【作者】 张娟；

【导师】 郑永飞； 赵子福；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2011， 博士

【摘要】 大别-苏鲁造山带是华南-华北陆块在三叠纪经过大陆碰撞形成的,广泛分布有中生代岩浆岩。研究这些造山带碰撞后岩浆岩的形成时代、岩石类型、地球化学特征、源区物质来源及其形成的地球动力学背景,对于认识俯冲陆壳物质再循环和碰撞造山带的构造演化具有十分重要的意义。前人对大别造山带碰撞后岩浆岩成因与俯冲陆壳再循环进行了大量研究,但是对苏鲁造山带中生代岩浆岩的研究相对薄弱。苏鲁造山带是中国东部中生代岩浆活动的重要区域之一,形成时代上主要属于晚三叠世、晚侏罗世和早白垩世。本文研究对象主要包括苏鲁造山带东部(胶东地块)和胶北地块的花岗岩以及苏鲁造山带西部(苏北-胶南地块)的酸-基性岩墙。本学位论文对这些中生代岩浆岩进行了岩相学观察、全岩主量、微量元素和Sr-Nd同位素、全岩和单矿物氧同位素以及锆石微量元素、U-Pb和Lu-Hf同位素分析。结果不仅能够对这些中生代岩浆岩的源区特征和成因机制提供可靠的地球化学制约,而且有助于正确理解造山带岩石圈的化学分异及其壳幔相互作用过程。本文研究的四个花岗岩体包括胶北地块出露的玲珑和郭家岭岩体以及胶东地块出露的昆嵛山和三佛山岩体。SHRIMP和LA-ICPMS锆石U-Pb定年结果表明,昆嵛山和玲珑岩体侵位于晚侏罗世(141±3-157±2Ma),而郭家岭和三佛山岩体侵位于早白垩世(111±2-133±3Ma)。大多数花岗岩具有相似的稀土和微量元素分配模式,表现出明显的LREE和LILE富集,HREE和HFSE亏损以及Pb正异常。它们大多具有高的Sr含量、低的Yb和Y含量以及高的Sr/Y和(La/Yb)N比值,没有明显的铕负异常(Eu/Eu*=0.69-1.58)。这些地球化学特征与埃达克质岩石相似,表明石榴石或石榴石+角闪石可能是部分熔融的残留相。此外,这些埃达克质花岗岩还具有较低的MgO、Cr、Ni含量和Mg#。它们具有高的全岩初始87Sr/86Sr比值(0.7080-0.7139)和低的εNd(t)值(-22.3--10.9)以及低的锆石εHf(t)值(-39.6--5.4)。它们大多具有高的锆石δ180值(5.91-8.93‰)。全岩主量和微量元素、放射成因同位素以及单矿物氧同位素结果表明,这些中生代花岗岩是古老陆壳物质重熔的产物。它们的锆石εHf(t)值具有较大变化范围,其中三佛山岩体两阶段锆石Hf模式年龄为古元古代(2097±63-2387±21Ma),其它三个岩体两阶段锆石Hf模式年龄主体为古元古代-晚太古代(22774±39-2856±44Ma)。此外,在晚侏罗世花岗岩中发现U-Pb年龄为769±19-790±21Ma的中新元古代继承锆石。古元古代中期地壳生长和新元古代中期岩浆活动是俯冲华南陆壳物质的特点,而华北陆块缺乏古元古代中期地壳生长和新元古代中期岩浆活动。虽然华北存在大量古元古代变质作用和岩浆活动,但是尚未发现古元古代中期新生地壳生长事件。因此,胶东半岛中生代花岗岩源区中俯冲华南陆壳物质的贡献是非常明显的。但是,胶北与胶东花岗岩在源区性质上可能存在差异。相对于胶东花岗岩,胶北地体玲珑花岗岩中发现了古太古代锆石Hf模式年龄(3310±96 Ma),表明胶北花岗岩源区可能有古老的华北陆壳物质参与。因此,三叠纪华南与华北陆块之间碰撞使两边的地壳物质卷入俯冲带形成加厚地壳,为这些碰撞后花岗岩提供了源区物质。本文研究的酸-基性岩墙样品采自苏鲁造山带西部的胶南(莒南-五莲)地区,包括二长花岗岩、闪长岩、含斜长石角闪石岩和辉长岩。LA-ICPMS锆石U-Pb定年给出了111±3-129±1 Ma的岩浆结晶年龄,在二长花岗岩样品中发现了新元古代的继承锆石年龄(743±9-773±9Ma)。它们富集LILE和LREE,亏损HFSE,具有高的初始87Sr/86Sr比值(0.7079-0.7100)和低的εNd(t)值(-21.6--13.6)以及低的锆石εHf(t)值(-36.7--10.6),对应的两阶段Hf模式年龄为1.84-3.47Ga。它们的锆石δ180值变化较小,为4.57-5.98‰,个别样品的锆石δ18O值略高于或低于正常地幔值。基性岩墙具有明显的弧型陆壳微量元素和放射成因同位素特征表明,其岩浆来源于相对富集的地幔源区,可能是由俯冲陆壳来源的熔体与上覆的大陆岩石圈地幔(SCLM)相互作用形成的。然而,辉长岩和角闪石岩之间在元素和同位素特征上的差异表明,它们来源于不同的SCLM源区。辉长岩具有相对较高的MgO、Cr、Ni含量和较低的Al2O3含量,表明它们来源于富含斜方辉石的SCLM源区。含斜长石角闪石岩具有相对较高的Al2O3、K2O含量和较低的MgO、Cr和Ni含量,表明它们来源于富含角闪石的SCLM源区。中酸性岩墙(二长花岗岩和闪长岩)与大别-苏鲁造山带广泛分布的早白垩世花岗岩具有相似的元素和同位素组成,表明它们来源于相似的源区,即俯冲的华南陆壳。一些中酸性岩墙具有较高的Mg#(高达66),表明它们可能是由基性岩浆和酸性岩浆混合形成的。因此,胶南地区酸-基性岩墙是早白垩世造山带岩石圈不同部位部分熔融形成的。综上所述,苏鲁造山带中生代岩浆岩是俯冲华南陆壳物质重熔和再循环的产物。大陆岩石圈拉张引起的地幔热扰动能够使造山带根部地壳及其下伏造山带岩石圈地幔发生部分熔融,形成不同类型的碰撞后岩浆岩。更多还原

【Abstract】 The Dabie-Sulu orogenic belt formed by the Triassic continental collision between the South China Block and the North China Block, with voluminous occurrence of postcollisional magmatic rocks. These mgamtic rocks contain valuable information on chemical geodynamics of continental subduction. Therefore, studies concerning the timing, rock type, geochemical characteristics, magma source, and geodynamic setting of postcollisional magmatic rocks have important implications for understanding the recycling of subducted continental crust and the tectonic evolution of collisional orogen. While much attention has been paid to postcollisional magmatic rocks in the Dabie orogen with respect to the recycling of the deeply subducted continental crust, fewer studies and progresses have been made on this topic for