冈底斯铜矿带埃达克质含矿斑岩的源区组成与地壳混染:Nd、Sr、Pb、O同位素约束

Source Compositions and Crustal Contaminations of AdakiticOre-bearing Porphyries in the Gangdise Copper Belt:Nd, Sr, Pb and O Isotope Constraints

本文通过对冈底斯铜矿带甲马、拉抗俄、南木、厅宫、冲江及洞嘎 6个矿区含矿斑岩的全岩 Nd、Sr、Pb、O同位素分析 ,发现它们具有比较清楚的变化规律。Sr、Pb同位素组成总体上表现为放射成因组份自西向东逐渐增高 ,87Sr/ 86 Sr、2 0 6 Pb/ 2 0 4 Pb、2 0 7Pb/ 2 0 4 Pb和 2 0 8Pb/ 2 0 4 Pb值变化范围分别为 0 .70 4 6 35～ 0 .70 792 0 ,18.315～18.6 6 1,15 .5 0 1～ 16 .6 2 6和 38.175～ 38.96 0 ;Nd同位素比值自西向东则逐渐降低 (1 43Nd/ 1 44 Nd=0 .5 12 313～0 .5 12 931)。综合分析显示这些含矿斑岩主要产生于俯冲到深部的雅鲁藏布江洋壳在榴辉岩相条件下的部分熔融 ,同时有少量俯冲沉积物参与了源区混合。上述同位素比值的区域变化与沉积物混入量沿成矿带自西向东不断增多有关 ,大体的比例是西段洞嘎矿区 <1% ;中段冲江、厅宫、南木和拉抗俄各矿区在 1%～ 5 %之间 ;东段甲马矿区为10 %～ 15 %。与 Nd、Sr、Pb同位素不同 ,氧同位素缺少上述变化规律。它们的 δ1 8O值在整个成矿带上都比较稳定 ,从 5 .5‰～ 9.8‰ ,平均为 7.7‰ (冲江和厅宫矿区以石英斑晶为准 ) ,明显高于亏损 MORB源区的 δ1 8O值 (5 .70‰ )。这说明在沉积物源区混合很少的情况下 。

Based on isotopic data of 6 copper deposits of Jiama, Lakang'e, Nanmu, Chongjiang, Tinggong and Dongga in the Gangdise copper belt, it is found that a good spatial variation regularity of Nd, Sr, Pb and O isotope compositions exists in the ore-bearing porphyries of the copper belt. Their Sr and Pb isotope compositions exhibit a radiogenic component increasing from the west to east along the belt. 87Sr/ 86Sr, 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb have a range of 0.704635～0.707920, 18.315～18.661, 15.501～16.626 and 38.175～38.960, respectively. At the same time, Nd isotopic composition displays a gradual decrease of 143Nd/ 144Nd from the west to east (with a range of 0.512313 to 0.512931). A comprehensive study demonstrates that these ore-bearing porphyries were mainly derived from partial melting of the subducted Yarlung Zangbo oceanic crust under eclogite facies condition with a minor mixing of subducted sediments in the magma source. These isotopic variations are related to such a gradual increase in mixing sediments from the west to east along the copper belt. The approximately mixing ratios in different portions of the copper belt are <1% of the Dongga deposit from the west segment, 1%～5% of the Chongjiang, Tinggong, Nanmu and Lakang'e deposits from the middle segment and 10%～15% of the Jiama deposit from the east segment. Unlike Nd, Sr and Pb isotopes, O isotope lacks the above regular variations. The δ 18O values of the ore-bearing porphyries are relatively stable along the whole copper belt (ranging from 5.5 ‰ to 9.8 ‰ with a mean of 7.7 ‰) and notably higher than that of depleted MORB (5.70 ‰). These variations indicate that the ore-bearing porphyries from the middle and west segments of the copper belt underwent more crustal contamination during their intrusion process in consideration of their less source mixing of subducted sediments. A local sharp decrease of δ 18O values in the copper belt (Chongjiang and Tinggong deposits with a range of -1.8‰ to 0.5‰ and a mean of -0.5 ‰) is probably related to meteoric water metasomatism immediately after the porphyry consolidation and suggest a penetrating-down 'funnel' present in this area.