西藏冈底斯带中段晚三叠世侵入岩年代学、岩石地球化学特征与地质意义

Geochronology and petrogeochemistry of the Late Triassic intrusive rocks in central part of the Gangdise belt, Tibet, China and their geological significances

研究区内晚三叠世侵入岩岩性主要为斑状二长花岗岩和中细粒花岗闪长岩,锆石U-Pb定年显示,斑状二长花岗岩的形成年龄为(204±1.7)Ma,为晚三叠世岩浆活动产物。岩石地球化学特征上,里特曼指数σ为1.70~2.71,为高钾钙碱性-钾玄岩系列,铝饱和指数A/NK和A/CNK比值分别为1.22~1.85和1.03~1.24,属过铝质花岗岩。相对富集大离子亲石元素Rb、K和不相容元素Zr、Hf,亏损Ba、Ta、Nb、Sr、P、Ti等元素,轻稀土元素明显富集,轻重稀土元素分馏较强,具有较强的Eu负异常,岩石地球化学特征显示属S型花岗岩。斑状二长花岗岩锆石εHf(t)值为-15.83^-3.24,Hf同位素二阶段模式年龄变化范围为1444~2240Ma,表明可能主要由中—古元古代地壳物质部分熔融形成,源岩以砂岩为主和少量泥质岩,岩浆在演化过程中存在分离结晶作用。晚三叠世侵入岩是新特提斯洋向北俯冲的产物,在俯冲的过程中,幔源岩浆的不断上涌,诱发古老地壳物质部分熔融,形成花岗质岩浆,揭示新特提斯洋的俯冲时间不晚于204 Ma。

The main Late Triassic intrusive rocks are porphyritic monzonitic granite and fine to medium grain sized granodiorite in the study area.The zircon U-Pb dating yielded an age of 204±1.7 Ma for the porphyritic monzonitic granite,indicating a product of Late Triassic magmatism.The lithogeochemical characteristics of these rocks reveal that they have Rittmann Index values(σ)of 1.70–2.71,The A/NK values of 1.22–1.85,and A/CNK values of 1.03–1.24,respectively.They belong to the high-K calc-alkalic-shoshonite series and are peraluminous granite.They are relatively enriched in LILE(Rb,K),obviously enriched in LREE,and depleted in Ba,Nb,Ta,Sr,P,Ti,with the relatively strong fractionation between LREE and HREE and negative Eu anomaly.The lithological and geochemical characteristics of intrusive rocks indicate that they belong to S-type granite.TheεHf(t)values of-15.83–-3.24 and Hf isotopic two-stage model ages of 1444–2240 Ma for zircons from porphyritic monzonitic granite indicate that the granite could be formed by partial melting of the Mesoproterozoic-Paleoproteozoic crust with source rocks of major sandstone and minor mudstone.The fractional crystallization occurred during the magma evolution process.It is believed that these Late Triassic intrusive rocks could be products of the northward subduction of the Neo-Tethyan ocean.In the subduction process,the continuous upwelling of mantle-derived magma into the lower crust resulted in the partial melting of ancient crust materials to form the granitic magma.The time of subduction of the Neo-Tethyan ocean should be no later than 204 Ma.