High-grade Metamorphic Terranes Research Articles

Metasomatism of mafic-intermediate rocks in the Motloutse Complex, Botswana: Effect on ore mineralization and implication for the Paleoproterozoic Large Alkali Metasomatic Province (LAMP) in southern Africa

Alkali metasomatism is a common overprint in Archean medium- to high-grade metamorphic terranes. Since mafic-intermediate rocks are able to host ore mineralization, it is important to understand the effect of metasomatism on its distribution. This study presents field, petrographic, mineralogical, geochemical, and U-Pb/40Ar-39Ar geochronologic characteristics of metasomatized amphibolite, amphibole gneiss and gabbro along a regional E-W section of the southeastern Motloutse Complex in eastern Botswana. The progressive metasomatism accompanies shear deformation and is visually manifested by varying degrees of pinkish discoloration of the host rocks. The metasomatic reactions in amphibolite, amphibole gneiss and gabbro are manifested in replacement of amphibole, clinopyroxene, ilmenite and plagioclase by the assemblages of chlorite, epidote, titanite, magnetite, sericite, albite and K-feldspar, up to near complete albitization and K-feldspathization with few relics of earlier minerals. Mineral compositions reflect the effects of a progressive metasomatic overprint. The metasomatic overprint resulted in the break down and dispersal of primary Cu-Ni sulphides. The pseudosection modelling (PERPLE_X) in terms of lg(aK2O) and lg(aH2O) parameters for representative compositions of leucocratic and melanocratic amphibolites, amphibole gneiss and gabbro indicates that the metasomatic process involved an increase of potassium activity followed by hydration. The modelling explains variable extent of metasomatism in different rock types. Whole-rock compositional variations indicate a general enrichment in Na2O, K2O and SiO2, and depletion in FeO and MgO in rocks during the metasomatism. 40Ar/39Ar amphibole and U-Pb titanite geochronology from metasomatized amphibolite and gabbro gave comparable ages of c.2.01–1.96 Ga. Alkaline fluids producing metasomatism were likely of crustal origin and moved along the reactivated regional c.2.01–1.95 Ga shear zones at the boundaries of the Motloutse-Limpopo complexes with the adjacent Zimbabwe and Kaapvaal cratons. Implications for a Paleoproterozoic Large Alkali Metasomatic Province (LAMP) in southern Africa, and its relation to the Bushveld large igneous province (LIP) is explored.

Delineating the dismembered remnants of Archean layered complexes in medium-high-grade terranes: Scenario from the Motloutse Complex, eastern Botswana

Ultramafic-mafic rocks in medium-to high-grade metamorphic terranes commonly occur as fragmented units of once continuous layered intrusions. A number of factors can complicate the delineation of the dismembered remnants and reconstruction of the original lithostratigraphy. This study focuses on remnant ultramafic rocks of the Mesoarchean Lechana layered complex in the southern Motloutse Complex, eastern Botswana. In view of the limited exposures, including the prominent cover rocks, geologic characterization was supplemented with geophysical techniques. The NE-SW traverse covered in the study includes the Tamasane and Sapolamorori hills, on either side of the cover rocks. Both hills are comprised of layered ultramafic rocks, with tonalite gneiss and amphibolite forming the surrounding rocks. The inferred layered lithostratigraphy includes serpentinized dunite, peridotite, peridotite-pyroxenite, pyroxenite-peridotite, and pyroxenite. Magnetitite seams constitute the main mineralization, and are associated with the serpentine-rich rocks. Aeromagnetic data provide insights into the bulk magnetic susceptibility of the ultramafic lithologies along the transect. The highest magnetic susceptibilities occur as semi-circular closed patches. The two anomalies at either end of the transect correlate with the layered ultramafic bodies at the Tamasane and Sapolamorori hills. The rest approximately correlates with high Ni ± Cr ± Cu soil anomalies reported from the region. This redefines the known extent of the Lechana layered complex beneath the cover rocks. The study highlights the potential of integrating geological and geophysical data to delineate the dismembered remnants of Archean layered complexes.

