Emplacement of Young Island Arc Crust Over Older Mantle Along a Cratonic Foreland: Constraints From Multiple Isotopes and Elemental Geochemistry

Abstract

AbstractMany 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.