滇西凤庆地区小湾花岗岩年代学、地球化学特征及其弧-陆俯冲向碰撞汇聚作用

刘军平; 杨爱平; 王小虎; 罗光明; 田素梅; 朱勋早; 殷伟; 张坤

doi:10.12029/gc20200424003

滇西凤庆地区小湾花岗岩年代学、地球化学特征及其弧-陆俯冲向碰撞汇聚作用

Zircon geochronolgy, geochemistry characteristics and arc-continent subduction to collision of the Xiaowan granite in Fengqing area, western Yunnan Province

摘要

摘要:
研究目的 滇西临沧地块岩浆岩带内风庆县小湾花岗岩体的形成机制对限定临沧壳源岩浆弧成因、洋盆俯冲消减及碰撞闭合时间等问题具有重要意义。
研究方法 本文首次报道了小湾花岗岩体的锆石U-Pb年代学、岩相学、全岩主量和微量元素地球化学特征。
研究结果 小湾花岗岩主要由黑云二长花岗岩组成，锆石LA-ICP-MS U-Pb定年显示其形成年龄为（260.4±1.3）Ma（MSWD=0.32，n=18），表明小湾岩体形成于中二叠世晚期，而非前人认为的中元古代。全岩地球化学分析显示黑云母二长花岗岩A/CNK大于1（1.02~1.47），表明小湾花岗岩体为一套弱过铝质花岗岩组成。该岩体具有高硅（SiO₂=67.54%~78.66%）、富碱（K₂O+Na₂O=6.20%~8.52%）、贫钙（CaO=0.25%~3.20%）、镁（MgO=0.11%~1.95%）、磷（P₂O₅=0.03%~0.16%）等元素的特征；富集Rb、Th、Ce等大离子亲石元素（LILE），相对亏损Ta、Nb、Zr、Ti、Hf、Y等高场强元素（HFSE），在(Y+Nb) vs.Rb、Yb vs.Ta图解中具有弧花岗岩及同碰撞花岗岩双重地球化学特征。
结论结合区域资料，本文认为小湾花岗岩体应属于“三江”昌宁—孟连古特提斯洋向东俯冲向汇聚转换背景下，上地壳泥质岩夹部分杂砂岩成分发生部分熔融的产物。本研究为青藏高原西南“三江”昌宁—孟连古特提斯洋弧-陆俯冲向碰撞汇聚的转换提供了重要的年代学及岩石学证据。

Abstract:
This paper is the result of geological survey engineering.
Objective The formation mechanism of the Xiaowan granitic pluton, located in the magmatic belt of the Lincang terrane in the Fengqing County, western Yunnan province, is very important for constraining the petrogenesis of the Lincang magmatic arc derived from crust, subduction of the ocean basin, and the time of collision closure.
Methods This paper reports for the first time the zircon U-Pb chronology, petrography, and geochemical characteristics of major and trace elements in the Xiaowan granitic pluton.
Results The Xiaowan granite is mainly composed of biotite monzogranite. Zircon U-Pb LA-ICP-MS dating shows that its formation age is (260.4±1.3) Ma (MSWD=0.32, n=18), suggesting that the Xiaowan granite was formed in the Late Mid-Permian, rather than the Middle Proterozoic as previously considered. Whole-rock geochemical analysis shows that the A/CNK of biotite monzogranite is greater than 1 (1.02-1.47), indicating that the Xiaowan granitic pluton is composed of weakly peraluminous granite. These rocks are characterized by high silica (SiO₂ = 67.54%-78.66%), alkali rich (K₂O+Na₂O = 6.20%-8.52%), poor in the elements of calcium (CaO=0.25%-3.20%), magnesium (MgO content 0.11%-1.95%) and phosphorus (P₂O₅ content 0.03%-0.16%). Meanwhile, they are enriched in large ion lithophile elements (LILE) such as Rb, Th, Ce, relatively depleted in high field strength elements (HFSE) such as Ta, Nb, Zr, Ti, Hf, Y. In the (Y+Nb) vs. Rb, Yb vs. Ta diagrams, they exhibit dual geochemical characteristics of arc and collisional granite.
Conclusions Based on regional geological data, we suggest that the Xiaowan granitic pluton is the product of partial melting of the mudstone mixing with sandstone at the upper crustal level, under the setting of the eastward subduction shifting to the convergence of the Changning-Menglian ocean (also called as "Three Rivers" ancient Tethys ocean). This study provides important chronological and petrological evidences for the transition of the Changning-Menglian ocean from arc-continental subduction to collision in the southwestern Qinghai-Tibet Plateau.

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