中条山-吕梁山前寒武纪变质杂岩的独居石电子探针定年研究

中条山和吕梁山前寒武纪变质杂岩是华北克拉通中部带南段的重要组成部分,它们的变质作用及其相关的花岗质岩浆作用是我们全面认识华北克拉通中部带的性质与演化的关键,具有重要的科学意义。中条山前寒武纪变质杂岩两个样品的独居石电子探针Th-U-Pb化学法定年表明,永济和横岭关岩体变质二长花岗质岩石记录了两个主要峰年龄范围,分别为1884.7~1849.9Ma和1743.5~1738.8Ma;前者为变质年龄,后者为流体活化的年龄。吕梁山五个样品的独居石电子探针Th-U-Pb化学法定年表明,石榴石二云母片岩和花岗质片麻岩的记录了>1902Ma、1883.3~1865.6Ma和1731.3Ma。>1902Ma年龄代表早期岩浆和深熔事件,1883.3~1865.6Ma代表峰期变质事件,1731.3Ma代表晚期流体活动事件。石榴石花岗岩记录了1882.8~1850.9Ma深熔岩浆事件。未变质花岗岩脉记录的1742.6Ma和石榴石二云母片岩记录的1731.1Ma为晚期岩浆-流体活动事件。综合上述独居石电子探针定年的结果,不难看出中条山—吕梁山前寒武纪变质杂岩主期变质作用发生在1885~1849Ma,并伴生有同期的S-型花岗质岩浆活动,与恒山—五台山—阜平杂岩的变质变形作用同时发生,进一步证明华北克拉通中部带的拼合作用发生在古元古代晚期。     

阿尔金淡水泉花岗质高压麻粒岩P-T演化及年代学研究

岩相学研究表明阿尔金南缘淡水泉一带出露的花岗质片麻岩的峰期矿物组合为Grt+Kfs+Ky+Rt+Qz,为典型的高压麻粒岩相矿物组合。利用Thermocalc3.33程序计算的P-T视剖面图和矿物温压计,确定其峰期变质温压条件为T=875~925℃,P=14.5~15.9kbar,该岩石后期还经历了以矿物组合Grt+Kfs+Bt+Pl+Sill+Ilm+Qz为代表的麻粒岩相和以Sill+Bt+Pl+Mu+Qz±Ilm为代表的角闪岩相的退变质作用,具有一个早期快速等温降压,后期近等压降温的顺时针型的退变质P-T演化轨迹。锆石的U-Pb同位素定年结果显示其原岩的时代为866±5Ma,峰期变质时代为505±5Ma。其中该岩石原岩的形成时代与阿尔金及其周缘地区广泛发育的由Rodinia超大陆裂解事件导致的岩浆活动时代一致,而峰期变质时代则与南阿尔金高压/超高压变质岩的峰期变质时代(486~509Ma)一致,说明花岗片麻岩原岩的形成与罗迪尼亚超大陆裂解有关,并在早古生代卷入了陆壳深俯冲事件,构成了南阿尔金高压/超高压变质带的一部分。     

中大别腹地榴辉岩锆石U-Pb年龄及其类型归属

根据对黄岗-牛凸岭地区榴辉岩样品JS249和JS250的岩相学研究表明,该地区榴辉岩具有明显的高压变质特征,石榴石中包含大量的早期矿物,成分环带显著,其峰期矿物组合主要为Grt+ Omp+ Phn+ Rt+ Qtz+ Ep+ Ky,角闪岩相退变矿物主要为Di+ Mg-Hbl+ Ab+ Prg+ Bt.对样品JS249和JS250温压条件进行计算,样品JS249为T=594±61℃,P=2.58±0.27GPa;样品JS250为T=616±56℃,P=2.14±0.14GPa.同时利用LA-ICPMS以及激光拉曼对这两个样品进行锆石U-Pb年代学研究和锆石包裹体分析,结果显示样品JS249锆石包体主要为角闪石、长石等退变矿物,其加权平均年龄为217.3±3.5Ma,该组年龄代表了角闪岩相退变质年龄;样品JS250锆石包含了石榴石、金红石、绿辉石等峰期矿物包体,其加权平均年龄为235.2±4.2Ma,该年龄应为峰期变质年龄.综合榴辉岩的岩石学、热力学和年代学的研究表明,黄岗-牛凸岭地区应属于低温高压单元,与南大别变质块体类似.     

