Tectonic evolution of the North Qinling Orogenic Belt, Central China: Insights from metamafic rocks of the Songshugou Complex
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High‐pressure (HP) and ultrahigh‐pressure (UHP) rocks have been widely recognized in the North Qinling Orogenic Belt (NQOB). The nature and age of their protoliths can provide insights into the formation and evolution of the NQOB. The Songshugou Complex, which crops out along the Shangdan Suture Zone, mainly consists of high‐grade metamorphosed mafic and ultramafic rocks, and thus, this area is an important study site for understanding the tectonic evolution of the NQOB. The metamafic rocks that enclose the ultramafic rocks are overwhelmingly composed of amphibolites that primarily exhibit amphibolite‐facies metamorphism, and minor garnet amphibolites (retrograde eclogite) and HP mafic granulites occur as discontinuous lenses or blocks within these amphibolites. Laser inductively coupled plasma mass spectrometry (LA‐ICPMS) U–Pb dating on zircons yielded ages of 499 ± 6, 507 ± 5, and 504 ± 10 Ma for the HP mafic granulite, garnet amphibolite (retrograde eclogite), and amphibolite, respectively. Those ages were interpreted to represent timing of the HP granulite or amphibolite metamorphism that occurred during later exhumation. Relict omphacites discovered in both garnet and zircon as well as in the matrix of the garnet amphibolite indicate that the rock underwent eclogite‐facies metamorphism and subsequent rapid exhumation. However, the timing of the different metamorphic stages cannot be distinguished from the zircon ages obtained using the LA‐ICPMS technique. The protolith age of the complex is constrained to be the early Neoproterozoic. Geochemical data indicate that the protoliths of the Songshugou metamafic rocks are of continental origin. These Neoproterozoic continental mafic rocks most likely formed during the breakup of the supercontinent Rodinia and were mainly derived from low‐degree partial melting of the asthenosphere with some lithospheric mantle contribution, within an extensional regime. All the available data reveal that the NQOB was one of the components of the Rodinia supercontinent and that it was likely involved in the Grenville orogenic belt and subsequently separated from the supercontinent at approximately 830–740 Ma.Keywords:
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High‐pressure (HP) and ultrahigh‐pressure (UHP) rocks have been widely recognized in the North Qinling Orogenic Belt (NQOB). The nature and age of their protoliths can provide insights into the formation and evolution of the NQOB. The Songshugou Complex, which crops out along the Shangdan Suture Zone, mainly consists of high‐grade metamorphosed mafic and ultramafic rocks, and thus, this area is an important study site for understanding the tectonic evolution of the NQOB. The metamafic rocks that enclose the ultramafic rocks are overwhelmingly composed of amphibolites that primarily exhibit amphibolite‐facies metamorphism, and minor garnet amphibolites (retrograde eclogite) and HP mafic granulites occur as discontinuous lenses or blocks within these amphibolites. Laser inductively coupled plasma mass spectrometry (LA‐ICPMS) U–Pb dating on zircons yielded ages of 499 ± 6, 507 ± 5, and 504 ± 10 Ma for the HP mafic granulite, garnet amphibolite (retrograde eclogite), and amphibolite, respectively. Those ages were interpreted to represent timing of the HP granulite or amphibolite metamorphism that occurred during later exhumation. Relict omphacites discovered in both garnet and zircon as well as in the matrix of the garnet amphibolite indicate that the rock underwent eclogite‐facies metamorphism and subsequent rapid exhumation. However, the timing of the different metamorphic stages cannot be distinguished from the zircon ages obtained using the LA‐ICPMS technique. The protolith age of the complex is constrained to be the early Neoproterozoic. Geochemical data indicate that the protoliths of the Songshugou metamafic rocks are of continental origin. These Neoproterozoic continental mafic rocks most likely formed during the breakup of the supercontinent Rodinia and were mainly derived from low‐degree partial melting of the asthenosphere with some lithospheric mantle contribution, within an extensional regime. All the available data reveal that the NQOB was one of the components of the Rodinia supercontinent and that it was likely involved in the Grenville orogenic belt and subsequently separated from the supercontinent at approximately 830–740 Ma.
