中甸弧西斑岩带印支期中酸性侵入岩成因与成矿意义

摘要

位于义敦岛弧带南端的中甸弧广泛发育印支期中-酸性浅成-超浅成复式杂岩及与之有关的斑(玢)岩型矿床；在其西斑岩带内,主要分布有阿热、春都、雪鸡坪和烂泥塘等4个斑(玢)岩体.阿热岩体的特征与埃达克岩相似:SiO2＞58.06％,平均58.64％,高Al2O3(14.77％～17.29％)、Sr(490× 10-6～1409×10-6)、Sr/Y(37.44～83.79)和La/Yb(4.62～31.48,平均28.51)值,低Y(13.1 ×10-6～16.82×10-6)和Yb(1.20～1.61)值,δ Eu为0.90～1.02,REE球粒陨石配分曲线呈平滑的右倾形态,富集轻稀土,严重亏损重稀土,具较高的MgO(0.91％～3.77％)及Mg#(37.84～70.00)值,与其他含矿岩体特征基本一致.通过对主量和稀土及Sr元素研究对比表明,研究区岩石可能由于俯冲的甘孜—理塘洋壳发生断离,先后引发地幔楔及洋壳部分熔融,初始熔浆与地壳发生不同混染叠加并随区域性收缩沿现今格咱断裂东侧短时间内多次脉动侵位形成具岛弧岩石向埃达克质岩石过渡特征的线性串珠状岩体,其源区残留组合有石榴石+单斜辉石+角闪石,初始熔浆上侵过程中受到地壳不同程度的混染叠加可能是导致该带成矿差异性的原因之一.%The west porphyry belt in the Zhongdian island arc is mainly composed of four Indosinian intermediate-acidic intrusive complex bodies, i.e., A're, Chundu, Xuejiping and Lannitang, related somewhat to Cu mineralization. Geochemical characteristics of the A're porphyry are similar to those of Adakite: SiO2>58.06% (averagely 58.64%) ,Al2O3 (14.77%-17.29%), Sr (490× 10-6-1409×10-6) , Sr/Y (37.44-83.79) and La/Yb (4.62-31.48,averagely 28.51) are high, Y (13.1×10-6-16.82×10-6) and Yb (1.20×10-6-1.61×10-6) are low, 8Eu is not distinct (0.90-1.02), Mg (0.91%-3.38%) and Mg# (37.84-70.00) are high, chondrite-normalized REE patterns are rightly-oblique, and there exist enrichment of LREE and heavy depletion of HREE. All these features are similar to characteristics of the other three porphyries. Based on the studies of major oxides, REE and Sr, the authors hold that the intermediate-acidic intrusive complex bodies in the west porphyry belt characterized by island -arc rocks to adakitic rocks were most probably formed by the rifting of the subducting Ganzi -Litang oceanic crust, which caused the successive partial melting of the mantle wedge and oceanic crust, during which the partially melted primitive magma exerted multi-staged upward intrusion along the present Geza fault on the eastern side and was contaminated by the crust in varying degrees. The garnet +clinopyroxene +amphibole association remained in the magma source area. There is no distinct difference between the four porphyries in magmatic evolution. Various degrees of mantle -crustal contamination might have played a certain role in the diversities of mineralization.