TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 6), pp. 551-569, 2017

Adakitic rocks explained by the deep accumulation of amphibole and apatite near the crust-mantle boundary: a case study from the Tongling region, Lower Yangtze River belt, eastern China

Le Hu1,2* Yang-Song Du,1 An-Jian Wang2 and Zi-Long Yue1

1China University of Geosciences, Beijing, 100083, P.R. China
2Research Center for Strategy of Global Mineral Resources, Chinese Academy of Geological Sciences, P.R. China

(Received February 12, 2017; Accepted July 4, 2017)

Abstract: Experimental studies have indicated that water-saturated magmas fractionate voluminous amphibole as a cumulus phase at depth, which is considered as a mechanism by which adakitic compositions can be produced. In the Tongling region, Lower Yangtze River belt (LYRB), widespread Late Mesozoic intrusions contain large amounts of amphibole-dominant cumulate enclaves that show adakitic affinities. Among them, the newly dated Early Cretaceous (138 Ma) Guihuachong quartz diorite intrusion is a representative with high Sr/Y (47–61) and La/Yb (22–26) ratios as well as weak Eu anomalies. The whole-rock geochemistry and the zircon Hf isotopic characteristics of the quartz diorites indicate a mixed crust-mantle source. Petrographic observations and geochemical analyses of the cumulate enclaves indicate significant amphibole and apatite fractionation during the early stages of magma evolution. In this paper, we highlight the roles of amphibole and apatite in the petrogenesis of rocks with adakitic affinities. The high-aluminum amphiboles crystallized under high-temperature conditions from a water-rich magma near the crust-mantle boundary and preferentially partitioned the middle-heavy rare earth elements (M-HREEs) over light REEs (LREEs). Under conditions of high water content and pressure, plagioclase crystallization was suppressed, which led to high concentrations of Sr and Eu in the melts. The crystallization of apatite consumed a large proportion of the Y (77.52–97.71 ppm in apatite), resulting in fractionated melts that were depleted in Y. Hence, the deep accumulation of amphibole and apatite led to the adakitic signatures of the derivative melts, which were the parent melts of the Tongling adakite-like rocks. The results of our study also indicate that continental adakites are not generated exclusively by partial melting of garnet-bearing sources during crustal thickening.
Key words: adakitic affinities, cumulate enclave, apatite, amphibole, Lower Yangtze River belt

*Corresponding author E-mail: hulecugb@126.com

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