Magma Mixing Genesis of the Mafic Enclaves in the Qingshanbao Complex of Longshou Mountain, China: Evidence from Petrology, Geochemistry, and Zircon Chronology

Abstract

The Qingshanbao complex, part of the uranium metallogenic belt of the Longshou-Qilian mountains, is located in the center of the Longshou Mountain next to the Jiling complex that hosts a number of U deposits. However, little research has been conducted in this area. In order to investigate the origin and formation of mafic enclaves observed in the Qingshanbao body and the implications for magmatic-tectonic dynamics, we systematically studied the mineralogy, petrography, and geochemistry of these enclaves. Our results showed that the enclaves contain plagioclase enwrapped by early dark minerals. These enclaves also showed round quartz crystals and acicular apatite in association with the plagioclase. Electron probe analyses showed that the plagioclase in the host rocks (such as K-feldspar granite, adamellite, granodiorite, etc.) show normal zoning, while the plagioclase in the mafic enclaves has a discontinuous rim composition and shows instances of reverse zoning. Major elemental geochemistry revealed that the mafic enclaves belong to the calc-alkaline rocks that are rich in titanium, iron, aluminum, and depleted in silica, while the host rocks are calc-alkaline to alkaline rocks with enrichment in silica. On Harker diagrams, SiO2 contents are negatively correlated with all major oxides but K2O. Both the mafic enclaves and host rock are rich in large ion lithophile elements such as Rb and K, as well as elements such as La, Nd, and Sm, and relatively poor in high field strength elements such as Nb, Ta, P, Ti, and U. Element ratios of Nb/La, Rb/Sr, and Nb/Ta indicate that the mafic enclaves were formed by the mixing of mafic and felsic magma. In terms of rare earth elements, both the mafic enclaves and the host rock show right-inclined trends with similar weak to medium degrees of negative Eu anomaly and with no obvious Ce anomaly. Zircon LA-ICP-MS (Laser ablation inductively coupled plasma mass spectrometry) U-Pb concordant ages of the mafic enclaves and host rock were determined to be 431.8 5.2 Ma (MSWD (mean standard weighted deviation)= 1.5, n = 14) and 432.8 4.2 Ma (MSWD = 1.7, n = 16), respectively, consistent with that for the zircon U-Pb ages of the granite and medium-coarse grained K-feldspar granites of the Qingshanbao complex. The estimated ages coincide with the timing of the late Caledonian collision of the Alashan Block. This comprehensive analysis allowed us to conclude that the mafic enclaves in the Qingshanbao complex were formed by the mixing of crust-mantle magma with mantle-derived magma due to underplating, which caused partial melting of the ancient basement crust during the collisional orogenesis between the Alashan Block and Qilian rock mass in the early Silurian Period.

Magma mixing in the genesis of the Qingshanbao granitoids in the Longshoushan area: Implications for the tectonic evolution of the North Qilian orogenic belt, NW China

“…The zircon grains from the K‐feldspar granite ( n = 14) have Th and U average content of 315 and 378 ppm, respectively, and yielded concordia age of 442 ± 4.2 Ma (MSWD = 1.4; Figure 4b). The zircons from the granodiorite ( n = 16) have higher concentrations of Th (avg = 551 ppm) and U (avg = 630 ppm) than the ones from the K‐feldspar granite and diorite and yielded a concordant age of 433 ± 4.2 Ma (MSWD = 1.8; Figure 4c; Liu et al, 2019a). The diorite zircon grains have average concentrations of U and Th of 369 and 249 ppm, respectively, and yielded a concordia age of 439 ± 4.7 Ma (MSWD = 1.1; Figure 4d).…”

“…(b and d) Concordia diagram for studied samples. Data for the granodiorite (QSB‐15) are from Liu et al (2019a)…”

“…One sample from each rock type was selected for U–Pb zircon dating and Lu–Hf isotope analyses (samples QSB‐02 and QSB‐08 from the K‐feldspar granite and diorite, respectively). Zircons from the granodiorite were recently studied by Liu et al (2019a), and their isotope data are included in this manuscript for comparison. Zircon concentrates were obtained at Langfang Yuneng Rock Mineral Separation Technology Service Co., Ltd., in the Hebei Province.…”

“…(b) Geological sketch map of the Longshoushan Metallogenic Belt and its adjacent blocks (modified from Song, Niu, Su, & Xia, 2013). (c) Geological map of the Qingshanbao complex (modified from Liu et al, 2019)…”

Tectonic Transformation and Metallogenesis of the Yanshan Movement during the Late Jurassic Period: Evidence from Geochemistry and Zircon U-Pb Geochronology of the Adamellites in Xingcheng, Western Liaoning, China

“…The limpen original lithospheric rocks were taken away by asthenospheric mantle, resulting in the thinning of the lithosphere, simultaneously the frontal side of the subduction slab began to roll back [28], leading of an extensional environment. There are few records of the metamorphic core complexes or the extensional deformation in Xingcheng-Xudapu area during the Middle Jurassic [39], while voluminous Late Jurassic metamorphic core complexes, as well as extensional sedimentary basins, spread widely in western Liaoning [17,47,[67][68][69][70][71], which indicate the extensional structures [34,35,72]. For example, Xu et al [73] found the Late Jurassic gneissic granite in the eastern part of the NCC, in which the interstitial materials developed directionally, and pointed out that the rocks were formed in the apotectonic and extensional setting.…”