Lattice-preferred orientations of late-Variscan granitoids derived from neutron diffraction data: implications for magma emplacement mechanisms

Abstract

The lattice-preferred orientation (LPOs) of two late-Variscan granitoids, the Meissen monzonite and the Podlesí dyke granite, were determined from high-resolution time-of-flight neutron diffraction patterns gained at the diffractometer SKAT in Dubna, Russia. The results demonstrate that the method is suitable for the LPO analysis of polyphase, relatively coarse-grained (0.1-6 mm) rocks. The Meissen monzonite has a prominent shape-preferred orientation (SPO) of the non-equidimensional minerals feldspar, mica and amphibole, whereas SPO of the Podlesí granite is unapparent at the hand-specimen scale. The neutron diffraction data revealed distinct LPOs in both granitoids. The LPO of the non-equidimensional minerals feldspar, mica and amphibole developed mainly during magmatic flow. In the case of the Meissen monzonite, the magmatic flow was superimposed by regional shear tectonics, which, however, had no significant effect on the LPOs. In both samples, quartz shows a weak but distinct LPO, which is atypical for plastic deformation and different in the syn-kinematic Meissen monzonite and the postkinematic Podlesí granite. We suggest that, first of all, the quartz LPO of the Meissen monzonite is the result of oriented growth in an anisotropic stress field. The quartz LPO of the Podlesí granite, which more or less resembles a deformational LPO in the flattening field of the local strain field, developed during magmatic flow, whereby the rhombohedral faces of the quartz crystals adhered to the (010) faces of aligned albite and to the (001) faces of zinnwaldite. Due to shape anisotropy of their attachments, the quartz crystals were passively aligned by magmatic flow. Thus, magmatic flow and oriented crystal growth are the major LPO-forming processes in both granitoids. For the Meissen monzonite, the solid-state flow was too weak to cause significant crystallographic re-orientation of the minerals aligned by magmatic flow. Finally, the significance of our results for the evaluation of the regional tectonic environment during magma emplacement is discussed. The discussion on the regional implications of the more methodologically oriented results provides the basis for future, more regionally aimed studies in view of the fabric characteristics of such plutons and their developing mechanisms.

Potential of full pattern fit methods for the texture analysis of geological materials: implications from texture measurements at the recently upgraded neutron time-of-flight diffractometer SKAT

“…Neutron diffraction has proven to be the most suitable and promising method for texture analysis of polymineralic rock samples (e.g. Brokmeier, 1994;Siegesmund et al, 1994;Wenk, 1991;Wenk et al, 2001;Leiss et al, 2002;Xie et al, 2003;Pehl & Wenk, 2005;Mü ller et al, 2010). Because of the low attenuation of thermal neutrons in matter large sample volumes can be investigated and good counting statistics are achieved.…”

Structure of the Castellaccio Pluton (Asinara Island, Italy)

“…Upper-crustal, fine-grained plutons are often characterized by weak shape preferred orientation (SPO) of K-feldspar phenocrysts, microgranular mafic enclaves (mme) and other flow markers (Johnson et al, 2003;Paterson, Pignotta, & Vernon, 2004), yielding to poorly constrained magmatic fabric. In these cases, the orientation of magmatic flow trajectories can be obtained by combining microstructural analysis (Mainprice & Nicolas, 1989;Muller, Leiss, Ullemeyer, & Breither, 2011;Cuccuru, Casini, Oggiano & Cherchi, 2012), geophysical methods (Kratinova et al, 2010;Román-Berdiel, Pueyo-Morer, & Casas-Sainz, 1995;Schöpa, Floess, de Saint Blanquat, Annen, & Launeau, 2015) and geochemical mapping (Puccini et al, 2014). On the other hand, flow markers are generally widespread in coarse-grained plutons emplaced at deep crustal levels; therefore, the internal structure can be thoroughly determined from field-structural analysis (Johnson et al, 2003;Marre, 1986).…”