In a small saucepan, heat the sparkling wine and 1/2 cup of the tangerine juice. Add the sugar and stir until dissolved. Place the mixture in a bowl and add the remaining tangerine juice. Chill completely in the fridge.
Fresh the mixture in an ice cream machine according to manufacturer's instructions. Sorbet will be soft. Place the sorbet in the freezer for several hours to firm up.
To serve, scoop the sorbet into chilled wine glasses or champagne flutes. Pour champagne over the sorbet. Serve immediately.
I used Minneola Tangelos but any kind of tangerine, or any other orange citrus could be used.