Geochronology and geochemistry of Chuzhou and Shangyaopu adakitic intrusive rocks in the eastern area of Anhui province:Implications for petrogenesis and mineralization

Abstract

The Chuzhou and Shangyaopu plutons are located in the eastern area of the Yangtze Craton and to the east of the Tan-Lu Fault Belt,and associated with Cu(Au or Fe)mineralization.They have ~(40)At-~(39)Ar ages(biotite)of 127.17±0.40Ma,129.90± 0.23Ma,respectively.The Chuzhou and Shangyaopu intrusive rocks are geochemically similar to adakites,e.g.,SiO_256%,high Al_2O_3(14.84%～16.38%),Sr(369×10~(-6)～1335×10~(-6)),Sr/Y(43～185)and La/Yb(22～44),but low Y(5.51×10~(-6)～ 11.0×10~(-6))and Yb(0.51×10~(-6)～1.09×10~(-6)),with negligible to positive Eu anomalies(δEu=(0.75～1.28).Moreover, most samples have relatively high MgO(2.23%～5.25%),Mg~#(53～68),Cr(89.6×10~(-6)～206×10~(-6))and Ni(43.0×10~(-6)～ 72.0×10~(-6))values,similar to high-Mg andesites.The Chuzhou and Shangyaopu adakitic rocks exhibit high(~(87)Sr/~(86)Sr)_i(0.7060～ 0.7067)and low e_(Nd)(t)(-11.53～-14.07),indicating that they were impossibly derived by partial melting of subducting oceanic crust.We suggest that the Chuzhou and Shangyaopn adakitic rocks were most probably generated by partial melting of delaminated lower crust.The interaction between these adakitic magmas and surrounding mantle peridotites possibly took place during the ascent of magmas:on the one side,MgO,Cr and Ni contents of the adakitic magmas were remarkably elevated due to the contamination of mantle peridotites;on the other side,the Fe_2O_3 from the adakitic magmas was likely added into the mantle and the oxygen fugacites (f_(o_2))of the mantle were obviously elevated,which made metallic sulfide in the mantle oxidized and the chalcophile elements were incorporated into the adakitic magmas in the form of sulphate.En route to the shallow part of crust,the adakitic magmas enriched in metallogenetic materials possibly released Cu or Au owing to the rapid depressing of temperature,pressure and fugacites(f_(o_2)),which led to Cu or Au mineralization.