Two Distinct Fractional Crystallization Mechanisms of A-Type Granites in the Nanling Range, South China: A Case Study of the Jiuyishan Complex Massif and Xianghualing Intrusive Stocks

Abstract

The petrogenesis of A-type granites with different occurrences in the Nanling Range remains unclear. In this study, a case study of the Jiuyishan complex massif and Xianghualing intrusive stocks was conducted to determine this problem. The Jiuyishan complex massif is composed of four units (Jinjiling, Pangxiemu, Shaziling and Xishan). These four units have similar zircon U-Pb ages of approximately 153 Ma, with high Zr + Nb + Ce + Y contents (>350 ppm), high 10,000 Ga/Al ratios (>2.6), and a high crystallization temperature, indicating A-type affinities. They show a gradual change in lithology and geochemistry, implying a fractional crystallization process. These units also have similar εNd(t) values (−8.2 to −5.8) and zircon εHf(t) values (−7.5 to −2.2) except for the Shaziling MMEs (mafic microgranular enclaves) (−14.2 to 4.8), demonstrating their lower crustal source. However, the Shaziling unit may have contributed mantle-derived magma based on the geochemical data of its hosted MMEs. In comparison, the two Xianghualing intrusive stocks have similar geochemical features but exhibit highly evolved features (high Rb, U, Y, Ta and Nb contents and low Eu, Ba, Sr, P, Ti, Ca, Mg and Fe contents, with V-shaped REE distribution patterns). They have different zircon U-Pb ages of approximately 160 Ma and 155 Ma. The two stocks also have similar whole-rock εNd(t) values (−6.5 to −5.7) and zircon εHf(t) values (−7.6 to −2.7) and equally illustrate a lower crustal source region. Combining with their vertical zonation, they may have experienced remarkable fractional crystallization with possible assimilation processes. We propose that the Jiuyishan complex and Xianghualing stocks have two distinct fractional crystallization mechanisms during their formation. The Jiuyishan complex was formed by in situ crystal mush fractionation, while the Xianghualing stocks were formed by flowage differentiation during magma ascent or gravitational settling during magma solidification after emplacement. However, more than one mechanism affected the fractional crystallization processes of these granitic rocks.

Geochemical characteristics, hazards impact assessment and radiogenic heat production of the alkaline rocks

“…It is important to note that U and Th behaves as incompatible elements due to their large ionic radii and charge, and thus tend to concentrate in the residual molten material during the crystallization of magma 42 . This support the enrichment of the studied rocks, especially granitic samples, with these radioactive elements, as they are typically formed from residual magma 25 , 43 . Moreover, alkaline rocks are generally contained accessory minerals like zircon, monazite, apatite, rutile, allanite and xenotime, which can incorporate large amounts of U and Th into their crystal structures 44 , 45 .…”

“…Significant U and Th deposits can be found in regions like Canada (Athabasca Basin) 21 , Kazakhstan (Inkai deposit) 22 and Niger (Arlit deposit) 23 . Magmatic rocks can be important sources of radioactive elements, conventionally, trapped within rare metal and radioactive minerals (e.g., zircon, thorite, uraninite, and monazite) in these rocks 24 , 25 . Uranium and thorium, natural radioelements, are lithophile elements found extensively throughout crustal rocks, with a notable preference for accumulating in silicic magmatic rocks over their intermediate, mafic, and ultramafic counterparts.…”

A Complex Meso–Cenozoic History of Far-Field Extension and Compression: Evidence from Fission Track Analysis in the Helanshan Mountain Tectonic Belt, NW China

“…The HTB occurred along the western margin of the Ordos Block, and their geolog processes were interconnected according to basin-range theory. Prior research sugg that the HTB and Ordos Block experienced the subsequent phases: (1) the N-S to NN SSW trending extension in the Early Jurassic [8,15], which was induced by the postlapse after the Triassic collision between the North China Craton with the South Ch Block [16]; (2) the multi-directional compression in the Late Jurassic (mainly in the E direction [1,6,17]), which was effected by the subduction of the Pacific Plate in the east, growth of the Qiangtang-Lhasa Block in the southwest and Siberian Block in the north a WNW-trending extension in the Early Cretaceous [3,8,15], correlated to the roll-bac subduction of the east Pacific Plate; (4) the NW-SE trending compression in the Late C taceous [4,8], transmitted from the growing subduction boundary of the Pacific Plate;…”

“…These newly acquired FT data will be utilized to enha comprehension of the far-field impact resulting from the subduction of the Pacific Plat the eastern region, as well as the collision between the Indian Plate and the Eurasian Pl The HTB occurred along the western margin of the Ordos Block, and their geological processes were interconnected according to basin-range theory. Prior research suggests that the HTB and Ordos Block experienced the subsequent phases: (1) the N-S to NNE-SSW trending extension in the Early Jurassic [8,15], which was induced by the postcollapse after the Triassic collision between the North China Craton with the South China Block [16]; (2) the multi-directional compression in the Late Jurassic (mainly in the E-W direction [1,6,17]), which was effected by the subduction of the Pacific Plate in the east, the growth of the Qiangtang-Lhasa Block in the southwest and Siberian Block in the north;…”