Leucogranite Magmatism Research Articles

Thermal pulse induced by emplacement of Ramba leucogranites in southern Tibet

AbstractThe thermal histories of Himalayan leucogranites provide critical information for unravelling the post‐collisional geodynamics of the Himalayas. The Ramba Dome is located at the intersection of the Tethyan Himalayan leucogranite belt with the Yadong–Gulu Rift and hosts several generations of granitic intrusions. Of these intrusions, the 8‐Ma two‐mica granites and garnet leucogranite dykes are the youngest of Himalayan leucogranites. In this study, we focus on the carbonaceous staurolite schist located ~1.3 km from the intrusion to constrain the thermal history of the aureole that marked the cessation of leucogranite magmatism. The schist contains euhedral garnet and staurolite porphyroblasts in a foliated matrix of muscovite + biotite + chlorite + plagioclase + quartz + graphite. The staurolite shows minor compositional variations from the inclusion‐free core to the inclusion‐rich rim. By contrast, the garnet features a distinctive bell‐shaped Mn profile and increasing Mg# from the garnet core to rims. In a graphite‐bearing equilibrium phase diagram for a modified bulk composition with garnet cores removed, the garnet rim composition suggests a peak temperature of ~550°C, consistent with an independent thermometer based on the Raman spectra of carbonaceous materials (RSCM; 548 ± 9°C). The P–T condition lies within the narrow low‐variance field bracketed by the staurolite‐in and chlorite‐out boundaries, indicating minimal overstepping of staurolite nucleation and growth. On the other hand, the garnet core composition indicates 520°C at 2.5 kbar, about 40°C higher than the predicted garnet‐in boundary (~480°C). This apparent temperature overstep corresponds to a small chemical affinity (<5 kJ/mol 12 O) for garnet nucleation, comparable to previous estimates. The sharp boundaries of the high‐Ca sector zoning in the core indicate limited diffusion modification (~1.5 Ma if at the peak temperature). The short thermal pulse involves advective heat transfer by leucogranite emplacement, followed by rapid cooling toward the end of Himalayan magmatism and rapid exhumation likely facilitated by the Yadong–Gulu Rift.

Paleomagnetism of the Greater Indian passive margin sediments from the Upper Cretaceous succession: Evidence for long-delayed remagnetizations and implication for the India-Asia collision

Paleomagnetism of the ∼87 Ma Greater Indian passive margin limestones indicates two episodes of prefolding remagnetization, which occurred at ∼11°N (Group A remagnetization) and ∼27°N (Group B remagnetization) paleolatitude positions, and are likely associated with the early Eocene collision between Greater India and an intraoceanic arc subduction system and the early Miocene leucogranite magmatism in the Tethyan Himalaya, respectively. • Two prefolding remagnetization directions were isolated from upper Coniacian limestones. • One was remagnetized at ∼ 11°N during early Eocene India-intraoceanic arc collision. • The second was acquired near its present position, associated with magmatism ∼ 20 Ma. • A minimum estimate of 610 ± 622 km was yielded for the northern extent of Greater India. The Late Cretaceous and Paleogene sediments from the Tethyan Himalaya can provide paleomagnetic constraints on the pre-collision size of Greater India. However, recent studies have shown that these sediments experienced significant chemical remagnetization. To resolve the controversy over the timing of remagnetization, we performed a paleomagnetic study of the Upper Cretaceous (upper Coniacian) Gangbacunkou Formation limestones from Duela, Yadong County in Tibet. A total of 109 cores were collected from three short sections, among which 90 specimens yielded interpretable results. After removal of a scattered remanence at 200–310 °C, linear trajectories were well defined at intermediate to high temperatures up to 400 °C to 500 °C, which fall into two groups of directions. Group A directions are all reversed and inconsistent with the normal polarity of Coniacian sedimentation, indicating a late stage remagnetization; although they passed the inclination-only fold test with an average inclination of − 21.0° ± 5.1° (n = 42, k = 19.5). The in situ Group B directions are distributed around north, with shallow positive or negative inclinations, and the tilt-corrected directions pass the McElhinny fold test. The group A remagnetization event is attributed to an intraoceanic arc–continent collision at 10.9° ± 5.1°N, ∼50 Ma. Previous estimates of the size of Greater India based on data from Gamba and Duela are likely overestimated due to the influence of this collision event. The Group B remagnetization was acquired at 27.4° ± 5.6°N, close to the present latitude, and is likely related to the metamorphism caused by leucogranite intrusion, ∼20 Ma. The latter provides us a minimum estimate of 610 ± 622 km for the northern extent of Greater India.

