Zircon U-Pb Geochronology, Geochemistry and Petrogenesis of Miocene Syenite in Chazi Area, Tibet
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摘要: 查孜正长岩体是拉萨地块中段新发现的中新世钾质-超钾质侵入岩，岩性主要为中粗粒石英角闪正长岩和斑状石英正长岩，有暗色包体发育.对两种正长岩进行了岩石学、锆石U-Pb年代学和地球化学研究.结果显示，两种岩石的锆石U-Pb年龄分别为10.37±0.24 Ma、11.06±0.39 Ma，代表它们形成于中新世，是拉萨地块后碰撞岩浆作用的产物.查孜正长岩具有相对高钾（K2O=6.75%~7.39%）、低镁（MgO=1.44%~2.97%）的特征，K2O/Na2O>1，属钾质岩；具有与超钾质岩石相似的微量元素特征，强烈富集Rb、Th、U、K等大离子亲石元素（LILE）和轻稀土元素（LREE），相对亏损Nb、Ta、Zr、Hf、Ti、P等高场强元素（HFSE）和重稀土元素（HREE），但Cr（22.7×10-6~64.6×10-6）、Ni（18.9×10-6~46.6×10-6）含量明显偏低.其中，斑状石英正长岩的SiO2含量相对较高，但MgO、K2O、Cr、Ni、REE和Y等元素含量比石英角闪正长岩的低.综合分析认为，查孜正长岩主要形成于岩浆混合作用，是富集岩石圈地幔部分熔融形成的超钾质岩浆与地壳物质部分熔融形成的酸性岩浆混合的结果，两种岩石的地球化学差异主要是岩浆混合的程度和比例不同导致的；它的形成可能与岩石圈地幔的减薄作用有关.Abstract: The Chazi syenite mass is a newly discovered Miocene potassic-ultrapotassic intrusive rock in the middle Lhasa block, and its lithology is mainly composed of medium-coarse grained quartz hornblende syenite and porphyraceous quartz syenite. Mafic magmatic Enclaves were found in the syenite body. Petrography, zircon U-Pb dating and geochemical analysis were done, in order to study the petrogenesis of the Chazi syenite. Zircon U-Pb ages of 10.37±0.24 Ma and 11.06±0.39 Ma were obtained for these two rock types respectively, which represent that they were formed in the Miocene Epoch and product of post-collision magmatism in the Lhasa block. The Chazi syenite rocks have relatively high K2O (6.75%-7.39%) and low MgO (1.44%-2.97%), with K2O/Na2O > 1, so they are potassic rocks. They have similar characteristics of trace elements to the ultrapotassic rocks:strongly enriched in large ion lithophile elements (LILE) such as Rb, Th, U and K, and light rare earth elements (LREE), relatively depleted in high field strength elements (HFSE) such as Nb, Ta, Zr, Hf, Ti and P, and heavy rare earth elements (HREE), but the contents of Cr (22.7×10-6-64.6×10-6) and Ni (18.9×10-6-46.6×10-6) of the syenite are obviously lower. Compared with porphyraceous quartz syenite rocks, the quartz hornblende syenite rocks have relatively higher SiO2, and lower elements sucn as TiO2, FeOT, MgO, K2O, Cr, Ni, REE and Y. By comprehensive analyses, it is proposed that the Chazi syenite was mainly formed by magma mixing, and produced by the mixing between the ultrapotassic magma that originated from the lithosphere mantle partial melting, and acid magma that originated from the continental crust partial melting. The chemical differcence of two rock types is mainly due to the different degrees of magma mixing and ratio of mixed magma.The Chazi syenite possibly resulted from convective thinning or delamination of over thickened mantle lithosphere.
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Key words:
- middle Lhasa Block /
- Chazi area /
- Miocene /
- syenite /
- zircon U-Pb age /
- geochemistry /
- magma mixing
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图 1 拉萨地块构造简图(a)和查孜地区地质简图(b)
图a据Zhu et al. (2011)修改. BNSZ.班公湖-怒江缝合带；SNMZ.狮泉河-纳木错蛇绿混杂岩带；LMF.洛巴堆-米拉山断裂带；YZSZ.雅鲁藏布江缝合带；NL.北拉萨地块；ML.中拉萨地块；SL.南拉萨地块
Fig. 1. Tectonic sketch of the Lhasa Block (a) and simplified geological map of the Chazi region (b)
图 2 查孜正长岩的野外和显微镜下照片
a.斑状石英正长岩; b.石英角闪正长岩、斑状石英正长岩及暗色包体; c.石英角闪正长岩显微照片(正交偏光); d.斑状石英正长岩显微照片(正交偏光); e.石英正长斑岩显微照片(正交偏光); f.正长石中的角闪石嵌晶和针状磷灰石(正交偏光). Q.石英; Or.正长石; Hb.普通角闪石; Px.辉石; Bt.黑云母; Ap.磷灰石
Fig. 2. Field photos and photomicrographs of the syenites in Chazi
图 3 样品SS01-N和AMX02-N的锆石CL图像和U-Pb年龄谐和图
Fig. 3. CL images and U-Pb age concordia diagrams for zircons from samples SS01-N and AMX02-N
图 4 样品的TAS (a)、A/CNK-A/NK (b)、SiO2-K2O (c)和Na2O-K2O (d)图解
图中文献数据来自Ding et al. (2003)、Gao et al.(2007, 2009)、Guo et al. (2013)、Liu et al. (2017)、Tian et al. (2017)和Zeng et al. (2017).拉萨地块超钾质岩石范围据刘栋等(2011)和Liu et al. (2017)修改
Fig. 4. Diagrams of TAS (a), A/CNK-A/NK (b), SiO2-K2O (c) and Na2O-K2O (d) for the samples
图 5 样品稀土元素球粒陨石标准化配分曲线图(a)和微量元素原始地幔标准化蛛网图(b)
球粒陨石标准化值据Boynton(1984); 原始地幔标准化值据Sun and McDonough (1989).拉萨地块超钾质火山岩和埃达克质岩石的微量元素配分范围参考刘栋等(2011)
Fig. 5. Chondrite-normalized REE pattern (a) and primitive mantle-normalized trace element spider diagram (b) for the samples
图 6 SiO2-Sr/Y (a)、SiO2-Y/Yb (b)、SiO2-Dy/Yb (c)和La-La/Yb (d)图解
HPFC.高压分异结晶; LPFC.低压分异结晶.文献数据同图 4
Fig. 6. Diagrams of SiO2-Sr/Y (a), SiO2-Y/Yb (b), SiO2-Dy/Yb (c) and La-La/Yb (d)
图 7 MgO-FeOT (a)、Ni-MgO (b)、MgO-CaO/Na2O (c)和Y-Sr/Y (d)图解
图d中的岩浆混合过程模拟曲线据Liu et al. (2017); 文献数据同图 4
Fig. 7. Diagrams of MgO-FeOT (a), Ni-MgO (b), MgO-CaO/Na2O (c) and Y-Sr/Y (d)
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