Partial Melting Of Metapelites Research Articles

Petrogenesis of granitic pegmatite veins: Perspectives from major element and B isotope in tourmalines, Chakabeishan, Northern Tibetan Plateau

The petrogenesis of regionally zoned granitic pegmatite veins remains debated. Because of the economic significance, we carried out a study on the Chakabeishan (CKBS) pegmatite-type Li-Be deposit, eastern North Qaidam Tectonic Belt, Northern Tibetan Plateau, by means of in-situ major element and B isotope compositions of tourmalines in the beryl-bearing and spodumene-bearing pegmatite veins. Tourmalines (Tur-Be) from the beryl-bearing pegmatite are homogeneous schorl with low Mg/(Mg + Fe), high Na/(Na + Ca) and YAl, suggesting that they are of magmatic origin. Two generations of tourmalines (Tur-Li) from the spodumene-bearing pegmatite are identified: (i) the crystal cores (mostly elbaite and Li-rich schorl with subordinate schorl) are consistent with being of magmatic origin crystallized at the magmatic stage; (ii) the crystal rims (schorl) are best understood as the overgrowth at the later hydrothermal stage. Tur-Be and Tur-Li show an obvious difference in core-to-rim B isotopic variation trend with δ11B decrease in Tur-Be and increase in Tur-Li. The core-to-rim δ11B decrease in Tur-Be results from degassing during its host pegmatitic melt evolution, whereas the core-to-rim δ11B increase in Tur-Li is related to fluid exsolution. The estimated δ11B values for the initial melts of the beryl-bearing and spodumene-bearing pegmatites are −10.46‰ and −10.78‰, respectively, indicating that they most likely originate from protracted fractional crystallization/differentiation of granitic intrusions rather than partial melting of metapelite. Both Mg/(Mg + Fe) ratios and Li abundances in the cores of Tur-Be are lower than those of Tur-Li, suggesting that Tur-Li crystallizes from chemically more evolved melts.

Paleo-Mesoproterozoic magmatic-sedimentary events in eastern Bainaimiao micro-block: Implications for Precambrian tectonic evolution of a fragment from Columbia supercontinent

Identifying tectonic positions and origins of Precambrian micro-blocks within the Central Asian Orogenic Belt (CAOB) is essential for revealing global paleogeography and evolution of the early Earth. The Bainaimiao micro-block in the southeastern CAOB preserves multistage geologic records which can better reveal Precambrian tectonic history of micro-blocks within the Phanerozoic accretionary orogenic belt. This study presents integrated research of geochronology, geochemistry, and zircon Hf isotopes on the Chenjiatun metamorphic complex in the eastern Bainaimiao micro-block, which consists of meta-sedimentary rocks (plagioclase gneisses and mica schists), amphibolites and two-mica granites. The protoliths of amphibolites, two-mica granites and meta-sedimentary rocks were formed at 1.75 Ga, 1.38 Ga and 1.39–1.34 Ga, respectively, suggesting that they represent Paleo-Mesoproterozoic basement of the Bainaimiao micro-block. The detrital zircon age spectra of meta-sedimentary rocks show two representative age peaks at 1.80 Ga and 1.40 Ga, with zircon Hf isotopes varying from −11.11 to +10.67. Characteristic mostly positive zircon Hf isotopes and geochemical features of the Chenjiatun complex indicates that the Bainaimiao micro-block probably has tectonic affinity with the Laurentia-Baltica continent. Paleoproterozoic amphibolites with positive εHf(t) (+5.67 to +15.42) were derived from metasomatized mantle wedge mixed with sediment melts. They are characterized by arc-like geochemistry such as high Th/Yb (0.73–2.77), enrichment in LILEs and depletion in HFSEs (eg. Nb and Ta), similar to coeval arc magmatism in the Laurentia-Baltica continent, which indicate the Bainaimiao micro-block was possibly in a subduction convergent setting in the late Paleoproterozoic. By contrast, Mesoproterozoic two-mica granites, with mostly positive εHf(t) (−0.29 to +11.53), belong to typical S-type granites and could be generated from partial melting of meta-pelites with participation peritectic garnets. These two-mica granites, together with ∼1.40 Ga juvenile A-type grantioids from nearby and other Precambrian micro-blocks in the southern CAOB, exhibit relatively high alkalis and HFSEs contents, young Hf model ages (1.50–1.40 Ga) and were generated at high-temperature environment. These findings reveals that the Bainaimiao micro-block was in an extensional setting at 1.40 Ga coeval with the breakup evolution of the Columbia supercontinent.