Contrasting fertility of metapelite vs diorite gneiss to produce leucogranite melts: Example from the Mahalapye Complex, eastern Botswana

High-grade terranes showing multiple episodes of leucogranite magmatism are ideal to assess the fertility of crustal protoliths. Two episodes of Paleoproterozoic leucogranite magmatism occur in the Mahalapye Complex high-grade terrane, at the northern edge of the Kaapvaal Craton. The older one (leucogranite 1) at ∼2.05 Ga represented by leucosomes, thin veins and dykes is related to the melting of ∼2.06 Ga biotite-bearing diorite gneiss. In comparison, the younger episode (leucogranite 2) at ∼2.03 Ga forms massive bodies. Leucogranite 1 is associated with the migmatitic diorite gneiss and crosscut by the ∼2.04 Ga granodiorite. Leucogranite 2 contains xenoliths of diorite gneiss and granodiorite, as well as inclusions of quartz-poor and quartz-rich metapelites (Bt + Crd + Pl + Kfs + Spl ± Sil ± Grt + Ilm + Qz). Phase equilibria modelling of a representative quartz-poor metapelite indicate that its partial melting occurred at ∼810 °C and ∼4.8 kbar in the presence of aqueous fluid. The peraluminous, ferroan and alkalic composition of the predicted melt is comparable to the chemistry of leucogranite 2. The larger volume of anatectic melt for the estimated P-T condition corresponds to the occurrence of leucogranite 2 as massive bodies. The lower temperature of in-situ anatexis (<750 °C) at a deeper level (∼7.3 kbar) and lack of additional fluid, likely did not generate significant melt fraction during the formation of leucogranite 1. The contrast in the fertility of the two protoliths with respect to production of leucogranite melts was also related to different setting of melting. Melting of diorite gneiss is related to the local influence of mafic dykes and sills, whereas melting of metapelite is related to regional collision. It follows that high pressures, water-deficient regimes and localized heat sources produce limited volumes of partial melt, whereas high temperatures, fluid-present regimes and a large-scale heat source produces higher volumes of melts.

Geochemical evidence for partial melting of progressively varied crustal sources for leucogranites during the Oligocene–Miocene in the Himalayan orogen

Crustal anatexis in collisional orogens has great bearing on geochemical differentiation of the continental crust. However, it is often uncertain what kind of crustal rocks were partially melted for felsic magmatism at convergent plate boundaries. To address this issue, a combined study of in-situ monazite U–Th–Pb ages, in-situ monazite and allanite SmNd isotopes and whole-rock SmNd isotopes was carried out for Higher Himalayan metamorphic rocks and leucogranites in the Himalayan orogen. Metapelite, metagreywacke and granitic gneiss are the dominant constituents of the Higher Himalayan Crystallines, and they experienced upper amphibolite to granulite facies metamorphism at ca. 26–13 Ma. Although the three rock types show consistently negative εNd(t) values at t = 20 Ma, their Nd isotope compositions become less and less enriched from metapelite through metagreywacke to granitic gneiss. The metapelite has the lowest εNd(t) values of −19.9 to −15.7, the metagreywacke has intermediate εNd(t) values of −17.4 to −12.7, and the granitic gneiss has the highest εNd(t) values of −14.1 to −7.7. Leucogranitic magmatism occurred at ages from ca. 26 to 7 Ma, coeval with anatectic metamorphism of the three rock types during the Oligocene–Miocene. Relict zircon UPb age distributions, whole-rock trace element patterns, initial Nd isotope compositions and two-stage Nd model ages for the Higher Himalayan leucogranites are comparable with those for the Higher Himalayan metamorphic rocks, confirming that the metapelite, metagreywacke and granitic gneiss would have their compositional counterparts at structurally deeper positions to serve as the crustal sources of the leucogranites. This is also verified by forward phase equilibrium modelling for partial melting of the metamorphic rocks with respect to the differences in their composition and fertility. In addition, the leucogranites show a decreasing trend in εNd(t) values with their ages, indicating that the dominant crustal sources would gradually change from the least fertile granitic gneiss through the intermediately fertile metagreywacke to the most fertile metapelite during the protracted crustal anatexis from the late Oligocene to the Miocene. Therefore, the deep crust in the Himalayan orogen would consist of the metamorphic rocks with similar compositions to the shallow crust. The combined geochronological and geochemical study of accessory minerals and host rocks can provide the genetic link between the crustal sources and their melting products in collisional orogens. Nevertheless, the Nd isotope variation in these leucogranites may also be related to incongruent melting of the crustal sources, which has a potential to result in the Nd isotope disequilibrium between melt and residue.

