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Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm−1and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm−1. These data combined with previously reported diamond band data (1331–1337 cm−1) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disorderedsp3-carbon, but show downshifting of the Raman band from the ideal value (1332 cm−1), and have a broader diamond band (FWHM >3 cm−1) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm−1. Our study together with previously reported C-isotopic compositions (δ13C, −17 to −27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of theP–Ttime of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane. [ABSTRACT FROM AUTHOR]

The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high-P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T-time path, suggesting exhumation rates of 5-10 km/Myr. A similar P-T-time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that δ18O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km³ along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric water-rock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks. [ABSTRACT FROM AUTHOR]

Abstract: The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality among numerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocks are interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granite gneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion, sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of zoned zircons from migmatites at the Kumdy-Kol region was performed to constrain the age of partial melting of the Kokchetav UHP metamorphic rocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206Pb/238U ages for core and rim domains of zirconsare nearly identical within analytical error. All SHRIMP analyses of zircons from four samples fall in the range 508–538 Ma with the weighted mean age for all zircon domains at 526±2.1 (MSWD=1.7). Our data show that migmatization of UHP pelites occurred later than the peak metamorphism (537±9 Ma) and the decompression partial melting took place during exhumation of diamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels (507±8 Ma). [Copyright &y& Elsevier]

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that abundant and complicated mineral inclusions occur in zoned zircon separates from paragneisses in drill holes CCSDPP2, ZK-2304, and CCSD-MH, at Donghai, southwestern Sulu terrane, China. Many low-P mineral inclusions are preserved in inherited zircon cores. Coesite + quartz within a single inclusion, and some low-P minerals (e.g., quartz and albite) are identified in zircon mantles. Textures showing the quartz to coesite transformation in zircon indicate that both core and mantle of the zircon are detrital. Inclusions of coesite, together with garnet, jadeite, and phengite, are common in zircon rims formed during ultrahigh-pressure (UHP) metamorphism. SHRIMP U-Pb analyses of these zoned zircon grains identify three discrete and meaningful age groups. Proterozoic ages of 840 to 658 Ma for zircon cores indicate that the detrital zircons have a variety of sources. An early Paleozoic group of 505 to 455 Ma ages for zircon mantles constrain neither the timing of a discrete UHP metamorphic event, nor the transformation age from quartz to coesite. The 227 ± 9 Ma (the weighted mean age) obtained from coesite-bearing zircon rims represents the UHP metamorphic age. Thus, one Late Triassic UHP metamorphic event, rather than two UHP metamorphic events, is well established in Sulu UHP terrane by the method of U-Pb SHRIMP dating for coesite-bearing zircon rims in garnet-biotite-amphibole-albite gneisses. [ABSTRACT FROM AUTHOR]

Examines the chronological evidence of Indo-Chinese strike-slip movement in the Altyn Tagh fault zone. Illustration on the product of the syntectonic anatexis resulting from ductile transpression; Types of zircon from mylonite; Measurement of zircon by ion microscopes.

Abstract: Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SHRIMP dates, and in situ trace element and Lu–Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz+Kfs+Pl+Ap inclusions, which record a Neoproterozoic protolith age of ∼790Ma, (2) mantles with Coe+Phe+Ap inclusions that record Triassic UHP age at 227±3Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at ∼210±3Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe+Phe+Ap inclusions record Triassic UHP age of 227±3Ma, and (2) outer magmatic areas with Qtz+Kfs+Ab+Ap inclusions that record partial melting time of 212±2Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz+Kfs+Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212±2Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201±2Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism. Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform 176Hf/177Hf of 0.28187±0.00003 (2SD; standard deviation) corresponding to εHf(790) and Hf model ages (T DM2) of about −16.3 and 2.41Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust. In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (<0.006), low Th/U (<0.1) and considerably higher, 176Hf/177Hf (0.28233±0.00002; 2SD) than the pre-metamorphic cores. The uniform but significantly different Hf isotope composition between the UHP (εHf(227) =−14.6±0.8; 2SD) and pre-metamorphic (εHf(227) =−27.7) domains indicates equilibration of the Lu–Hf isotope system only within the UHP metamorphic mineral assemblage. The disequilibrium between whole rock and UHP zircon suggests that about two thirds of the whole rock Hf retained in the pre-metamorphic zircon domains. Zircon domains crystallized during partial melting at 212Ma in granitic leucosome and pegmatites have a Hf isotope composition indistinguishable from that of the UHP zircon domains. This suggests that only Hf (and Zr) equilibrated during UHP metamorphism was remobilized during partial melting while pre-metamorphic zircon remained stable or was not accessible. In contrast, the magmatic zircon edges from pegmatite have somewhat lower 176Hf/177Hf (∼0.28216) and εHf(t) (−17.6±1.2; 2SD) indicating some release of less radiogenic Hf for instance by dissolution of pre-metamorphic zircon during late regional amphibolite-facies retrogression. [Copyright &y& Elsevier]