Autor(es):
Lopes, José Carrilho 
; Munhá, José ; Pin, Christian ; Mata, João
Data: 2011
Identificador Persistente: http://hdl.handle.net/10174/3645
Origem: Repositório Científico da Universidade de Évora
Assunto(s): alkaline rocks; geochemistry
Descrição
Geochemical characterization of alkaline gneissic rocks of Alentejo (Portugal)
J. Carrilho Lopes1, J. Munhá2, C. Pin3, J. Mata2
1Departamento de Geociências, Universidade de Évora & Centro de Geologia da
Universidade de Lisboa, Portugal.
2Departamento de Geologia & Centro de Geologia da Universidade de Lisboa, Portugal.
3Département de Géologie, C.N.R.S., Université Blaise Pascal, France.
carrilho@uevora.pt
This study presents and interprets, on a petrological/petrogenetic point of view, a set of
mineral and whole-rock geochemical data collected from the so called “Alkaline Province of
Northeast Alentejo”, a group of gneisses that outcrops in lithostratigraphic domains known as
Ossa-Morena Zone and Blastomilonitic Belt. It’s composed by felsic gneissic rocks of
(per)alkaline type, represented by syenites with sodic inossilicates (riebeckite and/or
aegirine), nefelinic syenites and hastingsitic syenites, as well as hedenbergitic granites. Most
of riebeckitic syenites presents (Zr/Nb)<10, (Y/Nb)<0.7 e (Th/Nb)<0.3, while hastingsitic ones
and hedenbergitic granites reveal higher values of these ratios (15.0, 2.0 e 0.6, respectively).
The highest contents of Zr (4800 ppm) are also observed on peralkaline terms, with minimum
values measured on alkaline granites (135 ppm). Maximum contents of F (6100 ppm) and Cl
(7233 ppm) have been determined on riebeckitic and nefelino-sodalitic syenites, respectively,
and seems that halogenous contents may be correlated with devolatilization processes,
deformation/micro-fracturation and REE mobility. Even though irregular crystallization of
phases which consume high contents of REE (e.g. allanite) can disturbe the correspondent
geochemical signatures, it is still possible to identify, in most of the cases, distinctions
between maximum values of (La/Sm)N , (La/Lu)N and (Gd/Lu)N of peralkaline rocks (29.6,
11.6, 2.4), hastingsitic syenites (14.8, 4.8, 2.0) and alkaline granites (4.0, 3.1, 1.7).
Riebeckitic and nefelinic facies present, simultaniously, the sharpest negative anomalies of
Ti and the less marked negative anomalies of Nb (means of 0.9 and 0.7), which can be
interpreted as a result of differentiation processes with small to moderate contributions of
crustal contamination; comparatively, this anomaly is higher in hastingsitic (0.6) and granitic
terms (0.4). Obtained in a small set of mafic and felsic samples, Rb-Sr and Sm-Nd isotopic
data, show the vulnerability of the first system to post-magmatic processes. Peralkaline rocks
show (+2.5<eNd480<+4.9) values which reflect the origin of these magmas from timeintegrated
depleted mantle sources, that were enriched in LREE at the time of, or shortly
before, the igneous episode in an intracontinental rift setting. Sr-Nd petrogenetic modelling
adds complementary information: i.) intracontinental alkaline character of (primary) basaltic
magmas as precursors of this alkaline province; ii.) low to moderate crustal contamination
during differentiation processes, namely 7% to 20% for peralkaline syenites and about 26%
for alkaline granites.