A New Method for Relating Zircon Crystallisation to Petrogenetic Events

Abstract The trace element contents of zircon can provide unique insights into tectonothermal events, however, interpreting these data and identifying correlations with specific magmatic/metamorphic events can be challenging. This limits our ability to construct temporally constrained petrogenetic histories of complex metamorphic terranes. Unlocking the information that the rare earth element (REE) patterns of zircon contain is difficult because of the need to quantify differences. We have parametrised the shape of zircon REE patterns in terms of three independent parameters: average abundance, slope, and curvature. Quantifying REE patterns using independent shape parameters is similar to the use of REE ratios but is an improvement as (1) it uses information from all 14 REE rather than just two; (2) the use of two independent parameters (e.g. slope and curvature) is a more robust discriminant than the use of a single ratio; and (3) subtle variations in shape are easily distinguished enabling trends in the REE patterns of large datasets to be identified. Quantitative models were constructed showing how the shapes of the REE patterns of zircon change due to the co-crystallisation of other metamorphic minerals (monazite, apatite, and garnet). Diagnostic changes in shape enable the REE contents of zircon crystals or crystal zones to be accurately related to the growth of specific minerals and hence metamorphic events. The results were used to interpret the REE patterns of zircons from high-grade metamorphic terranes, which have experienced multiple deformation events (Val Malenco, Italy; Betic Cordillera, Spain; Seram, Indonesia; Lewisian Gneiss Complex, Scotland; Napier Complex, East Antarctica) and clearly identified zircon that crystallised in the presence of garnet. Quantitative comparison enabled zircon that crystallised prior to, synchronously with, or after garnet to be identified. Similar models can be used to interpret the REE patterns of monazite. This allows the relative timing of the growth of these minerals to be accurately constrained, which given the importance of zircon for geochronology and garnet for geobarometry has the potential to provide insights into the evolution of a metamorphic event.

Contribution of Precambrian basements to the Mesozoic ore-fluid system: An illustration using the Majiayao gold deposit, Jiaodong, China

In contrast to most Mesozoic granite-hosted gold deposits in the giant Jiaodong gold province (China), the Majiayao deposit represents a significant example hosted in the Archean high-grade metamorphic terrane. The mineralization is characterized by auriferous quartz-sulfide veins together with notable Fe-bearing carbonates (ankerite and siderite), iron oxides and barite. In this study, the comprehensive in-situ analyses of vein-hosted hydrothermal minerals (such as: xenotime U-Pb dating, ankerite Rb-Sr isotopes and monazite Sm-Nd isotopes) and sulfur isotopic analyses of gold-bearing sulfides are used to constrain the timing of mineralization and to better understand the ore genesis. The syn-ore hydrothermal xenotime yielded a U-Pb age of 120.0 ± 1.2 Ma marking timing of gold mineralization, which is much younger than wallrocks’ latest metamorphism at 1950 to 1850 Ma. The relatively higher and variable initial 87Sr/86Sr ratios (0.715809–0.718753) of ore-related ankerite may be leached from ore-hosting Archean metamorphic rocks and/or regional Neoproterozoic Penglai Group low-grade metasedimentary rocks via extensive fluid-rock interactions. This inference is also supported by the newly reported near-zero δ34S values (–1.2 to 1.4‰) of pyrite from altered wallrocks, which are significantly comparable to the surrounding Archean plagioclase amphibolite. The monazite has relatively homogenous εNd(t=120Ma) values ranging from –19.3 to –17.6, which are much higher than the Nd isotope compositions of the ore-hosting Archean high-grade metamorphic rocks. Combining the widespread mafic dykes and previous relevant geochemical studies, it is proposed that the ore-forming fluids were probably derived from deeply mantle-related contents, followed by intense interaction with Archean basements at the shallow crustal level. In addition, the progressive decrease of δ34S values of pyrite and more appearances of iron oxides and barite from early to late mineralization stages, are ascribed to gradual fluid oxidation by extensive carbonatization of Fe-rich Archean wallrocks.

