赞比亚索卢韦齐地区新元古代石英二长岩的成因:年代学、地球化学和Sr-Nd-Hf同位素约束

Petrogenesis of Neoproterozoic Quartz Monzonite in Solwezi Region,Zambia:Constraint from Geochronology,Geochemistry and Sr-Nd-Hf Isotopes

研究卢菲利安弧地区新元古代与裂谷作用有关的基性–中酸性岩浆作用,对了解区域地壳生长和演化具有重要意义。研究表明,卢菲利安弧地区发育有大量新元古代与裂谷作用有关的基性岩类,但相关的中酸性岩岩浆作用却鲜有报道。笔者首次在赞比亚索卢韦齐地区发现有新元古代的石英二长岩体,锆石U–Pb年龄为(707.1±3.0)Ma。地球化学特征显示该岩体具有较低的MgO(0.46%~0.76%)、CaO(1.63%~1.76%)、K2O(0.49%~0.56%)、Mg#值(8~13)和Sr/Y值(1.14~2.50),较高的Al_(2)O_(3)(15.61%~16.02%)。岩体富集轻稀土和高场强元素HFSEs(Nb、Ta、Hf),(La/Yb)N值为6.64~7.86,亏损P、Ti、Zr和大离子亲石元素LILEs(Rb、Ba、Sr、K)。此外,石英二长岩具有低的初始87Sr/86Sr值(0.7058~0.7060),正的εNd(t)值(1.89~2.03)和锆石εHf(t)值(1.30~5.67),该特征与索卢韦齐地区新元古代早期辉长岩相似,推测石英二长岩可能为新生的镁铁质下地壳在中–低压条件下部分熔融形成的。综合地质年代学和岩石成因研究,笔者认为卢菲利安弧地区在新元古代经历了多阶段的地壳生长作用,后期侵位的地幔岩浆加热早期就位于下地壳的镁铁质岩石并导致其部分熔融,从而达到对地壳的改造作用。

The study of mafic–intermediate and felsic magmatism related to Neoproterozoic rift in the Lufilian Arc is of great significance for understanding the crustal growth and secular evolution of the region.Studies have shown that there are a large number of Neoproterozoic mafic rocks which are related to rifting in the Lufilan arc,but a few of related intermediate and felsic magmatism are discovered.A Neoproterozoic quartz monzonite with a zircon U–Pb age of 707.1±3.0 Ma was first discovered and reported in the Lufilian Arc.The plu-ton is characterized by relatively low MgO(0.46%~0.76%),CaO(1.63%~1.76%),K2O(0.49%~0.56%),Mg#values(8~13)and Sr/Y ratios(1.14~2.50),as well as high Al_(2)O_(3) content(15.61%~16.02%).REE-normal-ized patterns show enrichment in LREE with(La/Yb)N of 6.64~7.86 and their primitive mantle-normalized trace element patterns are characterized by depletion of LILEs(Rb,Ba,Sr,K)and P,Ti,Zr and enrichment of HFSEs(Nb,Ta,Hf).They have a low initial 87Sr/86Sr ratios(0.7058~0.7060)with positiveεNd(t)values(1.89~2.03)and their zirconεHf(t)values range from 1.30 to 5.67,their isotopic data are similar to those of the Neoproterozoic mafic intrusions in the Solwezi region,suggesting that the quartz monzonite were generated by partial melting of newly emplaced mafic lower crust.In combination with the studies of geochronology and pet-rogenesis,it is concluded that the Lufilian Arc experienced a multi-stage crustal growth in the Neoproterozoic,the late intrusive mantle magma heated the mafic rocks emplaced in the lower crust at early stage,resulting in partial melting and reworking the early crust.