Showing papers on "Metamorphism published in 2015"

On ultrahigh temperature crustal metamorphism: phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

Abstract: Ultrahigh temperature (UHT) metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 °C. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as ‘normal’ in the spectrum of regional crustal processes. UHT metamorphism is typically identified on the basis of diagnostic mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite ± quartz and osumilite in Mg–Al-rich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes: (1) development of a ferric iron activity–composition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; (2) quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; (3) more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; (4) recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; (5) more strongly linking U–Pb geochronological data from zircon and monazite to P–T points or path segments through using Y + REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and (6) improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.

The fall and rise of metamorphic zircon

Abstract: Zircon geochronology and geochemistry are increasingly important for understanding metamorphic processes, particularly at extreme conditions, but drivers of zircon dissolution and regrowth are poorly understood. Here, we model Zr mass balance to identify P-T regions where zircon should dissolve or grow. Zirconium contents of major metamorphic minerals were assessed from published data and new measurements, and models were constructed of mineralogical development and zircon abundance for hydrous MORB and metapelitic compositions along representative P-T paths. Excluding zircon, the minerals rutile, garnet, and hornblende strongly influence Zr mass balance in metabasites, accounting for as much as 40% of the whole-rock Zr budget. Clinopyroxene and garnet contain more Zr than plagioclase, so breakdown of plagioclase at the amphibolite to eclogite facies transition, should cause zircon to dissolve slightly, rather than grow. Growth of UHP zircon is predicted over a restricted region, and most zircon grows subsequently at much lower pressure. In metapelites, zircon is predicted to undergo only minor changes to modal abundance in solid state assemblages. Partial melting, however, drives massive zircon dissolution, whereas melt crystallization regrows zircon. From a mass-balance perspective, zircon growth cannot be attributed a priori to the prograde amphibolite-eclogite transition, to UHP metamorphism, or to partial melting. Instead, zircon should grow mainly during late-stage exhumation and cooling, particularly during oxide transitions from rutile to ilmenite and melt crystallization. As predicted, most zircons from HP/UHP eclogites of the Western Gneiss Region and Papua New Guinea substantially postdate eclogite formation and maximum pressures.

Developing plate tectonics theory from oceanic subduction zones to collisional orogens

Abstract: Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products (i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.

Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales

Abstract: The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises melange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Melanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimenta...

High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin

Abstract: An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbona-ceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40 Ar– 39 Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomar-gins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamor-phism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauleon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cre-taceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamor-phism are intimately associated with the areas where subcon-tinental mantle rocks have been unroofed or exhumed.

Metabasalts as sources of metals in orogenic gold deposits

Abstract: Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 ± 0.5 ppb) and upper greenschist facies (1.0 ± 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 ± 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

