藏南错那洞穹隆早渐新世含绿柱石花岗伟晶岩的成因机制及其地质意义

The early Oligocene beryl-bearing pegmatite in the Cuonadong dome，southern Tibet: Its forming mechanism and geological significances

关键词:  错那洞  穹隆  伟晶岩  部分熔融  特提斯  喜马拉雅

Key words:  Cuonadong dome  pegmatite  partial melting  Tethyan Himalaya

摘要:

错那洞穹隆位于特提斯喜马拉雅东段，发育钨锡-铍稀有金属成矿作用。错那洞穹隆由上（边部）、中（幔部）、下（核部）3个构造层组成，分别以上、下拆离断层为分界线，其中在幔部强变形带中发育一套同构造变形的含绿柱石花岗伟晶岩。锆石U-Pb年代学表明，该套伟晶岩形成于33.7±0.4Ma（MSWD=1.12），为早渐新世岩浆活动的产物，明显早于穹隆中目前发现的淡色花岗岩（20~14Ma）。岩石地球化学和Sr-Nd-Hf同位素测试结果显示：（1）错那洞早渐新世花岗伟晶岩为过铝质高钾富钠花岗质岩石，具有较高SiO₂（>69.74%）、高Al₂O₃（>14.58%）及较低的CaO、MgO、MnO、TiO₂的特征；（2）高场强元素及大离子亲石元素均呈现高度变化特征，富集轻稀土元素，亏损重稀土元素；（3）Sr同位素初始值（0.696308～0.751604）与Nd同位素初始值（-11.48～-12.05）总体在角闪岩与泥质片麻岩之间，ε_Hf（t）值介于-5.4～0.1之间（主要集中在-5.4～-1.8）。综合研究表明，错那洞早渐新世含绿柱石伟晶岩是角闪岩与泥质片麻岩混熔的结果，其中泥质片麻岩的部分熔融起主导作用，其形成与藏南拆离系（STDS）的活动密切相关，表明错那洞地区新生代地壳深熔作用主要源岩在早渐新世已完成了从角闪岩向泥质片麻岩的转变。该同构造变形含绿柱石伟晶岩的发现，揭示错那洞穹隆的成穹作用至少在早渐新世便已开始。铍稀有金属可能在早渐新世已有了初始富集，而在中新世大规模岩浆活动中实现了巨量富集。

Abstract:

The large-scale tungsten-tin-beryllium mineralization was newly discovered in the Cuonadong Dome，which is located in the east of the Tethyan Himalaya. Cuonadong Dome was divided into the upper part (the edge)，the middle part (the mantle)，and the lower part (the core) by the upper and lower detachment faults. A set of syntectonic deformed beryl-bearing granitic pegmatite was occurred in the mantle，of which the rocks were strongly deformed. The zircon U-Pb isotopic data show that the pegmatite was formed at 33.7±0.4 Ma (MSWD=1.12)，which was the Early Oligocene magmatism and obviously older than the Miocene leucogranites(20-14Ma)in the Dome. The characteristics of the whole-rock geochemistry and isotopic geochemistry show that: (1) the Early Oligocene granitic pegmatite is the peraluminous granitic rock with high potassium and sodium. The characteristics of the major elements show that the granitic rocks have higher SiO₂ (>69.74%) and Al₂O₃ (>14.58%)，lower CaO，MgO，MnO and TiO₂than common granitic rocks; (2) high field strength elements(HFSE) and large ion lithophile elements (LILE) all exhibit high variation characteristics，enriching light rare earth elements and depleting heavy rare earth elements; (3) The values of the initial Sr isotope (0.696308～0.751604) and the initial Nd isotope (-11.48～-12.05) range between those of amphibolite and metapelites，and the values of zircon εHf(t) are between -5.4 and 0.1 (mainly concentrated in -5.4～-1.8). According to comprehensive research，the Early Oligocene beryl-bearing pegmatite was derived from the partial melting of source consisting dominantly of metapelites and subordinately amphibolite，which was related to the activities of the South Tibetan Detachment System (STDS)，and indicate that the primary partial melting source of the Cenozoic crustal anatexis in the Cuonadong Dome changed from amphibolite to metapelites at the Early Oligocene. The discovery of the syntectonic deformed beryl-bearing pegmatite reveals that the Cuonadong Dome began to form at least in the Early Oligocene. The initial enrichment of beryllium mineralization likely occurred in the Early Oligocene，whereas the giant enrichment occurred in the Miocene associated with Miocene leucogranites.