Zircon U-Pb geochronology, geochemistry and Hf isotopes of the Late Cretaceous Hongshan intrusion, western Yunnan, SW China

The Hongshan quartz monzonite porphyry is one of the Yanshanian intrusions in the southern part of the Yudun Arc. Detailed zircon U-Pb data of four samples yielded ages of 78.8-80.7Ma, indicating that the Hongshan intrusion was emplaced during the late stage of Late Cretaceous. The Hongshan intrusion shows shoshonitic and high-K calc-alkaline, with A/CNK=0.64-1.14. The rocks show an obvious fractionation between light and heavy rare-earth elements (average [La/Yb](N)=38.85), with negative Eu anomalies (Eu/Eu*=0.60-0.87), enrichment in large-ion lithophile elements (Rb, Th, U and K) and depletion in high field-strength elements (Nb, Ta and P). Rocks have high Sr and low Y content which are characteristics of adakitic rocks, suggesting magma derivation from thickened lower crust. In order to evaluate the nature of the source region, Hf isotope data of zircons were acquired through LA-MC-ICPMS. The negative and variable epsilon(Hf)(t) values demonstrate that the Hongshan intrusion was derived from ancient crust, without mantle-derived components and is significantly different from the Triassic intrusions in the southern part of the Yudun Arc. The three Yanshanian intrusions in Hongshan, Relin and Tongchangou are remarkably similar in terms of geochronology, geochemistry and Hf isotopes. We therefore infer that these intrusions had the same magmatic source and we correlate the tectonics with northward subduction of Tethys underneath the Asian continent. Copyright (c) 2015 John Wiley & Sons, Ltd.