Tourmaline Leucogranites Research Articles

From source to emplacement: The origin of leucogranites from the Sikkim-Darjeeling Himalayas, India

Himalayan leucogranites are important for understanding the tectonic evolution of collision zones in general and the causes of crustal melting in the Himalayan orogen in particular. This paper aims to understand the melt source and emplacement age of the leucogranites from Sikkim in order to decipher the deep geodynamic processes of the eastern Himalayas. Zircon U-Pb analysis of the Higher Himalayan Sequence (HHS) metamorphic core reveals a prolonged period of crustal melting between > 33 Ma and ca. 14 Ma. Major and trace element abundances are presented for 27 leucogranites from North Sikkim that are classified into two-mica and tourmaline leucogranite types. They are peraluminous in composition, characterized by high SiO2 (70.91–74.9 wt.%), Al2O3 (13.69–15.82 wt.%), and low MgO (0.13–0.74 wt.%). Elemental abundances suggest that Sikkim Himalayan leucogranites are derived from crustal melts. The two-mica leucogranites are derived from a metagreywacke source, whereas the tourmaline leucogranites are sourced from metapelitic sources, with inherited zircons indicating an HHS origin for both types. U-Pb zircon geochronology of the two mica leucogranites indicates ages of ca. 19–15 Ma, consistent with crustal melting recorded in HHS gneisses from Darjeeling. Monazites from both the two-mica and tourmaline leucogranites yield a crystallization age of ca. 15–14 Ma, coeval with movement on the Main Central Thrust and South Tibetan Detachment System which further provides constraints on the timing and mechanism of petrogenesis of leucogranites in the Sikkim Himalayas.

Metasomatic origin of the Himalayan banded tourmaline leucogranite

Metasomatic origin of the Himalayan banded tourmaline leucogranite

Petrogenesis of monzonites and leucogranites in the Ama Drime Massif: Implications for sources and differentiation of Himalayan leucogranites

Determining the sources and the evolution of the Himalayan leucogranites is crucial to deciphering the teotono-magmatic evolution history of the Himalayan Orogen. Various magmatic rocks, including the monzonites, two-mica leucogranites, and tourmaline leucogranites, are exposed as dykes in the Paleoproterozoic orthogneisses of the Ama Drime Massif (ADM), providing new insights into the Himalayan magmatism. This contribution presents systematic petrological, geochronological, and geochemical studies of these magmatic rocks from the ADM to constrain their emplacement timing, protolith, and petrogenesis. Geochronological results reveal two episodes of magmatism in the ADM, represented by the 13.5 Ma monzonites and the 10 Ma two-mica and tourmaline leucogranites. Despite having different ages, the two episodes of magmatism in the ADM both originated from the melting of the Paleoproterozoic orthogneisses. However, various magmatic rocks underwent different differentiation processes during emplacement. Based on mineral assemblages and geochemical features, the monzonites were formed by the accumulation of plagioclase, biotite, zircon, and monazite. For the leucogranites, the systematic variations in mineral assemblages, whole-rock geochemistry, and mineralogical chemistry demonstrate that the two-mica leucogranites gradually evolved into the tourmaline leucogranites by the separation of plagioclase, biotite, zircon, and monazite. Combining this contribution and previous studies, the Himalayan leucogranites might evolve from the peraluminous melts produced by the melting of Himalayan anatectic rocks according to fractional crystallization, with the formation of complementary cumulate rocks. The involvement of orthogneisses in the source suggests that the Himalayan leucogranites can not be simply classified into S-type granites.

Significant boron isotopic fractionation in the magmatic evolution of Himalayan leucogranite recorded in multiple generations of tourmaline

