Late Jurassic Maofengshan two‐mica granites in Guangzhou, South China: fractional crystallization products of metasedimentary‐rock‐derived magmas

Abstract

Late Jurassic (~ 160 Ma) granitoids are widely distributed across the South China Block. These granitoids are mainly biotite granites, with subordinate amphibole-bearing and two-mica or muscovite-bearing granites. Previous studies have shown that a clear negative correlation exists between SiO2 and P2O5 contents in the amphibole-bearing, biotite, two-mica, and muscovite-bearing granites, and suggested that these granitoids are I-type (metaigneous-rock-derived) or fractionated I-type granites. However, whether the Late Jurassic two-mica granites were formed by fractional crystallization of S-type (metasedimentary-rock-derived) granitic magmas in addition to fractional crystallization of I-type granitic magmas, and the role of mantle-derived magma in the generation of the Late Jurassic granitoids, are still debated. We present data for two-mica granites in the Maofengshan area of Guangzhou, Guangdong Province, southeastern China. Zircon U–Pb ages show that these rocks were formed during the Late Jurassic (159 Ma). The Maofengshan two-mica granites have high SiO2 contents (73.1–74.1 wt.%) and peraluminous compositions (A/CNK = 1.08–1.11). They are enriched in Rb, Th, U, and Pb, depleted in Ba, Sr, Nb, and Ti, and have pronounced negative Eu anomalies. The Maofengshan granites also have enriched Sr–Nd–Hf isotopic compositions [(87Sr/86Sr)i = 0.7103–0.7118, eNd(t) = − 10.3 to − 10.1, and eHf(t) = − 14.2 to − 7.5)] and heavy zircon O isotopic compositions (δ18O = 8.2‰–10.2‰). We suggest that these granites were formed by fractional crystallization of metasedimentary-rock-derived granitic magmas in the middle crust (16.5–20.0 km), as they have similar major and trace element and isotopic characteristics to S-type granites and show relatively higher FeOT/MgO (5.2–5.6 > 4), (K2O + Na2O)/CaO (8.2–9.3 > 7), Rb/Sr (5.4–6.4), and Rb/Ba (1.2–1.5) and lower Nb/Ta (5.6–7.3 < 11) ratios than normal unfractionated granites. Mantle-derived mafic magma mainly provided the heat for their generation.