Miocene granitic magmatism constrains the early E-W extension in the Himalayan Orogen: A case study of Kung Co leucogranite

Himalayan leucogranites record the tectonic evolution of the Himalayan Orogen and provide a window to explore the deep dynamic processes. In this study, we conducted petrological, geochronological, and geochemical investigations of the Kung Co leucogranites that intrude into the shallow Tethys Himalayan Sequence along the north-south trending Nyima-Tingri rift. Whole-rock major and trace elements, and SrNd isotope geochemical investigations of the Kung Co leucogranites revealed that they: (1) had high SiO2 (> 71.5 wt%) and Al2O3 (> 14.8 wt%), relatively high CaO (> 1 wt%) and TiO2 (avg. 0.2 wt%), high Na2O and K2O, and uniform K2O/Na2O ratios (0.98–1.17); (2) were enriched in large-ion lithophilic elements but depleted in high-field-strength elements, and had relatively high Sr (> 114 ppm), low Rb (< 249 ppm), and low Rb/Sr ratios (0.66–1.64); (3) had moderate total REEs, enriched light REEs, and flat heavy REEs patterns, and no or slightly negative Eu anomalies (Eu/Eu* = 0.65–0.86); and (4) had homogeneous initial 87Sr/86Sr ratios (0.738047 to 0.739784) and εNd(t) values (−14.4 to −15.3). Geochemical characteristics and comparisons with Himalayan magmatic sources indicated that the Kung Co leucogranites originated from the partial melting of metagraywacke in the Greater Himalayan Crystalline Complex. Zircon and monazite U-Th-Pb dating confirmed leucogranitic magmatism at ca. 16.2 Ma. Zircon saturation thermometry and monazite saturation thermometry showed that the Kung Co leucogranites had higher melting temperatures than the Miocene Great Himalayan Granites, suggesting an anomalous geothermal gradient. Combined with the previous model of the Miocene evolution of the Himalayan Orogen and geophysical data, we proposed that Himalayan lithospheric delamination and upwelling of the asthenosphere provided additional heat that enabled the partial melting of metagraywacke, while the lithospheric delamination caused east-west extension and associated rifting activity, with magma rapidly intruding upward along the rift to the shallow crust.

Miocene high-temperature leucogranite magmatism in the Himalayan orogen

AbstractHimalayan leucogranites of Cenozoic age are generally attributed to partial melting of metasedimentary rocks at low temperatures of &amp;lt;770 °C. It is unknown what the spatial distribution and characteristics of high-temperature (&amp;gt;800 °C) leucogranites are in the Himalayan orogen. The present study reports the occurrence of such leucogranites in the collisional orogen. We use the Ti-in-zircon thermometry in combination with the thermodynamically calibrated relationships of T-aSiO2-aTiO2 to retrieve crystallization temperatures of Miocene (ca. 17 Ma) two-mica granites from Yalaxiangbo, in the eastern Himalaya, SE Tibet. The results give the maximum temperature as high as ∼850 °C for granite crystallization, providing a significant constraint on the nature of thermal sources. Phase equilibrium modeling using metasedimentary rocks as the source rocks indicates that felsic melts produced at ∼850 °C and 6–10 kbar can best match the target leucogranites in lithochemistry. In this regard, the anatectic temperatures previously obtained for the production of Himalayan leucogranites would probably be underestimated to some extent. Such high temperatures are difficult to explain purely by the internal heating of the thickened orogenic crust. Instead, they require an extra heat source, which would probably be provided by upwelling of asthenospheric mantle subsequent to thinning of the orogenic lithospheric mantle by foundering along the convergent plate boundary. Therefore, the Himalayan leucogranites of Miocene age would be derived from partial melting of the metasedimentary rocks in the post-collisional stage.

Highly fractionated leucogranites in the eastern Himalayan Cuonadong dome and related magmatic Be–Nb–Ta and hydrothermal Be–W–Sn mineralization