The late-stage evolution of leucogranite magmas and related magmatic-hydrothermal processes which could lead to rare-metal mineralization are still poorly studied in the Himalayas. Since tourmaline is among the minerals that are present in evolved granites of the Himalayas, the geochemistry of this mineral, and in particular the boron isotopic composition, is a powerful tool to trace evolution processes and sources of these magmas. This study focuses on the analysis of tourmalines in the Gurla Mandhata tourmaline leucogranites (GMTL) from the northwest of the Himalayan orogen. Two generations of tourmaline show clear differences in texture, major element, and B isotopic compositions. Early-stage tourmalines have high-Mg# [Mg/(Mg + Fe) 0.39–0.45] and occur as inclusions in late-stage tourmalines [Mg# <0.34] and other minerals. The δ11B values are in the range of −7 ~ −8‰ for the early-stage tourmaline and in the range of −12 ~ −15‰ for the late-stage tourmaline. The occurrence of tourmalines with contrasting B isotopic compositions from the same leucogranite body (even in a same specimen) can be explained either by mixing of magmas from different sources with different B isotopic compositions or by remarkable B isotopic fractionation during magma differentiation. As indicated by mineral textures and compositions, the mixing of different magma batches can be ruled out as a major cause in the case of GMTL. Alternatively, the bimodal distribution of tourmaline δ11B most likely suggests the occurrence of multiple-stage B isotopic fractionation during the magma evolution. Our modeling based on Rayleigh fractionation shows that this fractionation may have been induced mainly by extraction of fluid at a late magmatic stage, but crystallization of tourmaline may have also played a minor role. Interestingly, a compilation of δ11B values (>260 data points) in magmatic tourmaline from eight Himalayan leucogranite bodies systematically shows a similar bimodal distribution (i.e., with peaks at −7‰ and − 13‰), implying that this significant B isotopic fractionation may be a common scenario during magma evolution of the Himalayan leucogranites. However, multiple magma sources with contrasting B isotopic compositions cannot be fully ruled out for explaining the bimodal distribution of B isotopic composition. Combining our results with SrNd isotopic and other geochemical characteristics of the Himalayan leucogranites, we propose that the granitic magmas with high δ11B originated either from fluid-present melting involving boron-rich fluids, or more probably from dehydration melting of the Greater Himalayan Sequence that had been metasomatized by boron-rich fluids. The geological context implies that these fluids were probably derived from dehydration of mica-rich rocks from the Lesser Himalayan Sequence underlying the Greater Himalayan Sequence.

Petrogenesis of deformed tourmaline leucogranite in the Gurla Mandhata metamorphic core complex, Southwestern Tibet

The Himalayan leucogranites, which were derived from crustal anatexis during the India-Asia continental collision, have recorded the tectono-magmatic evolution process of the Himalayan orogenic belt. In the Gurla Mandhata area of the western Himalaya, tourmaline leucogranites accompanied by variable deformation are observed along the Gurla Mandhata detachment fault. Whole-rock compositions show that these leucogranites have high SiO2, alkaline elements and low CaO, MgO, FeO, TiO2 contents, and are strongly peraluminous with A/CNK value of 1.13–1.27. For trace elements, these leucogranites are characterized by low Ba Sr, Nb, Nd concentrations and relatively high Rb, U concentrations with significant negative Eu anomalies. Zircon UPb dating on the tourmaline leucogranites reveals a crystallization age of 11–12 Ma. Field observations, mineral assemblage, and geochemical features indicate that the Gurla Mandhata tourmaline leucogranites experienced high-degree fractional crystallization, which mainly induced by the long-distance migration from generation to emplacement, and the enrichment of volatile component (B, F, Cl, and H2O) facilitates the transportation ability of the leucogranite magma. Two types of tourmaline, Tur I and Tur II, are identified in the tourmaline leucogranites, enclosing relationships and mineral compositions suggest that these tourmalines should have formed at late and early stage in terms of magma evolution, respectively. The occurrence of early-stage tourmaline suggests that the primitive magma of the Gurla Mandhata tourmaline leucogranites is enriched in boron and H2O, which is most likely derived from fluid-flux melting of metasedimentary rocks from the Greater Himalayan Crystalline. The existence of the inherited zircon with ages of Paleoproterozoic and Neoproterozoic in combination with previous whole-rock SrNd isotopic study implies that the source region of the Gurla Mandhata tourmaline leucogranites is a two-component mixture between the Greater Himalayan Crystalline and the Lesser Himalayan Sequence.

