Search

Garnet-bearing mafic granulites occur as lenses within TTG gneiss or metapelite in the Fuping Complex, middle Palaeoproterozoic Trans-North China Orogen. They are composed mainly of garnet, clinopyroxene, orthopyroxene, hornblende, plagioclase, quartz and accessary zircon and titanite. Four stages of metamorphic mineral assemblages have been found in these mafic granulites: (a) the prograde assemblages (M1) are inclusions preserved in the garnet porphyroblasts, including clinopyroxene, hornblende, plagioclase and quartz; (b) the metamorphic peak assemblages (M2) consist of garnet porphyroblasts plus matrix minerals clinopyroxene + orthopyroxene + hornblende + plagioclase + quartz + zircon + titanite; (c) the first retrograde assemblages (M3) are represented by clinopyroxene + orthopyroxene + plagioclase + quartz symplectites rimming the garnets; and (4) the final retrograde assemblages (M4) are hornblende + plagioclase + quartz symplectites rimming or adjacent to the garnets. The retrieved metamorphic P-T paths pass from 530–600 °C/6.5–7.0 kbar (M1) through 680–820 °C/7.2–11.7 kbar (M2) to 730–760 °C/3.2–3.9 kbar (M3) and finally to 600–640 °C/3.2–3.7 kbar (M4). All the P-T paths of the mafic granulites are clockwise, two of these even contain near isothermal decompression (ITD) and subsequent near isobaric cooling (IBC) segments. SHRIMP U-Pb dating of metamorphic zircons of the mafic granulites yields a mean weighted age of ∼1849 ± 9Ma, which is consistent with the metamorphic time of surrounding TTG gneisses. Therefore, the Fuping Complex records the collision event between the Eastern and Western Blocks of the North China Craton that led to the final assembly of this craton during the late Paleoproterozoic.

Geothermobarometry and phase equilibria modeling were conducted for garnet-bearing amphibolite of the Inyoni Shear Zone (ISZ), southwestern Barberton Greenstone Belt, South Africa. The prograde (M1), metamorphic peak (M2) and retrograde (M3) metamorphic P-T conditions were estimated to be ∼560 °C/∼8 kbar, 660–730 °C/7–9.5 kbar and 570–630 °C/5–6 kbar, respectively. The amphibolite records clockwise P-T paths, and the metamorphic peak P-T conditions belong to the medium P/T facies series. The retrograde segment of the P-T path documents near isothermal decompression which may reflect relative fast exhumation. The estimated apparent geotherm is 20–30 °C/km. SIMS U-Pb dating of metamorphic zircon suggests that the metamorphism of the ISZ amphibolite have occurred during ∼3.26–3.22 Ga, and the metamorphic peak might occur at ∼3.24 Ga. Furthermore, 40Ar/39Ar dating of hornblende indicates that the cooling stage might occur at ∼3.20 Ga. It is anticipated that the protolith of the amphibolite of the ISZ may have been involved in a subduction-like tectonic process along with the TTG plutons and was metamorphosed during 3.26–3.22 Ga, then uplifted from within the surrounding TTG plutons at ∼3.20 Ga.

The discrete mafic granulite and amphibolite enclaves within the Pingyang trondhjemitic pluton, Fuping metamorphic terrane, middle Trans-North China Orogen, ubiquitously record three stages of metamorphic assemblages. For the mafic granulite enclaves, the prograde metamorphic assemblage (M1) is represented by mineral inclusions (clinopyroxene + hornblende + plagioclase + quartz ± apatite) preserved in the garnet porphyroblast, the metamorphic peak (M2) assemblage consists of garnet and matrix minerals of clinopyroxene + orthopyroxene + hornblende + plagioclase + quartz ± magnetite ± ilmenite ± zircon, and the retrograde (M3) stage is represented by symplectite assemblage of orthopyroxene + plagioclase ± clinopyroxene ± hornblende ± quartz rimming the garnet porphyroblast. The metamorphic P-T paths of the mafic granulite enclaves are defined to pass from 560–750 °C/6–9 kbar (M1) through 740–880 °C/11–14 kbar (M2) to 720–800 °C/6–8 (M3), indicative of a collisional orogenic process. The metamorphic P-T path of the amphibolite enclave experienced from 670–730 °C/9.3–11.8 kbar to 710–730 °C/3.9–5.5 kbar, further certify such metamorphic feature of an orogenic belt. SIMS U-Pb dating of zircons of the trondhjemitic gneiss yields the magmatic crystallization age of ∼2.52 Ga, and both the trondhjemitic gneiss and the mafic granulite enclaves yield the metamorphic age of ∼1.89–1.86 Ga. Therefore, it can be concluded that the Fuping terrane was involved in the late Palaeoproterozoic subduction-collision-uplifting tectonic process caused by amalgamation of the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen.

