北大别早白垩纪花岗岩类的Sm-Nd和锆石Hf同位素及其构造意义  

Early Cretaceous granitoids in the North Dabie and their tectonic implications:Sr-Nd and zircon Hf isotopic evidences.

作　　者：续海金[1,2,3] 叶凯[3] 马昌前[1,2] 

机构地区：[1]中国地质大学地球科学学院,武汉430074 [2]中国地质大学地质过程与矿产资源国家重点实验室,武汉430074 [3]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2008年第1期87-103,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40603002;40334037和40472046);地质过程与矿产资源国家重点实验室开放基金项目(GPMR2007-1)资助

摘　　要：大别造山带产出两期早白垩纪造山后花岗岩类:早期(≈132Ma)变形的角闪石英二长岩和斑状二长花岗岩具高钾的类埃达克岩的地球化学特征,形成于增厚地壳(>50km)的下地壳部分熔融;晚期(≈128Ma)未变形的花岗岩包括细粒二长花岗岩和钾长花岗(斑)岩,属于正常安山岩-英安岩-流纹岩系列岩石,它们形成于相对薄的地壳(<35km)的下地壳部分熔融。早期变形的花岗岩类中,角闪石英二长岩的(^(87)Sr/^(86)Sr)_i=0.7066～0.7076,ε_(Nd)(t)=-20.3～-27.8,类似于扬子下地壳基性麻粒岩的Sr-Nd同位素特征,其中白垩纪岩浆锆石(≈132Ma)Hf同位素初始比值(ε_(Hf)(t))和两阶段Hf模式年龄(t_(DM2))分别为-29.30.5和303065Ma;我们认为角闪石英二长岩源于锆石Hf模式年龄约3.0Ga的基性下地壳部分熔融。早期斑状二长花岗岩的(^(87)Sr/^(86)Sr)_i=0.7078～0.7083,ε_(Nd)(t)=-15.8～-20.0,类似于北大别英云闪长质-花岗质片麻岩的Sr-Nd同位素特征,其白垩纪岩浆锆石(≈132Ma)的ε_(Hf)(t)和t_(DM2)分别为-24.80.5和274434Ma。我们认为宽状二长花岗岩源于约2.7Ga的中酸性下地壳部分熔融。晚期未变形的花岗岩的(^(87)Sr/^(86)Sr)_i=0.7069～0.7105,ε_(Nd)(t)=-16.4～-22.1,类似于北大别英云闪长质-花岗质片麻岩的Sr-Nd同位素特征;其白垩纪岩浆锆石(≈128Ma)的ε_(Hf)(t)具有较大的变化范围(-25.2～7.4),主要峰值为-22.5±0.5,次要峰值为-16.3±0.7,并有少量正的ε_(Hf)(t)值5.8～7.4,与ε_(Hf)(t)值对应的t_(DM2)分别为2600±40Ma、2211±68Ma和743±130Ma。我们认为晚期未变形的花岗岩源于锆石Hf模式年龄约2.6～2.2Ga的中酸性下地壳部分熔融,源区夹杂新元古代新生的陆壳。在大别造山带垮塌之前(>132Ma),增厚地壳(>50km)的下地壳存在双层结构:锆石Hf模式年龄约为3.0Ga的基性下地壳基底和约2.7Ga的中酸性(英云闪长质-花岗质)上部下地壳.在大别造山带伸展垮塌的晚期阶段(≈128Ma),减�Two stages of the early Cretaceous post-orogenic granitoids are identified in the Dabie orogen, eastern China, which recorded processes of the extensional collapse of the orogen. The early stage （≈132Ma） granitoids consists of hornblende quartz monzonites and porphyritic monzogranites, they are pervasively deformed and have high-K adakitic geochemical characteristics. Their eochemical features indicate that they were formed by partial melting of over-thickened （ 〉 50km） lower continental crust. The late stage （ ≈ 128Ma） undeformed granites consists of fine-grained monzogranites, fine-grained K-feldspar granites and coarse-grained K- feldspar granite-porphyry, which have no adakitic geochemical signatures. Their eochemical features indicate that they were produced by partial melting of the lower continental crust that already uplifted into much shallower levels （〈35km）. The initial 87Sr/86Sr ratios and end （t） values of the early stage foliated hornblende quartz monzonites range between 0. 7066 - 0. 7076 and - 20.3 - - 27.8, respectively, resembling that of the mafic granulites of the Yangtze lower continental crust. The cretaceous zircons have very negative initial Hf isotope compositions （ εHf（t） = - 29.3 ± 0.5 ） and very old two-stage Hf model ages （ tDM2 = 3030 ± 65Ma）. We suggest that the foliated hornblende quartz monzonites were derived from partial melting of the mafic lower continental crust with about 3.0Ga zircon Hf model ages. The initial 87Sr/86 Sr ratios and end （t） values of the early stage foliated porphyritic monzogranites range between 0.7078-0.7083 and -15.8 - -20.0, respectively, resembling those of the tonalitic to granitic gneisses in the North Dabie. Their Cretaceous zircons also have negative εHf （ t ） values of - 24. 8 ± 0. 5 and old tom of 2744 ± 34Ma. We regard that the foliated porphyritic monzogranites were derived from the partial melting of the felsic lower continental crust with about 2.7Ga zircon Hf model ages. These features su

关 键 词：早白垩纪 花岗岩类 锆石HF同位素 地壳结构 大别造山带 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]