EN
We investigate a frustrated Lieb-Mattis-like spin-1/2 model that is a reference model for the corresponding square-lattice Heisenberg model describing the unusual magnetic properties of Ba_{2}Cu_{3}O_{4}Cl_{2}. Due to frustration we obtain a rich magnetic phase diagram. We find two critical temperatures in accordance with recent experiments on Ba_{2}Cu_{3}O_{4}Cl_{2}.