Neoproterozoic Nb-Ta-W-Sn bearing tourmaline leucogranite in the western part of Jiangnan Orogen: Implications for episodic mineralization in South China

There are four episodes (Neoproterozoic, early Paleozoic, early and late Mesozoic) of Sn-W-Nb-Ta mineralization in South China, accounting for more than 50% W and 20% Sn reserves of the world and hosting considerable Nb and Ta resources. The genesis for such large-scale mineralization that may have seen multiple phases of recycling of ore elements is not well unterstood. The study of the earliest mineralized granites in South China, however, may be helpful to better understand the mechanism of mineralization. We present detailed mineralogical, geochemical, and isotopic data from two typical Neoproterozoic, mineralized intrusions, i.e., the Fanjingshan and Yuanbaoshan tourmaline leucogranites. There is a characteritstic mineral succession of diverse magmatic (columbite group minerals [CGMs], Nb-Ta-bearing cassiterite) and hydrothermal (wolframite, wolframoixiolite, Nb-Ta-poor cassiterite) Sn-W-Nb-Ta oxides. Hydrothermal cassiterite and W-bearing oxides (wolframite, wolframoixiolite) in the Yuanbaoshan intrusion were deposited separately by two types, i.e., Sn-rich and Nb-W-rich fluids. In-situ U-Pb dating of zircon, columbite and wolframoixiolite yields ages of 830 ± 5, 832 ± 5 and 821 ± 5 Ma, respectively. Whole-rock εNd(t) values of −2.5 to −6.4 correspond to crustal Nd model ages of 1.6–1.9 Ga. Paleoproterozoic model ages (~1.6–2.2 Ga) for most episodes of W-Nb-Ta mineralization in South China indicate that (i) Paleoproterozoic or older sediments may have been involved in the formation of W-Nb-Ta bearing granites and (ii) contributions of juvenile material was not important for these W-Nb-Ta mineralization.Neoproterozoic peraluminous granites along the Jiangnan Orogen formed in post-collision environments, but only those in the western part show Sn (-W-Nb-Ta) mineralization. The mineralized Neoproterozoic granites of the western part have experienced more extensive fractional crystallization (lower Nb/Ta, Zr/Hf ratios and more depleted in Fe, Mg, Ca, Sr, Ba, Eu, Ti) and are richer in volatiles (H2O, B) than the barren peraluminous intrusions of the eastern part. As similarly evolved Mesozoic metaluminous granites of the Nanling area have considerably more important tin mineralization than the mineralized Neoproterozoic granites, fractionation is not the only factor controlling mineralization. Instead, other factors, such as protolith and conditions of partial melting may account for the more important Mesozoic tin mineralization.

Genesis of early cretaceous leucogranites in the Central Sanandaj–Sirjan zone, Iran: Reworking of Neoproterozoic metasedimentary rocks in an active continental margin

New U–Pb ages and geochemical and isotopic data from leucogranites within the central Sanandaj–Sirjan zone (SSZ) of the Zagros orogen in Iran enable an evaluation of a migrating magmatic system in response to subduction of Neo-Tethyan lithosphere. The Esfajerd leucogranites, from the Golpaygan region of the Muteh–Golpaygan metamorphic complex, are composed mainly of tourmaline and two-mica leucogranites and are characterized by strongly peraluminous compositions. SIMS U–Pb zircon ages indicate that these rocks were emplaced over a 7 Myr period between 138 and 131 Ma. In multi-element variation diagrams, all samples display pronounced negative Eu anomalies, enrichment in light rare-earth elements and large-ion lithophile elements, and depletion in high-field-strength elements, Sr, and Ba. The tourmaline and two-mica leucogranites record positive zircon εHf values of 1.7–5.6 and 2.3–8.9, respectively, suggesting a predominantly Neoproterozoic crustal source. Zircon Hf–O isotopic compositions, together with the whole-rock geochemical data, show that both leucogranite types were produced by muscovite dehydration melting of metasedimentary sources under fluid-absent conditions, followed by fractional crystallization of the parental melt. The relatively high δ18O values (8.68–10.32‰) of the tourmaline leucogranites are consistent with their derivation from a psammite-dominated source, such as metagraywackes, whereas the wider range and higher maximum of the δ18O values (6.99–12.32‰) of the two-mica leucogranites correspond to mixed pelitic–psammitic sources, with little or no input of mantle-derived material. We suggest that the leucogranites were formed by melting of metasedimentary rocks during a stage of flat-slab subduction initiated by plate reorganization.

