摘要
本博士论文涉及两个研究领域:(1)大别山超高压变质岩普通Pb同位素体系,和(2)大别山榴辉岩和石榴辉石岩及所含金红石的微量元素地球化学。首先本文对整个东大别造山带三个主要的超高压岩片(北大别高温超高压杂岩带、中大别中温超高压变质带和南大别低温榴辉岩带)中的正片麻岩进行了系统的长石普通铅同位素研究,探讨大别山超高压变质岩普通Pb同位素组成以及Plumbotectonic模型在俯冲陆壳超高压变质造山带的适用性。主要研究目标是:(1)通过对罗田穹窿和北大别东部地区超高压正片麻岩的长石普通铅同位素研究,并结合前人对该区域岩石学,地球化学和年代学研究,观察罗田穹窿与北大别东部是否存在剥蚀深度的差异;(2)通过对三个超高压岩片的普通铅同位素组成的研究,探讨俯冲陆壳内部上、中、下地壳的Pb同位素演化和对Zartman的Plumbotectonic模型在俯冲陆壳超高压变质带应用的补充;(3)通过比较大别高压—超高压正片麻岩和苏鲁地区大陆科学钻探(CCSD)中主孔超高压变质岩的普通铅同位素组成,探讨大别与苏鲁超高压变质带之间的对比关系。本论文中的超高压榴辉岩和石榴透辉石岩及所含金红石主、微量元素地球化学研究研究包括:对大别-苏鲁地区70余件榴辉岩(石榴辉石岩)进行金红石激光原位分析,寻找适合于U-Pb定年的高U金红石,分别于中大别山双河地区发现了含高U金红石的榴辉岩透镜体和毛屋变镁铁质-超镁铁质岩中发现含高U金红石的石榴透辉石岩。在此基础上,分别对两个地区榴辉岩主、微量和单矿物及金红石的主、微量元素分析,并对部分高U含量金红石进行Cameca U-Pb定年工作,探讨了俯冲陆壳在进变质和退变质作用中微量元素的行为,和板块俯冲过程中可能存在具有不同Nb/Ta比特征以及低镁同位素组成的地幔楔流体交代作用。本博士论文获得主要研究成果如下:1.罗田穹窿作为北大别西南段一个特殊的构造单元,由于其经历了燕山期穹窿构造作用及后期的剥蚀,可能出露较北大别东部地区更深的超高压岩片。从普通Pb同位素组成的数据来看,这两个地区的花岗片麻岩具有类似的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb组成:罗田穹窿正片麻岩的普通Pb同位素组成为206Pb/204Pb=16.312-17.708(平均值为16.941),207Pb/204Pb=15.289~15.473(平均值为15.373),20sPb/204Pb=37.152~38.013(平均值为37.558)以及208Pb/206Pb=2.143~2.282(平均值为2.218);北大别带东部正片麻岩普通Pb组成为206Pb/204Pb=16.213-17.442(平均值为16.762),207Pb/204Pb=15.294-15.445(平均值为15.382),208Pb/204Pb=37.162~37.867(平均值为37.515)以及208Pb/206Pb=2.168~2.292(平均值为2.239)。两个区域之间并没有明显的Pb同位素差异,说明了罗田穹窿的剥蚀深度尚没有达到更深层次的超高压岩片,因此罗田穹窿和北大别东段可作为一个连续的超高压单元(北大别带)。2.大别山三个超高压岩片的普通铅同位素表现出如下特征:(1)在207Pb/204Pb vs.206Pb/204Pb图上中、南大别以及部分北大别正片麻岩数据(206Pb/204Pb>17.0)落在0.8-0.23Ga古等时线之间,另一些北大别数据落在1.6-0.8Ga(206Pb/204Pb4)特征。北大别206Pb/204Pb<17.0的数据主要沿下地壳演化线(k=5.9)分布,而南大别和中大别正片麻岩主要沿着4Two research fields were involved in this doctoral dissertation:(1) common Pb isotopic system of Dabie Ultra-High Pressure Metamorphic (UHPM) rocks, and (2) geochemistry of trace elements of eclogites and garnet pyroxenites, as well as rutiles in them from Dabie orogen.A systematic research on the Pb isotopic compositions of feldspars in Dabie UHPM orthogneisses from the whole eastern Dabie orogen (including Northern Dabie Zone (NDZ), Central Dabie Zone (CDZ) and Southern Dabie Zone (SDZ)) is present in this dissertation. One objective of this dissertation is to explore the pre-metamorphic Pb isotopic evolution of Dabie UHPM rocks and the suitability of the plumbotectonic model for UHPM belt in the collisional orogen. The main purposes are:(1) to examine differences of the erosion depths between Luotian dome and the eastern part of the NDZ,(2) to develop a common Pb isotope map of the subducted upper, middle and lower continental crusts, and to provide constraint on the pre-metamorphic evolution of the three main units from the Dabie orogen, and to investigate the suitability of the plumbotectonic model for UHPM belt in the collisional orogen, and (3) to discuss the relationship between Dabie UHPM terrane and Sulu UHPM terrane during the subduction and exhumation processes.The other main part of this dissertation is to constrain the behavior of trace elements in eclogite (garnet pyroxenite) and rutiles in them, based on a combined study of petrography, whole-rock major and trace elements, mineral major and trace elements as well as rutile trace elements, Cameca in-situ U-Pb dating. The major contents include:Trace elements compositions of rutiles from more than70eclogites and garnet pyroxenites from Dabie-Sulu orogen were obtained to find the rutiles with high U (U/Pb) which are suitable for Cameca in-situ U-Pb dating. Based on the major and trace elements compositions of eclogites, garnet pyroxenites and high U rutiles in Shuanghe and Maowu from