b'Don Emersons best of Exploration GeophysicsFeaturethe lower crust has distinctive geochemical characteristics.Much of the iron, however, is always sequestered within These basement terranes are often poorly correlated with theparamagnetic silicate minerals. If the rocks are paramagnetic, tectonostratigraphic terranes that are defined from the surfacethe susceptibility decreases monotonically with increasing silica geology. content. This occurs if the iron oxidation ratio of the rocks is low, particularly in the more evolved rocks. In that case, silicates Geological factors that control magnetisation oftake up the predominantly ferrous iron and the relatively small amounts of ferric iron can also be accommodated in silicates, intrusions mainly in hydrous phases. As the Fe oxidation ratio of the rocks Iron content and oxidation ratio increases, the silicates are obliged initially to take up more ferric iron. Once the oxidation ratio exceeds the maximum amount Many petrological studies of intrusive igneous rocks haveof ferric iron that can be accommodated in silicates, the excess been made that are relevant to the problem of definingferric iron is forced to appear as magnetite.the geological controls on magnetic properties in theseFigure 12(c) shows, for the same suite of granitoids considered rocks. To a good approximation, the magnetic susceptibilityin Figure 12(b), how the Fe oxidation ratios of hornblende and of intrusive igneous rocks is simply proportional to theirbiotite are correlated with oxidation ratio of the whole rock, magnetite content. The directly relevant chemical parametersindicating that these phases start to become saturated with are the total iron content of the rock, which constrains theferric iron at rock oxidation ratios above ~20%. Maximum ferric theoretical maximum attainable susceptibility, and theiron contents in these silicates are attained when the oxidation oxidation ratio (ferric/total iron), which essentially determinesratio of the rock is ~30%. When this ratio is exceeded, there is a the partitioning of iron between silicates and oxides (mainlysteady increase in magnetite content, until it constitutes ~20% of magnetite, in fresh igneous rocks). Figure 12(a) shows thethe mafic minerals, as the oxidation ratio increases up to ~70%. typical trend for major elements with increasing silica for aAbove this value, the whole rock ferric iron would be in surplus series of igneous rocks derived from a basic parental magma.for forming magnetite, especially when the large proportion Total iron tends to decrease steadily, but it is important toof ferrous iron in silicates is considered, and haematite or note that even the most felsic members of common igneousmaghemite would be present in addition to magnetite.rock series would contain sufficient iron to make them at least moderately to strongly ferromagnetic, provided that allIn mafic anhydrous rocks without amphibole or mica, however, the iron was contained in magnetite. Figure 12(b) gives anthe anhydrous silicates can accommodate much less ferric iron example of a total iron versus differentiation index trend for athan hornblende and biotite, and magnetite appears in such comagmatic suite of granitoids, showing that even the mostrocks at lower oxidation ratios. This explains why many gabbros evolved members of this suite have at least 0.5 wt %, andand norites are strongly magnetic, in spite of lower oxidation generally more than 1 wt %, total iron. ratios than for the granitoids considered in Figure 12(b)-(d).Figure 12.(a) Harker diagram showing typical differentiation trends for major elements in a comagmatic igneous rock suite. (b) Total iron versus differentiation index for a suite of granitoids from the Sierra Nevada Batholith (after Dodge 1972). (c) Oxidation ratio (%) for hornblende and biotite from the Sierra Nevada granitoids versus oxidation ratio (%) of whole rock. (d) Opaque mineral (essentially magnetite) contents as proportion of total iron-bearing mineral assemblage of the Sierra Nevada granitoids versus oxidation ratio of rock.APRIL 2020 PREVIEW 56'