CAJ | 학술논문

哀牢山带内零星发育一些变基性岩,如斜长角闪岩、变辉长岩和变辉绿岩等,它们主要呈似层状、透镜状、豌豆状或碎片状分布于哀牢山群中。这些变基性岩对研究扬子板块西南缘的大地构造属性具有重要意义。目前关于它们的形成时代、岩石成因及其大地构造意义尚不明确。本文对采自哀牢山群中的斜长角闪岩进行了系统的地球化学分析和锆石 U-Pb 年代学测试。岩石地球化学分析表明,斜长角闪岩的 SiO2含量为46.85%~49.03%,具有较高的 FeOt(11.56%~13.09%)、MgO(6.83%~11.57%)和TiO2(1.71%~2.22%)含量,样品的全碱(K2O+Na2O)含量为1.61%~2.26%, Na2O/K2O比值为1.88~9.25。样品相对富集轻稀土元素,轻、重稀土元素分馏程度相对较强((La/Yb)N=6.55~7.52),稀土总量为112~130μg/g。样品的Sr、P呈负异常,无明显Eu异常(δEu=0.97~1.03)；Nb、Ta亏损不明显, Nb含量变化于14.28~18.43μg/g,与世界上一些典型的富Nb玄武岩类似。定年结果显示斜长角闪岩的形成年龄为814±12 Ma,表明哀牢山带在新元古代时期(~814 Ma)存在基性岩浆活动。综合分析认为该富Nb岩石形成于弧后盆地环境,初步认为其岩石成因与新元古代俯冲体制下导致的弧后拉张作用有关。
애뢰산대내령성발육일사변기성암,여사장각섬암、변휘장암화변휘록암등,타문주요정사층상、투경상、완두상혹쇄편상분포우애뢰산군중。저사변기성암대연구양자판괴서남연적대지구조속성구유중요의의。목전관우타문적형성시대、암석성인급기대지구조의의상불명학。본문대채자애뢰산군중적사장각섬암진행료계통적지구화학분석화고석 U-Pb 년대학측시。암석지구화학분석표명,사장각섬암적 SiO2함량위46.85%~49.03%,구유교고적 FeOt(11.56%~13.09%)、MgO(6.83%~11.57%)화TiO2(1.71%~2.22%)함량,양품적전감(K2O+Na2O)함량위1.61%~2.26%, Na2O/K2O비치위1.88~9.25。양품상대부집경희토원소,경、중희토원소분류정도상대교강((La/Yb)N=6.55~7.52),희토총량위112~130μg/g。양품적Sr、P정부이상,무명현Eu이상(δEu=0.97~1.03)；Nb、Ta우손불명현, Nb함량변화우14.28~18.43μg/g,여세계상일사전형적부Nb현무암유사。정년결과현시사장각섬암적형성년령위814±12 Ma,표명애뢰산대재신원고대시기(~814 Ma)존재기성암장활동。종합분석인위해부Nb암석형성우호후분지배경,초보인위기암석성인여신원고대부충체제하도치적호후랍장작용유관。
Some small-sized outcrops of metabasic rocks (e.g., amphibolite, metagabbro and metadiabase) occur mainly as lens, pods and fragments in the Ailaoshan Complex along the Ailaoshan zone, which were previously mapped as Proterozoic basement. These rocks provide possibility for better understanding the tectonic setting of the SW margin of the Yangtze Block in South China, however, their formation time, petrogenesis and tectonic implications are poorly documented so far. The petrological, geochemical and zircon U-Pb geochronological data for the plagioclase amphibolites in the Ailaoshan Complex are reported in this paper. Geochemical analyses show that the amphibolites have SiO2 contents ranging from 46.85% to 49.03% and with relatively high FeOt (11.56%~13.09%), MgO (6.83%~11.57%) and TiO2 contents (1.71%~2.22%), K2O+Na2O vary from 1.61%to 2.26%with Na2O/K2O ratios of 1.88~9.25. The rocks are characterized by enriched LREE ((La/Yb)N=6.55~7.52) and Nb (Nb=14.28~18.43 μg/g). On primitive mantle-normalized trace element diagrams, the rocks show no obvious Nb and Ta anomalies and significant Sr-P depletion, displaying geochemical characteristics similar to those of the Yunkai (South China) back-arc basin basalt. Zircon U-Pb dating of the amphibolite vields a mean age of 814±12 Ma. In conjunction with other geological observations, it is proposed that there was an Early Neoproterozoic (~814 Ma) arc-back-arc setting along the Ailaoshan zone in the SW margin of the Yangtze Block.