ABSTRACT

The Triassic Lincang granites make up the largest peraluminous granitic batholith in the Changning-Menglian Paleo-Tethys orogenic belt, southeastern Tibetan Plateau. It is undoubtedly very important to investigate the role of crustal magmatism in the evolution of the Changning-Menglian Paleo-Tethys orogenic belt. The Late Triassic Mengsong pluton is composed of strongly peraluminous two-mica monzogranites, and is located in the southernmost part of the Lincang batholith. Despite all being peraluminous granites, they differ markedly in dimensions, petrographic and geochemical characteristics. Mengsong two-mica monzogranites have low contents of TiO₂, FeO^tot, MgO, and CaO, but high values of SiO₂ and Na₂O, compared to biotite monzogranites of the Lincang batholith. The Lincang biotite monzogranites show an enrichment in light rare earth elements (LREE) and weak Eu anomalies (δEu = 0.3–0.8). However, the Mengsong two-mica monzogranites show a clear tetrad effect and contain pronounced negative Eu anomalies (δEu = 0.1–0.3). Geochemical compositions and mineral textures of two-mica monzogranites indicate that they are of highly fractionated type affinity. The two-mica monzogranites are characterized by negative ε_Hf (t) values (−9.3 to −1.7) and ε_Nd (t) values (−9.6 to −7.9), while the biotite monzogranites have lower ε_Hf (t) values (−15.5 to −6.1) and ε_Nd (t) values (−14.5 to −11.5). Isotopic compositions suggest that they were cogenetic and derived from partial melting of the lower crust. Our petrogenetic and zircon U–Pb age data support models that relate the Lincang and Mengsong granites to post-collisional magmatism in the Changning-Menglian Paleo-Tethys orogenic belt during the Middle to Late Triassic. We interpret the Mengsong pluton to represent highly fractionated melts from crystal mush reservoirs and the Lincang granites to be complementary residual.

摘要

三叠系临沧花岗岩构成了青藏高原东南部长宁-孟连古-特提斯造山带最大的过铝质花岗岩基体。研究地壳岩浆作用在长宁-孟连古-特提斯造山带演化中的作用无疑具有十分重要的意义。晚三叠世勐松岩体由强过铝二云母二长花岗岩组成，位于临沧岩基的最南端。尽管它们都是过铝花岗岩，但它们在尺寸、岩石学和地球化学特征方面存在显着差异。勐松二云母二长花岗岩TiO ₂、FeO ^tot、MgO、CaO含量低，SiO ₂、Na _2含量高O，与临沧岩基的黑云母二长花岗岩相比。临沧黑云母二长花岗岩富集轻稀土元素（LREE）和弱Eu异常（δEu=0.3~0.8）。然而，勐松二云母二长花岗岩表现出明显的四分体效应，并含有明显的负Eu异常（δEu = 0.1-0.3）。双云母二长花岗岩的地球化学成分和矿物结构表明它们具有高度分馏类型的亲和力。二云母二长花岗岩具有负ε _Hf ( t ) 值（-9.3 至 -1.7）和ε _Nd ( t ) 值（-9.6 至 -7.9），而黑云母二长花岗岩具有较低的ε _Hf ( t) 值（-15.5 到 -6.1）和ε _Nd ( t ) 值（-14.5 到 -11.5）。同位素组成表明它们是共生的，源自下地壳的部分熔融。我们的岩石成因和锆石 U-Pb 年龄数据支持模型，将临沧和勐松花岗岩与中晚三叠世长宁-孟连古-特提斯造山带的碰撞后岩浆作用联系起来。我们将勐松岩体解释为来自结晶糊状储层的高度分馏熔体，而临沧花岗岩则为互补残余。