桂岭岩体及其暗色微粒包体的岩石成因和地质意义

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SiO2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(δE...

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

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. K₂O content, (⁸⁷Sr/⁸⁶Sr)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 SiO₂ 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 (with δEu=0.92-0.94). On the characteristics of Sr-Nd isotopic composition, the host granite has lower εNd(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 (87Sr/86Sr)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.