasing temperature in the crust, which inhibited extensive mixing processes with the upwelling melts. At the same time, the presence of felsic intrusions in the shallow crust would have prevented the rise of more primitive basic magmas, which would have consolidated at depth. Finally, the high radiogenic character of Sarrabus mafic products, compared to coeval Variscan mafic rocks of Corsica and northern Sardinia, may be indicative of previous fractionation and mixing processes, possibly related to magmatic underplating of the lower crust.