Once in a while, you walk into a club and find something that so exceeds your expectations, you remember why you keep going back into clubs in the first place. There’s an excitement steaming off the bandstand, a supercharged atmosphere–just what you’d have found if you wandered into Oz about six weeks ago, when keyboardist Jim Trompeter debuted this quartet. Trompeter, who was part of the Miami Sound Machine before returning to his jazz roots a year or two ago, is a ferocious improviser. And in hooking up with Pat Metheny’s rhythm section–Chicagoans Steve Rodby (bass) and Paul Wertico (drums)–he’s crafted a trio that’s emblematic of jazz in the 90s: a virtuosic blend of eclectic elements, under the banner of a New Mainstream. But the catalyst, and the real surprise, turned out to be saxophonist Rick Margitza, one of Trompeter’s college chums, who flew in from New York for the gig. Neither Margitza’s short stint with Miles Davis nor his debut album on Blue Note really foretold his authoritative command of the music’s legacy: his meaty solo on a top-speed version of “Cherokee,” the bebop warhorse, was a revelation. The quartet coalesces again this weekend, and the air-conditioning could be sorely taxed. Tonight and Saturday, Oz, 2917 N. Sheffield; 975-8100.