Magma Mixing of the Eocene Quxu Batholith from the Gangdese Magmatic Belt, South Tibet: Evidence from Cathodoluminescence Characteristics and Composition Changes of Plagioclase
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摘要: 斜长石作为主要造岩矿物，是研究岩石成因、示踪岩浆演化和岩浆混合过程的有效工具.对冈底斯带曲水岩基始新世花岗闪长岩、二长花岗岩、闪长岩脉和暗色包体中的斜长石进行了阴极发光图像结构特征、电子探针主量元素和LA-ICP-MS微量元素成分的分析，揭示了斜长石复杂环带的成因和相关的岩浆过程.该区斜长石的阴极发光图像呈现出多种颜色且与其An值相对应，随着An值降低依次为绿色、蓝色和暗灰色或暗红色等，并发育补丁状环带、筛状环带、韵律环带等.花岗闪长岩、二长花岗岩中斜长石的An值具有相似的变化范围(20~55)，而闪长岩脉和暗色包体中An值的变化范围较大(25~85)，表明曲水岩基经历了复杂的开放过程.微量元素结果表明：花岗闪长岩与闪长岩脉和暗色微粒包体具有相同的Sr含量范围(600×10-6~1 100×10-6)；而二长花岗岩的Sr含量(1 000×10-6~2 400×10-6)整体高于前者.以上研究表明，花岗闪长岩中阴极发光呈现绿色的核部或幔部是偏中性岩浆注入寄主岩岩浆混合的结果；具有高Sr含量的二长花岗岩认为是高Sr含量的岩浆结晶形成的；闪长岩脉和暗色微粒包体中的筛状结构斜长石为寄主岩捕掳晶.Abstract: As the main rock-forming mineral, plagioclase is an effective tool for studying petrogenesis, magma evolution and magma mixing. The cathodoluminescence image, electron probe micro-analysis and LA-ICP-MS composition analysis were carried out for the plagioclase from the granodiorite, monzogranite, diorite dykes and mafic microgranular enclaves (MMEs) in the Quxu batholith in the Gangdese magmatic belt, which can reveal the formation mechanisms and relative magma evolution process of plagioclase complex zoning. The cathodoluminescence images of the plagioclase from the Quxu batholith shows that their color displays a corresponding relationship with the An value. With the decreasing with the An values, the colors are green, blue and dark gray or dark red in turn. The plagioclases have obvious three types of zoning:patchy zonation, sieve texture and oscillatory zoned. The An values of plagioclase from the Quxu granodiorite and monzonite have similar ranges (20-55), while the An values in diorite dikes and MMEs vary widely (25-85), all indicating that the Quxu batholith has undergone a complex opening process. Insitu trace elements analyses show that the granodiorite has similar Sr content (600×10-6-1 100×10-6) with the diorite dykes and MMEs; the Sr content of monzogranite (1 000×10-6-2 400×10-6) is higher than the granodiorite, diorite dykes and MMEs. The above studies show that:the green luminescence of cores and mantle in granodiorite is the result of the mixing of the intermediate magma and felsic magma; the high Sr content of monzogranite is considered to be derived from a Sr-enriched melt. The above studies show that the complex zonings of the plagioclase are the result of the injection of mafic magma into felsic magma. The core of sieve texture plagioclase in diorite vein and MMEs could be xenocrystals, which are captured from the host rocks.
