拉萨地块中北部晚白垩世(约90Ma)拔拉扎含矿斑岩地球化学特征及其成因

摘要

拉萨地块中北部形成于90～88Ma的拔拉扎含矿斑岩具有明显的埃达克质岩特征:高SiO2(＞69％)、Al2O3(平均为15.89％)、Sr(平均为354×10-6),低Y(平均为12.97×10-6)、Yb(平均为0.95×10-6)含量,轻重稀土强烈分异((La/Yb)N平均为19.8)；同时它们有着高Mg#(平均为65)、Cr(平均为107×10-6)、Ni(平均为13×10-6)含量.研究区这些具有埃达克质岩特征的含矿斑岩并非源于俯冲洋壳、底侵或加厚下地壳部分熔融的产物,也不是玄武质岩浆结晶分异的产物,而很可能是拆沉下地壳部分熔融的结果.另一方面,南向俯冲的Slainajap洋壳或班公湖-怒汪洋壳的断离也可能诱发板片窗上部的壳幔物质发生部分熔融而形成研究区的含矿斑岩.%The ore-bearing porphyry of Balazha in mid-northem Lhasa terrane, with a forming age of 90 ～ 88 Ma, shows some typical geochemical characteristics of an adakite, such as high SiO2 ( > 69% ), Al2O, (15. 89% in average) , Sr (354 × 10 -6 in average ) , but low Y (12. 97 × 10 -6 in average) , Yb (0. 95 × 10-6 in average) contents, and strongly fractionation between LREE and HREE ( (La/Yb)N = 19. 8 in average). Meanwhile, it is of relatively higher Mg# (65 in average), Cr ( 107 ×10-6 in average) and Ni ( 13 × 10-6 in average). These geochemical features, combined with other characteristics of Late Cretaceous lavas in mid-northern Lhasa terrane, indicate that they did not result from melting of young oceanic crust, assimilation and fractional crystallization, or from either underplated or thickened continental crust Instead, they were possibly derived from partial melting of delaminated lower continental crust On the other hand, they are also likely to the result of partial melting of crust-mantle material, triggered by slab break-off of south-dipping subduction of Slainajap ocean or Bangong Co-Nujiang ocean.