Petrogenesis and redox state of late Mesozoic granites in the Pingmiao deposit: Implications for the W–Cu–Mo mineralization in the Dahutang district

The Pingmiao W (Cu) deposit is located in the central part of the newly-discovered giant Dahutang ore-concentrated district, South China. Field relationships and mineral assemblages confirm that the W (Cu) mineralization in the Pingmiao deposit is genetically related to multiphase late Mesozoic granites, which include the porphyritic two-mica granite, biotite granite porphyry, fine-grained muscovite granite, and Li-mica albite granite. Zircon U–Pb dating shows that these granites emplaced at ca. 146–143 Ma, which are coeval with the W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district. The strongly peraluminous (A/CNK > 1.1) feature, negative correlations between Rb and Th and between Rb and P2O5, low Zr + Nb + Ce + Y values (mostly lower than 200), and relatively low Zr saturation temperatures (lower than 800 °C) suggest that these granites are typical S-type granite. These granites show similar Nd isotopes (−8.91 to −4.61), CaO/Na2O (0.2–0.57), and Al2O3/TiO2 (mostly ranging of 50–90) ratios with those W–Cu–Mo-bearing granitoids from other ore deposits in the Dahutang district, suggesting that they derived from partial melting of metapelites in the Shuangqiaoshan Group (or the Proterozoic metamorphic basement geochemically similar to the Shuangqiaoshan Group), with involving of minor mafic–ultramafic volcanic rocks. Geochemical fractionation trends recorded by whole-rocks and micas permit to distinguish the evolutionary trend from the porphyritic two-mica granite and biotite granite porphyry to the fine-grained muscovite granite and Li-mica albite granite. The apparent rare earth element tetrad effect, high Li and F contents, low K/Rb, Zr/Hf, and Nb/Ta ratios, as well as the zoned features of micas suggest that the melt–fluid interaction occurring during the formation of Li-mica albite granite. The Pingmiao granites have reduced redox state, revealed by their low whole-rock Fe2O3/FeO ratios (mostly <0.5), biotite Fe3+/Fe2+ ratios (ranging of 0.02–0.07), zircon Ce4+/Ce3+ ratios (median value of 18.70), and oxygen fugacity (near or below the FMQ buffer). The data in this study indicate that the late Mesozoic granitoids in the Dahutang district are favorable for W, Cu (<1 Mt), and Mo (<0.3 Mt) mineralization. Notably, the more evolved Li-mica albite granite has potential for W, Sn, Nb and Ta mineralization.

Genesis of S‐type Granites in the Pengshan Sn‐polymetallic Ore Field, Northern Jiangxi Province and its Implications

AbstractThe Pengshan Sn‐polymetallic ore field is located in the southeastern part of the Yangtze block, spanning the southeast edge of the MLYDZ and the northern edge of the mid‐segment of the Jiangnan Uplift, and on one side of the MLYDZ. The studies of LA–ICP–MS zircon U–Pb chronology and petrogeochemistry for Early Cretaceous acid granites from the Pengshan ore field were carried out in this paper. We report zircon U–Pb geochronology and whole‐rock geochemistry for acid granites in the Pengshan ore field. The zircon U–Pb ages of the muscovite‐granite, biotite adamellite and granite‐porphyry are 127.6 ± 1.7 Ma, 126.9 ± 1.6 Ma and 126.6 ± 2.0 Ma, respectively. The granites in Pengshan are characterized by a high silicon content and are rich in alkali. They belong to high‐potassium, calc‐alkaline, peraluminous granite. The rocks have a relatively high Rb/Ba ratio, and the data points for muscovite‐granite and biotite adamellite all fall within the clay‐rich sources region, near the pelite‐derived end‐member, showing that the Pengshan muscovite‐granite and biotite adamellite mainly originated from the partial melting of metapelites with high maturity. The transformation of the compressional and extensional tectonics in this region approximately 128 Ma obviously lags behind that in the mid‐segment of the Jiangnan Uplift (135 Ma), but occurred earlier than the MLYDZ (126 Ma). The Pengshan ore field extends from the mid‐segment of the Jiangnan Uplift to the MLYDZ. Although the tectonic stress field is constrained by the combination of the two secondary tectonic units, the time of tectonic system transformation is closer to the MLYDZ because the spatial orientation of the area is enclosed in the MLYDZ. Relevant geophysical and drilling data confirm the rationality of Pengshan–Ao'xia as a multi‐center vertical zoning ore field, and show the scientificity of the prospecting idea of abutting joint between the north‐west of Pengshan area and the south‐east of Ao'xia area.

