Lemuria is a mythological place said to exist in Indian ocean, below current indian sub-continent.Was it ever mentioned in texts.
Lemuria is an imaginary story.
Yes. There is no concept of Kumari Kandam aka Lemuria before 1600s. During 17th century, it was placed between Australia and North America. Later it was renamed as MU and another continent was placed below the Indian ocean adjoining with Madagascar. Due to similarities between lands in Madagascar, Australia and southern India, it is believed that in ancient times, there may be a continent joining India and Africa(Madagascar).It was named as Lemuria. But today we discovered continental drift theory. As per this theory, India, Australia and Africa including Madagascar were joined many millions years ago. As the continents of Africa and India were joined, that's the reason why we have similarities between Africa and India. So, we don't need any continents in-between.
Coming to your question, let's see whether Lemuria is mentioned in texts before 1600s:
kumarikodum kodunkadal kolla.....
The mighty sea at the end of kumari(kanyakumari,not Kumari Kandam) submerged. Here the author speaks about sea around kumari submerging the puhar(keveri pattinam) port. Silappadikaram'also describes Kadal Vadimpalampa Nindra Pandyan said to have thrown his spear towards the sea. The sea retaliated by swallowing a large area including Pahruli river and Panmalai Adukkam.
It Records the same incident of the puhar being engulfed by sea. Silapathikaram and manimekalai both not being eyewitness accounts and known for gross exageration of facts clearly talk sea engulfing the city of puhar.
Sangam literary work, `Kalithogai' calls it Kadal vowal. The poem says that when tidal waves swept away his land, the Pandyan monarch did not despair, but forged ahead into the territories of Cheras and Chozhas and brought the invaded country under his sway, thus making good the loss of territory due to the sea-swell.
It records say sea taking the land (Tsumani, in which a smaller portion of land in Sri Lanka, not Kumari Kandam) in 4th century BC.
If you look at the various arguments, Lemuria is just another attempt to increase the antiquity of Tamil to pre-historic times.