Geochemical characteristics of the organic matter in UPPMRs and the implication for fluid–rock exchange due to the retrograde metamorphism in the Dabie–Sulu orogenic belt, North China

The samples were collected from ultrahigh-pressure para-metamorphic rocks (UPPMRs) around 500–2000 m deep by Chinese Continental Scientific Drilling. The combined method of ultrasonic disintegrator and Soxhlet was strictly conducted to extract the indigenous hydrocarbons from the UPPMRs, to obtain non-contaminated and entire organic matter to reveal the geochemical characteristics and origin of hydrocarbons in the UPPMRs. Through gas chromatography–mass spectrometry analysis, the ratios of Pr/Ph in the soluble hydrocarbon were from 0.04 to 0.87. It was inferred that the precursor of extracts would be deposited in the anoxic setting. Based on the relative content among C27, C28 and C29 steranes, it was found that the main source of organic matter was marine algae. But the isomer ratios of C31 hopane 22S/(22S + 22R) were 0.39–0.69, and the distribution range of C29 sterane 20S/(20S + 20R) was 0.41–0.63, both of which reflected the equivalent maturity of organic matter was no more than 1% (Ro%). Therefore, the immature organic matter derived from algae obviously conflicted with its host rock, which experienced ultrahigh-pressure metamorphism. Therefore, this sort of immature organic matter probably is of secondary origin. Furthermore, the chromatography of n-alkane almost assumes bimodal distribution, and the data of Tmax ranges from 394 to 565°C, as S1/total organic carbon (TOC) ratios systematically increase with the corresponding Tmax. Therefore, it could be further proved that the organic matter in the UPPMRs probably is of mixed origin and is mainly derived from immigrant hydrocarbons during the stages of subduction and retrograde metamorphism. The emergence of Zr-in-rutile also indicates that the geological temperature was lower than that during the peak of metamorphism. Therefore, it could be inferred that the kerogen associated with rutile might go through diagenesis regularly at the stage of retrograde metamorphism. It was also shown that the δ18O value suddenly reduced up to −10‰ corresponding to the highest TOC (1820 μg/g) around the subduction fault. The relationship between the TOC and δ18O value indicated that the fluid–rock exchange reaction was the main reason for the immature organic matter present in the para-metamorphic rock. The returned subduction and retrograde metamorphism resulted in the activity of the formation fluid, which could prominently impact the geochemical characteristics of the para-metamorphic rock and should also be considered in the geodynamics research of the metamorphic orogenic belt.