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Abstract U–Pb zircon geochronology and trace element analysis was applied to eclogites and (ultra)high-pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the Orlica-Śnieżnik complex, Bohemian Massif. Under favourable circumstances such data may help to unravel protolith ages and yet-undetermined aspects of the metamorphic evolution, for example, the time span over which eclogite-facies conditions were attained. By means of ion-probe and laser ablation techniques, a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences. The 206 Pb/ 238 U dates for zircons of all samples show a large variability, and no single age can be calculated. The protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra. The spread in apparent ages is interpreted to be mainly caused by variable and possibly multi-stage Pb-loss. Further complexities are added by metamorphic zircon growth and re-equilibration processes, the unknown relevance of inherited components and possible mixing of different aged domains during analysis. A reliable interpretation of igneous crystallization ages is not yet possible. Previous studies and the new data document the importance of a Carboniferous metamorphic event at c. 340 Ma. The geological significance of this age group is controversial. Such ages have previously either been related to peak (U)HP conditions, the waning stages of eclogite-facies metamorphism or the amphibolite-facies overprint. This study provides new arguments for this discussion because, in both rock types, metamorphic zircon is characterized by very low total REE abundances, flat HREE patterns and the absence of an Eu anomaly. These features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the Carboniferous ages document late-stage eclogite-facies metamorphism, and not amphibolite-facies overprinting.
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Abstract An integrated study of petrology, mineralogy, geochemistry, and geochronology was carried out for contemporaneous mafic granulite and diorite from the Dabie orogen. The results provide evidence for granulite‐facies reworking of the ultrahigh‐pressure (UHP) metamorphic rock in the collisional orogen. Most zircons from the granulite are new growth, and their U‐Pb ages are clearly categorized into two groups at 122–127 Ma and 188.2 Ma. Although these two groups of zircons show similarly steep HREE patterns and variably negative Eu anomalies, the younger group has much higher U, Th and REE contents and Th/U ratios, much lower εHf(t) values than the older group. This suggests their growth is associated with different types of dehydration reactions. The older zircon domains contain mineral inclusions of Grt, Cpx and Qz, indicating their growth through metamorphic reactions at high pressures. In contrast, the young zircon domains would have grown through peritectic reaction at low to medium pressures. The younger granulite‐facies metamorphic age is in agreement not only with the adjacent diorite at 125.1 Ma in this study but also the voluminous emplacement of coeval mafic and felsic magmas in the Dabie orogen. Mineral separates from both mafic granulite and its adjacent diorite show uniformly lower δ18O values than normal mantle, similar to those for UHP eclogite‐facies metaigneous rocks in the Dabie orogen. In combination with major‐trace elements and zircon Lu‐Hf isotope compositions, it is inferred that the protolith of mafic granulites shares with the source rock of diorites, both being a kind of mafic metasomatites at the slab‐mantle interface in the continental subduction channel. This provides a direct link in petrogenesis between the granulitic, migmatic and magmatic rocks in the collisional orogen to active continental rifting, whereby high heat flow was transferred from the asthenospheric mantle into the thinned orogenic lithosphere for partia melting.
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We report zircon U–Pb ages and Lu‐Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu‐Hf isotopic of ε Hf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.
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The protolith of retrograded eclogite samples occurred in Chicheng,northern Hebei Province is of tholeiitic oceanic crust with geochemical characteristics of mid-ocean ridge(MORB) or island arc(IAT) environment.Three main metamorphic evolutional stages are recognized and characterized by the early peak eclogite facies stage,the later granulite facies retrogressive metamorphism stage and amphibolite facies retrogressive metamorphism stage.Zircon SHRIMP isotopic dating of two different textural varieties of retrograded eclogite samples show four different ages.The 206Pb/238U age of ~438 Ma from an inherited magmatic zircon core in sample L3 implies that it may be the age of the igneous precursor.The age of ~355 Ma from a metamorphic zircon in sample L1 may be interpreted as the peak metamorphic age of the eclogites.A weighted mean age of ~341 Ma and 322 Ma~326 Ma may represent the later granulite facies and amphibolite facies metamorphic ages respectively.The confirmation of time sequence for the retrograded eclogite is important for the understanding of tectonic evolution in the central part of the northern margin of North China Craton(NCC).
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