摘要
产于陆内环境的含矿斑岩往往具有高Sr/Y特征,多数学者认为与岩浆起源于加厚或拆沉的下地壳有关。然而,目前这一模式仍存在较大争议。本文以敖仑花斑岩矿床为例,新报道了矿区内不成矿岩体的岩石学和地球化学特征,与同期成矿岩体进行对比,探讨含矿斑岩高Sr/Y原因及控制是否成矿的可能因素。相对于含矿斑岩,敖仑花贫矿黑云母花岗岩具有高SiO2（74%~78%）、低MgO（〈0.2%）和CaO（0.2%~1.1%）、贫Sr（30×10^-6~251×10^-6）和Cr（1×10^-6~6×10^-6）的特征,显示准铝质到过铝质性质（A/CNK=0.9~1.2）,Mg#值（2~30）与玄武岩实验熔体成份相似,说明岩浆可能主要来自基性下地壳源区的部分熔融。而已有的研究表明含矿斑岩为壳幔岩浆混合成因,富集地幔起源的偏基性岩浆的加入提升了寄主长英质岩浆的Mg#值、大离子亲石元素（Sr、La等）含量及其氧逸度（fO2）。我们认为幔源岩浆的混合可能是导致陆内环境含矿斑岩高Sr/Y及有利成矿的根本原因;相反,来自纯壳源的花岗质熔体因缺乏富水、高fO2幔源岩浆的参与而不利于形成斑岩Cu±Au±Mo矿床。
A Ore-bearing porphyries formed in non-arc environments （such as continental collision zones and intra-continents） often have high Sr/Y ratios. Most researchers advocate that intermediate to felsic magmas for these ore-bearing porphyries originated from a thickened or delaminated continental lower crust source. However, this model is still under discussion. Here we report a case study to provide new insights into this issue and give a different interpretation. The Aolunhua porphyry Mo-Cu deposit from the eastern Central Asian Orogenic Belt formed in an intra-continental extensional setting during the Early Cretaceous. Two types of granitic intrusions have been recognized in the mining area. One is the fertile monzogranite-porphyry （formed at 132Ma by zircon U-Pb dating）, and the other one is the barren biotite granite （formed at 126Ma by zircon U-Pb dating）. In this paper, we present new petrological and geochemical data of the barren biotite granite, and compare these data with that of the fertile monzogranite-porphyry reported by previous studies. Our results show that the barren biotite granite has relatively high SiO2 （74.4% -78. 3% ）, K2O （4. 3% -4. 9% ）, Y （ 18.4 × 10^-6 -27.6×10^-6） andYb （2.4×10^-6 -3.0×10^-6）, but lowMgO （〈0.2%）, CaO （0.2% -1.1%）, Sr （30×10^-6 -251× 10^-6） and Cr （1.1 × 10^-6- 5.8× 10^-6） contents, and Mg# values （2 - 30）. They are metaluminous to peraluminous （A/CNK = 0. 9 - 1.2 ）, and show relatively flat REE patterns and pronounced negative Eu anomalies （ Eu/Eu＊ = 0. 05 - 0. 2 ） . These compositions are comparative to those of metabasaltic experimental melts （1-4.0GPa）, suggesting that magmas of the biotite granite probably derived from partial melting of a lower crust source. In contrast, the fertile monzogranite-porphyry contains plenty of mafic microgranular enclaves （ MMEs）, and is enriched in St, Ba, LREE and other LILE. They show concave REE patterns with negligible Eu anomalies, and have high Sr and La, but are low in Y and Yb concentrations, which are comparative to that of typical adakitic rocks. All samples plot in the ＂adakite＂ field in the Sr/Y vs. Y and （La/Yb） N VS. （Yb） N diagrams. Compositional and textural disequilibrium of plagioclase phenocrysts, variable εHf （t） values for zircons, together with high Mg# values （45 - 52 ） and Cr abundances, indicating that the fertile monzogranite-porphyry was mainly derived from partial melting of a lower crustal source, but with remarkable addition of enriched mantle-derived materials. The mafic end-member is represented by MMEs hosted in the ore-bearing monzogranite-porphyry. They are characterized by high Mg# values （56 - 63） and enrichment of Sr, Ba, LREE and other LILE, as well as moderate radiogenic Nd and Hf, implying that their parental magmas were derived from a fluids/melts-metasomatized mantle source. We propose that high Sr/Y ratios of the fertile monzogranite-porphyry could be caused by a process of crust-mantle magma mixing plus subsequent fractionation as follows： First, mafic magma derived from partial melting of the lithospheric mantle previously metasomatized by subduction zone fluids/melts have high LILEs （ e. g. , Sr, La and LREE）. Mixing of such mafic magma with crustal melts in the lower crust resulted in the formation of hybrid parental magmas for the fertile monzogranite-porphyry. As revealed by the Sr abundances and Mg# values, all ore-bearing porphyry and MMEs samples define a linear and positive correlated trend. By magma mixing, enriched mantle-derived wet mafic magmas could provide abundant St, La and other incompatible elements to the host felsic magmas, and meanwhile, raise the H2O content and oxygen fugacity （fo2） of the resultant magmas. Therefore, the high Sr and highSr/Y ratios of the porphyry are partially attributed to the addition of enriched mantle-derived mafic magma. Second, under water-rich condition, the crystal fractionation of hornblende, titanite and other accessory minerals （ all these fractionated minerals have low Dsr （0. 03 -0. 516） and high Dy ） from the parental magma was earlier than that of plagioclase （because plagioclase crystallization was suppressed by the high water content of the melt）, consequently leading to the enhanced high Sr/Y ratios for the evolved magmas. The Mo-Cu mineralization is attributed to the high H2O andfo2 nature of hybrid magmas. In contrast, the barren biotite granite with normal Sr/Y ratios, H2O-deficient and low fo2 signatures, are not favorable for the Mo-Cu mineralization. Therefore, the mixing of water-rich mafie magmas from an enriched lithospheric mantle with the felsic lower crustal melts is significant to the formation of high Sr/Y porphyries and related deposits in non-arc environments.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第7期2020-2030,共11页
Acta Petrologica Sinica
基金
国家“973”项目(2013CB429804)
国家青年科学基金项目(41202033)联合资助
关键词
斑岩矿床
埃达克岩
壳幔岩浆混合
陆内环境
Porphyry deposit
Adakite
Crust-mantle magma mixing
Non-arc environments