The role of titanite in shaping the geochemistry of amphibolite-derived melts

Titanite, one of the major accessory phases in middle to lower crustal rocks, could play an important role in shaping trace element as well as radiogenic isotope compositions in amphibolite-derived melts. Petrographic, zircon U/Pb, whole-rock element and isotope (Sr and Nd) data on a suite of leucosomes, and titanite Nd isotope compositions from the migmatitic amphibolite complex, southern Liaoning, indicate that (1) the amphibolite with a protolith age of ~2450 Ma underwent low-degree partial melting at ~160 Ma and produced two groups of leucosome; (2) these leucosomes preserved a large magnitude of Nd isotope disequilibrium with up to ~10 epsilon units of difference with respect to their sources; (3) differential dissolution of titanite during partial melting might lead to negative correlations between Nd isotope compositions and respective Ti or V contents (proxy of the amount of titanite presented). These data indicates that during low-degree partial melting of mafic rocks, titanite stays as a residue phase due to either unfavorable texture or melting (temperature or fluid) conditions and leads to the development of melts with depletion in Ti-affinity elements, REE, and a large positive shift in Nd isotope compositions. Similar to other accessory phases (e.g., zircon, apatite, and monazite) in metasedimentary sources, this contribution presents another case that could be common in high-grade metamorphic terranes in which titanite could be a key phase that controls key trace element (e.g. NbTa, V, and REE) budget and Nd isotope compositions in melts derived from low-pressure melting of amphibolite or mafic rocks in various tectonic environments.

Miocene-Pleistocene crustal anatexis in the Namche Barwa massif, southern Tibet

位于喜马拉雅东构造结的南迦巴瓦地块经历了复杂的构造变形、强烈的变质和深熔作用，是研究碰撞造山过程中地壳深熔作用的重要对象。完整地厘定新生代晚期岩浆作用期次对于揭示南迦巴瓦地区的构造演化历史和深部过程具有重要意义。南迦巴瓦地块3件淡色花岗岩样品的锆石U-Pb定年结果显示该地块经历了11.30±0.16Ma和2.59±0.04Ma两期地壳深熔作用，可能与南迦巴瓦地块晚新生代快速隆升和剥蚀相关。南迦巴瓦地块保存了大量的~11Ma变质作用和地壳深熔作用记录指示该时间段为构造活动剧烈期。上新世晚期的淡色花岗岩表明，穹窿的隆升和剥蚀所导致的岩浆作用至少持续到了~2.59Ma，代表了南迦巴瓦地区一次年轻的构造岩浆事件。;The Namche Barwa massif, located in the eastern tectonic junction of Himalayan Orogen, has experienced complex tectonic deformation, intensive metamorphism and anatexis which resulted in various types of leucogranites to investigate the nature of crustal anatexis during the tectonic evolution of collision orogenic belts. Knowledge of the timing of such crustal anatexis in rapidly exhumed high grade metamorphic terranes like the Namche Barwa is important to understand the feedback of rapid near-surface processes (erosion and denudation) and those relatively deep processes (high-T metamorphism and partial melting). Zircon U-Pb dating of three leucogranite samples from the Namche Barwa yields that this block has experienced two episodes of young crustal anatexis (11.30±0.16Ma and 2.59±0.04Ma, respectively), which may be related to rapid denudation and uplift of the Namche Barwa in the Late Cenozoic. A number of studies have demonstrated that various rock types in the Namche Barwa have experienced a major episode of metamorphism and crustal anatexis at~11Ma, indicating intense tectonic activities in this period. Occurrence of leucogranite of late Pliocene in age further suggests that the anatectic processes caused by rapid denudation and uplift in this massif lasted at least to~2.59Ma.

Sediment-derived origin of the putative Munnar carbonatite, South India

Metacarbonate assemblages in high-grade metamorphic terranes often pose challenges when trying to distinguish between mantle-derived carbonatite and sedimentary carbonate protoliths. We present a study of granulite-facies metacarbonate samples of the putative Munnar carbonatite described as decimeter-thick dikes and veins, and layers of a meter-thick metacarbonate and calc-silicate assemblage, respectively. Thin sections of the metacarbonate dike samples show absence of pyrochlore and ubiquitous scapolite, titanite, wollastonite, and detrital zircons are compatible with impure limestone protoliths. Nd and Sr isotope compositions indicate protoliths with Paleoproterozoic crustal residence times which contrast the mantle sources of Indian and global carbonatites. Trace-element patterns display the characteristics of upper crust, and Ce- and Y-anomalies in a number of samples suggest protolith formation under marine conditions. Carbon and oxygen isotope compositions of the metacarbonate samples interlayered with calc-silicate rocks are similar to those in marine limestone. The metacarbonate dikes, however, show mantle-like compositions which are interpreted as reflecting equilibration with mantle-derived CO2 during granulite-facies metamorphism. The dikes yielded a U-Pb zircon crystallization age of 1020 ± 70 Ma and a cross-cutting quartz syenite, thought to be cogenetic, a magmatic age of 620 ± 35 Ma; the hosting gneiss provided a magmatic age of 2452 ± 14 Ma. We conclude that the layered metacarbonate and calc-silicate rocks represent a former marine limestone and marl sequence and the metacarbonate dikes and veins small-volume melts of crust-derived carbonate-rich sediment.

