Ultrahigh‐temperature metamorphism in the Tuguiwula area,Khondalite Belt,North China Craton

In this study, sapphirine‐bearing granulites and sapphirine‐absent garnet–sillimanite gneisses from the Tuguiwula area in the eastern segment of the Khondalite Belt, North China Craton (NCC) are interpreted to show a P–T evolution involving cooling at pressures of 8–9 kbar from >960°C to the solidus (~820°C) and late subsolidus decompression. This interpretation is based on the sequence of mineral appearance and thermodynamic modelling of phase equilibria. Sapphirine is observed to coexist with spinel within the peak assemblages. This observation conflicts with the traditional view that spinel generally appears prior to sapphirine and thus indicates pre‐T_max compression. For ultrahigh‐temperature (UHT) metapelites at Tuguiwula, a clockwise P–T path may be more likely, which would be consistent with the clockwise P–T evolution of the extensive “normal” granulites (T_max <900°C) and UHT granulites at other localities in the eastern segment of the Khondalite Belt. At Tuguiwula, for UHT metapelites with low bulk‐rock Mg/(Mg+Fe^T), the oxidation state/Fe³⁺ content is interpreted to be a significant factor in controlling the mineral assemblages. We find that these compositions tend to contain sapphirine under oxidized conditions but spinel (without sapphirine) under reduced conditions. This difference may account for the simultaneous presence of both sapphirine‐bearing UHT granulites and sapphirine‐absent garnet–sillimanite UHT gneisses at Tuguiwula. LA‐ICP‐MS U–Pb dating of metamorphic zircon in the UHT metapelites yields mean ²⁰⁷Pb/²⁰⁶Pb ages of c. 1.92 Ga (two samples), which are interpreted to record the timing of cooling of the UHT rocks to the solidus. The UHT metamorphism is interpreted to have been generated by mantle upwelling and emplacement of mafic magmas within a post‐orogenic setting.     

Petrology,phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex,North China Craton

The Fuping Complex is one of the important basement terranes within the central segment of the Trans‐North China Orogen (TNCO) where mafic granulites are exposed as boudins within tonalite–trondhjemite–granodiorite (TTG) gneisses. Garnet in these granulites shows compositional zoning with homogeneous cores formed in the peak metamorphic stage, surrounded by thin rims with an increase in almandine and decrease in grossular contents suggesting retrograde decompression and cooling. Petrological and phase equilibria studies including pseudosection calculation using thermocalc define a clockwise P–T path. The peak mineral assemblages comprise garnet+clinopyroxene+amphibole+quartz+plagioclase+K‐feldspar+ilmenite±orthopyroxene±magnetite, with metamorphic P–T conditions estimated at 8.2–9.2 kbar, 870–882 °C (15FP‐02), 9.6–11.3 kbar, 855–870 °C (15FP‐03) and 9.7–10.5 kbar, 880–900 °C (15FP‐06) respectively. The pseudosections for the subsequent retrograde stages based on relatively higher H₂O contents from P/T–M(H₂O) diagrams define the retrograde P–T conditions of <6.1 kbar, <795 °C (15FP‐02), 5.6–5.8 kbar, <795 °C (15FP‐03), and <9 kbar, <865 °C (15FP‐06) respectively. Data from LA‐ICP‐MS zircon U–Pb dating show that the mafic dyke protoliths of the granulite were emplaced at c. 2327 Ma. The metamorphic zircon shows two groups of ages at 1.96–1.90 Ga (peak at 1.93–1.92 Ga) and 1.89–1.80 Ga (peak at 1.86–1.83 Ga), consistent with the two metamorphic events widely reported from different segments of the TNCO. The 1.93–1.92 Ga ages are considered to date the peak granulite facies metamorphism, whereas the 1.86–1.83 Ga ages are correlated with the retrograde event. Thus, the collisional assembly of the major crustal blocks in the North China Craton (NCC) might have occurred during 1.93–1.90 Ga, marking the final cratonization of the NCC.     

