Geoscience Frontiers (May 2023)
The effects of the source composition on the origin of orthopyroxene-bearing adakitic granitoid in West Qinling, Central China
Abstract
Orthopyroxene-bearing granodiorite (sometimes referred to as ‘charnockite’) with an adakitic affinity is a rare type of granitoid. It is generally accepted that the stabilization of orthopyroxene in igneous charnockites essentially requires low aH2O and/or high temperatures in a closed system. However, orthopyroxene can be an antecryst in a trans-crustal magmatic system. In this regard, orthopyroxene-bearing granitoids are somewhat analogous to pseudo-charnockites, where the orthopyroxene stems from a mafic reservoir. On the other hand, the source compositions of continental adakites can vary, which is often ignored in the interpretation of their contribution to the adakitic geochemical signature. In this study, we have investigated a rare orthopyroxene-bearing felsic pluton from the Zhuyuan area of West Qinling, Central China. The Zhuyuan pluton was emplaced in the Middle–Late Triassic (222 Ma–217 Ma) and is mainly composed of metaluminous to weakly peraluminous granodiorites belonging to the high-K calc-alkaline series. Moreover, they are characterized by high Mg# values (49.7–60.9) and Sr contents (471–697 ppm), low Y (12.2–15.4 ppm) and Yb (1.03–1.24 ppm) contents, high Sr/Y (33.2–46.2) and (La/Yb)N (15.3–21.4) ratios, and weakly negative Eu anomalies (Eu/Eu* = 0.78–0.89). The Zhuyuan adakitic granodiorites exhibit fairly limited Sr–Nd–Pb isotopic ratios and variable zircon initial Hf isotopes, indicating a major contribution from the Neoproterozoic basement of the Qinling Orogenic Belt. There is no evidence of any formation through high-pressure magmatic processes, and we propose that the adakitic signature of the Zhuyuan pluton could have been inherited from its source rocks (i.e., from the Neoproterozoic basement). The orthopyroxenes in the Zhuyuan granodiorites display poikilitic textures with high MgO, NiO and Cr2O3 contents, indicating that they have an antecrystic origin. Studies of regional tectonic evolution have shown that the Zhuyuan granodiorites formed during the tearing stage of the A'nimaque–Mianlue oceanic subduction slab. Therefore, this study emphasizes the effect of source inheritance on the formation of pseudo-charnockite with an adakitic signature.