Geochronology and Geochemistry of Archean TTG and Tremolite Schist Xenoliths in Yemadong Complex: Evidence for ≥3.0 Ga Archean Continental Crust in Kongling High-Grade Metamorphic Terrane, Yangtze Craton, China

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units and the familiar metabasite xenoliths they host in the Yangtze Craton, China, remain controversial, and resolving these issues is important if we are to understand the evolution of the early Yangtze Craton. We focused on biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).

Decoupled U-Pb date and chemical zonation of monazite in migmatites: The case for disturbance of isotopic systematics by coupled dissolution-reprecipitation

Monazite is an abundant accessory mineral in metasedimentary rocks and their anatectic products. Trace element analysis combined with U-Pb dating of monazite is widely used to reconstruct P-T-t histories of high-grade metamorphic terranes. This approach relies on interpreting U-Pb dates, which requires understanding the processes that cause isotopic disturbances. We present in-situ laser ablation split-stream (LASS) results of U-Pb dating and trace element spot analyses of single monazite grains from anatectic granite samples from a HT-LP migmatitic turbidite terrane exposed in the Sierra de Quilmes, NW Argentina. This terrane was part of the back-arc and remained close to anatectic temperatures for most of the ∼510–440 Ma Famatinian Orogeny, in the Western Gondwana margin. Sampled outcrops display evidence of remelting of the anatectic granites, marked by leucosomes and confirmed by zircon chronology that define multiple thermal peaks and anatexis events during ∼60 Myr duration of the arc. U-Pb monazite chronology of these granite samples reveals a continuum of dates between ∼500 and 450 Ma recording most of the duration of the Famatinian event and matching the range of zircon growth ages, but without their multiple, well-defined age populations. Despite each sample yielding a reasonable Famatinian monazite mean date, the results reveal that the isotopic system has been perturbed. Data for each rock sample yield similar date spread for both the cores and the well-defined rims of single monazite grains, indicating a disconnect between U-Pb dates and chemical zonation. We interpret these random within-grain date variations as a result of coupled dissolution-reprecipitation reactions between monazite grains and ambient fluids, most likely silicate melts. These reactions occurred during a single orogeny marked by a long-lasting, high-temperature metamorphic history. During coupled dissolution-reprecipitation reactions, ingrown radiogenic Pb was remobilized and/or redistributed within the grain giving rise to unsupported Pb. This process was likely significant here because dissolution of apatite into silicate melts during migmatization stabilized monazite, preventing their dissolution but not preventing their reaction with the melt. Redistribution of radiogenic Pb caused by the coupled dissolution-reprecipitation reactions gave rise to meaningless individual dates spreading along the 50 Myr duration of the thermal event associated with the orogeny. In this case, rather than reacting with fluids from a distinct event, monazite dates were perturbed during the same orogeny.

A Review of Boron-Bearing Minerals (Excluding Tourmaline) in the Adirondack Region of New York State

Boron is a biologically important element, but its distribution in the natural environment and its behavior during many geological processes is not fully understood. In most metamorphic and igneous environments, boron is incorporated into minerals of the tourmaline supergroup. In high-grade metamorphic terranes like that of the Adirondack region of northern New York State, uncommon rock compositions combined with unusual and variable geologic conditions resulted in the formation of many additional boron-bearing minerals. This paper reviews the occurrences and geological settings of twelve relatively uncommon boron-bearing minerals in the southern Grenville Province of upstate New York and provides new chemical and Raman spectral data for seven of these minerals. The boron minerals range from relatively simple metal borates (e.g., vonsenite), to chemically complex borosilicates (e.g., prismatine), to a relatively rare borosilicate-carbonate (e.g., harkerite). Some are of primary igneous origin, while others are formed by a variety of prograde and retrograde metamorphic processes or by metasomatic/hydrothermal processes. Most of the boron minerals are formed within, or adjacent to, metasedimentary lithologies that surround the anorthositic massifs of the central Adirondacks. The metasedimentary rocks are thought to be the source of most of the boron, although additional boron isotope studies are needed to confirm this and to constrain the mechanisms of the formation of these unusual minerals.