the CDZ, the origin and constraining factors of the rutiles with high U, and potential application for rutile in-situ U-Pb dating, as well as the behavior of trace elements during pro-and retro-grade metamorphisms were investigated. Studies of two types of high U rutiles with distinctively different features of trace element compositions (especially Nb/Ta ratios) from Maowu garnet pyroxenites were taken to trace the multi-stages metasomatisms of the overlying mantle wedge in subduction region.The main results related to this thesis are given in details in the following:1. Although the LT dome as a special geologic feature in the south-western segment of the NDZ is a deeply eroded area with abundant felsic and mafic granulite. our data show the common Pb isotopic ratios of feldspar from the Luotian dome and the ENDZ orthogneisses are remarkably uniform in spite of the wide variations in206Pb/204Pb,207Pb/204Pb and208Pb/204Pb of them:Pb isotopic compositions of the LT dome orthogneisses show large variation with206Pb/204Pb ranging from16.312to17.708(average:16.941),207Pb/204Pb ranging from15.289to15.473(average:15.373),208Pb/204Pb ranging from37.152to38.013(average:37.558) and208pb/206pb ranging from2.143to2.282(average:2.218). The orthogneisses of the ENDZ show similar Pb isotope compositional characteristics as the LT dome, with206pb/204pb ranging from16.213to17.442(average:16.762).207Pb/204Pb ranging from15.294to15.445(average:15.382),208Pb/204Pb ranging from37.162to37.867(average:37.515) and208Pb/206Pb ranging from2.168to2.292(average:2.239). The uniformity of the common Pb isotopic compositions of the Luotian dome and the ENDZ orthogneisses indicates no deeper UHPM slice was exposed in the Luotian dome during post-collision orogeny. Thus, the LT dome and the ENDZ could be treated as a coherent unit (the NDZ) experiencing Triassic UHP metamorphism.2. The characteristics of common Pb isotopic compositions of three Dabie UHPM sices are present in the following:(1) on207Pb/204Pb vs206Pb/204Pb diagram, the arrays of the orthogneisses from part of the NDZ (206Pb/204Pb>17.0), the SDZ and he CDZ lie between the0.23Ga and0.8Ga paleogeochrons. while others from the NDZ (206Pb/204Pb<17.0) lie to the left of the0.8Ga paleogeochron and two of them to the left of the1.2Ga paleogeochron and right of the1.6Ga paleogcochron. This suggests that the Pb isotopic evolution of the feldspars was ended at0.23Ga and the common Pb data of the feldspars can only record premetamorphic Pb evolutions of three UHPM units, as well as Pb isotopic homogeni/ation did not take place among the three units during the UHP metamorphism, even in an individual unit. Two paragneisses from the CDZ lie above the model upper crust evolution curve and display significantly more radiogenic than the orthogneisses. corresponding to the sedimentary protolith with high u values. With respect to Pb isotopic compositions of the orthogneisses. however, only one sample (09TH-FZL-1) from the SDZ lie adjacent to the curve for the evolution line of the average orogen. The remainders are centred at the mantle evolution curve, with most data of the NDZ and SDZ arrays spreading between the mantle and lower crustal evolution curves far away from the model orogen evolution curve of Zartman et al.