新疆西南天山木扎尔特河一带低压泥质麻粒岩岩石学特征、独居石U-Th-Pb定年及其地质意义

古南天山洋闭合过程中,由于洋壳俯冲产生的岛弧岩浆作用加热大陆地壳,在新疆西南天山木扎尔特一带形成了一套低压高温泥质麻粒岩相变质岩石.本文用Theriak-Domino热力学软件对该套岩石中的堇青石榴夕线石黑云母片麻岩和含夕线石堇青石榴黑云母片麻岩进行了岩石学相平衡计算研究,得到它们峰期变质的温压条件分别是：T=630～674℃,P=5.2～5.5kbar和T=645～684℃,P=5.4～5.7kbar.并采用独居石Th-U-Pb电子探针定年方法,对样品WQ006中的3颗独居石进行了原位年龄测定(38个分析点),得到2组等时线年龄,分别是376±8Ma和280σ8Ma(2σ).结合独居石的岩相学特征,提出了新疆西南天山低压高温麻粒岩相峰期变质作用的时代为280±8Ma,而376±8Ma(2σ)可能为原沉积岩的原岩/成岩年龄.表明西南天山洋壳开始俯冲发生在晚古生代,进一步证明了西南天山造山带俯冲碰撞发生在晚二叠纪之后的观点.     

佳木斯地块花岗质片麻岩的独居石年龄及其地质意义

依据电子探针独居石Th-U-Pb化学法(CHIME)对分布于佳木斯地块北部花岗质片麻岩进行定年,其中独居石415 Ma年龄值报道了佳木斯地块北部存在志留纪晚期的岩浆热事件,(494.35±23.6)Ma、(506.96±52.20)Ma(、481.04±23.19)Ma的独居石年龄值表明,佳木斯地块北部存在500 Ma(晚泛非期)的高级变质事件,结合南部的锆石SHRIMP年龄数据,认为整个佳木斯地块都存在500 Ma(晚泛非期)的高级变质事件。该时期佳木斯地块和松嫩地块可能已经拼贴到一起,共同遭受了500 Ma(晚泛非期)变质事件的影响。     

Determination of Early Paleozoic metamorphic event in the Longshan Complex and its geological implicationSCIEI北大核心CSCD

长期以来,陇山杂岩的归属问题一直存在争议。本文对出露于秦岭-祁连山结合部位的陇山杂岩中石榴黑云斜长片麻岩和石榴斜长角闪岩进行了详细的岩石学、P-T温压计算、独居石和榍石U-Pb年代学研究。通过详细的岩相学观察,石榴黑云斜长片麻岩的变质峰期矿物组合为石榴子石+黑云母+斜长石+石英;石榴斜长角闪岩中则识别出了以石榴子石+单斜辉石+角闪石+斜长石+石英为峰期的变质矿物组合。通过传统温压计计算,石榴黑云斜长片麻岩(样品21LS40)和石榴斜长角闪岩样品(样品21LS42-1)的峰期变质P-T条件分别为700℃、0.72GPa和710℃、0.74GPa。激光剥蚀电感耦合等离子体质谱(LA-ICP-MS) U-Pb数据表明,石榴黑云斜长片麻岩中独居石的206Pb/238U的加权平均年龄为407~435Ma。石榴斜长角闪岩中榍石的下交点年龄分别为410±7Ma、409±5Ma和426±10Ma,榍石中Zr含量温度计的计算结果分别为750℃、751℃和748℃(假定压力为0.7GPa)。本文从变质作用的角度出发,将陇山杂岩与秦岭杂岩进行温压条件和变质时代对比研究,认为陇山杂岩与东秦岭杂岩高压-超高压岩石的最后一期退变质作用和西秦岭天水地区的秦岭杂岩麻粒岩相变质作用类似,可能为北秦岭造山带的西延。     

