The petrogenesis and tectonic significance of the Early Cretaceous intraplate granites in eastern China: The Laoshan granite as an example

Abstract

The Laoshan granite is an example of the widespread intraplate granitoids of early Cretaceous age in eastern continental China. The petrogenesis of these granitoids remains in dispute and we choose the Laoshan granite as the representative case to study these granitoids. Zircons from the Laoshan granite give a crystallization age of ~120 Ma, which is consistent with the emplacement age of ~ 126 Ma given by the bulk-rock Rb-Sr isochron. Representative samples from the granite show a large range of major element compositional variation (e.g., SiO2/MgO = 64 to 1937), reflecting a varying degree of fractional crystallization of plagioclase, alkali feldspar, amphibole, biotite and accessory phases as observed. The samples are enriched in light rare earth elements, Rb, Th and U, but depleted in Ba and Sr with negative Eu anomalies. The high 87Sr/86Sr (0.7083 to 1.2265) is largely caused by variably high Rb/Sr (~0.31 to 91 with an average of ~13) due to feldspar fractionation. The low ƐNd(t) (−13.8 to −19.5), εHf(t) (−14.6 to −24.4), and (206Pb/204Pb)i (16.244 to 17.304) are consistent with significant contributions of the lower continental crust to magmas parental to the Laoshan granite. The origin of the parental magmas is best understood as resulting from anatexis of the lower crust (~20% − 25% partial melting of the mafic granulite) triggered by and mixed with the underplating and intruding basaltic magmas. The basaltic magmas were likely derived from melting of the thinning lithosphere being transformed into the asthenosphere as the result of “basal hydration weakening” with the water ultimately coming from dehydration of the stagnant paleo-Pacific slab in the mantle transition zone.