= Solution
For a laterally extensive <Bouguer slab> of thickness $h$ and <geophysical density contrast> $\Delta\rho$,
$$
\Delta g=2\pi G\Delta\rho h,\qquad \rho_{\rm gs}=2670+\frac{\Delta g}{2\pi Gh}.
$$
The <greenstone belt> reaches from roughly $+1.0$ to $-0.5\ \mathrm{km}$ near its deepest central part, so use $h\simeq1500\ \mathrm m$. 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 $\Delta g\simeq15\ \mathrm{mGal}=1.5\times10^{-4}\ \mathrm{m\,s^{-2}}$. Thus
$$
\boxed{\rho_{\rm gs}\simeq2670+\frac{1.5\times10^{-4}}{2\pi(6.6743\times10^{-11})(1500)}\simeq2.91\times10^3\ \mathrm{kg\,m^{-3}}}.
$$
This 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.
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