The Hadamengou K-feldspar–quartz vein-type Au mineralization in the northern margin of the North China Craton (China): an Indosinian oxidized intrusion-related Au–(Mo) system?

K-feldspar–quartz vein-type Au deposits, represented by the Hadamengou super-large Au deposit, are widely developed in the Archean high-grade metamorphic terrane in the western segment of the northern margin of the North China Craton. However, the formation processes of these Au deposits have long been debated, and their genetic relationship with magmatic activity remains unclear. In this study, by examining the geological and geochemical characteristics of the Hadamengou Au deposit and several related intrusions, we find that the Au–(Mo) mineralization system may be related to Indosinian alkaline magmatic activity. The Dongshadegai and Xishadegai intrusions, which are related to the formation of the Hadamengou Au deposit, formed in an Indosinian extensional tectonic environment, and both are characterized by high silica and alkaline contents and strong oxidizing properties, indicating that the parent magmas of these intrusions were capable of dissolving and transporting metallic elements during magma formation and evolution. The Indosinian Hadamengou Au deposit contains minerals such as magnetite and barite, and the H, O, S and Pb isotopic characteristics of this deposit indicate that the ore-forming fluids originated from magmatic–hydrothermal fluids and had oxidizing properties. These observations suggest the presence of an oxidized intrusion-related Au–(Mo) mineralization system in the studied metamorphic terrane.

A rare Phanerozoic amphibolite-hosted gold deposit at Danba, Yangtze Craton, China: significance to fluid and metal sources for orogenic gold systems

The Danba gold deposit is located in a poorly-documented gold province on the north-western margin of the Yangtze Craton. It is sited in Devonian sequences in a high-grade metamorphic terrane that includes an extensional metamorphic core complex. Around the deposit, peak metamorphic conditions of 6 ± 0.5 kbar and 650 ± 50 °C at ca. 193 Ma were followed by retrograde sillimanite-grade conditions of 4.5 ± 0.5 kbar and 550 ± 50 °C. The deposit is hosted in a broadly strata-bound ductile-brittle shear zone with high-T proximal alteration assemblages of biotite-amphibole-plagioclase and ore assemblages dominated by pyrrhotite, but with a strong association between gold and bismuth tellurides. Alteration mineral thermobarometers, together with heating/freezing studies of low-salinity H2O–CO2–CH4 fluid inclusions, indicate P-T conditions of early ore deposition of approximately 4–5 kbar and 500–650 °C at around 185 ± 9 Ma indicated by Re-Os geochronology on ore-related molybdenite. In conjunction, all data demonstrate that Danba represents a Lower Jurassic hypozonal orogenic gold deposit that formed during post-peak metamorphic retrogression. The primary high P-T nature of the deposit, combined with its late-metamorphic timing, negate that the ore fluid was sourced via devolatilization of the hosting supracrustal sequences. A deep externally-derived ore-fluid source is required. The most likely source is the K–H2O–CO2 and ore-metal fertilized lithospheric mantle that was metasomatized during Neoproterozoic subduction. It is proposed that transition from lithospheric transpression to extension in the Jurassic triggered the devolatilization of this metasomatized lithosphere to cause the formation of this rare Phanerozoic amphibolite-hosted gold deposit at Danba.

Petrogenesis of ultramafic rocks of komatiitic composition from the Central Zone of the Limpopo Belt, South Africa: Evidence from O and H isotopes