华北克拉通桑干地区高压麻粒岩的锆石SHRIMP U-Pb年龄及其地质含义

为了揭示华北克拉通桑干地区古元古代高压麻粒岩变质峰期时限,对选自该区的两个高压麻粒岩样品(DST02,XYS01)进行了锆石SHRIMP U-Pb测年。锆石样品的阴极发光图像显示为球形和无内部结构,Th/U比值变化为0.01~0.93。这些特征表明两样品的锆石应属于变质成因锆石。两样品的SHRIMP测年结果分别给出(1 792±12)Ma和(1 891±46)Ma。根据前人的年代学方面的研究成果,特别是未变质强过铝花岗岩中获得的1 900~1 850 Ma的锆石U-Pb年龄(郭敬辉等,2002)结果来看,本区高压麻粒岩峰期变质时代不会晚于此。因此本文获得的1 850~1 800 Ma的锆石SHRIMP U-Pb年龄应代表退变年龄。而(1 891±46)Ma的年龄限定了峰期高压变质年龄上限。     

Ultra‐high temperature metamorphism recorded in Fe‐rich olivine‐bearing migmatite from the Khondalite Belt,North China Craton

We document the first occurrence of Fe‐rich olivine‐bearing migmatitic metapelite in the Khondalite Belt, North China Craton. Petrological analyses revealed two exotic assemblages of orthopyroxene+spinel+olivine and orthopyroxene+spinel+cordierite. Phase relation modelling suggests that these assemblages are diagnostic of ultra‐high temperature (UHT) metamorphism in the Fe‐rich system, with temperatures from 1,000 to 1,050°C at 0.6 GPa. U–Th–Pb SIMS analyses on zircon reveal a similar age of c. 1.92 Ga for the olivine‐bearing migmatite and an adjacent gabbronoritic intrusion that is therefore identified as the heat source for the UHT metamorphism. These results, coupled with additional analysis of the famous Tuguiwula sapphirine‐bearing granulite, lead to a re‐appraisal of the P–T path shape and heat source for the UHT metamorphism. We suggest that UHT metamorphism, dated between 1.92 and 1.88 Ga, across the whole Khondalite belt, proceeded from a clockwise P–T evolution with an initial near‐isobaric heating path at ~0.6–0.8 GPa, and a maximum temperature of 1,050°C followed by a cooling path with minor decompression to ~0.5 GPa. Considering our results and previous works, we propose that the orogenic crust underwent partial melting at temperature reaching 850°C and depth of ~20 to ~30 km during a period of c. 30 Ma, between 1.93 and 1.90 Ga. During this time span, the partially molten crust was continuously intruded by mafic magma pulses responsible for local greater heat supply and UHT metamorphism above 1,000°C. We propose that the UHT metamorphism in the Khondalite belt is not related to an extensional post‐collisional event, but is rather syn‐orogenic and associated with mafic magma supplies.     

华北克拉通孔兹岩带东端孤山剖面石榴石基性麻粒岩的变质作用及年代学研究

大同孤山石榴石基性麻粒岩出露在华北克拉通孔兹岩带与中部带的构造接触部位,以大小不等的透镜体形式产于孔兹岩带内的夕线石榴片麻岩和紫苏二长片麻岩中.根据岩相学观察、矿物化学研究、P-T视剖面图和传统温压计计算结果,揭示孤山石榴石基性麻粒岩经历了4个阶段的变质演化:早期进变质阶段(M1)的主要矿物组合为石榴石核心及其内部包体矿物单斜辉石+角闪石+斜长石+石英+钛铁矿±金红石.反环带斜长石富钠核部记录了早期压力可达11k bar;峰期变质阶段(M2)的矿物组合是石榴石斑晶和基质中的单斜辉石+斜方辉石+斜长石+钛铁矿+石英,记录的温压条件为850～900℃、9～10kbar;峰期后降压阶段(M3)的标志是石榴石外围发育的后成合晶和冠状环,主要有单斜辉石+斜长石、斜方辉石+斜长石和角闪石+斜长石组合,其形成温压条件为760 ～820℃、5～8kbar;晚期低角闪岩相角闪石的生长表明岩石又经历了降温冷却的过程(M4),温度降至690℃以下.石榴石基性麻粒岩记录了含有近等温降压(ITD)阶段的顺时针变质作用P-T轨迹,揭示了阴山地块与鄂尔多斯地块之间俯冲碰撞加厚下地壳的折返过程.石榴石基性麻粒岩的变质锆石LA-ICP-MS U-Pb定年得到了两组变质年龄数据,分别为1945±25Ma和1828±36Ma,它们与阴山地块、鄂尔多斯地块碰撞形成孔兹岩带的时代及华北克拉通东、西陆块碰撞形成中部带的时代一致.结合该地区其他研究结果推断,石榴石基性麻粒岩在～ 1.95Ga鄂尔多斯地块与阴山地块碰撞过程中俯冲进入下地壳底部,经历早期的高压麻粒岩阶段,随后缓慢地抬升到下地壳上部;之后在～1.85Ga东、西部陆块碰撞过程中,石榴石基性麻粒岩折返到中上地壳.     

