Abstract
Calc-alkaline andesitic rocks are a major product of subduction-related magmatism at convergent margins. Where these melts are originated, how long they are stored in the magma chambers, and how they evolved is still a matter of debate. In this study, we present new data of whole-rock elemental and Sr-Nd-Pb isotope compositions, and zircon U-Pb-Th isotopes and trace element contents of Nageng (basaltic-)andesites in the East Kunlun Orogen (NW China). The similar age and whole-rock elemental and Sr-Nd-Pb isotope contents suggest that the Nageng andesite and basaltic andesite are co-magmatic. Their low initial 87Sr/86Sr (0.7084–0.7086) but negative εNd(t) values (−10.61 to −9.49) are consistent with a magma source from the juvenile mafic lower crust, possibly related to the mantle wedge with recycled sediment input. The U-Pb age gap between the zircon core (ca. 248 Ma) and rim (ca. 240 Ma) reveals a protracted magma storage (~8 Myr) prior to the volcanic eruption. When compared to the zircon rims, the zircon cores have higher Ti content and Zr/Hf and Nb/Ta ratios, but lower Hf content and light/heavy rare earth element ratios, which suggests that the parental magma was hotter and less evolved than the basaltic andesite. The plagioclase accumulation likely resulted in Al2O3-enrichment and Fe-depletion, forming the calc-alkaline signature of the Nageng (basaltic-)andesites. The magma temperature, as indicated by the zircon saturation and Ti-in-zircon thermometry, remained low (725–828 °C), and allowed for the magma chamber to survive over ~8 Myr. The decreasing εHf(t) values from zircon core (avg. 0.21, range: −1.28 to 1.32) to rim (avg. −3.68, range: −7.30 to −1.13), together with the presence of some very old xenocrystic zircons (268–856 Ma), suggest that the magma chamber had undergone extensive crustal contamination.
Highlights
Calc-alkaline rocks are generally considered to have formed at convergent Margins, and they contain higher Al2 O3 and lower FeOT /MgO ratios than tholeiitic rocks [1,2,3,4,5].Calc-alkaline andesitic rocks are extensively distributed on Earth and they represent an important constituent of the continental crust [6,7]
Understanding the formation mechanism of calc-alkaline andesites is important in unravelling the continental crust evolution [8], which has remained a key debate in igneous petrology
We propose that the Magma source May have been the juvenile Mafic lower crust because: (1) The Hf model ages (1.2–1.4 Ga) of zircon cores from the studied rocks are much younger than ancient lower crust rocks in the EKO (e.g., Proterozoic gneiss and schist; 1.6–2.2 Ga) [49,51,58], which suggests the involvement of juvenile components
Summary
Calc-alkaline rocks are generally considered to have formed at convergent Margins, and they contain higher Al2 O3 and lower FeOT /MgO ratios than tholeiitic rocks [1,2,3,4,5].Calc-alkaline andesitic rocks are extensively distributed on Earth and they represent an important constituent of the continental crust [6,7]. Many models were proposed to explain the formation of these melts: (1) the partial melting of hydrous Mantle [6,9]; (2) fractional crystallization of Mantle-derived basaltic Magma [10]; and, (3) mixing between Mafic and felsic Magmas [4,11,12]. These models are built up around the fundamental question of whether the calc-alkaline signature is inherited from the source region or induced by Magmatic processes, including Magma fractionation, crustal contamination, and mixing with Si-rich, Fe-poor crustal Materials [13,14]. Based on whole-rock geochemistry and Sr-Nd-Pb isotopes, in combination with zircon U-Pb geochronology, trace element geochemistry, and zircon saturation and Ti-in-zircon thermometry on the Nageng (basaltic-)
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