八达岭基性岩和高Ba-Sr花岗岩地球化学特征及成因探讨:华北和大别-苏鲁造山带中生代岩浆岩的对比

Geochemical characteristics and petrogenesis of the Badaling high Ba-Sr granitoids: a comparison of igneous rocks from North China and the Dabie-Sulu Orogen.

八达岭杂岩侵位于华北北部 ,由辉长闪长岩岩、石英闪长岩、石英二长岩、二长闪长岩、二长花岗岩、碱长花岗岩和石英正长岩等组成 ,主要属高钾钙碱性系列。除了含 V- Ti磁铁矿的堆晶辉长闪长岩 ,整套岩石的主量元素变化范围较大 ,Si O2 =4 6 .5 %～ 75 .3% ,Mg O=5 .6 %～ 0 .2 % ,中酸性岩石的 K2 O/Na2 O为 0 .5 9～ 1.0 9。碱长花岗岩和少数石英二长岩 Ba和Sr含量较低 ,且具有明显 Eu负异常。大多数中酸性岩石 (高 Ba- Sr花岗岩 )具有如下显著的微量元素地球化学特征 :Ba,Sr和轻稀土 (L REE)富集 ,Y和重稀土 (HREE)亏损 ,L REE/ HREE强烈分离 ,Sr/ Y和 L a/ Yb比值较高 ;在原始地幔标准化的蛛网图中具有显著的 Nb,Ta和 Ti亏损 ,不具明显的 Sr和 Eu亏损。在 Harker图解中 ,基性岩和高 Ba- Sr花岗岩的主量元素相关性明显 ;两者还具有相似的微量元素和稀土 (REE)分配特征 ,并且 ,REE,Y,Sr,P和 Ti含量从基性到酸性逐渐降低。辉长闪长岩和高 Ba- Sr花岗岩的 Sr- Nd同位素初始值呈 EM1特征 (ISr=0 .70 5 1～ 0 .70 6 8,εNdi=- 8.2～ - 2 0 .2 ) ,大致呈负相关。地球化学特征表明基性岩浆为富集的大陆岩石圈地幔部分熔融形成 ,而高 Ba- Sr花岗岩则为基性岩浆通过陆壳混染和结晶分离形成 ;富 P和

The Mesozoic Badaling complex, emplaced in the northern part of the North China block (NCB), is composed of gabbrodiorite, quartz diorite, quartz monzonite, monzodiorite, adamellite, moyite and quartz syenite, which are predominantly high-K calc-alkaline and subordinately shoshonitic. Their major elements, except that of cumulative gabbrodiorite with substantive magnetite, range widely, e. g., SiO2 = 46.5% similar to 75.3%, MgO = 5.6% similar to 0.2%. K2O/Na2O ratio varies from 0.59 to 1.09 for the intermediate-felsic rocks. The moyite and quartz monzonite have low Ba and Sr abundances and strong negative Eu anomaly. However, most intermediate-felsic rocks are consistently characterized with high Ba, Sr and LREE and low Rb, Y and HREE abundances, high Sr/Y and La/Yb ratios, distinct Nb, Ta and Ti depletion, as well as absence of negative Eu anomalies, and are comprehensively called high Ba-Sr granitoids. In addition, the mafic rocks and high Ba-Sr granitoids have covariant major elements, and similar incompatible trace element and rare earth element (REE) patterns, with REE, Y, Sr, P and Ti abundances consistently decreasing from basic to felsic rocks. Initial Sr-87/Sr-86 (0.7051 similar to 0.7068) and epsilon(Ndi) (-8.2 similar to -20.2) of mafic rocks and high Ba-Sr granitoids show EM1-type signature, and are negatively correlated. Geochemical data indicate that mafic rocks were derived from partial melting of an enriched subcontinental lithospheric mantle, and high Ba-Sr granitoids were produced from the mafic magmas mainly through fractional crystallization with some crustal contamination. Fractionation of P- and Ti-bearing accessory minerals, such as apatite and titanite, played an important role in causing the decrease of the REEs with progressive differentiation. The Yanshanian mafic rocks, high Ba-Sr and low Ba-Sr granitic rocks with comparable geochemical characteristics to their correspondents in the Badaling complex are widespread in the eastern part of NCB and the Dabie-Sulu Orogen (DSOB). Hence we propose that the occurrence of Yanshanian igneous rocks in NCB and DSOB had common geodynamic mechanism, and the geochemical characteristics of the Yanshanian igneous rocks from DSOB were not necessarily due to the interaction between the subducted continental crust (Yangtze block) and mantle peridotite. Delamination or lithospheric thinning probably caused the partial melting of the enriched subcontinental lithospheric mantle beneath NCB and DSOB. Mantle enrichment beneath NCB and DSOB may occur early in Proterozoic, implicated by the similarity of Nd depleted mantle model ages (T-DM).