皖南中生代高钾钙碱性埃达克岩地球化学特征及岩石成因

Geochemical Characteristics and Petrogenesis of the Mesozoic Adakite in South Anhui Province

皖南晚侏罗世-早白垩世早期酸性岩浆岩产于扬子陆块江南古隆起东段,岩体类型为花岗闪长岩,主矿物组合为斜长石+石英+钾长石+角闪石±黑云母,副矿物有磷灰石、锆石、榍石、磁铁矿、钛铁矿。岩石主量元素含量(%)具有高SiO2、Al2O3、K2O/Na2O,低TiO2、MgO、CaO、P2O5的特征,A/CNK≈1.06,σ≈1.97,为弱过铝钙碱性系列。稀土元素表现为LREE富集,HREE亏损,具弱的负Eu异常,元素Y和Yb(×10-6)含量低,但具有较高的Sr/Y和(La/Yb)N比值;微量元素表现为富集LILE(RbL、a、Ba、Sr)和亏损HFSE(Nb、Ta、ZrY、)(,87Sr/86Sr)i>0.704,εNd<0。通过岩石学及地球化学特征分析,认为皖南晚侏罗世-早白垩世早期酸性岩浆岩为"C"型、高钾钙碱性埃达克岩。晚侏罗世-早白垩世早期,皖南地区大地构造环境处于由挤压到伸展的转折时期,埃达克岩浆来源于印支期加厚的下地壳受地幔物质上涌影响而拆层熔融形成。皖南埃达克岩的发现,可为该地区寻找与之有密切成因联系的Au、Ag、Mo及斑岩铜矿等提供理论依据。

The Acidic magmatic rocks of late Jurassic-early Cretaceous in South Anhui are indigenous to the eastern part of Jiangnan paleouplift in Yangtze landmass,and the rock type is calc-alkaline granodiorite with the major mineral assemblage being plagioclase＋quartz＋potassium feldspar＋amphibole±biotit and some accessory minerals being apatite,zircon,titanite,magnetite,ilmenite.The magmatic rocks belong to calc-alkaline series with geochemical characteristics of higher SiO2,Al2O3,K2O/Na2O and lower TiO2,MgO,CaO,P2O5.They are also obviously rich in LREE and LILE（Rb,La,Ba,Sr）,and deplete in HREE and HSFE（Nb,Ta,Zr,Y） with slightly negative δEu,higher Sr/Y and La/Yb,lower Y and Yb,and meanwhile（87↑Sr/86↑Sr）i〉0.704,εNd〈0.Through the analysis of petrological and geochemical characteristics,the magmatic rocks of late Jurassic-early Cretaceous in South Anhui are ＂C＂ type,high-K calc-alkaline adakite（HKCAA）,which formed from partial melting of the thickened lower crust by delamination in Mesozoic in eastern China.Discovery of the Adakite in south Anhui can provide theoretical evidence for the exploration of its closely related Au,Ag,Mo and porphyry copper in this area.