【作者】 谢成龙；

【导师】 朱光；

【作者基本信息】 合肥工业大学 ， 矿物学、岩石学、矿床学， 2008， 博士

【摘要】 本论文对发育于郯庐断裂带南段上的一些晚中生代岩浆岩,包括滁州火山岩、巢湖侵入岩和庐江火山岩,进行了主量元素、微量元素、Sr-Nd-Pb同位素等地球化学研究和LA-ICPMS法单颗粒锆石U-Pb年代学研究。通过这些研究,首先确定了这些岩浆岩的具体形成时间,认识了它们的岩石学和地球化学特征;随后讨论了岩浆形成的深部过程和岩浆源区组成,并结合年代学结果讨论了岩浆演化特征;最后在区域对比的基础上,分析了郯庐断裂带内岩石圈减薄的方式和特点,并讨论了该断裂带在华北克拉通岩石圈减薄中的作用。滁州火山岩位于张八岭隆起北段的东侧边界上,该火山岩形成于早白垩世,具体喷发时期在132～116Ma之间,经历了约16Ma的喷发历史。132Ma的喷发年龄是目前已知的郯庐断裂带内最早的钙碱性岩浆岩形成年龄,该年龄应最接近郯庐断裂带下岩石圈伸展活动的开始时间。该火山岩是一套中—酸性的、粗安岩/粗面岩/英安岩为主的岩浆岩,岩石属于高钾钙碱性系列、准铝质、硅过饱和类型的岩石。火山岩富集LILE和LREE、相对亏损HFSE和HREE,具有富集的Sr-Nd-Pb同位素组成。地球化学特征显示,早期（～132Ma）和晚期火山岩（123～116Ma）具有一般钙碱性火山岩的特征,中期火山岩（129～125Ma）则具有类似于埃达克岩的特征。该火山岩的岩浆来源于华北克拉通,早期火山岩是EM l型富集地幔和古老华北下地壳源区混合的结果,中期火山岩是底侵的幔源物质与古老地壳物质发生部分熔融并混合形成的,晚期火山岩的岩浆源区则主要以古老的下地壳端元为主。地球化学特征显示,滁州火山岩的岩浆起源于壳—幔过渡带的深度范围,随喷发时代的变新岩浆中壳源比例逐渐增加,岩浆源区逐渐变浅。滁州火山岩的地球化学及岩浆演化特征不支持岩石圈整体拆沉作用模型,岩浆应形成于岩石圈由下至上逐渐减薄的背景下。区域对比发现,滁州火山岩比华北克拉通内部岩浆活动具有更早的开始时间和较长期的活动历史,暗示郯庐断裂带内具有更强烈的岩石圈减薄程度和更长期的减薄历史。与华北克拉通内部相比,滁州火山岩具有源区熔融程度更高、熔融规模更大、壳—幔相互作用更为强烈、源区深度更浅等特征,一致证明了郯庐断裂带是岩石圈内的强减薄带。巢湖侵入岩位于张八岭隆起南段的郯庐断裂带内,五个样品给出的岩体侵位时间在127～103Ma之间,总体经历了约24Ma的侵位历史。该套岩石是一套铝和硅均过饱和的、准铝质向过铝质过渡的岩浆岩,岩石富钾且全碱含量较高。早期和中早期侵入岩富集LILE、LREE,亏损HREE,无Eu异常或具有正的Eu异常,是一套埃达克质的二长花岗岩;晚期和中晚期侵入岩富集LILE但亏损Ba、Sr,REE总量较低且轻、重稀土分馏较弱,具有负的Eu异常,是一套A型花岗岩。先期的埃达克岩代表了该处岩石圈伸展活动的开始阶段,而后期的A型花岗岩则代表了岩石圈强烈伸展的时间。巢湖侵入岩的岩浆也来源于古老的华北克拉通,且主要为古老的华北克拉通下地壳,早期岩浆包含少量的富集地幔端元,而晚期岩浆则为更浅的地壳源区。地球化学特征表明该套侵入岩在岩浆侵位过程中没有明显的中、上地壳混染,岩浆源区经历了不同程度的部分熔融,并经历了不同造岩矿物和锆石、磷灰石、褐帘石等副矿物的分离结晶作用。随岩浆侵位时代的不同,岩浆源区的残留相明显不同,由早到晚源区残留从石榴子石到斜长石再向角闪石过渡,且由早到晚岩浆源区是逐渐变浅的。岩浆演化规律暗示岩浆活动的深部动力学过程为:华北克拉通岩石圈底部的逐渐拆沉减薄造成了软流圈顶面抬升,导致岩石圈内热流升高,化学作用逐渐加强,从而出现了不同深度下的下地壳源区部分熔融。与远离断裂带的克拉通内部相比,郯庐断裂带具有更强烈的岩石圈伸展程度、侵入岩具有更浅的岩浆源区、岩浆源区具有更强烈的演化程度和更大程度的部分熔融、岩浆活动具有更长期和复杂的历史,这一系列的证据也均证明郯庐断裂带是岩石圈减薄中的强减薄带。庐江火山岩位于华北克拉通最东南缘的郯庐断裂带内,岩浆喷发时间在125Ma～93Ma之间,经历了约32Ma的喷发历史。火山岩具有双峰式火山岩和玄粗岩岩系的特征,岩石富碱、富钾,相对贫MgO、TiO₂,具有富集的Sr-Nd-Pb同位素组成。玄武岩和粗面岩样品富集LILE、LREE并相对亏损HFSE、HREE,无明显Eu异常;流纹岩样品则相对亏损Ba、Sr、P、Ti,具有明显的Eu负异常。岩浆岩地球化学特征表明,该火山岩具有四类不同的源区背景:早期基性岩（125Ma）的源区主要为华北克拉通岩石圈地幔,兼有地壳源区的贡献;流纹岩（120Ma）源区主要为华北中、下地壳源区;粗面岩（100Ma）为华北下地壳和岩石圈地幔的混合源区;晚期基性岩（93Ma）主要为华北岩石圈地幔源区,兼有下地壳和软流圈物质的贡献。庐江火山岩的地球化学特征也不支持岩石圈地幔和下地壳的整体拆沉作用模型,岩浆活动应是在岩石圈底部的持续减薄作用下,岩石圈内热流逐渐升高、岩浆源区的热—化学状态的变化所造成的。不同时期的岩浆活动其地球化学特征差异较大,反映了岩浆源区随岩石圈热—化学状态的改变而不断演化和迁移的规律。岩石组合、同位素年代学、地球化学特征对比等—系列的证据表明,位于郯庐断裂带上的庐江火山岩,形成于强烈的伸展构造背景下,是岩石圈强烈减薄的产物。表明郯庐断裂带是岩石圈内的强减薄带,这与滁州火山岩和巢湖侵入岩揭示的信息是一致的。本文的研究结果表明,郯庐断裂带南段的晚中生代岩浆岩虽然具有不同的岩石类型和地球化学特征,但岩浆源区均属于华北克拉通,且岩浆往往具有壳—幔混源的特征。综合目前已有的华北克拉通晚中生代岩浆岩资料来看,郯庐断裂带内各部位的岩浆活动持续时间最长、结束时间最晚,且岩浆活动指示的往往是更为强烈的伸展构造背景。这些岩浆岩具有清晰的岩浆演化与源区迁移规律,显示岩浆源区组成和温压条件的不断改变,源区的迁移规律和一系列的地球化学证据均不支持岩石圈的整体拆沉作用观点,岩石圈应是以由下至上逐渐拆沉进入软流圈的方式发生减薄的。一系列的地球化学及地质学证据均表明,郯庐断裂带是岩石圈内的强减薄带,这与现今地球物理资料揭示的现象是一致的。分析认为,郯庐断裂带对华北克拉通岩石圈的整体性具有破坏作用,断裂带的存在和活动在岩石圈减薄过程中不仅具有增强和促进作用,还可能具有控制意义。更多还原

【Abstract】 In this paper, Chronological, geochemical and Sr-Nd-Pb isotopic analyses have been carried out on the Late Mesozoic igneous rocks including Chuzhou volcanic rocks, Chaohu intrusive rocks and Lujiang volcanic rocks along the southern segment of the Tan-Lu fault zone. Based on these results, this paper demarcated formation time of these igneous rocks, summarized nature of their petrology and geochemistry, discussed deep processes of the magmatism, compositions of magma sources, and magmatic evolution features. Based on regional comparsions, this paper also analyzed patterns and characteristics of the lithosphere thinning beneath the Tan-Lu fault zone, revealed the importance and function of such a large-scale fault zone during the lithosphere thinning of the North China Craton.Chuzhou volcanic rocks are located on the eastern boundary of the the northern segment of the Zhangbaling uplift, were erupted during the Early Cretaceous, the eruption ages varied from 132Ma to 116Ma with a long period of 16Ma. 132Ma is the