Extreme Mg and Zn isotope fractionation recorded in the Himalayan leucogranites

High-silica granites (typically > 70 wt.% SiO2) represent the products of extreme crustal differentiation, but whether their distinctive chemical compositions reflect source lithology or are produced via magmatic differentiation is commonly difficult to discriminate. To provide new insights into this issue, here we present high-precision Mg and Zn isotope data for high-silica, peraluminous leucogranites from the Himalayan orogen. The samples are subdivided into two-mica leucogranite, tourmaline leucogranite and garnet-bearing leucogranite based on their mineral assemblages. The two-mica leucogranites, representing the least evolved Himalayan leucogranites, have similar δ66ZnJMC 3-0749L (mean = 0.31 ± 0.06‰) but heavier δ26MgDSM-3 values (mean = −0.01 ± 0.12‰) relative to more mafic igneous rocks. This indicates that they were formed by anatexis of weathered silicates, consistent with the well-acknowledged metasedimentary source of the Himalayan leucogranites. In contrast, the more evolved tourmaline leucogranites and garnet-bearing leucogranites have lower δ26Mg and higher δ66Zn values compared with the two-mica leucogranites. The extremely low δ26Mg (−1.32‰ to −0.54‰) and high δ66Zn values (0.35–0.69‰) of garnet-bearing leucogranites vary systematically with indices of granitic differentiation (e.g., Zr/Hf, K/Rb, Eu/Eu*, 1/TiO2). Although exsolution of chlorine-rich fluid may result in elevated δ66Zn values, it is unlikely to explain the low δ26Mg signatures of the same samples. Analysis of major Mg-bearing minerals suggests that substantial segregation of tourmaline and/or Fe-Ti oxide could have driven the differentiated leucogranites towards very low δ26Mg values. In this regard, the slightly lower δ26Mg values (mean = −0.17 ± 0.06‰) of tourmaline leucogranites relative to the two-mica leucogranites may also reflect that the former were more differentiated than the latter. Thus, although source heterogeneity may be responsible for the Mg isotopic variations observed in some high-silica granites, our study implies that high-silica granites could be remarkably heterogeneous in terms of Mg isotopes primarily as a result of prolonged fractional crystallization at the late stage of melt evolution. The anomalously light Mg and heavy Zn isotopic signatures of garnet-bearing leucogranites highlight that Mg and Zn isotopes may be treated as important makers of highly fractionated granites in future studies.

Syn-metamorphic B-bearing fluid infiltrations deduced from tourmaline in the Main Central Thrust zone, Eastern Nepal Himalayas

Mode of occurrence and chemical composition of tourmaline in pelitic schists from the Main Central Thrust (MCT) zone of the Lesser Himalayan Sequences (LHS) are described in detail with the aim of deducing the chemical characteristics of tourmaline formed through B-bearing fluid infiltration and of estimating the composition of the syn-metamorphic fluids. Metasomatic tourmalines from the tourmalinized wall rocks show significant increases in XCa [=Ca/(Ca + Na)] at almost constant XMg [=Mg/(Mg + Fe2+)] from the cores or mantles to the rims. Tourmaline in tourmaline-rich (> 1.0 vol%) pelitic schists from the biotite zone to the kyanite zone also show marked increase in XCa at almost constant XMg, and are interpreted as a product of B-bearing fluid infiltration. Abundant margarite and anorthite formed in the pelitic schists intercalated with the metadolostone layers suggesting that the B-rich fluid became Ca enriched as it interacted with metadolostone layers, and metasomatically introduced Ca into the pelitic schists. Infiltration of such B- and Ca-rich fluids into pelitic schists likely resulted in production of abundant tourmaline with the compositional trend of increasing XCa at almost constant XMg. Most of the tourmaline in tourmaline-rich pelitic schists are in equilibrium with plagioclase, suggesting that the fluid composition was buffered by the pelitic schists. Composition of tourmaline in the pelitic schists with <1 vol% tourmaline mostly show increase in XCa and wider range of XMg values, reflecting its growth during prograde metamorphism. The composition of fluids that coexisted with tourmaline is estimated by applying experimentally-determined fluid/tourmaline chemical relationships to the composition of tourmaline from veins associated with tourmalinization, pelitic schists with <1 vol% tourmaline and tourmaline-rich pelitic schists. Assuming coexisting anion to be Cl, the salinity estimated for these rock types was ∼0.44–0.59 mol/l NaCl + CaCl2 that is similar to or slightly lower than the present-day seawater. Veins associated with tourmalinization and tourmaline-rich pelitic schists are sporadically distributed in the MCT zone of the LHS, suggesting that the syn-metamorphic, B-bearing saline fluid infiltrations took place widely in the MCT zone, and the fluid pathways were localized and channeled. Our observation supports the scenario whereby infiltration of fluid into the High Himalayan Crystallines (HHC) caused vapor-saturated partial melting of the HHC to give tourmaline leucogranite melts contemporaneous with inverted metamorphism in the MCT zone.

