A group of small granitic stocks outcrop in an area of 4 km2 at Punta Falcone (northern Sardinia, Italy). The stocks, which belong to the Sardinia Corsica Batholith, were emplaced during the late tectonic phase of the Hercynian orogeny. Detailed field observations, and studies of the petrography, mineral- and whole-rock geochemistry were carried out on these stocks: systematic differences in both whole-rock and mineral compositions were found between the stocks. The granitic stocks which outcrop in the southwestern and northern sector of the Punta Falcone area were examined but briefly since they have fairly homogeneous compositions. The granitic stock which outcrops in the central sector, on the other hand, shows evidence of a complex evolutionary history. This intrusion shows compositional zoning from granodiorite to leucogranite. A two-stage model for the evolution of granitic magmas by an unmixing process between a solid and a liquid phase within a single magmatic batch is proposed on the basis of the geochemical characteristics of this granitic stock. In the first stage, the parental magma undergoes a 30–35% in-situ fractional crystallization, developing the solid and liquid phases represented by early crystallized minerals and residual liquid, respectively. Then, in the second stage, the relative proportions of the solid and liquid phases are modified by a filter pressing segregation mechanism which squeezes the liquid phase inward to the centre of the intrusion. Therefore, each sample represents a mixture between the early crystallized mineralogic assemblage (solid phase) and the residual liquid (liquid phase). The bulk chemical composition of the liquid phase was determined using bivariate diagrams with Sr as differentiation index. The bulk chemical composition of the solid phase was computed using a self-consistent “feedback system” which handles variations in the modal composition of the solid phase accompanied by variations in trace-element bulk distribution coefficients.
A geochemical approach to the evolution of granitic plutons: a case study, the acid intrusions of Punta Falcone (northern Sardinia, Italy)
POLI, Giampiero;
1991
Abstract
A group of small granitic stocks outcrop in an area of 4 km2 at Punta Falcone (northern Sardinia, Italy). The stocks, which belong to the Sardinia Corsica Batholith, were emplaced during the late tectonic phase of the Hercynian orogeny. Detailed field observations, and studies of the petrography, mineral- and whole-rock geochemistry were carried out on these stocks: systematic differences in both whole-rock and mineral compositions were found between the stocks. The granitic stocks which outcrop in the southwestern and northern sector of the Punta Falcone area were examined but briefly since they have fairly homogeneous compositions. The granitic stock which outcrops in the central sector, on the other hand, shows evidence of a complex evolutionary history. This intrusion shows compositional zoning from granodiorite to leucogranite. A two-stage model for the evolution of granitic magmas by an unmixing process between a solid and a liquid phase within a single magmatic batch is proposed on the basis of the geochemical characteristics of this granitic stock. In the first stage, the parental magma undergoes a 30–35% in-situ fractional crystallization, developing the solid and liquid phases represented by early crystallized minerals and residual liquid, respectively. Then, in the second stage, the relative proportions of the solid and liquid phases are modified by a filter pressing segregation mechanism which squeezes the liquid phase inward to the centre of the intrusion. Therefore, each sample represents a mixture between the early crystallized mineralogic assemblage (solid phase) and the residual liquid (liquid phase). The bulk chemical composition of the liquid phase was determined using bivariate diagrams with Sr as differentiation index. The bulk chemical composition of the solid phase was computed using a self-consistent “feedback system” which handles variations in the modal composition of the solid phase accompanied by variations in trace-element bulk distribution coefficients.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
