THE SUMMARY AND COMMENT ON RESEARCH ON A NEW KIND OF IGNEOUS ROCK—ADAKITE
Received date: 2000-06-28
Revised date: 2000-08-21
Online published: 2001-04-01
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%, Al2O3≥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.
Key words： Adakite; Petrogenesis types; Slab subducting; Underplating geodynamics.
WANG Qiang, XU Ji-feng,ZHAO Zhen-hua . THE SUMMARY AND COMMENT ON RESEARCH ON A NEW KIND OF IGNEOUS ROCK—ADAKITE[J]. Advances in Earth Science, 2001 , 16(2) : 201 -208 . DOI: 10.11867/j.issn.1001-8166.2001.02.0201
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