Formation and Evolution of Archean Continental Crust of the North China Craton

Abstract: The North China Craton (NCC) has had a long geological history back to ca. 3.8 Ga ago. In the Anshan area, northeastern part of the craton, three distinct complexes with ages of 3.8–3.1 Ga (Baijiafen, Dongshan, and Shengousi) have been identified, along with widespread 3.1–2.5 Ga rocks of different origins and ages. In eastern Hebei Province, abundant 3.88–3.4 Ga detrital zircons were obtained from metasedimentary rocks of the Caozhuang Complex, and the oldest rock identified is a 3.4 Ga gneissic quartz diorite. The oldest zircons that may originally have been derived from the NCC are 4.1–3.9 Ga grains in Paleozoic volcano-sedimentary rocks in the northern Qinling Orogenic Belt bordering the NCC in the south. 3.0–2.8 Ga rocks occur in Anshan, eastern Hebei, eastern Shandong, and Lushan. ca. 2.7 Ga rocks of igneous origin are exposed in eight areas of the NCC, but ~2.7 Ga supracrustal rocks have so far only been identified in western Shandong. ca. 2.5 Ga intrusive and supracrustal rocks and associated regional metamorphism occur in almost all Archean areas of the NCC. Banded iron formations contain the most important ore deposit of the Archean in the NCC and mainly formed during the late Neoarchean. Ancient crustal records obtained from deep crust beneath the NCC are similar to those in the exposed areas, with the oldest ca. 3.6 Ga rock enclaves occurring in Xinyang near the southern margin of the NCC. This synthesis is based on the compilation of a large database of zircon ages as well as whole-rock Nd isotopic and Hf-in-zircon isotopic data in order to understand the formation and evolution of the early Precambrian basement of the NCC. Considering the craton as an entity, there is a continuous age record from 3.8 to 1.8 Ga, and two tectono-thermal events are most significant in the late Neoarchean to the earliest Paleoproterozoic and late Paleoproterozoic history, with age peaks at ~2.52 and ~1.85 Ga, respectively. Whole-rock Nd and Hf-in-zircon isotopic data show similar features, documenting the addition of juvenile material to the continental crust at 3.8–3.55, 3.45, 3.35–3.3, 2.9, and 2.85–2.5 Ga with the late Mesoarchean to early Neoarchean being the most important period. Crustal recycling began as early as 3.8 Ga and continued until 3.25 Ga and appears to have played a more important role than juvenile additions between 3.25 and 2.90 Ga. After outlining the general geological history of the NNC basement, we discuss several issues relating to Archean crust formation and evolution and arrive at the following major conclusions: (1) Similar to several other cratons, the late Mesoarchean to early Neoarchean was the most important period for rapid production of continental crust, and the most intensive and widespread tectono-thermal event occurred at the end of the Neoarchean. (2) In our new tectonic model, we define and outline three ancient terranes containing abundant 3.8–2.6 Ga rocks, namely the Eastern Ancient Terrane, Southern Ancient Terrane, and Central Ancient Terrane. (3) Vertical magmatic growth is seen as the main mechanism of crust formation prior to the Mesoarchean. We favor a multi-island arc model related to subduction/collision and amalgamation of different ancient terranes in the late Neoarchean. (4) The NCC may already have been a large crustal unit as a result of cratonic stabilization at the end of the late Neoarchean, probably due to magmatic underplating.

Progresses and overviews of voluminous meta-sedimentary series within the Paleoproterozoic Jiao-Liao-Ji orogenic / mobile belt,North China Craton

