Magma Mixing in a Granite and Related Rock Association: Insight From Its Mineralogical, Petrochemical, and “Reversed Isotope” Features
Magma mixing commonly takes place between isotopically depleted mafic and enriched felsic magmas. Here we present isotopic evidence exhibiting the opposite behavior in the Early Cretaceous Siling complex (South China), which comprises gabbro, quartz diorite, granodiorite, and alkali feldspar granite with locally many mafic microgranular enclaves. Field observations and zircon U–Pb dating indicate that all of the rock units crystallized contemporaneously at ca 127–129 Ma. Mineralogical and petrochemical analyses indicate that the Siling quartz diorite and granodiorite crystallized from hybrid magmas of temporally and spatially coexisting gabbroic and granitic melts. The Siling gabbro, characterized by variable yet enriched Sr–Nd–Hf isotopic compositions [(87Sr/86Sr)i = 0.70788 to 0.70833; ɛNd(t) = –7.6 to –3.5; ɛHf(t) = –6.8 to –1.9], exhibits Th/Nb, Nb/Nb*, and Sm/Yb vs eNd(t) correlations, indicating that the gabbro represents variable mixing of magmas derived from deep-level pyroxenite and shallow-level peridotite sources. The Siling alkali feldspar granite, which has typical A-type characteristics, exhibits less enriched Sr–Nd–Hf isotopic signatures [(87Sr/86Sr)i = 0.70650; eNd(t) = –3.2 to –2.5; ɛHf(t) = –1.3] than the coexisting gabbro, indicating its derivation from the re-melting of juvenile lower crust. “Reversed isotope” feature of the Siling gabbro and alkali feldspar granite means that the quartz diorite and granodiorite recorded “reversed isotope” mixing between isotopically enriched mafic and relatively depleted felsic magmas. The results indicate that the injection of mantle-derived mafic magma does not necessarily imprint relatively depleted isotopic signatures on the host felsic melt, and that the vertical growth of the continental crust by the input of isotopically enriched mafic magma should be concerned.
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