A database of plagioclase crystal preferred orientations (CPO) and microstructures – implications for CPO origin, strength, symmetry and seismic anisotropy in gabbroic rocks

Abstract

Abstract. This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow- and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.

Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

“…9a, d). The axial-[100] CPO symmetry in both minerals was attributed to deformation producing prolate shape fabric geometries (Tomassi et al, 1999;Satsukawa et al, 2013;Chatzaras et al, 2016). Wrenchdominated transtension in Baja California (Legg et al, 1991;Umhoefer, 2011) could have induced constrictional strain in both the upper mantle and lower crust, leading to the formation of the observed axial-[100] CPO symmetries.…”

“…9a). The poles to (010) (010)[100] and (001)[100] plagioclase slip systems and may be associated with prolate shape fabric geometry (Satsukawa et al, 2013). Two xenoliths (SQW-110 and SQW-115) have poles to (010) and (001) planes at a high angle to the foliation and a small angle to the lineation, respectively (Fig.…”

Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks

“…All of these three end‐members are of transverse isotropy with its unique axis parallel to X (slow, Type III), Y (slow, Type I), or Z (fast, Type II). The laminated anorthosites from Sept Iles, Quebec (SI‐1) and the Glen Mountains layered complex in the Wichita Mountains of Oklahoma [89ANOK, Seront et al ., ], and the gabbroic anorthosite from the Britt Domain of the Grenville Province (LB‐80, this study) all display Type II seismic anisotropy which is caused by type A plagioclase CPO (Figure a) which is characterized by a single concentration of the poles to (010) at Z and complete girdles of [100] and [001] parallel to the foliation [e.g., Lamoureux et al ., ; Morales et al ., ; Satsukawa et al ., ; Seront et al ., ]. This type of plagioclase CPO could not be produced by nucleation and early growth of plagioclase because the plagioclase crystals enclosed in pyroxene oikocrystals display random CPO or SPO [ Higgins , ].…”

“…It is also possible, although less common, for the [001] directions of plagioclase crystals to be aligned parallel to the magma flow direction and for parallel (010) planes to form foliation in the flow plane [Type C fabric, Figure c; Brothers , ; Satsukawa et al ., ]. This fabric pattern results in seismic anisotropy with V p (Z) > V p (X) > V p (Y), which lies below the k p = 1 line (Figure b).…”

Hyperextension of continental to oceanic-like lithosphere: The record of late gabbros in the shallow subcontinental lithospheric mantle of the westernmost Mediterranean

“…2b) and EBSD data are more coherent with oriented crystallization of clinopyroxene within the gabbro dikes, than with syn-or postmagmatic plastic deformation of the gabbro. Based on the plagioclase CPO data we carried out Eigenvalue analysis of (010) and [100] pole figure symmetry in plagioclase by calculating BA-indices as proposed by Satsukawa et al (2013), defined as BA = 0.5 × [2 − (P (010) / (G (010) + P (010) )) − (G [100] /(G [100] + P [100] ))], where P and G stand for point-and girdle-like distribution of a given axis, respectively. The calculated values (0.58 and 0.61 for samples SA93A-1A and SA93A-3, respectively) correspond to a P-type plagioclase fabric, which is frequent in rocks that record magmatic texture with minor, if any, overprint of plastic deformation.…”