The Mencué Batholith: Permian episodic arc-related magmatism in the western North Patagonian Massif, Argentina

ABSTRACT The Mencué Batholith, western North Patagonian Massif, includes three major bodies. The Mencué Granodiorite, Cura Lauquén Granite and La Blancura Syenogranite. There are dikes of granitic and basaltic rocks cutting the above-cited rocks. The Mencué Batholith represents several episodes of magmatism, with ages varying between 294 and 239 Ma. The Mencué Granodiorite and the Cura Lauquén Granite are solid-state deformed and are S-type. They have high-K and normal calc-alkaline affinities. These rocks contain significant quantities of subduction-zone chemical components that decrease towards younger lithofacies. La Blancura Syenogranite lack subduction zone chemical components and represent A-type granite, typical of within-plate magmatism. The partial melting of metapelites could be the process of formation of these bodies. The older lithofacies of the Mencué Batholith, found westernmost, display a stronger deformation, but there is a progressive eastward change to younger and mildly deformed bodies and even non-deformed bodies. We conclude that the evolution of the Mencué Batholith start in the Sakmarian-Roadian (Early Permian) period. At this time, a subduction zone was active to the west and its thermal influence affected sedimentary or metamorphic rocks producing S-type granites During the Wordian and Capitanian, (Middle Permian) periods, the Mencué Batholith was mildly deformed, possibly in the process of the vanishing of the deformation and has a minor subduction chemical signature. Between the Wuchiapingian and Olenekia periods, the alkaline facies of the Mencué Batholith show an absence of deformation and the characteristics of within-plate magmatism. The Early Permian magmatic events in the western North Patagonian Massif are represented by the older bodies of the Mencué Batholith and were produced by subduction in the western margin of the Gondwanan continent. The Late Permian-Early Triassic magmatic events show a noticeable decreasing influence of subduction and an increasing influence of within-plate chemical components.

Contrasting Ba-Sr Granitoids from Bamenda Area, NW Cameroon: Sources Characteristics and Implications for the Evolution of the Pan African Fold Belt

The basement rocks of Bamenda town are mainly covered by volcanic rocks and are made up of two distinct types of granitoids: granites and leucogranites. These basement rocks belong geochemically to the same granite field. The studied granitoids are silica rich, with concentrations ranging from 70 to 73% in granite and 73 to 76% in leucogranite. Both belong to high-K calc-alcaline series with granite being magnesian and more potassic (Na2O/K2O 2O/K2O >1). Leucogranites are low Ba-Sr granitoids, strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, while granites are high Ba-Sr granitoids, slightly peraluminous and plot in the field of I-type granitoids. The chondrite-normalized REE patterns show more LREE enrichment in granites (>100*chondrite) than in leucogranites (>10*chondrite). Major and trace element compositions of the leucogranites and granites indicate crustal derivation from the partial melting of metapelite and metagreywacke respectively. The REE patterns display pronounced negative Eu anomalies (Eu/Eu* = 0.23-0.36) in leucogranites due to low degree of partial melting of a plagioclase depleted crustal source, and weakly negative to null Eu anomalies (Eu/Eu* = 0.81-1.08) in granites due to high degree of partial melting. The estimated temperatures of Bamenda granitoids magma range between 800°C and 950°C. Bamenda I-type granites are syn-tectonic and are similar to the other granitoids of the central domain of the Pan-African North-Equatorial Fold belt in Cameroon. Bamenda S-type peraluminous leucogranites are post-tectonic and chemically similar to the Himalayan peraluminous leucogranites. Furthermore, decompression model may be related to Bamenda leucogranite formation. Tectonically, the studied granitoids are related to crustal delamination characterizing the post-collisional event within the Pan African Fold Belt.

