The geochronology and geochemistry of Bomi granitoids in eastern Tibet and its significance
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摘要:
波密岩体位于西藏东南部，处于班公湖-怒江蛇绿混杂岩带与印度河-雅鲁藏布蛇绿混杂岩带之间，整体呈NW向延伸，面积约29km2。本文对波密花岗岩体进行了系统的岩石学、地球化学及同位素年代学研究。结果显示，波密岩体主要以黑云母石英闪长岩与黑云母花岗闪长岩为主，两个花岗闪长岩的LA-ICP-MS锆石U-Pb年龄分别为113.1±2.3Ma和113.4±2.1Ma。花岗岩类SiO2介于64.11%~66.63%，K2O+Na2O=6.36%~7.54%，全碱含量较高，属于高钾钙碱系列。稀土元素含量为114.9×10-6~182.8×10-6，分配曲线均呈右倾趋势。另外，A/CNK为0.80~0.95，波密岩体有高K，高Si，低P的特点，大离子亲石元素(Rb、K)富集，高场强元素(Nb、Ta、P、Ti)亏损，属于偏铝质I型花岗岩。2件样品37个测点的锆石εHf(t)值不均一，几乎全为负值(-13.72~-0.08)，反映岩浆可能源于古老地壳；地壳模式年龄(tDMC)集中于891~2047Ma之间。综合分析波密岩体地球化学及锆石年龄数据，均与中冈底斯及弧背断隆带中早白垩世的岩体特征相似。因此本文推断波密岩体的成因为中冈底斯早白垩世带状岩浆大爆发事件在东部的延续。
Abstract:Bomi granitoids in the southeast of Tibet located between Bangong Co-Nujiang suture zone and Yaluzangbujiang suture zone. Bomi granitoids are distributed in the north-west direction, with area of 29km2. Based on the petrological, geochemical and isotopic studies, Bomi granitoids are lithologically dominated by biotite granodiorite and biotite quartz diorite, LA-ICP-MS zircon U-Pb data for two samples of granodiorite give the ages of 113.1±2.3Ma and 113.4±2.1Ma respectively. The chemical analyses show that SiO2 varies from 64.11%~66.63% for the granitoids. The total alkali contents are high in the granitoids (K2O+Na2O=6.36%~7.54%). The REE pattern is flat and slightly declining without clearly Eu anomaly with the total content of 114.9×10-6~182.8×10-6 in the granitoids. In addition, A/CNK varies from 0.80~0.95. The rocks are enriched in silica and alkali, but the content of phosphorus is low. Bomi granitoids are petrochemically rich in large-ion incompatible elements (Rb, K) and depleted in high field strength elements (Nb, Ta, P, Ti), and show essential characteristics of arc magmatism. The rocks belong to medium-high K calc-alkaline I-type granite. 37 analyses from 2 samples give a wide range zircon εHf(t) values (-13.72~-0.08) and Hf-isotope crustal model ages (891~2047Ma). It shows that the Bomi granitoids are probably being derived from ancient anomaly crustal materials. The characteristics of petrological, geochemical and isotopic data are similar to middle Gangdese and Gangdese Retroarc Uplift Belt (GRUB). Therefore, it was concerned that Bomi granitoids are related to the zonal magmatic flare-up of middle Gangdese at Early Cretaceous.
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Key words:
- Bomi granitoids /
- Early Cretaceous /
- Hf isotope /
- Lhasa Terrane /
- Tibet
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图 3
波密早白垩世侵入岩稀土元素球粒陨石标准化图解(a，据Boynton, 1984)和微量元素蛛网图(b, 据Sun and McDonough, 1989)
Figure 3.
Chondrite-normalized REE patterns (a, after Boynton, 1984) and primitive mantle-normalized trace element patterns (b, after Sun and McDonough, 1989) for the Bomi granitoids
图 4
波密花岗岩类样品(12BM08-1和12BM08-2)锆石U-Pb年龄谐和图和阴极发光图
Figure 4.
U-Pb age concordia plots and cathodoluminescence images of zircon grains (12BM08-1 and 12BM08-2) from the Bomi granitoids
图 5
波密岩体花岗岩类εHf(t)-U-Pb年龄图解(据常青松等，2011)
Figure 5.
εHf(t) values vs. U-Pb ages plot of the Bomi granitoids (after Chang et al., 2011)
图 6
波密岩体班公湖-怒江缝合带岛弧型花岗岩Rb-(Y+Nb)、Nb-Y、Ta-Yb和Rb-(Yb+Ta)构造环境判别图(据杜德道等，2011)
Figure 6.
Diagrams of Rb-(Y+Nb), Nb-Y, Ta-Yb and Rb-(Yb+Ta) for the arc granitoids in the east segment of Bangonghu-Nujiang suture zone (after Du et al., 2011)
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