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Sediments or clastic rocks can record the evolution history of basins, orogenic processes, crustal uplift and erosion, and even paleo-environments. Detrital minerals such as zircon, garnet, and apatite are useful media for studies of sedimentary sources and basin evolution. Detrital zircon has been widely taken as an indicator for provenances and tectonic evolution of geological terrenes via age distribution patterns. Apatite can remain stable during erosion and transportation and is also considered as an ideal object for source tracing. This mineral normally contains high Sr and negligible Rb. Its Sr isotopic composition can remain almost unchanged after crystallization. Unlike isotopic ages of detrital minerals, Apatite isotopic compositions have been less frequently used for tracing the provenance of sedimentary rocks in the last few decades. In the present study, we report on the Sr isotopic composition of individual apatite grains of Neogene and Jurassic conglomerates from the Hui-Cheng basin in the South Qinling orogenic belt, obtained via thermal ionization mass spectrometry. Detrital apatite grains of Jurassic rocks have narrow ranges of 87Sr/86Sr values from 0.7076 to 0.7100, but those of Neogene rocks gave variant 87Sr/86Sr values from 0.7055 to 0.7534, providing distinct evidence for complex sources of Neogene sedimentary rocks. Analytical results show that the distribution patterns of 87Sr/86Sr values of single-grain detrital apatite fit the distribution patterns of detrital zircon U-Pb from the isotopic ages very well. Detrital apatite Sr isotopic composition can provide essential information for tracing the origins and evolution of sedimentary sources. [ABSTRACT FROM AUTHOR]

Magma mixing or mingling between mafic and felsic magmas is a common phenomenon in granitoid magmatism, and chemical and mechanical interactions between the magmas are frequently observed in granitic rocks. The Dongjiangkou pluton is an example of magma mixing among the numerous early Mesozoic granitoid plutons that are exposed in eastern South Qinling. This pluton is predominantly composed of granodiorite and contains abundant mafic/intermediate microgranular enclaves (MMEs) and is sporadically intruded by mafic dykes. Both the granodiorite and MMEs were crystallized at 224–214 Ma and have inherited zircons of similar age ranges. The granodioritic rocks had metaluminous to weak peraluminous features and adakitic signatures and belong to the high-K calc-alkaline series, whereas the MMEs and dioritic dykes had low SiO 2 contents and metaluminous features and belong to the high-K calc-alkaline to shoshonite series. All of the rocks had high Mg# values and similar geochemical characteristics regarding trace elements and Sr isotopes, but they had diverse whole-rock Nd-Pb and zircon Hf isotopic compositions. The Dongjiangkou granodiorite was produced by partial melting of old basement rocks and Paleozoic juvenile crust beneath South Qinling triggered by asthenospheric upwelling. Magma(s) of the Dongjiangkou MMEs may have been derived from deep mantle and injected into and mixed with the magma(s) of the granodiorite, but chemical and isotopic homogenization was not achieved during the magma mixing process. Clastic rocks may have played a major role in the formation of crust-derived magma(s), causing strong Nd-Hf isotopic decoupling of several granitoid plutons in South Qinling. • Dongjiangkou granite witnesses insufficient mixing of felsic and mafic magmas. • Nd and Hf isotopic equilibrium did not reach during the magma mixing. • Felsic magma originated from old basement and Paleozoic rocks in South Qinling. • Input of clastic rocks to granitic magma source caused Nd-Hf isotopic decoupling. [ABSTRACT FROM AUTHOR]

• Precambrian paragneisses from the Foping complex in South Qinling have a Neoproterozoic sedimentary protolith. • Neoproterozoic felsic igneous rocks in South Qinling and the Yangtze Block were the major sources of the sedimentary protolith. • Crustal growth in South Qinling occurred at ca. 1000–900 Ma, followed by reworking of ancient crust after ca. 900 Ma. • Tectonic shift from an island arc to a continental arc in the northern Yangtze Block occurred at ca. 900–860 Ma. Basement rocks within orogenic belts are useful for reconstructing crustal evolution. In the present study, we report the U-Pb ages and Hf isotopic compositions of detrital zircon and the geochemical of whole rock for Precambrian paragneiss from the Foping complex in South Qinling. The paragneiss samples had TiO 2 /Al 2 O 3 ratios of 1.84–5.70, Th/Sc ratios of 0.67–1.20, and Zr/Sc ratios of 11.0–18.1, suggesting felsic rocks as the major source. The immobile trace elements indicate that the source rocks were mostly derived from a continental arc environment. The U-Pb ages of detrital zircon are notably concentrated around two major peaks at ∼850 Ma and ∼700 Ma and two subordinate peaks at ∼2500 Ma and ∼2000 Ma with ε Hf (t) values varied widely from –26.0 to +12.4. The predominantly Neoproterozoic ages of the detrital zircon grains show a similar age distribution to those of Neoproterozoic igneous rocks in South Qinling and the Yangtze Block. Combining the results with the existing data for Neoproterozoic rocks in the study area, we propose that South Qinling was generally characterized by growth of the crust in the early Neoproterozoic (ca. 1000–900 Ma) that could have resulted from an intra-oceanic subduction along the northern margin of the Yangtze Block. Reworking of the ancient crust strengthened thereafter, which could be attributed to tectonic shift from an island–arc setting to a continental arc environment at ca. 900–860 Ma. Extensive formation of igneous rocks in the continental arc provided the main source for the Precambrian paragneisses in the Foping dome. [ABSTRACT FROM AUTHOR]

The Qinling orogen was ultimately formed by suturing of the South Qinling and Yangtze blocks, but the exact timing of the final amalgamation of the two blocks has not been well established so far. Partial melting of the Qinling orogenic continental crust resulted in the generation of migmatites, and such rocks may help to decipher the chronology of such event. In this paper, we report U–Pb ages, trace element, and Hf isotopic compositions of zircons from migmatites and diorite gneisses of the Foping dome, South Qinling. Zircons from migmatites form anhedral grains of variable sizes that are characterized by complex trace element compositions. Based on zircon U–Pb ages, the migmatites can be subdivided into two groups: Group 1 migmatites mainly retain Triassic zircons with U–Pb ages of 214–211 Ma and Hf model ages of ~ 1.46 Ga in core and rim domains; zircons from Group 2 migmatites record both Triassic (~ 210 Ma) and Neoproterozoic U–Pb ages, analogous to igneous rocks of the Wudang and Yaolinghe Groups exposed in South Qinling. Zircons from the diorite gneisses yield U–Pb ages of 216–210 Ma with Hf isotopic composition (T DM2 ages of ~ 1.46 Ga) similar to the migmatites. Evidence from whole-rock Nd isotopic analyses also points to a similar genesis between migmatites and diorite gneisses. It is proposed that Group 1 migmatites were derived by melting of Triassic diorites, while Group 2 migmatites were derived from Neoproterozoic igneous rocks, a major basement lithology of South Qinling. Partial melting of the orogenic crust took place at ~ 214–210 Ma, approximately consistent with the retrograde metamorphism of granulites exposed along the suture zone between the South Qinling and Yangtze blocks. We suggest that the collision of these two blocks occurred prior to ~ 215 Ma and that the Foping dome resulted from rapid collapse of an overthickened crust followed by partial melting enhanced by asthenospheric influx. [ABSTRACT FROM AUTHOR]