Abstract: Three Paleoproterozoic tectonic belts have been identified within the North China Craton,including the Jiao-Liao-Ji belt in the Eastern Block,the Khondalite belt in the Western Block,and the Trans-North China Orogen between the Eastern and Western blocks. In the last two decades,extensive structural, metamorphic, magmatic, geochemical, geochronological and geophysical investigations have been carried out on these Paleoproterozoic tectonic belts,producing an abundant amount of new data and competing interpretations,which have resulted in significant reinterpretations of the formation and evolution of three tectonic belts and the Paleoproterozoic amalgamation of the North China Craton. The Jiao-Liao-Ji belt is the most important Paleoproterozoic orogenic / mobile belt in the North China Craton,which has not only received huge mount of Paleoproterozoic continental crust deposits,but also experienced a very complex tectonic evolution, accompanied with multi-stage magmatic-metamorphic events simultaneously. As previous studies,the Jiao-Liao-Ji orogenic / mobile belt located in the Eastern Block subdivides the block into the Longgang and Langrim blocks. The belt consists mainly of voluminous meta-sedimentary and volcanic successions with associated granitic and mafic intrusions.The sedimentary and volcanic successions,including the Macheonayeong Group in North Korea,the Ji'an and Laoling groups in the southern Jilin,the North and South Liaohe groups in the southeastern Liaoning Peninsula,the Fenzishan and Jingshang groups in the Jiaobei massif,and the Wuhe Group in Anhui Province,are transitional from a basal clastic-rich sequence and a lower bimodalvolcanic sequence,through a middle carbonate-rich sequence,to an upper pelite-rich sequence. Associated with the sedimentary and volcanic rocks in the Jiao-Liao-Ji orogenic / mobile belt are abundant Paleoproterozoic granitoid and mafic intrusions,of which the granitoid plutons are composed of deformed A-type granites and undeformed alkaline syenites and rapakivi granites,and mafic intrusions consist of meta-gabbros and-dolerites. In general,the Jiao-Liao-Ji orogenic / mobile belt is NE-trending and is ca. 1000 km long. Based on the rock assemblage and spatial distribution,the Jingshan Group is comparable with the South Liaohe and Ji'an groups,whereas the Fenzishan group is comparable with the North Liaohe and Laoling groups. However,the original stratigraphic sequences and contact relationships for each group / formation have been completely destroyed due to the multi-stage intensive deformation,which outcrop currently as varying scale of tectonic slices separated by faults or ductile shear zones. The massive sedimentary rocks are mainly sourced from the Paleoproterozoic granitic rocks within the orogenic / mobile belt and the metamorphic basement of the surrounding ancient blocks,and the protoliths were formed between 2. 15 Ga and 1. 95 Ga. Previous studies show that the Paleoproterozoic metamorphism is very heterogeneous across the Jiao-Liao-Ji orogenic / mobile zone,in which the( medium to high pressure) granulite-facies metamorphism is only constrained within the Jingshan Group and adjacent rocks. However,the Fenzishan,South and North Liaohe,Ji'an and Laoling groups only experienced amphibolite-facies and locally greenschist-facies metamorphism.The metamorphic evolution for the Fenzishan,North Liaohe and Laoling groups are characterized by similar clockwise metamorphic P-Tt paths,whereas the Jingshan,South Liaohe and Ji'an groups are featured by counterclockwise P-T-t paths. It is noted that various petrographic evidence for the Paleoproterozoic granulite-facies metamorphism have been identified throughout the South Liaohe and Ji'an groups,which have similar clockwise P-T-t paths with near-isothermal decompression( ITD) to those of the pelitic and mafic granulites of the Jingshan Group. Abundant geochronological data show that the peak metamorphic age of the granulite-facies metamorphism is around 1. 90 ~ 1. 95 Ga throughout the Jiao-Liao-Ji orogenic / mobile belt. Field observations and petrographic studies show that the pelitic granulites of the Jingshan,South Liaohe and Ji 'an groups have experienced widespread anatexis( or partial melting),and various felsic veins outcrop as irregular veinlets,stockworks and lenticular in the host rocks with gradual transition relationships. Abundant U-Pb dating results of anatexis zircons show that an regional partial melting event happened at 1. 84 Ga to 1. 86 Ga,indicating that the widespread anatexis should occur at the post-peak low-pressure granulite-facies metamorphic stage during exhumation of the Jiao-Liao-Ji orogenic / mobile belt.Up to now,there have been distinct disagreements and controversies about spatial distribution,north and south boundaries,extension pattern and tectonic setting of the Jiao-Liao-Ji orogenic / mobile belt. Recent studies show that the outcrops,and granitic,mafic granulite and Al-rich gneissic cores in the Bengbu-Huoqiu area recorded a granulite facies-metamorphism occurring at ~ 1. 85 Ga to 1. 95 Ga,suggesting that the belt likely extends across the Tan-Lu fault,through the southwestern Shandong Province and to the Bengbu-Huoqiu metamorphic basement beneath the Quaternary coverage. A widespread overprint of a Paleoproterozoic( ~ 1. 85 Ga to1. 95Ga) metamorphic event and two stages of magmatic events at ~ 2. 1Ga and 1. 8Ga to 1. 9Ga in the Southern Liaoning and Langrim blocks suggest that both of them( at least part of the metamorphic basement) have been involved in the Paleoproterozoic tectonic evolution of the Jiao-Liao-Ji orogenic / mobile belt. A variety of models have been proposed for the Paleoproterozoic tectonic evolution of the Jiao-Liao-Ji orogenic / mobile belt,including those invoking arc-continent collision and those involving the opening and closing of an intra-continental rift. Recently,a newly model for the Paleoproterozoic Jiao-Liao-Ji orogenic / mobile belt have been proposed,including the opening of a rift basin,followed by the development of an initial ocean basin,and the final closure of the ocean basin through subduction and collision. However,it is difficult to adopt any of these models to explain the extremely complex and massive volcanicsedimentary rocks,multi-stage magmatic events,different types of metamorphism and various metamorphic P-T-t paths and multi-stage deformation within the Jiao-Liao-Ji orogenic / mobile belt. Thus,further studies should be carried out about determination of its southern boundary,tectonic attribution of the Southern Liaoning and Langrim blocks and especially the tectonic setting and evolution for the JiaoLiao-Ji orogenic / mobile belt at the Paleoproterozoic.

