Genesis of the Cretaceous HFSE-enriched Hongliuxia andesitic dike on the northern margin of the Qinghai-Tibet Plateau：Anorthite fractionaL crystallization and enhanced dissolution of zircon

Abstract：The Hongliuxia volcanic field is located at the junction of the Altyn Tagh strike-slip fault and the northern Qilian Shan frontal trust fault. In this area, an andesitic dike, ~85 Ma old, cuts through ~105 Ma shoshonitic and basaltic lavas. Major and trace element studies and Sr and Nd isotopic analyses of this suite of rocks show that this younger dike has anomalous geochemistry, as compared to nearby basalts. This dike has the following salient features：(1) peralkaline with Na2O+K2O=11.8% and A/CNK=0.5540；(2) highly enriched in HFSE (e.g. Zr=1421 μg/g, Hf=26.4 μg/g, Y=48.2μg/g, Nb=104.0μg/g and Ta=8.5μg/g)；(3) a pronounced positive Ce anomaly (Ce/Ce*=1.128)；(4) nearly flat chondrite-normalized MREE and HREE distribution patterns；and (5) similar initial Sr but notably low Nd (by >2 εNd units) isotope ratios, as compared to those in the Hongliuxia shoshonitic basalts. These differences reflect not only the difference in geochemistry of their respective source regions but also the difference in behaviors of dissolution or crystallization of dominant minerals and accessory minerals during partial melting and magmatic evolution.fractionaｌ crystallization of anorthite and enhanced dissolution of zircons in its source regions are the main controlling factors responsible for the anomalous geochemistry of this dike. The geochemical processes reｃｏｒｄｅｄ by this dike may help the interpretation of similar intermediate-acid alkaline volcanic rocks since the Mesozoic on the Qinghai-Tibet Plateau.