胶北南山口含榴辉石岩岩石学与锆石U-Pb定年的初步研究

胶北南山口镁铁-超镁铁质杂岩主要由含榴辉石岩和含榴基性麻粒岩所组成,且以不规则透镜体的形式赋存于太古宙英云闪长质片麻岩之中。岩相学观察、矿物相转变分析与矿物化学研究结果表明,胶北南山口含榴辉石岩不仅普遍发育近等温减压反应结构,即石榴石+富钙流体→单斜辉石+葡萄石+榍石±钠长石与石榴石+富钙流体+二氧化碳→角闪石+葡萄石±钠长石±方解石±榍石,指示其早期可能经历了高压麻粒岩相变质作用。而且,与南山口含榴基性麻粒岩类似,在晚期降温退变过程中,它们还经历了强烈的钙质交代作用,形成富钙矿物组合:富钙铝榴石的石榴石+次透辉石质单斜辉石+钙质角闪石+葡萄石+钠长石+方解石+榍石。SHRIMP锆石U-Pb定年结果表明,胶北南山口含榴辉石岩中的岩浆锆石记录了2900~2850Ma的207Pb/206Pb年龄,指示胶北地体在中太古代晚期存在一次重要的岩浆事件,而其变质锆石还记录了1950~1800Ma的207Pb/206Pb年龄,说明胶北南山口镁铁-超镁铁质杂岩曾卷入了胶北古元古代晚期地壳造山作用。结合研究区其它地质研究资料,本文推断胶北南山口镁铁-超镁铁质杂岩可能形成于古元古代,是华北克拉通胶-辽-吉带古元古代岩系的重要组成部分,并于1950~1800Ma期间,卷入了胶-辽-吉带古元古代造山作用,先后经历了高压麻粒岩相变质作用和晚期降温与钙质交代的联合退变质作用。     

华北克拉通北缘裂谷渣尔泰群LA-ICP-MS碎屑锆石U-Pb测年及地质意义

华北克拉通自1.85Ga形成之后,经历了广泛的拉伸,形成了一系列中-新元古代裂谷。其中北缘裂谷由渣尔泰群、白云鄂博群及化德群组成,发育了一系列大型-超大型多金属矿床。该裂谷系形成演化的研究对认识华北克拉通中-新元古代演化及区域找矿具有重要意义。但是对于该裂谷系中渣尔泰群的时代,一直存在争议。早期认为渣尔泰群属于中元古代,主要分布于狼山地区和渣尔泰山地区。但最新的研究已将狼山地区的渣尔泰群限定为新元古代,更名为狼山群,并据此确定华北克拉通北缘存在新元古代裂谷。因此,必须对渣尔泰山地区渣尔泰群的时代进行重新限定。在此基础上可综合分析华北克拉通北缘裂谷的形成与演化。系统的LA-ICP-MS锆石U-Pb测年结果表明,渣尔泰山地区渣尔泰群碎屑锆石年龄峰值主要为1.8~1.9Ga和2.5Ga。与北缘裂谷长城系,包括白云鄂博群下部及化德群下部的碎屑锆石年龄组成特征一致。而蓟县系、待建系及青白口系存在1.1~1.35Ga、1.5~1.6Ga等较年轻的碎屑锆石年龄峰值,如白云鄂博群上部、新元古代狼山群及化德群上部。因此,渣尔泰山地区渣尔泰群整体可与长城系对比。碎屑锆石年龄组成特征表明,渣尔泰群物质主要来自华北克拉通内部的太古宙-古元古代结晶基底。综合前人对白云鄂博群、化德群及狼山群地层年龄及岩浆岩的研究成果,可确定北缘裂谷是中-新元古代多期裂解事件形成的复杂裂谷。     

