HARQ mechanism is used for forward error correction. MAC layer start HARQ process and control by PHY layer. whereas ARQ has significance at RLC layer only. In case of NACK received for the transmitted data, PHY layers doesn't retransmit same data. PHY send some different data(good enough) so that receiver take this re-transmitted data with the earlier received corrupted data and try to decode and then send either NACK or ACK to receiver.
In ARQ mechanism, same data is used for re-transmission. In LTE, for the better resource optimization, HARQ has been introduced.