  We analyze instabilities of the collinear up-up-down state of a
two-dimensional quantum spin-S spatially anisotropic triangular lattice
antiferromagnet in a magnetic field. We find, within large-S approximation,
that near the end point of the plateau, the collinear state becomes unstable
due to condensation of two-magnon bound pairs rather than single magnons. The
two-magnon instability leads to a novel 2D vector chiral phase with alternating
spin currents but no magnetic order in the direction transverse to the field.
This phase breaks a discrete Z_2 symmetry but preserves a continuous U(1) one
of rotations about the field axis. It possesses orbital antiferromagnetism and
displays a magnetoelectric effect.
