Micro-zircon: origin and evolution during metamorphism

The size and distribution of zircon within a garnet-mica-schist from the Scottish Highlands were assessed using scanning electron microscopy. The study reveals that abundant 0.2–3.0 μm sized zircon is preferentially concentrated within garnet and biotite porphyroblasts. The micro-zircon has grown during regional metamorphism and represents >90% of the total number of zircon in the schist. It is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution, and more rarely, small metamorphic outgrowths. The sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified. Zircon is reactive and soluble, and responds to medium-grade metamorphism in a series of dissolution and crystallization events, linked to possible changes in fluid composition. Deformation also has a significant influence on the distribution of zircon, allowing inclusions previously trapped within biotite to react. About 8 × 10⁶ micro-zircon occur as inclusions within a typical individual 5-mm garnet porphyroblast and their presence must be considered prior to trace-element or isotopic analysis of such metamorphic phases.     

U–Pb isotopic constraints on diachronous metamorphism in the northern Monashee complex, southern Canadian Cordillera

U–Pb isotopic data from the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera, indicate that the age of metamorphism varies according to structural position in a 6 km thick section. This metamorphism resulted in an unusual sequence in which rocks with the lowest-grade mineral assemblage (kyanite–sillimanite–staurolite–muscovite) are underlain and overlain by higher-grade rocks. Xenotime and monazite U–Pb dates vary progressively from 64 Ma in the structurally highest rocks to 49 Ma in the deepest rocks. Discordant U–Pb ages from Proterozoic and Cretaceous monazite and titanite are used to interpret the thermal significance of the early Tertiary dates. The discordant analyses define linear arrays with lower intercepts that broadly overlap with early Tertiary, and the amount of discordance varies with structural level; it is least in the deeper rocks and greatest in higher rocks. Electron microprobe work showed that the monazite discordance in the deeper rocks resulted from Tertiary mineral overgrowth and recrystallization rather than Pb diffusion. We use previous studies of Pb diffusion and the fact that Proterozoic monazite and titanite suffered only negligible to moderate amounts of diffusive Pb loss to contend that elevated temperatures (c. 600–650 °C are inferred from pelitic mineral assemblages) existed in the deeper rocks for a short duration, perhaps a few million years. The downwards younging 64–49 Ma U–Pb dates are interpreted as closely reflecting xenotime and monazite growth ages rather than cooling ages or substantially reset ages based on the lack of Pb diffusion in monazite and the previously obtained ⁴⁰Ar/³⁹Ar data which suggest that rapid cooling occurred immediately after the U–Pb dates. In addition, growth ages are interpreted as thermal peak ages based on U–Pb dates from coeval kyanite-bearing leucosomes, the consistent nature of the U–Pb dates throughout the study area, and petrographic relationships which suggest that monazite grew before or during development of the syn-metamorphic foliation. These interpretations lead us to conclude that metamorphism was diachronous according to structural level, with higher rocks attaining peak temperatures and cooling rapidly while deeper rocks were heating towards a thermal peak that was attained a few million years later. This thermal scenario requires that higher rocks cannot have been the heat source for the deeper metamorphism, as was previously proposed.     

Low-pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low-pressure metamorphism in the Lachlan Fold Belt, south-eastern Australia. It comprises metamorphosed Ordovician quartz-rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K-feldspar and sillimanite–K-feldspar. Migmatites occur in the sillimanite–K-feldspar zone, but large bodies of S-type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T = 700°C and P = 3.5 kbar, indicating a very high P–T gradient of 65°C/km.
The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre-metamorphic back-arc basin environment, and intrusion of hot, mantle-derived magmas into the lower and middle crust.
Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro-Devonian, accompanied by intrusion of many I-type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.     

Unpairing metamorphic belts: P–T paths and a tectonic model for the Ryoke Belt, southwest Japan

Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high-pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest-grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin-like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak-T , and near-isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high-P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt-parallel ductile stretching, recorded as syn-metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left-lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record.     

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.     

贵州瓮安玉山煤矿煤质特征及其影响因素

玉山煤矿含煤地层为吴家坪组,可采煤层2层,具有中灰、中等挥发分、高硫分、低磷分、特低—低氯,低砷、高氟,中高发热量的煤质特征,煤类丰富,呈条带状分布。通过对区内沉积环境和煤的变质作用2个方面的分析,阐述了形成煤层高硫、中灰特征的影响因素,解释了煤类分带现象,并指出了主要煤质指标的变化规律。     

喜马拉雅造山带中段亚东地区石榴角闪岩的成因：岩石学和锆石U- Pb年代学

喜马拉雅造山带中段出露的基性麻粒岩是理解印度大陆前喜马拉雅期演化历史和新生代碰撞造山作用的理想研究对象。本文对亚东多庆湖地区的石榴角闪岩进行了岩石学、全岩主微量元素地球化学和锆石U- Pb年代学研究,揭示了其原岩类型和新生代的变质作用过程。石榴角闪岩的原岩很可能为新元古代(~890 Ma)的玄武岩,具有E- MORB型岩石的地球化学特征。石榴角闪岩具有三期矿物组合：① 进变质矿物组合可能为石榴子石+角闪石+斜长石+钛铁矿+石英,即石榴子石核部及其中包裹体;② 峰期矿物组合为石榴子石+角闪石+斜长石+黑云母+石英,即石榴子石边部和基质矿物;③ 退变质矿物组合为角闪石+斜方辉石+斜长石+黑云母+石英,包括退变质域和石榴子石边部的后成合晶矿物。矿物温压计和相平衡模拟表明,石榴角闪岩进变质、峰期和退变质条件分别为609~621℃和0. 59~0. 65 GPa、805~845℃和0. 91~1. 04 GPa、825~840℃和0. 61~0. 68 GPa,经历了峰期高压麻粒岩相的变质作用。锆石U- Pb年代学研究表明,石榴角闪岩的峰期变质时间为34. 8~20. 3 Ma,退变质时间为18. 1~17. 7 Ma,可能经历了一个较长期的部分熔融过程。本文研究认为,亚东石榴角闪岩是印度板块向欧亚板块长期俯冲、地壳增厚成因的基性麻粒岩,原岩可能与Rodinia超大陆拼合相关;其以加热埋藏、近等温降压为特征的顺时针P- T轨迹指示了喜马拉雅造山带中段的大喜马拉雅岩系上部构造层位经历了长期持续的地壳增厚和高温麻粒岩相变质作用,以及早中新世(21~17 Ma)相对快速的减压抬升和随后(17 Ma之后)相对缓慢的折返至地表的演化过程。     

阿尔泰大东沟铅锌矿的碳质流体及其成因

大东沟铅锌矿是阿尔泰南缘泥盆纪克朗火山-沉积盆地的块状硫化物矿床之一,在石炭—二叠纪同造山的区域变质过程中,受到热液叠加改造作用,层状铅锌矿体发育脉状石英和矿化。本文对阿勒泰大东沟铅锌矿区石英脉中的包裹体进行了详细的岩相学和显微测温研究,估算出包裹体形成时的物理化学条件,并采用激光拉曼、同步辐射X射线荧光(SRXRF)对流体包裹体进行了成分测试。结果显示,石英脉中的包裹体主要为碳质流体包裹体,多以面状、带状分布,最低捕获温度在209～459℃之间,密度为0.75～1.15g/cm3,最低捕获压力在110～540MPa之间。初步研究表明碳质流体的来源与同造山的变质作用有关,而与海底喷流沉积无关。激光拉曼测试结果表明包裹体气液主要成分为CO2和N2。SRXRF测试碳质包裹体中金属微量元素显示低Cu、Zn、Pb,而富集Au。     

Mineralogical and Petrological Characteristics of the Neoproterozoic Orthoamphibolite and Orthogneisses in the Mutki Area, the Bitlis Massif, Southeast Turkey

<正>The rocks form as amphibolite±garnet±epidote and orthogneisses in the Pan-African basement of the Bitlis Massif.The petrochemical data of the studied metamorphic rocks suggest different igneous protoliths ranging from calcalkaline basalt to andesite in composition. Petrochemically,the rocks can be classified as group 1(low Zr and La) and group 2(high Zr and La), all showing various enrichments in large ion lithophiles and light rare earth elements,and a depletion in high-field strength elements,suggestive of a destructive plate margin setting.The protoliths of the all samples might have formed mostly by the partial melting of an enriched source,possibly coupled with the fractional crystallization of plagioclase,apatite,and titaniferous magnetite±olivine±clinopyroxene±amphibole in relation with subduction-related magmatism neighboring the Andeantype active margins of Gondwana.The group 2 samples could,however,be generated by a relatively lower degree of the partial melting of an inhomogeneous source with a preponderance of a high-level, fractional crystallization process in comparison to group 1.The protoliths of the samples were metamorphosed up to amphibolite facies conditions,which destroys original igneous texture and mineral assemblages.Geothermobarometric calculations show that the metamorphic rocks are finally equilibrated between 540 and 610℃and～5 kbars,following a clockwise P-T-t path.     

从地壳上地幔构造看大陆碰撞带岩石圈的克拉通化

本篇讨论超级大陆汇聚后逐渐变为克拉通或扩大克拉通的作用过程,即指经及大陆碰撞地体汇聚后新形成的大陆块逐渐转变为刚性克拉通的作用过程。增生大陆岩石圈的克拉通化的作用后果,包括大陆地壳密度的增加,岩石圈地幔的增厚和大地热流值的下降,使大陆岩石圈逐渐刚性强化。大陆碰撞后形成的大陆块必须经过克拉通化的过程,才能逐渐成为刚性克拉通。作用过程主要包括:①上地壳沉积碎屑岩石结晶岩化和中地壳岩石角闪岩化;②下地壳岩石基性化;③大陆碰撞带下凹莫霍面的磨平;④岩石圈地幔底侵加厚形成陆根。从大陆碰撞带转变为克拉通的过程也是岩石圈地幔不断增厚而地壳缓慢变硬变冷的过程。这个过程包含以下作用:区域变质作用,交代作用和岩石圈幔源岩浆的底侵。这个过程时间尺度比碰撞造山作用大一个级次。长期的底侵作用使地壳岩石密度和强度不断加大,改变岩层的矿物成分和局部结构。当大陆岩石圈克拉通化到一定程度之后,由于下方软流圈的热能供应逐渐减缓,使岩石圈地温梯度缓慢下降,最终结果会形成大陆根。由于显生宙大陆碰撞带岩石圈强度弱,大陆碰撞时更容易造成岩石圈变形,因此大陆碰撞的板内效应主要发生在大陆内的显生宙碰撞带。显生宙大陆碰撞带如果再次受到大陆碰撞板内效应的作用,其克拉通化的过程必然会推迟。