蚌埠隆起区古元古代钾长花岗岩的成因:岩石地球化学、锆石U-Pb年代学与Hf同位素的制约

Petrogenesis of the Paleoproterozoic K-Feldspar Granites in Bengbu Uplift:Constraints from Petro-Geochemistry,Zircon U-Pb Dating and Hf Isotope

对蚌埠隆起区庄子里和磨盘山钾长花岗岩进行了系统的年代学和地球化学以及锆石Hf同位素的研究,以便对其岩石成因进行约束.研究结果表明,庄子里和磨盘山钾长花岗岩中锆石发育震荡生长环带,且具有较高的Th/U比值(0.13～1.47),反映了岩浆成因特征.对庄子里和磨盘山钾长花岗岩中岩浆锆石进行的LA-ICP-MSU-Pb定年结果(上交点年龄)分别为2104±20Ma和2196±190Ma,这表明蚌埠隆起区钾长花岗岩的形成时代为古元古代.钾长花岗岩的SiO2和K2O含量分别介于69.65%～77.95%和4.98%～5.17%之间;该类岩石富集轻稀土元素和Zr、Hf、Rb、Th、U等元素,明显亏损Ba、Sr、Eu、P和Ti等元素;它们的εNd(t)值变化于-3.4～+3.2之间,Nd的模式年龄变化于2.31～2.79Ga之间;钾长花岗岩中锆石的εHf(t)值和Hf同位素两阶段模式年龄分别介于-5.1～+7.8和2.26～2.83Ga之间.上述特征表明,蚌埠隆起区钾长花岗岩的原始岩浆起源于有少量古老地壳物质涉入的新生下地壳的部分熔融.庄子里和磨盘山钾长花岗岩为A型花岗岩,形成于伸展的构造背景.

The study on the chronology, geochemistry and zircon Hf isotopes of the Zhuangzili and Mopanshan K-feldspar granites in Bengbu uplift was carried out in order to constrain their petrogenesis Zircons from the intrusions display oscillatory zoning and high Th/U ratios（0. 13-1.47）, implying their magmatic origin. Zircon LA-ICP-MS, U-Ph dating results of the Zhuangzili and Mopans-han K-feldspar granites indicate that they formed in Palcoproterozoic with the upper intercept ages of 2104±20 Ma and 2196±190 Ma, respectively. Their SiO2 and K2O contents range from 69.65 to 77.95% and 4.98 to 5.17%, respectively. Moreover, the K-feldspar granites are enriched with light rare earth dements as well as Zr, Hf, Rb, Th, and U, while markedly depleted in Ba, Sr, Eu, P and Ti. Their εNd（t） values vary from-3.4 to ＋3.2 and Nd model ages range from 2.31 to 2.79 Ga. Their initial Hf isotope ratios and Hf two-stage model ages range from-5.1 to ＋7.8 and 2.26 to 2.83 Ca, respectively. Taken together, it is suggested that the primary magma for the K-feldspar granites could be mainly derived from partial melting of the Paleoproterzoic and Neoarchean juvenile crust and that a small amount of ancient crustal material was involved in their origin. Chemically, these K-feldspar granites are A-type ones, implying that they formed under an extensional tectonic setting.