Ferrocolumbite from fractionated P-rich tourmaline leucogranite linked to the Vydra Pluton (Moldanubian Batholith, Šumava Mts., Czech Republic)

Ferrocolumbite from fractionated P-rich tourmaline leucogranite linked to the Vydra Pluton (Moldanubian Batholith, Šumava Mts., Czech Republic)

Geological, geochemical and Rb–Sr isotopic studies on tungsten mineralised Sewariya–Govindgarh granites of Delhi Fold Belt, Rajasthan, NW India

Neoproterozoic granites are widespread in the Delhi Fold Belt of the Aravalli craton, some of which are associated with tungsten mineralisation. In one such instance, the volcano-sedimentary sequence of Barotiya Group in the South Delhi Fold Belt is intruded by a pluton of biotite granite gneiss known as Sewariya Granite (SG) and later by stocks and dyke swarm of tourmaline leucogranite known as Govindgarh Granite (GG). GG magmatism was associated with wolframite mineralisation in hydrothermal quartz veins occurring along the sheared contact between SG pluton and Barotiya mica schist. SG pluton shows the evidence of ductile and brittle deformations, whereas GG is by and large undeformed. Apart from quartz and feldspars, SG contains biotite and muscovite, and GG contains muscovite, tourmaline and garnet. Although both SG and GG are peraluminous, SG has a wide range of \(\hbox {SiO}_{2}\) and narrow range of alkalis, and GG has a narrow range of \(\hbox {SiO}_{2}\) and a wide range of alkalis. REE (rare Earth elements) modelling shows that the parent magma of SG and GG was derived from partial melting at different crustal levels. Rb–Sr isotope data of GG yield a mineral isochron age of \(860\pm 7.4\,\hbox {Ma}\) which represent the time of igneous crystallisation and cooling of the granite to less than 400\({^{\circ }}\)C.

Structure of Masuleh Shear Zone: Evidence for Early–Middle Jurassic Dextral Shear Along Paleo-Tethys Suture Zone in the Western Alborz, NW Iran

The Paleo-Tethys suture zone in northern Iran was formed when the Paleo-Tethys Ocean, (between Gonwana-derived Alborz Microcontinent and the Turan Plate), closed during the Eocimmerian orogeny and after they collided together in the Mid-Late Triassic. The NW-striking Boghrov-Dagh basement Fault Zone that lies in the vicinity of Masuleh village and the southern boundary of Gasht Metamorphic Complex is a part of the Eocimmerian suture zone in the Western Alborz. Along this part of the suture zone, tourmaline leucogranites intruded in metamorphic rocks. We recognize three distinct deformation stages (D1 to D3) in the study area especially in the Masuleh Shear Zone. D1 which was synchronous with formation of the main metamorphic minerals, such as sillimanite and staurolite under medium- to high-grade metamorphic conditions probably during the Hercynian event and a NE-directed shortening. The slaty cleavage in metamorphosed Upper Paleozoic rocks and crenulation cleavage and folds in the older rocks were produced due to D2 deformation during the Eocimmerian event under greenschist facies conditions. The Masuleh Shear Zone formed as a result of a ductile strike-slip shear during the Early-Middle Jurassic Mid-Cimmerian D3 event with a pure dextral to transtension shear sense at low to locally medium-grade conditions. All of the D3 structural features agree with a NNW-directed compression and an ENE-directed extension caused by overall dextral shear parallel to the Masuleh shear zone and the Boghrov-Dagh Fault Zone. Based on the available evidence, especially cross-cutting relationships between structural fabrics and rock units, emplacement of the Gasht-Masuleh leucogranites occurred after the D2 collisional event coeval to the possible slab break-off and before the D3 event, between Eocimmerian and Mid-Cimmerian movements.

