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 is composed of 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 versus εNd(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; εNd(t) = −3.2 to −2.5; ɛHf(t) = −1.3] than the coexisting gabbro, indicating its derivation from the remelting 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 magma should be concerned.

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Many studies have revealed that mantle‐derived mafic rocks can be more isotopically enriched than coexisting crustal felsic rocks. However, its origin remains to be elucidated. Here, we report on some Early Cretaceous (ca. 130 Ma) mafic rocks in Gan‐Hang Belt (South China) that are more isotopically enriched than the temporally and spatially associated granitoids. Relatively depleted Nd‐Hf isotopic signatures [εNd(t) = −0.4 to +0.6, εHf(t) = 3.4 to 4.8, εHf(t)zircon = −2.3 to +9.3], coupled with low zircon δ18OSMOW values (5.33–6.84 ‰) and geographical proximity to the exposed island arc rocks, indicate that the isotopically depleted Daixi alkali feldspar granites were dominantly derived from the relatively young Neoproterozoic accreted Shuangxiwu island arc sequences. Yet new and published whole‐rock Sr‐Nd‐Hf‐Pb‐O isotopic data [87Sr/86Sr)i = 0.70788–0.70833; εNd(t) = −7.6 to −3.5; εHf(t) = −6.8 to −1.9; (206Pb/204Pb)i = 18.028–18.108, (207Pb/204Pb)i = 15.565–15.577, (208Pb/204Pb)i = 38.198–38.341; δ18OSMOW = 4.5–5.5 ‰] highlight that the isotopically enriched Siling hornblende gabbros were predominantly sourced from an older, metasomatically enriched SCLM of adjacent Yangtze cratonic affinity. The arc crustal allochthon is assumed to have been tectonically emplaced onto the southeast Yangtze cratonic foreland, resulting in isotope inversion with younger crust on top of the older lithospheric mantle. Our results reveal a juxtaposition geometry of continental material after collision and accretion of island arcs by using isotope methods, which is independently supported by tectonic geological and geophysical studies. It suggests that ancient lithospheres should exist at depth within certain orogens associated with island arc accretion.

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