Abstract :
[en] We present preliminary results of a field and petro-structural study of the c. 419 ± 13 Ma [1] microdioritic Lessines intrusion (Belgium). It is part of a discrete magmatic belt located in the south-west margin of the Brabant Massif [2]. Up to now, the morphology of the Lessines intrusion and its relation with synchronous tectonics has been poorly constrained. Therefore, we performed detailed petro-structural analysis to unravel the mode and dynamics of its emplacement.
40 sites were investigated for detailed fabric analysis on oriented samples. Firstly, we measured on field stretching directions of basic enclaves and lineations of preferred orientation of feldspar phenocrysts. Secondly, the anisotropy of magnetic susceptibility was determined using low field KLY-4S Kappabridge susceptibilimeter (@ Université de La Rochelle, France). And thirdly, image analysis has been carried out on scans of three orthogonal sample faces using the Intercept method [3] to determine shape preferred orientations (SPO) of two sets of phenocrysts (melanocratic and leucocratic subfabrics).
The magnetic fabrics and the melanocratic subfabric defined by shape preferred orientations of ferromagnesian phenocrysts (chloritized amphiboles and pyroxenes) appear strongly correlated. These are mainly marked by prolate shaped ellipsoid (T<0), NE trending lineation with a plunge ranging from 60° to subvertical. They also define foliation oriented E-W to N120 and subvertically north-dipping. The bulk magnetic susceptibility (Km) ranges between 1957 and 3745 µSI, in agreement with a magnetic source mainly controlled by ferromagnetic s.l. phases. The magnetic anisotropy degree (P' parameter) is moderate (less than 1.10).
On the other hand, the leucocratic subfabric determined by SPO of feldspar phenocrysts shows similar foliation strikes (N090 to N120 striking) but generally subhorizontal to low plunging (~30°) E-W trending lineations.
We also focused on magma cooling features. Cooling prisms are dominantly trending N034 and plunging on average of 62° to the NE, normal to the host rock bedding and thus, to the cooling front. However, subsequent dykes (~ 50m wide) crosscut the magmatic body, creating their own cooling prisms. These dykes are E-W striking with subvertical dip.
To sum up, this preliminary investigation allows us to conclude that:
- The Lessines intrusion geometry is composite. The emplacement history of the main body has to be distinguished from E-W oriented subsequent subvertical dykes, compatible with a multi-phase emplacement of the diorite.
- All subfabrics foliations (feldspar and ferromagnesian image analysis combined to AMS) show dominant strike E-W to N120, suggesting a unique direction for the magmatic flow.
- However, two fabrics lineations can be distinguished: a first one defined by magnetic subfabric and the melanocratic subfabric and a second one marked by the leucocratic subfabric. This discrepancy could be due to differential record of the subvolcanic phenocrysts during a syntectonic emplacement.
- In light of these observations, the prominent high dips of the fabric are consistent with a subvertical magma flow. This latter could be attributed to local sill-like internal flowing features (intrusion vertical border); or, it could be related to a bigger intrusive body with a dyke-like morphology that provides magma in lateral sills.
[1] André, L. et Deutsch, S., (1984)
[2] Linnemann et al, (2012)
[3] Launeau et al. (2010)