Leucogranitic rocks, including two-mica granite, muscovite granite, and granitic pegmatite (aplite), crop out in the eastern Himalayan Cuonadong dome, southern Tibet. These rocks have high SiO2 (∼73 wt.%), Al2O3 (∼16 wt.%), and total alkali contents (∼9 wt.%), and are peraluminous with A/CNK ratios of 1.0–1.1. Whole-rock major and trace element data and mineral compositions reveal strong differentiation and fractionation trends for the two-mica granite, muscovite granite, and pegmatite–aplite dikes. Albite, muscovite, and garnet contents increased as the leucogranitic magma evolved, whereas K-feldspar and biotite contents decreased. Muscovite in aplite has higher Rb2O (∼0.4 wt.%), Nb (∼219 ppm), and W + Sn (∼737 ppm) contents, and lower Nb/Ta ratios (∼6) than muscovite in the two-mica and muscovite granites. Garnet varies in composition from almandine in the leucogranites to spessartine in the aplites. The muscovite granite and pegmatite are associated with rare-metal mineralization. Rare-metal-bearing minerals include beryl, columbite-group minerals, tapiolite, Nb–Ta-bearing rutile, cassiterite, and wolframite. Skarn and altered (i.e., chloritized) rocks along the margins of the leucogranites are the products of hydrothermal reaction between leucogranite and marble. Beryl, scheelite, and cassiterite occur in the skarn and altered rocks. Rare-metal mineralization in the Cuonadong dome can be divided into two types: magmatic Be–Nb–Ta mineralization related to pegmatite–aplite dikes and hydrothermal Be–W–Sn mineralization hosted in the skarn and associated with the two-mica and muscovite granites. Monazite U–Th–Pb dating reveals two episodes of leucogranitic magmatism in the Cuonadong dome at ca. 20 and 17 Ma. Columbite U–Pb dating indicates two stages of rare-metal mineralization at ca. 17 and 14 Ma (including tapiolite). It is inferred that magmatism was related to the exhumation of the Higher Himalayan Crystalline Basement, with the degree of differentiation of the associated leucogranites being the key factor controlling the type of mineralization.

Structural evolution of the Eastern Anatolian Basins: an example from collisional to postcollisional tectonic processes, Turkey

The Eastern Anatolian Basins (EABs) started to develop on a mosaic, made up of forearc deposits of the Eastern Pontide Arc (EPA), continental metamorphic rocks of the Anatolian-Iranian Platform (AIP), and obducted ophiolitic units of the North Anatolian Ophiolitic Belt (NAOB) and Southeastern Anatolian Ophiolitic Belt (SAOB). The basement rocks of the East Anatolian Region represent a continental fragment positioned between the EPA to the north and the Arabian Platform to the south. In appearance, it can be said that there are many different basins in the Eastern Anatolian Region. This paper reports on three of them that were studied as a part of the work, defining the setting and structural evolution of the EABs as a whole. These are, from north to south, the Oltu-Balkaya Basin, the Tekman-Karayazi Basin, and the Mus Basin. The EABs started to develop in the beginning of the Maastrichtian age with tectonic subsidence. They then evolved as collisional foreland basins on the AIP and ophiolitic units of the NAOB and SAOB during the Paleocene and Early Eocene period, as well. The fill of these basins is represented by basal conglomerate, shallow marine limestone, and clastic rock units. During deposition of this fill, syncollisional Paleocene peraluminous leucogranitic magmatism developed along the Eastern Pontides. From the Middle Eocene to the Quaternary age, different superimposed postcollisional basins developed over the collisional foreland basin fill. In this time interval, a shallow marine-continental molasse deposition was accompanied with high-K calc-alkaline volcanism, which may be attributed to postcollisional magmatism in the Eastern Anatolian Region. As a result, for the structural evolution of EABs, it can be said that the EABs represent a foreland basin in the Maastrichtian-Early Eocene and superimposed postcollisional basins in the Middle Eocene-Quaternary interval.

Garnet-bearing low-Sr and high-Sr Singelele leucogranite: A record of Neoarchean episodic melting in collisional setting and Paleoproterozoic overprint in the Beit Bridge complex, southern Africa

The study presents field, petrographic, mineral chemical, bulk-rock geochemical, U–Pb zircon and 40Ar/39Ar amphibole-biotite geochronologic characteristics of the Singelele leucogranite, which constitutes a prominent Neoarchean magmatic event in the Beit Bridge Complex terrane of the Limpopo Complex, southern Africa. The Singelele leucogranite is magnesian to ferroan, calcic to alkalic and peraluminous. Two groups of granites are distinguished on the basis of bulk-rock Sr content and garnet chemistry: low-Sr with low-XMg garnet and high-Sr with high XMg garnet. Major and trace element characteristics show that these two groups were, probably, generated by a fluid-absent melting of crustal rocks like metagreywacke at different depths in collision tectonic setting. Spatial distribution and available geochronologic data indicate that these two groups represent two episodes of Singelele leucogranite magmatism: low-Sr at ~2.68–2.65 Ga and high-Sr at ~2.63–2.62 Ga. The Singelele leucogranite occurring close to major fold structures and faults/shear zones are locally affected by anatectic and metasomatic Paleoproterozoic overprint at ~2.0 Ga (2037 ± 21 Ma and 2015 ± 15 Ma U-Pb for zircon overgrowths). Zoning of coarse garnet grains with a decrease of XCa in the melting-overprinted rocks reflects decompression of about 3–4 kbar. The metasomatic overprint produces Cl-F-rich biotite-2 and amphibole. Phase equilibria modeling shows a lack of equilibrium between these minerals and earlier garnet, and indicates modification of the garnet composition by metasomatism. 40Ar/39Ar ages for amphibole (2043 ± 18 Ma, 1994 ± 12 Ma) and biotite-2 (1985 ± 11 Ma, 1967 ± 11 Ma) are consistent with the zircon overgrowth ages.

