I am not an expert on mango trees, as they do not grow in the area of the United States where I am from, but based on the pictures you've sent I feel confident in saying that you are dealing with a nutritional disorder rather than a disease.  The fact that the leaves are turning yellow and the veins remain green and the fact that it is happening on the new foliage at the top of the plant and not the older foliage near the bottom (or at least happening less at the bottom) are classic signs of iron deficiency. The specific yellowing of the leaves is often referred to as iron chlorosis. 

Now, there may be two reasons why this is happening.  One is that there is not enough iron in the soil where the tree is planted . The other is that there is iron in the soil, but the soil pH is not optimum, which effects the soil chemistry and means that the iron is not in a form that is chemically available to the plant.  It's also possible that the plants are not getting enough water, as that would affect its ability to take up iron from the soil, but I'd suspect you'd see additional symptoms if that were the case.

In the US, we offer soil testing services through many of our state departments of agriculture and/or agricultural universities. Those services tell you the pH of your soil and the observed levels of key nutrients such as iron, and provide recommendations for treatment based on your specific case. I'm not sure if such a service is available in Dubai, but if it is I'd recommend you take advantage of it.  If not, your other option is to give the plant a fertilizer with iron in it and see if the plant responds. If it does your issue is solved, at least for the moment (you'll probably want to continuously monitor it in case it recurs), but if it doesn't you may need to adjust the pH level and/or change your watering practices.