SF340 CREW ENCOUNTERS NMAC DURING GAR IN MIA AIRSPACE.

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SEVERAL MI FROM RWY 9R; GIVEN CLRNC TO LAND AND TO HOLD SHORT OF RWY 12. FOLLOWING A DASH 7. OUR RELATIVE DISTANCE BTWN EACH ACFT WAS DECREASING; BTWN 100-200 FT AGL. ADVISED THE FO TO PLAN FOR A GAR SINCE THE DASH 7 WAS STILL ON THE RWY. A FEW SECONDS LATER; I TOLD THE FO TO COMMENCE THE GAR. AS THE PNF BEGAN THE PROCESS OF 'CLEANING UP' THE ACFT AND ADVISED THE TWR WE WERE 'GOING AROUND;' TWR REPLIED WITH A R TURN WHEN SPD AND ALT PERMITTED. CLBING THROUGH 400 FT; MY ATTN WAS DRAWN TO AN OBJECT ON THE L. SIGHTED A B737 CLBING OFF RWY 12. IT WAS SLIGHTLY BELOW OUR ALT AND CONVERGING. THERE WAS NO RELATIVE MOVEMENT BTWN OUR 2 ACFT. I TOOK CTL OF THE ACFT AND BEGAN A HIGH ANGLE BANK TO THE R. GLANCING BACK OUT THE SIDE WINDSHIELD; I NOTED THE B737 STARTING TO TURN TO THE L. THE TWR ISSUED A TURN TO THE SW. ADVISED THE FO I HAD THE ACFT AND FINISHED THE BALKED LNDG PROC AND CLB CHKS. RETURNED TO LAND RWY 9R UNEVENTFULLY. OTHER THAN THE INITIAL LAND AND HOLD SHORT CLRNC INDICATING OPS IN PROGRESS ON RWY 12 WE DID NOT RECEIVE ADVISORIES REGARDING TFC. DO NOT RECALL HEARING THE TKOF CLRNC FOR THE B737. TCASII DID NOT PROVIDE GUIDANCE.