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.

Geochronological and mineral chemical constraints on the age and formation conditions of the leucogranite in the Mawat ophiolite, Northeastern of Iraq: insight to sync-subduction zone granite

Daraban leucogranite occurs as cactus-like dykes hosted in serpentinized harzburgite in the mantle section of the Mawat ophiolite, Kurdistan region, northeastern Iraq. It has a primary mineral assemblage of quartz + orthoclase + albite + tourmaline + muscovite, with Mg-rich biotite, phlogopite, zircon, ilmenite–titanohematite exsolution, xenotime, and monazite as the most abundant accessory minerals. New laser ablation inductively coupled plasma mass spectrometry U–(Th)–Pb dating of zircon, monazite, and xenotime reveal a single episode of leucogranite magmatism in the Mawat ophiolite at 92.6 ± 1.2 Ma. These data indicate that the intrusion ages of leucogranite rocks postdate the 105 ± 5 Ma formation age of Mawat ophiolite obtained by K–Ar hornblende method. The leucogranite magma originated by anatexis of pelagic sediments during the late Cretaceous subduction in the Neo-Tethys Ocean, leading to the formation of the Mawat ophiolite as part of the main Zagros ophiolite belt in Iraq and Iran. Tourmaline and biotite from leucogranite dykes were examined regarding their microchemistry and formation environment. Electron microprobe studies show that the tourmalines form mainly dravite–schorl solid solutions with a tendency to schorl compositions. Biotites in the leucogranite have bimodal composition represents by phlogopite and Mg-rich biotite. The tourmaline and biotite compositions, as well as field observations, appear to exclude a late-stage magmatic differentiation origin for the leucogranite. A probable source is S-type granitic magma rich in boron that resulted from the anatexis of silica-rich Ca-poor subduction wedge sediments like those of the Qulqula group. This intruded the ophiolites during the subduction stage. Calculated biotite and Fe–Ti oxide equilibria indicate that the parent magma formed along the subduction zone and solidified in the mantle wedge at a pressure 3.8–4.2 kbar, equivalent to 12.5–13.8 km depth.

Petrogenesis of the Ramba leucogranite in the Tethyan Himalaya and constraints on the channel flow model

Abstract The Himalayan leucogranites are critical for understanding the geodynamic process of Himalayan orogenesis. In this article, we present petrological, geochronological and geochemical results of the Ramba leucogranites that crop out in the center of the Ramba dome within the Tethyan Himalaya. U–(Th)–Pb dating of zircon, monazite and xenotime revealed three episodes of leucogranitic magmatism in the Ramba dome at ca. 44 Ma, ca. 28 Ma and ca. 8 Ma, respectively. The ca. 44 Ma and ca. 28 Ma magmatisms are both represented by strongly deformed porphyritic two-mica granite gneiss dykes that intruded into the margin of the dome. These granite gneisses have high CaO and Sr contents, relatively high Na/K, Sr/Y and low Rb/Sr ratios, and much lower initial 87Sr/86Sr ratios but higher eNd(t) and eHf(t) values than the ancient Indian crustal material. These results indicate that the granite gneisses were not derived from metasedimentary rocks of the High Himalayan Sequence but most likely were derived from partial melting of mafic rocks at a thickened crustal condition. The ca. 8 Ma leucogranites consist of a two-mica granite pluton in the core of the dome and garnet-bearing granite dykes in the margin of the dome. Major and trace element compositions indicate that these Miocene leucogranites are typical S-type granites and were most likely derived from muscovite and/or biotite dehydration melting of metapelites. The relatively lower TiO2, TFeO, MgO and CaO contents, higher Rb concentration and Rb/Sr ratio, and more strongly negative Eu anomaly of the garnet-bearing granite indicate that it was more evolved than the coeval two-mica granite. The non-CHARAC (CHarge-And-Radius-Controlled) trace element behavior and the tetrad effect of the REE distribution of the garnet-bearing granite further suggest that it is a highly fractionated granite. Besides, our study detected some Asian-like materials in the Ramba leucogranites, such as Mesozoic-aged inherited zircons and the relatively depleted Sr–Nd–Hf isotopic compositions of some leucogranite samples. These features of leucogranites in the Tethyan Himalaya have been previously considered as direct geological evidence for the channel flow model. However, detailed geochronological and geochemical studies of the country rocks suggest that these Asian-like materials are most likely sourced from the country rock rather than the Asian lithosphere, thus they may not be used as solid evidence for the channel flow model.