江西庐山星子群片麻岩的变质年龄和变质机制:独居石和锆石SIMS年代学

江西庐山地区星子群片麻岩的构造属性对认识江西甚至华南的大地构造具有重要意义。对星子群片麻岩穹窿不同构造部位的4个片麻岩中独居石进行高精度SIMS U-Pb年龄测定,结果显示它们的²⁰⁶Pb/²³⁸U加权平均年龄分别为141.0±1.0 Ma、143.0±1.0 Ma、143.5±1.0 Ma和144.2±1.6 Ma,因此将星子群的峰期变质时代限定于141.0～144.2 Ma。同时本文首次报道了庐山观音尖辉长岩的锆石SIMSU-Pb定年,确定其侵位年龄为141.1±1.5Ma,这与星子群片麻岩独居石获得的峰期变质年龄在误差范围内一致。结合星子群的变质温度和变质分带特征,推测庐山片麻岩穹窿可能是中生代岩浆热穹窿,其角闪岩相的变质作用可能是岩浆热动力变质的产物。本研究不仅为庐山地区构造变形属性的厘定提供了详实的定年证据,还能为中国东部中生代板内强烈变质变形的研究提供重要参考。     

南天山榆树沟高压麻粒岩地体多期变质定年研究

通过详细的矿物学及岩石学研究,特别是对角闪石的系统研究,确定榆树沟麻粒岩地体至少经历过高压麻粒岩相、中压麻粒岩相、角闪岩相和绿片岩相四期变质作用的改造。总结了各期变质作用的期次和特点。在此基础上,采用~(40)Ar-~(39)Ar同位素定年获得368.2±4.8Ma坪年龄和360±10Ma等时线年龄。采用Sm-Nd同位素矿物等时线定年获得Gra+Pl+Ⅰlm+全岩的等时线年龄为310±5Ma。结合已发表的定年结果综合分析认为,前者可代表榆树沟地体峰期变质-高压麻粒岩相变质作用年龄;后者为峰期后经受中压麻粒岩相变质变形的叠加改造年龄。讨论了多期变质作用中同位素的均一化和封闭温度问题。     

东喜马拉雅构造结泥质高压麻粒岩的锆石和独居石定年与地质意义

东喜马拉雅构造结的南迦巴瓦杂岩含有广泛分布的高压麻粒岩,但由于以前获得了许多不同的年龄,对这些麻粒岩的变质与深熔时代、持续时间和成因存在不同认识。本文对泥质高压麻粒岩（蓝晶石榴黑云片岩）中的锆石和独居石进行了系统的内部结构、U-（Th）-Pb定年和微量元素分析,以求揭示这些岩石是否具有相同的演化过程。所研究的6个蓝晶石榴黑云片岩由石榴石、蓝晶石、黑云母、石英、钾长石、斜长石、夕线石、白云母、石墨和副矿物金红石、钛铁矿、锆石和独居石组成,峰期矿物组合是石榴石+蓝晶石+斜长石+钾长石+黑云母+石英+金红石。6个样品中的锆石均由继承碎屑核+变质（深熔）幔+变质（深熔）边组成。其中3个样品中的锆石幔和边较宽,均可进行原位定年,幔部给出了类似的较老年龄范围（39.6~31.6Ma、40.8~32.0Ma和38.1~31.3Ma）,而边部给出了类似的较年轻年龄范围（26.8~17.3Ma、28.3~18.6Ma和28.4~18.8Ma）。另外3个样品的锆石幔部较窄,不能进行分析,其边部给出了与前3个样品锆石边部类似的年轻年龄范围（22.0~17.0Ma、20.9~16.9Ma和22.2~16.6Ma）。一个片岩样品中的独居石给出了与其锆石幔部+边部年龄类似的较宽年龄范围（38.1~17.5Ma）,而另外3个样品中的独居石获得了与其锆石边部年龄相似的年轻年龄范围（26.0~18.8Ma、22.3~16.9Ma和26.4~19.4Ma）。随着年龄的减小,锆石和独居石的Th/U比值增大,Eu/Eu^*减小,独居石的HREE和Y含量减小。基于这些分析结果,笔者认为所研究的6个片岩记录了相同的、从~41Ma持续到~17Ma的进变质与深熔过程。但是,由于某些样品中的锆石和独居石在早期变质和深熔过程中形成的结晶域（锆石幔部）很窄,无法定年,导致不同的样品获得了不同的年龄范围。结合现有研究成果,笔者推测南迦巴瓦杂岩中的高压麻粒岩经历了相似的长期进变质与深熔过程。     