postcollisional magmatic rocks in the Sulu orogen. The Sulu orogen is one of most important regions in east-central China that crop out Mesozoic magmatic rocks, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. This study deals with the Mesozoic granitoids from both the eastern Sulu orogen (the Jiaodong terrane) and the Jiaobei terranes as well as mafic to felsic dykes in the western Sulu orogen (the Subei-Jiaonan terrane).This dissertation presents a combined study of petrography, whole-rock major, trace elements and Rb-Sr and Sm-Nd isotopes, whole-rock and mineral O isotopes as well as zircon trace elements, U-Pb and Lu-Hf isotopes for the Mesozoic magmatic rocks in the Sulu orogen. The results not only provide insight into the source nature and petrogenesis of Mesozoic magmatic rocks in the Sulu orogen, but also place important constraints on the chemical differentiation and crust-mantle interaction of orogenic lithosphere.Four plutons were investigated, which are the Linglong and Guojialing plutons from the Jiaobei terrane and the Kunyushan and Sanfoshan plutons from the Jiaodong terrane. SHRIMP and LA-ICPMS zircon U-Pb dating yields Late Jurassic ages of 141±3 to 157±2 Ma for the Linglong and Kunyushan plutons, but Early Cretaceous ages of 111±2 to 133±3 Ma for the Guojialing and Sanfoshan plutons. Most of the granitoids have similar REE and trace element patterns with LREE and LILE enrichement but HREE and HFSE depletion as well as positive Pb anomaly. Most of them have high Sr, low Yb and Y contents as well as high (La/Yb)N and Sr/Y ratios with negligible to positive Eu anomalies (Eu/Eu*=0.69 to 1.58), which are similar to common adakites. This indicates the presence of garnet or garnet and amphibole as residual phases during partial melting. On the other hand, they have relatively low MgO, Cr, Ni contents and thus low Mg#. In addition, they have high initial 87Sr/86Sr ratios of 0.7080 to 0.7139 and lowεNd(t) values of-22.3 to-10.9 as well as zirconεHf(t) values of-39.6 to-5.4. They mostly have high zirconδ18O values of 5.91 to 8.93‰.The results of whole-rock major and trace elements, radiogenic isotopes and mineral O isotopes indicate that these Mesozoic granitoids were derived from partial melting of ancient continental crust. All the granitoids have variably negative zirconεHf(t) values of-39.6 to-5.4, with Paleoproterozoic Hf model ages of 2097±63 to 2387±21 Ma for the Sanfoshan pluton, but mainly Paleoproterozoic to Neoarchean Hf model ages of 2277±39 to 2856±44 Ma for the other three plutons. In addition, some zircon cores from the Linglong and Kunyushan granitoids have middle Neoproterozoic U-Pb ages of 769±19 to 790±21 Ma. The middle Paleoproterozoic crustal growth and middle Neoproterozoic magmatism are diagnostic features that distinguish the South China Block from the North China Block. Therefore, the Mesozoic granitoids were mainly derived from partial melting of the subducted continental crust of the South China Block. However, there are some differences between the Jiaobei and Jiaodong plutons. Compared to the Jiaodong granitoids, the Jiaobei granitoids