Interplay of deformation and magmatism in the Pangong Transpression Zone, eastern Ladakh, India: Implications for remobilization of the trans-Himalayan magmatic arc and initiation of the Karakoram Fault

In the eastern part of Ladakh, the right-lateral Karakoram Fault Zone (KFZ) bifurcates into two strands called the Pangong Strand and the Tangtse Strand. These two strands bound a region called the Pangong Transpression Zone (PTZ), which consists of migmatitic dioritic gneiss, calc-silicates and the Durbuk Pluton; a pluton of two-mica leucogranite. Outcrop scale observations suggest pervasive migration of leucogranitic melt through the existing tectonic structures of the gneiss. Magnetic fabric from both the tectonized and undeformed parts of the Durbuk Pluton show parallelism with the tectonic fabric of the host gneiss, which, along with pervasive melt migration, indicates syn-kinematic relationship between deformation along the KFZ, leucogranite magmatism and emplacement of the Durbuk Pluton. U–Pb geochronology of zircons from the dioritic gneiss yields a crystallization age of 63.6 ± 1.5 Ma and also shows younger zircon growth down to ∼13 Ma, which suggest arc magmatism at ∼65 Ma followed by partial melting and leucogranite magmatism in the KFZ till ∼13 Ma. One two-mica leucogranite sample from the Durbuk Pluton gives a crystallization age of 22.7 ± 0.5 Ma. As the Durbuk Pluton is syn-tectonic with deformation along KFZ, it is inferred that the KFZ initiated at least ∼23 Ma ago.

The geochemical and temporal evolution of the continental lithosphere and its relationship to continental‐scale faulting: The Karakoram Fault, eastern Karakoram, NW Himalayas

New laser ablation multicollector–inductively coupled plasma–mass spectrometry and isotope dilution‐thermal ionization mass spectrometry U‐Pb ages, coupled with Sm‐Nd isotope and geochemical analysis, define the temporal and geochemical evolution of the continental lithosphere in the eastern Karakoram, India, NW Himalaya. Our analysis demonstrates that magmatism occurred between ~108 and 69 Ma and ~22 and 13 Ma. The new age data, coupled with geochemical examination of the granitoids, confirm a parallel evolution with the western Karakoram in Pakistan and supports a model of regional continental crustal thickening and related metamorphism. Middle to Late Cretaceous magmatism immediately adjacent to the Karakoram fault suggests that crustal melting and associated metamorphism are unrelated to shearing along the fault. Miocene leucogranite magmatism occurred almost exactly concomitant with the emplacement of the Baltoro batholith in Pakistan. These trans‐Karakoram leucogranites also display similar geochemical evolution trends. Our new data clearly link the leucogranites along the fault to the regional Baltoro batholith and related metamorphic complexes to the west. This supports previous work suggesting that magmatism and metamorphism were not syn‐kinematic with continental‐scale faulting. The data demonstrate that the Karakoram fault could not have accommodated lateral offset in this region prior to ~16 Ma, limiting the long‐term averaged slip rate to a maximum of ~10 mm/yr.

Prolonged Paleozoic Magmatism in the East Greenland Caledonides: Some Constraints from U-Pb Ages and Hf Isotopes

New U, Pb, and Hf isotopic data from thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry from intermediate calc-alkaline rocks in the Caledonides of central East Greenland provide new evidence of a prolonged period of Paleozoic subduction-related magmatism. The new data point toward two groups of plutons: one older generation crystallizing at ca. 450 Ma and a younger generation at ca. 423 Ma. There is additionally a titanite-forming event at ca. 440 Ma. The Hf isotopes of the younger group have a larger variation and less juvenile character than those of the older group, but data indicate a mixed source with both crustal and mantle components for both groups. These new data combined with already published geochronological data testify to a prolonged and complex magmatic history starting markedly earlier in the southernmost segment of the East Greenland Caledonides than farther north. The varied geochemical compositions of the plutons and the prolonged intrusive activity are in contrast to the short-lived homogeneous S-type leucogranitic magmatism found elsewhere in the East Greenland Caledonides and bear more similarities to arc-related terranes now found in the Irish, Scottish, and Scandinavian Caledonides. It is speculated that the earlier plutons in East Greenland represent the northernmost expression of continental arcs formed in a west-directed subduction system along the full length of the Laurentian margin in the Paleozoic. The later plutons formed concomitant with leucogranite intrusions within a collisional setting.

