Magma Mixing of the Heimahe Pluton in the Qinghai Nanshan Tectonic Zone: Evidence from Petrology, Geochemistry and Geochronology, and Its Tectonic Implications
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摘要: 黑马河岩体位于西秦岭北缘青海南山构造带内，岩体中发育大量闪长质包体，但对其成因却有一定的争议.从岩石学、矿物学、地球化学和年代学等方面对黑马河岩体花岗闪长岩及其中的闪长质包体进行了详细研究.包体的野外产出状态、形态、结构构造和矿物学特征均显示出，它们是基性岩浆注入到中酸性岩浆中快速冷凝结晶的产物.在主量元素协变图解中，花岗闪长岩和闪长质包体显示出壳幔岩浆混合作用的趋势.另外，两者稀土总量和相似的稀土元素配分模式及Eu负异常程度，也显示二者具有岩浆混合的特征.LA-ICPMS锆石U-Pb定年结果显示，花岗闪长岩形成于246 Ma，闪长质包体形成于245 Ma，两者具有几乎一致的结晶年龄，排除了包体为源区难熔残余或围岩捕掳体的可能性，也排除了基性岩浆在花岗质岩浆固结后再侵入的可能性.结合区域地质资料，认为黑马河岩体形成于西秦岭北缘有限小洋盆向南的俯冲阶段，青海南山构造带与天峻南山一带具有相同的构造岩浆演化历史.Abstract: The Heimahe pluton is located in the Qinghai Nanshan tectonic belt, northern margin of the West Qinling orogeny. Mafic microgranular enclaves (MMEs) are widely distributed in the Heimahe pluton. However, the petrogenesis of the MMEs is an issue of debate. We present the petrology, mineral chemistry, rock geochemistry and geochronology of the granodiorite and MMEs from the pluton in this paper. The outcrop conditions, shapes, structural and mineral characters of the MMEs reflect that they were products of rapid crystallization when mafic magma injected into the intermediate-acid magma. In the binary diagram of major elements, the granodiorites and MMEs show a mixing trend between mantle- and crust-derived magmas. Besides, their total REE contents and similar REE patterns and Eu anomalies also support the magma mixing model. LA-ICPMS zircon U-Pb dating results show that the granodiorite formed at 246 Ma, and the MMEs formed at 245 Ma. The same crystallization age suggests that the MMEs were not restites or host-rock xenoliths. Combined with regional geology, we suggest that the Heimahe pluton formed by the southward subduction of the limited oceanic basin in the northern margin of the West Qinling, and the Qinghai Nanshan tectonic belt experienced the same evolutionary process with the Tianjun Nanshan belt.
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图 1 青海南山构造带大地构造位置(a)、区域地质简图(b)及黑马河岩体地质简图(c)
图a据Li et al.(2014)修改
Fig. 1. Regional geological map of China showing the location of the Qinghai-Nanshan tectonic zone (a), geological sketch map of the Qinghai-Nanshan tectonic zone (b), and geological map of the Heimahe pluton(c)
图 2 黑马河岩体野外露头及显微照片
a.闪长质包体中的反向脉；b.包体边部的冷凝边；c.寄主花岗闪长岩中的斜长石斑晶进入包体；d.花岗闪长岩中斜长石的环带结构（寄主岩基质2号斜长石）；e.花岗闪长岩中角闪石包裹斜长石；f.花岗闪长岩中黑云母包裹石英；g、h、i.闪长质包体中的斜长石成分环带（分别为包体基质中1号、3号和包体1号斑晶斜长石）.Hb.角闪石；Bi.黑云母；Pl.斜长石；Kfs.钾长石；Qtz.石英；Ap.磷灰石
Fig. 2. Outcrop and microphotographs of the Heimahe pluton
图 3 黑马河岩体花岗闪长岩及闪长质包体代表性锆石阴极发光图像及年龄值
图中白色圆圈为35 μm
Fig. 3. Representative zircon CL images of granodiorite and mafic microgranular enclaves from the Heimahe pluton
图 4 黑马河岩体花岗闪长岩及闪长质包体锆石U-Pb年龄谐和图
Fig. 4. U-Pb concordia diagrams of zircon in the granodiorite and mafic microgranular enclaves from the Heimahe pluton
图 5 黑马河岩体岩石类型判别图解
a. SiO2-K2O图解，据Peccerillo and Taylor（1976）；b. A/CNK-A/NK图解，据Maniar and Piccoli（1989）；虚线所代表的I型和S型花岗岩的分界线据Chappell and White（1992）
Fig. 5. Chemical classification diagrams for the samples from the Heimahe pluton
图 6 黑马河岩体主微量元素Harker图解
Fig. 6. Harker plots of selected major and trace elements for the Heimahe pluton
图 7 黑马河岩体稀土元素球粒陨石标准化配分曲线图（a）及微量元素原始地幔标准化蛛网图（b）
图a中球粒陨石和图b中原始地幔标准化数据均引自Sun and McDonough（1989）
Fig. 7. Chondrite-normalized REE patterns (a) and primitive mantle normalized trace element multi-variation diagrams for the samples from the Heimahe pluton
图 8 黑马河花岗闪长岩及闪长质包体中斜长石An值变化剖面
Fig. 8. Profile showing variations of An values in different plagioclase grains from the Heimahe granodiorite and mafic microgranular enclaves
图 9 黑马河岩体花岗闪长岩源区判别图解
Fig. 9. Source rock discrimination diagram for the granodiorites from the Heimahe pluton
图 10 黑马河岩体主量元素协变图解
a. Al2O3/CaO-Na2O/CaO图解；b. Al2O3/CaO-FeOT图解；c. MgO/Al2O3-SiO2/CaO图解；d. MgO-Fe2O3T图解，据Zorpi et al.（1989）
Fig. 10. Binary diagram of major elements of the Heimahe pluton
图 11 黑马河岩体花岗闪长岩构造环境判别图解
a. R1-R2图解，据Batchelor and Bowden（1985）；b. Yb-Ta图解，据Pearce（1984）
Fig. 11. Tectonic setting discrimination diagrams for the granodiorites from the Heimahe pluton
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