【作者】 邓新；

【导师】 杨坤光；

【作者基本信息】 中国地质大学 ， 构造地质学， 2013， 博士

【摘要】 大别造山带是扬子板块与华北板块三叠纪陆陆碰撞的产物,它不仅是世界上出露规模最大、保存最好的高压-超高压变质区,而且也是陆-陆碰撞之后,在超高压和高压岩石剥露过程中岩浆活动最为强烈的地区之一。大别造山带自超高压变质矿物发现便开始受到国内外众多学者的关注。大别造山带东端的郯庐断裂带将其与苏鲁造山带左行错开约500kmm,商城-麻城断裂将大别造山带分为东西两段,西段以南阳盆地与秦岭造山带相隔。东大别南界为襄樊-广济断裂,北界的信阳-舒城断裂带将造山带与合肥盆地分隔。大别造山带经历了中三叠世陆陆碰撞,发生深俯冲及超高压岩石变质过程；晚三叠世至早侏罗世超高压岩石折返至中上地壳的折返阶段和早白垩世(140-120Ma)造山后伸展,巨量花岗岩形成阶段。北大别处于大别造山带北部,北为磨子潭-晓天断裂,南为五河-水吼剪切带,东为郯庐断裂带,西为商城-麻城断裂带,是大别造山带出露面积最大的构造单元,由于出露新元古代变质侵入岩、超高压变质矿物、镁铁质-超镁铁质岩块、白垩纪巨量中酸性花岗岩,花岗岩的侵位年龄主要发生在140-120Ma,少数为中侏罗世(160-140Ma),与东部苏鲁造山带发育晚三叠世、侏罗纪及白垩纪岩浆具有明显的差别。岩石学、地球化学、地球物理研究表明大别造山带在侏罗纪仍然存在造山根,在早白垩世发生造山根拆沉垮塌,这与大别造山带由挤压构造体制转为伸展构造体制密不可分。大别山构造体制转换的具体时间、机制及其岩石圈减薄的过程是探讨大别造山带碰撞后造山过程与动力学的重要窗口。本论文将研究重点集中在北大别出露面积最大的、包含侵位次序最多的天堂寨复式岩体。通过野外宏观构造变形分析、接触带特征分析、岩体与剪切带变形关系分析、岩体的捕掳体的上升定位空间分析、显微构造与岩石磁组构分析、锆石LA-ICP-MS和SHRIMPU-Pb定年、全岩主量微量和稀土分析、Sr-Nd同位素分析、锆石Hf同位素分析,并结合岩石矿物温压计和地球物理资料对北大别天堂寨复式岩体进行了侵位次序、岩浆源区物质成分与深度、源岩的年龄、岩浆上升与定位的方式、花岗岩侵位机制、构造体制转换的时间、壳幔相互作用、岩石圈伸展减薄的过程与不同期次花岗岩侵位的对应关系的研究,并在此基础上探讨了大别造山带白垩纪花岗岩事件的构造属性。以大陆动力学与板块构造为指导理论,本文对天堂寨复式岩体(石鼓尖片麻状石英二长闪长岩、鹅公包眼球状二长花岗岩、九资河条带状二长花岗岩、天堂寨斑状二长花岗岩、二湾片麻状石英二长花岗岩和薄刀锋细粒花岗岩)进行侵位机制研究,结合构造地质学、显微构造地质学、岩石组构学、构造年代学、地球化学和变质地质学等学科最后综合前人研究成果系统讨论了北大别白垩纪花岗岩的侵位特征与晚中生代中国东部岩石圈减薄机制下巨量花岗岩侵位的耦合性,并在此基础上提出了大别造山带白垩纪花岗岩侵位模式图,探讨了大别造山带及整个中国东部晚中生代岩石圈减薄的动力学机制。取得的主要进展与结论如下：1.野外宏观构造变形与接触带研究得出天堂寨复式岩体侵位次序由早至晚为：石鼓尖片麻状石英二长闪长岩→鹅公包眼球状花岗岩→九资河条带状二长花岗岩→天堂寨斑状二长花岗岩→二湾片麻状石英二长花岗岩→薄刀锋细粒二长花岗岩,Si02含量逐渐增加,变形逐渐减弱,岩体中斑晶结晶形态逐渐完好,指示大别造山带伸展垮塌、地壳减薄过程中部分熔融形成花岗岩的源岩深度逐渐变浅。可以划分为三期岩体侵位过程,①早白垩世早期石鼓尖岩体构造变形特征与NE向韧性剪切带一致,属同构造变形产物,剪切带作为岩浆上升侵位的通道,对岩体形态、侵位特点、应力状态进行控制,并改造石鼓尖岩体；②鹅公包、九资河、天堂寨及二湾岩体内变形较弱,仅在NE、NW向剪切带发育局部发育韧性变形；③而薄刀锋内部变形弱,岩体中不发育剪切带。第一期和第二期岩体呈“网脉状”侵入于大别杂岩中和先期侵入的岩体,而并非典型的“气球膨胀型”岩浆上升侵位方式；而第三期薄刀锋细粒二长花岗岩中无大别杂岩,二者呈突变接触,是以“气球膨胀型”或岩墙形式侵入的。2.显微构造与岩石磁组构表明石鼓尖岩体为同构造侵入体,是在挤压构造背景下侵位的,锆石U-Pb年龄为141±2.3Ma,全岩主微量特征显示岩体侵位时具有加厚的地壳结构,角闪石Ti温压计指示源区深度是36km的下地壳的部分熔融,因此,北大别由挤压构造体制转为伸展构造体制和岩石圈减薄启动的时间在141Ma之后。3.锆石U-Pb定年显示天堂寨复式岩体三期侵位的年龄分别为141Ma,139-132Ma和132-127Ma,分别代表着构造体制转换的上限年龄、岩石圈减薄巨量花岗岩侵位的启动年龄和岩石圈减薄结束的时间。锆石中具有古元古代、中元古代、新元古代、早古生代及三叠纪的继承核,其中古、中元古代锆石年龄代表源岩的变质事件；新元古代年龄(830-695Ma)与扬子板块北缘新元古代构造岩浆事件年代相似,指示花岗岩的源岩来自俯冲的扬子板块北缘；早古生代年龄目前确切的地质意义还尚不明确；三叠纪年龄(243～203Ma)与大别造山带陆陆碰撞过程、深俯冲及折返过程年代相似,记录着碰撞造山过程。大别造山带晚中生代构造过程为：141Ma构造体制转换为伸展背景→135Ma伸展垮塌程度达到最大,绝大部分白垩纪花岗岩在此时期形成→125Ma伸展垮塌结束,大别造山带回归正常地壳厚度,形成未变形的细粒花岗岩和酸性、基性岩脉,大别造山带岩石圈减薄垮塌仅在几个Ma之内就完成,是一个快速的垮塌过程。4.岩石地球化学特征表明,天堂寨复式岩体的物源均来源于陆壳的部分熔融,由早至晚表现为地壳贡献增强的趋势,岩浆房和侵位深度逐渐变浅。石鼓尖岩体形成于加厚地壳,代表构造体制由挤压向伸展背景的转换的开始：鹅公包、九资河、天堂寨和二湾岩体代表构造体制转换过程中部分熔融的产物；薄刀锋岩体形成于转换后正常地壳厚度的熔融。5.锆石Hf结果显示,eHf(t)值均小于0,暗示花岗岩的源岩均来源于地壳的部分熔融,而无新生地幔的参与。北大别天堂寨复式岩体侵位时代由早至晚,eHf(t)值逐渐变小,向负值减小的方向变化,指示源区基性物质组分的减少,酸性物质组分增加。锆石Hf的两阶段模式年龄指示北大别三期花岗岩的源区分别来自2.9-3.1Ga加厚的下地壳基性岩、2.6-3.0Ga加厚的中酸性下地壳上部和2.2Ga减薄后正常厚度地壳的中酸性岩。6.岩体构造变形特征、岩石地球化学特征、锆石Hf同位素示综及角闪石温压计表明,石鼓尖岩浆源区深度为36km,上侵距离大于25kmm；天堂寨峰期岩体源区深度为45.5km,上侵距离为38.5km；薄刀锋岩体源区深度为15～17km,上侵距离约2.5km。反映了石鼓尖岩体侵位时大别造山带仍然为加厚的地壳,存在造山带山根,岩浆起源于下地壳约36km深度基性物质,以主动膨胀和岩浆侵吞作用为主上升获得定位空间；天堂寨峰期岩体(九资河、天堂寨、二湾岩体)形成时造山带山根开始垮塌,来源于造山带垮塌拆沉掉入岩石圈地幔的下地壳中酸性物质,以岩浆侵吞和构造扩展方式上升定位；而晚期薄刀锋细粒花岗岩则来源于垮塌结束后约12～17km深度的正常地壳的中酸性物质,以岩墙扩展方式侵位的。7.天堂寨复式岩体侵位机制是在挤压构造体制转为伸展构造体制后,加厚的岩石圈减薄、垮塌、拆沉,不同深度、不同源区的下地壳受到软流圈地幔上涌提供的热量发生部分熔融的结果。大别造山带岩石圈伸展减薄与晚中生代中国东部岩石圈减薄处于同一动力学背景,即特提斯构造域转为太平洋构造域之后,中国东部整体处于伸展背景,太平洋板块向下俯冲于欧亚板块之下的俯冲角度、方向的改变导致地幔加速对流,软流圈底侵于下地壳底部,造成下地壳不同层次、不同物质成分的部分熔融。更多还原

