冈底斯带南部桑日高分异Ⅰ型花岗岩的岩石成因及其动力学意义

摘要

Long-term subduction of the Neo-Tethyan oceanic lithosphere beneath an active continental margin gave rise to prolonged plutonic activity,which is illustrated by the vast chain of Gangdese batholith in the south margin of Lhasa Terrane,Southern Tibet.Although extensive studies have performed on the source regions,spatial-temporal associations and geotectonic implications of the granitic rocks within the batholith,the magmatic evolution,petrogenesis of highly fractionated Ⅰ-type granite and the geodynamical evolution process in 70 ～ 65Ma have not been well studied.Here we present zircon U-Pb geochronology,Hf isotope and bulk-rock geochemistry of the Late Cretaceous Sangri granites in order to shed light on this issue.Zircon U-Pb geochronology demonstrates the plutonic activity emplaced at 67 ～ 66Ma.The Sangri granites belong to high-K calc-alkaline series,displaying highly fractionated Ⅰ-type signature with high content of SiO2 (74.26％ ～ 76.93％),K2O + Na2O (7.87％ ～ 8.56％),but low content of CaO (0.28％ ～ 1.00％) and P2O5(0.02％ ～0.04％) and pronounced depletion in Nb,P and Ti,and enrichment in K,Rb,Th.The affinity of mantle component in the Sangri granites that was elucidated by positive zircon εHf (t) of + 4.6 ～ + 10.9,which makes it indistinguishable from other granitic rocks within Gangdese batholith.Tectonic models of evolution of the Neo-Tethyan Ocean suggest that the rollback of the oceanic slab is required in order to generate magmatism in Gangdese batholith during 70 ～ 65Ma.Combined with the previous research in Gangdese,we suggest the magma of Sangri granites had been derived from juvenile lower crust mingled with mantle component upwelling through the window of Neo-Tethyan oceanic slab,and were subjected to high degree of fractionation in the magma chamber.%冈底斯岩浆岩带位于西藏南部的拉萨地体南缘,它形成于特提斯洋和印度-亚洲大陆长期俯冲碰撞过程中,是青藏高原花岗岩最发育的地区.前人对冈底斯岩浆带中各类型花岗岩的成因、源区、时空分布以及其地球动力学意义进行了详细大量的研究,但是对高分异花岗岩的具体成因、演化过程以及在70 ～ 65Ma拉萨地块的地球动力学演化过程研究较少.本文选择冈底斯南缘白垩纪末桑日花岗岩进行研究,揭示了桑日花岗岩的岩石学特征、锆石U-Pb年龄、锆石Hf同位素特征和地球化学特征.本文样品LA-ICP-MS测得的锆石U-Pb年龄为67 ～ 66Ma.桑日花岗岩属于高钾钙碱性系列,具有高SiO2(74.26％～76.93％)、高K2O +Na2O(7.87％ ～8.56％),低P2O5(0.02％ ～0.04％)和CaO (0.28％ ～ 1.00％),以及富集K、Rb、Th,亏损Nb、P、Ti的高分异Ⅰ型花岗岩的特征.在锆石Hf同位素上,桑日花岗岩εHf(t)＞0(+4.6～+10.9),且具有Hf不均一的特征.结合前人研究,本文认为桑日花岗岩是高分异Ⅰ型花岗岩,在特提斯洋板块北向俯冲过程中,板片回转,俯冲洋壳脱水产生的流体进入地幔楔,引发地幔楔发生部分熔融产生镁铁质幔源物质并底侵上涌,导致浅部地壳发生部分熔融,并与慢源岩浆混合,从而在浅部形成混源岩浆房,最终在侵位与成岩后期经历高程度的分异演化形成的.