  Understanding how super-massive black holes form and grow in the early
Universe has become a major challenge since the discovery of luminous quasars
only 700 million years after the Big Bang. Simulations indicate an evolutionary
sequence of dust-reddened quasars emerging from heavily dust-obscured
starbursts that then transition to unobscured luminous quasars by expelling gas
and dust. Although the last phase has been identified out to a redshift of 7.6,
a transitioning quasar has not been found at similar redshifts owing to their
faintness at optical and near-infrared wavelengths. Here we report observations
of an ultraviolet compact object, GNz7q, associated with a dust-enshrouded
starburst at a redshift of z=7.1899+/-0.0005. The host galaxy is more luminous
in dust emission than any other known object at this epoch, forming 1,600 solar
masses of stars per year within a central radius of 480 parsec. A red point
source in the far-ultraviolet is identified in deep, high-resolution imaging
and slitless spectroscopy. GNz7q is extremely faint in X-rays, which indicates
the emergence of a uniquely ultraviolet compact star-forming region or a
Compton-thick super-Eddington black-hole accretion disk at the dusty starburst
core. In the latter case, the observed properties are consistent with
predictions from cosmological simulations and suggest that GNz7q is an
antecedent to unobscured luminous quasars at later epochs.
