A MAGNETIC FABRIC STUDY ASSESSING THE STRUCTURAL AND TECTONIC CONTROLS ON THE EMPLACEMENT OF THE MAFIC GREENDALE IGNEOUS COMPLEX, ANTIGONISH HIGHLANDS, NOVA SCOTIA

The ca. 607 Ma Greendale Igneous Complex (GIC) is a predominantly mafic pluton located in the Antigonish Highlands within Avalonia of the Appalachian orogen. The complex is dominated by sheets of appinitic (hornblende-rich) mafic to ultramafic rocks with minor conjugate syn- to late felsic dykes and lenses. The appinites exhibit highly variable textures (aplitic to pegmatitic) and fabrics (strongly lineated; foliated to unfoliated; equigranular sheets). Sheet emplacement may have been controlled by dextral movement along NE-SW faults that bound the complex. New Anisotropy of Magnetic Susceptibility (AMS) data assess the structural and tectonic controls on the emplacement of the GIC and allow the identification of fabrics that may not be visible through traditional field methods.

Oriented core samples were collected from appinite sheets from a total of 68 sites at two localities (A and B). The rocks contain Ca-rich hornblende and plagioclase, minor pyrrhotite, titanite and rutile, and trace amounts of titanomagnetite and pyrite. Curie point measurements indicate that pyrrhotite is the dominant ferromagnetic phase along with trace titanomagnetite. Kmean values range between 3.17×10^-4 and 1.73×10^-3 (SI units) and isothermal remanent magnetization acquisition curves reach 95% saturation at ≤ 0.2T. The absence of significant amounts of moderate to high coercivity phases has inhibited further paleomagnetic analysis. AMS data yield tensors that are generally consistent along strike within sheets but vary from sheet to sheet, suggesting that the fabrics have not undergone a tectonic overprint. Magnetic fabrics in locality A have a strong correlation with the orientation of the hornblende, which are preferentially oriented perpendicular to the sheet contacts. The magnetic lineations are typically sub-parallel to the c-axis of the hornblendes. Fabrics in locality B are weaker due to a less pronounced orientation of the hornblende crystals and generally exhibit fabrics parallel to the sheet contacts. The differing fabrics suggest different emplacement regimes; locality A was dominated by extension, facilitating large, oriented crystals, whereas locality B was influenced by a stronger magmatic flow component. The overall results indicate dynamic and variable modes of emplacement within the GIC.