滇西漕涧地区崇山杂岩带始新世花岗岩的发现及其地质意义

The Discovery of the Eocene Granite in the Chongshan Complex Belt of Caojian Area in Western Yunnan and Its Geological Significance

崇山杂岩带为滇西三江造山系西侧边缘的一条重要边界构造,属青藏高原造山带的南东缘。带内岩浆活动强烈,前人认为其主要由晚元古代、中生代的花岗岩组成;而据地质调查发现,带内新生代的岩浆作用十分发育。本文对带内新发现的灰白色黑云二花岗岩进行研究,获得了锆石U-Pb LA-ICP-MS年龄为34.88 Ma和35.25 Ma,表明其侵位于古近纪始新世。岩石地球化学表明,黑云二花岗岩为钾玄质-高钾钙碱性系列的过铝质花岗岩类,稀土元素配分曲线为右倾,其中LREE明显富集,HREE为不同程度亏损,具明显的负Eu异常;富集Rb、Th、Ce、Sm和亏损K、Ba、Ta、Hf、Y等元素,且为淡色花岗岩特征;据锆石Hf同位素测试分析,εHf(t)值为–2.35~+2.33和TDM2为962~1259 Ma,表明了花岗岩岩浆源区为壳幔混合的产物,以及源岩主要源自中元古代—新元古代陆壳基底物质的部分熔融。综合研究认为,始新世黑云二长花岗岩形成于喜马拉雅期碰撞造山向造山晚期转换阶段,是一套典型的“超厚地壳”部分熔融和壳幔相互作用的淡色花岗岩;以及其侵位时间代表了漕涧地区喜马拉雅碰撞造山阶段的完成和造山晚期阶段的开始,且转换时间约为35 Ma。

The Chongshan complex belt is an important boundary structure of the Sanjiang orogenic system in western Yunnan and belongs to the southeast edge of the Tibetan Plateau orogenic belt.Magmatic activities within the belt is strong,which was regarded by previous researchers as being mainly composed of granite in Late Proterozoic and Mesozoic magmatism.However,according to the geological survey,Cenozoic magmatism in the belt is well developed.In this paper,the newly discovered grayish white biotite granite in the belt,with the zircon U-Pb LA-ICP-MS ages being 34.88 Ma and 35.25 Ma,indicates that it intruded in Eocene of Paleogene.Rock geochemical studies show that the biotite granites belong to the peraluminous leucogranites series of potassium-high-potassium calc-alkaline nature,and the REE distribution curve is right-inclined.In the biotite granites,LREE is obviously enriched,HREE is depleted to varying extent with obvious negative Eu anomaly,Rb,Th,Ce,Sm are enriched but K,Ba,Ta,Hf,Y and other elements are depleted.According to zircon Hf isotope test and analysis,εHf(t)values are–2.35~+2.33 and TDM2 values are 962~1259 Ma,indicating that the magma source area of granite was the product of crust-mantle mixing,and the source rock mainly came from the partial melting of Mesoproterozoic continental crust basement material.According to comprehensive research,the Eocene biotite monzogranite was formed in the transition stage from Himalayan collision orogeny to late orogeny,and it was a set of typical pale granites with“ultra-thick crust”partially melted and experienced crust-mantle interaction.The emplacement time represents the completion of Himalayan collision orogeny and the beginning of late orogeny in Caojian area,and the transition time is about 35 Ma.