【Abstract】 The Dabie orogenic belt in eastern China is formed as a result of a Triassic collision between the North China and Yangtze cratons. Since the discovery of coesite and micro-diamond inclusions in eclogites from the Dabie orogen in Central East China, this region is one of the most important targets for studying UHP metamorphism that documents continental subduction to mantle depths. After continental collision, it appears the strongest magmatism. Cretaceous post-collisional intrusive and volcanic rocks comprise47%of the surface exposure of the Dabie orogen. Dabie orogenic belt is bounded by the Xinyang-Shucheng fault to the north, Xiangfan-Guangji fault to the south, Shangcheng-Macheng fault to the west, and Tancheng-Lujiang fault to the east. Tancheng-Lujiang fault is a left-lateral syn-orogenic transform fault formed in236Ma and its major strike-slip offset in the Cretaceous period. It caused the Dabi-Sulu orogenic belt sinistral offset more than500km. The Dabie orogen belt experienced Triassic continental collision, deep subduction, Ultra-high pressure(UHP) metamorphism, exhumation of the UHP rocks and crustal extension and large-scale magmatism during the Early Cretaceous period (140-120Ma).North Dabie Complex(NDC) is the biggest tectonic units in the northern of the Dabie orogenic belt. It is bounded by the Xiaotian-Mozitan fault to the north, Wuhe-Shuihou fault to the south, Shangcheng-Macheng fault to the west, and Tancheng-Lujiang fault to the east. NDC was intruded by metamorphic intrusive rocks in Neoproterozoic, voluminous granites and mafic-ultramafic rocks in the early Cretaceous period. Geologists have made great efforts on dating emplacement age of granite. Most zircon U-Pb ages for the granites in the North Dabie fall between140Ma and120Ma, and a few ages is middle-Jurassic(160-140Ma). It is greater difference between the Sulu orogenic belt for it has Late Triassic, Jurassic and Cretaceous igneous intrusion.Many petrology, geochemistry, geophysics and geochronology studies were used to reveal the structure of the Dabie orogenic belt. The Dabie orogenic belt has crustal root during the Jurassic period and experienced extension-collapse tectonics, lithospheric thinning and lower crust delamination in the Early Cretaceous. Thest geological process are interwovenness connect with the tectonic regime transforms from compression to extension. The time and mechanism of transformation and the process of the lithospheric thinning are the key to discuss the orogenetic process and dynamics after collision.This thesis was based on the Tiantangzhai