西藏邦铺矿区辉绿玢岩成因及对区域构造岩浆演化的指示

Petrogenesis of diabase porphyrite in Bangpu deposit of Tibet and its instructions to the regional tectonic and magmatic evolution

邦铺是继驱龙、甲玛之后在冈底斯斑岩成矿带发现的又一大型的斑岩型矿床。该矿床的成矿作用以斑岩Mo—Cu矿化为主，伴生矽卡岩型Pb—Zn矿化。矿区出露岩体面积较广，种类较多，约占矿区总面积的50％。辉绿玢岩位于矿区的北侧，呈独立脉体产出。其SiO2含量变化于48．39％-50．08％之间，（K2O＋Na2O）平均为3．50％，铝饱和指数A／CNK为1．23，属于过铝质系列，MgO=7．92％-9．12％，Mg^#=61．72～64．79。稀土总量∑REE变化于44．93x10～-67．58×10^-6之间．轻重稀土间的分馏不明显，富集Sr、Ba、U，强烈亏损Rb、K等元素。相容元素Ni、Cr含量平均值分别为89×10^-6和256×10^-6，相对于原生玄武岩浆范围较小，说明邦铺辉绿玢岩经历了一定程度的橄榄石、单斜辉石等镁铁质矿物的分离结晶，但程度不高，可以近似认为辉绿玢岩是岩浆源区平衡部分熔融的产物。锆石U-Pb年龄为14．46±0．38Ma，与成矿年龄14．92Ma相近，锆石原位εHf（t）平均值为4．7。邦铺辉绿玢岩岩石地球化学特征与达孜玄武岩具有高度的一致性，表明二者具有相同的源区，即由于印度板片发生断离，软流圈物质上涌并部分熔融而形成。其成岩年龄的差距是印度板片向北俯冲到不同位置的结果。邦铺辉绿玢岩在成岩过程中受到一定程度地壳物质的混染。它的发现可以作为后碰撞期岩浆活动的一个基性端元，从而为限定地幔性质、岩浆活动时期、以及探索地幔物质对区域岩浆活动及对成矿的影响提供了机会.

Bangpu is found as a large-scale porphyry deposit after Qulong and Jiama in Gangdese porphyry copper metallogenic beh which contains mainly porphyry Mo-Cu with skarn Pb-Zn accompanying. There are different species of rock masses in Bangpu mining area, which possess about half of the total area. The diabase porphyrite locates in the north of the mining area as independent vein. The contents of SiO2 are between 48.39% - 50. 08% , the average content of （K2O ＋ Na2O） is 3.50% , A/CNK is 1.23 and it is belonging to the peraluminous series. MgO =7. 92% -9. 12% , Mg^# =61.72 -64. 79. The REEs vary from 44. 93 x 10-6 to 67. 58 x 10-6 without detense differentiation between LREE and HREE. The average contents of Ni and Cr are 89 x 10-6 and 256 x 10-6 respectively which are lower to native basaltic magma, indicating the diabase porphyrite undergoes mafic minerals such as olivine and clinopyroxene fractional crystallization but not strong, so the diabase porphyrite can be considered as product of equilibrium partial melting. The zircon U-Pb age is 14.46 ±0. 38Ma which is similar to the mineralization age. The average ear（t） value in situ is 4. 7. There is a high degree of consistency between the diabase porphyrite and Dazi basalt in geochemical characteristics which showing the same source region they share. That is the upwelling and partial melting of the asthenospheric because of the break off of Indian plate. The gap between their diagenetic ages is the result of continued northward subduction of Indian plate. The diabase porphyrite has been contaminated by crustal material to a certain extent. It is discovered as a magma-mafic-end member in post collision stage which gives us the opportunity to limit the character of mantle-derived magma and its effect to regional magmatism and mineralization.