Chris Ofili, Tate Britain, London

Chris Ofili has his famous signature, of course: the ball of African elephant dung. But don't let that distract you from his achievement. The paintings in his retrospective at Tate Britain are among the most marvellous of the last 20 years. It's a public art, an art of luminous colour, an art of wild imagination. If that sounds a peculiar combination, then look at it this way: William Blake's influence on modern art has taken many forms, but none so strange or true.

Grounding it all there is the Ofili format – the very original means he devised for conveying pictorial meaning. The canvas isn't hung on the wall, but stood up against it, propped on two balls of elephant dung. The painting's title is often written on them in map pins. More pieces of dung are attached to the canvas. And the picture's surface is further adorned and decorated with dots, glitter, layers of resin, pieces of collage, which are arranged in radiant and symmetrical designs.

Opulent, dense and dazzling, it's a contemporary version of a gilded altar-piece. It makes painting into an impersonal medium. It is a kind of amber in which a subject is held suspended – and Ofili's paintings can accommodate almost any theme, however socially sensitive or filthy-minded.

Ofili is black. His material is black. Dung is made into a general symbol for blackness. The dots are inspired by Zimbabwean cave paintings. Rap and blaxploitation films are invoked, but recast into a new imaginary world. There's the extraordinary vision of Seven Bitches Tossing their Pussies Before the Divine Dung. There's the cartoon super-hero, Captain Shit.

Race and sex stereotypes are held up – for glorification or derision or what? Does the image revel in black masculinity, say, or mock it, or mock white fear of it? Ofili toys with attitudes in ways that are all-round ironic and provocative, and which conclude in sheer explosion: moral anarchy matching the visual energy.

His language is also capable of grief, just about. No Woman, No Cry, a severe, weeping female profile, pays homage to Stephen Lawrence's mother. And then there's religion. The Holy Virgin Mary, embellished with obscene collage, is certainly blasphemous, which is fine – but the blasphemy seems pointless.

The climax of work in this format is the installation The Upper Room. A darkened chamber holds 13 lit paintings. Each has an image of a monkey, holding a cup. It's a union of a chimps tea party and The Last Supper. Each one is allotted a colour, named on its dung-props in Spanish and glowing from its surface. This work holds a mysterious mixture of absurdity and reverence. It has no creed, but performs an act of adoration. It worships colours.

More affirmations followed. Ofili is rather unusual among contemporary artists in celebrating love, happiness and heterosexuality. A series of love scenes, painted in the colours of Marcus Garvey black nationalism (red, green and black) and with a beguilingly complex patterning, are probably his most intensely beautiful works.

But at some point Ofili was going to have to cut the crap. In the last five years all his devices have been shed: the adornments, patterns, dung attachments and props. The last two rooms display pure paintings, paint on canvases, hung on the wall. It becomes clear that what gave Ofili's art such power was also a way of playing safe. With all those devices, nothing could go seriously wrong. These new paintings are exposed things.

The first group protect themselves in another way, with darkness. They're painted in the narrowest range of deepest blues. The subject is often barely discernible, though when it is, a startling imagination is again revealed – see, if you can, Iscariot Blues. But in the final room, with its bright, loopy, expressive figuration, I don't know if I'd recognise these works as Ofili's if I didn't know, or that I'd stop to look. Ah well: mid-career retrospectives often end on a doubtful note. Nothing diminishes the riches that came before.

To 16 May (020 7887 8888)