赣西蒙山岩体地球化学、锆石U-Pb年龄、Hf同位素特征及地质意义

Geochemistry,Zircon U-Pb Ages,and Hf Isotopes of the Mengshan Rock Mass in Western Jiangxi Province and Their Geologic Implications

以赣西地区蒙山岩体为研究对象,通过对该岩体地球化学、锆石U-Pb年龄及Hf同位素的研究,探讨其岩浆物质来源及构造背景。该岩体经历了印支期、燕山期岩浆侵入活动,形成六种岩石类型,LA-ICP-MS锆石U-Pb年龄分别为224.0±2.10 Ma(n=19,MSWD=1.9)、223.0±2.20 Ma(n=21,MSWD=2.6)、222.0±2.80 Ma(n=21,MSWD=2.7)、220.0±1.90 Ma(n=16,MSWD=1.5)、217.0±3.70 Ma(n=8,MSWD=1.8)、142.0±0.98 Ma(n=21,MSWD=1.4),表明岩体经历了印支期、燕山期两期岩浆活动。印支期花岗质岩石地球化学具高硅富钾、准铝-过铝质特征;微量元素富集高场强元素Zr、Hf、Th、U和稀土元素La、Ce、Sm、Nd、Y,明显亏损Rb、Ba、Sr、Ti、P;稀土元素分馏程度不高,δEu介于0.03~0.15,具强Eu负异常,Ce异常不明显,稀土配分曲线呈“V”型;为同源岩浆演化的特点。CaO/Na_(2) O比值<0.3、Rb/Sr>5,岩石二阶段模式年龄T DM2主体介于868~1465 Ma,为中-新元古代,εHf(t)对应的T DM2值显示,岩体物源起源于成熟地壳,后期有大量新生地壳物质的加入,表明岩石来源于含水矿物(如白云母、绿帘石/黝帘石)的脱水反应导致成熟度高地壳物质的熔融而形成。在花岗岩构造环境判别图上,投于碰撞大地构造背景上的花岗岩和造山后花岗岩类(POG)区域,锆石U-Pb年龄表明成岩时段滞后于印支运动高峰期(258~243 Ma),为受印支运动造成华南地壳加厚滞后,地壳进入伸展阶段,减压熔融形成的花岗质岩浆。

Based on the study of the geochemistry,zircon U-Pb ages and Hf isotopes,the source of magmatic materials and tectonic setting of the Mengshan rock mass in western Jiangxi Province are discussed.Research suggests that this rock mass has experienced magma intrusion in Indosinian-Yanshanian periods,forming six rock types,of which the LA-ICP-MS zircon U-Pb ages are as follows:224.0±2.10 Ma(n=19,MSWD=1.9),223.0±2.20 Ma(n=21,MSWD=2.6,222.0±2.80 Ma(n=21,MSWD=2.7),220.0±1.90 Ma(n=16,MSWD=1.5),217.0±3.70 Ma(n=8,MSWD=1.8)and 142.0±0.98 Ma(n=21,MSWD=1.4),indicating two stages of magmatism in Indosinian and Yanshanian times.The geochemistry of these Indosinian granitic rocks is characterized by high silicon,high potassium and quasi aluminum-peraluminous.Trace elements are rich in Zr,Hf,Th,U and rare earth elements(REE)La,Ce,Sm,Nd and Y,but are obviously depleted in Rb,Ba,Sr,Ti and P.REE fractionation is not high,in which theδEu is between 0.03 and 0.15 with strong an Eu anomaly but an inconspicuous Ce anomaly,presenting a“V”type of the REE distribution curve.It is a feature of the evolution of homologous magma.The ratio of CaO/Na_(2) O is less than 0.3 and Rb/Sr is more than 5.The major T DM2 of the second-stage model age yields a middle Neoproterozoic period between 868 and 1465 Ma.The T DM2 value corresponding toεHf(t)indicates that the rock material was originated from the mature crust,and a large number of new crustal materials were added in the later period,proving that the rocks are derived from the dehydration reaction process of rich basic rocks and hydrated minerals(such as muscovite,epidote/zoisite),leading to the melting of high mature crust materials.The projection of numerical points on the discrimination map of the granitic tectonic setting reflects a position within granites and post orogenic granite(POG)region under a collision tectonic setting.Its U-Pb zircon age indicates that the diagenetic time of the rock mass lagged behind the peak of Indosinian movement(258~243 Ma).It can be inferred that the South China crust began to extend after the Indosinian movement caused its crustal thickening,and then decompressed and melted forming granitic magma.