Komatiites are MgO-rich ultramafic volcanic rocks, mostly interpreted to be essentially anhydrous at the time of extrusion, and are often characterized by spinifex texture. They mainly occur in Archean or Proterozoic greenstone belts within cratons. However, in the Central Zone of the Limpopo Belt in South Africa, ultramafic rocks of komatiitic composition occur within a high-grade metamorphic terrane. Their chemical composition overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, but unlike komatiites, they are mainly composed of pargasitic amphibole, along with olivine, orthopyroxene, Cr-rich spinel and magnetite. The studied rocks have a narrow range of whole rock δ18O values (+5.4 to +6.3‰), unlike δ18O values from komatiites, which are generally altered and have δ18O values from ∼+2 to +9‰. However, they are within the range estimated for unaltered komatiites (+5.4 to +6.0‰). Amphibole separates from the studied rocks have δ18O values between +5.5 and + 6.1‰, similar to that of the bulk rock. The δD values of amphiboles range between −78 and −83‰, similar to primary amphiboles from other komatiites and many other mantle-derived mafic igneous rocks. The δ18O values, along with textural and mineralogical evidence, are consistent with the ultramafic rocks of komatiitic composition from the Limpopo Belt being intrusive and unaffected by low-temperature alteration. The O and H isotope composition of the amphiboles are consistent with them being primary magmatic in origin. It cannot be distinguished with certainty whether the water was introduced to the magma through partial melting of hydrated mantle or during ascent through hydrous mantle subsequent to their formation. The minerals observed at present most likely crystallized from komatiite magma that intruded into the lower crust and subsequently re-equilibrated at the ambient P-T conditions.

Indosinian gold mineralization and magmatic-hydrothermal evolution of the Hadamengou gold deposit at the northern margin of the North China Craton: Constraints from K-feldspar laser 40Ar/39Ar dating

The Hadamengou gold deposit, which is located in a high-grade metamorphic terrane at the northern margin of the North China Craton, is characterized by K-feldspar-quartz vein gold mineralization. The 40Ar/39Ar dating of hydrothermal K-feldspar indicates that the mineralization formed over 45 Ma commencing at 243.5 ± 1.3 Ma and ending at 199.0 ± 0.8 Ma. The Pb, C and O isotope compositions and the geochronology of the gold-bearing K-feldspar-quartz veins imply a close genetic association between the gold mineralization and alkaline magmatism, which are both products of the Indosinian alkaline magmatic event caused by the extensional tectonic setting that prevailed during the late Hercynian collisional orogeny. We propose an Indosinian phase of gold mineralization in addition to the abundant mineral deposits formed during the Yanshan Epoch.

Evolution of the Archean continental crust in the nucleus of the Yangtze block: Evidence from geochemistry of 3.0 Ga TTG gneisses in the Kongling high-grade metamorphic terrane, South China

Archean Tonalite-Trondhjemite-Granodiorite (TTG) rocks are scattered within the Kongling high-grade metamorphic terrane (KHMT) in the northern South China block. A comprehensive geochronological and geochemical study is carried out on the Taoyuan granitic gneisses, a newly recognized TTG suite in the northwestern KHMT. This suite has long been regarded as a Mesoproterozoic magmatic pluton, but U-Pb zircon ages of 2994 ± 22 Ma and 2970 ± 15 Ma are obtained by LA-ICP-MS method in this study. The Taoyuan gneiss suite is trondhjemitic in composition, and has high SiO2 (67.80–74.93 wt.%), Na2O (5.11–5.81 wt.%) contents with Na2O/K2O ratios greater than unity, and low Ni (2.56–7.61 ppm), Cr (1.26–7.67 ppm), Yb (0.32–0.82 ppm) and Y (4.48–11.5 ppm) contents. Plots show large variation in La/Yb and Sr/Y ratios and pronounced depletion in Nb, Ta and Ti in the primitive mantle-normalized spiderdiagram. The gneiss suite also displays two-stage Nd model ages close to its crystallization age with corresponding εNd(t) values of −2.5 to +3.5. It is thus suggested that the Taoyuan gneisses, in fact, is part of the Archean Kongling basement complex.Geochemical evidence implies that the TTG rocks may be derived from partial melting of subducted oceanic crust from a garnetiferous amphibolite source with residual assemblage of garnet + amphibole + plagioclase. Our study further indicates that the nucleus of the Yangtze block might experience a juvenile continental crustal growth during Mesoarchean. We also suggest that the Yangtze block may have its own crustal evolutionary history independent from the North China craton and the Tarim block before Paleoproterozoic.

