西藏班公湖-怒江缝合带美日切错组中酸性火山岩锆石U-Pb年龄、Sr-Nd-Hf同位素、岩石成因及其构造意义

Petrogenesis,Zircon U-Pb Geochronology and Sr-Nd-Hf Isotopes of the Intermediate-felsic Volcanic Rocks from the Duolong deposit in the Bangonghu-Nujiang Suture Zone,Tibet,and Its Tectonic Significance

西藏班公湖-怒江缝合带是我国重要的斑岩铜金矿成矿带,成矿地质条件极为优越。多龙矿集区是班公湖-怒江缝合带西段典型斑岩-浅成低温热液型铜(金)超大型矿床,区内广泛分布着美日切错组喷溢相火山岩,但其岩石成因、源区属性、形成年代及地球化学动力学背景尚不明确。本文对美日切错组安山岩及流纹岩进行锆石U-Pb测年,获得结晶年龄分别为108.2±2.6Ma(MSWD=0.39)和109.3±2.2Ma(MSWD=1.70)。矿区安山岩及流纹岩具高硅(SiO2=60.89%~72.00%)、富钾(K2O=3.08%~5.53%)、富碱(K2O+Na2O=6.88%~8.96%)、准铝质-过铝质(A/CNK=0.92~1.28)特征,属于高钾钙碱性及钾玄岩系列岩石。其明显均富集轻稀土(LREE)及大离子亲石元素(LILE:Th、U、K、Pb及Rb),亏损重稀土(HREE)及高场强元素(HFSE:Ta、Nb、Ti及Zr),稀土总量(ΣREE)为141.52×10-6~236.05×10-6之间,LaN/YbN为10.42~11.05,δEu为0.65~0.80,具有中等负铕异常,显示出典型岛弧岩浆岩的特征。(87 Sr/86 Sr)i值为0.7050~0.7053,(143 Nd/144 Nd)i为0.5124~0.5126,εNd(t)值为-1.51~1.29,具有高Sr低Nd的特征。锆石Hf同位素分析结果表明,流纹岩εHf(t)为+11.6^+15.5,平均值为+13.8,两阶段模式年龄平均值为288.0Ma;安山岩εHf(t)为+3.4^+8.0,平均值为+4.8,两阶段模式年龄平均值为813.1Ma;表现出明显的幔源特征。综合研究表明,美日切错组火山岩为班公湖-怒江新特斯洋洋壳向北的俯冲背景下,由俯冲板片脱水产生流体交代地幔楔发生部分熔融形成原始玄武质岩浆,并在上升后,滞留在壳幔边界形成新生下地壳,新生下地壳与持续底侵幔源玄武质岩浆混合而部分熔融形成。其形成于典型岛弧的构造背景下,暗示班公湖-怒江洋在早白垩世晚期(108~109Ma)仍正在向北俯冲于羌塘地块之下,可能尚未关闭。

The Bangonghu-Nujiang suture zone in Tibet of China. has been is an important porphyry copper gold metallogenic belt, with superior geological condition of mineralization, The Duolong ore-concentrated deposit, the first superlarge epithermal porphyry Cu-Au deposit discovered in the western Bangonghu- Nujiang suture zone, is widely covered with plenty of effusive volcanic rocks. However, the petrogenesis, magma source and geodynamie setting of the granitoids are still constrained loosely. In this paper, LA- ICP-MS zircon date yields U-Pb age of 108.2 - 2. 6Ma（MSWD= 0. 39） for the dacite, regarding to the similar geochronological data of 109.3±2.2Ma（MSWD= 1.70） for the rhyolite. The dacite and rhyolite are characterized by high silica （SiO2 - 60. 89 % - 72.00 %）, potassium-rich （K2O - 3. 08 % ~ 5. 53 %）, alkali-rich （K20-Na20= 6.88% -8. 96%）,peralkaline to peraluminous （A/CNK= 0. 92-1. 28） and High calc-alkaline to shoshonitie. The rocks are systematically more enriched in large-ion lithophile elements （LILE. Th,U,K,Pb and Rb） and LREE, relatively deplted in high strength elements （HFSE： Ta,Nb,Ti and Zr） and HREE. Total Coneentartion of REE range from 141.52 × 10.6 - 236.05- 10.6 , with LREE/HREE of 8.54-9.30, LaN/YbN of 10.42-11.05 and middle negative Eu anomalies（SEu= 0.65-0. 80）. Besides, trace elements compositions indicate that the magma is contaminated by crustal mantle when it rise and The isotopes of these rocks are characterized by high（87Sr/86 Sr）; （from 0. 7050 to 0. 7053）, low （143 Nd/144 Nd）i （from 0. 5124 to 0. 5126, low εNa （t） （from - 1. 51 to 1. 29）. The positive εHf （t） values of zircons from rhyolite whose second stage Hf mode mean ages are 288.0Ma vary from ＋11.6 to -ff15.5, contrasted with εHf（t）values of ＋3. 4-＋8. 0 from dacite whose second stage Hf mode mean ages are 813.1Ma. Based on the above discussions, we proposed that the geochemical characteristics of these volcanic rocks are consistent with those of arc-type magmas worldwide and the arc volcanic rocks magma were all generated by partial melting of mantle wedges replaced by the fluid which is formed the dehydration of the oceanic crust subduction plate, and its rock-forming resulted from the mixing between the juvenile lower crust-derived melts and mantle-derived marie melts under the background of Bangonghu- Nujiang Tethys subducting northward to Qiangtang massif. In addition, the volcanic activity suggests Bangonghu-Nujiang oeean was subducting at least during approximate 108-109Ma, and the closure time of this ocean should be later than early Cretaceous（108-109Ma）, which is different from early recognition of closure time of late Jurassic to early-Cretaceous.