complex granites which is the biggest intrusion and contains maximum emplacement sequence in the north Dabie mountain. The purposes of this paper are:(1) to present emplacement sequences,(2) to present the composition and the depth of granitic magma,(3) to constraint the age of magma,(4) to present the ascent and emplacement mechanism,(5) to present the transformation time from compression to extension,(6) Crust-mantle interactions,(7) to present the process of lithospheric thinning. Therefore, this study through integrated approach (eg. macroscopic structure deformation, contact zone, shear zone deformation, microstructure, anisotropy of magnetic susceptibility(AMS), LA-ICP-MS and SHRIMP zircon U-Pb dating, whole rock major, trace and rare earth element, Sr-Nd isotope, Zircon Hf isotope and geo-thermobarometers) to achieve the research. We then discuss the emplacement characteristic of Cretaceous granite in North Dabie and the coupling of enormous granite invasion under the mechanism of lithospheric thinning in eastern China during the late Mesozoic, on which basis, we proposed a mode chart of the granite invasion in North Dabie orogenic belt. We got following major conclusions.1. macroscopic structure on tectonic deformation and the contact zones in the field, we found that the emplacement sequence of the complex massif, from early to late, ranges from Shigujian gneissoid quartz monzodiorite to E’gongbao augen granite to Jiuzihe banding monzonitic granite to Tiantangzhai phyric monzonitic granite to Erwan gneissic granite to Bodaofeng fine-grained monzonitic granite. In this sequence, the content of SiO2is gradually increasing, the deformation is gradually weakening, while the crystal habit of the phenocrysts in the massif is gradually automorphic; which indicates that the deep source rock which partial melted to form granite gradually became thinner and thinner during the process of the extension and collapsing of Dabie orogenic belt and the reduction of crust.And the massif emplacement can be divided into three phases, i.e.,(1)in the early stage of early Cretaceous, the tectonic deformation features of Shigujian pluton are consistent with that of the NE ductile shear zone, both of them are the deformation products of syntectonic. The ductile shear zone, as the passage for the ascent of magma emplacement, controlled pluton shape, emplacement characteristic and stress state, as well as reformed Shigujian pluton;(2)the deformation in E’gongbao pluton, Jiuzihe pluton, Tiantangzhai pluton and Erwan pluton, comparatively weak, only took place in NE and/or NW ductile shear zone with partly ductile deformation;(3) the deformation in the interrior of Baodaofeng is weak and no ductile shear zone can be found there. The first-phase and second-phase pluton that intruded Dabie complex like a network and the pluton pre-intruded did not typically appear to be pumiceous inflation of