An investigation into the controls of granite plutonism in the Sierra da Freita region, Northern Portugal

Granite;

Reavy, Reginald John

University:University of St Andrews

Department:Earth & Environmental Sciences and Geography & Sustainable Development (Schools of)

关键词: Granite;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/2973/ReginaldJohnReavyPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

The Serra da Freita region of north central Portugal was chosen for studyas it displays the complex relationships between regional structure,plutonism, regional and contact metamorphism typical of this part ofIberia. The region was mapped on a scale of 1:10000.The Serra da Freita pluton, which intrudes the core of the Porto-Viseumetamorphic belt developed in the late Pre-Cambrian - Cambrian Beira Schists,is shown to lie in a sinistral transpressive shear zone, the Serra daFreita shear zone. Early structures are progressively modified over aprotracted period by shear zone deformation, during which timemetamorphism reached a peak and the granite was emplaced. Mapping of theintrusive contacts of the granite show that following initial intrusion ofa steeply inclined sheet of magma into the zone of highest strain, magmaswere injected into a region of progressively lower strain where the magmawas acconunodated as a nearly flat sheet. The distal end of this massballooned upvards to form the small intrusion of Castanheira which hasabundant biotite nodules which acted as near perfect strain markers.The main pluton is shown to intrude obliquely the core of a narrowmetamorphic belt characterized by parageneses of biotite,andalusite/staurolite, sillimanite, which maps distinctly from a youngercordierite sillimanite contact aureole around an adjacent quartz dioritebody.Several facies of granite within the pluton have been recognized;petrographical and structural studies allow the interpreted emplacemen~mechanism of these units to be integrated within a more general model forthe evolution of the shear zone.Geochemical analyses of major and trace elements show that certaincompositional trends within these facies cannot be simply related as partof a fractionation sequence. A model is put forward in which repeatedmelting of a heterogeneous source is followed by sequential emplacement ofdiscrete batches of magma as sheets and wedges wi thin the acti ve shearzone. A Rb-Sr whole rock isochron age of 324 Ma was obtained and thisdates not only the emplacement age of the syn-tectonic granite, but alsoconstrains the time of movement along the shear zone.Radiogenic and stable isotope data strongly point to the local high gradeBeira Schists as being sui table source rocks for generation of magmas wi thmarked S-type characteristics which now form the Serra da Freita pluton.180 values for the granites of 10.64 ± 0.24 - 13.00 ± 0.12 overlap thoseof the schists which lie in the range 12.38 ± 0.24 - 14.15 ± 0.4. Thewhole rock Rb-Sr isochron for the granite has an initial ratio of 0.7136 ±0.0008 (MSWD = 3.2).A regional and tectonothennal model is put forward in whichend-Palaeozoic oblique strike slip collision took place in theIbero-Armorican Arc. The resulting peturbation in continental heat flow,coupled with the possible effects of shear heating, fluid concentrationand local high ductility contrasts in the heterogeneous metasediments, areinvoked as being responsible for causing anatexis of the Beira Schists ata depth of 10-12 km, and the generation of granitic melts. Emplacement ofthese bodies gave rise to the Porto-Viseu metamorphic belt, into whichlater smaller higher-level melts were injected. It is argued that some ofthese later magmas which reached higher levels are now exposed as theconstituent facies of the Serra da Freita pluton. The Serra cia Freitashear zone, active throughout metamorphism, anatexis and magma emplacementwas a dominant feature of the geological history of the region.