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya

Petrogenesis of the Himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of Himalayan orogen. In this article, we present petrological, geochronological and geochemical results of the Lhozag leucogranites that crop out alongside the South Tibetan Detachment System (STDS) in the east of Himalaya. Zircon U-Pb dating revealed three episodes of leucogranitic magmatism in Lhozag at 17.8±0.1Ma, 15.1±0.1Ma, and 12.0±0.1Ma, respectively. The Lhozag leucogranites show relatively low εNd(t), low zircon εHf(t) and high initial 87Sr/86Sr ratios, which are similar to the High Himalayan Crystalline Series (HHCS), indicating that they were derived from the HHCS. The characteristics of relatively high Na2O and Rb contents, high Rb/Sr ratios and low CaO, MgO, TFe2O3, TiO2, and Sr contents indicate that both the ca. 18Ma Lhozag tourmaline leucogranites and the ca. 15Ma Lhozag two-mica granites were derived from fluid-absent muscovite-dehydration melting of metasediments. The opposite geochemistry characteristics of the ca. 12Ma Khula Kangri two-mica granites imply that these granites are derived from fluid-present melting of metasediments. Four Khula Kangri two-mica granite samples with relatively lower TiO2, TFe2O3, MgO, and CaO contents, higher Rb concentrations and Rb/Sr ratios could be evolved from the Khula Kangri two-mica granites with relatively lower Rb/Sr ratios. The melting behaviors of the Lhozag leucogranites varied from fluid-absent melting to fluid-present melting, implying that there were P-T-XH2O variations in the deep crust. The tectonic evolution would give rise to variation of P-T-XH2O variation, and subsequent transformation of melting behavior. Our new results display the transformation of melting behavior of the Lhozag leucogranites, which implies the tectonic evolution from earlier N–S extension to later E–W extension in the eastern Himalaya at ca. 12Ma.

Chemical and boron isotopic compositions of tourmaline from the Nyalam leucogranites, South Tibetan Himalaya: Implication for their formation from B-rich melt to hydrothermal fluids

The Miocene Nyalam leucogranites from the South Tibetan Himalaya consist of two-mica leucogranite and tourmaline leucogranite, in which tourmaline is an important constituent in nodular tourmaline-quartz segregations. Within the two-mica leucogranite these nodules display two basic morphologies: rounded (2mg-ro type) and radial or dendritic (2mg-ra type). The nodules in the tourmaline leucogranite share similar textural features with the 2mg-ro type tourmaline nodules, but without the leucocratic halo. Tourmaline crystals in the tourmaline leucogranite display overgrowth textures — most commonly in the form of homogeneous cores (Tg-I type) surrounded by oscillatory-zoned rims (Tg-II type). Based on the petrography, chemical and boron isotopic compositions of the various types of tourmaline, it is hypothesized that the 2mg-ro type tourmaline nodules crystallized during the transition from magmatic to hydrothermal evolution of the granite, while the 2mg-ra type tourmaline nodules likely formed during syn-magmatic crystallization from boron-rich melts. In this hypothesis, the Tg-I type tourmalines also formed during the transitional stage from late magmatic to early hydrothermal process. Tg-II type tourmalines were obviously formed after the Tg-I type tourmaline, and can be attributed to the mixing of juvenile fluids and volatiles coming from the wall rocks to the nodule in a post-magmatic hydrothermal environment where Tg-I type tourmaline has already crystallized. Thus, the origin of the tourmaline nodules is related to the different stages of syn-magmatic crystallization from a B-rich melt, or the subsequent transition from magmatic to hydrothermal environment, or crystallized in the post-magmatic hydrothermal fluids.The average δ11B values of the 2mg-ra type and 2mg-ro type tourmaline are −15.1‰ and −14.4‰, respectively. The tourmalines in tourmaline leucogranite have slightly higher δ11B values, with the Tg-I type tourmalines averaging −12.5‰ and the Tg-II type tourmalines averaging −12.1‰. The boron isotopic composition of the tourmalines has implications for the origin of the leucogranites. We suggest that the distinction of the boron isotopic compositions between the source rocks of the two types of leucogranite could account for this boron isotope distinction in tourmalines.

