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Early Paleozoic peraluminous granites are abundant in the eastern part of the Qilian orogen, northeastern margin of the Tibetan Plateau. A combined study involving geochronology, whole-rock geochemical and Sr–Nd–Hf isotopic compositions for three Early Paleozoic peraluminous granitic plutons (Jishishan, Ledu and Shichuan plutons) from the eastern Qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas. These granitic plutons have magma crystallization ages of 455–427 Ma and are moderately to strongly peraluminous (A/CNK = 1.03–1.18). Geochemical and Sr–Nd–Hf isotopic data indicate that they consist substantially of crust-derived melts. The Jishishan and Ledu peraluminous granites were dominantly produced by partial melting of Precambrian orthogneisses. The Shichuan monzogranites, with low HREE contents (e.g., Yb = 0.80–1.83 ppm) and slightly negative ε Nd ( t ) (− 5.3 to − 2.3) and positive ε Hf ( t ) (+ 1.6 to + 3.4), could be derived from immature crustal materials. Relatively high average zircon saturation temperatures (> 750 °C for each pluton), obvious negative Eu anomalies (Eu/Eu* = 0.28–0.80) and low Pb/Ba ratios (0.03–0.16) for the Jishishan, Ledu and Shichuan granites are consistent with crustal melting involving biotite breakdown under fluid-absent conditions. Our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions, while the processes such as mixing with mantle-derived magma, fractional crystallization, restite unmixing and peritectic assemblage entrainment were insignificant (or only play secondary roles) in their genesis. Late Ordovician to Middle Silurian crustal anatexis in the eastern Central Qilian was probably linked with slab break-off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post-collisional setting. [ABSTRACT FROM AUTHOR]

Voluminous Early Paleozoic intrusive rocks occur in the Qilian orogenic belt, northeastern margin of the Tibetan Plateau. Their petrogenesis can provide insights into the tectonic evolution of Qilian and its adjacent areas. This paper carries out an integrated study of U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions for the Bamishan and Heishishan plutons in the eastern Qilian orogen. Both the Bamishan and Heishishan plutons consist of mafic, intermediate and felsic intrusive rocks. U–Pb zircon dating yielded the magma crystallization ages of 459–449 Ma for the Bamishan pluton and 447–438 Ma for the Heishishan pluton. Their geochemical and Sr–Nd–Hf isotopic data show that the parental magmas of the Bamishan monzogranites and quartz diorites were generated by slab (MORB and overlying sediments) melting. The Bamishan hornblende gabbros have extremely high Cr (1159–1163 ppm), Ni (220–353 ppm) and Mg# (73–74), indicating a cumulate origin. The Heishishan granodiorite–granite has adakitic geochemical signatures and they were produced by partial melting of the mafic lower crust. The Heishishan hornblende gabbro and quartz diorites were derived from the enriched lithospheric mantle metasomatized by subduction-related fluid/melt. In light of the available data, we propose that there was an Early Paleozoic ocean basin between the Central-South Qilian and Qaidam–West Qiling belts. The northward closure of this basin and subsequent continental collision and post-collisional processes resulted in the generation of the Early Paleozoic magmatism in the eastern Central Qilian. The Late Ordovician to Early Silurian (~ 460–440 Ma) intrusive rocks in the eastern Central Qilian formed in a post-collisional setting. A slab break-off model may account for the magma generation. [ABSTRACT FROM AUTHOR]

Abstract: An integrated study of petrology, zircon U–Pb age, and geochemistry was reported for a Paleoproterozoic granitic pluton from the Kongling terrain in the northern part of the Yangtze Block. The Quanqitang pluton is composed mainly of alkali feldspar, quartz, biotite and plagioclase. Zircon U–Pb dating yields a 207Pb/206Pb age of ca.1.85Ga, which represents its intrusion age. The Quanqitang pluton has high SiO2 (72.6–74.4wt.%), K2O (5.09–5.56wt.%) and Na2O (3.02–3.36wt.%), and low Fe2O3 (2.56–3.19wt.%) and MgO (0.25–0.37wt.%). It is metaluminous (A/CNK=0.93–0.98) and belongs to the high-K calc-alkaline series. On the chondrite-normalized REE diagram, all the samples invariably show a relatively enrichment in light rare earth elements (LREEs) with high (La/Yb)N ratios (10.6–21.7) and strong negative Eu anomalies. Their primitive-mantle normalized spidergrams display negative Ba, Nb, Ta, Sr, P, Eu and Ti anomalies. The calculated zirconium saturation temperatures range from 802 to 861°C with an average of 845°C. Zircons from the granites have ε Hf(t) values ranging from −17.6 to −20.9, corresponding to two-stage Hf modal ages of 3.6 to 3.8Ga. Based on the geochemical features, it is inferred that the granites were generated by the dehydration melting of Archean tonalitic gneisses with a plagioclase-rich residual assemblage. The granites chemically belong to the A2-type granites, and have an intrusion age younger than the possible ca. 2.0Ga collisional event in the Yangtze Block. This suggests that they formed in a post-collisional tectonic setting, and thus a tectonic regime switch from collision to extension might have occurred in the Yangtze Block before ca. 1.85Ga. The occurrence of the A2-type granite indicates that the Kongling microcontinental block became a stabilized craton at ca. 1.85Ga. [Copyright &y& Elsevier]

