沙坪沟钼矿区橄榄角闪石岩岩石成因——矿物学特征约束

Petrogenesis of olivine hornblende in Shapinggou molybdenum ore district:Constraints from mineralogical characteristics

对位于秦岭-大别造山带的沙坪沟钼矿矿区出露的橄榄角闪石岩开展了系统的岩石学、岩相学及电子探针原位分析,厘定了其成岩物理化学条件,并探讨了岩石成因及形成的构造环境。橄榄角闪石岩主要造岩矿物为橄榄石、角闪石、金云母和辉石;副矿物为磷灰石、铬铁尖晶石、镁铁尖晶石、磁铁矿、磁黄铁矿、镍黄铁矿等。电子探针原位分析结果显示,橄榄石属于贵橄榄石系列,角闪石主要是韭闪石,辉石为透辉石,云母为金云母。造岩矿物结晶顺序为橄榄石-辉石-角闪石,结晶温度介于1 111. 64～1 223. 30℃之间,形成压力为350～740 MPa,形成于较高的氧逸度环境(角闪石结晶时的氧逸度范围为NNO+0. 59～NNO+2. 27)。角闪石和金云母的矿物学特征显示它们均是与幔源岩浆作用相关的原生矿物,表明该套基性-超基性岩与幔源岩浆作用有关。综合分析认为,在大别造山带中生代地壳减薄的伸展构造背景下,该橄榄角闪石岩母岩浆源于地幔,是经过地幔7. 92%部分熔融的玄武质岩浆经结晶分异作用形成的幔源岩浆底侵下地壳的产物。

The Shapinggou molybdenum deposit is the second largest porphyry molybdenum deposit in the world.The mafic-ultramafic rock widely exposed in the area of Shapinggou molybdenum ore deposit is an important medium to reveal the deep information of the deposit.In this paper,the olivine-bearing amphibolite outcropped in the mining area were studied based on petrology,petrography and in situ analysis so as to clarify physicochemical conditions of diagenesis and discuss the petrogenesis and tectonic setting.The main rock-forming minerals contain olivine,amphibole,phlogopite and pyroxene,and the accessory minerals are apatite,chromium-iron spinel,mafic spinel,magnetite,pyrrhotite and pentlandite.The results of in-situ analysis of EPMA show that olivine belongs to the chrysolite series,amphibole is parasite,pyroxene is diopside and mica is phlogopite.The order of crystallization is olivine-pyroxene-amphibole,the temperature ranges from 1 111.64 to 1 223.30℃ and the formation pressure ranges from 350 to 740 MPa,with high oxygen fugacity(the oxygen fugacity varies from NNO + 0.59 to NNO+ 2.27).The mineralogical characteristics of phlogopite and amphibole indicate that phlogopite was related to mantle-derived magmatism.The study shows that the parent magma of olivine-bearing amphibolite originated from the mantle and was formed through crystallization differentiation of basalt magma which was partially melted by 7.92% of the mantle.The olivine-bearing amphibolite outcropped in the north of Shapinggou mining area should be the product of partial melting of mantle-derived magma under the extensional tectonic background.