Reply to Zou et al.: Neoarchean eclogite-facies oceanic crust in the North China Craton.

We welcome comments on our report of Archean eclogite-facies oceanic crust and deep subduction. However, Zou et al. (1)’s claims concerning our geobarometry and petrography (2) are demonstrably misleading.

The garnet-clinopyroxene (Grt-Cpx) geobarometer (3) was calibrated at 2 GPa to 7 GPa and 900 °C to 1,550 °C, and, with other experiments, is extrapolatable to eclogitic rocks below 700 °C (3), not a minimum of 1,200 °C as misreported by ref. 1, who incorrectly redraw figure 4 of Beyer et al. (3), omitting original calibration data at low temperature and low AlIVCpx(the tetrahedral aluminum content in Cpx) in the range of our samples (the complete original figure 4 of Beyer is reproduced as our Fig. 1). Even if AlIVCpx is low (e.g., <0.05 per formular unit (p.f.u.)), the geobarometer yields accurate results (Fig. 1).

Fig. 1.

(A) Plot of temperature versus the Cpx tetrahedral aluminum (AlIVCpx) for the experimental eclogite assemblages in Beyer et al. (3) and other data resources [after figure 4 of Beyer et al. (3)]. Note that figure 1B of Zou et al. (1) fails to report the calibrated fields (in green colors), that are close to our data (in purple). (B) Diagram of the difference between the calculated pressure [using Grt-Cpx geobarometer of Beyer et al. (3)] and experimental pressure with AlIVCpx [after figure 7b of Beyer et al. (3)], showing that, regardless of the variation of AlIVCpx (even if it is less than 0.05), the calculated pressure is consistent with the experimental pressure within the error range (±4 kbar). A and B are adapted from Beyer et al. (3) with permission from Springer Nature, Contributions to Mineralogy and Petrology.

The rare earth element–based Grt-Cpx geothermobarometer (4) was developed for Mg-rich samples (e.g., Mg# of Grt > 0.54), yielding reliable results on many lower Mg# rocks (e.g., Mg# of Grt < 0.40) (4–7), establishing reliability for our samples. Zou et al. (1) claim they tested the above geobarometers using “published metamorphic studies” (their figure 1C and D), but no information about their tested data or validity of other pressure estimates is provided.

Zou et al. (1) claim that our exsolution textures indicate cooling from high temperature, “inconsistent with eclogite-facies metamorphism.” This is misleading. Numerous studies (2) show similar textures formed during decreasing pressure–temperature (P-T) conditions from eclogite and/or granulite facies (8). Exsolution textures themselves are not indicative of eclogite-facies metamorphism, but are consistent with and support our calculations, and statement “implying decreased pressure and temperature due to exhumation from HP–UHP (high pressure–ultrahigh pressure) metamorphic conditions (2).”

We considered but did not use pseudosections, because the peak mineral assemblage is too simple to provide useful constraints on P-T conditions. More importantly, the major minerals (Grt, Cpx) of Shangying garnet clinopyroxenite are compositionally zoned, which prominently affects the effective bulk-rock composition usable for calculating peak stage pseudosections. However, ref. 1 constructs a P-T pseudosection for the system NCKFMASHTO using our raw bulk composition, but with thermodynamic dataset, Fe3+ value, water content, and model conditions following literature, unrelated and not applicable to our case. Even if their pseudosection (1) is correct, the predicted mineral assemblages are inconsistent with the observed peak mineralogy, bearing no relationship to our rocks.

The Na2O contents in plagioclase are similar to values from retrogressed eclogites in Phanerozoic orogens. Low-sodic clinopyroxene is typically caused by 1) low protolith Na2O, indicated by bulk compositions (table S1 in ref. 2), similar to some normal mid-ocean ridge basaltic gabbro from the modern seafloor (9); or 2) dehydroxylation, and/or breakdown of clinopyroxene during exhumation, reported from other UHP eclogites (10, 11).

Zou et al. (1) write, “without robust evidence for eclogite-facies metamorphism, the peak metamorphic pressures would not exceed 12 kbar at T > 890 °C.” Applying reliable geothermometers, we retrieved peak temperatures of 792 °C to 890 °C, not > 890 °C as they misquote. Our estimations of P-T conditions and petrographic interpretations remain valid, providing clear documentation of peak eclogite-facies metamorphism of the Shangying garnet clinopyroxenite (2). The interpretation of the tectonic setting related to deep subduction, based on this and other evidence (2), stands.