Petrogenesis and Tectonic Implications of Peralkaline A-Type Granites and Syenites from the Suizhou-Zaoyang Region,Central China

In this study, we present systematic petrological, geochemical, LA-ICP-MS zircon U-Pb ages and Nd isotopic data for the A-type granites and syenites from Suizhou-Zaoyang region. The results show that the peralkaline A-type granites and syenites were episodically emplaced in Suizhou-Zaoyang region between 450±3 and 441±7 Ma which corresponds to Late Ordovician and Early Silurian periods, respectively. Petrologically, the syenite-peralkaline granite association comprises of nephefine normative-syenite and alkaline granite in Guanzishan and quartz normative syenite and alkafine granite in Huangyangshan. The syenite-granite associations are ferroan to alkali in composition. They depict characteristics of typical OIB （oceanic island basalts） derived A-type granites in multi-elements primitive normalized diagram and Ybffa vs. Y/Nb as well as Ce/Nb vs. Y/Nb binary plots. Significant depletion in Ba, Sr, P, Ti and Eu indicates fractionation of feldspars, biotite, amphiboles and Ti-rich augite. The values of eNd（t） in Guanzishan nepheline syenite and alkaline granite are ＋1.81 and ＋2.26, respectively and the calculated two-stage model age for these rocks are 1 040 and 1 003 Ma, respectively. On the other hand, the Huangyangshan alkaline granite has eNd（t） values ranging from ＋2.61 to ＋3.46 and a relatively younger two-stage Nd model age values ranging from 906 to 975 Ma, respectively. Based on these data, we inferred that the Guanzishan nepheline syenites and granites were formed from fractional crystallization of OIB-like basic magmas derived from upweliing of metasomatized lithospheric mantle. The Huangyangshan quartz syenite and granite on the other hand, were formed from similar magmas through fractional crystallization with low input from the ancient crustal rocks. Typically, the rocks exhibit Al-type granite affinity and classified as within plate granites associated with the Ordovician crustal extension and the Silurian rifting.

Origin of the Low δ^(18)O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China:Evidence from the Kulongshan Pluton

The origin of lowδ^(18)O signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed.It is uncertain whether the^(18)O-depleted features were inherited from high-temperature hydrothermal altered source rock or resulted from water-rock interaction after emplacement.In this paper,zircon oxygen isotopes in the~130 Ma Kulongshan A-type granites in the northern North China Craton are analyzed.The zircons could be subdivided into 5 types based on their luminescent intensity and internal structures in CL images.Theirδ^(18)O values also vary in different types and show negative correlation with U and Th contents and accompanying cumulativeα-decay doses,implying that theirδ^(18)O values may have been modified to various degrees by meteoric water-rock interaction after the accumulation of radiation damage.The idea is further confirmed by oxygen isotopic equilibrium calculation between co-existing mineral pairs.It is inferred that only the least-influenced zircons,with slightly elevatedδ^(18)O values than normal mantle,have preserved the magmatic oxygen isotopes.In combination with other evidences,it is proposed that the A-type granites are lower-crustal-derived,unnecessarily invoking a high-temperature hydrothermal altered source.The proposition is applicable to many other Cretaceous A-type granites that have similar zircon behaviors.