magma; the third-phase pluton, Bodaofeng fine-grained adamellite, with no Dabie complex inside, being abrupt contact with each other, intruded in the way of dyke or balloon distending.2.Evidences from microstructure and anisotropy of magnetic susceptibility fabric indicate that Shigujian pluton, paratectonic, emplaced under the compressional background. And the zircon U-Pb dating result is141±2.3Ma. The characteristics of major and trace elements of whole rocks present the pluton emplacement possessed a thinken crustal structure. Amphibole-Ti temperature and pressure gauge indicates that the source area, about36km deep, partially melted in the lower crust. Hence, the tectonic transformation of north Dabie, from squeezing to extenting, happened after141Ma.3. Zircon U-Pb dating results have shown that the three phases of composite pluton in Tiantangzhai emplaced in141Ma,139-132Ma,132-127Ma separately, representing the upper limit age of the tectonic transformation, starting point of enomorously granitic emplacement while lithosphere thinning and the completion of lithosphere thinning, respectively. In a zircon, inherited cores in late proterozoic, middle proterozoic, neoproterozoic, early paleozoic and triassic can have been discovered. Among which, zircon age in late and middle proterozoic represents metamorphic event of the source area; zircon age in neoproterozoic, from830Ma to695Ma, similar to the age of neoproterozoic tectonomagmatic event in the northern boundary of Yangtze plate, indicates that the source of granite is from the Yangtze plate; the geological significance of zircon age in early paleozoic remains unknown; and zircon age in triassic, from243Ma to203Ma, closing to the age of the continent-continent collision, deep thrusting and turning-back process of Dabie orogen, records the process of collisional orogeny. Dabie orogen experienced a rapidly collapsing tectonic process in late mesozoic. At141Ma, the tectonic regime transformed into extensional background; at135Ma, the extension reached to its fullest, and most north Dabie Cretaceous granite formed during this period, at125Ma, the extension and collapsion ended and Dabie orogen returned to normal crust in thickness, resulting in undeformed granitello and both acidic and basic veins. The thinning and collapsion of lithosphere in Dabie orogen completed just in a few million years.4. Results from lithogeochemistry state that all magma source of the Tiantangzhai complex pluton came from partial melting of the continental crust, and during the process, the crust increasingly contributed while the depth of magma chamber and emplacement became lower and lower. Shiguqian pluton, formed in the thickened crust, represented the beginning of tectonic regime transformation from compression to extension; pluton from E’gongbao, Jiuzihe, Tiantangzhai and Erwan represents the partialy melted product from the tectonic