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Architecture and construction mechanism of continental bimodal intrusions: insight from the stratified, multiply recharged Devonian Saint Jean du Doidt gabbro pluton, Brittany, France

English title Architecture and construction mechanism of continental bimodal intrusions: insight from the stratified, multiply recharged Devonian Saint Jean du Doidt gabbro pluton, Brittany, France
Applicant Bussy François
Number 116705
Funding scheme Project funding (Div. I-III)
Research institution Institut de Minéralogie et Géochimie Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geochemistry
Start/End 01.04.2007 - 31.03.2010
Approved amount 179'204.00
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Keywords (11)

gabbro; layered intrusion; magma chamber; Brittany; Variscan geodynamics; Armorica; stratified intrusion; zircon dating; alkaline granite; sill; Variscan

Lay Summary (English)

Lay summary
The aim of this project is to understand the construction mechanisms of a stratified gabbro-granite intrusion of late Devonian age in answering the following questions:
1. how did the magmas find the necessary space in the continentla crust?
2. How can we explain the temporal and spatial coexistence of mafic magmas of mantellic origin and granitic magmas of crustal origin?
3. What are the exact sources of the magmas?
4. What is the geotectonic context of the intrusion in the Variscan belt.
Pluton emplacement in the continental crust has long been a challenging question, in particular for km-sized granite intrusions. This "space problem", which is tightly connected to magma transport issues, has been addressed in different ways. Proposed mechanisms include, among others, en masse transport through diapirism, stoping of roof country rock and multiple injection of magma through vertical dikes, ponding in structural traps and resulting in the progressive construction of large bodies. The Devonian St Jean du Doigt gabbro pluton is a rare example in the Variscan belt of Western Europe of such a stratified, multiply recharged intrusion. Although essentially gabbroic, this pluton hosts volumetrically subordinate, but spatially omnipresent felsic rocks of unknown origin, which underline the sheet-like architecture of the intrusion. Spectacular rheologic features are developed at the interface of coeval mafic-felsic magma layers and hybrid rocks are common. Crosscutting dykes of basaltic to hybrid composition might be the feeders of the magma layers.
The St Jean du Doigt gabbro intrusion is also an important clue for the geodynamic reconstruction of Western Europe during Early- to mid-Variscan times. Indeed, the Late Carboniferous migmatization event and huge granite magmatism recorded in the inner zones of the belt have erased most traces of the early history of this orogen.
The aims and methods used in this project are: (1) to decipher the complex architecture of the gabbro intrusion through detailed mapping and reconstitute the magma dynamics and construction mechanisms of the pluton; (2) to precisely establish the time and duration of the St Jean du Doigt magmatism through high-precision U-Pb zircon dating; (3) to perform a complete chemical study of the various facies with XRF, LA-ICPMS, ID-TIMS, EPMA techniques in order to identify their respective source(s), including that of the felsic rocks and characterize the geotectonic setting of this segment the Variscan crust in Devonian times; (4) to quantify the small-scale, double-diffusive interactions between coexisting mafic and felsic magmas.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
130253 Architecture and construction mechanisms of continental bimodal intrusions: insight from the layered plutons of St-Jean-du-Doigt (Brittany, France) and Punta Falcone (Sardinia, Italy) 01.04.2010 Project funding (Div. I-III)