Chun-Ming Wu

Metamorphism of the Lüliang amphibolite: Implications for the Tectonic Evolution of the North China Craton

The Lüliang Complex is located in the southern sector of the Trans-North China Orogen, a continent-continent collisional belt along which the discrete Archean Eastern and Western blocks amalgamated to form the basement of the North China Craton. The complex consists of Paleoproterozoic supracrustal rocks and granitoid plutons of which the former include amphibolites and pelitic schists/gneisses. Petrological evidence from the Lüliang amphibolites indicates three distinct metamorphic mineral assemblages. The early inclusion assemblage (M1) is composed of hornblende + plagioclase + quartz + ilmenite, preserved as mineral inclusions within garnet porphyroblasts. The THERMOCALC program yielded maximum P-T conditions of 7.1 to 6.1 kbar and 668 to 655 °C for the M1 assemblage, based on the core compositions of garnet porphyroblasts and inclusion-type hornblende and plagioclase. The peak porphyroblast-matrix assemblage (M2) is garnet + hornblende + plagioclase + clinopyroxene ± quartz ± rutile, representing the growth of garnet porphyroblasts and matrix minerals. The P-T conditions of the M2 assemblage were estimated at 9.2 to 8.3 kbar and 753 to 748 °C, based on the rim compositions of garnet porphyroblasts and matrix plagioclase, hornblende and clinopyroxene. The post-peak symplectite assemblage (M3) is represented by cummingtonite + plagioclase symplectite surrounding embayed garnet grains, with the maximum P-T conditions of 5.1 to 4.1 kbar and 737 to 697 °C, based on the rim compositions of garnet and symplectic cummingtonite and plagioclase. A combination of petrographic textures, reaction relations, mineral compositions and P-T data define a clockwise P-T path involving near-isothermal decompression for the Lüliang amphibolites. This suggests that like those metamorphic complexes in the northern part of the Trans-North China Orogen, the Lüliang Complex in the central part of the orogen also underwent initial crustal thickening (M1 and M2), followed by erosional and/or extensional exhumation (M3), which is in accordance with collision between the Eastern and Western Blocks to form the coherent basement of the North China Craton at ∼1.85 Ga.

Empirical garnet–muscovite geothermometry in metapelites

Abstract Two empirical garnet–muscovite geothermometers, assuming no ferric iron (Model A) and 50% ferric iron (Model B) in muscovite, respectively, were calibrated under the physical conditions of P =3.0–14.0 kbar and T =530–700 °C. The input temperatures and pressures were determined by simultaneously applying the garnet–biotite thermometer [Am. Mineral. 85 (2000) 881.] and the GASP geobarometer [Am. Mineral. 86 (2001) 1117.] to natural metapelites. To confirm internal thermodynamic consistency, Holdaways [Am. Mineral. 85 (2000) 881.] garnet mixing properties were adopted. Muscovite was treated as a symmetric Fe–Mg–Al VI ternary solid solution, and its Margules parameters were derived in this work. The resulting two formulae reproduced the input garnet–biotite temperatures well within ±50 °C, and gave identical results for a great body of natural samples. Moreover, they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence, inverted metamorphic zone, and thermal contact aureole. Pressure estimation has almost no effect on the two formalisms of the garnet–muscovite geothermometer. Assuming analytical error of ±5% for the relevant components of both garnet and muscovite, the total random uncertainty of the two formulations will generally be within ±5 °C. The two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 °C, low- to high-pressure, in the composition ranges Xalm=0.51–0.82, Xpyr=0.04–0.22, and Xgros=0.03–0.24 in garnet, and Fe tot =0.03–0.17, and Mg=0.04–0.14 atoms p.f.u. in muscovite.

Zircon U–Pb geochronology of the Zanhuang metamorphic complex: reappraisal of the Palaeoproterozoic amalgamation of the Trans-North China Orogen

Amphibolites and metapelites exposed in the Zanhuang metamorphic complex situated in the south-middle section of the Trans-North China Orogen (TNCO) underwent upper-amphibolite-facies metamorphism and record clockwise P – T paths including retrograde isothermal decompression. High-resolution zircon U–Pb geochronological analyses indicate that the metamorphic peak occurred during ~ 1840–1860 Ma, which is in accordance with the ubiquitous metamorphic ages of ~ 1850 Ma retrieved by miscellaneous geochronologic methods throughout the metamorphic terranes of the northern TNCO, confirming that the south-middle section of the TNCO was involved in the amalgamation of the Eastern and Western Blocks of the North China Craton during the Palaeoproterozoic.

