鄂东南地区鄂城岩体的时代、成因及其对成矿作用的指示

鄂城岩体位于鄂东南地区的最北部，是鄂东南地区的六大岩体之一.在该岩体的南缘接触带上产出了长江中下游地区最大的矽卡岩型铁矿床——程潮铁矿床.众多研究表明，程潮铁矿化与鄂城杂岩体的岩浆演化密切相关，然而目前对于成矿作用究竟是与花岗质岩还是闪长质岩有关仍存在争议.通过对鄂城杂岩体开展系统的锆石U-Pb年代学、元素地球化学和Sr-Nd-Hf同位素研究，结果表明该岩体主要由花岗岩、石英二长岩、花岗斑岩以及小面积的闪长岩组成，最早侵位于140±1 Ma（中粒闪长岩），之后依次侵位形成了细粒闪长岩（132±2 Ma）、花岗斑岩（130±2 Ma）、花岗岩（中细粒花岗岩129±2 Ma，中粒花岗岩129±1 Ma）和石英二长岩（129±1 Ma）.根据全岩地球化学特征，鄂城杂岩体的岩石组成大致可以分为两组:（1）花岗岩类，包括花岗岩、花岗斑岩和角闪石英二长岩，钾质，具有高SiO₂，低TiO₂、FeO_t、MnO、MgO含量等特征；（2）闪长岩类，包括中、细粒闪长岩，钠质，具有低SiO₂，高TiO₂、FeO_t、MnO、MgO含量等特征.这些岩石均富集轻稀土元素（LREE）和大离子亲石元素（LILE，如Rb、Th等），亏损高场强元素（HFSE，如Nb、P、Ti）等，且花岗岩类具明显的负Eu异常，而闪长岩类则无此特征.在同位素组成方面，鄂城花岗岩类具有较负的全岩ε_Nd（t）值（-11.7~-10.1）和锆石ε_Hf（t）值（-22.91~-9.83），闪长岩类则具有稍高的全岩ε_Nd（t）值（-7.6）和锆石ε_Hf（t）值（-12.04~-4.69）.元素和同位素地球化学特征共同表明，鄂城花岗岩类属于高分异Ⅰ型花岗岩，且主要来源于古元古代基底物质的部分熔融作用，源区可能有少量幔源物质的加入；闪长岩类主要来源于富集岩石圈地幔，且经历了一定的分离结晶作用.年代学结果显示，鄂城花岗岩类和细粒闪长岩的侵位时间均与程潮铁矿床的主成矿期吻合.结合野外接触关系以及前人的研究，程潮铁矿化可能与上述两类岩石均密切相关.从整个鄂东南地区的成矿作用来看，随着岩浆源区壳源物质贡献的增大以及岩浆分异程度的增加，岩浆作用与铁矿化的关系也更加密切. 

Age and Petrogenesis of the Echeng Intrusion in Southeastern Hubei Province: Implications for Iron Mineralization

The Echeng pluton is one of the six large plutons in southeastern Hubei Province, which is located in the northernmost of the region. The Chengchao iron deposit, the largest skarn iron deposit in the Middle-Lower reaches of Yangtze River belt, has occurred in the southern contact zone of the Echeng pluton. Many researches have showed that the Chengchao iron mineralization was closely connected with the magma evolution of the Echeng complex, whereas whether the mineralization is related to granitic rocks or dioritic rocks is still controversial. In this paper, we present U-Pb age, geochemical, and Sr-Nd-Hf isotopic data to provide constraints on the petrogenesis of the Echeng intrusion and discuss the relationship of magma evolution and iron mineralization. The Echeng complex mainly consists of granite, quartz monzonite, granite porphyry and small-scale diorite. LA-ICP-MS zircon U-Pb data indicates that the Echeng complex began to emplace at 140±1 Ma by medium-grained diorite phase, subsequently fine-grained diorite (132±2 Ma), granite porphyry (130±2 Ma), granite including medium-fine grained (129±2 Ma) and medium-grained (129±1 Ma) phases and quartz monzonite (129±1 Ma) were emplaced in order. According to geochemical characteristics, all the rocks can broadly be divided into two groups:(1) granitoids including granite, granite porphyry, hornblende quartz monzonite are potassic and are characterized by high SiO₂, low TiO₂, FeO_t, MnO, MgO concentrations; (2) diorite including fine and medium grained diorites, are sodic and are characterized by low SiO₂, high TiO₂, FeO_t, MnO, MgO concentrations. These rocks are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILE) such as Rb, Th, and depleted in high field strength elements(HFSE) such as Nb, P and Ti. Moreover, the Echeng granitoids show obviously negative Eu anomalies but the diorites have no such a signature. The Echeng granitoids have more negative ε_Nd(t) values ranging from -11.7 to -10.1 and zircon ε_Hf(t) values ranging from -22.91 to -9.83, and the diorites have relatively high ε_Nd(t) value (-7.6) and zircon ε_Hf(t) values ranging from -12.04 to -4.69. The geochemical and isotopic data suggests that the Echeng granitoids belong to highly fractionated I-type granites and were mainly derived from partial melting of the Paleoproterozoic basement, possibly with the addition of a small amount of mantle-derived materials, whereas the diorites were mainly originated from the enriched lithospheric mantle, followed by a certain degree of crystallization fractionation. The new geochronological data suggests that the Echeng granitoids and fine-grained diorite broadly coeval with the main mineralization of the Chengchao iron deposit. Combining with field contact relationship and previous research results, the Chengchao iron mineralization was probably related to the Echeng granitoids and fine-grained diorite. Based on the mineralizations in southeastern Hubei Province, magmatism is more closely related to iron mineralization with the increases of differentiation degrees and crustal contributions in the magma source.