A-type leucogranite magmatism in the evolution of continental crust on the western margin of the Siberian craton

We discuss problems of the origin, settings, and age of Neoproterozoic A-type leucogranites widespread in the Yenisei Ridge. Combined analysis of geological, petrological, and geochemical (including isotope) data shows that some granitoids (Glushikha complex) were formed at the postcollisional stage (750–720 Ma), and others (Tatarka complex), in an anorogenic environment (680–630 Ma). The anorogenic complex contains diverse igneous rocks, including alkaline varieties and carbonatites. Leucogranites form separate plutons within different igneous complexes. They have high contents of potassium (up to ultrapotassic composition in the Glushikha complex), iron, and fluorine and are depleted in europium. Postcollisional granitoids show the highest concentrations of Rb, Th, and U, extremely low concentrations of Ba and Sr, whereas anorogenic granitoids are rich in Ta, Nb, Y, Sm, and HREE. The obtained data point to the augmented mantle contribution to the formation of continental crust of the Yenisei Ridge between 750 and 630 Ma. We also report new results of U-Pb zircon dating, including SHRIMP and Ar-Ar data.

Low-pressure metamorphism and leucogranite magmatism, northeastern Yeongnam Massif, Korea: implication for Paleoproterozoic crustal evolution

Abstract The northeastern Yeongnam Massif consists primarily of pelitic to psammopelitic metasedimentary rocks, together with garnet-bearing leucogranites and granitic gneisses. Metapelitic mineral assemblages define three progressive metamorphic zones of the low-pressure facies series: cordierite, sillimanite, and garnet zones with increasing temperature. Metamorphic grade ranges from lower-amphibolite to lower-granulite facies, reaching 750–800 °C and 4–6 kbar in migmatitic gneisses. Geochemical characteristics, such as the enrichments of Rb and Ba relative to Sr, high K contents, and negative e Nd (1.9 Ga) values, indicate that peraluminous leucogranites were produced primarily by the fluid-absent melting of supracrustal rocks. Trace element characteristics of leucogranites are generally compatible with those of volcanic arc granites, and the widespread crustal melting may be attributed largely to magmatic heat flux in an arc environment. Whole rock Pb–Pb ages suggest that leucogranite intruded at ca. 1.93–1.86 Ga into the granitic gneiss, formed by an earlier magmatic episode at ca. 1.95 Ga. These ages are compatible with those of felsic magmatism reported from other parts of the Yeongnam Massif. Our results, in conjunction with the available geochronologic data, suggest that four stages of Precambrian crustal evolution are represented in the Yeongnam Massif: (1) the formation of proto-crust at ca. 2.9–2.5 Ga, and felsic magmatic episodes primarily at (2) ca. 2.1 Ga, (3) 1.95 Ga, and (4) 1.9 Ga. In particular, the last tectonothermal event produced widespread low-pressure type metamorphism in association with the emplacement of leucogranite, and is correlative with the Luliangian Orogeny in North China.

The timing of W-Sn-rare metals mineral deposit formation in the Western Variscan chain in their orogenic setting: the case of the Limousin area (Massif Central, France)

Abstract New 40 Ar/ 39 Ar dating performed on Rare Metal Granites and W ± Sn deposits in the northern Limousin has provided evidence of two metallogenic episodes. An Early Namurian episode ( c. 325 Ma) was contemporaneous with the emplacement of the large peraluminous leucogranite bodies, which are associated with small W ± Sn deposits, but also with some larger deposits, at Puy-les-Vignes (323.4 ± 0.9 Ma) and Moulin-Barret (323.7 ± 0.8 Ma) formed at a shallower level above cryptic granite plutons. These new data indicate that the metallogenic potential of the Namurian leucogranites might have been underestimated. Most other W ± Sn deposits in the northern Limousin area are attributed to a Mid-Westphalian episode ( c. 310 Ma), and are contemporaneous with the emplacement of all the Rare Metal Granites. Both episodes were related to leucogranite emplacement and associated fluid circulations, but in two different geodynamic contexts. The Early Namurian episode may be related to syncollisional extension of the Variscan belt, whereas the Mid-Westphalian one occurs during generalized extension and rapid exhumation of the belt associated with the granulite-facies metamorphism of the lower lithosphere probably related to the delamination of the lower lithosphere. Thus, W ± Sn and rare metals (Ta, Nb, Be, Li) deposits are clearly temporally and probably genetically related to leucogranitic magmatism.

