Microdiamonds, their classification and tectonic implications for the host eclogites from the Dabie and Su-Lu regions in central eastern China

Abstract

We have found >10 in situ microdiamonds in thin sections of eclogites from the Dabie and Su-Lu regions of central eastern China since the first occurrence of microdiamond in eclogites from the Dabie Mountains (DMT) reported in 1992. The microdiamonds are found not only in the central part but also in the northern part of the DMT. Several free crystals have been recovered from the crushed eclogites from the central DMT. Most in situ microdiamonds are inclusions in garnets but a few larger ones are intergranular. Most of the diamondiferous eclogites in the central part of the DMT are associated with coesite. Most importantly, the observation of microdiamonds in northern Dabie lead us to question the supposition that this is a low-P metamorphic terrane. All the diamondiferous eclogites from both the north and central DMT are of continental affinity as demonstrated by their negative εNd values. Therefore, both the north and central eclogite belts in the DMT are considered to be from the deep subducted terrane. Five in situ microdiamonds and two free crystals are first reported in this paper. The dimensions of the in situ microdiamonds are 30–180 µm and the free crystals are up to 400–700 mm across. All the microdiamonds are confirmed as such by Raman spectroscopy. The results of an infrared spectroscopic investigation on two larger free crystals and two in situ microdiamonds show that all the microdiamonds from both the Dabie and Su-Lu regions are mixed types IaA and IaB diamonds and there is no indication of any synthetic microdiamonds in our samples because such synthetic microdiamonds are always rich in type Ib.

Diverse P–T paths of the northern Dabie complex in central China and its reworking in the early Cretaceous

“…Similar conditions of 1.1-1.4 GPa and 820-910°C were suggested by Liu et al (2001cLiu et al ( , 2005 and Xu et al (2002). Higher temperature conditions of 900-950°C and 0.7 GPa or >900°C were inferred by Xiao et al (2001) and from osumilite growth by Xu et al (2005). The pigeonite and monalbite exsolution in the host clinopyroxene of the retro-eclogite from Raobazhai indicates that the post-peak overprinting granulite metamorphism may have reached UHT conditions of 1000°C at 1-1.2 GPa (Tong et al, 2007a;Tong and Jahn, 2009) or 980°C (Liu and Jin, 2008).…”

“…Lower peak pressure conditions of 2.3 GPa at 790°C and 1.9 GPa at 750°C were based on garnet pyroxenite (retrograde eclogite) from Muzidian and Raobazhai by Zhang et al (2000) and Xiao et al (2001), possibly indicating the condition of post-peak HP stage. Higher peak UHP conditions of >4 GPa or >5-7 GPa at >900°C were inferred from the exsolved Rt-Cpx-Ap needles in garnet and the occurrence of microdiamond in the retro-eclogite relics or as zircon inclusions in the Luotian retro-eclogite and the Ta'erhe orthogneiss by Xu et al (2002Xu et al ( , 2003Xu et al ( , 2005 and Liu et al (2007a,b). Meanwhile, peak UHP P-T conditions of 3.5 GPa and P750-800°C or 4.0 GPa and 750°C were suggested from high-P experiments for the retro-eclogite relics in Shanguantuan (Malaspina et al, 2006) and from UHP CaEs-bearing Cpx exsolution in the retro-eclogite in Raobazhai (Tong et al, 2007a;Tong and Jahn, 2009).…”

“…1). These relics differ from those of the CDB, because they suffered intense overprinting of high-temperature granulite facies metamorphism, showing a strong retrogression to garnet pyroxenites or garnet granulites, with the development of granulite facies symplectite assemblages Xu et al, 2000Xu et al, , 2002Xu et al, , 2005Liu et al, 2001cLiu et al, , 2005Xiao et al, 2001;Malaspina et al, 2006;Tong et al, 2007a).…”

“…Thus, until recently, the tectonic nature of the NDC in particular whether Triassic subduction and HP/UHP metamorphism played a role has been an issue (Ernst et al, 2007;Jahn and Chen, 2007;Zhang et al, 2009). Different proposals have been put forward for the NDC such as: (1) it is the upper part of the subducted plate (Maruyama et al, 1994;Ernst and Liou, 1995); (2) it belongs to a Palaeozoic magmatic arc complex (Zhai et al, 1995) or part of the hanging wall of the North China block without UHP metamorphism; (3) it represents a Cretaceous extensional magmatic complex (Hacker et al, 1998Ratschbacher et al, 2000); (4) it represents a Jurassic migmatite dome (metamorphic core complex) involving Triassic subduction and UHP metamorphism (Faure et al, 1999;Zhong et al, 1999); (5) it is an extension of the Yangtze craton unaffected by the Triassic UHP metamorphism (Bryant et al, 2004); or (6) it belongs to part of the Yangtze plate that experienced Triassic deep subduction and UHP metamorphism Xu et al, 2000Xu et al, , 2005Liu et al, 2001cLiu et al, ,d, 2005Liu et al, , 2007aMalaspina et al, 2006).…”

Assemblages and Textural Evolution of UHP Eclogites from the Chinese Continental Scientific Drilling Main Hole

“…Zhang et al, 2000), and coesite inclusions in zircon from orthogneiss (Ye et al, 2000b;Liu et al, 2001). Microdiamond inclusions in garnet in eclogites have also been reported (S. Xu et al, 1992Xu et al, , 2005. Geochronological studies indicate that the UHP metamorphic event occurred during the Triassic (210-230 Ma; e.g., Li et al, 1993;Ames et al, 1996;Chavagnac and Jahn, 1996;Hacker et al, 2000;Jahn et al, 2003aJahn et al, , 2003bLiu et al, 2004aLiu et al, , 2005.…”