Four-stage building of the Cambrian Carion pluton (Madagascar)

Abstract

The 532±5 Ma old Carion pluton is a dark, porphyritic ferro-potassic granitoid emplaced near the late Pan-African Angavo mega-shear zone. A rough normal zoning from tonalitic to granitic compositions can be recognised in the field. Steep magmatic foliations are evidenced by K-feldspar megacryst preferred orientations. Microstructures are either magmatic or typical of incipient solid-state deformation in near solidus conditions. Magnetic susceptibility magnitudes (K) range from 11 to 111×10−3 SI in the pluton and can be correlated to the petrography (highest K values in the tonalites; lowest K in the granites; granodiorites in between). The susceptibility magnitudes display a complex zoning pattern. Combined with the arrangement of magnetic foliation trajectories, it is possible to delineate four nested sub-units, regarded as four magmatic pulses successively emplaced from the west to the east of the pluton. The four pulses are characterised by very similar magma geochemistry, but variable magmatic differentiation. The highest degrees of magnetic susceptibility anisotropies (up to 1·6) are observed along internal contacts between sub-units and along the borders of the pluton. The magnetic lineations are also steeply plunging in some places in each sub-unit, possibly imaging the different feeder zones. Magma emplacement occurred at the end of the activity of the Angavo shear zone, hence avoiding re-orientation of the magmatic structures by the late Pan-African transcurrent tectonics. The diachronicity of the four magmatic pulses is consistent with previously determined palaeomagnetic data, because only the two older sub-units display a magnetic reversal sequence, whereas the two youngest sub-units lack any reversion. Emplacement of these four magmatic batches was responsible for a strain aureole and suggests a diapiric mode of ascent.

Strathbogie batholith: field-based subdivision of a large granitic intrusion in central Victoria, Australia

“…layered aplitic to pegmatitic rocks near batholith roofs) or it may be more occult, discernible only in the geochemistry or isotope chemistry of the rocks. Magnetic imaging of some more magnetic granitic intrusions reveals the presence of what may be pluton-scale flow fabrics that trace the sites of magma upwelling and lateral flow in sheets (Cruden et al, 1999;Clemens and Benn, 2010;Razanatseheno et al, 2010).…”

The Shir-Kuh pluton (Central Iran): Magnetic fabric evidences for the coalescence of magma batches during emplacement

“…These successive magma pulses are also recorded by their internal structure marking either a continuous feeding, resulting in continuous structural trends, or a discontinuous feeding, resulting in abruptly more evolved magmas issued from different feeder zones, cross-cutting contacts and discordant structural patterns. Successions of magma batches in the infilling process of a pluton, using both petrographic and magnetic fabric data, is now frequently reported by authors such as, for example Razanatseheno et al (2010).…”

Anisotropy of magnetic susceptibility fabrics in syntectonic plutons as tectonic strain markers: the example of the Canso pluton, Meguma Terrane, Nova Scotia

“…There are also distinct fabric orientations in the nested portions of the pluton that would represent separate magma pulses, indicating the fabrics record the flow kinematics in the two pulses. The structural relationships in Figure 1A suggest that the pluton fabrics record magma flow that was decoupled from deformation in the host rocks, so they can be interpreted to indicate emplacement of the successive magma batches (Vigneresse & Bouchez 1997;Dehls et al 1998;Becker et al 2000;Molyneux & Hutton 2000;D'Eramo et al 2006;Razanatseheno et al 2009). This is an example of fabric in a post-tectonic pluton that can be used to infer an emplacement mechanism.…”