TECTONOTHERMAL EVOLUTION OF THE NORTHERN MINTO BLOCK, SUPERIOR PROVINCE, QUEBEC, CANADA

Previously considered one of Earth’s largest granulite terranes, the Minto block, in northern Quebec, consists dominantly of granitic rocks with sparse supracrustal remnants that record low-pressure, high-temperature metamorphism to the amphibolite and granulite facies at ca. 2.7 Ga. Much of the region is underlain by plutonic rocks, including igneous charnockitic bodies, that provide little evidence of metamorphism. In contrast, pelitic assemblages in low-grade greenstone belts and their higher-grade shoulders define four metamorphic zones: 1) sub-staurolite-grade slates, found only in the Duquet belt, 2) staurolite – andalusite – sillimanite – garnet-zone schists, present in all belts, 3) marginal garnet – sillimanite ± kyanite migmatites, and 4) garnet – sillimanite ± cordierite and garnet – orthopyroxene granulites, as enclaves within plutonic units. A prograde low-pressure, hightemperature regional facies-series is defined by andalusite – sillimanite assemblages, thermobarometric analyses that indicate peak conditions ranging from 3 kbar, 560°C to 8.4 kbar, 840°C, and short segments of anticlockwise P–T trajectories preserved in two of the lowest-grade areas. Kyanite occurs in migmatites in the vicinity of a ca. 2.81 Ga shear zone bounding two distinct supracrustal assemblages and marks an earlier Barrovian event. All belts are polydeformed, with a common, broadly synmetamorphic, NNW-striking, steep foliation and down-dip lineation related to regional-scale F2 folds. Distributed late-metamorphic dip-slip shear appears responsible in some areas for cross-strike juxtaposition of low- and high-grade metamorphic zones. Older, generally poorly preserved D1 deformation, recognized in several belts, may represent accretionary structures. Variably developed younger deformation includes NNW dextral transcurrent brittle–ductile shear zones (S3), variably developed east-trending F4 folds, and WNW-striking pseudotachylite zones. U–Pb monazite ages of 2702 Ma date high-grade metamorphism of a pelitic granulite, crystallization of two diatexites, and staurolite-zone metamorphism in one schist. Together with similar zircon ages of crust-derived granites, these data point to a major collisional orogeny at this time. Older titanite (2748–2765 Ma) indicates areas that escaped >650°C metamorphism during the 2.7 Ga event. Younger monazite occurs in a migmatite-grade rock (2688, 2672 Ma) and staurolite-zone schists (2679–2642, 2648, 2637, and 2642–2628 Ma), similar to titanite ages in greenstones (2643–2550 Ma) and 40 Ar/ 39 Ar hornblende ages (2.64–2.56 Ga). These post-peak metamorphic ages in the greenstone belts based on monazite and titanite are attributed to hydrothermal growth from fluids channeled along permeable zones; they may reflect late reheating events observed throughout the Superior Province as leucogranitic magmatism, metamorphism in the deep crust, and mineral growth in shear zones.

Timing of prograde metamorphism in the Zanskar Himalaya

The timing of the prograde metamorphism of continental crust buried during the collision of India and Eurasia is of importance to our understanding of the mechanics of mountain building and crustal metamorphism. Our knowledge of the Himalaya indicates that continental collision (at 55–50 Ma) culminated in crustal melting and leucogranitic magmatism at 24–18 Ma. However, the timing of events in the intervening 30 m.y. is poorly understood. Here we present Sm-Nd isotope analyses of garnets from the Zanskar Himalaya that also preserve information on crustal and orogenic evolution during that period. The prograde P-T-t paths indicate that heating and burial (from 6 to 10 kbar) continued until at least 25–30 Ma and imply peak temperatures (∼700 °C) close to those necessary to cause the crustal melting in the Himalaya at ca. 20 Ma, without a requirement for additional heat sources such as shear heating on major crustal shear zones. The persistence of high temperatures as late as 20 Ma in the Himalaya may be related to postcollisional thrusting south of the suture zone and within the Indian plate at 40–35 Ma.