Zircon U-Pb ages, Hf-O isotopes and trace elements of Mesozoic high Sr/Y porphyries from Ningzhen, eastern China: Constraints on their petrogenesis, tectonic implications and Cu mineralization

摘要

The relationship between high Sr/Y (adakitic) rocks and Cu mineralization has been long recognized but the mechanism remains unclear. The Cretaceous high Sr/Y porphyries in the Ningzhen area host major Cu polymetallic deposits in the Lower Yangtze River Belt (LYRB) of eastern China. These rocks exhibit some geochemical characteristics (e.g., non-radiogenic Pb isotope ratios) that differ from adaldtic rocks from adjacent locations in the LYRB. In this study, we present a study of the zircon U-Pb-Hf-O isotope and trace element compositions for five porphyries from Ningzhen to reveal their petrogenesis and how that correlates with Cu-Fe-Mo mineralization. Zircon U-Pb ages of Anjishan (Cu deposit), Tongshan (Cu-Mo deposit) and Xiangshan (Fe deposit) plutons in the Ningzhen area are 108.8 +/- 1.5 Ma, 105-107 Ma and 100-105 Ma, respectively, which are significantly younger than the ore-bearing adakites (140 +/- 5 Ma) in the western part of the LYRB. Zircon epsilon(Hf)(t) and delta O-18 values range from -23.4 to -10.6 and from 5.7 to 7.0 parts per thousand., respectively, falling between subduction-related adakites from the other regions in the LYRB and delamination-related adakitic rocks from the adjacent South Tan-Lu Fault Zone. The similarities of Ce4+/Ce3+ and Eu/Eu* ratios in zircons from Ningzhen and those from the western LYRB indicate higher oxygen fugacity in their magma sources. Ti-in-zircon thermometer yields magma temperatures of 550 to 700 degrees C (with an average of similar to 650 degrees C) for the Ningzhen porphyries, which are significantly lower than those of the South Tan-Lu Fault adakites (>750 degrees C), but similar to those for the LYRB adakites (<700 degrees C). In summary, the Ningzhen high Sr/Y porphyries have high Mg# (>50), non-radiogenic Pb, enriched Sr-Nd isotopic compositions, negative zircon epsilon(Hf)(t), mantle-like delta O-18 values, high oxygen fugacities and low magma temperatures. Mafic rocks that co-exist with ore-bearing porphyries or occur as xenoliths in porphyries are widespread. We proposed that the Ningzhen high Sr/Y porphyries originated from mixing of magmas from metasomatized mantle and ancient lower continental crust. The mantle-derived basaltic magmas supplied not only the heat input, but also the elevated fo(2) and water for the generation of Cu-Fe-Mo mineralization. The compiled ages of Mesozoic igneous rocks in the LYRB display an eastward decrease from inland towards the coast. This may suggest rolling back of a flat subducted oceanic plateau; this has been argued to account for the generation of igneous rocks and associated Cu-Fe-Mo mineralization in the LYRB. (C) 2014 Elsevier B.V. All rights reserved.