北苏鲁超高压变质带中斜长角闪岩的成因  

Genetic mechanism of garnet-bearing amphibolite in the north Sulu ultrahigh-pressure(UHP) metamorphic belt Acta Petrologica Sinica

作　　者：刘福来[1] 薛怀民[1] 刘平华[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2009年第7期1575-1586,共12页Acta Petrologica Sinica

基　　金：国家杰出青年基金项目(40725007);国家自然科学基金重大项目(40399143);中国地质调查局地质大调查项目(1212010811065)资助成果

摘　　要：在北苏鲁超高压变质带的成海地区的正片麻岩中,存在大量规模不一的含石榴石斜长角闪岩的透镜体或不规则团块。锆石中矿物包体的激光拉曼测试、阴极发光图像分析、稀土元素和微量元素以及Lu-Hf同位素的LA-(MC)-ICP-MS测试等综合研究结果表明,含石榴石斜长角闪岩(WH17)只存在一种成因类型的锆石,即变质锆石。该类锆石自核部到边部均保存了典型的超高压包体矿物组合:柯石英(Coe)+石榴石(Grt)+绿辉石(Omp)+多硅白云母(Phe)+金红石(Rt)+磷灰石(Ap),相应的阴极发光图像自核部到边部十分均匀,具有典型变质锆石的特点。锆石核部和边部的稀土元素含量特征和配分模式也十分相似,主要表现为轻稀土元素相对亏损,而重稀土元素相对平坦,无Eu异常(Eu/Eu~*=0.94～1.04),具较明显的正Ce异常(Ce/Ce~*=83.4～111.0),低的Th/U(<0.02)和Lu/Hf(<0.00010)比值以及极低的Th(1×10^(-6)～4×10^(-6))含量。锆石核部到边缘的重稀土元素亏损与石榴石稳定有关,而无Eu异常则与斜长石分解关系密切。上述特征表明,含石榴石斜长角闪岩中的锆石均是在超高压变质阶段形成的变质锆石,而研究样品中继承性岩浆结晶锆石的缺乏,是由于含石榴石斜长角闪岩在原岩形成时Zr的成分明显匮乏所致。SHRIMP U-Pb定年结果表明,含柯石英锆石自核部到边部记录了十分一致的^(206)Pb/^(238)U年龄,变化于240.2±3.3Ma到220.2±2.8Ma之间,加权平均年龄为229.8±2.0Ma,这组年龄与前人对苏鲁超高压变质带中榴辉岩、正片麻岩、副片麻岩和大理岩等含柯石英锆石微区记录的年龄完全吻合,应代表苏鲁地体超高压变质时代。Lu-Hf同位素研究结果表明,含石榴石斜长角闪岩中变质锆石的Hf同位素特征与苏鲁超高压变质带中榴辉岩的继承性岩浆锆石和变质锆石均存在本质差别,其中^(176)Lu/^(177)Hf=0.00001～0.00003,^(176)Hf/^(177)Hf_((t))=0.282052～0.282107,�In this study, we link mineral inclusion assemblage, trace element analyses, U-Pb SHRIMP age and Lu-Hf isotope composition obtained from coesite-bearing metamorphic zircons to unravel the origin and metamorphic evolution of garnet-bearing amphibolites from the north Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from garnet-bearing amphibolite (WH17) are characterized by homogeneous luminescence in cathodoluminescence (CL) images. These newly grown zircons contain a coesite eclogite-facies mineral inclusion assemblage of Coe + Grt + Omp + Phe + Rt + Ap from the core until rim, and are characterized by low Th/U ratios (<0.02) and Lu/Hf (<0.00010) and pronounced low contents of Th (1 x 10(-6) similar to 4 x 10(-6)). The REE patterns are remarkably flat for the HREE with no Eu anomaly (Eu/Eu* = 0.94 similar to 1.04). Depletion of HREE indicates coesite-bearing zircon growth (luring or after the crystallization of garnet, whereas the no Eu anomaly is compatible with the release of Eu(2+) due to the break-down of plagioclase. Both the mineral inclusions and the trace element systematics of coesite-bearing zircons are best interpreted as being UHP metamorphic in origin. The coesite-bearing zircon cores and rims yield consistent SHRIMP (206)Pb/(238)U ages of 240.2 +/- 3.3Ma similar to 220.2 +/- 2.8Ma with a weighted mean of 229.8 +/- 2.0Ma, interpreted as the age of coesite eclogite-facies metamorphism for the Sulu UHP terrane. Based on the LA-(MC)-ICP-MS analyses, the coesite-bearing zircons are characterized by pronounced low (176)Lu/(177)Hf (0.00001 similar to 0.00003). The (179)Hf/(177)Hf((t)) ratios range from 0.282052 to 0.282107, with epsilon(Hf) (t) of - 21.1 to -18.8 and t(DM2) ages of 2.14 to 2.24Ga. The Hf contents of the studied coesite-bearing zircons are very similar to those of the coesite-bearing zircon domains from orthogneisses surrounding the garnet-bearing amphibolite (WH17) outcrop, implying a significant input of Zr and Hf contents from the country rocks to the garnet

关 键 词：锆石 含石榴石料长角闪岩 U-PB SHRIMP定年 LU-HF同位素 威海 北苏鲁超高压地体 

分 类 号：P588.34[天文地球—岩石学] P597.3[天文地球—地质学]