earliest eruption age of the cal-alkaline magma within the Tan-Lu fault zone, and this age should represent the beginning of the lithosphere extensive movement beneath the fault zone. These volcanic rocks are metaluminous and silicon-saturation, belonging to intermediate-acidic high-K cal-alkaline series, and notably rich in LILE and LREE, depleted in HREE and HFSE, and rich in Sr-Nd-Pb isotope component. Geochemical characteristics show that the earlier （about 132Ma） and the later （123～116Ma） rocks were characterized by common cal-alkaline volcanic rocks, medium-term with eruption age （129～125Ma） would resemble in geochemical characteristics of adakitic rocks. The geochemical data indicate a magma source of the North China craton. Early magma reflects a mixing between the EMI-type mantle and ancient lower crust of the NCC. Medium-term magma is a partial melting and mixture of the underplating mantle-derived components mantle and ancient lower crust. The late magma are mostly ancient lower crust components. From early to late, ancient crustal components in volcanic rocks gradually increased, and magma source became shallower and shallower, which implied Chuzhou volcanic rocks were originated from the transitional zone between the crust and mantle. Characteristics of geochemistry and magma evolvement could not support lithosphere whole detachment model, however magma should be formed under this setting of lithosphere thinning gradually from top down. Compared with the geochronology data of the contemporary volcanic rocks from the inner North China craton, the volcanic activities on the Tan-Lu fault zone began earlier and have a comparatively long history. Therefore, Chuzhou volcanic rocks indicated an intensive degree and a long period of the lithospheric thinning within the fault zone accompanying with these characteristics of higher degree melting and scale. More intensive interaction between the crust and mantle, and shallower depth of magma source, further demonstrated the Tan-Lu fault zone may be an extensive lithosphere thinning zone in eastern North China Craton.Chaohu intrusions were obtained from the southern segment of the Zhangbaling uplift along the Tan-Lu fault zone. Five samples show an emplacment ages range of 127～103Ma with 24Ma emplacement period. These intrusions are aluminous and silicon-saturation, a set of transitional magmatic rocks from metaluminous to peraluminous characterized by rich in alkali, potassium. Early and middle-early period intrusions are rich in LILE and LREE, depleted in HREE, and no or positive Eu anomaly, belonging to a set of adakite-like monzonitic granite, in contrast, late and middle-late A-type granites characterized by rich in LILE, and showing negative Ba, Sr, Eu anomalies and lower REE with more weakly fractional degree of LREE/HREE. The earlier adakite should represent the beginning of the lithosphere extensive movement, however the later A-type granites indicate the intensive period of the lithosphere extensive movement. Chaohu intrusions magmas also derived from ancient NNC, particularly ancient lower crust of the NCC. Earlier magmas contained few enriched mantle endmember components, later magmas came from shallower crust source. Geochemical characteristics indicate these intrusions did not suffer from the middle-upper crustal contamination during magma ascent, while the mother magma area experienced different degree partial melting, and fractional crystallization of different rock forming minerals and accessory minerals, such as zircon, apatite and allanite. With different magma emplacement age, residues of magma source are obviously different. Namely, from early to late, residue of magma source are