藏南彭措林埃达克质岩脉的岩石成因及对区域成矿作用的启示  

The geochemical characteristics of the Pengcuolin adakitic dykes,southern Tibet:Petrogenesis and implications for regional metallogenesis

作　　者：裴英茹[1,2,3] 杨竹森[3] 郑远川[2] 侯增谦[4] 田世洪[3] 刘英超[4] 赵晓燕[3] 周金胜[4] 

机构地区：[1]中国地质科学院地质力学研究所,北京100081 [2]中国地质大学地球科学与资源学院,北京100083 [3]中国地质科学院矿产资源研究所,北京100037 [4]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2017年第2期515-528,共14页Acta Petrologica Sinica

基　　金：国家重点研发计划(2016YFC0600306);国家重点基础研究发展计划("973"计划)(2011CB403104);中国地质调查局项目(12120113037901);国家科技支撑计划(2006BAB01A04)联合资助

摘　　要：彭措林岩脉群位于藏南冈底斯斑岩铜矿带中段的西侧,宽约3~5m,呈近南北向穿截冈底斯岩基。两组样品的锆石U-Pb定年结果为9.7±0.2Ma和9.9±0.3Ma。岩石地球化学研究显示,岩石以高SiO_2(67.05%~69.96%)、K_2O(6.05%~6.88%)和低MgO(0.47%~1.27%)为特征,高度富集轻稀土元素(LREE)和大离子亲石元素(LILE),亏损高场强元素(HFSE),具有高Sr/Y和La/Yb比值,表现出埃达克岩地球化学亲合性。相对冈底斯中新世埃达克质斑岩而言,该岩脉更加富集放射性成因Sr、Pb同位素(^(87)Sr/^(86)Sr_((i))=0.7120~0.7123,^(206)Pb/^(204)Pb=18.812~18.844,^(207)Pb/^(204)Pb=15.705~15.728,^(208)Pb/^(204)Pb=39.424~39.523)、具更低的Nd同位素值(εNd(t)=-10.9^-9.8)和更为古老的Nd模式年龄(tDM=1.36~1.43Ga)。以上地球化学分析表明,彭措林岩脉很可能起源于加厚的古老下地壳,相较于冈底斯斑岩铜矿带内其他的中新世斑岩而言,其岩浆源区含有更少的幔源组分和更多的古老地壳组分。锆石微量元素结果显示,岩脉的氧逸度较低(ΔFMQ=-6.7^+2.1,平均值为-1.4)。故而,彭措林埃达克质岩脉不具备区域成矿潜力的原因可以归结如下:(1)下地壳岩浆源区中新生幔源组分含量较少,指示了古老下地壳中岛弧幔源岩浆注入量较少,因而岛弧期堆晶至下地壳的金属硫化物极为有限;(2)较低的氧逸度导致岩浆萃取金属的能力相对较弱。结合前人研究可知,下地壳中新生幔源组分的贡献率是影响冈底斯斑岩铜矿带后碰撞埃达克岩能否成矿的关键因素。The Pengcuolin dykes are located in the west part of the central Gangdese porphyry copper belt, southern Tibet, and they intruded into the Gangdese batholith with width of 3 -5m. LA-ICP-MS U-Pb dating of zircons from two dyke samples yield ages of 9.7 ±0. 2Ma and 9. 9 ±0. 3Ma. Geochemical analyses indicate that they are characterized by high SiO2 contents （67.05% - 69. 96% ）, K2O ranging from 6.05% to 6. 88%, MgO from 0.47% to 1.27%, enriched in light rare earth elements （LREE） and large ion lithophile elements （LILE） but depleted in high field strength elements （HFSE）, relatively high Sr/Y （45.5 -80. 0） and La/Yb （75.7 -110） ratios, showing adakitic affinities. Compared with the Miocene porphyries in the Gangdese porphyry copper belt, the dykes have higher initial 87Sr/86 Sr（i） （0. 7120 ± 0. 7123 ） , 206pb/204 Pb （ 18. 812 - 18. 844）, 207pb/204 Pb （ 15. 705 ± 15. 728）, 208 pb/204 Pb （ 39. 424 ±39. 523 ） ratios, but lower εNd （t） values （ - 10. 9 - - 9. 8） and older Nd model ages （ t DM = 1. 36 ±1.43Ga）. The above geochemical data indicates that the dykes were probably derived from an overthickened lower crust with minor mantle material, and more old crust material have been involved than the Miocene porphyries in the same belt. Meanwhile, the zircon trace element analysis show that they possess low fO2 values （△FMQ = -6.7 - ＋ 2. 1, average = -1.4）. Thus the absence of regional metallogenesis is likely attributed to ： （ 1 ） the limited sulfide cumulated in the lower crust as a result of the underplating of former arc basalitic magmas; （2） the lowfO2 that are not favor to ore-forming metal. Combining with previous study, it suggests that contribution of juvenile mantle components to the lower crust beneath the south Tibet is the key factor for mineralization in the Gangdese porphyry Cu belt.

关 键 词：埃达克质岩脉 中新世 岩石成因 成矿作用 藏南彭措林 

分 类 号：P315.2[天文地球—地震学]