摘要
赣东北地区沿江绍断裂北缘和江南造山带南侧有大量的前寒武纪火山侵入杂岩呈带状分布,主要由大面积的陆相火山岩和若干长英质镁铁质火成杂岩体组成,后者中出露面积最大的港边杂岩体在填图尺度上可划分为长英质岩、镁铁质岩和过渡性岩3大组成部分,长英质岩颗粒锆石U_Pb谐和不一致曲线上交点年龄为822±4Ma,代表杂岩体中岩浆混合作用发生的时间。港边岩体中长英质岩石的Nd模式年龄为1·49~1·68Ga,镁铁质岩中玄武质岩石为1·51~2·21Ga,辉长岩的Nd模式年龄与玄武质岩石基本相同,为1·54~2·13Ga,经计算的过渡性岩石的Nd模式年龄为1·58~1·90Ga,因此认为镁铁质岩石从源区分离的时间要远早于长英质岩石,而过渡性岩石含有前两个端员岩浆相互混合(和)的信息。长英质岩石的εNd(t=822Ma)值为-0·8^-4·4,87Sr/86Sr(822)比值为0·70368~0·70549;玄武质岩石的εNd(822)值为-2·6^+2·3,87Sr/86Sr(822)比值为0·70387~0·70527,这反映了镁铁质岩浆的源区接近总地球,长英质岩浆则来自具壳幔混合性质的EMⅠ型地幔的源区。可以推测,赣东北前寒武纪也曾发生过岩浆底侵作用。底侵作用对港边火成杂岩体岩浆的形成起着重要的作用。底侵于壳幔边界的玄武质岩浆使地壳熔融形成长英质岩浆并聚集于浅部岩浆房。随着长英质岩浆房降温,长英质岩浆的结晶度和粘度增加,其物理状态发生液态→(固+液)混合态→固态的变化;当玄武质岩浆穿透下地壳并不断进入物理状态变化的长英质岩浆房时,持续地发生两种岩浆的化学混合作用或两岩浆的机械混合作用或玄武质岩浆的侵入,其混合作用的时间以长英质岩锆石的U_Pb谐和不一致曲线上交点年龄822±4Ma界定。根据Nd模式年龄(tDM)和Nd富集系数(fSm/Nd)分布特征,可以确定中元古代江南造山带为太古宙扬子古陆核的横向增生体,而新元古代陆相火山侵入杂岩带则横向增生于江南造山带南缘。至此,扬子古陆横向增生作用结束,开始与华夏古陆拼贴,包括港边火成杂岩体在内的新元古代火山侵入杂岩带则成为两古陆连结的纽带。
Large quantities of Precambrian volcanic-intrusive complexes are distributed between the northern side of Jiangshao suture and the southern side of Jiangnan orogenic belt in northeastern Jiangxi Province. They con- sist mainly of Neoproterozoic volcanic rocks and felsic-mafic igneous complexes, of which the largest one is Gangbian body covering 120 km2 in Hengfeng County. This complex body can be divided into felsic, mafic and transitional rock units at the mapping scale. The age of the upper intersection point on the U - Pb incongruent curve of zircon from the felsic rocks is 822 ± 4 Ma. The Nd model ages of various rocks from Gangbian complex body are different from each other： felsic rocks are 1.49～1.68 Ga; mafic basaltic rocks are 1.51～2.21 Ga; Nd model ages of gabbro are approximately the same as those of basaltic rocks, being 1.54-2.13 Ga; the calculated Nd model ages of transitive rocks are 1.58～1.90 Ga. Therefore, the fractionation of mafic rocks in this body from the mantle must have occurred much earlier than that of the felsic rocks. Nevertheless, the Nd model ages of these transitive rocks contain some information on mutual mixing and mingling of the two end-member magmatic bodies. The authors obtained quite a few isotopic data for various rocks in Gangbian complex body： eNd（ t = 822） values of the felsic rocks are - 0.8- - 4.4, 87Sr/86Sr（822） ratios are 0.703 68～0.705 49, eNd （822） values of the mafic basaltic rocks are - 2.6～ ＋ 2.3, and 87Sr/86Sr（822） ratios are 0.703 87-0.705 27. These data show that the source of the basaltic magma was close to the bulk earth, whereas the source of Neoproterozoic felsic magma in the complex was derived from the EM I mantle characterized by the mixing of mantle and crust. It is thus considered that northeastern Jiangxi area developed magmatic underplating in Precombrian, and this process played an important role in the formation of the mama of Gangbian complex. The formation of the felsic magma was controlled by the melting of the crust because underplating of the basaltic magma along the boundary between the mantle and the crust supplied thermal energy which was accumulated in the magma chamber; temperature of the diapiric felsic magma chamber decreased in company with increasing extent of crystallization and viscosity in the felsic chamber; the physical state of the felsic magma chamber also changed simul- taneously in order of liquid→（soild ＋ liquid）→solid. The liquid basaltic magma entered the felsic magma chamber through the lower crust, in which the physical condition was changing. Finally, the successive chemical mixing and mechanical mingling of the two end-member magma bodies and the intrusion of magma took place. The time of magmatic mixing is defined by the age of the upper intersection point on the U-Pb incongruent curve of the zircon, i.e., 822 ± 4 Ma. Based on the data of the Nd model age （tDM） and the Nd enrichment coefficient （fSm/Na）, it can be concluded that the middle Proterozoic Jiangnan orogenic belt was accreted on the southern margin of the Archean land nucleus in Yangtze paleocontinent, and that the Neoproterozoic terrestrial volcanic-intrusive complex zone was later accreted on the southern side of the Jiangnan orogenic belt. At that time, the accretion of Yangtze paleocontinent drew to its end and its piecing together with the Cathaysia paleocontinent began. Therefore, the Neoproterozoic volcanic-intrusive complex zone including Gangbian complex body became the link of these two paleocontinents.
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2006年第5期357-376,共20页
Acta Petrologica et Mineralogica
基金
国家自然科学基金资助项目(40172024)