Diverse magma sources for the Himalayan leucogranites: Evidence from B-Sr-Nd isotopes

The Himalayan orogen is featured by widespread S-type leucogranites (ca. 45 to 9 Ma) formed primarily from the partial melting of metapelites (800–480 Ma), and minor from the partial melting of amphibolite with subordinate metapelites. The Ama Drime gneiss and Mabja leucogranite pluton are both located at the footwall of the NS-trending Xainza–Dinggye normal fault in the central part of the Himalayan orogen, with published εNd(t) values of −21.0 to −19.6 and − 19.3 to −18.3 and 87Sr/86Sr(t) values of 0.90954–0.92574 and 0.84853–0.85474, respectively. In this study, we presented new SHRIMP zircon U-Pb ages and laser ablation-multi-collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) tourmaline boron isotope data on the metamorphic rocks and leucogranites of the Himalayan orogen. The weighted mean 207Pb/206Pb age of the zircon cores of the Ama Drime gneiss (T0446-2-3) is 1854 ± 4 Ma. The tourmaline δ11B values of the Ama Drime gneiss (T0446-1-6) are −17.6 to −14.3‰, similar to those of the Mabja leucogranite (T0436-4, −18.9 to −17.4‰). In contrast, the tourmaline δ11B values of the Quedang metapelite (T0389-18) and its partial melting product – Malashan leucogranite (T0659-12A-3) are substantially higher, i.e., −15.3 to −12.5‰ and − 16.2 to −8.0‰, respectively. The tourmaline δ11B values of the Yadoi leucogranite (T0321-4), derived from partial melting of amphibolite, range from −8.4 to −5.4‰. Therefore, the tourmaline B-isotopes and Sr-Nd-isotopes results are consistent, and may have reflected an E-W extension along the southern Tibetan rift system, indicating a late Miocene anatexis in the North Himalaya region. The melts derived from the partial melting of metapelites and mature crustal materials (e.g., Paleoproterozoic Ama Drime gneiss) ascended along the N-S trending Xainza-Dinggye normal fault, and were subsequently emplaced in the Tethyan Himalaya sequence.

Lithium isotope behaviour during partial melting of metapelites from the Jiangnan Orogen, South China: Implications for the origin of REE tetrad effect of F-rich granite and associated rare-metal mineralization

We carried out combined Nd and Li isotopic studies on the Neoproterozoic biotite granites, late Mesozoic evolved two-mica/muscovite granites and associated the Shuangqiaoshan Group metapelites from the Jiangnan Orogen (South China), which, along with petrological and geochemical data, were used to constrain the lithium isotopic behavior during partial melting of metapelites, the origin of the REE tetrad effect in evolved granites, and implications for related rare-metal mineralization. The Neoproterozoic biotite granites show features typical of S-types (e.g., high A/CNK ratios), and have εNd = −6.9 to −10 and δ7Li = −2.2–−4‰, comparable to the Shuangqiaoshan Group metapelites (with εNd = −6 to −11 and δ7Li = −2–−3.7‰, respectively), suggesting that the biotite granites were derived from melting of the metapelites and the Li isotopic fractionation during the melting process appears negligible. The two late Mesozoic Shimensi and Zengjialong granites occur as small stocks, showing high silica, peraluminous, low CaO and Sr contents, variably enrichment in volatiles (F, B), and are accompanied by W-Cu and Sn mineralization. The Shimensi granites have εNd(t) values of −7.0–−8.6 and δ7Li = −2.7‰–−3.9‰, similar to the Shuangqiaoshan Group metapelites, whereas the Zengjialong granites have relatively high εNd(t) values (−5.1–−7.3) and heavy Li isotopic features (mostly δ7Li = −0.18‰–+1.53‰), and particularly, REE tetrad effect is seen in the latter. We argue that the late Mesozoic evolved granites formed as a result of re-melting of the granulitic residues left in the lower crust after extraction of the Neoproterozoic biotite granites from the metapelites. Such re-melting of the residues was triggered by underplating of basaltic magma, with more mantle input (and F, B as well) in the source of the Zengjialong granites. The elevated δ7Li values of the Zengjialong granites were probably caused by intense interaction between melt and external fluids during magma emplacement, which, in turn, resulted in the formation of the REE tetrad effect in the granites. Isotopic modeling suggests that fluid exsolution from evolved granites is minor, and is less likely to have been responsible for the extensive rare-metal mineralization as traditionally thought.