have very old zircon Hf model ages up to 3310±96 Ma, suggesting the possible involvement of very ancient continental crust of the North China Block. Thus, the continental collision between the two blocks would bring crustal materials from both sides into the subduction zone during the Triassic, yielding subduction-thickened crust as the magma source for the adakite-like granitoids.Postcollisional mafic to felsic dykes in the Jiaonan terrane consist of monzogranite, diorite, plagioclase-bearing hornblendite and gabbro. LA-ICPMS zircon U-Pb dating yields broadly consistent ages of 111±3 to 129±1 Ma for magma crystallization of these mafic to felsic rocks, with Neoproterozoic ages of 743±9 to 773±9 Ma for residual cores in some monzogranite samples. The mafic to felsic rocks are characterized by strong LILE and LREE enrichment and negative HFSE anomalies, high initial 87Sr/86Sr ratios of 0.7079 to 0.7100 and very lowεHf(t) values of-21.6 to-13.6. Zircon Lu-Hf isotope analyses also show highly negativeεHf(t) values of-36.7 to-10.6, corresponding to two-stage Hf model ages of 1.84 to 3.47 Ga. They have relatively limited range of zirconδ18O values of 4.57 to 5.98%o, some of them are slightly higher or lower than those of the normal mantle.The arc-like continental crustal signatures are evident for the mafic dykes, suggesting their derivations from enriched mantle sources, which may be generated by interaction between the overlying subcontinental lithospheric mantle (SCLM) and felsic melts derived from the subducted continental crust during the Triassic subduction. However, the difference in geochemical and isotopic features between gabbro and plagioclase-bearing hornblendite indicates that they were derived from different SCLM sources. The gabbros have relatively high MgO, Cr, Ni and low Al2O3 contents, indicating that they were derived from an orthopyroxene-rich SCLM source. The plagioclase-bearing hornblendites have relatively high Al2O3, K2O and low MgO, Cr, Ni contents, indicating that they were derived from a hornblende-rich SCLM source. The general similarity in geochemical and isotopic characteristics between the felsic-intermediate dykes (monzogranite and diorite) and widespread postcollisional Early Cretaceous granitoids in the Dabie-Sulu orogenic belt indicate their derivation from the similar source materials, i.e. the subducted continental crust of the South China Block. Some felsic-intermediate samples have high Mg# up to 66, suggesting that they were possibly formed by mixing between mafic and felsic magmas. Therefore, the mafic to felsic dykes were formed by partial melting of the collision orogenic lithosphere in the Early Cretaceous.In summary, the Mesozoic magmatic rocks in the Sulu orogen were derived from partial melting and recycling of the subducted continental crust of the South China Block. Mantle heat perturbation due to extension of the continental lithosphere triggered partial melting of orogenic crust and underlying SCLM, resulting in the postcollisional magmatic rocks with the chemical and isotopic compositions of arc-like continental crust.更多还原