班公湖带多不杂富金斑岩铜矿床斑岩-火山岩的地球化学特征与时代:对成矿构造背景的制约

Geochemistry of porphyries and volcanic rocks and ore-forming geochronology of Duobuza gold-rich porphyry copper deposit in Bangonghu belt,Tibet:Constraints on metaliogenic tectonic settings.

西藏多不杂铜矿床是班公湖-怒江带北侧新近发现的具有超大型远景的、典型的富金斑岩型铜矿床。本文对含矿斑岩、玄武质火山岩进行了系统的地球化学分析,甄别出三套岩石系列:埃达克岩、高 Nb 玄武岩和正常的岛弧玄武安山岩。三套岩石 SiO_2含量47%～68%,Al_2O_3含13%～18%,MgO 含量1.4%～8.5%,FeOt 含量2.3%～8.1%和 CaO 含量2.1%～ 10%,属于钙碱系列。MgO、CaO 和 FeOt与 SiO_2呈负相关,K_2O 与 SiO_2基本呈正相关。高 Nb 玄武岩和正常的岛弧玄武安山岩富 Na,Na_2O/K_2O 在0.9～7之间,而埃达克岩是相对富 K,Na_2O/K_2O比为0.8。稀土元素总量ΣREE 为29×10^(-6)-203×10^(-6),从基性到酸性岩∑REE 是逐渐减小的,高 Nb 玄武岩的稀土元素含量最高,而埃达克岩最低。球粒陨石标准化配分曲线为轻稀土富集型,LREE/HREE 为7.0～12.4,(La/Yb)_N 为3.2～13,δEu 为0.9～2.1。埃达克岩和正常的岛弧玄武岩富集大离子亲石元素(LILE:如 Rb、Ba、K、Sr)和活泼的高场强元素(如:U、Th),相对亏损其它高场强元素(HFSE:如 Nb、Ta、Zr、Hf、Ti),表明具有俯冲带之上岛弧岩浆的特征。而高 Nb 玄武岩具有明显 Nb、Ta 正异常,且 TiO_2含量高(>2%),(La/Nb)_(PM)<2。微量元素地球化学特征和 Ar、Nd 同位素结果表明该区埃达克岩直接来源于俯冲洋壳的部分熔融,但可能有俯冲沉积物成分的加入;而高 Nb 的玄武岩则可能来源于埃达克质熔体交代或者超临界流体交代而产生富 Nb、Ta 的地幔源区,可能有软流圈地幔的加入;而正常的岛弧火山岩则来源于俯冲流体交代过的地幔楔。另外,多不杂矿区埃达克岩和高 Nb 玄武岩(HNB)空间共生的"埃达克质岩浆交代的火山岩系列",表明多不杂铜矿床形成于典型的岛弧俯冲构造背景。对与成矿密切相关的花岗闪长斑岩进行精确的 SHRIMP 锆石 U-Pb 年代学研究,其锆石具有明显的岩浆结晶环带,Th/U 比值范围为0.51～0.90,均大于0.1,为岩浆成因锆石,其 SHRIMP U-Pb 年龄为121.6±1.9Ma,表明至少在大约120Ma 期间班公-怒江洋盆正在向北俯冲,洋盆闭合时间应晚于早白垩世中期。

Duobuza copper deposit, located in the north of Bangonghu-Nujiang suture, possesses giant perspective, representative gold-rich porphyry copper deposit in Tibet. On the base of geochemistry of ore-forming porphyry, basaltic volcanic rock, distinguish three series of rock ： adakites, high Nb basaltic rocks （HNB） and normal arc volcanic rocks. All rocks belong to calc-alkaline series, with SiO2 ranging from 47% to 68%, Al2O3 13% to 18%, MgO 1.4% to 8.5%, FeOt 2.3% to 8.1% and CaO 2.1% to 10%. MgO, MnO and FeOt abundance exhibit a negative correlation with SiO2, K2O exhibit a positive correlation with SiO2. The high Nb basaltic rocks and normal arc volcanic rocks are enrichment in Na, Na2O/K2O ranging from 0.9 to 7, whereas adakites have more enrich in K2O. Total concentration of REE, with ranging from 29 × 10^-6 to 203 × 10^-6, gradually decreases from uhramafic to felsic rocks, and high Nb basaltic rock （HNB） have the highest ∑REE concentration, whereas adakites have the lowest. Chondritenormalized REE patterns are LREE-rich, with LREE/HREE 7.0 - 12.4, （La/Yb） N 为 3.2 - 13, δEu 0.9 - 2.1. The adakites and normal arc basaltic andesites are systematically more enriched in large-ion lithophile elements （LILE： Rb, Ba, Sr, K）, more mobile highly incompatible lithophile elements （HILE. U, Th） and relatively depleted in high field strength elements （HFSE： Nb, Ta, Zr, Hf, Ti）, and these geochemical characteristics suggest they have the characteristics of subduction-related magma. However, high Nb basaltic rocks are characterized by Nb, Ta positive anomaly, high TiO2 （ 〉 2% ） and low（La/Nb） PM ratios （ 〈 2）. Trace element geochemistry and Sr-Nd isotope characteristics suggest that adakitic magma directly derived by partial melting of subduction ocean crust, with the addition of subduction sediments. High Nb basaltic rocks （HNB） are products of a mantle souce, enriched in Nb, Ta, metasomatized by a adakitic melt or supercritical fluid, likely with the infiltration of asthneosphere material. Normal arc basaltic andesites derived from mantle wedge metasomatized by hydrous fluids from subduction dehydration. In addition, the spatial association of the HNB with adakites ＂adakitic metasomatic volcanic series＂, indicate Duobuza porphyry copper deposit form in the typical island arc subduction setting. Ore-forming granodiorite porphyry zircon have obvious oscillatory zone, Th/U （0.51 - 0.90） ratio 〉 0. 1, indicate zircon is magmatic genesis, and its SHRIMPU-Pb gave an age of 121.6 ± 1.9Ma, suggests Banonghu-Nujiang ocean were subducting northward at least during approximate 120Ma, and the closure time of this ocean should later than early Cretaceous.