Search

The tectonic setting on both the northern and southern sides of the Dabie Mountains reveals that low-grade metamorphic rocks are important constituents produced by the subduction of the oceanic crust prior to collision between the Sino-Korean and Yangtze cratons. The Zhangbaling Group/Mulanshan schist is a pre-Ordovician oceanic crust. The Sujiahe and Xinyang/Foziling Groups are trench sediments of the Ordovician-Devonian age, and constitute an accretionary prism associated with subduction. The Yangshan coal measures/Meishan Group was a forearc basin sediment of Carboniferous age, and was overthrust by the accretionary prism during collision. The Susong Group is composed of passive continental margin sediments of the Yangtze craton. Backarc basin sediments are postulated to be concealed by Mesozoic–Cenozoic sediments to the north of the Dabie Mountains. High-ultrahigh pressure terrains are exotic tectonic slices exhumed from depths, located between low-grade metamorphic rocks, and disturb the integrity of the earlier subduction orogen. Subduction occurred during the Ordovician to Devonian periods, and collision initiated at the beginning of the Permian.

Zircon SHRIMP U–Pb ages and cathodoluminescence (CL) images reveal that most zircon separated from two tonalitic gneiss samples in the northern Dabie high T / P metamorphic zone (NDZ) have four different domains: (1) inherited core, with clear oscillatory zoning, low- P mineral inclusions and high Th/U ratios, indicating the Neoproterozoic age of the protolith; (2) inner-mantle, with homogeneous CL intensity, low Th/U ratios of ≤ 0.09 and occasionally ultrahigh pressure metamorphic (UHPM) mineral inclusions such as diamond, garnet and rutile, which defined a weighted mean 206 Pb/ 238 U age of 218 ± 3 Ma, corresponding to the age of ultrahigh-pressure metamorphism; (3) outer-mantle with lower Th/U ratio and retrograde mineral inclusions, which yields a retrogressive age of 191 ± 5 Ma; and (4) rim, which is black luminescence, with relatively high Th/U ratios of 0.12–0.40 and a weighted mean 206 Pb/ 238 U age of 126 ± 5 Ma, indicating an overprint of the Early Cretaceous migmatization. Consequently, four discrete and meaningful age groups have been identified. Remarkably, the U–Pb ages of both UHPM zircon and retrograded metamorphic zircon from the NDZ are significantly younger than the U–Pb ages of 238 ± 3 Ma–230 ± 4 Ma for UHPM zircon and U–Th–Pb ages of 218 ± 1.2 Ma–209 ± 3 Ma for rutile and monazite overgrowths (representing the cooling or retrograded metamorphic time corresponding to amphibolite-facies) from the southern Dabie UHPM zone (SDZ), respectively. Combined with reported ages for UHPM rocks from the NDZ, SDZ and Huangzhen low- T eclogite zone (HZ), we found that the metamorphic ages of these three UHPM units in the Dabie orogen gradually decrease from south to north. This age distribution suggests that the three UHPM units represent 3 exhumed crustal slices, which are decoupled from each other and have different subduction and exhumation histories. Firstly, the subducted Huangzhen crustal slice was detached from the underlying subducted continental lithosphere, and then exhumed because of the buoyancy. In the meantime, subduction of underlying continental plate continued. After the initiation of this process, lithologic separations or decoupling within subducted continental crust occurred repeatedly several times to form an imbricate megastructure in the Dabie UHPM belt.

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.

As a contribution to the discussion about the “in situ” vs. “exotic” origin of the ultrahigh-pressure metamorphic (UHPM) rocks, and to the real size of the exhumed tectonic units, the Wuhe–Pailou Unit (WPU) in eastern Dabie Shan was studied. The WPU typically consists of two micas—epidote gneiss, marble with minor eclogite, and jadeite-bearing granofels with rare garnetite and eclogite nodules; these lithologies are enclosed within leucocratic granitic gneiss. Jadeite-bearing granofels is the most peculiar lithology of the WPU, being the only felsic rock in Dabie Shan still preserving extensive relics of the UHPM mineral assemblage. The UHP peak assemblage is coesite+jadeite+garnet±epidote±phengite±kyanite, with accessory rutile, apatite, and zircon. All eclogites consist of garnet, omphacite, quartz/coesite, rutile or high-Al titanite±phengite±clinozoisite±kyanite. Two micas—epidote gneiss associated with the jadeite-bearing granofels typically consists of epidote, muscovite, biotite, plagioclase, K-feldspar, rare garnet, and accessory rutile, titanite, apatite, and zircon. Locally, the WPU paragneiss contains garnet with inclusions of phengite, kyanite, rutile, and amphibole+Na plagioclase symplectite after omphacite. A clockwise P–T path was inferred for the WPU, which consists of an early prograde part up to coesite–eclogite facies conditions (P∼3.3 GPa and T∼720 °C), an early decompression stage and a late part characterised by a near-adiabatic decompression down to medium pressure (P∼1.1 GPa, T∼670 °C) and to low-pressure amphibolite-facies conditions, followed by a final cooling to greenschist-facies conditions. Both field and laboratory data consistently suggest that the WPU, characterised by the close association of paragneiss, marble, eclogite, and granofels, is a supracrustal metasedimentary sequence, which underwent subduction, UHPM, and exhumation as a coherent tectonic unit. Therefore, within collisional orogens, large coherent slices of continental crust may be subducted at depths greater than 100 km.

