The in-situ gamma ray spectrometry is a sound and proven technique to map radioelements variations of rocks and has been satisfactorily used in geological surveys of late-Variscan plutons belonging to Sardinia batholith, where good contact exposures between intrusive units are often obliterated by the occurrence of covers and by pedogenesis of rocks. This leads to frequent issues, as the definition of geological continuity of isolated intrusive units when common field indicators (e.g. magmatic flow, unconformities and chilled margins), usually used to constrain the anatomy of plutons are not applicable or conclusive. The Sàrrabus igneous massif (400 km2) is a multi-pulse, composite intrusive complex recording a complicated evolutive history marked by coalescing intrusions dominated by granodiorites, monzogranites and leucogranites. A continuous contribution of mantle pulses is documented by several mafic episodes -predating or coeval to-, granodiorites. Mantle-derived pulses evolve towards diffuse diking, which resulted in several generations of mainly NNW dike swarms. A true peraluminous character is only recognized for garnet-bearing two mica granites occurring as minor intrusions and NE trending acidic dikes. Within this scenario, some unsolved issues mainly regard the discrimination of different rock-units of granodioritic, monzogranitic and leucogranitic composition, which in the field display quite similar geological styles. The geo-statistical processing of 100 in-situ measurements of 40K, 232Th and 238U abundances by mean of a gamma-ray spectrometer -equipped with a 1 liter NaI (Tl) scintillator- generated a radionuclides map, which mirrors seven intrusive units improving the previous geological reconstruction. Indeed, the combined geological-petrographical and radiological mapping, allows to better constrain the anatomy and the emplacement sequence of the several intrusions in Sàrrabus massif. In detail, from older to younger, main intrusive units are: Burcei Unit (gabbrotonalites) ? Monte Cresia Unit (granodiorites grading to monzogranites) ? Cala Regina Unit (granodiorites with mafic septa and monzosyenites) ? Monte Maria Unit (peraluminous leucogranites) ? Bruncu Nicola Bove Unit (monzogranites grading to leucogranites) ? San Priamo Unit (leucogranites) ? Sette Fratelli Unit (monzogranites grading to leucogranites). In this way, an early southward growth model for the Sàrrabus igneous massif can be envisaged for granodioritic intrusions; it seems to be controlled by mixing of different crustal melts and mantle batches, along an E-W crustal shear zone. Overall, the surveys performed in the Sàrrabus igneous massif, the portable gamma-ray spectrometer has been confirmed as a useful tool to improve field surveys when conventional geological discrimination techniques are not conclusive.
Application of in-situ gamma-ray spectrometry to mapping intrusive complexes: examples from Sàrrabus igneous massif (SE Sardinia, Italy)
A M Conte;S Naitza;F Secchi;
2019
Abstract
The in-situ gamma ray spectrometry is a sound and proven technique to map radioelements variations of rocks and has been satisfactorily used in geological surveys of late-Variscan plutons belonging to Sardinia batholith, where good contact exposures between intrusive units are often obliterated by the occurrence of covers and by pedogenesis of rocks. This leads to frequent issues, as the definition of geological continuity of isolated intrusive units when common field indicators (e.g. magmatic flow, unconformities and chilled margins), usually used to constrain the anatomy of plutons are not applicable or conclusive. The Sàrrabus igneous massif (400 km2) is a multi-pulse, composite intrusive complex recording a complicated evolutive history marked by coalescing intrusions dominated by granodiorites, monzogranites and leucogranites. A continuous contribution of mantle pulses is documented by several mafic episodes -predating or coeval to-, granodiorites. Mantle-derived pulses evolve towards diffuse diking, which resulted in several generations of mainly NNW dike swarms. A true peraluminous character is only recognized for garnet-bearing two mica granites occurring as minor intrusions and NE trending acidic dikes. Within this scenario, some unsolved issues mainly regard the discrimination of different rock-units of granodioritic, monzogranitic and leucogranitic composition, which in the field display quite similar geological styles. The geo-statistical processing of 100 in-situ measurements of 40K, 232Th and 238U abundances by mean of a gamma-ray spectrometer -equipped with a 1 liter NaI (Tl) scintillator- generated a radionuclides map, which mirrors seven intrusive units improving the previous geological reconstruction. Indeed, the combined geological-petrographical and radiological mapping, allows to better constrain the anatomy and the emplacement sequence of the several intrusions in Sàrrabus massif. In detail, from older to younger, main intrusive units are: Burcei Unit (gabbrotonalites) ? Monte Cresia Unit (granodiorites grading to monzogranites) ? Cala Regina Unit (granodiorites with mafic septa and monzosyenites) ? Monte Maria Unit (peraluminous leucogranites) ? Bruncu Nicola Bove Unit (monzogranites grading to leucogranites) ? San Priamo Unit (leucogranites) ? Sette Fratelli Unit (monzogranites grading to leucogranites). In this way, an early southward growth model for the Sàrrabus igneous massif can be envisaged for granodioritic intrusions; it seems to be controlled by mixing of different crustal melts and mantle batches, along an E-W crustal shear zone. Overall, the surveys performed in the Sàrrabus igneous massif, the portable gamma-ray spectrometer has been confirmed as a useful tool to improve field surveys when conventional geological discrimination techniques are not conclusive.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