regime transformation; and Bodaofeng pluton formed from the melting of crust in normal depth after the transforamtion.5. Zircon Hf isotope reveal that all the values of εHf(t) are under zero, suggesting that all the source rock of granite came from partial melting of the continental crust rather than the juvenile mantle. The values of εHf(t) gradually decreases with the development of Tiantangzhai complex pluton in North Dabie and changes in the direction of discreasing negative values. This indicates that the decreasing of basic components and increasing of acidic ones in the source area. The two-stage model age of Hf in zircon indicates the source area of three phases granites respectively came from (1)2.9-3.1Ga basic rock in the thickened lower crust,(3)2.6-3.0Ga intermediate acidity upper part of the thickened lower crust, and (2)2.2Ga intermediate acidity rock in the thinned crust with normal thickness.6. All the evidences above together have showed that the source area of Shigujian magma is36km deep, and its intrusion distance is greater than25km; the source area of Tiantangzhai granites is45.5km deep, and its intrusion distance is38.5km; the source area of Bodaofeng pluton is15-17km, and its intrusion distance is about2.5km. These factes indicate that Dabie orogen remained thickened crust during the emplacement of Shigujian pluton, orogenic root existed, and the magma origined from basic material about36km deep in lower crust and obtained room for placement by active expanding and intrusion; orogenic root began to collapse when Tiantangzhai granites (like Jiuzihe pluton, Tiantangzhai pluton and Erwan pluton) formed, resulting in intermediate acidity materials in the lower crust falling into the mantle in the lithosphere and they ascended by magmatic intrusion and structural expanding; late Bodaofeng granitello originated from intermediate acidity materials in normal crust about12-17km deep after the collapsion and emplaced in a way of dike expanding.7.After the tectonic regime transformed from compression to extension, the thickened lithosphere thinning, collapsed and delaminated. Due to the heat provided by the upwelling of asthenosphere in the mantle, the lower crust in different depth and different source area partly melted. And the Tiantangzhai complex pluton resulted from this process. The lithosphere of Dabie orogen extended and thinned under the same dynamic background with the late-Mesozoic thinning of lithosphere in the eastern China, that is, after the Tethys tectonic region transformed into the Pacific tectonic region, whole eastern China was under extensional setting, the convection of mantle speeded up as the angle and the direction of the Pacific plate subducting into Eurasian plate changed. The asthenosphere underplated into the bottom of the lower crust, causing partial melting of the lower crust at different levels and of different components.更多还原