Lifei Zhang's research while affiliated with Peking University and other places

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.

The South China Block (SCB) is among the large-scale W-Sn mineralized regions of the globe. The Laojunshan W-Sn-dominant ore area (LOA) in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam, and Yangtze to Cathaysia blocks, with coeval formation of giant metallic deposits. The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB. In this study, we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-scheelite mineralization. The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units. The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block (420–380 Ma) jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones. This event generated the Dulong-Song Chay granitoids, together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites (ca. 419 Ma) and pegmatites (ca. 389 Ma), which include the early-stage Sn-sulfide skarn (ca. 418 Ma) and the late-stage Sn-bearing schist (ca. 389 Ma). During the Late Permian to Late Triassic (260–220 Ma), with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east, the SCB and Indochina Block (IB) were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit. The subducted Paleo-Tethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume (270–259 Ma), which formed the Maguan diabase (ca. 260 Ma) that shows significantly older Re-Os model age of ca. 268 Ma, suggesting that the Nanwenhe mineralization is potentially derived from ca. 260 Ma source. Furthermore, the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism (ca. 230 Ma), with the main stage of scheelite-Sn-Mo mineralization (229.9, 229.8 and 219 Ma) and contemporary formation of the pegmatite (230.7 Ma). The Late Cretaceous involved two episodes of alternate extension and shortening, driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the Neo-Tethys seafloor. The evolution of the LOA consists of the NW–SE transpression ending ca. 100 Ma, the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma, the WNW–ESE transpression beginning at ca. 85 Ma and the N–S extension in the later episode during the latest Cretaceous, which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization, with ages in the range of 90–89 Ma, 87–85 Ma and 83–79 Ma. We also evaluate the implications of magmatic-metamorphic-metallogenic degassing on the regional paleoclimatic history.

BN is a commonly used pressure transmitting material in experimental petrology. It is often considered to be as inert as MgO or Al2O3, and its redox potential is seldomly discussed. It is generally implied that, when used as a capsule sleeve, BN may impose relatively reduced conditions, similar to the effect of the fayalite-magnetite-quartz (FMQ) buffer. However, sediment melting experiments performed at 1050C and 3 GPa with BN as the capsule sleeve, produced a hydrous rhyolitic melt with dissolved H2S and CH4 (Li et al. 2021). The resulting fO2 estimate is significantly more reduced than that for the magnetite-wustite (MW)-buffered experiment where H2S and CH4 were undetected (Li et al. 2021), possibly to the extent of the quartz-iron-fayalite (QIF) buffered conditions produced when BN is used as a capsule or crucible (Wendlandt et al. 1982). In order to establish an explanation for such a discrepancy, we have conducted further investigation to better constrain the fO2 imposed by BN, when used as a capsule sleeve. Here we report results on analyses of Fe content in Au capsules, a comparative experiment using a QIF buffer and an experiment with an Fe-(Mg,Fe)O sensor for direct analysis of fO2. The calibration of the equilibrium between FeO in melt and Fe in the Au capsule, from Ratajeski and Sisson (1999) appears to be inadequate in constraining fO2 for our experiments. However, we were able to obtain Fe diffusion coefficients in Au from the Fe diffusion profiles observed in the capsule of the Fe-(Mg,Fe)O sensor experiment, and both the inner and outer capsules of the MW-buffered experiment, with resulting values of 110-13 m2/s, 310-14 m2/s, and 510-14 m2/s, respectively. The QIF-buffered and Fe-(Mg,Fe)O sensor experiments provide several lines of evidence supporting the observation that BN imposes QIF-like redox conditions. Firstly, the Fe-(Mg,Fe)O sensor returned an fO2 value of QIF. Secondly, the “apparent” partition coefficients between FeO content in melt and Fe in the Au capsules are similar between the BN experiment and the QIF-buffered experiment. Thirdly, we observe CH4 and H2O peaks with similar intensities in the Raman spectra of melts from these two experiments, suggesting similar H2 and thus O2 fugacity. As our experiments were performed on a cubic press with the experimental assembly encased in a pyrophyllite cube, we interpret that the significantly reduced conditions imposed by BN are likely due to high H2O activity maintained by dehydration of pyrophyllite, which can be explained using the reaction 2BN+3H2O=B2O3+N2+3H2. Lower H2O activity will reduce or inhibit the oxidation of BN and its fO2 buffering ability. If heat-treated, BN acts as a highly efficient H2 barrier, as shown by Truckenbrodt et al. (1997). Through our efforts to determine the fO2 imposed by using BN as a capsule sleeve in our experimental assembly, we are able to demonstrate the reducing ability of BN as an assembly component, and furthermore shed light on the process by which BN imposes such reduced fO2. We hereby present what we have learnt during the course of this investigation, in the hope that the effect of BN on fO2 control is both recognized and further exploited in future experimental studies.