Thermal metamorphism of mantle chromites and the stability of noble-metal nanoparticles

Abstract: The Loma Baya complex in south-western Mexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite–olivine assemblage was replaced by a secondary, porous intergrowth of Fe2+-rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This third-generation chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromite-hosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS2, releasing minute particles of Ru–Os–Ir alloys <50 nm in diameter. The following short-lived thermal event that promoted dehydration in the chromitite had the opposite effect on nanoparticle stability, producing a significant coarsening of metal nanoparticles to dimensions larger than a micron. Based on such observations, we argue that IPGE nanoparticles can be exsolved and grown (or coarsen) from sulphide matrices during prograde metamorphism or heating and not exclusively upon cooling under magmatic conditions as it has been previously suggested. These results provide new insights on the relevant role of temperature and nanoparticle–host interaction phenomena in natural systems, shedding new light on the kinetic controls of nano- to micron-scale IPGE particle distributions during metamorphism.

The role of metamorphic fluids in the formation of ore deposits

Abstract: Many ore deposits are hosted by metamorphic rocks, and metamorphic fluids have been invoked as a source for various deposits, especially gold deposits. Metamorphic fluid compositions reflect original sedimentary environment: continental shelf sequences yield saline metamorphic fluids with little dissolved gas while metasediments from accretionary and oceanic settings host less saline fluids with significant CO2 contents. The principal difficulty in reconciling ore deposits with a metamorphic origin is that many form quickly (c. 1 Ma), whereas metamorphic heating is slow (c. 10–208/Ma). Gravitational instability means that fluid cannot be retained. Metamorphic ores may nevertheless form by: (a) segregation leading to enrichment of pre-existing concentrations; (b) infiltration of water-rich fluids from schists into marbles at high temperature overstepping decarbonation reactions and allowing fast reaction that locally draws down temperature; and (c) rapid uplift driving dehydration reactions owing to pressure drop. Some orogenic lode gold deposits fit well with a purely metamorphic origin during rapid uplift, but others are problematic. At Sunrise Dam, Western Australia, anomalies in Sr-isotope ratios and in apatite compositions indicate a partial mantle/magmatic source. Low salinity, H2O–CO2 fluids commonly associated with hydrothermal gold reflect the effect of salt on gas solubility, not the origin of the fluid. Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. Many hydrothermal ore deposits are hosted by metamorphic rocks, and so the possibility that ore deposits may form from metamorphic fluids has been discussed for many years. In this paper we will review what is known about metamorphic fluids from a chemical perspective, and the implications that this may have for their ore-forming potential, and then put this in the context of the history of regional metamorphism and the physical supply of fluid to possible ore systems. Possible natural examples of metamorphic fluids giving rise to ore deposits are also discussed with an emphasis on gold mineralization, because this is the type of mineralization for which metamorphic fluids have been most widely invoked. The term metamorphic fluid is used here in the strict sense to denote fluids present during prograde metamorphism. They will commonly include a component of pre-metamorphic formation waters as well as fluid released by breakdown of volatilebearing minerals, all modified by ongoing interactions with host rock. This definition is much more restrictive than that often used in stable isotope studies, which embraces all fluids that have isotopically equilibrated with metamorphic rocks, irrespective of their origin. Despite some clear evidence to the contrary (e.g. Roedder 1972), until recently crustal fluids were generally assumed to carry only small amounts of potential ore metal in solution. Thus the problem of understanding ore deposits was seen as one of accounting for extensive focussing of fluid flow. With the recognition in recent years that crustal fluids can carry very large dissolved loads (e.g. Yardley & Bottrell 1992; Heinrich et al. 1992; Campbell et al. 1995; Audetat et al. 2000; Yardley 2005; Heijlen et al. 2008; Newton & Manning 2008), this consideration is no longer such a dominant constraint for many metals, although some are always relatively insoluble. While attention has rightly focussed on mass balance considerations, the development of refined methods of geochronology has also highlighted the duration of ore-forming episodes, and in particular has demonstrated that the duration of ore-forming events is often very short in geological terms. Timing provides a further constraint for models of ore genesis, since the requisite volumes of fluid must flow or circulate for relatively short periods of time compared with the metamorphic evolution of the host rocks. This constraint is hardly novel to field geologists used to determining the relative, From: Jenkin, G. R. T., Lusty, P. A. J., McDonald, I., Smith, M. P., Boyce, A. J. & Wilkinson, J. J. (eds) 2015. Ore Deposits in an Evolving Earth. Geological Society, London, Special Publications, 393, 117–134. First published online October 7, 2013, http://dx.doi.org/10.1144/SP393.5 # The Authors 2015. