鲁西早白垩世岩石圈地幔的属性:大昆仑辉长岩和辉绿岩年代学、岩石地球化学和Sr-Nd-Pb-Hf同位素制约

Nature of the Early Cretaceous lithospheric mantle in western Shangdong: Constraints from geochronology,geochemistry and Sr-Nd-Pb-Hf isotopic data of Dakunlun gabbros and diabases

大昆仑辉长岩和辉绿岩锆石LA-ICP-MS U-Pb年代学、锆石Hf同位素和全岩地球化学及全岩Sr-Nd-Pb同位素的研究为探讨鲁西地区早白垩世岩石圈地幔的属性提供了制约。结果表明,大昆仑辉长岩中少量锆石和辉绿岩中锆石发育条痕状吸收,具有相对高的Th/U比值(0.11~3.12),暗示它们均为岩浆成因,辉长岩中多数为岩浆成因的捕获锆石。辉长岩和辉绿岩中岩浆锆石的^(206)Pb/^(238)U加权平均年龄分别为132±3Ma和112±1Ma;辉长岩中捕获锆石的谐和年龄主要介于2469~1752Ma之间,暗示古老地壳物质的存在。辉长岩和辉绿岩整体具有低Si O_2(49.08%~52.61%)、高MgO(4.52%~9.67%)、Na_2O含量(2.58%~3.49%)以及高Mg~#值(45~65)特征;富集轻稀土元素和大离子亲石元素(Rb、Ba、Th)、贫重稀土元素和高场强元素(Nb、Ta、Zr和Hf),其(La/Yb)_N=8.98~10.2,Eu负异常不明显(δEu=0.83~0.93)。辉长岩的初始^(87)Sr/^(86)Sr比值变化于0.7046~0.7060之间,ε_(Nd)(t)值介于-16.71^-15.54之间,^(206)Pb/^(204)Pb、^(207)Pb/^(204)Pb和^(208)Pb/^(204)Pb初始比值分别为16.596~16.776、15.255~15.290和36.328~36.648。辉绿岩中岩浆锆石的ε_(Hf)(t)值介于-12.0^-2.5之间。上述结果表明,大昆仑辉长岩和辉绿岩形成于早白垩世,与华北克拉通东部早白垩世岩石圈减薄的峰期时间一致;起源于受华北克拉通古老下地壳物质改造的富集岩石圈地幔的部分熔融,形成于岩石圈减薄的伸展构造背景。

The LA-ICP-MS zircon U-Pb dating,Hf isotope,whole-rock geochemical and Sr-Nd-Pb data are reported for the Dakunlun gabbros and diabases in western Shandong to understand their nature of the Early Cretaceous lithospheric mantle.Few zircons from Dakunlun gabbros and the zircons from diabases all display striped absorption in cathodoluminescence images.It also indicates a magmatic origin that the high Th/U ratio（0.11~3.12）.Moreover,most of zircons in the gabbros are captured and magmatic origin.The weighted mean^（206）Pb/^（238）U ages of the magmatic zircons are 132±3Ma and 112±1Ma in the gabbros and diabases,respectively,implying an Early Cretaceous in age.The concordia ages of the captured zircons range from 2469Ma to 1752Ma,suggesting the existence of ancient crustal material.Geochemically,the Dakunlun gabbros and diabases are characterized by low Si O_2（49.08%~52.61%）,high MgO（4.52%~9.67%）,Na_2O（2.58%~3.49%）and high Mg~#（45~65）.Moreover,they are enriched in light rare earth elements and large ion lithophile elements（e.g.,Rb,Ba,Th）,and are depleted in heavy rare earth elements and high field strength elements（e.g.,Nb,Ta,Zr,and Hf）.They have（La/Yb）_N=8.98~10.2 and weak negative Eu anomaly（δEu=0.83~0.93）.The（^（87）Sr/^（86）Sr）_iratios vary between 0.7046 and 0.7060 and theε_（Nd）（t）vary between-16.71 and-15.54 in the gabbros.Whole-rock initial^（206）Pb/^（204）Pb,^（207）Pb/^（204）Pb and^（208）Pb/^（204）Pb ratios of the gabbros range from 16.596 to 16.776,15.255 to15.290 and 36.328 to 36.648,respectively.Theε_（Hf）（t）values range from-12.0 to-2.5 in the magmatic zircons from diabases.The above data show that the Early Cretaceous Dakunlun gabbros and diabases are consistent with the period of lithospheric thinning beneath the eastern North China Craton.The primary magma was derived by partial melting of the enriched lithospheric mantle modified by the ancient lower crustal material.They were formed in an extensional tectonic environment related to the lithospheric thinning.