In-situ elemental and isotopic compositions of apatite and zircon from the Shuikoushan and Xihuashan granitic plutons: Implication for Jurassic granitoid-related Cu-Pb-Zn and W mineralization in the Nanling Range, South China

The Nanling Range, South China, was world-famous for hosting abundant granitoid-related copper-lead-zinc (Cu-Pb-Zn) polymetallic ore deposits and quartz vein-type wolframite (W) ore deposits, both of which have roughly similar formation ages predominately between 150 and 160 Ma with a peak of ca. 156 Ma. In this study, accessory minerals from typical Cu-Pb-Zn-bearing and W-bearing granitic rocks are compared in terms of elemental and isotopic compositions. Apatite from W-bearing granites in the Xihuashan pluton has lower εNd(t) value (−11.9 to −8.6) than that from Cu-Pb-Zn-bearing granitic rocks in Shuikoushan with εNd(t) value of −8.7 to −4.2 and initial 87Sr/86Sr ratio of 0.7097–0.7109. Zircon grains in Xihuashan, yielded εHf(t) value from −14.9 to −11.4, δ18O value from 8.6‰ to 10.4‰, and highly variable and negative δ7Li value (−45.8‰ to −3.8‰), whereas those in Shuikoushan have relatively higher εHf(t) value (−10.6 to −8.1), lower δ18O value (8.4‰–9.7‰), and highly variable δ7Li values from −12.7‰ to +17.6‰. In situ Hf-O-Li isotopic compositions of zircon and Sr-Nd isotopes of apatite, suggesting that the Shuikoushan granitic pluton was likely generated from dehydration melting of amphibolite from a metal-fertile mafic source in the middle-to-lower crust, whereas the Xihuashan granitic pluton could be derived from partial melting of metapelite with minor amphibolite in the middle to upper crust. The geochemical records in accessory minerals fingerprint that the Shuikoushan granitic magma was characterized by high Cl content (0.11–1.44 wt%) and logfo2 value (>ΔFMQ+1), whereas the Xihuashan granitic magma have elevated F (3.51–4.80 wt%) and Li (3.49–42.4 ppm) contents with low logfo2 value (<FMQ+0), which suggested moderately oxidized magmas with high Cl contents are in favor of the formation of magmatic-hydrothermal Cu-Pb-Zn deposits, whereas weakly reduced magmas with high F and Li contents could be involved in the formation of the quartz vein-type wolframite deposits. In conclusion, different source rocks and magmatic evolution processes are the key to the understanding of the Jurassic diverse granitoid-related mineralization in the Nanling Range.

The effects of lateral density gradients, slopes and buoyancy on channel flow: 1D analytical solutions and applications to the SE Canadian Cordillera

We present 1D analytical solutions for channel flow in orogens driven by various types of pressure gradients. Our calculations demonstrate that lateral density gradients in the upper crust, such as would occur across a suture zone separating arc rocks from pericratonic sediments provide a driving force for Poiseuille flow as large as topographic gradients observed in modern mountain belts. For cases for which the gradients are external (topographic and lateral density gradients) and internal (e.g. partial melting of channel material) to the channel, inclination decreases and increases the Poiseuille component of the average flow-velocity within the channel by the cosine and sine of the slope, respectively. The magnitude of the pressure gradient consequent upon the buoyancy generated by partial melting of metapelites in a channel with a 30° slope, such as would occur above an underthrusting basement ramp, is similar to that of topographic or lateral density gradients. Channel flow up a ramp could thus constitute an important exhumation mechanism in large hot orogens. Our calculations indicate that mid-crustal channel flow was a highly likely process in the Late Cretaceous-Paleocene setting of the southeastern Canadian Cordillera. The flow was first driven by the lateral density contrast between pericratonic sediments and the arc-related Intermontane terrane, then by combined effect of topographic gradient and melt-induced buoyancy of the Lower Selkirk Allochthon (part of the Shuswap Complex). Flow up the underthrusting basement ramp resulted in exhumation from mid- to upper-crustal levels. Channel flow then migrated downward to involve basement and overlying cover sequence rocks. Our results indicate that syn-convergent channel flow was a viable and very likely process in the southeastern Canadian Cordillera.