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics rather than absolute, duration of ore formation within a complex regional terrane, but is now quantified for a range of deposit types. When and where do fluids occur in metamorphic rocks? Metamorphic rocks in a classic orogenic belt evolve from relatively porous sediments, with correspondingly high water content, to crystalline rocks with very little porosity. Once they have recrystallized to a low-porosity metamorphic rock they can accommodate significant (percentage) levels of fluid only if they are fractured or as a result of specific metamorphic reactions which generate secondary porosity. Such reactions are known from certain rock types, but they are not widespread and the enhanced porosity is very transient. Furthermore, far from being a ubiquitous source of fluid, metamorphic rocks have the potential to act as a sink, rather than a source, for fluid, absorbing water or other fluid species in retrograde reactions as they cool. It summary, metamorphic rocks only release fluids during specific parts of their metamorphic history (Fig. 1), normally when they are being heated so that volatile-bearing minerals break down and are replaced by mineral assemblages with a lower volatile content. These reactions are strongly endothermic. During progressive heating, fluid is released and fluid pressure rises until the rock acquires sufficient permeability for the fluid to escape (Yardley 1986; Ingebritsen & Manning 2010). Conventionally, this is assumed to be at a value close to lithostatic pressure, and there is extensive evidence from both veins and from phase equilibrium calculations to indicate that this condition is widespread during prograde metamorphism. In contrast, on cooling retrograde reactions reverse the devolatilization and rapidly consume any remaining pore fluid so that the rock becomes essentially dry (Frost & Bucher 1996; Yardley & Valley 1997). After the onset of retrogression, the chemical potential of water or other volatile species in a rock is defined by the coexistence of peak metamorphic minerals and their retrograde products. Typically, the values are at least an order of magnitude lower than would be expected if a free fluid was present. To summarize the fluid history of a reactive metamorphic rock, we can identify three distinct types of fluid regime. During initial prograde metamorphism, fluid is pervasively released by reactions into the grain boundary network. Further fluid loss may arise from reduction in porosity. The composition of the pore fluid evolves progressively as fluid is released from minerals and is lost by flow owing to its low density. Fluid pressures are very high, indicating that the rock is very impermeable. Except for very low-grade rocks then, irrespective of the temperature reached, the onset of cooling is generally accompanied by incipient retrogression, lowering fluid pressure, unless lithostatic pressure also drops markedly (see below). This effectively consumes all free fluid from the pore space. The rock is now dry and a potential sink for fluid, and any fluid that infiltrates along fractures or elsewhere will tend to react to form retrograde assemblages. Many high-temperature minerals are prone to rapid retrograde reaction with infiltrated water (e.g. olivine, orthopyroxene, Al-silicates, biotite), whereas others are much more sluggish to respond. In the event of a subsequent orogenic episode, the response of a pre-existing metamorphic rock depends on the relative conditions attained in the two metamorphic episodes. Often, a later orogeny is marked by pervasive retrogression, deformation and shearing of earlier rocks and the rock continues to act as a fluid sink until it has been fully rehydrated under the new metamorphic conditions. The classic Laxfordian shear zones of the NW Highlands are an example of this pattern (Beach 1973). It is also possible for a later metamorphism to attain more extreme conditions and to drive further volatile loss. This pattern of fluid availability during the metamorphic cycle (Fig. 1) provides the fundamental framework for understanding the role that 0.001 0.01 0.1 1 10 Pelite (porosity) psammite (porosity) Basalt (porosity) Pelite (minerals) Psammite (minerals) Basalt (minerals) Water content (wt. %) Surface Surface T max Time Fig. 1. Schematic representation of the changes in the water contents of a sequence of supracrustal rocks during an idealized metamorphic cycle. The horizontal axis represents time and temperature, with the ends corresponding to surface conditions and the middle to amphibolites facies metamorphism at c. 650 8C. Separate trends are shown for pore water and water combined in minerals, for pelite, psammite and basalt lithologies. Modified from Yardley (1996). B. W. D. YARDLEY & J. S. CLEVERLEY 118