Zircon SIMS U-Pb geochronology of the Lushan terrane: dating metamorphism of the southwestern terminal of the Palaeoproterozoic Trans-North China Orogen

High-resolution SIMS U–Pb dating of metamorphic zircons of the TTG gneisses, gneissic granitoid and amphibolites of the Lushan terrane, Taihua metamorphic complex, suggests that the metamorphism had taken place at least as early as ~1.96–1.86 Ga. These new dates, along with reference data, demonstrate that the southern and middle terranes of the Trans-North China Orogen had been involved in the continent–continent collision between the Western Block and the Eastern Block of the North China Craton. This orogenic process started as early as 1.96 Ga and lasted as late as 1.80 Ga.

Metamorphic evolution and SIMS U-Pb geochronology of the Qingshigou area, Dunhuang block, NW China: Tectonic implications of the southernmost Central Asian orogenic belt

Garnet-bearing mafic granulite and amphibolite exposed as lenses, boudins, or interlayers within metasediments in the Qingshigou area, Dunhuang block, southernmost Central Asian orogenic belt, record important information for understanding the tectono-metamorphic evolution of subduction and collision zones in the southern Central Asian orogenic belt during the mid-Paleozoic. Three stages of metamorphic assemblages (M1, M2, M3) are recognized in the high- and medium-pressure mafic granulite and amphibolite. In the high-pressure mafic granulite, the prograde assemblage (M1) is represented by inclusion minerals (hornblende + plagioclase + quartz) preserved in garnet porphyroblasts; the metamorphic peak assemblage (M2) is characterized by garnet porphyroblasts and matrix minerals (garnet + clinopyroxene + plagioclase + quartz ± zircon ± titanite); and the retrograde assemblage (M3) is marked by coronitic symplectite (hornblende + plagioclase + quartz ± magnetite) rimming the garnet porphyroblasts. In the medium-pressure mafic granulite, the prograde assemblage (M1) of hornblende + plagioclase + quartz is included in the garnet porphyroblasts; the peak assemblage (M2) consists of garnet + orthopyroxene + clinopyroxene + plagioclase + quartz ± zircon ± titanite (M2) in the matrix; and the retrograde assemblage (M3) of hornblende + orthopyroxene + plagioclase + quartz (M3) surrounds the garnet porphyroblasts. In the amphibolite, the prograde assemblage (hornblende + plagioclase + quartz + ilmenite) is preserved as inclusions in garnet (M1); the peak assemblage (M2) is composed of garnet + hornblende + plagioclase + quartz ± zircon ± titanite; and the retrograde assemblage (M3), consisting of hornblende + biotite + plagioclase + quartz + epidote + magnetite, rings the garnet porphyroblasts. Geothermobarometric calculations suggest that the metamorphic pressure-temperature paths pass from 568 °C and 8.8 kbar through 607 °C and 10.6 kbar and 861 °C and 16.9 kbar and finally to 598 °C and 4.4 kbar for the high-pressure mafic granulite; from 756 °C and 9.0 kbar through 750–874 °C and 9.3–11.6 kbar to 675 °C and 4.7 kbar for the medium-pressure mafic granulite; and from 686 °C and 7.6 kbar through 715–766 °C and 10.6–11.2 kbar to 671 °C and 5.6 kbar for the amphibolite, and the paths show clockwise pressure-temperature loops typical of an orogenic process. The metamorphic peak of the high-pressure mafic granulite lies in the eclogite facies, which is indicative of a subduction zone environment. High-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon indicates that the metamorphism occurred in the Early Silurian (ca. 430 Ma) and lasted for at least 65 m.y. This study reveals a possible southward subduction history of a branch of the Paleo–Asian Ocean, the Liuyuan Ocean, from the Silurian to Late Devonian, which may be an important event in the accretionary history of the Central Asian orogenic belt.

