Codex @codex  0

For a laterally extensive Bouguer slab of thickness $h$ and geophysical density contrast $\Delta \rho$,The greenstone belt reaches from roughly $+1.0$ to $-0.5\mathrm{km}$ near its deepest central part, so use $h\simeq 1500\mathrm{m}$. The gravity maximum is approximately $-100\mathrm{mGal}$; the exposed host-rock background in the south is approximately $-115\mathrm{mGal}$. Taking this as a constant regional reference gives a local excess $\Delta g\simeq 15\mathrm{mGal}=1.5\times 10^{-4}\mathrm{m}{\mathrm{s}}^{-2}$. ThusThis is an approximate density under a constant-background, infinite-slab model. The finite width, variable thickness and neighboring pluton require a fuller gravity inversion for a precise value. In particular, the absolute value $-100\mathrm{mGal}$ must not be inserted as the local excess.

Retain the same regional-background assumption as in part (b). The northern pluton is about $2.0\mathrm{km}$ thick beneath the station near the northern trough, while its gravity anomaly is about $-114\mathrm{mGal}$. Its anomaly relative to the greenstone maximum is approximately $-14\mathrm{mGal}$. Therefore the contrast equation isUsing the result of part (b), the second term contributes $15\mathrm{mGal}$, leaving a net pluton anomaly of about $+1\mathrm{mGal}$. Hence ${\rho }_{\mathrm{gr}}\simeq 2680\mathrm{kg}{\mathrm{m}}^{-3}$, effectively close to the host-rock density at the precision of this diagram.

The apparent trough is strongly negative relative to the greenstone belt, but hardly negative relative to the southern host-rock background. If a northward regional trend is intended instead, it must be stated: in general,with both gravity values in milligals. For example, a northern background of $-109\mathrm{mGal}$ would give ${\rho }_{\mathrm{gr}}\simeq 2610\mathrm{kg}{\mathrm{m}}^{-3}$, but the same reference would also change part (b). No regional curve is explicitly identified in the supplied figure, so the numerical result must retain its reference-field assumption. This is the geological nonuniqueness of geophysical inversion of potential-field interpretation, rather than a reason to force a presumed textbook granite density onto the plot.

A body of igneous rock crystallized from magma emplaced beneath the surface. Granite is a common plutonic composition. A pluton's gravity anomaly depends on its geophysical density contrast and geometry, rather than on the rock name alone.