Behaviour of apatite during partial melting of metapelites and consequences for prograde suprasolidus monazite growth

The suprasolidus behaviour of apatite and monazite is examined for an average metapelite composition using phase equilibria modelling coupled with solubility equations of these minerals. Both closed- and open-system scenarios are considered. Partial melting above the solidus requires apatite and monazite breakdown in order to saturate the anatectic melt in phosphorus and the light rare earth elements. In general, melt loss is predicted to increase the stability of apatite and monazite at high temperature. Most apatite is predicted to survive up to ultrahigh temperature conditions except for rocks with low bulk phosphorus concentrations. By contrast, most monazite is expected to be consumed by UHT conditions. Thorium substitution in monazite is expected to increase the stability of monazite to higher temperatures. The presence of LREE-rich apatite decreases the stability of monazite above the solidus, but the breakdown of this apatite during anatexis may generate prograde monazite at the apatite–melt interface in local pockets of melt oversaturation. However, prograde suprasolidus monazite along grain boundaries is expected to be consumed during further partial melting or during melt homogenization when an interconnected melt network develops. Anatectic melts are predicted to be saturated with respect to apatite except at UHT conditions and for rocks with low initial P2O5 bulk concentrations.

Geochemistry and Nd - isotopic composition of high silica rhyolites in the neoarchaean Musoma-Mara greenstone belt, northern Tanzania: evidence for the presence of older continental crust

The high silica rhyolites of the Kibasuka Hill, although volumetrically minor, constitute a part of the greenstone sequence in the northern Musoma-Mara Greenstone Belt (MMGB). They are characterized by high silica contents (63.38 – 81.65 wt %), very low CaO (! 0.05 wt %),variable to unusually high K2O (2.24 – 13.43 wt %) most likely attributed to strong sericite alteration, and their major element composition are similar to those of melts derived from partial melting of metapelites. The samples have very low concentration of the transition elements, Sr (3.13 – 48.4 ppm) and very large negative Europium anomalies (Eu/Eu* = 0.27 –0.48). Their Nd isotopic composition reveals that the T-depleted mantle (DM) model ages of the samples are highly variable and range from 2867 Ma to 4015 Ma. These geochemical features are consistent with generation of the high silica rhyolites by partial melting of a heterogeneous source composed of juvenile Archaean rocks of the MMGB and older sedimentary rocks. The later imparts to the rocks very old mean crustal residence ages. Partial melting of this suite took place at low-pressure conditions within the continental crust where plagioclase was a stable phase. Such old TDM model ages of up to 4015 Ma recorded by the high silica rhyolites suggest the presence of very old continental crust in the MMGB which has so far not been identified anywhere else in the Tanzania Craton.

Partial melting of metapelites at ultrahigh‐pressure conditions, Greenland Caledonides