Zircon U-Pb ages and Hf isotopic systematics of charnockite gneisses from the Ediacaran–Cambrian high-grade metamorphic terranes, southern India: Constraints on crust formation, recycling, and Gondwana correlations

The Southern Granulite terrane, southern India, comprises a vast exposure of deep crust and forms a key region in the reconstruction of the Gondwana supercontinent. An E-W−trending crustal-scale shear zone, the Palghat-Cauvery suture zone system, which formed during the late Neoproterozoic−Cambrian (0.75−0.50 Ga), marks a prominent terrane boundary separating terranes with a predominantly late Neoarchean (ca. 2.5 Ga) regional granulite metamorphism to the north from those with an Ediacaran−Cambrian (0.63−0.50 Ga) regional granulite metamorphism to the south. Focusing on the younger granulite metamorphic domains, we present here new zircon U-Pb ages and Hf isotopic compositions for 11 charnockite orthogneisses from the Madurai, Trivandrum, and Nagercoil blocks and contribute to the resolution of the age of their magmatic protoliths. This study shows that the charnockite orthogneisses south of the Palghat-Cauvery suture zone relate to a minimum of four distinct episodes of felsic magmatism centered at: ca. 2.62−2.46 Ga, ca. 2.05−1.84 Ga, ca. 1.0−0.9 Ga, and ca. 0.80−0.76 Ga, pertaining to the Siderian, Orosirian, and Tonian Periods. Hafnium isotope analyses of zircon grains from the charnockite gneisses suggest that the protoliths of the ca. 2.05−1.98 Ga gneisses from the Trivandrum and Nagercoil blocks and the ca. 1.0−0.9 Ga gneisses along the southeastern Madurai block involved a significant juvenile magma component, while the protoliths of charnockite gneisses elsewhere in the Madurai block formed mainly through recycling of older crust up to ca. 3.2 Ga. A regional granulite-facies metamorphic imprint during the Ediacaran−Cambrian marked an advanced stage in the amalgamation of the Madurai, Trivandrum, and Nagercoil blocks into the East African orogen and its collision with the Dharwar craton.

Early Mesozoic retrograded eclogite and mafic granulite from the Badu Complex of the Cathaysia Block, South China: Petrology and tectonic implications

The high-grade metamorphic terrane in the Badu region along the northeastern Cathaysia Block in South China preserves retrograded eclogites and mafic granulites. Here we present the petrology, mineral phase equilibria and P-T conditions based on pseudosection computations, as well as zircon U-Pb ages of these rocks. Mineral textures and reaction relationships suggest four metamorphic stages for the retrograded eclogite as follows: (1) eclogite facies stage (M1), (2) clinopyroxene retrograde stage (M2), (3) amphibole retrograde stage (M3), and (4) chlorite retrograde stage (M4). For the mafic granulite, three stages are identified as: (1) plagioclase-absent stage (M1), (2) granulite facies stage (M2) and (3) amphibolite facies stage (M3). Metamorphic evolution of both of the rock types follows clockwise P-T path. Conventional geothermometers and geobarometers in combination with phase equilibria modelling yield metamorphic P-T conditions for each metamorphic stage for the eclogite as 500–560°C, 23–24kbar (M1), 640–660°C, 14–16kbar (M2), 730–750°C, and 11–13kbar (M3). The chlorite retrograde stage (M4) is inferred to have occurred at lower amphibolite to greenschist facies conditions. Phase equilibria modelling of the mafic granulite shows P-T conditions for each metamorphic stage as 600–720°C, >13kbar (M1) and 860–890°C, 5–6kbar (M2) and M3 at amphibolite facies conditions. LA-ICPMS zircon U-Pb dating and trace element analysis show that the high pressure metamorphism occurred at 245–251Ma. Protolith age of the mafic granulite is 997Ma, similar to that of the mafic to ultramafic rocks widely distributed in the Cathaysia Block and also along the Jiangnan belt. Subduction of ancient oceanic lithospheric materials (or crustal thickening) during Mesozoic and formation of eclogites suggest that the Cathaysia Block was perhaps in the Tethyan oceanic domain at this time. The granulite formation might have been aided by Mesozoic mafic magma underplating associated with lithospheric delamination, heating and retrogression of the eclogite accompanied by rapid uplift.

