  Partonic matter produced in the early stage of ultrarelativistic
nucleus-nucleus collisions is assumed to be composed mainly of gluons, and
quarks and antiquarks are produced at later times. To study the implications of
such a scenario, the dynamical evolution of a chemically nonequilibrated system
is described by the ideal (2+1)-dimensional hydrodynamics with a time dependent
(anti)quark fugacity. The equation of state interpolates linearly between the
lattice data for the pure gluonic matter and the lattice data for the
chemically equilibrated quark-gluon plasma. The spectra and elliptic flows of
thermal dileptons and photons are calculated for central Pb+Pb collisions at
the CERN Large Hadron Collider energy of $\sqrt{s_{_{\rm NN}}} = 2.76$ TeV. We
test the sensitivity of the results to the choice of equilibration times,
including also the case where the complete chemical equilibrium of partons is
reached already at the initial stage. It is shown that a suppression of quarks
at early times leads to a significant reduction of the yield of the thermal
dileptons, but only to a rather modest suppression of the $p_T$-distribution of
direct photons. It is demonstrated that an enhancement of photon and dilepton
elliptic flows might serve as a promising signature of the pure-glue initial
state.
