藏南冈底斯岩基东段石炭纪岩浆作用记录

藏南冈底斯岩基记录了大量中生代和新生代以来的岩浆作用信息，虽然晚古生代的岩浆岩报道较少，但对限定拉萨地块在新特提斯洋俯冲之前的构造作用具有重要意义。本文通过综合已有资料，进一步对加查县和朗县晚古生代花岗岩开展了锆石U-Pb地质年代学及全岩元素地球化学组成的研究。加查县和郎县花岗片麻岩的原岩年龄分别为~344.8Ma和344.0~362.0Ma，表明加查和朗县岩体都形成于石炭纪。这些岩石具有较高的SiO₂含量（67.46%~75.33%），Al₂O₃含量较低（12.66%~15.82%），CaO含量为0.79%~4.32%，FeO和MgO的含量分别为0.48%~3.00%和0.28%~1.64%。依据它们的K₂O/Na₂O比值，这些岩石可分为富钾和富钠两个演化系列。这些岩石富集轻稀土元素，亏损Nb、Ta和Ti，但Zr和Hf无明显异常。地球化学特征和年代学信息表明：（1）冈底斯岩基东段石炭纪花岗岩形成于弧后伸展环境，可能与古特提斯洋向冈瓦纳大陆北缘的俯冲作用有关；（2）镁铁质岩浆演化形成富钠花岗岩，幔源岩浆和中下地壳岩熔体的混合形成富钾花岗岩；和（3）石炭纪岩浆作用持续时间至少~30Myr。

Carboniferous magmatic records in the eastern Gangdese batholith, southern Tibet

The Gangdese batholith consists dominantly of magmatic rocks with ages from Mesozoic to Cenozoic (ca. 230~10Ma), however, increasing number of studies have documented that magmatic suites of Late Paleozoic age could provide critical constraints on the tectonic processes in the Lhasa terrane prior to the subduction of Neo-Tethyan oceanic lithosphere. Zircon U-Pb dating yields that the protoliths of the granitic gneisses from Jiacha and Lang formed at ~344.8Ma and ca.362.0~344.0Ma, respectively, which bracket the range of timing reported in the literature. Combined those literature data, new geochemical data show that these Carboniferous granites are characterized by high SiO₂ (67.46%~75.33%), but low Al₂O₃ (12.66%~15.82%), CaO (0.79%~4.32%), FeO (0.48%~3.00%) and MgO (0.28%~1.64%). Based on their K₂O/Na₂O ratios, they can be divided into two series: potassium-rich and sodium-rich, respectively. These rocks are characterized by (1) enrichment in light rare earth elements (LREE); (2) substantial negative Eu anomalies; and (3) depletion in Nb, Ta, and Ti, but no Zr and Hf negative anomalies. Geochemical and geochronological data suggest that: (1) the Carboniferous granites in the eastern Gangdese were formed in a back-arc extension setting, possibly related to the subduction of the Paleo-Tethys Ocean to the northern margin of Gondwana continent; (2) sodium-rich granites represent the differentiation products of the mafic parental magmas, whereas the potassium-rich ones from mixing of mantle-derived magma with middle-lower crustal; (3) this phase of magmatism lasted at least ~30Myr.