西藏冈底斯南带辉长岩及其所反映的壳幔作用信息

Gabbros from southern Gangdese:implication for mass exchange between mantle and crust.

在巨型冈底斯岩浆岩带南端产出有一个辉长岩带,与雅鲁藏布江蛇绿岩带平行共生。辉长岩带中以辉长岩为主,与冈底斯花岗质岩石空间上呈渐变过渡接触关系,表现为接触带附近岩石成分的规律变化,并出现暗色微粒包体。本文在项目组多年来对冈底斯岩浆岩带研究的基础上,对拉萨曲水县城—日喀则一带的辉长岩类进行了详细的野外观测,选择了典型露头系统采集了样品,系统的岩石学、地球化学及同位素地球化学分析和研究表明该辉长岩类的SiO_2 49%～55%、Mg~#49～66,近于原始基性岩浆,REE曲线平缓右倾,无明显负Eu异常,表明未发生明显的岩浆分异作用,其^(87)Sr/^(86)Sr比值变化在0.7036～07051、^(143)Nd/^(144)Nd为0.5128～0.5131,且ε_(Nd)为2.34～6.87,其^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb,^(208)Pb/^(204)Pb分别为17.9095,15.4986和38.0176。这些特征都表明该辉长岩属于PREMA源区物质局部熔融而成,并在上侵就位过程中受到壳源物质影响。已有SHRIMP锆石U-Pb年龄表明该辉长岩类形成于40～53Ma,与曲水岩体岩浆混合时代一致。因此可以推断印度-欧亚陆陆碰撞过程中,随着俯冲板片的折返断离,具有亏损特征的普通幔源物质在始新世局部熔融,其熔浆底侵、上升,不同程度地与壳源花岗质岩浆混合,形成冈底斯辉长岩带,其成因与大陆碰撞密切相关。

Gabbro as predominant rock of an ultramafic to mafic rocks belt is occurring in southern Gangdese, Tibet, paralleling with Yarlung Zangbo Suture. Field investigation has found the gradually transitional relationship between the gabbro and granitoid with abundant mafic macro-granular enclaves （MME）. Petrological, geochemical and isotopic studies have proved that the gabbro is directly similar to primary basaltic magma with SiO2 of 49% -55% and Mg^# of 49 -66. The REE pattern is flat and slightly declining with no Eu anomaly, showing less magma differentiation taking place. Ratio of ^87 Sr/^86 Sr varies in range of 0. 7036 - 07051, while ^143 Nd/^144 Nd ratio is 0. 5128 -0. 5131 with εNd of 2. 34 -6. 87. The ratio of ^206 pb/^204 Pb, ^207 pb/^204 Pb, ^208 pb/^204 Pb are averagely 17. 9095, 15.4986 and 38. 0176 respectively. All the characters indicated that the gabbro is likely resulted from partial melting in prevalent mantle reservoir （PREMA） and some magma mixing during intruding up. SHRIMP zircon U-Pb dating for gabbro samples gave 52. 5 - 40. 0Ma of crystallized age, which is quite similar with the gabbro dating of 47.0 - 48.9 Ma in Quxu. It probably represents that the gabbro intrusions are formed in 53 - 40Ma. The paper therefore inferred that partial melting was induced in mantle wedge due to decompression and heating following slab-breakoff during the continental collision in Eocene. Then the basaltic magma is underplating below and then intruding up into the low crust. It is the underplated marie magma that resulted in the partial melting in the crust and then magma mixing following gabbro magma invading up into the crust along Gangdese belt, forming the Gangdese giant magmatie belt.