近年的研究基本揭示,在冈底斯南缘存在一条晚三叠世株罗纪的花岗岩带,其成因认识有助于恢复青藏高原中生代演化历史,但却存在争议.为此,本文选择拉萨东部驱龙早侏罗世花岗斑岩开展Sr-Nd-Pb及锆石Hf同位素研究,调查岩浆源区特征,以期查明岩浆成因.分析结果显示,驱龙早侏罗世斑岩具有相对较低的87Sr/86 Sr初始比值(0.7029～0.7050)、正的εNd(t)(平均0.7)和锆石εHf(t)值(2.6～8.2),表明岩浆来自新生地壳物质的部分熔融.综合对比发现,冈底斯南部晚三叠世-早侏罗世花岗岩的锆石Hf同位素在500km宽的范围内显示出由西向东亏损程度逐渐减弱、甚至变得富集的规律,表明在此类花岗岩的形成过程中,成熟地壳组分的贡献自西向东逐渐增多.这既可因南部拉萨地体基底Hf同位素组成的东西向不均一所引起,也可能因东段驱龙、米林等地花岗岩更靠近古老的拉萨微陆块所致.%Recent investigations indicate there is a Late Triassic-Early Jurassic granite belt in the southern margin of the Lhasa terrane. Their petrogenesis is important to understand Mesozoic evolution history of Tibet plateau, but has not been well constrained by far. In the study, we report Sr-Nd-Pb and zircon Hf isotopic compositions of the Late Jurassic granitic porphyries at Qulong, Tibet. The results show that the granitic porphyries have relatively low initial 87Sr/86Sr values (0. 7029 ~ 0.7050) , slightly negative to positive εNd (t) values (-0. 1~1.2), and positive zircon εHf (t) values (2. 6 ~8. 2), suggesting the magma was derived from partial melting of juvenile crust By comparison with already-reported Hf (and part Sr-Nd) isotopic data of the Late Triassic-Early Jurassic granite, we find that the εHf (t) values of zircons from these granites decrease from west (Xietongroen) to east (Milin) in a 500km-wide belt, indicating contribution of mature crustal composition in formation of the granite increases from west to east This probably attributes to heterogeneous Hf isotopic compositions of crystalline basement of south Lhasa subterrane at Late Triassic-Early Jurassic. However, geographical factor should also not be excluded because that the eastern granites (e. G. , Qulong, Milin) are closer to the old central Lhasa subterrane, and thus easily result in involvement of more mature crustal material in their magma source.
展开▼
