华北陆块南缘小秦岭地区早白垩世埃达克质花岗岩的LA-ICP-MS锆石U-Pb年龄、Hf同位素和元素地球化学特征

LA-ICP-MS zircon U-Pb ages,Hf isotopic composition and geochemistry of adakitic granites in the Xiaoqinling region,the south margin of the North China block

文峪和娘娘山花岗岩体位于华北陆块南缘小秦岭地区, 侵位于太古宇太华岩群中, 主要岩性为二长花岗岩。LA-ICP-MS锆石U-Pb定年结果显示, 文峪和娘娘山黑云母二长花岗岩体形成时间分别为（135±7） Ma和（139 ±4） Ma, 普遍含有大量继承锆石。两个岩体均属于具有高硅（SiO2 = 64.80%~73.30%）、高铝（Al2O3 = 14.24%~19.21%）、富碱（K2O ＋ Na2O = 7.66%~9.37%）、低镁（MgO = 0.21%~0.73%）特征的高钾钙碱性、准铝质到过铝质花岗岩（ACNK = 0.93~1.24）, 富集轻稀土元素、亏损重稀土元素（ΣLREE/ΣHREE = 13.7~21.8）。除文峪岩体第5期侵入体外, 两个岩体均具有弱Eu负异常-Eu正异常（δEu = 0.64~1.27, 平均0.82）, 富集Sr、Ba, 亏损Y、Yb。文峪晚期侵入的第5期侵入体具明显的Eu负异常（0.63~0.70, 平均0.66）, 中等的Sr含量, 亏损Y、Yb。地球化学特征表明, 除文峪岩体第5期侵入体外, 两个岩体均属于高Sr、低Yb含量的埃达克质花岗岩, 源区残留石榴子石而无斜长石残留, 形成于压力较高的加厚下地壳; 文峪第5期侵入体的低Sr、Y含量的特征表明源区除石榴子石外还有斜长石残留, 形成于加厚地壳向正常地壳减薄的环境。文峪岩体岩浆锆石的εHf（t） = -19.92~ -12.25, 两阶段Hf模式年龄为1961~2442 Ma, 反映物质来源以古元古代陆壳物质为主; 娘娘山岩体的岩浆锆石εHf（t）值, 除一个点为-8.08外, 其余点介于-29.74~ -20.18之间, 两阶段Hf模式年龄除一个点为1699 Ma外, 其余点主要集中于2460~3057 Ma之间, 物质来源以新太古代陆壳为主。两者的继承锆石的εHf（t）值和tDM2年龄也均表明其来源于古元古代和新太古代的地壳物质, 与太华杂岩解体的古元古代和新太古代两个部分时限一致, 也与华北克拉通主要的地壳增长历史基本一致。结合两岩体地球化学特征和区域地质演化历史, 认为文峪和娘娘山岩体形成于幔源岩浆底侵加热, 促使下地壳部分熔融。从早期侵入体到晚期侵入体, 反映了花岗岩岩浆活动的源区或岩浆房逐渐向浅部迁移, 及地壳从挤压加厚向伸展减薄转换的过程。

Wenyu and Niangniangshan granites in the Xiaoqinling region, Henan Province, is tectonically located in the south margin of the North China Craton and intruded into the Archean Taihua Group. The two granites mainly consist of monzogranite. LA-ICP-MS zircon U-Pb dating of Wenyu and Niangniangshan granites give weighted average ages of （135±7） Ma and （139±4） Ma, with abundant inherited zircon. The similar ages of these two granites indicate they belong to the same magmatic event. The two granites are roughly contemporaneous and geochemically similar to each other, i.e., both of them are high K calc-alkaline and metaluminous-peraluminous, enriched in silica （SiO2 = 64.80%~73.30%）, aluminum （Al2O3 = 14.24%~19.21%）, alkali （K2O ＋ Na2O = 7.66%~9.37%） contents and low MgO （0.21%~0.73%） content, ACNK values （0.93~1.24）, LREE, relatively depleted in HREE （ΣLREE/ΣHREE = 13.7~21.8）. Except the fifth intrusive stage of Wenyu granite, both granites are enriched in Sr, Ba, depleted in low Y, Yb, with slightly negative to positive Eu anomalies （δEu = 0.64~1.27, average 0.82）, suggesting adakitic affinity formed by partial melting of the thickened lower crust, with garnet residual and no plagioclase residual. The fifth intrusive stage of Wenyu granite have medium Sr and Ba contents, and low Y and Yb contents, with obvious negative Eu anomaly （δEu = 0.63~0.70, average 0.66）, formed during the transition from thickened crust to normal crust. The Wenyu granite show εHf（t） values ranging from -19.92 to -12.25 and tDM2 ages of 1961~2442 Ma, suggesting that the primary magma originated from Paleoproterozoic continental crust; while the εHf（t） values of the Niangniangshan granite range mainly from -29.74 to -20.18, with the exception of one point, -8.08. Correspondingly, the tDM2 ages range between 2460 Ma and 3057 Ma, with the exception of one 1699 Ma, suggesting that the primary magma originated from Late Archean continental crust. The εHf（t） values and tDM2 of their inherited zircons also indicate both granites were formed by Paleoproterozoic and Late Archean crustal material, which is consistent with crustal growth histroy of the North China craton. The geochemistry data suggest that both granites were originated from partial melting of the thickened lower crust resulting from underplating of mantle derived magma. The differences between early stage granite and the late stage granite reflect the process of the shallow migration of granitic magmatism source or magma chamber and the transformation of crustal tectonic regimesfrom compression and thickening to extension and thinning.