西藏南部冈底斯带东段晚白垩世埃达克岩:新特提斯洋脊俯冲的产物?

Late Cretaceous adakites in the eastern segment of the Gangdese Belt,southern Tibet:Products of Neo-Tethyan ridge subduction?

西藏南部冈底斯带晚白垩世岩浆作用的性质及其产生的地球动力学机制仍存在不同看法。为进一步约束冈底斯带晚白垩世的构造岩浆演化历史,本文报道了南冈底斯东段朗县至米林一带晚白垩世花岗岩类的岩石学、锆石U-Pb年代学、全岩地球化学以及锆石Hf同位素数据。锆石U-Pb定年结果表明,这些花岗岩侵位于84～78Ma,以高Al2O3(15.69%～17.65%),低MgO(0.47%～1.24%),极低相容元素(Cr=1.34×10-6～3.27×10-6、Ni=0.69×10-6～3.32×10-6)含量和高Sr(542×10-6～774×10-6)以及高Sr/Y比值(48～397)为特征,显示埃达克质岩石的地球化学特征,属于中钾钙碱性偏铝质岩石(A/CNK=0.95～1.04),富集大离子亲石元素(LILE)、亏损高场强元素(HFSE),部分样品具显著的正铕异常(δEu=1.16～2.26)和中稀土元素亏损的特征,锆石εHf(t)值变化范围大(+0.2～+15.1)。这些晚白垩世埃达克岩可能是先期底侵的镁铁质下地壳在含水角闪岩相条件下发生部分熔融作用的产物。通过与冈底斯带110Ma和50Ma左右大规模带状岩浆作用的对比,本文赞成洋脊俯冲模式是用来解释南冈底斯带晚白垩世大规模带状岩浆作用成因的一种最可能机制。

Contrasting views remain exist so far for the nature and geodynamic mechanism of the Late Cretaceous magmatism in the southern Gangdese Belt,southern Tibet.To further constrain the tectonomagmatic evolution of southern Tibet during the Late Cretaceous,we present zircon LA-ICPMS U-Pb data,geochemical and Hf isotopic data for the granitoids from the eastern segment of the southern Gangdese Belt,southern Tibet.New age data indicate that the granitoids were emplaced at 84~78Ma.These rocks are characterized by high Al2O3(15.69%~17.65%),low MgO(0.47%~1.24%),very low abundances of compatible elements(Cr=1.34×10-6~3.27×10-6、Ni=0.69×10-6~3.32×10-6),high Sr(542×10-6~774×10-6)and high Sr/Y ratios(48~397),similar to those of adakite.The rocks are medium-K calc-alkaline,metaluminous(A/CNK=0.95~1.04),and are enriched in LILE,depleted in HFSE,and are heterogeneous in zircon εHf(t)values(+0.2^+15.1).Some of they show distinct positive Eu anomalies(δEu=1.16~2.26)and middle REE depletion.The Late Cretaceous adakites are interpreted as the products of partial melting of newly underplated mafic lower crust under hydrous amphibolite facies conditions.Compared with the zonal magmatic flare-up events at about 110Ma documented in the middle-northern Gangdese and at about 50Ma documented in the southern Gangdese,we argue that the Neo-Tethyan ridge subduction is the most probable mechanism to trigger the generation of the widespread zonal magmatism in the southern Gangdese during the Late Cretaceous.