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Key words:
- Quxu batholith /
- plagioclase /
- cathodoluminescence image analysis /
- magma evolution /
- magma mixing /
- petrology
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图 1 (a) 青藏高原地质简图、(b)拉萨地块地质简图和(c)曲水岩基地质图及采样点位置
a据Yin and Harrison（2000）修改；b据Chung et al.（2009）修改；c据Mo et al.（2005）修改
Fig. 1. A geological sketch map of the Tibetan plateau (a), a geological sketch map of the Lhasa block (b) and a geological map of the Quxu and sampling location (c)
图 2 曲水岩基的野外及岩相学特征
a.花岗闪长岩中的包体，呈卵状、水滴状，大小不一，大多数集中在20~30 cm；b.闪长岩脉体侵入花岗闪长岩中；c.D16T01，花岗闪长岩，主要矿物组成为斜长石、石英、钾长石、黑云母、角闪石；d.D16T14-2，似斑状二长花岗岩，斑晶为钾长石，基质为钾长石、石英、斜长石、角闪石、黑云母；e.D16T03，闪长岩，主要矿物组成为斜长石、角闪石、黑云母；f.D16T06，暗色包体，主要矿物组成为斜长石、角闪石、钾长石、黑云母.Qtz.石英；Pl.斜长石；Kfs.钾长石；Bt.黑云母；Hbl.普通角闪石
Fig. 2. Field and petrographic characteristics of the Quxu plution
图 3 花岗闪长岩中的特征斜长石环带阴极发光环带和成分趋势
a.D16T01-2-1，核-边结构，核部绿色斑块An值高，边部暗红色；b.D16T01-3-1，核-幔-边结构，核部和边部均为暗红色，An值相似，微量元素Sr、Ba含量有差别，幔部为蓝色斑块；c.核-边结构，核部蓝色斑块；d~f为电子探针和微区元素含量分析的An值的变化趋势图；g~i为微区元素含量分析Sr、Ba的变化趋势图
Fig. 3. The cathodoluminescence microphotogaphs and composition profiles of plagioclase zoning from granodiorite
图 4 二长花岗岩中的特征斜长石环带阴极发光环带和成分趋势
a.D16T07-1-1-5，核-边结构，核部绿色斑块An值高，边部暗红色；b.D16T11-1-1-1，核-边结构，整体Sr含量较低；c.D16T14-2-4-1，核-边结构，核部蓝绿色斑块；d~f为电子探针和微区元素含量分析的An值的变化趋势图；g~i为微区元素含量分析Sr、Ba的变化趋势图
Fig. 4. The cathodoluminescence microphotogaphs and composition profiles of plagioclase zoning from monzogranite
图 5 闪长岩脉和暗色包体中的特征斜长石环带阴极发光环带和成分趋势
a.D16T03-1-3，核-边结构，闪长岩脉中原生斜长石，核部亮绿色斑块An值高，可达80，边部暗红色；b.D16T03-4-2，核-幔-边结构，捕掳晶，幔部绿色筛状环带An值较核部和边部高；c.D16T10-7-1，核-幔-边结构，捕掳晶，幔部绿色环带An值较高，核部含有许多细小矿物；d~f为电子探针和微区元素含量分析的An值的变化趋势图；g~i为微区元素含量分析Sr、Ba的变化趋势图
Fig. 5. The cathodoluminescence microphotogaphs and composition profiles of plagiolase zoning from diorite and mafic microgranular enclave
图 6 An值变化范围
a.花岗闪长岩An值变化范围图，斜长石属于奥长石-拉长石系列；b.二长花岗岩An值变化范围，与花岗闪长岩范围相同；c.闪长岩脉An值变化范围，斜长石属于中长石-培长石范围；d.暗色微粒包体中斜长石An值变化范围，属于奥长石-培长石范围
Fig. 6. The variation range of An content
图 7 An-Sr图解(a、c)和Sr-Ba图解(b、d)
a.花岗闪长岩、闪长岩和MME的An-Sr图解；b.花岗闪长岩、闪长岩和MME的An-Ba图解；c.二长花岗岩的An-Sr图解，蓝色A区域代表花岗闪长岩的An-Sr分布范围，绿色B区域代表闪长岩和暗色包体的An-Sr分布范围；d.二长花岗岩的An-Ba图解
Fig. 7. An-Sr diagrams(a, c) and Sr-Ba diagrams(b, d)
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