胶北地块高压与低压泥质麻粒岩的相平衡关系与p-T演化轨迹

利用最新的内洽性热力学数据库和THERMOCALC3.21程序对胶北地块高压与低压泥质麻粒岩的相平衡关系进行了定量分析。计算了胶北地块高压泥质麻粒岩、低压泥质麻粒岩和夕线石榴黑云片岩等代表性富铝岩石KFMASH(K2O-FeO-MgO-Al2O3-SiO2-H2O)体系的p-T视剖面图,再现了这些岩石随温压条件变化可能出现的各种矿物组合与矿物成分变化,发现原岩成分不同的变质岩石,尽管变质演化过程有所差异,但在麻粒岩相变质条件下所形成的矿物组合一致。通过计算泥质岩石在高压(p=1.0GPa)和低压(p=0.5GPa)条件下的T-X视剖面图,发现极度富铁、贫镁的岩石,在高压麻粒岩相条件下并不会生成含蓝晶石的特征矿物组合,在低压麻粒岩相条件下也不会生成含堇青石的特征矿物组合。将样品实际观测结果与p-T视剖面图的计算结果对比,确定胶北地块高压泥质麻粒岩变质峰期的温压条件为830~860℃,1.25~1.4GPa,峰期后呈现顺时针样式的p-T演化轨迹,反映陆壳先碰撞增厚、后又快速减薄的地质动力学过程;确定胶北地块低压泥质麻粒岩变质峰期的温压条件为790~820℃,0.62~0.68GPa,峰期后呈现近等压冷却的p-T演化轨迹。     

华北克拉通大同-怀安地区古元古代泥质麻粒岩的矿物学特征及变质作用剖析

以往研究认为华北克拉通中北部古元古代泥质麻粒岩主体属于一套中-低压高温麻粒岩。最近我们在大同-怀安地区发现该区泥质麻粒岩早期经历了高压-高温麻粒岩相变质作用,与伴生的石榴基性麻粒岩具有一致的变质条件,但由于后期中-低压高温(超高温)变质条件的叠加,早期高压矿物组合被强烈改造。本文以大同-怀安地区的高压-高温泥质麻粒岩为例,对其进行详细的矿物学特征和变质作用剖析。根据岩相学,如石榴石包裹体、矿物反应结构和矿物化学特征,将研究区泥质麻粒岩划分为四个变质阶段:进变质阶段(M_1)、压力峰期阶段(M_2)、温度峰期阶段(M_3)和冷却阶段(M_4)。M_1主要以石榴石核部包裹体为特征,含大量石英,少量黑云母、金红石和白云母等;M_2以幔部包裹体为特征,包裹体含量少,有石英、蓝晶石、钾长石、金红石、黑云母和锌尖晶石等,有时可见多硅白云母;M_3以基质矿物或石榴石边部包裹体为特征,如石榴石、夕线石、钾长石、石英和钛铁矿等,有或无锌尖晶石、金红石、黑云母等;M_4以石榴石边部的分解反应结构为特征,伴随熔体的结晶。综合矿物温压计、金红石Zr温度计和视剖面图方法获得进变质晚期阶段的条件为~10kbar/661~779℃,压力峰期变质条件为10~15kbar/810~860℃,温度峰期条件为6~10.5kbar/850~925℃,冷却阶段变质条件约为5~8kbar/645~761℃。同时,本文亦根据视剖面图方法模拟并解释了泥质麻粒岩(样品13GS47)中不同变质阶段的反应结构和变质反应,并对其中的石榴石成分环带进行了正演模拟。研究区与泥质麻粒岩伴生的其它岩石在不同程度上均记录了降压和降温的反应结构。最近研究证实部分石榴基性麻粒岩记录了进变质阶段,支持研究区的泥质麻粒岩和石榴基性麻粒岩可能经历了一致的变质历史,它们有可能是在压力峰期变质阶段(ca.1.97~1.90Ga)之前通过造山作用进入下地壳叠置在一起,并非在晚期(ca.1.85~1.80Ga)叠置在一起。     

