Sr-Nd-Pb and zircon Hf isotopic constraints on petrogenesis of the Late Jurassic granitic porphyry at Qulong, Tibet
Sr-Nd-Pb and zircon Hf isotopic constraints on petrogenesis of the Late Jurassic granitic porphyry at Qulong, Tibet
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作者：
Yang Zhi
近年来的研究表明，拉萨南缘存在一条晚三叠世-早侏罗世花岗岩带。它们的成因对认识青藏高原中生代演化历史具有重要意义，但迄今为止尚未得到很好的制约。本文报道了西藏驱龙晚侏罗世花岗斑岩的Sr-Nd-Pb和锆石Hf同位素组成。结果表明，花岗斑岩具有较低的87 Sr/86 Sr比值（0.7029~0.7050），eNd（t）值为-0.1~1.2，锆石eHf（t）值为2.6~8.2，表明岩浆来源于年轻地壳的部分熔融。通过与已报道的晚三叠世-早侏罗世花岗岩的Hf（及部分Sr-Nd）同位素数据对比，发现在500 km宽的带内，这些花岗岩的锆石eHf（t）值由西（谢通门）向东（米林）逐渐减小，表明成熟地壳成分对花岗岩形成的贡献由西向东逐渐增大。这可能与拉萨南缘晚三叠世-早侏罗世结晶基底Hf同位素组成的不均一性有关。然而，也不应排除地理因素，因为东部花岗岩（例如，曲龙、米林）更靠近古老的拉萨次大陆中部，岩浆源区更容易有较成熟的地壳物质参与。
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 eNd(t) values (-0.1~1.2), and positive zircon eHf(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 eHf(t) values of zircons from these granites decrease from west (Xietongmen) 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.