Early Cretaceous fayalite-, ferrosilite-, and biotite-bearing rhyolitic porphyries in the Baishuizhai area, South China: Formation by fractional crystallization in the shallow crust

Abstract

Fayalite- and ferrosilite-bearing felsic igneous rocks are a distinctive category of A-type granitic magmas, and the origins of these rocks have proved controversial. In addition, the shallow-crust high-silica rhyolitic magmas responsible for such rocks are considered to have higher viscosities than their plutonic equivalents, which casts doubt on whether fractional crystallization could have occurred during the evolution of these high-silica melts. Here, we report a study of earliest Early Cretaceous (145–142 Ma) fayalite- and ferrosilite-bearing rhyolitic porphyries (FFBRPs) and closely associated biotite-bearing rhyolitic porphyries (BBRPs) from the Baishuizhai area, Guangzhou, Guangdong Province, South China. The Baishuizhai suite provides an excellent opportunity to establish the magmatic origins and evolutionary processes responsible for the FFBRPs. Rocks from the Baishuizhai suite have SiO2 contents of 73.2–77.7 wt%, Na2O + K2O contents of 7.83–9.42 wt%, and Fe2O3T contents of 1.52–2.97 wt%. They have A-type granite features including mineral assemblages (euhedral fayalite and ferrosilite in addition to anhedral amphibole and biotite) and geochemical characteristics [e.g., high 10,000 × Ga/Al values (3.7–4.7), high FeOT/MgO ratios (13.0–34.8), and high Zr (245–514 ppm) and Nb (56.4–96.5 ppm) contents]. The rocks have slightly enriched in situ plagioclase Sr [(87Sr/86Sr)i = 0.7070–0.7076] and whole-rock Nd [εNd(t) = −2.3 to −2.0] compositions and depleted zircon Hf isotopic compositions [εHf(t) = −0.9 to +7.0] relative to chondrite, as well as higher zircon δ18O values (6.3‰–7.8‰) and intermediate in situ plagioclase Pb isotopic compositions [(206Pb/204Pb)i = 18.368–18.727, (207Pb/204Pb)i = 15.488–15.673, and (208Pb/204Pb)i = 38.049–38.801] compared with depleted mantle. We infer that the primary magmas of this rock suite were formed by mixing of mantle-derived magmas with subordinate (20%–50%) metasedimentary-rock-derived magmas. Fayalite and ferrosilite in the FFBRPs were most likely crystallized from dry, hot, and reduced magma in a lower (<16.5 km depth) chamber. The lower magma gradually evolved during ascent to a water-enriched magma in an intermediate chamber at a shallow depth (3–4 km), forming the FFBRPs (elevation: 129–168 m). Finally, the residual melts migrated upward and resided in an upper (<3 km depth) chamber, forming the BBRPs (elevation: 196–824 m). According to the established temporal–spatial distribution of Early Cretaceous A-type granites and adakitic rocks in South China, we suggest that the Paleo-Pacific plate underwent diachronous rollback, starting during the earliest Early Cretaceous. This rollback enhanced the degree of crust–mantle interaction in the Guangzhou area.