Syn-metamorphic aqueous fluid infiltration and petrogenesis of tourmaline leucogranites in the MCT zone, Dhankuta area, Eastern Nepal

Syn-metamorphic aqueous fluid infiltration and petrogenesis of tourmaline leucogranites in the MCT zone, Dhankuta area, Eastern Nepal

Episodic crustal anatexis and the formation of Paiku composite leucogranitic pluton in the Malashan Gneiss Dome, Southern Tibet

The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite, two-mica granite and garnet-bearing leucogranite. Zircon U-Pb dating yields that (1) tourmaline leucogranite formed at 28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma; (2) two-mica granite formed at 19.8±0.5 Ma; (3) both types of leucogranite contain inherited zircon grains with an age peak at ∼480 Ma. These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions. As compared to the two-mica granites, the tourmaline ones have higher initial Sr and zircon Hf isotope compositions, indicating that they were derived from different source rocks combined with different melting reactions. Combined with available literature data, it is suggested that anatexis at ∼35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System (STDS), and led to the tectonic transition from compressive shortening to extension. Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt. Crustal anatexis at ∼28 Ma and ∼20 Ma represent large-scale melting reactions associated with the movement of the STDS.

Late Variscan magmatism in the Nevado-Filábride Complex: U-Pb geochronologic evidence for the pre-Mesozoic nature of the deepest Betic complex (SE Spain)

Orthogneisses crop out in different lithostratigraphic positions throughout the Nevado-Filábride metamorphic Complex (Betic Cordillera, southern Spain). Their abundance offers the opportunity to establish its origin and to date the ages of acid magmatism and of early deposition ages of metasedimentary host rocks of the orthogneisses, in this complex metamorphosed and deformed during the Alpine Orogeny. Petrography and bulk-rock chemistry indicate that the orthoderivates samples are peraluminous tourmaline leucogranites and strongly peraluminous biotite gneisses. Compositional similarity in different outcrops points to common genetic relationships. Although the studied gneisses are affected by metamorphic and deformational processes their compositional variations may be explained by differing degrees of crustal melting, source heterogeneity or minor feldspar fractionation. Variation in REE contents is attributed to accessory minerals fractionation. The geochemical data show that the protoliths originated by low degree of partial melting of biotite-rich, muscovite-plagioclase-bearing metasediments. U-Pb SHRIMP dating of magmatic zircons from six samples yielded ages between ~282±5 and 295±3Ma. Their ages and chemical composition indicate that these intrusive bodies strongly resemble the peraluminous, post-tectonic, leucogranites widespread in the Variscan Orogen, particularly those produced by crustal partial melting of the Central Iberian Zone. Zircons in host pelitic metasediments provide older ages ranging from ~336 to 2.050Ma. They testify to the recycling of Early Carboniferous and older granites, which are found in the Central Iberian zones of the Iberian Massif. The new geochronological data combined with detailed field observations allow drafting a new chronostratigraphy of the Nevado-Filábride Complex. Traditionally proposed Triassic ages for the metasedimentary upper series of the complex are not consistent with the ages reported here, which suggests that the entire complex consists of Paleozoic or older rocks. This conclusion has profound implications on the validity of the tectonic subdivisions proposed so far. Metamorphic zircon rims in two gneisses and a pelitic metasediment yield Miocene (17.3±0.4Ma) ages in line with what previously proposed for the high-pressure metamorphic peak in the Nevado-Filábride Complex. The new radiometric data moreover confirm independent metamorphic and tectonic evolutions for the Nevado-Filábride and the Alpujárride complexes of the Betic Cordillera.

The Himalayan leucogranites: Constraints on the nature of their crustal source region and geodynamic setting

Late Oligocene–Miocene leucogranites within southern Tibet form part of an extensive intrusive igneous province within the Himalayan orogen. The main rock types are tourmaline leucogranites (Tg) and two-mica leucogranites (2mg). They have high SiO2 (70.56–75.32wt.%), Al2O3 (13.55–15.67wt.%) and (87Sr/86Sr)i (0.724001–0.797297), and low MgO (0.02–0.46wt.%) and (143Nd/144Nd)i (0.511693–0.511906). Chondrite-normalized rare earth element (REE) patterns display strong negative Eu anomalies. Whole-rock major and trace element and Sr–Nd isotope data for the leucogranites suggest that their source region was a two-component mixture between a fluid derived from the Lesser Himalayan (LH) crustal sequence and the bulk crust of the Higher Himalayan (HH) sequence. Trace element and Sr–Nd isotope modeling indicate that the proportion of fluid derived from the LH sequence varied from 2% to 19% and the resulting metasomatised source experienced 7–16% melting. The amount of fluid derived from the LH sequence increases from north to south. Northward underthrusting of the Indian continent resulted in infiltration of the LH-derived fluid into the overlying HH sequence. Subsequent decompression melting of this metasomatised crust, mostly during the Miocene (25–9Ma), generated the leucogranites. This may be linked to steepening of the subducted slab of Indian lithosphere beneath the orogenic belt.