AbstractEclogite, orthogneiss and, by association, metapelite from an island at 78°N in North‐East Greenland experienced ultrahigh‐pressure (UHP) metamorphism at approximately 970 °C and 3.6 GPa, at the end of the Caledonian collision, 360–350 Ma. Hydrous metapelites contain abundant leucocratic layers and lenses composed of medium‐grained, anhedral, equigranular quartz, antiperthitic plagioclase and K‐feldspar with minor small garnet and kyanite crystals. Leucosomes are generally parallel to the matrix foliation, are interlayered with residual quartz bands, anastomose around residual garnet and commonly cross‐cut micaceous segregations. Textures suggest that the leucosomes crystallized from a syntectonic melt, but crystallized at the end of local high‐grade deformation. The metapelite outcrop is &lt; 1.5 km from kyanite eclogites with confirmed coesite, but the metapelites lack coesite and palisade textures diagnostic of coesite pseudomorphs. They do contain highly fractured garnet megacrysts with polycrystalline quartz inclusions (some surrounded by radial fractures) and Ti‐rich phengite inclusions that suggest the former presence of coesite. Polyphase inclusions in garnet contain reactants and products of the inferred dehydration melting reaction: Phe + Qtz = Ky + Kfs + Rt + melt. The reactants are thought to have been early inclusions of hydrous phases within garnet that melted and then crystallized new phases. Garnet surrounding these inclusions has patchy zoning with elevated Ca, consistent with experiments that produced similar patchy microstructures in garnet around inclusions with an unequivocal melt origin. The peak UHP metamorphic assemblage in these rocks is inferred to have been phengite, coesite, garnet, kyanite, rutile, fluid ± omphacite ± epidote. Phase diagrams indicate that dehydration melting of phengite in this assemblage would have occurred after decompression from peak pressure, but still above the coesite to quartz transition. Unusual crown‐ and moat‐like textures in garnet around some polycrystalline quartz inclusions are also consistent with the inference that melting took place at UHP conditions.

Partial melting of metapelites in the Gagnon terrane below the high-pressure belt in the Manicouagan area (Grenville Province): pressure–temperature (P–T) and U–Pb age constraints and implications

Metapelites of the parautochthonous Gagnon terrane at the footwall of the high-pressure (high-P) belt in the Manicouagan area (central Grenville Province) preserve an impressive textural record of partial melting reactions, mainly in polymineralic inclusions within garnet. The dominant textures were developed within the pressure–temperature (P–T) field of the continuous dehydration melting reaction biotite + kyanite (or sillimanite) + plagioclase + quartz = garnet + K-feldspar + melt, with sillimanite instead of kyanite in the southern part of the footwall. Inferred P–T paths have a hair-pin form in the range of 750–850 °C and 1000–1500 MPa for the kyanite-bearing rocks. Monazite crystallization ages are consistently late Grenvillian, either 995 or 985 Ma, and one sample contains monazite of both ages. Two of these samples also contain inherited monazite with ages of 1738 ± 5 and 1719 ± 30 Ma, indicative of an earlier metamorphic event. Tonalite and diorite from the same area yield Archean zircon crystallization ages and titanite ages of 961 ± 3 and 956 ± 4 Ma, the youngest in the Manicouagan region. The late Grenvillian metamorphism was of comparable intensity but ~50 Ma younger than in the overlying high-P belt in this area and therefore seems to be unrelated to the emplacement of the latter over the Parautochthonous Belt, as previously suggested. Rather, this younger metamorphism suggests a reactivation of the footwall by underthrusting of the Gagnon terrane during the waning stages of convergence, and a link with a major coeval post-tectonic thermal event farther south in the hinterland.

Petrogenesis of the Puerto Edén Igneous and Metamorphic Complex, Magallanes, Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial 87Sr/ 86Sr ratios of up to 0.7134 and εNd 150 values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial 87Sr/ 86Sr ratios between 0.7073 and 0.7089, and εNd 150 values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial 87Sr/ 86Sr ratios of 0.7048 and 0.7061, and εNd 100 values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.

The Pan-African high-K calc-alkaline peraluminous Elat granite from southern Israel: geology, geochemistry and petrogenesis

Calc-alkaline leucocratic granites that were emplaced at the late post-collision stage of the Pan-African orogeny are abundant in the northern half of the Arabian-Nubian Shield. Commonly, they are referred to as the Younger Granite II suite. In southern Israel such rocks are known as Elat granite. Studies of these rocks enable to recognize two types of granites: coarse-grained, massive Elat granite (EG), and fine- to medium-grained Shahmon gneissic granite (SGG). Both granite types are high-K and peraluminous ( ASI ranges from 1.03 to 1.16). They are similar in modal composition, mineral and whole-rock chemistry. Within the EG, a noticeable distinction in whole-rock chemistry and mineral composition is observed between rocks making up different plutons. In particular, the granite of Wadi Shelomo, as compared to the Rehavam pluton, is enriched in SiO 2, FeO∗, K 2O, Ba, Zr, Th, LREE and impoverished in MgO, Na 2O, Sr, and HREE. The Eu/Eu∗ values in the granite are low, up to 0.44. Mass-balance calculations suggest that chemical and mineralogical variations were caused by fractionation of ∼16 wt.% plagioclase from the parental Rehavam granite magma at temperature of 760–800 °C (muscovite-biotite geothermometer). The Rb-Sr isochrons yielded a date of 623 ± 24 Ma for the EG, although high value of age-error does not allow to constrain time of emplacement properly. The Rb-Sr date for SGG is 640 ± 9 Ma; however, it is likely that this date points to the time of metamorphism. A survey of the literature shows that peraluminous, high-K granites, similar to the EG, are abundant among the Younger Granite II plutons in the Sinai Peninsula and Eastern Desert, Egypt. They were emplaced at the end of the batholithic (late post-collision) stage. The most appropriate model for the generation of the peraluminous granitic magma is partial melting of metapelite and metagreywacke.

