Petrogenesis and Geochemistry of Meso-Cenozoic Granitic Rocks and Implication of Crustal Structure Changes in Shannan Area, Southern Tibet
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摘要: 西藏拉萨地块南部发育大规模东西带状展布的花岗质岩石，记录了新特提斯洋壳俯冲晚期及随后印度-欧亚大陆碰撞、后碰撞过程的重要信息，受到了学者的广泛关注.对拉萨地块南部山南地区采集的6件花岗质岩石样品进行了LA-ICP-MS锆石U-Pb年代学、Hf同位素及全岩主微量元素和Sr-Nd同位素地球化学分析，获得了~90 Ma、65 Ma及23 Ma三阶段的锆石年龄，显示区域内发育了三期岩浆活动事件.三个时代的岩石样品均为亚碱性系列，具类似埃达克质岩特征，富集高场强元素并亏损大离子亲石元素，稀土元素分布图呈右倾型，具有弧岩浆的地球化学特征.本文所有样品的锆石εHf(t)均为正值(+5.6~+14.6)，暗示它们可能来源于新生下地壳物质的部分熔融.结合前人已有数据，采用花岗质岩石的La/Yb比值定量还原了山南地区100 Ma以来的地壳厚度演化历史.从晚中生代开始，区域内地壳厚度由厚减薄，到新生代早期达到最薄，此后逐渐增厚.这与中新生代以来新特提斯洋俯冲至印度-欧亚大陆碰撞-后碰撞过程引起地壳结构变化较好地契合.Abstract: In the southern part of the Lhasa block in southern Tibet, large-scale zonal distribution of granitic rocks is developed, where the important information on the late subduction of the Neo-Tethys oceanic crust and subsequent collisions and post-collision processes in the Indian-Eurasia continent is recorded. In this paper, LA-ICP-MS zircon U-Pb geochronology, Hf isotope and total trace elements and Sr-Nd isotope geochemical analysis of six granitic rock samples collected in the southern Shannan area of the Lhasa block were obtained. The three-stage zircon ages of~90 Ma, 65 Ma and 23 Ma show that three phases of magmatic activity occurred in the area. The rock samples of the three eras are all subalkalic series, with similar adakite characteristics, enriched with high field strength elements and depleted with large ion lithophile elements. The distribution of rare earth elements is right-dip, geochemical with the feature of arc magma. The zircon εHf(t) of all samples in this paper are positive (+5.6-+14.6), suggesting that they may be derived from the partial melting of the new lower crust. Based on the previous data, the La/Yb ratio of granitic rocks is used to quantitatively show the evolution of crustal thickness in the Shannan area for 100 Ma. From the late Mesozoic, the thickness of the crust in the area was thinned from thick to thin in the Early Cenozoic, and gradually thickened thereafter. This is in line with the changes in the crustal structure caused by the subduction of the Neo-Tethys and the Indian-Eurasia collision-postcollision process since Mesozoic to Cenozoic.
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Key words:
- South Lhasa block /
- crustal thickness /
- quantitative calculation /
- La/Yb ratio /
- intermediate-acid magma /
- petrology
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图 1 青藏高原南部岩浆-构造分布简图及山南地区结巴乡地质图和样品情况
修改自Chung et al.(2009)；Jiang et al.(2014)；MBT.主边界逆冲断层；MCT.主中央逆冲断层；STDS：藏南拆离断层系；Kf.钾长石；Hb.角闪石；Q.石英
Fig. 1. Sketch map of southern Tibetan Plateau showing major blocks and temporal spatial distribution of magmatic rocks and geological map and photos of granitic rocks of Jieba Village in Shannan area
图 2 山南地区花岗质岩石锆石U-Pb年龄谐和图
Fig. 2. Zircon U-Pb concordia diagrams of the granitic rocks in Shannan area
图 3 山南地区花岗质岩石锆石年龄与εHf(t)图解
Fig. 3. Plot of εHf(t) versus age for the granitic rocks in Shannan area
图 4 山南地区花岗质岩石TAS图解
Fig. 4. Plot of total alkali versus SiO2 of the granitic rocks in Shannan area
图 5 山南地区花岗质岩石(La/Yb)N-(Yb)N图解
Fig. 5. Plot of (La/Yb)N versus (Yb)N of the granitic rocks in Shannan area
图 6 山南地区花岗质岩石全岩原始地幔标准化微量元素蛛网图和球粒陨石均一化稀土分配图
Fig. 6. Primitive mantle-normalized trace element patterns and chondrite-normalized rare earth element (REE) patterns of the granitic rocks in Shannan area
图 7 山南地区花岗质岩石Sr-Nd同位素组成图解
数据底图据Jiang et al.(2014)
Fig. 7. Plot of εHf(t) vs. (87Sr/86Sr)i of the granitic rocks in Shannan area
图 8 山南地区晚白垩世-中新世地壳厚度演化
Fig. 8. Crustal thickness evolution of Shannan area from Late Cretaceous to Miocene
表 1 山南地区花岗质岩石岩石Sr-Nd同位素分析结果
Table 1. Sr-Nd isotope analysis results of the granitic rocks in Shannan area
样品编号 87Rb/86Sr 87Sr/86Sr 2σ 87Sr/86Sri 147Sm/144Nd 143Nd/144Nd 1σ εHf(t) fSm/Nd tDM(Ma) tDM2(Ma) 13JBX-01 0.428 5 0.705 49 5 0.705 35 0.092 86 0.512 55 7 -1.1 -0.53 753 945 13JBX-02 0.263 8 0.704 38 5 0.704 03 0.099 98 0.512 77 8 4.8 -0.49 517 598 13JBX-03 2.431 3 0.706 13 5 0.703 88 0.102 98 0.512 75 9 3.7 -0.48 557 635 13JBX-04 1.558 1 0.705 06 5 0.703 03 0.092 52 0.512 76 6 4.8 -0.53 487 592 13JBX-05 0.816 4 0.704 90 6 0.703 86 0.095 62 0.512 76 8 4.7 -0.51 503 600 13JBX-06 0.638 1 0.705 18 6 0.704 39 0.101 09 0.512 74 10 4.1 -0.49 559 642 
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