胶东西北部北截岩体岩石成因:锆石U-Pb年龄、岩石地球化学与Sr-Nd-Pb同位素制约

胶东半岛西北部是我国最重要的金矿集区,95%以上的金资源储量赋存于晚侏罗世玲珑型和早白垩世郭家岭型花岗岩中。然而,对郭家岭型花岗岩形成的大地构造背景及其与金成矿的关系,尤其是岩石成因仍存有争议。本文通过对归属于郭家岭型花岗岩的北截岩体开展详细的岩相学、岩石地球化学、锆石U-Pb年代学和Sr-Nd-Pb地球化学研究,识别出细粒花岗闪长岩、含斑细粒花岗闪长岩、斑状中粒花岗闪长岩、斑状粗粒花岗闪长岩和花岗伟晶岩脉5种侵入岩类型,它们具有相似的地球化学特征和同位素组成,呈连续演化关系,为同一期岩浆活动的产物。电子探针分析显示,样品中的黑云母具有富FeOT、MgO、高Al2O3低TiO2的特点,为岩浆成因的富铁黑云母,Mg和Al元素的负相关性表明结晶过程中二者可能发生了置换。岩石地球化学研究表明,花岗闪长岩具有高的全碱(K2O+Na20=6.92%~9.31%)、Ba(518×10-6~3330×10-6)、Sr(305×10-6~1371×10-6),低的Al2O3(14.41%~15.82%)、MgO(0.19%~0.77%)、Rb(39.3×10-6~105×10-6)、Y(1.2×10-6~6.05×10-6)、Yb(0.08×10-6~0.40×10-6)含量特征,轻稀土(LREE)富集、重稀土(HREE)相对亏损(LREE/HREE=13.13-21.40),轻、重稀土元素分馏明显(La/Yb)N=18.39~45.76],Rb、Sr、Ba、Pb等大离子亲石元素(LILE)富集,Nd、Ta、Zr、Hf等高场强元素(HFSE)明显亏损,具有高Ba-Sr花岗岩的地球化学特征。锆石LA-ICP-MS定年结果表明,斑状粗粒花岗闪长岩形成于131.53±0.86Ma,花岗伟晶岩脉形成于127.4±1.3Ma,说明其形成于早白垩世早期,伟晶岩脉应该是本次岩浆活动后期的产物。Sr-Nd同位素分析表明,初始87Sr/86Sr(ISr)和εNd(t)分别为0.710658-0.711568和-18.53^-11.17,二阶段亏损地幔模式年龄(tDM2)为2107-2425Ma,与胶东群变质岩年龄一致,指示了其原始岩浆有部分熔融的胶东群参与;Pb同位素分析表明,初始206Pb/204Pb和初始208Pb/204Pb分别为17.047~17.945和37.744~38.389,说明有大量的下地壳物质参与成岩。上述数据分析表明,北截岩体是基底岩石胶东群变质岩部分熔融形成的下地壳酸性岩浆与幔源基性岩浆混合作用的结果,太平洋板块向欧亚板块俯冲构造背景下的软流圈物质上涌和下地壳的部分熔融可能是形成这套花岗岩的机制,是板块俯冲产生的欧亚板块边缘火山弧的组成部分;在区域金成矿方面,其为金成矿提供了有利的赋存空间,而应该不是成矿流体来源或热动力条件。

Petrogenesis of the Beijie pluton from the northwestern Jiaodong Peninsula: Constraints from zircon U-Pb age, petrogeochemistry and Sr-Nd-Pb isotopes

The northwestern Jiaodong Peninsula is the most important gold mineralization area in China.The majority of gold resources C> 95% D in Jiaodong are hosted in the Late Jurassic Linglong-type and Early Cretaceous Guojialing-type granitoids.However,there is still controversy over the geotectonic setting of Guojialing-type granitoids and its relationship with gold mineralization,especially its petrogenesis.This paper systematically investigated the Beijie pluton which belongs to Guojialing-type granitoids to analyse its petrography,petrogeochemistry,zircon U-Pb age,and Sr-Nd-Pb geochemistry.Fine-grained granodiorite,porphyritic finegrained granodiorite,porphyritic medium-grained granodiorite,porphyritic coarse-grained granodiorite and granite pegmatite are identified in Beijie pluton.They have the similar geochemical characteristics and isotope compositions,and present continuous evolution tendency.All these suggest the five intrusive rocks are the product of the same magmatic activity.Electron probe analysis shows the biotites in granodiorites have high MgO,FeOT,Al2 O3 and low TiO2,indicating the Fe-rich biotite is magmatic origin.Negative correlation with Mg and A1 implicit they may be displaced during crystallization.Petrogeochemistry studies show that:C1 D the Beijie granodiorites possess high total alkalis CK2 O+Na2 O=6.92%~9.31% D,Ba C518×10-6~3330×10-6 D,Sr C305×10-6~1371×10-6 D,with low Al2 O3 C14.41%~15.82% D,MgO CD.19%~0.77% D,Rb C39.3×10-6~105×10-6 D,Y C1.2×10-6~6.05×10-6 D,Yb G0.08×10-6~0.40×10-6 D,enrichment in LREEs,relative depletion in HREEs CLREE/HREE=13.13~21.40 D;C2 D the fractional distillation of the LREEs and HREEs is obvious CLa/YbDN=18.39~45.76];C3 D the LILE Ce.g.,Rb,Sr,Ba and PbD is enriched,while the HFSE Ce.g.,Nd,Ta,Zr and HfD is obviously depleted.All these features suggest that the rocks are similar to the Ba-Sr granites.Zircon LA-ICP-MS U-Pb dating yield 131.53±0.86 Ma for the granodiorite and 127.4±1.3 Ma for the granitic pegmatite,indicating they are formed in Early Cretaceous.The granitic pegmatite should be the product of the late magmatic activity.Sr-Nd isotope analysis shows that they have ISr and εNdCtD values of 0.710658~0.711568 and-18.53^-11.17 respectively,with LDM2 of 2107~2425 Ma which have the same age with the metamorphic rocks of the Jiaodong Group.This indicates that the partial melting of Jiaodong Group is involved in the primordial magma.Pb isotope analysis shows their Pb compositions are 17.047~17.945 CC206Pb/204 PbD;D and 37.744~38.389 CC208Pb/204 PbD;D,implying a large amount of lower crust material involved in the diagenesis.The above data analysis shows the Beijie pluton might be generated by acidic magmas from lower crust formed by partial melting of the metamorphic rocks in Jiaodong Group mixed with basic magma derived from mantle.Upwelling of asthenosphere material and partial melting of the lower crust under the tectonic background of subduction of the Pacific Plate to the Eurasian Plate may be the mechanism of the formation of this granite.It is part of the volcanic arc at the edge of the Eurasian Plate created by subduction.In terms of regional gold mineralization,it provides favorable occurrence space for gold mineralization,rather than the source of metallogenic fluid or thermal dynamic conditions.