赣南珠兰埠复式岩体成因及其构造意义

Petrogenesis of the Zhulanbu composite pluton and its implications for tectonic setting

会昌地区位于南岭北部,该区出露有富城岩体和珠兰埠岩体,其中后者的研究程度低且时代与成因方面存在较多争议。珠兰埠复式岩体包括主体粗粒黑云母二长花岗岩和补体细粒二长花岗岩,LA-ICP-MS锆石U-Pb定年结果显示两者分别形成于印支期(228 Ma)和燕山晚期(135~133 Ma),而富城复式岩体的主体和补体花岗岩均形成于印支期。对珠兰埠主体与补体花岗岩开展主量元素与微量元素分析,结果显示两者均富Si、Al、K,低Ca、Mg,属弱过铝-强过铝质,富集Rb、Th、U而亏损Sr、Ba、P、Ti、Eu等元素,∑REE(平均值<165×10-6)和Zr+Nb+Ce+Y含量较低(<350×10-6),表明两者均为S型花岗岩。全岩Sr-Nd同位素分析结果表明,珠兰埠主体粗粒黑云母二长花岗岩(87Sr/86Sr)i值大于补体细粒二长花岗岩(分别为0.715 6~0.726 0和0.710 5~0.716 4),但εNd(t)值基本一致(-10.0^-7.0),锆石原位Hf同位素分析结果显示两者εHf(t)值分别为-12.9^-7.7和-15.1^-10.5,tCDM分别为2.0~1.7 Ga和2.1~1.8 Ga。元素和同位素分析结果表明珠兰埠复式岩体可能源自古元古代的混合源区(变泥质岩为主,夹少量变质杂砂岩)部分熔融而成,印支期珠兰埠主体粗粒黑云母二长花岗岩和富城复式岩体均形成于古亚洲-特提斯构造域挤压构造背景,而燕山晚期珠兰埠补体细粒二长花岗岩则形成于古太平洋构造域的板内拉张环境。

There are two composite plutons, Fucheng and Zhulanbu, emplaced in the Huichang region of the north- ern Nanling Mountains. Nevertheless, very insufficient researches have been conducted on the Zhulanbu composite pluton, and its age and petrogenesis remain problems of much controversy. The Zhulanbu composite pluton consists of early intrusive coarse-grained biotite mozonite and late intrusive fine-grained mozonite, and LA-ICP-MS zircon dating suggests that they were formed in Indosinian （228 Ma） and Late Yanshanian （135 -133 Ma）, respectively; however, the Fucheng composite pluton was emplaced in Indosinian. Chemical analyses of major and trace elements indicate that both the early and late granitic intrusive bodies of the Zhulanbu composite pluton are characterized by intermediate-strong peraluminous nature, high content of Si, A1, K, low content of Ca, Mg, enrichment of Rb, Th, U, and depletion of Sr, Ba, P, Ti, Eu. They also have lower EREE （〈 165 ×10^-6 on average） and Zr ＋ Nb ＋ Ce ＋ Y content （〈 350 × 10^-6）. These geochemical characteristics suggest S-type granites. The whole rock Sr- Nd isotopic analysis results show that the early intrusive granite exhibits higher （^87Sr/^86Sr） i than the late counter- parts （0. 715 6 -0.726 0 and 0. 710 5 - 0. 716 4, respectively）, but they exhibit the same variation of εNd（t） values （ ranging from - 10. 0 to - 7.0）. Zircon in-situ Lu-Hf isotopic analysis indicates their similarities of eHf（t） values （ -12. 9 - -7.7 and -15.1 - -10.5, respectively） and tCM（2. 0- 1. 7 Ga and 2. 1 -1.8 Ga, respective- ly）. The elemental and isotopic results suggest that the Zhulanbu composite pluton probably originated from the partial melting of a mixed crustal source involving the Paleoproterozoic metapelite and minor metagreywacke. The Fucheng composite pluton and the early intrusive granite of Zhulanbu composite pluton were formed in a compressive background of the Tethys tectonic domain during the Indosinian; in contrast, the late intrusive granite of Zhulanbu composite pluton was formed in the intraplate extensional environment of the paleo-Pacific tectonic domain during the Yanshanian period.