赣西北蒙山岩体的锆石U-Pb-Hf、地球化学特征及成因

U-Pb-Hf Isotope of Zircons,Geochemistry and Genesis of Mengshan Granitoids in Northwestern Jiangxi Province

华南印支期花岗岩形成的构造背景和成因机制存在较大的争议,赣西北印支期岩浆活动的规模及该次岩浆活动是否与基性岩浆的底侵有关,仍缺乏可靠的资料.过去认为赣西北蒙山花岗岩形成于燕山期,而本次锆石LA-ICP-MS U-Pb年代学研究表明,蒙山3次花岗质岩浆活动的时间分别为236±3 Ma,220±3 Ma和217±1 Ma.蒙山花岗岩属准铝-过铝质,高硅、富钾,K2O+Na2O为7.53%~8.86%;稀土总量为213.09~380.75μg/g,轻稀土元素富集,δEu=0.07~0.40,富集大离子亲石元素,Nb-Ta弱亏损,P、Ti亏损.大部分花岗岩的εNd(t)值为-9.9^-6.1,Nd二阶段模式年龄tDM2为1.5~1.8 Ga.第1次形成的灰白色粗粒花岗岩锆石的εHf(t)值大部分集中在1.10~2.65,Hf同位素单阶段模式年龄tDM1集中分布于782~866 Ma,二阶段模式年龄tDM2集中分布在1 096~1 186 Ma,说明其物源主要为新元古代早期形成的地壳;第3次形成的细粒花岗岩锆石的εHf(t)值集中分布于1.71~4.98,tDM1为671~832 Ma,tDM2集中在932~1 139 Ma.蒙山细粒花岗岩锆石的Hf同位素组成暗示成岩过程中有基性岩浆加入,为华南部分印支期花岗岩的成因与基性岩浆底侵有关的关系提供了证据.蒙山花岗岩的Nd同位素和锆石Hf同位素出现了解耦现象,解耦原因可能与花岗岩的物源主要为新元古代早期形成的具有Nd-Hf解耦特征的弧源地壳有关.根据目前获得的有关华南印支期火成岩的资料,总结了华南印支期花岗岩类岩石的成因类型及其时空分布规律,据此讨论了华南印支期的动力学背景和华南印支期花岗岩的成因机制,认为华南印支期的构造-岩浆活动与古太平洋西北向俯冲于华南板块之下有关.

The petrogenesis and tectonic setting of the Indosinian granites within the South China Block （SCB） are controversial, and there is no sound geochronological proof on occurrence of Indosinian magmatism in northwestern Jiangxi Province. Mengshan granitoids in this region were thought to be a Yanshanian intrusion. However, our new geochronological results show that Mengshan granites consist of three Indosinian magmatisms, with ages of 236±3 Ma, 220±3 Ma and 217±1 Ma respectively. Researches on geochemistry and Hf isotope of zircons are carried out. The Mengshan granites are metaluminousperaluminous, having high SiO2 and K2O, with K2O＋Na2O 7. 53%- 8. 86%. The granites have ∑ REE of 213. 09- 380. 75μg/g and REE patterns of LREE-enrichment with moderate negative Eu anomalies （Eu/Eu^＊ = 0. 07-0. 40）. There is strong enrichment in LILE, but showing pronounced negative anomalies in HFSE （Nb, Ta and TiO2 ） relative to neighbouring elements. Calculated εNd （t） values for most of samples are -9. 9 to -6. 1 and corresponding two-stage Nd model ages （tDM2） are 1. 5-1. 8 Ga. Most of the zircons from the greyish-white coarse biotite granites which were formed firstly have εNf （t） of 1.10 to 2. 65, their single-stage Hf model ages tDM1 range from 782 to 866 Ma and two-stage Hf model ages tDM2 are 1 096- 1 186 Ma. The zircons from the fined-grained granites which were formed thirdly in Mengshan complex have εHf （t） values of 1. 71-4. 98, and Hf tDM1 of 671-832 Ma and Hf tDM2 mostly are 932-1 139 Ma. The compositions of zircon Hf isotope suggest an addition of newly mantle-derived magma during the diagenesis of Mengshan granites, which provide a direct proof for the relationship between basalts underplating and the genesises of some Indonisian granites in South China. Decoupling between Nd and Hf isotopes has been found in Mengshan granites, and it probably resulted from the involvement of Neoproterozoic island- arc crust. The regularity of genentic types and the temporal-spatial distribution of the Indosinian magmatism in SCB are sum marized. On this basis, the Indosinian tectonic setting and the petrogenesis of the Indosinian granites in SCB are discussed. The northwestward subduction of Pacific was probably responsible to the Indosinian magmatisms in SCB.