阳山金矿带安坝里南斑岩脉锆石U-Pb年龄及岩石地球化学特征

LA-ICP-MS Zircon U-Pb Dating and Geochemical of Porphyric Dykes at the South of Anbali Area,Yangshan Gold Belt

阳山金矿带安坝里南矿段位于碧口地块、松潘-甘孜褶皱系及秦岭微板块的结合部。对安坝里南矿段发育的斑岩进行了详细的岩相学、锆石U-Pb年代学、岩石地球化学特征等方面的分析。研究结果显示,斑岩的岩石类型主要为花岗闪长斑岩和花岗斑岩。斑岩形成于晚三叠世(T3),其锆石LA-ICP-MS U-Pb年龄为(218.7±3.7)Ma。斑岩属富铝高钾钙碱性强过铝质花岗岩,其主量元素化学成分表现为高SiO2(70.41%～74.89%)、富Al2O3(14.60%～16.09%)、高K2O(2.83%～3.52%)、较高的K2O/Na2O比值(平均17.61)和CaO/Na2O比值(平均8.43)、低Al2O3/TiO2(56.50～76.62)的特征。A/CNK比值为1.10～3.57(平均2.28),且出现大量的刚玉分子(1.46%～12.13%)。稀土元素配分模式表现为轻稀土元素强烈富集的右倾斜型,其(La/Yb)N比值为8.92～23.63,基本无铈异常和明显的负铕异常,其Ce/Ce*比值和Eu/Eu*比值分别为0.97～1.05和0.66～0.90。微量元素比值蛛网图上普遍显示出强烈的Nb、Ta、P、Ti的亏损。斑岩的源区物质显示出壳源的特征,Rb/Sr>0.32和Nb/Ta<12.0,其源区物质可能主要为碧口群。斑岩形成于晚三叠世时期扬子陆块与秦岭弧碰撞期后的地壳增厚向岩石圈拆沉与大规模伸展走滑转换阶段,岩浆起源于该环境下约800℃时碧口群发生的部分熔融,属弧-陆碰撞后阶段的产物。

The south of Anbali gold district was located at the junction of Qinling orogenic belt, Songpan-Garze orogenic belt and Bikou terrane. Based on mineralogy, LA-ICP-MS Zircon U-Pb dating and geo-chemical of porphyric dykes, the granite was high-K calc-alkalic series and strongly peraluminous grano-diorite porphyries and granite porphyries which shared similar characteristics with the typical muscovite-bearing peraluminous granitoids. Their U-Pb ages of zircon mainly range from 216 to 225 Ma and a weigh-ted mean ^206Pb/^238U age is （218.7±3.7）Ma, which indicated that the peraluminous porphyry was formed in Late Triassic. Porphyries are characterized by high SiOz （70. 41% to 74. 89%）, enriched in A1203 （14.60%-16.09%）, high in 20（2.83%-3.52%）, low in Na20 and higher ratio of CaO/Na20（average 8.43）. Alumina index A/CNK was higher arid ranges from 1. 10 to 3. 57 on the average of 2. 28. The quantity of Corundum widely ranged from 1.64% to 12.13% and on the average of 7.53% . REE patterns are strongly oblique to the HREE side with intensive enriched in LREE, and （La/Yb）N values are from 8. 92 to 23.63. Eu anomaly is quite visible, which Eu/Eu^＊ values from 0.66 to 0. 90 and while Ce anomaly is invisible （Ce/Ce^＊ ranging from 0.97 to 1.05）. In the primitive mantel-normalized spidergrams of trace elements, obvious negative anomalies showed in Nb, Ta, P, Ti and Ba. The ratio of Rb/Sr is higher than 0.32 and Nb/Ta is lower than 12. 0, reflecting upper crust source and Bikou Group may be the major source. The porphyries were possibly formed in regional strike-slip shear setting when Qinling thickened crust transferred to extension in Yangzi block-South Qinling magmatic arc post continent-collision.