Try something along the lines of :
string queryString = string.format("select {0} from {1}",fields,tableName);
List<ObjectParameter> someList= new List<ObjectParameter>();
ObjectParameter[] paramList = someList.ToArray();
ObjectQuery<tabletype> dynamicQuery = context.CreateQuery<tabletype>(queryString,paramList)</tabletype></tabletype>
Then you can do like:
var dataItems = from item in dynamicQuery select item;
return dataItems.ToList();
Good luck.