Granitic magmas with enriched isotopic compositions, from enriched mantle source: Implications for continental generation

Abstract

Granitoids with depleted isotopic compositions are generally considered beneficial to continental generation. However, most granitoids exhibit enriched isotopic characteristics (high initial Sr isotopic ratios, negative ƐNd and ƐHf values) and their role in continental generation is scantly explored. In this contribution, we report petrological and geochemical data of the Permian (ca. 270 Ma) Yabulai granites and hornblende gabbros in the Alxa Block of western North China Craton. Compared to initial mantle-derived basalts, the hornblende gabbros show low MgO (3.74 to 5.68 wt%), Cr (34.9 to 66.2 ppm), Ni (16.6 to 45.6 ppm) contents and Mg# (0.46 to 0.54), with zircon ƐHf values of +3.5 to +6.6 and −8.4 to −4.8. Relative to heavy rare earth elements (HREEs), the hornblende gabbros are enriched in light rare earth elements (LREEs) with the absence of Eu anomalies. In primitive mantle-normalized spidergrams, the hornblende gabbros exhibit notably negative NbTa and positive Pb anomalies. Elemental and isotopic data indicate that they might have originated from a shallow and heterogeneous lithospheric mantle source modified by subducted materials. The granites are metaluminous to weakly peraluminous and can be classified as high-K calc-alkaline I-type, with negative zircon ƐHf values of −8.3 to −5.3. These granitic rocks were likely to be derived from a newly underplated mafic crustal source. Using our data and published data from the granitic rocks in the Alxa Block, we suggest that a Permian crustal generation event has occurred in the Nuru-Langshan belt of the Alxa Block. Our current results reveal that the crustal generation rates at the convergent plate margins might have been underestimated due to the unintended omission of the mantle contribution to the granitoids with enriched isotopic compositions.