Petrogenesis and tectonic implications of the Neoarchean North Liaoning tonalitic-trondhjemitic gneisses of the North China Craton, North China

Tonalitic-trondhjemitic-granodioritic (TTG) gneisses are dominant lithological assemblages in Archean high grade metamorphic terranes in the world. These TTG gneisses preserve important information in formation and evolution of Archean continental crust. Tangtu-Majuanzi microblock in North Liaoning Province (NLP) is one of the major Neoarchean metamorphic basement terranes in the northeastern margin of the North China Craton (NCC). The Tangtu-Majuanzi microblock is composed mainly of Neoarchean tonalitic-trondhjemitic (TT) gneisses, subordinate granodioritic to monzogranitic association (GMA) and minor supracrustal rocks. The tonalitic-trondhjemitic gneisses are divided into high MgO Group (HMG) and low MgO Group (LMG) based on their chemical compositions. Detailed petrogenetic investigations suggest that the magmatic precursors of the HMG samples were derived from partial melting of subducted slabs and contaminated by the overlying mantle wedge during its ascent; whereas, magmatic precursors of the LMG samples were derived from the juvenile lower crust. LA-ICPMS zircon U-Pb isotopic dating analyses reveal that the magmatic precursors of the HMG samples were formed at 2553–2531 Ma. An older HMG tonalitic gneiss sample was discovered at Sandaoguan in the southmost of the studied area, with its magmatic precursor emplaced at 2680Ma. The magmatic precursors of the LMG samples emplaced at 2595–2583Ma.Combined with previous credible chronological data, our newly obtained zircon U-Pb dating and Lu-Hf isotopic data indicate that three episodes of magmatism at ∼2700–2680Ma, ∼2600–2570Ma and ∼2550–2510Ma occurred in the Tangtu-Majuanzi microblock, and the TT gneisses in this microblock were subjected to generally amphibolite-facies metamorphism at ∼2520–2470Ma.Based on the above Neoarchean crust-mantle thermal-dynamic processes, we propose that the Neoarchean magmatism and metamorphism in the Tangtu-Majuanzi microblock of North Liaoning Province occurred in an active continental margin.

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: A case study from the Lewisian Gneiss Complex of north-west Scotland

Many rare metals used today are derived from granitic pegmatites, but debate continues about the origin of these rocks. It is clear that some pegmatites represent the most highly fractionated products of a parental granite body, whilst others have formed by anatexis of local crust. However, the importance of these two processes in the formation of rare-metal pegmatites is not always evident.The Lewisian Gneiss Complex of NW Scotland comprises Archaean meta-igneous gneisses which were highly reworked during accretional and collisional events in the Palaeoproterozoic (Laxfordian orogeny). Crustal thickening and subsequent decompression led to melting and the formation of abundant granitic and pegmatitic sheets in many parts of the Lewisian Gneiss Complex. This paper presents new petrological, geochemical and age data for those pegmatites and shows that, whilst the majority are barren biotite–magnetite granitic pegmatites, a few muscovite–garnet (rare-metal) pegmatites are present. These are mainly intruded into a belt of Palaeoproterozoic metasedimentary and meta-igneous rocks known as the Harris Granulite Belt.The rare-metal pegmatites are distinct in their mineralogy, containing garnet and muscovite, with local tourmaline and a range of accessory minerals including columbite and tantalite. In contrast, the biotite–magnetite pegmatites have biotite and magnetite as their main mafic components. The rare-metal pegmatites are also distinguished by their bulk-rock and mineral chemistry, including a more peraluminous character and enrichments in Rb, Li, Cs, Be, Nb and Ta. New U–Pb ages (c. 1690–1710Ma) suggest that these rare-metal pegmatites are within the age range of nearby biotite–magnetite pegmatites, indicating that similar genetic processes could have been responsible for their formation.The peraluminous nature of the rare-metal pegmatites strongly points towards a metasedimentary source. Notably, within the Lewisian Gneiss Complex, such pegmatites are only found in areas where a metasedimentary source is available. The evidence thus points towards all the Laxfordian pegmatites being formed by a process of crustal anatexis, with the formation of rare-metal pegmatites being largely controlled by source composition rather than solely by genetic process. This is in keeping with previous studies that have also challenged the widely accepted model that all rare-metal pegmatites are formed by fractionation from a parental granite, and raises questions about the origin of other mineralised pegmatites worldwide.

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

Many rare metals used in a range of in high-technology and clean energy applications are derived from granitic pegmatites, but debate continues about the origin of these rocks. It is clear that som...