湖南骑田岭花岗岩及其暗色微粒包体的地球化学与壳幔岩浆的混合作用

Geochemistry and Crust_Mantle Magmatic Mixing of the Qitianling Granites and Their Dark Microgranular Enclaves in Hunan Province

骑田岭岩体是南岭地区燕山早期具幔源组分贡献的花岗岩典型代表。其主体岩性为角闪黑云二长花岗岩和(角闪石)黑云正长花岗岩,其中发育暗色微粒包体和由暗色矿物组成的团块或条带。暗色微粒包体具有岩浆混合的大部分岩相学证据。如包体的浑圆状外形、塑性形变、冷凝边、斜长石An的“双峰式”分布、似环斑长石、针状磷灰石等。包体属于准铝质(A/KNC=0·72~1·00,平均0·85)钾玄岩系列岩石,寄主岩石为准铝或弱过铝质(A/KNC=0·89~1·06,平均0·97)高钾钙碱性系列的岩石。二者在主量和微量元素上表现出岩浆混合成因的演化趋势。包体与寄主岩石的同位素组成具趋同性,它们的ISr和εNd(t)值分别为0·71041~0·71263、-6·9^-5·3和0·70854~0·71416、-9·2^-5·1,均表现出壳幔混源花岗岩类岩石的特点。包体K_Ar年龄为152Ma,与其寄主岩石的形成年龄(155~161Ma)接近,显示岩浆混合作用发生的时间大致为晚侏罗世早期。对包体及其寄主岩石产出的构造背景和地球化学特征的综合分析表明,该岩体中的暗色微粒包体是在伸展作用的大地构造背景下,上涌的幔源基性岩浆及其诱发的长英质酸性岩浆混合作用的产物。

The Qitianling pluton is one of the typical granites with the contribution of mantle-derived components in the Nanling region, southern China. The amphibole biotite monzogranite and (amphibole) biotite syenogranite, which commonly contain abundant dark microgranular enclaves and dark mineral bands or nodules, have formed the main body of the pluton. The dark microgranular enclaves have most typical macro- and micro-textures of magma mixing such as ovoid and plastic deformation shape, chilled margin, bimodal plagioclase, rapakivi-like feldspar and apatite needles. The enclaves belong to metaluminous (A/KNC=0.72～1.00, 0.85 on average) shoshonitic rock, and the host rocks show metaluminous to weakly peraluminous attribute(A/KNC=0.89～1.06, 0.97 on average). Between them, main and trace elements display the magmatic mixing evolution trend. They also display close Sr, Nd isotopic compositions with I- Sr and ε- Nd(t) values of 0.71041～0.71263, -6.9～-5.3 for the enclaves and 0.70854～0.71416, -9.2～-5.1 for the host rocks, which are all indicative of mixing of crustal materials with mantle components. The formation age(152 Ma)of the enclave is close to that(155～161 Ma) of the host rock, suggesting that the mixing event took place during the early period of Late Jurassic. According to a comprehensive analysis of their tectonic settings as well as geological and geochemical data, it is suggested that the dark microgranular enclaves in the Qitianling pluton were generated by mixing of mantle-derived basic magma and its induced crustal felsic magma in an extension environment.