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

Abstract

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply 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 investigated by a combined petrological and isotopic study. Four dierent zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P>43 kbar; T 950 °C). Domain 3 is characterised by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P 10 kbar; T 800 °C). 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 without coexisting garnet, most probably at amphibolite-facies conditions (P 5 kbar; T 600 °C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 5275, 5288 and 5265 Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 5283 Ma, indicating that 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 at 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 slab from depth >140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.

Raman mapping of coesite inclusions in garnet from the Kokchetav Massif (Northern Kazakhstan)

“…Garnet in these gneisses displays a large unzoned core and narrow rims with decreasing Ca and increasing Mg contents [28,29]. Similar zoning has been observed in some dolomitic metacarbonates [20]. It was suggested that the observed garnet zoning reflects decompression [20] from UHP to granulite-facies conditions.…”

“…The peak metamorphic temperature is constrained at 950-1000 • C [17][18][19][20]. A minimum pressure of 4.3 GPa is given by the presence of diamond and the maximum pressure of about 6.0 GPa is defined by the stability of dolomite [21,22].…”

“…A minimum pressure of 4.3 GPa is given by the presence of diamond and the maximum pressure of about 6.0 GPa is defined by the stability of dolomite [21,22]. A summary of the geology, tectonic setting and geochronological data for this region has been provided by [17,18,20,[23][24][25][26]. The diamondiferous rocks are present in the UHP unit I of the Zerenda Series.…”

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

“…2). They have homogeneous cores whereas slight variations in their composition occur in narrow ($100 mm) external zones (Shatsky et al, 1995;Korsakov et al, 1998;Hermann et al, 2001;Korsakov et al, 2002;Korsakov & Hermann, 2006). Diamond and coesite inclusions identified in the garnet porphyroblasts point to peak metamorphic conditions of P$4-6 GPa and T$950-1000 C (Sobolev & Shatsky, 1990;Hermann et al, 2001;Korsakov et al, 2007).…”

“…BSE images of garnet porphyroblasts and P-T paths for coesite-(a-d) and diamond-grade (e-f) metamorphic rocks from the Kokchetav massif plotted on metamorphic facies diagram (after Liou et al, 1998;Okamoto & Maruyama, 1999). The P-T stages indicated along the path of the coesite-and diamond-grade metamorphic rocks are based on petrologic data of Parkinson (2000), Theunissen et al (2007) and Hermann et al (2001), Korsakov et al (2007), respectively. Thick arrows indicate the different stages of garnet growth.…”