Yilong Li

Late Devonian to early Carboniferous arc-related magmatism in the Baolidao arc, Inner Mongolia, China: Significance for southward accretion of the eastern Central Asian orogenic belt

The Central Asian orogenic belt, formed in response to consumption of the Paleo–Asian Ocean, is one of the largest and most complex accretionary collages in the world and was responsible for considerable Phanerozoic juvenile crustal growth in Central Asia. The timing of subduction-accretion processes and closure of the Paleo–Asian Ocean is controversial. The Xilingol complex, composed of deformed quartzofeldspathic rocks and lenticular or quasi-lamellar amphibolites, is located on the northern section of the eastern Central Asian orogenic belt in Inner Mongolia, China. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of an amphibolite and an epidote amphibolite from the complex. The protolith of the amphibolite is a gabbro or gabbroic diorite with a laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb age of 382 ± 4 Ma and uniform eHf(t) values (–1.06 to +1.25). The protolith of the epidote amphibolite is a quartz diorite with a LA-ICP-MS zircon U-Pb age of 327 ± 5 Ma and uniform positive eHf(t) values (+0.78 to +4.11). The primitive magma of the Devonian gabbroic dike was generated by partial melting of a spinel lherzolite lithospheric mantle that was modified by fluids and melts from subducted slab components. A newly enriched lithospheric mantle is a possible source region for the Devonian mafic rocks. Fractionation of olivine and hornblende played a dominant role in magma differentiation with little or no crustal contamination. Amphibolite-facies metamorphism affected the Devonian gabbroic rocks at 321.6 ± 3.1 Ma, and the quartz diorite underwent epidote amphibolite-facies metamorphism at 279.4 ± 5.3 Ma, based on hornblende 40Ar/39Ar dating. The Devonian to Carboniferous intrusive rocks in the eastern Central Asian orogenic belt likely formed during the collapse of a mature arc at the southern margin of the South Mongolian microcontinent. Combining our results with previous data, we identify an initial phase of postcollisional extension (382–340 Ma) that occurred after earlier compression related to intra-oceanic subduction (484–469 Ma), ridge subduction (440–434 Ma), and arc-continent collision (427–383 Ma). We also constrain increasing extension accompanying extensive collapse of the mature arc (340–309 Ma), northward subduction of the forearc oceanic crust (322–274 Ma) coeval with development of the Hegenshan back-arc basin (354–269 Ma), and final collision (246–228 Ma). The presence of an accretionary belt along the southern margin of the South Mongolian microcontinent reflects the importance of continental growth by accretion of an arc chain during the Late Cambrian–Middle Triassic in the eastern Central Asian orogenic belt.

Lower Crustal Accretion and Reworking Beneath the North China Craton: Evidences from Granulite Xenoliths

How has the Earth’s deep continental crust evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in igneous rocks can be an important source of information. The North China Craton (NCC) is one of the oldest cratons in the world and Phanerozoic igneous rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continental crust. Abundant data of U–Pb ages and Hf isotopes in zircons were obtained for lower crustal xenoliths from over ten localities to constrain the history beneath the craton. The oldest components of the NCC may be ~4.0 Ga. The craton experienced complex accretion and reworking processes in its deep crust, accompanied by the formation and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. In the lower crust, the ancient components of the craton were reworked in Paleoarchean (3.80–3.65 Ga) time. The craton also experienced two important accretionary episodes, in the Neoarchean (2.8–2.5 Ga) and the Paleoproterozoic (2.3–1.8 Ga). Asthenospheric upwelling in Neoproterozoic time (0.6 Ga) locally modified the lower crust. Subduction and collision of the surrounding blocks, such as the Yangtze Craton, in Paleozoic and in early Mesozoic time also strongly modified the lower crust, especially along the cratonic margins. Accretion and modification of the lower crust during late Mesozoic–Paleogene were obvious due to the addition of depleted-mantle materials (underplating).

Metamorphic P-T path differences between the two UHP terranes of Sulu orogen, Eastern China: petrologic comparison between eclogites from Donghai and Rongcheng.

The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice I and two UHP slices II and III with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrography, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC(K)NFMASHO system. Eclogites from Slice III mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice II show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice II witness a peak metamorphism of eclogite-facies under conditions of 3.1–3.3 GPa and 660–690 ºC, and a retrograde cooling decompression process. The eclogites from Slice III record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840 ºC and a peak-T stage of 2.4 GPa and 950 ºC, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to ~100–105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice II exhumed after the eclogitic peak-pressure conditions earlier than that of Slice III. The existence of Slice II diminished the buoyancy force on Slice III, resulting in a granulite-facies overprint on Slice III. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.

Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group, Central Qilian Block, NW China

The Central Qilian Block is a Precambrian block in the Qilian Orogen, which has long drawn international attention for the study of orogeny and continental dynamics. The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north. Based on detailed fieldwork, this study presents a systematic study of petrography, mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones. The garnet metamorphic zone is composed of micaschist, garnet-bearing micaschist and felsic leptynite, with interlayered plagioclase amphibolite. The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist, sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the outer rim, the contents of almandine and pyrope slightly decrease and grossular slightly increase. Biotite in both metamorphic zones is ferro-biotite. Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone. Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage (4.5–5.0 kbar, 520–530 °C), a peak P stage (9.8–10.2 kbar, 560–570 °C), a stage of thermal relaxation (8.0–8.5 kbar, 580–590 °C) and finally a retrograde stage (6.8–7.0 kbar, 560–580 °C). Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage (5.5–6.0 kbar, 540–550 °C) and a peak stage (7.8–8.5 kbar, 660–690 °C). The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism, which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.