柴北缘早古生代中低压麻粒岩相变质作用——以乌兰地区泥质麻粒岩为例

柴北缘乌兰地区出露有大量早古生代中低压泥质麻粒岩。据岩石学和矿物化学研究,将乌兰地区早古生代泥质麻粒岩变质作用划分为两期。其中峰期变质阶段矿物组合为Grt₁+Bt₁+Sil+Kfs+Pl+Qtz+melt,退变质阶段矿物组合为Bt₂+Ms+Grt₂+H₂O±Pl±Qtz。通过对泥质麻粒岩样品进行锆石LA-ICP-MS U-Pb定年,获得泥质麻粒岩峰期变质时代为(482±3)Ma。为厘清乌兰地区早古生代泥质麻粒岩变质演化历史,揭示麻粒岩变质机制,通过相平衡模拟技术和黑云母Ti温度计,获得乌兰地区早古生代泥质麻粒岩峰期和退变质温压条件分别为:710℃～730℃/5～6.2 kbar和560℃～650℃/5～6.2 kbar。由于岩浆的冷却使其退变质阶段经历等压降温变质作用。     

阿拉善地块东部早古生代(约420 Ma)变质事件的发现及其地质意义:来自独居石与锆石U-Pb定年的新证据

前寒武纪变沉积岩系是阿拉善地块重要组成部分,准确测定其原岩成岩时代和变质时代,对于进一步认识阿拉善地块起源、形成、演化与亲缘性具有十分重要的科学意义.本文对阿拉善地块东部阿拉坦敖包地区德尔和通特组含蓝晶石石榴云母石英片岩开展了系统的野外地质调查、岩相学观察和同位素年代学研究.碎屑锆石U-Pb定年和野外地质调查表明,阿拉坦敖包地区德尔和通特组和祖宗毛道组碎屑锆石年龄介于3 306~1 146 Ma,并以出现中元古代锆石年龄（1 800~1 100 Ma）高频区为特征,最小一组碎屑锆石年龄约为1 123 Ma,结合区域上它们被约900 Ma花岗质片麻岩侵入的野外关系,阿拉坦敖包地区德尔和通特组与祖宗毛道组变沉积岩系的沉积时代被限定在1 123~900 Ma之间,为一套中元古代晚期?新元古代早期陆缘沉积建造,具有亲华北板块的特点.此外,变质独居石和变质锆石U-Pb定年和微量元素分析表明,阿拉坦敖包地区德尔和通特组含蓝晶石泥质片岩中发育大量变质独居石,它们具有典型变质独居石的稀土元素配分模式（轻稀土元素强烈富集而重稀土元素强烈亏损）,其加权平均年龄为419±3 Ma（MSWD=7.1,n=40）,类似地,一个锆石变质边也记录了406±7 Ma的206Pb/238U年龄.这些变质年龄被解释为阿拉善地块东部阿拉坦敖包地区德尔和通特组含蓝晶石石榴云母石英片岩遭受早古生代末角闪岩相变质?变形作用的时代,可能是阿拉善地块与周缘微陆块早古生代末碰撞造山作用的响应.      

Late Neoproterozoic,Ordovician and Carboniferous events recorded in monazites from southern-central Madagascar

The evolution of the basement of southern Madagascar north and south of the Ranotsara shear zone was investigated using (U + Th)/Pb electron probe monazite age dating in combination with petrographic constraints. Several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic, metamorphic and/or fluid events. The oldest ages, ranging from 630–2400 Ma, occur relatively rare in relic cores. A first, clear age-population is dated at 550–560 Ma. Most ages fall in two populations at 420–460 and 490–500 Ma, which in some samples overlap in error. We interprete these ages as dating low-pressure and high-temperature metamorphism. We have also clear evidence for Carboniferous (300–310 Ma) monazite overgrowth rims, which can not directly be related to macroscopic structures or metamorphic parageneses. In combination with literature data, we propose that the observed monazite age populations are related to Gondwana amalgamation and subsequent rifting events during the break up of Gondwana. Our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of Madagascar.     

Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

The paragenesis of monazite in metapelitic rocks from the contact aureole of the Harney Peak Granite, Black Hills, South Dakota, was investigated using zoning patterns of monazite and garnet, electron microprobe dating of monazite, bulk-rock compositions, and major phase mineral equilibria. The area is characterized by low-pressure and high-temperature metamorphism with metamorphic zones ranging from garnet to sillimanite zones. Garnet porphyroblasts containing euhedral Y annuli are observed from the garnet to sillimanite zones. Although major phase mineral equilibria predict resorption of garnet at the staurolite isograd and regrowth at the andalusite isograd, textural and mass balance analyses suggest that the formation of the Y annuli is not related to the resorption-and-regrowth of garnet having formed instead during garnet growth in the garnet zone. Monazite grains in Black Hills pelites were divided into two generations on the basis of zoning patterns of Y and U: monazite 1 with low-Y and -U and monazite 2 with high-Y and -U. Monazite 1 occurs in the garnet zone and persists into the sillimanite zone as cores shielded by monazite 2 which starts to form in the andalusite zone. Pelites containing garnet porphyroblasts with Y annuli and monazite 1 with patchy Th zoning are more calcic than those with garnet with no Y annuli and monazite with concentric Th zoning. Monazite 1 is attributed to breakdown of allanite in the garnet zone, additionally giving rise to the Y annuli observed in garnet. Monazite 2 grows in the andalusite zone, probably at the expense of garnet and monazite 1 in the andalusite and sillimanite zones. The ages of the two different generations of monazite are within the precision of chemical dating of electron microprobe. The electron microprobe ages of all monazites from the Black Hills show a single ca. 1713 Ma population, close to the intrusion age of the Harney Peak Granite (1715 Ma). This study demonstrates that Y zoning in garnet and monazite are critical to the interpretation of monazite petrogenesis and therefore monazite ages.     

Discovery of Ordovician–Silurian metamorphic monazite in garnet metapelites of the Alpine External Aiguilles Rouges Massif

The pre-Mesozoic, mainly Variscan metamorphic basement of the Col de Bérard area (Aiguilles Rouges Massif, External domain) consists of paragneisses and micaschists together with various orthogneisses and metabasites. Monazite in metapelites was analysed by the electron microprobe (EMPA-CHIME) age dating method. The monazites in garnet micaschists are dominantly of Variscan age (330–300 Ma). Garnet in these rocks displays well developed growth zonations in Fe–Mg–Ca–Mn and crystallized at maximal temperatures of 670°C/7 kbar to the west and 600°C/7–8 kbar to the east. In consequence the monazite is interpreted to date a slightly pressure-dominated Variscan amphibolite-facies evolution. In mylonitic garnet gneisses, large metamorphic monazite grains of Ordovician–Silurian (~440 Ma) age but also small monazite grains of Variscan (~300 Ma) age were discovered. Garnets in the mylonitic garnet gneisses display high-temperature homogenized Mg-rich profiles in their cores and crystallized near to ~800°C/6 kbar. The Ordovician–Silurian-age monazites can be assigned to a pre-Variscan high-temperature event recorded by the homogenised garnets. These monazite age data confirm Ordovician–Silurian and Devonian–Carboniferous metamorphic cycles which were already reported from other Alpine domains and further regions in the internal Variscides.     

