摘要
埃达克岩 (adakite)是一种中酸性富钠火成岩 (安山岩、英安岩、钠质流纹岩及相应的侵入岩 ) ,其突出的地球化学特征就是 Si O2 ≥ 56% ,Al2 O3≥ 1 5% ,亏损重稀土元素 (HREE)与 Y(如 Yb≤ 1 .9× 1 0 -6,Y≤ 1 8× 1 0 -6) ,高 Sr(大多数 >40 0× 1 0 -6、La/Yb(≥ 1 0 .0 )与 Sr/Y(>2 0 .0～ 40 .0 ) ,一般具有正铕异常 (少数具有极弱负铕异常 )。埃达克岩存在两种成因类型 :一种由俯冲的年轻 (≤2 5～ 30 Ma)大洋板片熔融形成 ( 类埃达克岩 ) ;另一种由增厚地壳环境中的玄武质下地壳熔融形成 ( 类埃达克岩 )。对两类埃达克岩的特征、形成机制、成矿作用、动力学意义以及我国埃达克岩的研究现状进行了评述 ,并指出了埃达克岩研究中所存在的问题。
Adakite is a kind of mediate and acid igneous Na rich rock (andesite, dacite, sodiumic rhyolite and homologus intrusive rock ), with the stickup characteristics of SiO 2≥56%, Al 2O 3≥15%,depletion in HREE and Y(Yb≤1.9×10 6 ,Y≤18×10 6 ), high Sr(>400×10 6 ,rarely<400×10 6 )、La/Yb(10.0) and Sr/Y(>20.0~40.0), as well as positive Sr anomaly but absent or positive negative Eu anomaly. There are two types of adakites with different petrogenesis: one type is formed by the melting of subducting young (≤25~30 Ma) slab (Ⅰ type adakite), and the other type is formed by the melting of basaltic lower crust under thickened setting (Ⅱtype adakite). Ⅰ type adakite has also these characteristics: sometimes associated with “high Nb” or “Nb enriched” basalt; usually reacting with the mantle during the adakitic magma uprising; closely related to high pressure, ultra pressure eclogite and shallow earthquakes; sometimes associated with Au、Ag、Cu、Mo hydrotherm and porphyry deposit. In addition, Ⅰ type adakite occurs when the age of subducted slab is less than 25~30 Ma and the subducting slab reaches 70~80 km under the arc. The Mg#(Mg/(Mg+Fe)molar) of Ⅱtype adakite is generally less than 0.50. In the condition of aqua undersaturation and the setting of thickened(>40 km) lower crust, Ⅱtype adakite is apt to be formed by the melting of newly underplating basaltic lower crust. Two types of adakites are very significative to studying geodynamics, crust mantle interaction, and the growth and evolution of continent crust. We found the Ⅰ type adakite in north Dabie and Qingling mountains and the Ⅱtype adakite in Shaxi region of Anhui province, and thought Shaxi porphyry Cu deposit could be related to Ⅱtype adakite.
出处
《地球科学进展》
CAS
CSCD
2001年第2期201-208,共8页
Advances in Earth Science
基金
国家重点基础研究发展规划项目!"大规模成矿作用及大型矿集区预测"(编号 :G19990 432 0 2 )
国家攀登计划预选项目!"与寻找超大型矿