Metamorphic evolution and geochronology of pelitic granulites were reported for the first time in the Taihua metamorphic complex, Mts. Huashan area, southern segment of the trans-North China Orogen (TNCO). Three generations of metamorphic mineral assemblages are recognized: (1) the prograde metamorphic mineral assemblages (M1) are represented by mineral inclusions within the garnet porphyroblasts; (2) the metamorphic peak assemblage (M2) are the garnet porphyroblasts and minerals in the matrix (biotite + orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + zircon + magnetite); and (3) the retrograde metamorphic mineral assemblages (M3) are represented by the symplectic assemblages around the garnet porphyroblasts, resulted from decomposition reactions between garnet rims and neighboring minerals in the matrix. Calculated by both conventional thermobarometry and pseudosection modeling in the NCKFMASHT system using the Perple_X technique, the P–T conditions of these three metamorphic stages are constrained to be of 4–5 kbar/520–530 °C for the M1 stage, 6.8–8.6 kbar/730–810 °C for the M2 stage and 4.1–6.4 kbar/570–740 °C for the M3 stage, respectively. The derived clockwise P–T paths imply that the Mts. Huashan terrane involved in the subduction and subsequent collision between the Eastern and Western Blocks of the North China Craton (NCC) along the Palaeoproterozoic Trans-North China Orogen (TNCO). High resolution SIMS U–Pb dating of metamorphic zircons reveals the metamorphic ages of 1.85–1.82 Ga. Combined with geochronological data from the literature, it is concluded that the tectonothermal evolution between the Eastern and Western Blocks started as early as ∼1.97 Ga and lasted as late as 1.80 Ga. Furthermore, the eastern Taihua complex records older metamorphic ages and higher peak metamorphic pressures than those of the western Taihua complex, possibly suggesting an eastward subduction model for the TNCO.

The Taihua metamorphic complex exposes in the southern Trans-North China Orogen (TNCO) and records at least three episodes of magmatism. The Shanxian TTG gneisses obtained at 2.54–2.48 Ga have high SiO 2 , variable Mg# with high (La/Yb) N and Sr/Y ratios (15.08–93.26 with average of 37.31 and 7.33–115.05 with average of 49.59, respectively). They show negative Nb and Ta but positive Zr and Hf anomalies with low Nb/Ta and Gd/Yb, and high Zr/Hf and Zr/Sm ratios (4.97–18.8, 1.92–6.37, 23.71–50.26, and 29.18–75.23, respectively), and have positive e Hf (t) values (+1.7 to +7.9) and e Nd (t) values (+0.72 to +2.55), indicating that they were possibly produced by partial melting of lower crust derived from metasomatized lithospheric mantle with the residue of minor garnet. The Sihe granitic gneisses recorded at 2.35–2.30 Ga are enriched in silica, Al 2 O 3 contents (13.59–15.33), K 2 O/Na 2 O ratios (1.13–1.7), defined as high-Al TTG. They exhibit high Sr/Y, Zr/Sm and Zr/Hf ratios and low Nb/Ta and Gd/Yb ratios (44.71–83.09, 47.79–62.58, 36.05–37.43, 12.04–16.36, and 2.95–3.68, respectively) with negative Eu anomaly. Combined with Hf and Nd isotopes (−2.5 to +4.7 and −0.74 to +0.36), the Sihe granitic gneiss was possibly generated from partial melting of lower crust with minor ancient material contributions, leaving residual amphibole and minor garnet in the source. The Muce monzonitic gneisses emplaced at 2.17–2.16 Ga have high Mg#, Ni and Cr contents, constant Sr/Y and (La/Yb) N (42.39–50.33, 19.8–41.4, 33.6–74.2, 31.32–34.14, and 15.32–16.7, respectively), with relatively low and constant Dy/Yb ratio (1.82–1.85). Together with Hf and Nd isotopes (−6.5 to −0.9 and −2.07 to −2.86), the Muce monzonitic gneisses might be derived from partial melting of Neoarchean pre-existing crustal materials, possibly in an arc setting. It is therefore found that four episodes of crustal growth and/or reworking in the southern segment of the TNCO have occurred at 2.85–2.72 Ga, 2.57–2.48 Ga, 2.35–2.30 Ga and 2.2–2.0 Ga, respectively, likely to connect with the subduction–collision between the Eastern and Western Blocks along the TNCO in the North China Craton.

