华北地块中部早白垩世富碱侵入岩的成因:锆石U-Pb年代学和Sr-Nd-Hf同位素制约

Petrogenesis of the Early Cretaceous alkali-rich intrusive rocks in the central North China Block: Constraints from zircon U-Pb chronology and Sr-Nd-Hf isotopes

本文对华北地块中部早白垩世富碱侵入岩:山西的二峰山霓辉二长岩、塔儿山二长闪长斑岩、狐偃山石英二长岩、万荣石英二长闪长岩和河南的黄龙垴霓辉正长岩、水冶霓霞正长岩、东冶角闪闪长岩进行了岩石地球化学和Sr-Nd同位素分析以及4个侵入岩中锆石LA-ICP-MS U-Pb年代学和Hf同位素的测试。结果表明,二峰山、狐偃山、万荣和黄龙垴侵入岩中锆石有两种,多数为自形-半自形晶、内部结构均匀、发育震荡环带和条纹状吸收以及具有相对高Th/U比值(0.13~4.40)的岩浆锆石,其206Pb/238U加权平均年龄分别为130±1Ma、130±3Ma、127±1Ma和127±2Ma,即富碱侵入岩形成于早白垩世;少量为继承锆石,发育核-边结构,核部呈浑圆状和弱的震荡环带,给出了三叠纪(207~226Ma)和古元古代-新太古代(1791Ma、1979Ma、2088Ma、2370Ma和2559Ma)的年龄;岩石的SiO 2=55.33%~65.00%、MgO=0.21%~7.66%、Mg#=19~62、(Na2O+K2O)=5.89%~14.38%,属于弱碱性-过碱性岩石;富集大离子亲石元素(Rb、Ba、Sr)和轻稀土元素,亏损高场强元素(Nb、Ta、Ti)和重稀土元素,具有明显的Pb正异常和弱的Eu异常;其87Sr/86Sr初始值多数介于0.7049~0.7062之间,εNd(t)值变化于-20.1^-13.3之间,岩浆锆石的Hf同位素组成为εHf(t)=-28.9^-17.4。上述结果结合符山高镁闪长岩及其中地幔橄榄岩包体的研究暗示,华北地块中部早白垩世富碱侵入岩起源于受拆沉陆壳物质熔融的熔体改造的富集岩石圈地幔物质的部分熔融,其形成于古太平洋板块西向俯冲动力学背景下的岩石圈伸展环境。

LA-ICP-MS zircon U-Pb dating and Hf isotopic data,whole-rock geochemical and Sr-Nd isotopic data have been determined for the Early Cretaceous alkali-rich intrusive rocks in central North China Block（ NCB）,including Erfengshan aegirineaugite monzonite, Taershan monzodioritic porphyry, Huyanshan quartz monzonite, Wanrong quartz monzodiorite in the Shanxi Province,and Huanglongnao aegirine-augite syenite,Shuiye aegirine-augite nepheline syenite in the Henan Province. Most zircons from these alkali-rich intrusive rocks are euhedral-subhedral,they display clear oscillatory zoning in CL imagines and high Th / U ratios of 0. 13 ～ 4. 40,which indicate magmatic origin; these zircons yield weighted mean206 Pb /238 U ages of 130 ± 1Ma,130 ± 3Ma,127 ±1Ma,and 127 ± 2Ma,suggesting that these rocks were emplaced in the Early Cretaceous. Additionally,several inherited zircon with core-rim textures yield Triassic（ 207 ～ 226Ma） and Paleoproterozoic to Neoarchean ages（ 1791 Ma,1979Ma,2088 Ma,2370Ma,and2559Ma）. Geochemically,these intrusive rocks belong to alkaline series with 55% ～ 65% SiO 2,0. 2% ～ 7. 7% MgO and 5. 9% ～14. 4%（ Na2 O ＋ K2O）. They are enriched in large ion lithophile elements（ e. g.,Rb,Ba,and Sr） and light rare earth elements,depleted in high field strength elements（ e. g.,Nb,Ta,and Ti） and heavy rare earth elements,and display prominent positive Pb anomalies and weakly Eu anomalies. Isotopically,the（87Sr /86Sr）ivalues of these intrusive rocks range from 0. 7049 to 0. 7062,εNd（ t） values from- 20. 1 to- 13. 3,and εHf（ t） values from- 28. 9 to- 17. 4. Our combined data,combined with the study on the Fushan high-Mg#diorites and the mantle peridotite xenoliths,imply that the Early Cretaceous alkali-rich intrusive rocks were derived from partial melting of the enriched lithospheric mantle modified by the delaminated lower continental crust,geodynamically related to extensional environment in response to the westward subduction of the Paleo-Pacific plate.