The Bushveld Complex, South Africa

Abstract: The mafic rocks of the Bushveld Complex, South Africa, were emplaced into a stable cratonic shield some 2.06 b.y. ago, and have remained remarkably well preserved from deformation, metamorphism and low temperature alteration, at least partly by its isostatic impact on the entire crustal thickness. The generation and emplacement of possibly 1 million km3 of magma within 65,000 years and the lateral continuity of layering (for up to 100 km) remain intriguing challenges to understanding the evolution of this igneous body. The intrusion is exposed as a three-lobed body, up to 7 km thick, with inward dipping layers that range from dunite to monzonite. Platinum-group element-rich orthopyroxenite, chromitite and vanadiferous magnetitite layers contain vast proportions of the World’s deposits of these commodities. Modal layering, on scales from mm to tens of m, ranges from well-developed in some vertical sections to virtually absent in others. Distinctive layers (ranging from mm to many tens of m) can be identified in two or even three lobes testifying to their connectivity. Feeders to the intrusion cannot be identified, and the exact compositions (and numbers) of the parental magmas are still debated. Rates and effectiveness of their mixing also require resolution. Models of magma additions and extents of mixing lead to very conflicting interpretations in terms of rapidity and vertical extents of the sequence affected. As the largest known mafic intrusion it represents an end-member in terms of magmatic chamber processes.

Sediment and weathering control on the distribution of Paleozoic magmatic tin–tungsten mineralization

Abstract: The formation of major granite-hosted Sn and/or W deposits and lithium–cesium–tantalum (LCT) type pegmatites in the Acadian, Variscan, and Alleghanian orogenic belts of Europe and Atlantic Northern America involves weathering-related Sn and W enrichment in the sedimentary debris of the Cadomian magmatic arc and melting of these sedimentary source rocks during later tectonic events, followed by magmatic Sn and W enrichment. We suggest that within this, more than 3,000-km long late Paleozoic belt, large Sn and/or W deposits are only found in regions where later redeposition of the Sn–W-enriched weathered sediments, followed by tectonic accumulation, created large volumes of Sn–W-enriched sedimentary rocks. Melting of these packages occurred both during the formation of Pangea, when continental collision subjected these source rocks to high-grade metamorphism and anatexis, and during post-orogenic crustal extension and mantle upwelling. The uncoupling of source enrichment and source melting explains (i) the diachronous occurrence of tin granites and LCT pegmatites in this late Paleozoic orogenic belt, (ii) the occurrence of Sn and/or W mineralizations and LCT pegmatites on both sides of the Rheic suture, and (iii) the contrasting tectonic setting of Sn and/or W mineralizations within this belt. Source enrichment, sedimentary and tectonic accumulation of the source rocks, and heat input to mobilize metals from the source rocks are three unrelated requirements for the formation of Sn and/or W granites. They are the controlling features on the large scale. Whether a particular granite eventually generates a Sn and/or W deposit depends on local conditions related to source melting, melt extraction, and fractionation processes.

