滇西那邦地区地壳变质深熔过程及其构造意义:来自渐新世混合岩年代学和地球化学约束

Crustal Metamorphic Anatexis in the Nabang Region,Yunnan Province,and Tectonic Significance:Constraints from Geochronology and Geochemistry of Oligocene Migmatites

滇西腾冲地块西侧的那邦地区作为东喜马拉雅造山带的重要组成部分和藏南冈底斯带的南延,不仅记录了印度‒欧亚大陆碰撞过程的重要信息,也记录了高原岩石圈向南挤出的详细过程。近年来的研究显示,该区部分原划分为早前寒武基底的高黎贡群形成于新生代,并经历过多期次的变质深熔作用,但关于变质深熔过程和变质机制并未引起足够重视。本文对滇西那邦地区新生代混合岩开展了年代学和地球化学特征分析。LA-ICP-MS锆石U-Pb年龄显示,混合岩重结晶锆石年龄为29 Ma。根据岩石地球化学特征,暗色混合岩基体可以分为两类:第一类暗色体,富铝低钙,富集LREE,亏损HREE,其ε_(Nd)(t)值为-4.31和-4.69,显示岛弧岩石的地球化学特征,与盈江始新世变基性岩地球化学特征相似,暗示其源区为受到俯冲板片流体改造的富集岩石圈地幔;第二类暗色体,富集LILE和HREE,而低轻稀土和亏损高场强元素,ε_(Nd)(t)值为-11.35,指示其源区为更古老的基性堆晶下地壳,结合始新世锆石具有低HREE特征,表明该暗色体原岩经历过加厚下地壳水平的石榴石角闪岩相的变质作用。浅色体为花岗闪长岩,具有低钾准铝质特征,富集HREE,亏损高场强元素,全岩(^(87)Sr/^(86)Sr)i值(0.7124)高,ε_(Nd)(t)值(-13.71)低,表明来源于与其共生的暗色体原岩的深熔,并有少量沉积物熔体的混染。综合区域构造‒岩浆‒变质作用的研究结果表明,腾冲地块在始新世早期~41 Ma经历过一次重要地壳加厚事件,随后发生了涉及地幔岩石圈水平的向南挤出和伸展。而40~20 Ma期间则发生以挤压为主的仅地壳层次的向南挤出和减薄,并引发了一系列中下地壳变质深熔作用。腾冲地块那邦地区的混合岩代表的是始新世‒渐新世早期变质深熔作用的产物。

The tectonic deformation mechanism of the eastern and southeastern Qinghai-Tibet Plateau has been a hot topic for decades.As an important part of the East Himalayan orogenic belt and the southern extension of the South Gangdese belt,the Nabang region in the western Tengchong Block recorded important information concerning the India-Eurasia collision and process of the extrusion of the plateau lithosphere to the south as well.In recent years,it has been determined that a portion of the metamorphic rocks in this region,which was formerly classified as the Gaoligong Group of the earlier Cambrian basement,belongs to the Cenozoic and underwent multiple stages of metamorphic anatexis.However,research on the process and mechanism of metamorphic anatexis is limited.This paper aims to investigate the geochronology and geochemistry of the Oligocene migmatites in the Nabang region of the western Yunnan province.LA-ICP-MS U-Pb dating showed that zircon grains from the migmatites have Th/U ratios ranging between 0.13 and 0.59 and yielded a recrystallization age of 29 Ma.Based on geochemical characteristics,the melanosomes of the migmatites can be divided into two main types.The first type of melanosomes resembles island-arc rocks,showing high aluminum,low calcium,LREE enrichment,HREE depletion,and negativeε_(Nd)(t)values of-4.31 and-4.69 which are similar to those of the Early Eocene metabasic rocks in the western Tengchong Block.It is suggested that these melanosomes originated from the lithospheric mantle modified by subduction-related fluids.The second type of melanosome has negativeε_(Nd)(t)value of-11.35 and is enriched in LILE and HREE but depleted in HFSE,indicating an ancient mafic lower crust source formed through from cumulus of enriched mantle-derived magma.Considering the low HREE contents of these Eocene zircons in the second type of melanosome,the protolith of the melanosome might have undergone a garnet hornblende facies metamorphic overprint during the Eocene.The leucosome of the migmatites is granodiorite,with characteristics of low potassium,quasi aluminum,HREE enrichment,HFSE depletion,a high initial Sr ratio of 0.7124,and a negativeε_(Nd)(t)value of-13.71,indicating that it was derived from anatectic melanosome and mixed with melt of sediment.Combined with regional tectonic-magmatic-metamorphism,we propose that the Tengchong Block experienced a significant crustal thickening in the early Eocene(ca.41 Ma),followed by mantle lithosphere involved southward extrusion and extension.During 40 Ma to 20 Ma,the crust underwent primarily southward extrusion and thinning,which triggered a series of metamorphic anatexis in the middle and lower crust that led to the formation of the migmatites in the Nabang region of the Tengchong Block.