The significance of mafic microgranular enclaves in the petrogenesis of the Dehno Complex, Sanandaj–Sirjan belt, Iran

Quartz-monzodioritic and granodioritic enclaves occur in the Upper Cretaceous–Paleocene Dehno Complex, NW of Isfahan, Iran. They are hosted by hornblende-granodiorite and granodiorite–monzogranite, respectively. The presence of xenocrystic plagioclase, biotite, quartz, magnesio-hornblende, occasionally with relics of clinopyroxene (in quartz-monzodiorites), minor K-feldspar and the lack of peraluminous minerals dominate the mineralogy of these rocks and suggest a metaluminous source. The plagioclase in the quartz-monzodioritic enclaves has higher An-content than that of the respective plagioclase of their host (hornblende-granodiorite) indicating crystallization from more mafic magma. The quartz-monzodioritic enclaves generally contain higher values of Fe 2O 3, MgO, CaO, MnO, Na 2O, transition elements (Ni, Cr, Co, V), REE and lower values of SiO 2, K 2O, P 2O 5 and Zr than their host which are in consistent with occurrence of abundant ferromagnesian minerals in these rocks. Compare to their host, the granodioritic enclaves have higher amount of Fe 2O 3, MgO, MnO, TiO 2, P 2O 5, Rb, Zr, Y, Ni, Cr, Co and lower concentration of SiO 2, K 2O and Ba. Aside from hornblende-granodiorite and granodiorite–monzogranite, the Dehno Complex comprises leucogranodiorite and tourmaline-leuco granites. Geochemically, the rocks of the Complex are metaluminous to strongly peraluminous and are enriched in LILE (e.g. Rb, K, Sr, Ba) and depleted in HFSE (e.g. Nb, Ta, P, Ti, Zr, Hf) with medium to high-K calc-alkaline nature. These geochemical features are proposed for the volcanic arc granites emplaced in an active continental margin. Whole-rock and quartz δ 18O values for all rocks are high (9.7–14.4‰ and 10.1–14.9‰, respectively), suggesting the interaction of magma with crustal-derived components. The strongly peraluminous composition can be interpreted in terms of this hypothesis. All these mineralogical and geochemical features support the idea that the granitoid rocks of the Dehno Complex developed through variable degrees of mixing/mingling between a basic magma and a supracrustal melt during subduction of the Neotethys oceanic crust beneath Central Iranian microcontenent. The quartz-monzodioritic enclaves most likely represent the nearest composition to the original more mafic magma. The granodioritic enclaves result from the mixing/mingling of globules basic magma with host granitic magma.

Trace element composition and degree of partial melting of pelitic migmatites from the Aoyama area, Ryoke metamorphic belt, SW Japan: Implications for the source region of tourmaline leucogranites

Degree of partial melting of pelitic migmatites from the Aoyama area, Ryoke metamorphic belt, SW Japan is determined utilizing whole-rock trace element compositions. The key samples used in this study were taken from the migmatite front of this area and have interboudin partitions filled with tourmaline-bearing leucosome. These samples are almost perfectly separated into leucosome (melt) and surrounding matrix (solid). This textural feature enables an estimate of the melting degree by a simple mass-balance calculation, giving the result of 5–11 wt.% of partial melting. Similar calculations applied to the migmatite samples, which assume average migmatite compositions to be the residue solid fraction, give degree of melt extraction of 12–14 wt.% from the migmatite zone. The similarity of the estimated melting degree of 5–11 wt.% with that in other tourmaline–leucogranites, such as Harney Peak leucogranite and Himalayan leucogranites, in spite of differences in formation process implies that the production of tourmaline leucogranites is limited to low degrees of partial melting around 10 wt.%, probably controlled by the breakdown of sink minerals for boron such as muscovite and tourmaline at a relatively early stage of partial melting. Because the amount of boron originally available in the pelitic source rock is limited (on average ∼100 ppm), ∼10 wt.% of melting locally requires almost complete breakdown of boron sink mineral(s) in the source rock, in order to provide sufficient boron into the melt to saturate it in tourmaline. This, in turn, means that boron-depleted metapelite regions are important candidates for the source regions of tourmaline leucogranites.