(1981). This indicates the source origins of three UHP units were derived from reworking of the ancient lower continental crust with Neoproterozoie mantle component. The CDZ and SDZ arrays show collinear with each other, while the SDZ is more radiogenic, meaning they may have similar premetamorphic Pb evolution history;(2) on-208Pb/204Pb vs206Pb/204Pb diagram. the NDZ data with (206Pb/204Pb16.8) of the NDZ. the data display a mixing trend from lower crust (high k and low radiogenic Pb) to an endmember (relatively low k and high radiogenic Pb). In addition, a mixing tendency was observed for both the CDZ and the SDZ arrays, which have a common endmember related to the NDZ. This can be explained by the influence of the retro-grade metamorphic fluid, released when the NDZ UHPM slice exhumed to middle crust level.The common Pb isotopic compositions of the Dabie UHPM orthogneisses are elearlv different from the modal orouenic evolution curve in Plumbotectonic model. The main reason is that, Plumbotectonic model was based on the assumption that older upper crust components contributed4/10of the remaining amounts of each older segment to the orogene during every orogeny, while this proportion of older lower crust was1/10. With respect to Pb isotopic compositions of the Dabie UHPM orthogneisses, they mainly inherit U-Pb isotopic features of their origin sources as dominant older lower continental crust and relatively less upper crust components. In addition, due to the subsequent Pb isotopic evolution from2.0-0.8Ga in the lower crust level, Pb compositions of the Dabie orthogneisses are distinct from modal orogen of Plumbotectonic model.Common Pb isotopic data of Dabie UHPM orthogneisses were simulated by a four-stages Pb evolution model. These four stages refer to:first stage is mantle Pb isotopic evolution from4.56to2.8Ga; second stage is Archean Pb evolution of juvenile upper and lower continental crusts from2.8to2.0Ga; third stage is the Pb isotopic evolution of the source origin of Dabie orthogneisses in lower crust level from2.0to0.8Ga; fourth stage is granitic magma derived from reworking of the source origin emplaced in different depths of continental crust at0.8Ga. The magmatic emplacement depths determine the U/Pb and Th/U values in subsequent Pb isotopic evolution from0.8-0.23Ga, leading to the different evolution histories of subducted upper and lower continental crust. The modeling results of orthogneiss data from three UHP units display that the NDZ arrays (206Pb/204Pb<16.8) have relatively low μ (3.4-9.6) for the0.8-0.23Ga evolution, corresponding to the observed high k character and magmatic intrusive emplacement of lower continental curst. The CDZ and SDZ arrays have relatively higher μ (calculated approximately10.9-17.2), corresponding to their magmatic emplacements into the middle and upper crust. Pb isotopic mixing among different slices would occur during the UHPM rocks exhumated to middle crust level with amphibolite facies retrograde metamorphism.Thus. U-Th-Pb isotopic system of UHPM subduction continental crust in collisional orogenic belt is mainly controlled by their magmatic sources and Pb isotopic evolution history, as well as emplacement depths in continental crust. The Pb isotopic compositions and specific multi-stages evolution histories of the source region should be taken into consideration. Moreover, the lower, middle and upper crust could be able to maintain their individual μ characters despite high-grade metamorphism. Generally, the slice emplaced in lower crust level (such as the NDZ) may be able to maintain the characters of its source origin and emplacement depth despite high-grade metamorphism.3. The comparison of common Pb isotopic compositions between the Dabie and the CCSD