滇西攀天阁地区花岗岩类年代学、地球化学特征及其地质意义

Chronology and Geochemical Characteristics of Granitoids in Pantiange Area,the Western Yunnan,China and Their Geological Significance

江达—维西岩浆岩带的构造背景还存在争议,有龙木措—双湖—昌宁—孟连古特提斯主洋盆向东俯冲、金沙江古特提斯洋盆向西俯冲及随后的碰撞过程两种不同认识。该构造背景的争议影响了古特提斯构造演化模型的建立。在江达—维西岩浆岩带南部攀天阁地区新发现一套侵入岩,主要成分为闪长质、花岗闪长质和花岗质等,岩体糜棱岩化明显。花岗质岩石中的锆石U-Pb年龄为(243.0±4.8)Ma,代表其结晶年龄,属于中三叠世。上述3类岩石明显富集Rb、Th、U等大离子亲石元素(LILE),相对亏损Nb、Ta、P、Ti等高场强元素(HFSE)和Ba、Sr等大离子亲石元素,轻、重稀土元素分馏明显,球粒陨石标准化稀土元素配分模式均表现为右倾型。花岗质岩石锆石εHf(t)值全为负,主要介于-11.92~-8.54,全岩初始87Sr/86Sr值为0.717229~0.725994;εNd(t)值全为负,介于-14.2~-7.8。这些特征表明,闪长质岩浆是由古元古代玄武质下地壳部分熔融形成的;花岗闪长质岩浆是由古元古代上地壳变质泥岩部分熔融形成的,并有地幔物质的加入;花岗质岩浆起源于古元古代上地壳变杂砂岩的部分熔融,再经过结晶分异形成,并有地幔物质的加入。综合区域构造背景及岩石地球化学特征认为,江达—维西岩浆岩带系古特提斯主洋盆向东俯冲而成。

The tectonic setting of Jiangda-Weixi magmatic belt is still controversy.Two different interpretations include the eastward subduction of Longmucuo-Shuanghu-Changning-Menglian Paleo-Tethys main ocean basin,and the westward subduction and subsequent collision of Jinshajiang Paleo-Tethys main ocean basin.These controversies concern different models for tectonic evolution of Paleo-Tethys.A new set of intrusive complex has been discovered in Pantiange area,the southern part of Jiangda-Weixi magmatic belt.The main components include dioritic,granodioritic and granitic rocks,which have been mylonitized.The zircons from granitic rocks have U-Pb mean age of(243.0±4.8)Ma.The age is considered as crystallization age of granitic rocks.Dioritic rocks have high contents of Na2O and Al2O3,and moderate content of K2O,suggesting they belong to metaluminous and calc-alkaline series.Granodioritic rocks has high contents of K2O and Al2O3,but low content of Na2O,suggesting they belong to peraluminous and shoshonite,high potassium calc-alkaline series.Granitic rocks have moderate content of Na2O and low content of K2O,and belong to metaluminous-peraluminous and low-potassium series.All three types of rocks are enriched in LILEs(such as Rb,Th,U),depleted in HFSEs(such as Nb,Ta,P,Ti)and LILEs(such as Ba,Sr),with high values of LREE/HREE ratio.The dioritic rocks have no obvious Eu anomaly,whereas granodioritic and granitic rocks show weak negative Eu anomaly.The granitic rocks have zirconεHf(t)values of-11.92--8.54,whole rocks initial87Sr/86Sr ratio of 0.717229-0.725994,andεNd(t)values of-14.2--7.8.These characteristics show that dioritic magmatism is formed by partial melting of Paleoproterozoic basaltic lower crust;granodioritic magmatism is formed by partial melting of metamorphic mudstones of Paleoproterozoic upper crust with the addition of mantle material;granitic magmatism originates from partial melting of metamorphic graywacke in Paleoproterozoic upper crust,and then underground crystallization fractionation with the addition of mantle material.Combined with regional tectonic setting and petrogeochemical characteristics,it is proposed that Jiangda-Weixi magmatic belt is formed by the eastwards subduction of Paleo-Tethys.