As a key accessory mineral in metapelite, monazite forms in the metamorphic event and therefore, places important direct constraint on the timing of the tectono-metamorphic event. In this study, we investigate four representative metapelites from different metamorphic terranes in the central-south Trans-North China Orogen (TNCO), applying secondary ion mass spectrometry (SIMS) U–Th–Pb dating on monazite. The dating results reveal that monazite collected from the Huashan, Luoning, Zanhuang and Wutai metamorphic complexes record the metamorphic ages of ~1.83 Ga, ~1.95 Ga, ~1.85 Ga and ~1.91 to 1.83 Ga, respectively. Combined with the literature data of metamorphic geochronology on the discrete terranes within the TNCO, it is suggested that the amalgamation between the Western and Eastern Blocks of the North China Craton along the nearly NS-trending TNCO stared as earlier as ~1.97 Ga and continued to as late as ~1.80 Ga, which was in fact a long-lived tectono-metamorphic event.

Garnet-bearing amphibolites and metapelites of the Luoning metamorphic terrane, Taihua metamorphic complex, southern terminal of the Palaeoproterozoic Trans-North China Orogen (TNCO), preserve different metamorphic mineral assemblages formed at three metamorphic stages in the Palaeoproterozoic. The prograde mineral assemblages (M1) comprise of hornblende + plagioclase + quartz ± biotite inclusions preserved in the garnet porphyroblasts in the amphibolites or quartz + biotite + plagioclase inclusions preserved in the garnet porphyroblasts in the metapelites. The metamorphic peak mineral assemblages (M2) are represented by the garnet porphyroblasts as well as the matrix minerals plagioclase + hornblende ± biotite + quartz in the amphibolites or quartz + biotite + plagioclase ± hornblende in the metapelites, respectively. The retrograde mineral assemblages (M3) are the worm-like, fine-grained “white-eye socket” symplectitic assemblages formed between the relict garnet porphyroblasts and the matrix minerals, consisting of plagioclase + hornblende ± biotite intergrowths in the amphibolites or plagioclase + biotite + quartz intergrowths in the metapelites, respectively. Geothermobarometric computation shows that the Luoning terrane records clockwise P – T trajectories including near isothermal decompression (ITD) processes, from 630–790 °C/5.3–8.9 kbar through 720–820 °C/8.9–10.7 kbar to 690–740 °C/7.1–7.7 kbar. SIMS U–Pb dating of zircons separated from the amphibolites and metapelites presents that the age of the protolith of the amphibolites is ∼2.32 Ga and the metamorphism occurred during 1.97–1.94 Ga, suggesting that the subduction–collision and the subsequent exhumation occurred in the late Palaeoproterozoic. Combined with the literature data of the different discrete terranes within the Trans-North China Orogen, it is further deciphered that the tectono-metamorphic event of the Trans-North China Orogen started as early as 1.97 Ga and lasted as late as 1.80 Ga, suggesting that the tectono-metamorphic event of the orogen is a long and complex history.

Exposed in the southernmost terminal of the Palaeoproterozoic Trans-North China Orogen (TNCO), the garnet-bearing amphibolites in the Mts. Huashan metamorphic complex preserve three generations of mineral assemblages including the prograde, peak and retrograde metamorphic stages. The prograde metamorphic stage (M1) is recognized as inclusion-type mineral assemblages (hornblende + plagioclase + quartz ± biotite ± ilmenite) preserved in the garnet porphyroblasts. The peak metamorphic assemblages (M2) are represented by the garnet porphyroblasts and matrix minerals (hornblende + plagioclase + quartz ± biotite ± ilmenite). The retrograde metamorphic assemblages (M3) are characterized by the symplectitic intergrowth minerals (hornblende + plagioclase + quartz ± orthopyroxene ± biotite ± ilmenite) rimming decomposed, relict garnet porphyroblasts, known as the “white-eye socket” reaction textures. Calculated by geothermometers and geobarometers, the P–T conditions of these three metamorphic stages are estimated to be of 5.7–6.0 kbar/660–760 °C for the prograde stage (M1), 6.6–7.7 kbar/700–820 °C for the peak stage (M2) and 4.5–6.7 kbar/720–760 °C for the retrograde stage (M3), respectively. Therefore, these three metamorphic stages define clockwise P–T paths including nearly isothermal decompression (ITD) processes. Geochronological studies of zircons from the amphibolites indicate that the protolith of the amphibolites formed at ∼2.3 Ga, and the metamorphism occurred at ∼1.87–1.85 Ga, and the metamorphic period was further deciphered to be of 1.96–1.80 Ga when considering our previous reports. It is therefore proposed that the southernmost terminal of the TNCO was also involved in the subduction and collision processes between the Eastern and Western Blocks of the North China Craton at least as early as 1.96 Ga and continued as late as 1.80 Ga, followed by rapid uplift and exhumation. The available data suggest that the Palaeoproterozoic tectono-metamorphic event of the southernmost terminal of the Trans-North China Orogen not only started earlier but also continued longer than the other terranes in the middle and northern sections elsewhere in the orogen.