Search

Tectonic affinity between the Qaidam‐Qilian Block and the Western Qinling Block is an outstanding issue that has not been fully understood. Detrital zircon U–Pb dating and Hf isotope data for the Precambrian and Permian–Triassic strata around the Gonghe Basin area in both the Western Qinling and Qaidam‐Qilian blocks were studied. Detrital zircons from Precambrian strata range in age from Archean to Late Neoproterozoic, with age peaks at ~2.7–2.5, ~1.8–1.6, and ~0.95–0.75 Ga. Except for Precambrian age groups, Early Palaeozoic (550–500 Ma and ~450 Ma) and Late Palaeozoic–Triassic (~250 Ma) age peaks were also detected in the Triassic strata. The juvenile accreted crustal materials were detected at 1.85–1.5 Ga and over ~2.5 Ga based on zircon Hf isotope studies. Different Precambrian and Triassic strata in both the Western Qinling Block and the Qaidam‐Qilian Block show parallel age distribution patterns and Hf isotopes. A Qaidam‐Qilian source contributed to the majority of detritus in Precambrian strata. Besides sources from the Qaidam‐Qilian Block, Late Permian intrusions around the Gonghe Basin area also provided inputs for the Triassic strata. Detrital zircon studies show a strong connection of the Qaidam‐Qilian Block and the majority of detritus in the Gonghe Basin area of the Western Qinling Block. Tectonomagmatic and provenances comparison of the Western Qinling and Qaidam‐Qilian blocks are supportive of their tectonic continuity at least in three stages, that is, in the Precambrian, Middle Palaeozoic, and Permian to Early Triassic. [ABSTRACT FROM AUTHOR]

The premise to discuss the evolution of an orogenic belt is to understand the origins and tectonic affinities of various blocks that constitute the orogen. The Baoshan Block in the SE Tibetan Plateau is an ultimate window to observe the orogenic evolution, but its origin and eastern boundary remains debated. In this contribution, we report new detrital zircon U–Pb ages and Hf isotopic data from the Cambrian Baoshan Formation in the middle–western part of the Baoshan Block. Detrital zircon ages range from the Archean to Early Palaeozoic, with age peaks at ~2.5, ~0.95, and ~0.55 Ga. Zircon εHf(t) values exhibit a wide range from negative to positive for each of the three major age groups, indicating diverse magma sources. The eastern boundary for the Baoshan Block should be extended from the Kejie‐Nandinghe Fault to the Changning‐Menglian suture belt, based on not only comparisons of Early Palaeozoic magmatism, sedimentary facies, and provenances for the Palaeozoic strata that developed on both sides of the Kejie‐Nandinghe Fault but also on the regional tectonic framework correlation. The Baoshan Block should be along the Indian margin as the Qiangtang, Tengchong, and Simao‐Indochina blocks in Early Palaeozoic according to the provenance analyses. [ABSTRACT FROM AUTHOR]

Detrital zircon U–Pb dating, Hf isotopic data, and Nd isotopic compositions of river sediments are used to constrain crustal evolution processes of their provenance in the South Qilian orogenic belt. The detrital zircons from the Yuka, Tataleng, and Bayinguole rivers range in age from Neoarchean (2,772 Ma) to Permian (261 Ma), with age peaks at ca. 2.4 Ga, ca. 1.95 Ga, and ca. 0.45 Ga. Hf isotopic compositions of Paleoproterozoic, Pan‐African, and Caledonian zircon age groups display a wide range of εHf(t) values, suggesting diverse sources of host magma of the zircon grains. The basement formation of the South Qilian orogenic belt from depleted mantle occurred at two stages; ca. 3.1–2.5 Ga and ca. 2.0–1.6 Ga. Meanwhile, the Nd model ages of silt‐mud from the region are between 1.98 Ga and 2.27 Ga with an average value of 2.12 Ga. Our study indicates that the Qilian oceanic plate formed between the Qaidam and Alxa blocks from late Neoproterozoic to Silurian. The South Qilian orogenic belt is a product of the Qilian Ocean closure and subsequent collision between the Qaidam Block and central Qilian belt during Ordovician–Silurian times. Combining regional tectonothermal events with new results of river sediments suggests that rifting from a continent and opening of the Qilian Ocean occurred at ca. 780–520 Ma, subduction of the oceanic crust at ca. 520–440 Ma, ocean closure and continental subduction at ca. 440–420 Ma, exhumation of subducted crust at ca. 420–400 Ma, and orogenic collapse and extension at ca. 400–360 Ma. [ABSTRACT FROM AUTHOR]

The North Qilian Orogenic Belt (NQOB) is an important element of the northern edge of the Tibetan Plateau, and several rivers run along it. Zircon U–Pb dating, trace elements, and Hf isotopic data derived from the sediments carried by the rivers have been used to deduce the crustal evolution of the NQOB. U–Pb ages obtained from detrital zircons found in the sediments of the Zamu, Zhuanglan, and Beida rivers can be divided into five major age groups, that is, 2,600–1,500, 1,500–1,100, 1,100–650, 550–400, and 360–150 Ma, and the corresponding peaks occur at ~2,450, ~1,750, ~950, ~450, and ~250 Ma. Archean components, which may be derived from the ancient continental nucleus in the region, are rare in the river sediments. The zircon grains with ages of 360–150 Ma are most likely related to crustal thickening and decompression melting in a post‐collisional tectonic setting. The age distribution patterns indicate that the sediments carried by the Zamu, Zhuanglang, and Beida rivers are derived from the NQOB. Furthermore, the Hf isotopic compositions of Meso‐ and Neoarchean, Mesoproterozoic, Grenvillian, Pan‐African, Caledonian, and Hercynian zircon age groups exhibit a wide range of εHf (t) values, suggesting diverse sources. The existence of strongly negative εHf (t) values among the Grenvillian and Paleoproterozoic zircons indicates that the source magma included reworked Palaeoarchean crustal materials. New results indicate that continental rifting and the opening of the Qilian Ocean occurred at 775–520 Ma; the subduction and closure of the Qilian Ocean occurred at 520–440 Ma; arc–continent collision and continental subduction occurred at 440–420 Ma; and orogen collapse and extension occurred at 400–360 Ma. Our study indicates that the formation of the NQOB was mainly related to the evolutionary history of the Proto‐Tethys Ocean. [ABSTRACT FROM AUTHOR]