南祁连拉脊山构造带早古生代岩浆混合作用: 以马场岩体为例

拉脊山构造带位于南祁连构造带北缘，发育大量早古生代岩浆岩。其南缘马场岩体主要由黑云母花岗岩和花岗闪长岩组成，花岗闪长岩中发育大量的镁铁质微粒包体。黑云母花岗岩、花岗闪长岩和镁铁质包体LA-ICP-MS锆石U-Pb年龄分别为467±7Ma、468±6Ma、466±6Ma。黑云母花岗岩具有低K₂O/Na₂O（0.28~0.37）、高Sr/Y（125~168）比值特征，为埃达克质岩；同时，黑云母花岗岩还具有相对低的MgO、Cr、Ni含量和Mg^#值，富集轻稀土、大离子亲石元素（Ba、K、Pb、Sr）以及Zr、Hf，亏损重稀土和高场强元素（Nb、Ta、Ti），具有Eu正异常（Eu/Eu^*=1.30~1.58），亏损Sr-Nd和锆石Hf同位素组成（（⁸⁷Sr/⁸⁶Sr）_t=0.7044~0.7046，ε_Nd（t）=+2.05~+2.21，ε_Hf（t）=+8.2~+10.2），指示黑云母花岗岩是新生地壳物质重熔的产物。镁铁质微粒包体与寄主花岗闪长岩之间呈过渡-截然接触界线，发育反向脉，角闪石嵌晶结构、斜长石不平衡生长结构、镁铁质凝块以及刀刃状黑云母、针状磷灰石等显微结构反映典型的岩浆混合特征。镁铁质微粒包体与寄主花岗闪长岩均富集大离子亲石元素（Cs、K、Pb、Sr），亏损高场强元素（Nb、Ta、Zr、Hf），具有与黑云母花岗岩一致的Sr-Nd同位素组成，但镁铁质微粒包体具有更高的MgO含量（6.15%~9.12%）、Mg^#值（57~60）和Cr（271×10^-6~424×10^-6）、Ni（54.7×10^-6~86.6×10^-6）含量，暗示镁铁质微粒包体与花岗闪长岩是由受俯冲流体交代的地幔熔体与新生地壳物质重熔形成的熔体经岩浆混合而成。结合区域背景分析，本文认为马场岩体的形成与南祁连洋俯冲过程中幔源岩浆底侵加热新生地壳以及岩浆混合作用相关。

Early Paleozoic magma mixing in the Lajishan tectonic belt of South Qilian: An example from the Machang pluton

Early Paleozoic granitoid rocks are widespread in the Lajishan tectonic belt, northern margin of the South Qilian Belt. The Machang pluton, located in the southern edge of the Lajishan tectonic belt, is mainly composed of biotite granite, granodiorite, and mafic microgranular enclaves (MMEs). LA-ICP-MS zircon U-Pb dating yields the Middle Ordovician crystallization ages of 468~466Ma for those rocks. The biotite granites exhibit low K2O/Na2O (0.28~0.37) and high Sr/Y (125~168) and (La/Yb)N ratios, typical features of adakitic rocks. They are enriched in LREEs, LILEs, Zr, and Hf, but depleted in HREEs and HFSEs (Nb, Ta, Ti) with positive Eu anomalies. In addition, they also have relatively low contents of MgO, Cr, and Ni and Mg# values, and slightly depleted Sr-Nd isotopic composition of whole-rocks and depleted Hf isotopic composition of zircons ((87Sr/86Sr)t=0.7044~0.7046, εNd(t)=+2.05~+2.21, εHf(t)=+8.2~+10.2), overlapping with those of the Late Cambrian to Early Ordovician arc-related mafic igneous rocks of the Lajishan tectonic belt, hinting an derivation from anatexis of juvenile crustal materials. Most of the MMEs are sharp contact with their host granodiorites, but a few shows transitional contacts. The MMEs and host granodiorites are characterized by enrichment of LILEs (Cs, K, Pb, Sr) and depletion of HFSEs (Nb, Ta, Zr, Hf), and have similar Sr-Nd-Hf isotopic composition, which are slightly enriched relative to those of biotite granites. However, the MMEs have lower SiO2, higher MgO (6.15%~9.12%), Mg# (57~60), Cr (271×10-6~424×10-6), and Ni (54.7×10-6~86.6×10-6) contents than the host rocks. Moreover, the MMEs are featured by disequilibrium textures, such as back veining, poikilitic hornblende, disequilibrium-textured plagioclase, mafic clot, blade-shaped biotite, and acicular apatite. It is therefore inferred that the MMEs and granodiorites are generated by the mixing of subduction-related metasomatized mantle-derived mafic magmas and felsic magmas originating from partial melting of the juvenile crust. In combination with evidence of regional geology, it can be concluded that the Machang pluton were formed during the subduction of the South Qilian Ocean and related to partial melting of the juvenile crust by the upwelling mantle and magma mixing.