Multiple zircon growth during fast exhumation of diamondiferous, deeply subducted continental crust (Kokchetav Massif, Kazakhstan)

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deepl y subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism, and subsequent retrogression during exhumation , zircons from diamondiferous gneisses and metacarbonates have been investi gated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, ca thodoluminescence, trace element contents and mineral inclusions. Mineral i nclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain consists of roun ded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond , coesite, omphacite and titanian phengite inclusions providing evidence th at it formed at UHP metamorphic conditions (P > 43 kbar; T similar to 950 d egreesC). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granu lite-facies overprint of the gneisses (P similar to 10 kbar; T similar to 8 00 degreesC). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr- saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed wi thout coexisting garnet, most probably at amphibolite-facies conditions (P similar to 5 kbar; T similar to 600 degreesC). The metamorphic zircon domai ns were dated by SHRIMP ion microprobe and yielded ages of 527 +/- 5, 528 /- 8 and 526 +/- 5 Ma for domains 2, 3 and 4 respectively. These indistingu ishable ages provide evidence for a fast exhumation beyond the resolution o f SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 538 +/- 3 Ma, indicating t hat decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust a t rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P-T-t path. Fast exhumation on top of the subducted s lab from depth > 140 to similar to 35 kin was driven by buoyancy and facili tated by the presence of partial melts. A period of near isobaric cooling w as followed by a second decompression event probably related to extension i n a late stage of continental collision.