乳山地区中生代花岗岩及捕虏体年代学、Hf同位素特征对胶东半岛地壳演化的启示

Hf Isotopic and Geochronological Characteristics of Mesozoic Granites and Xenoliths in Rushan Area and Its Implication on Crustal Evolution of Jiaodong Peninsula

花岗岩是大陆地壳主要组成部分,胶东半岛中生代花岗岩的出现是推演俯冲板片岩浆演化与构造演化的重要依据.因此,通过研究乳山地区出露的花岗岩及其捕虏体(变质基性岩),有助于更好的了解中生代胶东半岛的岩浆演化与地壳演化.该研究为胶东地区提供了新的主微量元素数据、U‐Pb和Lu‐Hf同位素数据.岩石地球化学表明黑云母二长花岗岩具有高钾钙碱性特征,相对贫钛、铁、锰、镁等元素,岩体有可能是分异程度较高的I型或者M型花岗岩.大离子亲石元素Ba和Sr明显富集,高场强元素Zr无明显亏损.斜长角闪岩SiO_(2)、TiO_(2)、Fe_(2)O_(3)T和MgO含量分别为48.9%、0.68%、12.64%和7.33%,为拉斑玄武岩成分,全碱ALK(K_(2)O+Na_(2)O)较低.大离子亲石元素Ba和Sr无明显富集,高场强元素Zr弱亏损,与石榴斜长角闪岩地球化学性质相似.锆石CL图像中花岗岩为岩浆锆石,斜长角闪岩为变质重结晶锆石.黑云母二长花岗岩锆石U‐Pb定年获得年龄为118.5±2.7 Ma,ε_(Hf)(t)值为−15.4~−27.7(Mean=−25.2±1.4),相应二阶段模式年龄(T_(DM2))为2.16~2.90 Ga,但大部分集中在~2.8 Ga.捕虏体斜长角闪岩锆石U‐Pb上交点年龄为1839±27 Ma,ε_(Hf)(t)值为0.5~5.1(Mean=3.23±0.74),相应的一阶段模式年龄(T_(DM1))为2.02~2.18 Ga.此外,念头村含榴花岗岩ε_(Hf)(t)值为−25.1~−27.1(Mean=−26.0±0.18),相应的二阶段模式年龄(T_(DM2))为2.75~2.87 Ga.其捕虏体含榴斜长角闪岩ε_(Hf)(t)值为3.7~4.4(Mean=3.93±0.21),相应的一阶段模式年龄(T_(DM1))为1.99~2.03 Ga.上述数据指示花岗岩为华北太古宙地壳重新熔融的产物;变基性岩属华北荆山群物质.因此,乳山地区中生代花岗岩及其捕虏体都具有华北板块的亲缘性.乳山地区模式年龄为太古宙的花岗岩暗示胶东半岛地壳演化的相关信息,并不具备拆沉作用产生岩浆岩的特点.捕虏体(变基性岩)可能为下地壳部分熔融后形成的新生地壳物质在短时间内携裹的产物.

Granite is the main component of continental crust,and the occurrence of Mesozoic granite in Jiaodong Peninsula is an important basis for deducing magmatic evolution and tectonic evolution of subduction plates.Therefore,the study of granite and its xenoliths(meta‐maficrocks)exposed in Rushan area is helpful to better understand the magmatic evolution and crustal evolution of Jiaodong Peninsula in Mesozoic.This study provides new data of major and traceelements,U‐Pb dataand Lu‐Hf isotopesfor Jiaodong Peninsula.Petro‐geochemistry shows that the biotite monzogranite has the characteristics of high potassium and calcium alkalinity,and is relatively poor in elements such as titanium,iron,manganese,and magnesium.The rock may be I‐type or M‐type granite with a high degree of differentiation.The large ion lithophile elements Ba and Sr are obviously enriched,and the high field strength element Zr has no obvious loss.The SiO_(2),TiO_(2),Fe_(2)O_(3)T,and MgO content of the amphibolite are 48.9%,0.68%,12.64%and 7.33%,respectively.It is a tholeiitic basalt composition,and the total alkali ALK(K_(2)O+Na_(2)O)is relatively low.The large ion lithophile elements Ba and Sr are not significantly enriched,and the high field strength element Zr is weakly depleted,which is similar to the geochemical properties of the garnet plagioclase amphibolite.The granite has magmatic zircon and the amphibolite contains metamorphic recrystallized zircon under zircon CL image.The zircon U‐Pb dating age of the biotite monzogranite is 118.5±2.7 Ma,and the value ofε_(Hf)(t)is−15.4 to−27.7(Mean=−25.2±1.4),and the corresponding two‐stage model age(T_(DM2))is 2.16 to 2.90 Ga,but most of them are concentrated in~2.8 Ga.The upper intersection age amphibolite(xenoliths)is 1839±27 Ma(zircon U‐Pb),and and the value ofε_(Hf)(t)is 0.5 to 5.1(Mean=3.23±0.74),corresponding to the one‐stage model age(T_(DM1))is 2.02 to 2.18 Ga.In addition,the value ofε_(Hf)(t)of the granite‐bearing granite in Niantou Village is−25.1 to−27.1(Mean=−26.0±0.18),and the corresponding two‐stage model age(T_(DM2))is 2.75 to 2.87 Ga.Theε_(Hf)(t)value of the xenoliths(garnet amphibolite)is 3.7 to 4.4(Mean=3.93±0.21),and the corresponding one‐stage model age(T_(DM1))is 1.99 to 2.03 Ga.The above data indicate that the granite is the product of Archean crust remelting in the Eastern North China Craton;the meta‐mafic rocks can be classified into the Jingshan Group of the North China Craton.Therefore,the Mesozoic granite and its xenoliths in the Rushan area have the same affiliation as the North China Craton.The granites with model ages of Archean age in the Rushan area suggest information about the crustal evolution of the Jiaodong Peninsula.It does not have the characteristics of magmatic rock produced by delamination.A short time after partial melting of the lower crust entrained,many ancient xenoliths(meta‐mafic rocks)were carried to the subsurface by re‐melting magma.