北秦岭构造带与华北板块关系探讨: 来自宽坪杂岩变碎屑岩锆石U-Pb年代学与变质作用证据

宽坪杂岩是连接北秦岭构造带和华北板块的重要岩石-构造单元之一,其物质组成和变质变形特征可为探讨北秦岭构造带与华北板块构造关系及演化提供重要证据。本文在已有研究基础上,对宝鸡-眉县-洛南一带宽坪变碎屑岩进行了系统的岩石学、矿物学和碎屑锆石U-Pb年代学研究。结果表明,宽坪碎屑岩最年轻碎屑锆石年龄峰值为~550Ma,最主要碎屑锆石年龄集中区为~2.5Ga和1.0~0.9Ga,次要年龄集中区为1.3~1.0Ga和850~750Ma。通过与周缘可能物源区的年代学特征对比揭示,宽坪碎屑岩物源主要来自北秦岭新元古代花岗岩与秦岭岩群副片麻岩,部分来自南秦岭和扬子板块北缘新元古代花岗岩以及太古宙基底岩系,缺乏华北板块物质;宽坪碎屑岩的碎屑锆石年龄谱特征与南侧的二郎坪碎屑岩近乎一致,暗示两者可能形成于同一沉积盆地。相平衡模拟结果显示,洛南红土岭宽坪杂岩含石榴子石石英片岩记录了顺时针P-T演化轨迹,峰期变质条件为P=7.17~7.92kbar,T=557~563℃,形成于大陆碰撞过程。结合前人报道宽坪杂岩~440Ma的变质年龄,我们认为在早志留世时期宽坪和二郎坪沉积盆地闭合,北秦岭构造带与华北板块碰撞形成现今的构造格局,在此之前北秦岭构造带与华北板块不具有亲缘性。

     

华北克拉通西南缘汝阳群沉积时代及其地质意义：来自碎屑锆石U-Pb年龄的证据

汝阳群分布在华北克拉通西南缘,位于河南-陕西-山西交界地区,主要为一套未变质的碎屑岩及碳酸盐岩地层,不整合于熊耳群火山岩系之上,其上被洛峪群整合覆盖。长期以来,其地质时代一直存有较大的争议。本文通过对汝阳群下部白草坪组4个石英砂岩样品中的碎屑锆石进行LA-ICP-MS U-Pb年龄测定,获得的²⁰⁷Pb/²⁰⁶Pb年龄分布范围为3 000～1 800 Ma,主要集中在2 600～2 400 Ma之间(约占67%),年龄主峰值为2 550～2 500 Ma,说明其沉积物质主要来源于新太古代末以及古元古代的地质体。其中,最年轻锆石的²⁰⁷Pb/²⁰⁶Pb谐和年龄值分别为1 817±22 Ma、1 838±23 Ma、1 924±17 Ma和1 829±28 Ma,说明汝阳群沉积时代不老于1 800 Ma,与其上覆洛峪群中近期获得1 611±8 Ma的年龄相吻合,因此其形成时代应为中元古代早期。     

乌拉山-集宁孔兹岩锆石激光探针等离子质谱（LA-ICP-MS）年龄——孔兹岩沉积时限的年代学研究

乌拉山-集宁地区由 TTG 片麻岩、麻粒岩、斜长角闪岩、孔兹岩等组成的高级变质地体,过去一直按有序地层划分,按变质程度定时代。现在识别出深成岩后,TTG 片麻岩已广泛采用锆石 U-Pb 测年,而表壳岩测年由于小于传统预想的地层年龄,所以往往仍按变质程度定时代:如只有中-新太古代年龄的麻粒岩(兴和岩群)被定为古太古代,只有古元古代年龄的孔兹岩被定成新太古代。尤其是孔兹岩,因为常规锆石(TIMS)年龄不能分辨碎屑年龄和变质年龄,所以其沉积时代是太古还是古元古更长期争论不决。为了解决这个问题,本文依据始于上世纪90年代的国内外区分岩浆锆石、变质锆石、碎屑锆石的研究成果,以锆石阴极发光(CL)和背散射(BSE)图象,首先查明孔兹岩除了含有均匀无内部结构的变质锆石还含有具结晶韵律环带结构的锆石,因为后一种结构是岩浆成因的有力证据,它产在沉积岩中就代表碎屑锆石。碎屑锆石有老有新,只有年龄最小的碎屑锆石才能限定沉积时代,所以沉积年龄必须以大量锆石测年统计。数以百计的锆石测年以 SHRIMP 进行固然最好,然而昂贵的费用本课题无力承担,所以我们改用灵敏度也较高,而且快速简便、费用低廉的激光探针(LA-ICP-MS)进行。结果得到:乌拉山大庙片麻状花岗岩的岩浆锆石年龄为2.49Ga,显示高级变质下原岩年龄仍能保存;乌拉山-集宁的孔兹岩变质锆石一致给出≈1.8Ga 的变质年龄,与已有的 TIMS 法≈1.85Ga 的变质年龄一致;乌拉山哈德门沟石墨厂、包白铁路桃儿湾、忽鸡沟窑子湾和集宁兴和黄土窑等四处孔兹岩给出的碎屑锆石年龄有老有新,最老的≈2.3Ga,多数谐和的碎屑锆石年龄在2.2～2.0Ga 之间,因为沉积时代以最年轻的碎屑锆石年龄限定,所以孔兹岩的沉积时代不是太古而是古元古。乌拉山-集宁地区的 TTG 片麻岩年龄比孔兹岩的碎屑锆石年龄大,表明孔兹岩不是由附近太古宙岩石提供碎屑物质的原地沉积,而是经吕梁运动碰撞进入下地壳,才与附近太古宙岩石产在一起的外来构造岩片。     

