Advances in Earth Science ›› 2018, Vol. 33 ›› Issue (10): 1058-1074. doi: 10.11867/j.issn.1001-8166.2018.10.1058.

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Identification of Magma Mixing: A Case Study of the Daocheng Batholith in the Yidun Arc

Ruigang Zhang( ), Xue Gao, Liqiang Yang *( )   

  1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
  • Received:2018-04-18 Revised:2018-08-13 Online:2018-10-10 Published:2018-11-16
  • Contact: Liqiang Yang;
  • About author:

    First author:Zhang Ruigang (1993-), male, Qingyang City, Gansu Province, Master student. Research areas include deposits and geochemistry of mineral deposits.

  • Supported by:
    Project supported by the National Key Basic Research and Development Plans "Typical composite metallogenic system and deep drive mechanism of southwest Tethys, in China"(No.2015CB452605);The Program of Academic Innovation and Intelligence Introduction in Iinstitutions of Higher Learning "Dynamics of ore-forming processes"(No.B07011).

Ruigang Zhang, Xue Gao, Liqiang Yang. Identification of Magma Mixing: A Case Study of the Daocheng Batholith in the Yidun Arc[J]. Advances in Earth Science, 2018, 33(10): 1058-1074.

The Daocheng batholiths, located in the east of the Yidun arc, consist of granite, granodiorite and K-feldspar granite. Abundant massive mafic microgranular enclaves (MMEs) mainly developed within the granodiorite and K-feldspar granite, and they have clear contacts with the hosted granites. The MMEs are characterized by the quartz eye structure, quenched apatite, and plagioclases phenocrysts with obvious oscillatory zones. Petrographical studies on MMEs and host granites, zoned plagioclase and whole-rock geochemical analysis were carried out to identify the presence of magma mixing. Combined with the previous studies on the whole-rock Sr-Nd-Hf isotopic signatures, the petrogenesis of Daocheng batholith was discussed. The zoned plagioclases from MMEs have An contents varying between 29 and 44, while those from the host granites have An contents of 21~50. The compositional variations and corrosion structure of plagioclase are probably related to magma mixing. Geochemically, the MMEs have relatively low SiO2 contents (56.34~60.91wt%), high Al2O3 contents of 16.06~17.98wt%, and are enriched in magnesium and iron, belonging to metalumnious series (A/CNK=0.82~0.98). The Daocheng batholith belongs to high-K calc-alkaline series, which have high alkaline contents (Na2O+K2O=6.25~7.79wt%) and low CaO contents (1.40~3.22wt%). Furthermore, both the MMEs and hosted granites are enriched in LILEs (K, Rb and Pb) and LREEs and depleted in HFSEs (Nb, Ta, Zr, Hf, P and Ti), showing affinities of typical arc magmas. Compared with the host granites, the MMEs are characterized by lower (La/Yb)N ratios of 1.99 to 2.46, and much more obvious Eu depletions (Eu/Eu*=0.30~0.50). The host granites have Rb/Sr ratios ranging from 1.0 to 1.9, and they are consistent with the crust-derived materials (Rb/Sr>0.5). Their Zr/Hf ratios range from 27.5 to 36.9, which are close to the transitional Zr/Hf ratios between mantle-and crust-derived materials. This indicates that the formation of Daocheng batholith is genetically related to the mixing between mantle-and crust-derived materials. In addition, the relatively low silica contents and high Mg# values, and the linear patterns of MgO, Al2O3 and Fe2O3 with SiO2 contents from the MMEs and host granites, show that the formation of MMEs is genetically related to magma mixing. Overall, the parent magmas of Daocheng granites are derived from the partial melting of Late Triassic arc lower crust, with the input of minor mantle-derived materials. The MMEs are generated by the mixing of the mafic magma with felsic magma.

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