  We study the relaxation of a single electron spin in a circular gate-tunbable
quantum dot in gapped graphene. Direct coupling of the electron spin to
out-of-plane phonons via the intrinsic spin-orbit coupling leads to a
relaxation time T_1 which is independent of the B-field at low fields. We also
find that Rashba spin-orbit induced admixture of opposite spin states in
combination with the emission of in-plane phonons provides various further
relaxation channels via deformation potential and bond-length change. In the
absence of valley mixing, spin relaxation takes place within each valley
separately and thus time-reversal symmetry is effectively broken, thus
inhibiting the van Vleck cancellation at B=0 known from GaAs quantum dots. Both
the absence of the van Vleck cancellation as well as the out-of-plane phonons
lead to a behavior of the spin relaxation rate at low magnetic fields which is
markedly different from the known results for GaAs. For low B-fields, we find
that the rate is constant in B and then crosses over to ~B^2 or ~B^4 at higher
fields.
