MAGNETIC SUSCEPTIBILITY OF GRANITOIDS: DEVELOPING NEW, QUANTITATIVE INTERPRETATIONS

Magnetic susceptibility (MS) of granitoids is the simplest, quickest, information-rich, quantitative analytical measurement a field geologist can make; however it is rarely adopted as a principle data target. Its limited employment is probably explained by the geoscientific landscape which hosted efforts of two earlier phases of measurement. I present new MS datasets, including those spanning the Scottish Caledonides and the Salinian segment of the late Cretaceous Southern California Batholith, in concert with regional and global geochemical and isotopic compilations to demonstrate the utility and potential of scalar MS when integrated systematically with geology.

1^st-generation paleomagnetists often reported MS, however these measurements preceded mature relevant interpretative frameworks in plate tectonics and petrology by decades. Later renewal by economic geologists, applying “iron series” (i.e, “magnetite-” or “ilmenite-series”) magmatic redox interpretation to granitoid MS, achieved widespread adoption as a resource prospective tool. However, today scalar granitoid MS is mostly a byproduct of pluton-scale anisotropy studies; its initial promise as an impactful scientific variable unto itself is unfulfilled.

A significant reason for this depreciation is that as petrographic opaque oxides were compared against field identification of mica and accessory minerals, the latter observations aligned closer with Sr and Pb isotopic indications of petrogenesis. Increasing maturity of the field of mineral magnetism also revealed that many factors might influence scalar MS of a rock. Justifiably, the “I-, S-type” classification scheme became preferred to iron-series as a simplistic assessment of melt origin. I suggest that several factors countervail these shortcomings:

Granitoid MS is measured across 7 orders of magnitude (commonly across 4), offering huge discrimination potential if used as a metric. Mineral magnetic and petrological caveats are addressed with high n and intentional sampling strategies. MS is poorly proxied by opaque petrography, and should not “classify” granitoids in a binary division. Examples show that detailed MS can reveal information about root-to-roof batholith architecture, crustal composition, and tectonic and geodynamic processes and evolution.