transition from garnet to plagioclase, then to hornblende, accompanying with shallower and shallower magma source. The evolutive disciplines imply deep dynamic process of magmatic activities: continual thinning of lithosphere from NNC bottom led to constantly uplifting of the asthenosphere surface and persistently ascending of isotherm plane within the lithosphere, chemical action became gradually strong, thus appeared the partial melting of the lower crustal source at different depth. Compared with inner craton, the Tan-Lu fault zone show more intensive lithosphere extensive degree, shallower magma source, stronger evolution, higher partial melting extent, more permanent magmatic activities and complicated history. A series of evidences demonstrated the Tan-Lu fault zone is an extensive thinning zone during lithosphere thinning.Lujiang volcanic rocks lies on the most southeastern segment of the Tan-Lu fault zone. The eruption ages ranged from 125Ma to 93Ma, which belong to the middle-late stage of the early Cretaceous to early stage of the late Cretaceous, indicating a 32Ma history of this volcanism. These rocks are rich in alkali and potassium, and relatively depleted in MgO, TiO₂, with enriched Sr-Nd-Pb compositions, belong to cal-alkaline to alkaline series, which having a characteristics of bimodal volcanism. Samples of basalts and trachytes show enriched in LILE, LREE, depleted in HFSE, no negative Eu anomaly. In contrast, rhyolites display negative Ba, Sr, P and Ti anomalies, and obviously negative Eu anomaly. Geochemical data indicate that magma come from four different types of magma source. Earlier basic rocks （125Ma） represent primary nature of NNC lithospheric mantle source accompanying with crustal source. Rhyolites （120Ma） of them come from the middle-lower crust of North China, in addition, trachytes （100Ma） is a mixing of lower crust North China and lithospheric mantle source. Later basic rocks （93Ma） show NNC lithospheric mantle source accompanying with the lower crustal source and asthenospheric component. Geochemic investigations can not support lithospheric mantle and lower crustal detachment model, the magma evolution should be ascribed to the increasing of lithospheric heat flow followed sustaining thinning of its lower part, the asthenosphere upwelling and lithosphere extension also play a key factors for the magmatism. In addition, Some geological, geochemical and geophysical features suggest a more intensive extensive setting, which indicate a more intensive lithospheric thinning beneath this fault zone, the lithospheric thinning may be trigged by the activity of the fault zone, according to results of Chuzhou volcanic rocks and Chaohu intrusions.Investigations demonstrate that although the late Mesozoic magamatic rocks show different rock-type and geochemical characteristics, their magma sources belong to the NNC with a mixing nature of the crust and mantle. Combined with previous data, different segment magmatic activities in the Tan-Lu fault zone have a relatively long duration and a later terminal time, and indicate an intensively extensional tectonic setting. Obvious magmatic evolvement feature represents incessant changes of magma source component and temperature-pressure environment. All evidences may explain lithosphere gradually thinning from the top down, though they could not support whole lithosphere delamination model. A series of evidences demonstrate that the Tan-Lu fault zone is an intensive thinning zone, its occurrence and activities provides an inducing and promoting role during the lithosphere thinning according to geophysical phenomena nowadays, and plays an important role in the destruction of the NCC.更多还原