华北克拉通北缘中生代高锶花岗岩类:地球化学与源区性质

The Mesozoic high-Sr granitoids in the northern marginal region of North China Craton: geochemistry and source region.

华北克拉通北缘广泛分布中生代高锶花岗岩类 ,其岩石类型主要包括石英闪长岩、花岗闪长岩、石英二长岩和二长花岗岩 ,以普遍发育条纹长石为主要岩相学特征。高锶花岗岩总体上具有富钠、高钾和高铝的岩石化学特征。根据 16个代表性岩体共 6 1个样品的分析结果 ,本区的高锶花岗岩可被明显地划分为低硅岩石组 (Si O2 =5 3%～ 6 0 % )和高硅岩石组(Si O2 =6 5 %～ 73% ) ,缺乏 Si O2 =6 0 %～ 6 5 %的岩石 ,因而具有“双峰”分布特点。绝大多数高硅岩石属于高钾钙碱系列 ,而低硅岩石既有属于高钾钙碱和普通钙碱系列的 ,也有属于岛弧拉斑玄武岩系列和钾玄岩系列的。其微量元素以高锶、亏损重稀土及不发育明显富铕异常为重要特征 ,并以此明显区别于非高锶花岗岩。根据高锶花岗岩的岩石地球化学特征 ,并将它们与近年来许多天然岩石在不同温压条件下脱水熔融实验所获熔体进行了广泛对比 ,认为该区高锶花岗岩类的低硅岩石的源岩应为玄武质角闪岩类 ,而高硅岩石的源岩则为变中酸性火成岩类。源区部分熔融应该是在高温 (T >85 0～ 90 0℃ )和高压(P≥ 1.5 GPa)条件下进行的。熔体从源区抽取后的岩浆过程中没有经历明显的长石矿物的分离结晶 。

The Mesozoic high-Sr granitoids (HSRG) are widely distributed in the northern marginal region of the North China Craton and mainly consist of quartz diorites, granodiorites, quartz monzonites and adamellites, which are petrographically characterized by common occurrences of varying types of perthites. Based on analyses of 61 samples from 16 HSRG intrusions, they exhibit markedly high Na(2)O and Al(2)O(3) contents in their bulk compositions and can readily be classified into two groups: (1) low-silica rocks with SiO(2) between 53%similar to60%, and (2) high-silica ones with SiO(2) between 65%similar to73%, but only one sample is observed between 60%similar to65% SiO(2). Thus HSRG in the region display an apparent 'bimodal' distribution. Majority of high-silica rocks are high-K calc-alkaline, but the low-silica rocks show a remarkable diversity in their affinity from low-K island tholeiitic, through calc-alkaline and high-K calc-alkaline, to shoshonitic. Their notable trace element geochemistry, including strongly-enriched strontium, strongly-depleted heavy rare earth elements (HREE) and nearly absence of negative europium anomalies, thus making them distinguishable from other granites occurred in the same region. By detailed comparison between HSRG and the partial melts generated experimentally from dehydration melting of various rocks commonly occurred within continental crust, it is suggested that the low-silica rocks from HSRG were derived from partial melting of basaltic amphibolites, but the high-silica rocks were generated from partial melting of intermediate-to-acid igneous protoliths, both under high pressure (assumedly greater than or equal to 1.5 GPa) and high temperature (assumedly > 850similar to900 degreesC) conditions. The partial melts extracted from the residual assemblages composed mainly of garnet + clinopyroxene +/- orthopyroxene +/- amphibole +/- quartz would unequivocally display the major- and trace-element geochemistry as the high-Sr granitoids do. The HREE depletions of HSRG could largely be ascribed to the residual garnet as exhibited by dehydration melting experiments, and both high-Sr abundance and absence of negative Eu anomalies could be largely contributed by nearly completely fusing of feldspar components of the protoliths into partial melts. Furthermore, they also indicate no significant plagioclase fractionation occurred in-between from melt segregation to magma emplacement. This implies that the HSRG were originated directly from mafic and/or intermediate-acid igneous protoliths in the overthickened continental lower crusts and may generally represent the primary granitic magmas with little differentiation before the onset of major phase of crystallization. The residual solid assemblages of granulite and/or eclogite facies are suggested to become new constituents of the continental lower crust below where HSRG emplaced, or to be delaminated down to the upper mantle by their density inversion.