Plate subduction links the Earth’s surface and interior and reshapes the redox state of the Earth’s mantle. Mantle wedges above subduction zones have high oxygen fugacity compared with other mantle reservoirs, but the cause is debated. Here we analyse high-pressure metamorphic rocks derived from ferromanganese pelagic sediments in the Qilian subduction complex, NW China. We show that progressive metamorphism is a process of reducing reactions, in which Mn4+ is reduced to Mn2+. On the global scale, such reactions would release significant amounts of oxygen (~1.27×1012 g year-1), estimated from the global flux of MnO in sediments passing into subduction zones. This budget is sufficient to raise the oxygen fugacity of the mantle wedge, hence arc magmas, to a higher level than other mantle reservoirs. In contrast, ferric iron (Fe3+) enters hematite, aegirine and garnet, without valence change, and plays little role in the oxidation of the mantle wedge. Fe3+ remains stable to depths of >100 km, but will transfer to the deeper mantle along with the subducting slab. The manganese reduction process provides a new explanation for high oxygen fugacity in the mantle wedge.

Impure dolomitic marble from the Great Himalayan Sequences (GHS) in Thongmön area, central Himalaya, is first systematically reported here concerning its petrographic features, textural relations, and fluid evolution. The Thongmön impure marble is characterized by the assemblage of calcite + dolomite + forsterite + spinel + phlogopite + clinohumite ± diopside ± retrograde serpentine. Three groups of calcite and dolomite occurring both as inclusions and in the matrix were identified: group I is represented by relatively magnesium-rich calcite (Cal) (CalI:XMg = 0.10–0.15) and almost pure dolomite (Dol) (DolI:XMg = 0.47–0.48), corresponding to the Cal-Dol solvus temperatures of 707–781 °C; group II is characterized by vermicular dolomite exsolutions (DolII:XMg = 0.45–0.46) in Mg-rich calcite and Mg-poor calcite (CalII:XMg = 0.05–0.08) adjacent to DolII, and the recorded solvus temperatures are 548–625 °C; group III is represented by nearly pure calcite (CalIII:XMg = 0.003–0.02) and Ca-rich dolomite in the matrix (DolIII:XMg = 0.33–0.44). Isobaric T-X(CO2) pseudosection at a peak pressure of 15 kbar in the system K2O-CaO-MgO-Al2O3-FeO-SiO2-H2O-CO2 suggests that the peak fluid composition of the Thongmön forsterite marble is restricted to X(CO2) < 0.04 at T > 780 °C due to a potential infiltration event of H2O-rich fluid. Alternatively, the forsterite marble is a retrograde product subordinated to the GHS exhumation process, and its fluid composition is relatively CO2-rich (0.6 < X(CO2) < 0.8 at 5 kbar, 750 °C) at a nearly isothermal decompression stage. In either case, we suggest that the carbon flux contributed by metacarbonate rocks in an orogen setting to the global carbon cycling must be considered.