Age constraints on Late Paleozoic evolution of continental crust from electron microprobe dating of monazite in the Peloritani Mountains (southern Italy): another example of resetting of monazite ages in high-grade rocks

In situ monazite microprobe dating has been performed, for the first time, on trondhjemite and amphibolite facies metasediments from the Peloritani Mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the Hercynian Belt. All samples show single-stage monazite growth of Hercynian age. One migmatite and one biotitic paragneiss yielded monazite ages of 311 ± 4 and 298 ± 6 Ma, respectively. These ages fit with previous age determinations in similar rocks from southern Calabria, indicating a thermal metamorphic peak at about 300 Ma, at the same time as widespread granitoid magmatism. The older of the two ages might represent a slightly earlier event, possibly associated with the emplacement of an adjacent trondhjemite pluton, previously dated by SHRIMP at 314 Ma. No evidence for pre-Hercynian events and only a little indication for some monazite crystallization starting from ca. 360 Ma were obtained from monazite dating of the metasediments, suggesting either a single-stage metamorphic evolution or a significant resetting of the monazite isotope system during the main Hercynian event (ca. 300 Ma). Rare monazite from a trondhjemite sample yields evidence for a late-Hercynian age of about 275 Ma. This age is interpreted as representing a post-magmatic stage of metasomatic monazite crystallization, which significantly postdates the emplacement of the original magmatic body.     

西秦岭勉县北部光头山二长花岗岩独居石电子探针U—Th—Pb化学法定年及其地质意义

南秦岭勉略缝合带北侧光头山花岗岩体由早期的石英闪长质-英云闪长质岩石和晚期的S型二长花岗岩组成。其中早期侵入相明显地经历了韧性剪切带的改造,形成于勉略构造带闭合之前。晚期的S型二长花岗岩侵位于早期的石英闪长岩、英云闪长岩和韧性剪切带中．是与勉略洋盆闭合密切相关的同碰撞-后碰撞阶段侵位的花岗岩。所以光头山岩体的形成年龄可以较好地限定西秦岭勉略洋盆闭合、扬子板块与微秦岭板块碰撞的时代。2个二长花岗岩样品的独居石电子探针U—Th—Pb定年分析,分别得到3组年龄和4组年龄。结合前人的研究成果、独居石BSE图像和表观年龄分布,确定S型花岗岩的结晶年龄为209-196Ma,它限定了勉略构造带闲合隆升的时代为晚三叠世-早侏罗世;较老的240Ma±4Ma、318Ma±5Ma和249Ma±4Ma记录了早期与勉略缝合带闭合洋壳俯冲相关的残留独居石的年龄,年轻的184Ma±3Ma和154Ma±3Ma反映了后期的变形作用和流体活动等的改造作用。     

Prograde destruction and formation of monazite and allanite during contact and regional metamorphism of pelites: petrology and geochronology

The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and ²⁰⁸Pb/²³²Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th-Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th-Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3Dž.5 Ma in the Buchan terrain, 352.9NJ.9 Ma in the Barrovian terrain, and 403.4LJ.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1Nj.1 Ma. Interpretation of Th-Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.     

西秦岭勉县北部光头山二长花岗岩独居石电子探针U-Th-Pb化学法定年及其地质意义

南秦岭勉略缝合带北侧光头山花岗岩体由早期的石英闪长质—英云闪长质岩石和晚期的S型二长花岗岩组成。其中早期侵入相明显地经历了韧性剪切带的改造,形成于勉略构造带闭合之前。晚期的S型二长花岗岩侵位于早期的石英闪长岩、英云闪长岩和韧性剪切带中,是与勉略洋盆闭合密切相关的同碰撞—后碰撞阶段侵位的花岗岩。所以光头山岩体的形成年龄可以较好地限定西秦岭勉略洋盆闭合、扬子板块与微秦岭板块碰撞的时代。2个二长花岗岩样品的独居石电子探针U-Th-Pb定年分析,分别得到3组年龄和4组年龄。结合前人的研究成果、独居石BSE图像和表观年龄分布,确定S型花岗岩的结晶年龄为209～196Ma ,它限定了勉略构造带闭合隆升的时代为晚三叠世—早侏罗世;较老的240Ma±4Ma、318Ma±5Ma和249Ma±4Ma 记录了早期与勉略缝合带闭合洋壳俯冲相关的残留独居石的年龄,年轻的184Ma±3Ma和154Ma±3Ma反映了后期的变形作用和流体活动等的改造作用。     

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists

A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.