indicates the magmatic source of unit1contains lower and upper crustal components, with the emplacement at middle or upper crust levels as mentioned before. However, Pb data of unit3represent less radiogenic and similar k compared with the NDZ range, which could be predictable if greater amounts of old lower crust are involved. The least radiogenic samples occur in unit5, which is consistent with significantly more amounts of old lower crust contributed to the magmatic source than the NDZ. Thus, on the base of above modelling interpretation, relatively lower second stage μ of unit5than the respective μ of unit3and the NDZ could be suggested that the magmatic source of unit5emplaced at a deeper location than unit3and the NDZ. Such a unit has not been observed in the surface of the Dabie orogen. Because the extremely low radiogenic Pb has been observed in post-collisional granitoids in Dabie orogen, a hidden slab with such extremely low radiogenic Pb may exist as a covert exhumed UHPM slice below the NDZ unit. In this case, the Su-Lu orogen should have experienced a stronger intensity of the erosion to a deeper position when compared to the Dabie orogen, leading to the exposure of unit5.4. Compared with previous research of eclogites from thick formation in Shuanghe, the unretrograded eclogite in small lense display higher Th and U, as well as LREE-rich features. The decouplings of riched Th-U-LREE and depleted Rb-K-Ba, Th-U-LREE and MgO (SiO2) are not consistent with magmatic fractional crystallization or crustal contamination of their protoliths, which could be interpretated by the released fluid and the formation of metamorphic mineral with riched Th-U-LREE (monazite or allanite) during prograde metamorphism. With respect to the subchondritic Nb/Ta of the Th-U-LREE enriched eclogite, the fluid may be released in pro-metamorphic amphiblite-face. The above display that, Th-U-LREE could be held in some metamorphic rocks consisting of some minerals which have obviously high mineral-fluid partition coefficients during the activity of enriched metamorphic fluid in prograde metamorphism stage. In addition, research of retrograde metamorphic profiles of Shuanghe eclogite lenses with high U rutiles suggests that, topical retrograde metamorphism could lead to obvious reduction of whole-rock Th-U and LREE contents, as well as U compositions in rutiles. Furthermore, U contents in the rim of rutiles would fall further during rutile replacement by retrograde metamorphic titanite.5. It is well documented that Maowu UHPM ultramafic massive is a slice from the mantle wedge that emplaced into subducted continental crust during the Triassic continental collision, while the meta-mafic rocks (garnet pyroxenites) would also be derived from mantle wedge as pyroxene dikes for characteristic of their element compositions (high Mg#, Cr, Ni and Th-U-LREE enriched) and geological occurrence (interbedded with meta-ultramafic rocks). The U-enriched rutiles from the Th-U-LREE enriched Maowu garnet pyroxenites record two episodes of mantle metasomatism with low Nb/Ta (n=85,5samples) and high Nb/Ta (n=89,1sample), respectively. Based on zircon U-Pb upper intercept age of447±80Ma reported by Rowley et al.(1997) for the Maowu garnet pyroxenite, the mantle metasomatism may occur in the Paleozoic during oceanic subduction. This observation documents that low Nb/Ta mantle metasomatism occurred in the mantle wedge during oceanic subduction is the major reason for low Nb/Ta feature in the continental crust.