The origin of the K-feldspar megacrysts is still controversial and how magmas move in crystal-rich reservoir and the mechanism of the reactivation of crystal mushes after rheological locking are still elusive. K-feldspar megacryst in granitoids is a good candidate to constrain these issues. We carried out an integrated study of field geology, crystal size distribution (CSD), LA-ICP-MS zircon U Pb geochronology, whole-rock geochemistry, and in-situ major and trace elements and Pb isotopic compositions of K-feldspar megacrysts from the monzogranites and mafic microgranular enclaves (MMEs) of the Quxu batholith in the Gangdese belt, southern Tibet. The Quxu monzogranites have a magma crystallization age of ∼50 Ma. Some elements (e.g., Al, Sr, Cr and Ni) of the Quxu monzogranite and MMEs show scatter and no linear variations with increasing SiO 2 , indicating that simple magma mixing between mafic and felsic endmembers is unconspicuous for the formation of the Quxu monzogranites. The K-feldspar megacrysts from the Quxu monzogranites and MMEs can be divided into three types: normal zoning, reverse zoning and oscillatory zoning or no zoning. In-situ major and trace element of the K-feldspar megacrysts, combined with their variation in-situ Pb isotopic composition (206Pb/204Pb = 18.41–18.80, 207Pb/204Pb = 15.42–15.75 and 208Pb/204Pb = 38.29–39.06), indicate that the Quxu monzogranites were formed by the assembly of multiple heterogeneous magma pulses. The CSD results suggest that K-feldspar megacrysts have a longer residence time of 0.14–0.33 k.yr to 4.1–52 k.yr. We suggest that K-feldspar megacrysts could grow prolongedly via crystals transfer or magma injection of different batches in melt-present environments, and magma mush in cold-stored could be rejuvenated by the repeatedly recharging of hot magma pulses. • The Quxu monzogranites were formed by assembly of many heterogeneous magma pulses. • The K-feldspar megacrysts is the results of prolonged growth via crystal transfer or injection into different magma batches. • Rejuvenation of cold-stored mush is caused by repeatedly injection of multiple hot magma pulses. [ABSTRACT FROM AUTHOR]

Abstract: The early history of the Yangtze Block has not been well constrained yet, due to the scarce outcrops of Archean to Paleoproterozoic rocks. In this study, we report an integrated study of zircon U–Pb age, major and trace element data and Sr–Nd–Hf isotope compositions for gray gneisses from the Houhe complex in the northwestern part of the Yangtze Block. Zircon U–Pb dating yields a weighted mean 207Pb/206Pb age of 2081±9Ma for a gneiss. This age is interpreted as the formation age of the Houhe complex and thus the Houhe complex represents the oldest rocks found in the western part of the Yangtze Block. The gray gneisses from the Houhe complex range in SiO2 contents from 58.63% to 68.59% and Na2O from 3.88% to 5.28%, and have relatively high Fe2O3 contents of 2.86–6.69%, Al2O3 of 16.01–18.88%, and MgO of 0.97–2.65%. These rocks show low Sr (149–390ppm), Cr (9.07–45.1ppm) and Ni (4.97–21.3ppm) contents, but high Y (12.9–32.7ppm) and Yb (0.95–2.25ppm). They are characterized by a relative enrichment in LILEs and LREEs, but a depletion in HFSEs. These features are similar to those of calc-alkaline granitoids, suggesting that their formation might be related to a subduction-related process or remelting of preexisted arc rocks. Compiled age spectra of Archean to Paleoproterozoic zircon grains reveal that the western and eastern parts of the Yangtze Block have similar Archean and early Paleoproterozoic age ranges, indicative of the occurrence of an old continental nucleus in both parts. On the other hand, there are large amounts of ca. 2050–2400Ma zircon grains revealed in the western part of the Yangtze Block, implying that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. Because of the poor outcrops of Archean to Paleoproterozoic rocks in the Yangtze Block, it is no possible to establish the exact nature of such an active margin yet. The occurrence of ca. 2.0Ga khondalitic and metasedimentary rocks has been documented in the eastern part of the Yangtze Block, implying that there might be a passive-type continental margin in the eastern part of the Yangtze Block. At ca. 2.0Ga, the collision between the western microcontinent and the eastern part may have resulted in the final arc-continental collision and the amalgamation of the Yangtze Block. [Copyright &y& Elsevier]