五台地区高凡群形成时代新证据:锆石SHRIMP U-Pb定年

五台地区高凡群对于华北克拉通早前寒武纪变质地层层序建立具有重要意义。本文对高凡群磨河组开展了地球化学和锆石年代学研究。变质沉积岩稀土总量为(132.78~231.84)×10^-6(样品W1927的稀土总量为1016.55×10^-6),轻重稀土分离弱((La/Yb)_N=4.5~12.5),具明显负铕异常(δEu=0.37~0.62),稀土模式与太古宙后泥砂质碎屑沉积岩十分类似。一个白云石英片岩样品(D004)的变质原岩中存在大量2.7 Ga、2.54~2.5 Ga、2.3 Ga和2.18 Ga碎屑锆石,最年轻锆石可能来自同时代火山岩。一个含黄铁矿白云片岩样品(W1927)的最年轻锆石年龄为2194 Ma,被认为代表了岩石形成时代。结合前人研究,可把高凡群形成时代限定在<2176 Ma和2350 Ma之间,碎屑物质主要来自恒山、五台、阜平、云中山、吕梁地区新太古代—古元古代变质基底或更远的地区。     

洛南-卢氏地区与铁铜多金属矿床有关的中酸性侵入岩锆石U-Pb定年及其地质意义

位于华北克拉通南缘的洛南-卢氏地区出露大量与成矿密切相关的中酸性小侵入体,主要岩性包括石英闪长岩和钾长花岗斑岩,其中大部分是该区矽卡岩型和斑岩型铁-铜-钼-铅锌多金属矿床的成矿母岩,但迄今为止对这些岩体的侵位时代及岩浆活动构造背景的研究还很薄弱.文章利用激光剥蚀电感耦合等离子体质谱法对该区典型侵入岩(夜长坪、柳关、后瑶...     

Oldest zircon xenocryst (4.17 Ga) from the North China Craton

Prior to this work, the existence of crustal materials older than 4.0 Ga has not been reported from the North China Craton (NCC) – one of the few global terrains where crustal rocks from ～3.8 Ga have been identified. Here we report the first occurrence of a xenocrystic zircon with a ²⁰⁷Pb/²⁰⁶Pb age of 4174 ± 48 Ma, from the Anshan–Benxi Archaean supracrustal greenstone belt, based on laser ablation–inductively coupled plasma–mass spectrometry. The 4.17 Ga zircon xenocryst is hosted within ～2523 ± 12 Ma massive fine-grained amphibolites which were subsequently metamorphosed at ～2481 ± 19 Ma. The xenocryst age is ca. 350 million years, older than the oldest zircon previously identified in the NCC, and is consistent with prior zircon Lu–Hf isotopic studies. Documentation of 4.17 Ga xenocrystal zircon not only provided a geochronological record of the oldest known crustal materials in the NCC, but also identified the geologic environment for further search for the rocks that formed during Earth’s earliest recorded evolution.     

华北克拉通孔兹岩带古元古代凉城石榴石花岗岩成因机制及其岩石学意义

在华北克拉通孔兹岩带东段凉城地区分布有大规模古元古代石榴石花岗岩。凉城石榴石花岗岩是产于麻粒岩相变沉积岩中的原地-半原地花岗岩,伴生有徐武家岩体为代表的辉长苏长岩小侵入体群。凉城石榴石花岗岩富含石榴子石（5%~15%,局部可达25%）,常见紫苏辉石,推测形成于高温（>850~900℃）、高压（8~10kbar）条件下,不同于典型的S型花岗岩。地球化学特征上,石榴石花岗岩具有强过铝的地球化学属性（A/CNK=1.1~1.6）,显示低SiO₂（55%~75%）和富MgO+FeO^T（5%~14%）的特征,已经偏离了正常S型花岗岩的成分范围。凉城石榴石花岗岩低硅富镁铁的成分特征很可能是由残留体（石榴石岩）带入和辉长苏长岩物质添加造成,残留体和辉长苏长岩的物质贡献比例约占20%~40%。年代学研究表明凉城石榴石花岗岩形成时代为1.93~1.92Ga,与辉长苏长岩形成时代（1.93Ga）和变质沉积岩记录的超高温变质作用时代（1.92Ga）一致,指示凉城石榴石花岗岩是幔源基性岩浆侵入麻粒岩相变质沉积岩中引起变沉积岩在高温-超高温条件下大规模部分熔融的产物。大规模出露的高温石榴石花岗岩及多点分布的超高温麻粒岩共同反映了集宁-凉城区域上超高温的特点。

     

华北陆块南缘汝阳群次火山岩SHRIMP锆石U-Pb年龄及其地质意义

华北陆块南缘汝阳群下部的次火山岩一直被看作是与汝阳群同时代的火山喷发夹层。通过研究测得玄武岩SHRIMP锆石U-Pb年龄为213.5±2.4Ma,并在邻近该次火山岩层上部的紫红色泥岩中发现了褪色的角岩化现象,说明该火山岩是晚三叠世沿汝阳群下部顺层侵入的次火山岩岩床。通过对该次火山岩岩石及其地球化学特征的研究,认为该次火山岩起源于有早期俯冲洋壳或陆壳参与再循环的大陆富集型地幔的部分熔融,它与秦岭造山带几乎同时代形成的超高压榴辉岩、埃达克质岩石、高钾钙碱性花岗岩、环斑花岗岩等共同揭示了秦岭造山带自中三叠世全面碰撞造山之后,由碰撞挤压逐渐转变为伸展拉张的深部动力学过程。     

华北陆块南缘汝阳群次火山岩SHRIMP锆石U-Pb年龄及其地质意义

华北陆块南缘汝阳群下部的次火山岩一直被看作是与汝阳群同时代的火山喷发夹层。通过研究测得玄武岩SHRIMP锆石U-Pb年龄为213.5±2.4Ma,并在邻近该次火山岩层上部的紫红色泥岩中发现了褪色的角岩化现象,说明该火山岩是晚三叠世沿汝阳群下部顺层侵入的次火山岩岩床。通过对该次火山岩岩石及其地球化学特征的研究,认为该次火山岩起源于有早期俯冲洋壳或陆壳参与再循环的大陆富集型地幔的部分熔融,它与秦岭造山带几乎同时代形成的超高压榴辉岩、埃达克质岩石、高钾钙碱性花岗岩、环斑花岗岩等共同揭示了秦岭造山带自中三叠世全面碰撞造山之后,由碰撞挤压逐渐转变为伸展拉张的深部动力学过程。     

High-P granulites of the Songshugou area (Qinling Orogen,east-central China): Petrography,phase relations,and U/Pb zircon geochronology

High-pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging-wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution.     

华北克拉通北部长城系底界年龄小于1670Ma:来自北京密云花岗斑岩岩脉锆石LA-MC-ICPMS U-Pb年龄的约束

近期在北京密云沙厂北东的大龙门村附近发现一条花岗斑岩岩脉,该岩脉侵入于新太古代密云群角闪斜长片麻岩当中,其顶部则与片麻岩一起被常州沟组含砾砂岩沉积不整合覆盖.对该岩脉采用LA-MC-ICPMS进行锆石U-Pb同位素年龄测定,获得了(1 673±10)Ma的侵位年龄.这表明华北北部的常州沟组底界(也即长城系的底界)年龄小...