桂岭岩体及其暗色微粒包体的岩石成因和地质意义

Petrogenesis and Significance of the Guiling Pluton and Its Mafic Microgranular Enclaves

文章通过桂岭岩体及其暗色微粒包体的岩石学和地球化学研究,探讨了南岭西段早古生代壳幔相互作用及其构造背景。研究结果表明,桂岭岩体主要岩石类型为黑云母二长花岗岩,其SiO_(2)含量为70.24%~75.13%,全碱含量为7.95%~8.44%;(La/Yb)_(N)值为5.0~8.4,具有轻稀土富集、重稀土亏损和明显负Eu异常(δEu=0.37~0.58)的稀土元素组成特征;铝饱和指数、K_(2)O含量、(^(87)Sr/^(86)Sr)_(i)值和较高的Rb/Sr值指示其为准铝—弱过铝质高钾钙碱性系列高分异S型花岗岩。暗色微粒包体SiO_(2)含量为48.38%~51.95%,镁、铁氧化物含量明显高于寄主岩体,Mg#值为59~67,全碱含量为3.81%~5.05%,为中钾钙碱性系列辉长岩和辉长闪长岩;轻重稀土分异明显,(La/Yb)_(N)值为11.74~12.12,负Eu异常不明显(δEu=0.92~0.94)。在Sr-Nd同位素组成上,寄主岩体具有较低的ε_(Nd)(t)值(-13.6~-5.0),T_(DM)^(2)=1.6~2.3 Ga;暗色微粒包体的(^(87)Sr/^(86)Sr)_(i)值变化范围较小(0.706~0.708),ε_(Nd)(t)值相对较高(3.4~3.6),T_(DM)^(2)=0.88~0.89 Ga。综合岩石学和岩石地球化学特征,文章研究认为暗色微粒包体源区为新元古代底侵到下地壳底部的岩石圈地幔组分,该组分在早古生代后碰撞构造环境下发生高度熔融形成基性岩浆,基性岩浆侵位引发中元古代中、下地壳变质杂砂岩部分熔融形成花岗质岩浆并上侵定位形成桂岭岩体。侵位过程中,基性岩浆与花岗质岩浆发生强烈的机械混合和低程度的化学混合作用形成暗色微粒包体。

This paper reports petrological and geochemical studies of the Guiling pluton and its mafic microgranular enclaves(MMEs)and discusses the early Paleozoic crust-mantle interaction and tectonic setting in the western Nanling Mountains.The results show that the Guiling pluton is mainly composed of the biotite monzonitic granite,with silica content of 70.24%-75.13%and total alkali content of 7.95%-8.44%.Chondrite-normalised rare-earth element(REE)of the granite shows light REE enrichment(with(La/Yb)_(N)=5.0-8.4)and significant negative europium anomalies(with δEu=0.37-0.58).The A/CNK index of the granite shows metaluminous-weak peraluminous characteristics.K_(2)O content,(^(87)Sr/^(86)Sr)_(i) values and high Rb/Sr values indicate that it is highly fractionated S-type granite of high potassium calc-alkaline series.The MMEs have lower SiO_(2) content(48.38%-51.95%)and higher contents of Mg and Fe oxides than the host granite,with Mg#values of 59-67 and total alkali content of 3.81%-5.05%,indicating that they are gabbro and gabbroic diorite of medium potassium calc-alkaline series.The light and heavy REEs of the MMEs have significant fractionation(with(La/Yb)_(N)=11.74-12.12)and only slight negative europium anomalies(withδEu=0.92-0.94).On the characteristics of Sr-Nd isotopic composition,the host granite has lower ε_(Nd)(t)values(-13.6--5.0)than the MMEs(3.4-3.6),and the crustal model age(1.6-2.3 Ga)of the granite is older than that(0.88-0.89 Ga)of the MMEs which show slight variation(0.706-0.708)of(^(87)Sr/^(86)Sr)_(i) values.The geochemical obtained data in this study suggest that the MMEs were derived from mafic material that was likely associated with the Neoproterozoic lithospheric mantle underplating.Highly partial melting of the material caused by post-collisional extension during the Early Paleozoic time generated mafic magmas.Ascent and emplacement of the mafic magmas triggered partial melting of the Mesoproterozoic metagreywacke at the middle and lower crust levels,producing voluminous granitic magmas that were emplaced to form the Guiling pluton.During ascent and emplacement of the granitic magmas,the mafic magmas were intensively mingled and moderately chemically mixed with the host granitic magmas forming MMEs throughout the pluton.