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The Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four‐stage metamorphic evolution, including a peak eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, a subsequent MP‐UHT granulite facies heating stage (M3) at 8–10 kbar and >840°C and a final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N‐MORB and arc‐related volcanics, with depleted whole‐rock εNd(t) values of 3.4 to 4.2, and are inferred to have formed in a back‐arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent. [ABSTRACT FROM AUTHOR]

Deciphering the geodynamic evolution and compositions of the Earth's mantle is crucial for understanding the origin and evolution of the Earth, however, the original geodynamic setting and significance of tectonically emplaced mantle in ancient orogens is always ambiguous. Orogenic garnet peridotites can shed light on the understanding of upper mantle petrology and the tectonic processes that operate at convergent plate boundaries. Petrographical evidence and P-T conditions of two contrasting types of orogenic garnet peridotites in the South Altun–North Qaidam (SAT–NQD) HP/UHP metamorphic belt suggest that they experienced a notably different geological history. Geochemical characteristics show that the garnet peridotites in the Bashiwake unit represent ultramafic complexes emplaced into the continental crust before subduction, while those in the Luliangshan unit originate from the mantle wedge above a deep subduction zone. Sr Nd isotopes for the two contrasting ultramafic rocks indicate that both have been modified either by mantle metasomatism or crustal contamination. The distinct petrochemical and isotopic characteristics of these contrasting orogenic garnet peridotite types demonstrate spatio-temporal variation in the tectonothermal-evolution of the SAT–NQD HP/UHP orogen. • There are two contrasting garnet peridotite types in the SAT–NQD HP/UHP belt. • These garnet peridotites record distinct initial tectonic settings and evolution. • They have been modified either by mantle metasomatism or crustal contamination. [ABSTRACT FROM AUTHOR]

In the northern West Qinling orogen (WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma (MSWD=0.88) and 433±4 Ma (MSWD=0.27), respectively. Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups: 424±3 Ma (MSWD=0.45) and 402±3 Ma (MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma (MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event. [ABSTRACT FROM AUTHOR]

The petrogenesis and geodynamic implications of the early Paleozoic adakitic rocks in the North Qilian orogenic belt remain topics of debate. In this study, petrology, zircon U–Pb age and Lu–Hf isotopic composition, whole-rock geochemistry and Sr–Nd isotopes were carried on both low-Mg and high-Mg adakitic rocks from the North Qilian orogenic belt. These results will provide significant constraints on the evolution of the North Qilian orogenic belt. In general, the low-Mg adakitic granites mainly consist of plagioclase (50–55%), alkali feldspar (20–25%), quartz (20–25%) and biotite (5%). In contrast, the high-Mg adakitic granodiorites are generally composed of alkali feldspar (30–35%), plagioclase (30–45%), quartz (20–25%) and amphibole (∼10%). The low-Mg adakitic granitoids (446–457 Ma) are characterized by high SiO 2 (69–72 wt.%), low Mg # (39–42) and low Cr and Ni contents. However, the high-Mg adakitic granitoids (430–431 Ma) have relatively lower SiO 2 (65–66 wt.%), higher Mg # (50–59) and higher Cr and Ni contents. The low-Mg adakitic rocks have high initial 87 Sr/ 86 Sr ratios (0.7068–0.7080), negative ε Nd ( t ) (−2.3 to −3.3) and ε Hf ( t ) values (−6.8 to 1.9) with old zircon Hf model ages (1.2–1.6 Ga). However, the high-Mg adakitic rocks show lower initial 87 Sr/ 86 Sr ratios (0.7044–0.7066), higher ε Nd ( t ) (−1.8 to 3.0) and positive ε Hf ( t ) values (2.3–7.7) with younger zircon Hf model ages (0.9–1.1 Ga). These results suggest that the low-Mg rocks were generated by partial melting of thickened crust, whereas the high-Mg rocks were derived from anatexis of delaminated lower crust, which subsequently interacted with mantle magma upon ascent. The data obtained in this study provide significant information about the geological and tectonic processes after the closure of the Qilian Ocean. The continent–continent collision and thickening probably occurred during 440–467 Ma with formation of low-Mg adakitic rocks; and the transition of the tectonic regime from compression to extension probably occurred at ∼430 Ma with formation of high-Mg adakitic rocks. [ABSTRACT FROM AUTHOR]

The Qilian orogenic system in the northern Tibetan Plateau records the complete tectonic cycle from opening of Proto-Tethys Ocean, initial subduction, to continental subduction and collision, and extensional collapse during Neoproterozoic to Devonian. Based on a compilation of recently published high-quality data, this contribution provides an overview of the composition, distribution, nature and ages of the principal tectonic elements, including ophiolites, multiple magmatic rocks, as well as high-grade metamorphism and anatexis, with a view to trace the tectonic evolution of the Qilian orogenic system with respect to the opening, subduction, accretion and closure of Proto-Tethys Ocean. Three main suture zones resulting from amalgamation involving microcontinental blocks within the Proto-Tethys Ocean are identified in the Qilian orogenic system, including the North Qilian subduction-accretion suture (NQLS), South Qilian subduction-accretion suture (SQLS) and North Qaidam subduction-collision zone (NQDS) from north to south that sandwich the Alxa, Qilian, Oulongbuluke and Qaidam Blocks. Two contrasting types of convergent orogeny in the Early Paleozoic includes the accretionary orogeny between Alxa, Qilian and Oulongbuluke Blocks (NQLS and SQLS), and the collisional orogeny between the Oulongbuluke Block and Qaidam Block (NQDS). Both Mariana-type and Andean-type subduction systems were identified in the NQLS based on the occurrence of Early Paleozoic accretionary complexes, ophiolites, high-pressure/low temperature metamorphic rocks and arc-related volcanic rocks and intrusions. The northward subduction beneath a Mariana-type intra-oceanic subduction system is represented by the Cambrian-Ordovician island-arc related volcanic rocks, back-arc volcanic rocks and ophiolite complex, and the southward subduction of the Proto-Tethys North Qilian Ocean beneath the Qilian Block generated an Andean-type continental arc, characterized by coeval arc magmatism and high-temperature metamorphism. Although oceanic subduction-related high-pressure metamorphism has not yet been recognized, the SQLS is also a typical accretionary orogen composed of island arc rocks, back-arc magmatic assemblages, ophiolite and accretionary complexes. The NQDS is a typical collisional orogen, characterized by ultrahigh pressure pelitic/granitic gneisses intercalated with minor eclogites and garnet peridotite blocks, although the previous subduction of oceanic crust can not yet be precluded, supported by the existence oceanic-type eclogite and arc-related magmatism and high-temperature metamorphism. Integrating the geological, petrological, geochemical and geochronological evidence, we reconstruct the Neoproterozoic-Paleozoic tectonic history of the Qilian orogenic system with respect to Proto-Tethys Ocean, which includes opening of the Proto-Tethys Qilian Ocean (580-550 Ma), subduction (520-460 Ma), and closure with subsequent deep continental subduction (455-430 Ma), and multi-stage exhumation of deeply subducted continental slab (425-400 Ma), as well as final extensional collapse (400-360 Ma) [ABSTRACT FROM AUTHOR]