青藏高原东南缘腾冲来利山A型花岗岩地球化学特征、锆石U-Pb定年及其构造意义

摘要

The Tengchong block located in the conjunction of West Yunnan composite orogenic belt and southeastern margin of the Tibetan Plateau, and the granites in the block are important way to explore its tectonic evolution. U-Pb dating of zircon from the monzogranites in Lailishan at Tengchong county yields an age of 50.4±0.4 Ma, indicating that the pluton is related to the Early Cenozoic Indian-Eurasian continental collision. The Lailishan granites are characterized by high-K calc-alkaline and metaluminous, with SiO2=67.52%‒68.99%, K2O=5.13%‒5.70%, Al2O3=14.03%‒14.98%, K2O/Na2O=1.66‒1.89, A/CNK=0.95‒0.99, A/NK=1.32‒1.40. The rocks have high FeOT/MgO (4.03‒4.54) and 10000×Ga/Al (5.18‒5.59) ratios, enrich in Rb, Th, U, K, Pb (LILE) and Zr, Hf (HFSE), but deplete in Ba, Nb, Sr, P, Ti, Eu, and right oblique REE type. All these characteristics suggest that the Lailishan granites are A-type granite and belonging to A2 sub-type. The εHf(t) values ( –11.44‒–8.01) and the two-stage model ages (tHf2) (1.63–1.85 Ga) of the Lailishan granites suggest that the granites were mainly derived from remelting of the thickened Mesoproterozoic continental crust. At the Early Eocene, around 50 Ma, the Tengchong-Lianghe area in West Yunnan was not in post-orogenic environment that traditionally represented by A2 type granites, but just in the transition tectonic setting of Indian-Eurasian plate from syn-collision to post-collision, revealing a geodynamic processe of enduring from extrusive mechanism to tensive mechanism in this area. Therefore, we consider that southeastern margin of the Tibetan Plateau are in the syn-collision tectonic setting since the peak of Indian-Eurasian continental collision to the Early Eocene, following in the short-lived relaxation of post-collision setting, then the Lailishan A-type Granites was formed in the Tengliang area of Western Yunnan.%腾冲地块处于滇西复合造山带和青藏高原东南缘,研究区发育的花岗岩携带了大量的构造信息。腾冲来利山二长花岗岩锆石 U-Pb 年龄为50.4±0.4 Ma,是新生代早期印度‒欧亚陆陆碰撞的产物。花岗岩 SiO2含量较低(67.52%~68.99%),高钾(K2O=5.13%~5.70%), AR 值为2.02~2.14, A/CNK 值为0.95~0.99,属于准铝质高钾钙碱性系列；FeOT/MgO(4.03~4.54)、10000×Ga/Al(5.18~5.59)比值较高,相对富集 LILE(Rb、Th、U、K、Pb)和 HFSE (Zr、Hf),相对亏损 Ba、Nb、Sr、P、Ti、Eu,而且稀土总量较高(∑REE=381.38×10–6~471.47×10–6),较富集轻稀土,稀土元素配分曲线呈右倾型。以上特征均指示来利山二长花岗岩具有A型花岗岩的特征,且属于A2亚型花岗岩。岩石的εHf(t)=–11.44~–8.01, Hf 同位素二阶段模式年龄 tDM2=1625~1845 Ma,反应其源岩物质单一,主要由地壳增厚引起中元古代的古老大陆地壳熔融形成。在早始新世(50 Ma 左右),滇西腾冲-梁河地区正好处于印度‒欧亚板块同碰撞‒后碰撞的过渡构造环境,揭示了该地区从挤压机制向拉张机制转变的动力学过程。因此，作者推断自印度‒欧亚板块陆陆碰撞高峰期至早始新世，青藏高原东南缘的板块应处于同碰撞挤压构造环境，之后进入后碰撞短暂拉张构造环境，并在滇西腾梁地区形成了来利山A型花岗岩。