The in-situ gamma ray spectrometry is a sound and proven technique to map radioelements variations of rocks. Recently this technique has been satisfactorily used in geological surveys of the Variscan basement and late-Variscan plutons belonging to the northern part of the Sardinia batholith, dominated by coalescing intrusions of peraluminous monzogranites, which are hardly distinguishable each other in the field. In addition, at the scale of the entire Sardinia batholith, good contact exposures between intrusive units are often obliterated by the occurrence of colluvium covers and by regolith. This leads to frequent mapping issues, as the geological definition of single 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 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 were followed by diffuse diking, which resulted in several generations of mainly NNW mafic dike swarms. Peraluminous character is only recognized for garnet-bearing two mica granites occurring as minor intrusions and NE trending acidic dikes. In this scenario, some unsolved issues mainly regard the discrimination within different rock-units of similar composition and geological style.The geo-statistical processing of 100 in-situ measurements of 40K, 232Th and 238 U abundances by mean of a gamma-ray spectrometer -equipped with a 1-liter NaI (Tl) scintillator generated three radionuclides maps, which mirror 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: Burcèi 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) > Monte 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 has confirmed as the portable gamma-ray spectrometer is 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)
Naitza S.;
2019-01-01
Abstract
The in-situ gamma ray spectrometry is a sound and proven technique to map radioelements variations of rocks. Recently this technique has been satisfactorily used in geological surveys of the Variscan basement and late-Variscan plutons belonging to the northern part of the Sardinia batholith, dominated by coalescing intrusions of peraluminous monzogranites, which are hardly distinguishable each other in the field. In addition, at the scale of the entire Sardinia batholith, good contact exposures between intrusive units are often obliterated by the occurrence of colluvium covers and by regolith. This leads to frequent mapping issues, as the geological definition of single 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 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 were followed by diffuse diking, which resulted in several generations of mainly NNW mafic dike swarms. Peraluminous character is only recognized for garnet-bearing two mica granites occurring as minor intrusions and NE trending acidic dikes. In this scenario, some unsolved issues mainly regard the discrimination within different rock-units of similar composition and geological style.The geo-statistical processing of 100 in-situ measurements of 40K, 232Th and 238 U abundances by mean of a gamma-ray spectrometer -equipped with a 1-liter NaI (Tl) scintillator generated three radionuclides maps, which mirror 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: Burcèi 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) > Monte 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 has confirmed as the portable gamma-ray spectrometer is a useful tool to improve field surveys when conventional geological discrimination techniques are not conclusive.
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