西秦岭阳山金矿带花岗斑岩元素及Sr-Nd-Pb同位素地球化学

Element and Sr-Nb-Pb isotope geochemistry of ganite-porphyry dykes in the Yangshan gold belt,western Qinling orogen.

甘肃阳山金矿带的花岗斑岩脉中含有石榴子石,A/CNK=1.65～3.65,属于强过铝质花岗岩类。花岗斑岩相对富集 LILE 和 LREE,亏损 Ba、Sr、Nb、Ta、P、Ti 等,配分模式类似于典型同碰撞型花岗岩类;花岗斑岩∑REE=54.35～124.01×10^(-6),(La/Yb)_N=9.72～27.80,δEu=0.70～0.89,表明其岩浆形成时部分斜长石进入熔体,而非完全残留。花岗斑岩 I_(Sr)值为0.70806～0.71756,平均0.71107;ε_(Nd)(t)值变化于-2.9～-5.0,平均-3.4;Nd 模式年龄(T_(2DM))为1.24～1.41Ga,平均1.34 (Ga)。以上同位素特征表明花岗斑岩岩浆应源自成熟度较低的中元古代基底地壳物质。花岗斑岩的(^(206)Pb/^(204)Pb)_(220Ma)、(^(207)Pb/^(204)Pb)_(220Ma)和(^(208)Pb/^(204)Ph)_(220Ma)的平均值分别为17.875、15.604和38.296,与秦岭微陆块的中元古代基底和碧口地体碧口群的 Pb 同位素组成一致。考虑到前人获得碧口群的年龄为1.235～1.367Ga,而秦岭微陆块沿勉略缝合带向南仰冲到碧口地体之上,我们认为由碧口群等组成的俯冲板片的变质脱水熔融作用导致了阳山金矿带花岗斑岩的形成。因此,阳山金矿带的花岗斑岩是扬子与华北大陆中生代碰撞造山过程中形成的同碰撞花岗岩类。

The garnet-bearing granite-porphyry dykes in the Yangshan gold belt, Gansu Province, are peraluminum granitoids with A/CNK ratios ranging 1.65 - 3.65. The granite-porphyries are rich in LILE and LREE, and depleted in Ba, Sr, Nb, Ta, P and Ti, showing similar trace element pattern to typical syn-collision granites. With ∑REE = 54.35 - 124.01× 10^-6, （La/Yb） N = 9.72 27.80, and δEu = 0.70 - 0.89, the granitic magmas were originated from partial-mehing with little of plagioclase as residual phase. The porphyries have Is, ratios of 0. 70806 -0. 71756 with average of 0.71107, εnd （t） values of -2.9 - -5.0 with average of -3.4, and Nd-model ages （T2DM ） of 1.24 - 1.41 Ga with average of 1.34 Ga, which suggests that the magmas must be sourced from lowlymatured Mesoproterozoic basement. The averages of （206^Pb/204^Pb）220Ma, （^207Pb/^204^Pb）220Ma and （^208Pb/^204Pb）220Ma ratios are 17. 875, 15. 604 and 38. 296, respectively, similar to those of both the Bikou Group of the Bikou terrane and the Mesoproterozoic basement of the Qinling micro-continent. Considering that the Bikou Group has been dated to have developed during 1. 235 - 1. 367 Ga in previous studies, and that the Qinling micro-continent, which hosts the Yangshan gold belt at its southern margin, southwardly overthrusted onto the Bikou terrane along the Mian-Lue suture, we suggest that the interested granite-porphyries were generated by metamorphic devolatilization-related partial melting of the underthrust slab mainly consisting of the Bikou Group. Hence the granite-porphyries in the Yangshan gold belt are typical syn-collision granitoids formed during the Mesozoic collision between the Yangtze and North China continents.