中拉萨地块晚白垩世曲桑格勒花岗岩的成因: 地球化学、锆石U-Pb年代学及Sr-Nd-Pb-Hf同位素的约束

目前关于班公湖-怒江缝合带的构造演化时限及俯冲方向仍存在较多争议.中拉萨地块北缘发育众多白垩纪岩浆岩，是认识拉萨地块白垩纪时期的岩浆成因机制和构造动力学过程的有效探针.在对尼玛县曲桑格勒花岗正长岩体详细的野外地质调查基础上，对其岩相学、地球化学、锆石U-Pb年代学及Sr-Nd-Pb-Hf同位素特征开展了综合研究.曲桑格勒花岗岩的地球化学特征显示其具有较高的SiO₂（75.23%~77.66%）、总碱（K₂O+Na₂O）（8.41%~8.94%）含量，低A1₂O₃（11.96%~12.38%）、CaO（0.18%~0.55%）含量，里特曼指数（σ₄₃）为2.05~2.33，铝指数（A/CNK）为0.99~1.03；富集大离子亲石元素（LILE）如Rb、Th、U、K、Pb，亏损高场强元素（HFSE）如Nb、Ta、P、Ti；轻稀土元素（LREE）富集，重稀土元素（HREE）相对亏损，轻重稀土分异偏低（LREE/HREE=2.42~5.00），呈现典型的“海鸥型”配分模式，具有强烈的负Eu异常（δEu=0.048~0.078），无明显Ce异常（δCe=0.739~1.471）.锆石饱和温度计算结果显示曲桑格勒花岗岩的结晶温度为763.50~850.65℃，平均795.2℃，这些特征与A型花岗岩相符.通过LA-ICP-MS方法测得正长花岗岩锆石206Pb/238U年龄加权平均值为101±1 Ma（MSWD=0.45），表明曲桑格勒花岗岩的岩浆作用发生在晚白垩世早期，具有正的ε_Hf（t）值（4.44~5.85），一阶段、二阶段模式年龄分别为536~592 Ma、702~781 Ma；（87Sr/86Sr）_t为0.706 2~0.710 6、（143Nd/144Nd）_t为0.512 315~0.512 441、ε_Nd（t）为-6.27~-3.82，（206Pb/204Pb）_t为18.653~18.794、（207Pb/204Pb）_t为15.709~15.731、（208Pb/204Pb）_t为38.960~39.100；构造环境判别图解显示其具有拉萨地块与羌塘地块后碰撞阶段上地壳的变质泥质岩部分熔融的特点，因此曲桑格勒花岗岩应该是碰撞后伸展阶段岩浆活动的产物.结合区域构造演化历史认为曲桑格勒花岗岩源自加厚的拉萨地块中上地壳，受减薄减压效应影响深部前寒武纪变质基底发生部分深熔作用，形成具有以上地壳为主要岩浆源特点的A型花岗岩. 

Genesis of Late Cretaceous Qusang 'gele Granitie in Central Lhasa Block,Tibet:Constraints by Geochemistry,Zircon U-Pb Geochronology,and Sr-Nd-Pb-Hf Isotopes

There are still many controversies about the tectonic evolution time and subduction direction of the Bangong Co-Nujiang suture zone. Many Early Cretaceous magmatic rocks lie in the north margin of the central Lhasa Block, it is helpful to understand the setting of magma activity and tectonic dynamics by researching the genesis and formation mechanism of the magma in the Cretaceous period of the Lhasa Block. Based on detailed field investigation upon the Qusang'gele granite, a comprehensive research of their petrography, geochemistry, zircon U-Pb geochronology and Sr-Nd-Pb-Hf isotope was carried out. Qusang'gele granite has high contents of SiO₂ (75.23%-77.66%), total (K₂O+Na₂O) (8.41%-8.94%), lower contents of A1₂O₃ (11.96%-12.38%) and CaO (0.18%-0.55%), with Reitman index of 2.05-2.33, and A/CNK values of 0.99 to 1.03; it is enriched in LILE such as Rb, Th, U, K, Pb, and depleted in HFSE such as Nb, Ta, P, Ti; all samples are enriched in LREE and depleted in HREE, the LREE/HREE ratios is 2.42 to 5.00, with typical seagull-shaped distribution pattern, strong negative Eu anomalies of 0.048 to 0.075, without obvious negative Ce anomalies (δCe=0.739-1.471).Caculated zircon saturated temperatures of Qusang'gele granite range from 763.50 to 850.65℃, averaged at 795.2℃.Combined with the geochemical characteristics above, the Qusang'gele granite should be defined as A-type granite. The syenogranite has a weighted mean 206Pb/238U age of 101±1 Ma (MSWD=0.45) by LA-ICP-MS zircon U-Pb analysis, suggesting that the Qusang'gele magmatism took place during the Early Late Cretaceous. It has positive ε_Hf(t) values of 4.44 to 5.85 and Hf model T_DM1 ages of 536-592 Ma and T_DM2 ages of 702-781 Ma; Qusang'gele granite is characterized with the (87Sr/86Sr)_t values of 0.706 2 to 0.710 6, (143Nd/144Nd)_t values of 0.512 315 to 0.512 441, and ε_Nd(t) values of -6.27 to -3.82; (206Pb/204Pb)_t values of 18.653 to 18.794, (207Pb/204Pb)_t values of 15.709 to 15.731, and (208Pb/204Pb)_t values of 38.960 to 39.100. It is showed that the partial melting of metamorphic mud in the upper crust during the post-collision stage between the Lhasa Block and the Qiangtang Block in the discrimination diagrams of tectonic setting. Therefore, the Qusang'gele granite occurred in post-collision extension stage when the deep Precambrian metamorphic basement underwent partially deep-melting due to the effect of thinning and decompression, which resulted in the A-type granite originated from the middle-upper crust of the thickened Lhasa Block.