Exhumation of high-pressure rocks of the Kokchetav massif: facts and models

The exhumation of ultrahigh-pressure (UHP) metamorphic units from depths more than 100–120 km is one of the most intriguing questions in modern petrology and geodynamics. We use the diamondiferous Kumdy-Kol domain in the Kokchetav Massif to show that exhumation models should take into consideration initially high uplift velocities (from 20 down to 6 cm/year) and the absence of the deformation of UHP assemblages. The high rate of exhumation are indicated by ion microprobe (SHRIMP) dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds. Diamondiferous rocks in the Kumdy-Kol domain occur as steeply dipping (60°–80°) thin slices (few hundred metres) within granite–gneiss. Using geological, petrological and isotopic–geochemical data, we show that partial melting of diamondiferous metamorphic rocks occurred; a very important factor which has not been taken into account in previous models. Deformation of diamondiferous rocks at Kumdy-Kol is insignificant; diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation. All these facts could be explained by partial melting of metapelites and granitic rocks in the Kumdy-Kol domain. The presence of melt is responsible for an essential reduction of viscosity and a density difference (Δ ρ) between crustal rocks and mantle material and reduced friction between the upwelling crustal block, the subducting and overriding plates. Besides Δ ρ, the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle. We construct different models for the three stages of exhumation: a model similar to “corner flow” for the first superfast exhumation stage, an intermediate stage of extension (most important from structural point of view) and a very low rate of exhumation in final diapir+erosional uplift.

Water as a Petrologic Catalyst Driving 18O/16O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ18O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ18O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable 18O/16O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of 18O/16O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ18O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in 18O/l6O must be a result of exchange with magmatic or metamorphic H2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ18O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system. LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H2O, aided by internal muscovite dehydration at ≈8 kbar and ≈750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (≈30°) partial melting of the metapelites produced magmas near H2O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H2O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H2O. The released H2O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H2O was released and again re-used for a final episode of 18O/l6O exchange with the unmelted metamorphic lithologies. In addition to the direct l8O/16O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H2O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.

Trace element distribution among rock-forming minerals in Black Hills migmatites, South Dakota; a case for solid-state equilibrium

Proterozoic migmatites in the Black Hills, South Dakota, were examined to determine if partitioning of trace elements hosted by rock-forming minerals between melanosomes and leucosomes preserves residue-melt equilibrium that was presumably established during partial melting of pelitic lithologies. Granitic leucosomes in the Black Hills, as is often the case elsewhere, have positive Eu anomalies, are highly enriched in Sr and Ba, and are depleted in Rb and Cs relative to granites derived by partial melting of metapelites. Distributions of these trace elements between melanosomes and leucosomes cannot be simulated using mineral-melt distribution coefficients. A metamorphic reaction-progress method is used to demonstrate that the distributions approach mineral-mineral equilibrium rather than mineral-melt equilibrium. Application of published diffusion data for the relevant elements in feldspars shows that solid-state equilibrium is unlikely to have been established during subsolidus cooling. Instead, it is suggested that partial melts maintained chemical equilibrium with melanosomes during crystallization that lead to the migmatites.

Behavior of accessory phases and redistribution of Zr, REE, Y, Th, and U during metamorphism and partial melting of metapelites in the lower crust: an example from the Kinzigite Formation of Ivrea-Verbano, NW Italy

This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th, and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5–6 to 14–15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ≈ 2.5–3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust.