Effect of metamorphism and partial melting of host rocks on zircons

Abstract Zircons have been studied from different layers of migmatites (from Arvika, western Sweden and Nelaug, southern Norway) and from a paragneiss (from Arvika) associated with one of the migmatites. The main purpose of the investigation is to establish whether or not information about zircons can help in the elucidation of the parentage and rock-forming processes of migmatites.
The elongation ratio of zircons from all layers is small and characteristic of sedimentary zircons. Further, the absence of characteristic colours and the growth trends of the zircons (indicated by the reduced major axes) observed in the various samples both support a sedimentary parentage for these rocks. The zircons of all layers exhibit secondary growth (overgrowth, outgrowth and multiple growth) due to metamorphism. Compared with the zircons from the paragneiss, those of the migmatite layers are more clouded and less rounded, some of them becoming opaque or even skeletal; this is especially true of the zircons from the leucosomes. These observations indicate an alteration of the original sedimentary zircons in the migmatite, especially in the leucosomes, in response to the migmatization process, previously interpreted as partial melting.     

Roles for fluid and/or melt advection in forming high-P mafic migmatites, Fiordland, New Zealand

A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.     

Zircon U–Pb ages and Hf isotope compositions of migmatites from the North Qinling terrane and their geological implications

Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra‐high‐P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.     

辽吉东部层状混合岩的成因

辽吉东部的层状混合岩与上、下围岩始终保持整合接触,反映它的体系基本上是封闭的,没有外来组份的加入。其组构、岩石化学、微量元素和稀土资料等表明层状混合岩不足岩浆型的,而是重熔岩浆型的,其原岩是沉积变质岩。     

Isocon analysis of migmatization in the Front Range, Colorado, USA

Isocon analysis has been applied to five sets of leucosome, mafic selvages and immediately adjacent mesosome in the migmatites from a 15-m outcrop in the Colorado Front Range. The results show: (i) mafic selvages formed from the adjacent mesosome by loss of felsic components and therefore the mesosomes are indeed palaeosomes or protoliths; (ii) the leucosomes did not form in a closed system from the palaeosome (in which case the material lost from the palaeosome during selvage formation would become the leucosome). The observed volumes and compositions of leucosomes require that the present leucosome must contain some material in addition to the felsic components lost from the selvages. The materials that must be added are leucotonalitic to granitic in composition, varying greatly in K/(Na + Ca) ratio. The trend in leucosome composition can be reproduced by assuming that a metasomatic exchange, KNa + Ca, modified originally leucotonalitic leucosomes to more K-rich compositions. These leucosomes most likely formed by injection of silicate melts accompanied, or followed, by metasomatism. The trend of leucosome compositions in this study reflects the general trend in the leucosome compositions which have been published from other areas, indicating that the proposed mechanism can be applicable to other regional migmatites.     

赣东加里东变质混合岩带金地球化学特征和地球化学行为

赣东加里东变质混合岩带 ,是以混合岩体为主体 ,受韧性剪切带控制的多相、多型、递增变质带 ,是受区域深构造控制的热变质带。该带产有茅排式金矿。通过对该带金地球化学特征和地球化学行为研究 ,认为带中金丰度 0 .83× 10 - 9,呈峰式分布 ,为对数及双对数正态分布型式 ,Au与主元素成分无关 ,与微量元素组合是Au -Zn -Li-Pb -Cs ,在韧性剪切过程中Au具活化迁移富集特点     

陕西周户地区“秦岭群”的解体

根据区内传统秦岭群包含不同原岩建造、不同变质岩石组合以及变形变质、副矿物和微量元素特征的差异，将其解作为丹凤岩群、深成侵入体、浅成一超浅成侵入体、片麻岩套等5个部分，首次区分出两类不同性质的混合岩，确认了秦岭群在该区的存在。     

Migmatite formation at subsolidus conditions–an alternative to anatexis

Abstract This contribution discusses the formation of stromatic high-grade migmatites. Volume considerations require that most of the leucosome material is not added from outside the system. A segregation mechanism is necessary except in those cases where the protolith of the migmatite already had a banded structure. Although partial melting is most often advocated to provide the segregation mechanism, several arguments can be raised against high degrees of melting: mineral compositions and even zoning patterns are similar in both mesosomes and leucosomes; sufficient degrees of melting at reasonable temperatures require more than the available amounts of water; the leucosomes do not always approximate to a minimum melt composition; high degrees of melting cannot occur without an appreciable volume increase; etc. Diffusion works as a segregation mechanism at low temperatures. As diffusion rates increase exponentially with temperature diffusion must become still more important as a segregation mechanism at high temperatures. A model is suggested based on the diffusion of components in response to the gradient δσ/δx, where σ= 1/3∑³_i=1 σ_i is the mean pressure. In homogeneously strained rocks, σ₃ is larger in rock parts rich in incompetent phases than in rock parts depleted in incompetent phases. Accordingly, mechanically competent but chemically incompetent high-volume phases like quartz and feldspars stressed in micadominated parts of a rock (high σσ) migrate to parts of the rock that are depleted in mica (low σ¯). It is suggested that hornblende occurring in many leucosomes may be premigmatitic or early syn-migmatitic and due to its mechanical competency it initiates the segregation. Diffusion occurs along grain boundaries and is enhanced by small amounts of ‘intergranular fluid’;. At the best, semiquantitative estimates of diffusion rates and distances indicate that the process should work over geological times.     

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.