Late Triassic Beryllium and Rubidium Rare Metals Mineralization in the Central Gangdise Metallogenic Belt, Tibet, China: Evidence from Monazite U-Pb Geochronology
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摘要: 在西藏南木林县北部地区新发现了Be-Rb±（Nb-Ta）稀有金属找矿线索，但由于含矿岩体中锆石U含量过高，导致对其进行U-Pb年代学研究不能获得可靠的岩浆结晶年龄.采用LA-ICP-MS方法对赋矿岩体的独居石进行了U-Pb年代学研究，获得了207.0±0.7 Ma（n=22，MSWD=0.101）的独居石206Pb/238U加权平均年龄和207.0±0.8 Ma（MSWD=0.107）的下交点年龄，能够更加可靠地代表赋矿岩浆岩的结晶年龄.该独居石U-Pb年龄表明，含矿岩体结晶于晚三叠世，晚于区内的花岗闪长岩形成年龄，可能代表了尚未能完全识别出的冈底斯印支期造山活动晚期一次重要的成岩事件.该期赋存稀有金属岩体的发现进一步丰富了中冈底斯成矿带的矿种和矿床类型，对更加深入地认识冈底斯成矿带中生代构造－岩浆演化与成矿作用，丰富和完善与青藏高原多岛弧盆演化有关的造山与成矿理论有重要意义.Abstract: Beryllium and rubidium mineralization is newly found in northern Namling, Tibet, China. However, the U-Pb ages are not reliable representation of magmatic emplacement age since U concentration in zircons from the mineralized pegmatitic muscovite monzogranite is too high. LA-ICP-MS is taken on monazites from this pyrogenic rock using 24 μm spot 193 nm laser, yielding reliable age. The weighted average age of 206Pb/238U is 207.0±0.7 Ma (n=22, MSWD=0.101), and the lower intercepted age is 207.0±0.8 Ma (MSWD=0.107). These ages obtained by LA-ICP-MS analysis show the mineralized pegmatitic muscovite monzogranite emplaced in Late Triassic and later than granodiorite in this region. It may represent an important diagenetic event that has not yet been fully identified in the late stage of Indosinian orogeny in Paleo-Gangdise. This new discovery may expand minerals and deposit types for further regional prospecting in the central Gangdise metallogenic belt. Furthermore, this discovery also has great importance for the re-understanding of the Mesozoic tectonic evolution, magmatic evolution and mineralization in this belt. And further enhances the orogenic and metallogenic theories related to the evolution of multi-island-arc-basin in the Qinghai-Tibet Plateau.
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图 1 冈底斯弧背断隆带Be矿化产出的大地构造位置及区内主要矿产分布概况
据Zhu et al.(2011)和李应栩等(2018)修改
Fig. 1. Tectonic setting and simplified geological and mineral resources map of the beryllium mineralization in the Gangdise back-arc uplift zone
图 2 南木林北铍铷矿化点区域地质概图
Fig. 2. Sketched regional geological map of the beryllium and rubidium mineralization site in northern Namling
图 3 南木林北铍铷矿化点露头照片(a)和剥土NBT01素描图(b)
Fig. 3. Field photograph of the beryllium and rubidium mineralized outcrops in northern Namling (a) and geological sketch of sampling in NBT01 (b)
图 4 南木林北铍铷矿化露头及围岩蚀变野外照片
a.发育于岩脉外接触带上的电气石；b.围岩中的红柱石；c.围岩裂隙中的石榴石；d.堇青石角岩化石英砂岩；e.伟晶岩包裹的红柱石角岩角砾，其中的红柱石结晶增大；f.角岩化砂岩中的石英脉；g.含矿白云母二长花岗岩及其中的铌钽铁矿；h.样槽(NBT01-H1)
Fig. 4. Field pictures showing the wall-rock alteration of the beryllium and rubidium mineralized outcrops in northern Namling
图 5 南木林北铍铷矿化伟晶状白云母二长花岗岩薄片透光镜下照片
a和c.单偏光；b和d.正交偏光；Kfs.钾长石；Mnz.独居石；Ms.白云母；Pl.斜长石；Q.石英；Zir.锆石
Fig. 5. Lithofacies photos of pegmatitic muscovite monzogranite bearing beryllium and rubidium mineralization in northern Namling
图 6 南木林北铍铷矿化伟晶状白云母二长花岗岩中锆石阴极发光图像
Fig. 6. Cathodoluminescence image of zircons in pegmatitic muscovite monzogranite bearing beryllium and rubidium mineralization in northern Namling
图 7 南木林北铍铷矿化伟晶状白云母二长花岗岩中独居石背散射图像
Fig. 7. Backscatter image of monazites in pegmatitic muscovite monzogranite bearing beryllium and rubidium mineralization in northern Namling
图 8 南木林北铍铷矿化伟晶状白云母二长花岗岩中锆石(a)和独居石(b)U-Pb谐和图
Fig. 8. U-Pb concordia diagrams of zircons (a) and monazites (b) in pegmatitic muscovite monzogranite bearing beryllium and rubidium mineralization in northern Namling
图 9 南木林北铍铷矿化伟晶状白云母二长花岗岩中锆石和独居石表观U-Pb年龄与U含量关系
Fig. 9. U-Pb age vs U concertration diagram of zircons and monazites in pegmatitic muscovite monzogranite bearing beryllium and rubidium mineralization in northern Namling
表 1 南木林北剥土NBT01白云母二长花岗岩脉刻槽样品分析结果(10-6)
Table 1. Analysis result of muscovite monzogranite samples from NBT01 in northern Namling (10-6)
样号 Li2O BeO Rb2O Cs2O Nb2O5 Ta2O5 样长(m) H1 29.11 716.2 581.8 62.13 123.5 20.47 1.35 H2 40.04 660.6 1159 191.3 164.5 26.63 1.70 注：分析结果据中国冶金地质总局山东局测试中心检测报告HF2017-205-2. 
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