Abstract
Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote–blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C–O–H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.
Highlights
The discovery of ultrahigh-pressure (UHP) minerals such as coesite and microdiamond in crustal metamorphic rocks has revolutionized our geodynamic view of the continental crust, which is considered as buoyant and not able to be subducted very deeply[1,2,3,4]
Temperature condition estimated by Raman microspectroscopy of graphite
The peak metamorphic temperature of the metapelite was estimated to be 450 °C according to Raman microspectroscopy of graphite[31,34]
Summary
The discovery of ultrahigh-pressure (UHP) minerals such as coesite and microdiamond in crustal metamorphic rocks has revolutionized our geodynamic view of the continental crust, which is considered as buoyant and not able to be subducted very deeply[1,2,3,4]. The SMB is a high pressure (HP)–low temperature (LT) metamorphic belt of Cretaceous in age, occurring along the Median Tectonic Line in the Outer Zone of Southwest Japan[25] (Fig. 1a). Radial cracks have developed around the inclusion, and in one example cracks are filled with quartz emerging from the quartz inclusion, which are dammed by chlorite derived from the matrix (Fig. 2d) This texture strongly suggests the transformation from coesite to quartz during exhumation of the Yukinoura mélange. Electron probe microanalysis-soft X-ray emission spectroscopy (EPMA-SXES) analysis showed a broad band derived from the sp3-bonded carbon of diamond (SI Fig. S3) in the spectra collected from inclusions in pyrite These lines of evidence strongly suggest that the present mineral assemblage of the metapelite represents a retrograde one except microdiamond
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