The kyanite quartzite from the ultrahigh-pressure metamorphic belt in the eastern Dabie Mountains is composed mainly of quartz, kyanite, talc and epidote, which contains a typical mineral assemblage of white schist. It suffered an eclogite facies metamorphism ofP = (2.4 ±0.3) GPa andT = 630 ° C together with coexisting eclogite. The protolith may be an aplite replaced by high-pressure fluid of eclogite facies.

This note reports the preiswerkite that was found as the first occurrence in the UHP metamorphic belt of the Dabie Mountain. It occurs in the quartz jadeitite, and coexists with plagioclase, taramite and magnetite as retrograde symplectite. The calculated temperature of formation for preiswerkite is about 600 – 620 °C (at P=1.0 – 1.5GPa), suggesting that the preiswerkite is an indicator mineral of high-grade greenschist facies retrograded from eclogitefacies rocks.

Under experimental conditions, characteristic fracture patterns can be produced on cleavage plates on mica by using a blunt tool. If stress is applied rapidly by striking the surface in a controlled way, a pattern known as the “percussion figure” is produced. When the stress is applied by steady pressure on the tool, a different but complementary pattern of fracture is formed. In sum, these induced fractures constitute the “pressure figure”. The orientation of each of these two sets of fractures with respect to the optical axial plane (OAP) of mica is different and therefore diagnostic of the manner in which they are produced. These patterns are distinct from those formed as a result of exsolution of Fe-Ti oxides which are commonly visible in sections of biotite cut parallel to the basal plane (001). A description is given of percussion figures produced by natural deformation in biotites from mylonite belts cutting the Proterozoic metasediments of the Feidong Group in eastern Anhui Province and another from Yunnan Province, China. The principal fracture of the natural percussion figure evidently is parallel to the (OAP) of the biotite and the other two sets are quite distinct as well, thus identifying it really as a percussion figure. Microscopic inclusions of sphene also are located along the crystallographically controlled fracture planes of the percussion figures. The data indicate that high strain rates would be required to form these natural percussion figures and that a special history of deformation must have affected the mylonites in which they occur. It is proposed that the homogeneous deformation of the mylonite in a ductile regime was complicated by strain hardening which led to episodes of abrupt stress itself relief (stick-slip) at rates of strain high enough to induce the formation of percussion figures in the biotites.

We report the first occurrence of a talc-kyanite assemblage typical of whiteschists in felsic ultrahigh-pressure metamorphic rocks of Dabie Shan, China. The whiteschist assemblage occurs in a leucocratic layer, less than half a meter thick, which crosscuts a coesite-bearing eclogite. Both its geologic setting and geochemistry suggest that the protolith was a felsic dyke, which suffered loss of some elements, in particular alkalies. The whiteschist consists of quartz, minor epidote, kyanite, talc and omphacite. Common accessories are rutile, apatite and zircon. Epidote is zoned, and the core includes quartz pseudomorphs after coesite and contains up to 5 wt% REE. Kyanite includes fresh omphacite with the highest jadeite content ( Jd ≤ 0.60). The host eclogite consists of omphacite (X Jd = 0.44 to 0.37 from core to rim), garnet, quartz, kyanite, epidote, phengite (Si ≤ 3.47 a.p.f.u.) and amphibole, and accessory rutile, apatite, zircon and ilmenite. Fresh coesite occurs in epidote, and its polycrystalline quartz pseudomorphs are included in both omphacite and kyanite. The whiteschist and the host eclogite share the same tectonometamorphic evolution, and show three metamorphic stages: (I) coesite-eclogite-facies stage at P ≥ 2.6–2.7 GPa and T = 710 ± 20°C; (II) early decompression stage at P ≥ 1.5–2.0 GPa and T = 650 ± 30°C; (III) late adiabatic decompression to P = 0.7 GPa and T = 670 ± 40°C.