UHP Metamorphism Documented in Ti-chondrodite- and Ti-clinohumite-bearing Serpentinized Ultramafic Rocks from Chinese Southwestern Tianshan

Abstract: The southwestern Tianshan (China) metamorphic belt records high-pressure (HP) to ultrahigh-pressure (UHP) conditions corresponding to a cold oceanic subduction-zone setting. Serpentinites enclosing retrogressed eclogite and rodingite occur as lenses within metapelites in the UHP unit, which also hosts coesite-bearing eclogites. Based on the petrology and petrography of these serpentinites, five events are recognized: (1) formation of a wehrlite–harzburgite–dunite association in the mantle; (2) retrograde metamorphism and partial hydration during exhumation of the mantle rocks close to the seafloor; (3) oceanic metamorphism leading to the first serpentinization and rodingitization; (4) UHP metamorphism during subduction; (5) retrograde metamorphism during exhumation together with a second serpentinization. The peak metamorphic mineral assemblage of the serpentinized wehrlite comprises Ti-chondrodite + olivine + antigorite + chlorite + magnetite + brucite. A computed pseudosection for this serpentinized wehrlite shows that the Al content in antigorite is mostly sensititive to temperature but can also be used to constrain pressure. The average XAl = 0·204 ± 0·026 of antigorite (XAl = Al (a.p.f.u.)/8, where Al is in atoms per formula unit for a structural formula M48T34O85(OH)62, and M and T are octahedral and tetrahedral sites, respectively) included in Ti-chondrodite and average XAl = 0·203 ± 0·019 of antigorite in the matrix result in a well-constrained peak metamorphic temperature of 510–530°C. Peak pressures are less precisely constrained at 37 ± 7 kbar. The Tianshan serpentinites thus record UHP metamorphic conditions and represent the deepest subducted serpentinites discovered so far. The retrograde evolution occurs within the stability field of brucite + antigorite + olivine + chlorite and formation of Ti-clinohumite at the expense of Ti-chondrodite has been observed, suggesting isothermal decompression. The resulting P–T path is in excellent agreement with the metamorphic evolution of country rocks, indicating that the UHP unit in Tianshan was subducted and exhumed as a coherent block. To refine the metamorphic path of the ultramafic rocks, we have investigated the stability fields of Ti-chondrodite and Ti-clinohumite using piston-cylinder experiments. A total of 11 experiments were conducted at 25–55 kbar and 600–750°C in a F-free natural system. Combined with previous experiments and information from natural rocks we constructed a petrogenetic grid for the stability of Ti-chondrodite and Ti-clinohumite in F-free peridotite compositions. The formation of Ti-chondrodite in serpentinites requires a minimum pressure of about 26 kbar, whereas in Ti-rich systems it can form at considerably lower pressures. A key finding is that at UHP conditions, F-free Ti-chondrodite or Ti-clinohumite breaks down in the presence of orthopyroxene between 700 and 750°C, at temperatures that are significantly lower than those of the terminal breakdown reactions of these humite minerals. These breakdown reactions are an additional source of fluid during prograde subduction of serpentinites.

Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent

Abstract: The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic.

Timing of Partial Melting and Cooling across the Greater Himalayan Crystalline Complex (Nyalam, Central Himalaya): In-sequence Thrusting and its Implications

Abstract: The timing of crustal melting and cooling has been investigated across the migmatites of the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, central Himalaya. Monazite U–Pb ages vary from 32 to 14 Ma and are linked to metamorphic conditions on the basis of monazite internal zoning, mineral inclusions, and changes in heavy rare earth element and Y composition. Metamorphic temperatures were estimated by Zr-in-rutile thermometry and cooling rates were further constrained by rutile U–Pb ages. The results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC experienced higher peak metamorphic temperatures (730–750 � C) and a higher degree of melting (15–25%). Partial melting was dominated by muscovite dehydration melting, which lasted from � 32 to 25 Ma, possibly until � 20 Ma. The lower GHC experienced lower peak metamorphic temperatures (640–675 � C) and a lower degree of melting (0–10%) mainly via H2O-saturated melting from 19 to 16 Ma. At different times, both upper and lower blocks experienced initial slow cooling (rates 35 6 8 and 10 6 5 � CM a –1 , respectively) followed by rapid cooling (100 6 20 � CM a –1 ). The documented diachronous metamorphism implies the presence of the ‘High Himalayan Thrust’ that was active at � 25–16 Ma within the GHC of the central Himalaya. Different degrees and durations of partial melting in the investigated section suggest that a channel flow process dominated the exhumation of the upper GHC migmatites at 25–16 Ma, whereas a critical taper process dominated the exhumation of the relatively lower-grade lower GHC rocks and cooled upper GHC migmatites at 16–10 Ma. We suggest that propagating thrusts along large tectonic boundaries together with low-viscosity lateral crustal flow could contribute to exhumation of high-grade metamorphic rocks in the Himalaya and other similar collisional orogens.

