Peraluminous granitoid magmatism from isotopically depleted sources: The case of Jing'erquanbei pluton in Eastern Tianshan, Northwest China

Abstract

The Eastern Tianshan region is a critical segment of the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic belts in the world. Several pulses of isotopically depleted Late Palaeozoic calc‐alkaline and high‐K calc‐alkaline granitoid magmatism are known to be associated with the collisional orogeny in the CAOB. However, no systematic studies have been conducted on the peraluminous granites that were formed during the Late Palaeozoic collisional orogeny in Eastern Tianshan. Here, we present major and trace element characteristics, whole‐rock Sr–Nd isotopes, and zircon U–Pb and Lu–Hf data from the Jing'erquanbei pluton in the eastern part of Eastern Tianshan. Zircon grains from the granite yield a weighted mean U–Pb age of 250 ± 4 Ma, which is consistent with the previously determined 40Ar–39Ar age of muscovite in the associated ore body, suggesting Late Permian magmatism. The pluton shows a wide range of Sr isotope ratios (87Sr/86Sr = 0.705777–1.308836) but depleted Nd (εNd(t) = +6.71 to +8.21) and Hf (εHf (t) = +12.6 to +14.6) isotopic values. The Nd and Hf isotopic values are also consistent with the Nd and Hf isotopic values of the Carboniferous mafic volcanics (εNd(t) = +5.50 to +9.66, εHf (t) = +6.3 to +13.2) in Eastern Tianshan. The whole‐rock Nd model age (TDM2 = 350–480 Ma) and zircon Hf model age (TDM2 = 342–451 Ma) as well as the Carboniferous inherited zircon (341 ± 5 Ma) suggest that original magma was derived from young and depleted juvenile crust source. The granite is characterized by high SiO2 (74.50%–75.80%), enriched alkali contents with high sodium values (K2O + Na2O = 5.65%–8.20%, Na2O/K2O = 1.24–2.74), concentration of peraluminous elements (Al2O3/(CaO + Na2O + K2O) [A/CNK] = 1.18–1.24), high large‐ion lithophile element content, low Nb/Ta (9.97–10.64) ratio, variable Rb/Sr (0.18–16.11) ratios, and negative Eu anomalies (Eu/Eu* = 0.05–0.81). These features suggest that some minerals, such as biotite, plagioclase, and apatite, underwent fractional crystallization during the formation and evolution of the pluton. Considering the regional magmatic activity as well as tectonic evolution, we speculate that peraluminous granites of the Huangshan–Jing'erquan area of Eastern Tianshan were generated during the postcollisional extension in the Late Permian. Dehydration and melting of the Carboniferous mafic volcanics through heat input from mafic–ultramafic magma in the Middle Permian is considered to have generated the peraluminous granites.