I believe story was written between 1950-1970. I read it in a large paperback anthology in the 1980s. It was a short story - deep space encounter with alien intelligence. The first astronaut to experience alien presence is sublimely moved. Back on human spaceship doubts are raised - have the aliens bewitched the astronaut? Final revelation is that aliens & humans complement one another, become as one and both species reborn into new life.
It's a rather short, nonspecific question, but here's a go at what matches there are.
It was a short story -
I believe story was written between 1950-1970.
"Common Time" by James Blish was published in 1953.
I read it in a large paperback anthology in the 1980s.
It was extensively anthologized; a few of many large popular and reprinted anthologies available by the 1980s were "The Penguin Science Fiction Omnibus" and "The Arbor House Treasury of Modern Science Fiction".
...deep space encounter with alien intelligence.
A round trip to Alpha Centauri:
The ship was keeping ship-time, which was identical with observer-time. It would arrive at the Alpha Centauri system in ten months.
But he would live, if that were the case. His mind would arrive at Alpha Centauri six thousand years older, and perhaps madder, than his body, but he would live.
encounter with alien intelligence.
The thing (or things) which had restored him to consciousness, however, was — it was what? It made no sense. It was a construction, a rather fragile one, which completely surrounded his hammock. No, it wasn't a construction, but evidently something alive — a living being, organized horizontally, that had arranged itself in a circle about him. No it was a number of beings. Or a combination of all of these things.
How it had gotten into the ship was a mystery, but there it was. Or there they were.
The first astronaut to experience alien presence is sublimely moved.
The offer the clinesterton beademung had just made was enormously hearted, and he in turn was much minded and of love, to his own delighting as well as to the beademungen; that almost went without saying.
Privately Garrard did not faith as much, but he said, "Yes, we-they will make a new wooing of the beademungen at some other radiant. With all of love."
The book. The clinesterton beademung had dropped it there. But what under God was a clinesterton beademung? And what was he, Garrard, crying about?
Back on human spaceship doubts are raised - have the aliens bewitched the astronaut?
The protagonist Garrard asks the same question himself, reflecting on how profoundly he was affected.
When I met the Centaurians — if I did, and I'm not at all sure of that — they became the most important thing in my world, and my personality changed to accommodate and understand them. That was a change about which I couldn't do a thing.
"Possibly I did understand them. But the man who understood them wasn't the same man you're talking to now, Adolph. Now that I'm back on Earth, I don't understand that man. He even spoke English in a way that's gibberish to me. If I can't understand myself during that period — and I can't; I don't even believe that that man was the Garrard I know — what hope have I of telling you or the Project about the Centaurians? They found me in a controlled environment, and they altered me by entering it. Now that they're gone, nothing comes through; I don't even understand why I think they spoke English!"
The story ends shortly thereafter, without revelation, rebirth, or a conclusion about the alien life. It ends with the profound effect on Garrard, when his project leader tells him that there is no way that the project will ever let him make the trip and meet the aliens again.
Garrard nodded, but he knew that Haertel could see the slight movement of his eyebrows and the wrinkles forming in his forehead, the contractions of the small muscles which stop the flow of tears only to make grief patent on the rest of the face.
Garrard, however, could say nothing more. He had returned to humanity's common time, and would never leave it again. Not even, for all his dimly remembered promise, with all there was left in him of love.