Extreme variation of sulfur isotopic compositions in pyrite from the Qiuling sediment-hosted gold deposit, West Qinling orogen, central China: an in situ SIMS study with implications for the source of sulfur

Abstract: High spatial resolution textural (scanning electron microscope (SEM)), chemical (electron microprobe (EMP)) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)), and sulfur isotopic (secondary ion mass spectrometry (SIMS)) analyses of pyrite from the Qiuling sediment-hosted gold deposit (232 ± 4 Ma) in the West Qinling orogen, central China were conducted to distinguish pyrite types and gain insights into the source and evolution of sulfur in hydrothermal fluids The results reveal an enormous variation (−271 to +696 ‰) in sulfur isotopic composition of pyrite deposited during three paragenetic stages Pre-ore framboidal pyrite, which is characterized by low concentrations of As, Au, Cu, Co, and Ni, has negative δ34S values of −271 to −76 ‰ that are interpreted in terms of bacterial reduction of marine sulfate during sedimentation and diagenesis of the Paleozoic carbonate and clastic sequences, the predominant lithologies in the deposit area, and the most important hosts of many sediment-hosted gold deposits throughout the West Qinling orogen The ore-stage hydrothermal pyrite contains high concentrations of Au, As, Cu, Sb, Tl, and Bi and has a relatively narrow range of positive δ34S values ranging from +81 to +152 ‰ The sulfur isotope data are comparable to those of ore pyrite from many Triassic orogenic gold deposits and Paleozoic sedimentary exhalative (SEDEX) Pb-Zn deposits in the West Qinling orogen, both being hosted mainly in the Devonian sequence This similarity indicates that sulfur, responsible for the auriferous pyrite at Qiuling, was largely derived from the metamorphic devolatization of Paleozoic marine sedimentary rocks Post-ore-stage pyrite, which is significantly enriched in Co and Ni but depleted in Au and As, has unusually high δ34S values ranging from +374 to +696 ‰, that are interpreted to result from thermochemical reduction of evaporite sulfates in underlying Cambrian sedimentary rocks with very high δ34S values The variations in Au content and sulfur isotopic compositions across a single ore-stage pyrite grain may reflect displacement of indigenous groundwater with low δ34S values by auriferous metamorphic fluids with high δ34S values The very low-grade metamorphism of the host rocks and the metamorphic derivation of sulfur for the ore pyrite indicate that the Qiuling sediment-hosted gold deposit is an epizonal manifestation of an orogenic gold system in the West Qinling orogen

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal.

Abstract: The Himalayan mountain belt is characterized by the impressive continuity over 2500 km of tectonic units, thrusts and normal faults, as well as large volumes of high-grade ‐metamorphic rocks and granite exposed at the surface (Visona et al., 2012). Although there are many studies on metamorphism, melt generation and deformation concentrated in the same time span as activity on the MCT and STD (~ 23 - 17 Ma; Godin et al. 2006), much fewer data on deformation, metamorphism, melt generation and geochronology in the Greater Himalayan Sequence (GHS) is available for the large time span (~ 30 M.y.) between collision at ~ 55 Ma and MCT-STD activities (Carosi et al., 2010; 2013; Montomoli et al., 2013). Melt generation and granite emplacement have been mainly related to the exhumation stage of the GHS during Early Miocene (Harris and Massey, 1994). Very few evidences of partial melting during prograde metamorphism have been reported until now in the southern part of the belt (Godin et al. 2001; Imayama et al. 2012). The GHS of eastern Nepal and Sikkim (India) shows the occurrence of melting at the bottom of the GHS (Barun gneiss) at nearly 33-27 Ma (Imayama et al., 2012; Ferrero et al., 2012; Rubatto et al., 2012). Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (Fig. 1) along Kali Gandaki valley, Central Nepal, nearly ~ 1 km structurally above the Main Central Thrust. In situ U-Th-Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed to constrain partial melting starting at ~36-41 Ma (Carosi et al., in press).