Thermal expansion of andalusite and sillimanite at ambient pressure: a powder X-ray diffraction study up to 1000°C

Abstract The unit-cell parameters of andalusite and sillimanite have been measured by high-T powder X-ray diffraction up to 1000°C at ambient pressure. Within the temperature range investigated, all the unit-cell parameters varied smoothly, indicating no phase transition. The volume-temperature data were fitted with a polynomial expression for the thermal expansion coefficient (αT = a0 + a1 T + a2 T-2), yielding a 0 = 2.55(2) × 10-5K-1, a1 = 0 and a2 = 0 for andalusite, and a0 = 1.40(4) × 10-5K-1, a1 = 7.1(8) × 10-9K-2 and a2 = 0 for sillimanite. Using the new thermal expansion data determined in the present study and compressional data from the literature, the P-T phase relations of the kyanite-andalusite-sillimanite system were calculated thermodynamically, with the invariant point located at ∼523°C and 3.93 kbar.

Tectonic mélange records the Silurian–Devonian subduction-metamorphic process of the southern Dunhuang terrane, southernmost Central Asian Orogenic Belt

The Hongliuxia tectonic melange of the southern Dunhuang terrane, northwestern China, southernmost Central Asian Orogenic Belt (CAOB), consists of eclogite, mafic granulite, and amphibolite as puddingstones within a matrix of metapelitic gneiss and marble; these rocks are interpreted to be part of an ancient subduction zone setting. Secondary ion mass spectrometry U-Pb dating of metamorphic zircons obtained from the puddingstones and matrix metapelite suggests that the metamorphism occurred at ca. 428–391 Ma. The metamorphic rocks all record similar clockwise metamorphic pressure-temperature-time ( P - T - t ) paths of the western Alpine type. However, remarkable differences between metamorphic peak P - T conditions ranging from 830 °C and 24.2 kbar for the eclogite puddingstone to 700 °C and 10.2 kbar for the metapelite matrix were found in the melange rocks. This indicates the mixing of rocks from significantly different depths to create a tectonic melange in a subduction channel, possibly juxtaposed during the uplift stage. These data suggest that the southernmost CAOB underwent subduction and subsequent exhumation caused by subduction of the Paleozoic Hongliuxia ocean during the middle Silurian to middle Devonian.

Paleozoic subduction of the southern Dunhuang Orogenic Belt, northwest China: metamorphism and geochronology of the Shuixiakou area

Abstract Amphibolites in the Shuixiakou area of the southern Dunhuang Orogenic Belt, southernmost Central Asian Orogenic Belt (CAOB), occur as lenses within hornblende-biotite-plagioclase gneiss or pelitic schist, exhibiting block-in-matrix feature of tectonic mélange. Three generations of metamorphic mineral assemblages (M1, M2, and M3) have been recognized in the garnet-bearing amphibolite lenses. The metamorphic prograde assemblage (M1) is documented with inclusion trails (hornblende + plagioclase + quartz) within garnet porphyroblasts, and are estimated to be formed under 610–690 °C and 6.5–10.2 kbar. The metamorphic peak assemblage (M2) consists of garnet + hornblende + clinopyroxene + plagioclase + quartz in the matrix and records metamorphic peak P-T conditions of 720–750 °C and 13.4–14.7 kbar. The retrograde assemblage (M3) is represented by the symplectic assemblage (hornblende + plagioclase + quartz ± biotite ± magnetite) rimming the garnet porphyroblast, formed in the decompression stage under P-T conditions of 630–730 °C and 3.8–7.2 kbar. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. SIMS dating of metamorphic zircons from the amphibolites confirm that the high-pressure metamorphism (M2) occurred at ca. 438–398 Ma.

Calibration of the garnet – biotite – Al2SiO5 – quartz (GBAQ) geobarometer for metapelites

A garnet-biotite-Al2SiO5-quartz (GBAQ) geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under P-T conditions of 450−950 °C and 1−17 kbar. In the calibration, activity models of garnet and biotite identical to those in the garnet-biotite geothermometer of Holdaway (2000) were used. Therefore, the GBAQ geobarometer and the garnet-biotite geothermometer can be simultaneously applied to iteratively estimate metamorphic P-T conditions. Successful application of the GBAQ geobarometer to natural metapelites certify its validity. Most importantly, when plagioclase is absent or CaO in garnet and/or plagioclase are deficient, this geobarometer may prove useful for estimating metamorphic pressures. The random error of the present GBAQ geobarometer is inferred to be around ±1.8 kbar. An electronic spreadsheet is available as Table S4 to apply the GBAQ geobarometer in combination with the garnet-biotite geothermometer. This article is protected by copyright. All rights reserved.