Kohistan Arc Research Articles

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

Exploring the tectono-magmatic evolution of intraoceanic fore-arc setting during subduction initiation: perspectives from trace and platinum group element systematics of the Jijal ultramafic arc system, NE Pakistan

ABSTRACT The Jijal Complex is considered one of the largest Neo-Tethyan remnants in the intra-oceanic Cretaceous–Palaeogene Kohistan Arc of Pakistan. This complex largely consists of gabbros, peridotites, and chromitites. Previous studies documented the genesis of gabbros and chromitites in detail; however, detailed geochemistry and magma genesis were lacking. In this investigation, we present whole-rock geochemical and platinum group elements (PGEs) data of Jijal peridotites (dunites and harzburgites). The objectives of this study are to elucidate magmatic processes and tectonic regimes involved in the formation of peridotites, factors influencing the concentration and dispersion of PGEs in them, and their tectonic evolution. The investigated peridotite samples are depleted in magmaphile major elements (Al2O3: 0.23–0.57, TiO2: 0.01–0.04, and CaO: 0.08–0.69 wt%), relative to the primitive mantle values, reflecting melt-rock interaction during their magmatic evolution. In addition, rare-earth element (REE) contents in harzburgites are relatively higher compared to the dunites; however, both rock types exhibit depletion with respect to the chondrite-normalized values. The obvious negative anomalies of Nb, Zr, and enriched LREE and large ion lithophiles collectively substantiate their association with depleted arc-type mantle components, metasomatism, and melt/fluid-rock reactions after partial melting episodes. Bulk-rock and geochemical modelling of PGEs data of the peridotites suggest their shallow depth spinel-bearing depleted heterogeneous mantle source and generation from refractory mantle residuum along with boninitic signatures in a supra-subduction zone (SSZ). The PGE’s geochemistry additionally reveals that partial melting, fractionation, metasomatism, and sulphur under-saturation were the key factors that controlled the concentration and distribution of PGEs in the studied rocks. The boninitic features of the Jijal peridotites are equated with intraoceanic fore-arc followed by subduction initiation, and melt-rock reactions in a hydrated forearc mantle, indicating robust evidence of mantle depletion and pervasive refertilisation in an embryonic Neo-Tethyan arc system. Tectonically, the investigated rocks encapsulate vestiges of the Cretaceous Tethyan Ocean and record multifaceted history from boninitic to slab-proximal island arc affinity in compliance with an intraoceanic SSZ fore-arc regime coherent with the subduction inception.

Formation of lower arc crust by magmatic underplating revealed by high-precision geochronology

Abstract In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth—largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust—have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution–thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ~20 m.y., corresponding to a rate of lower crust formation of up to 200–260 km3 km–1 Ma–1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust.

Metamorphic and intrusive history of the Hindu Raj region, northern Pakistan

AbstractThe Hindu Raj region of northern Pakistan is situated between the Karakoram to the east and the Hindu Kush to the west. Both the Karakoram and the Hindu Kush are better studied and have well-documented, distinct geological histories. Investigation of the Hindu Raj region has been mainly limited to reconnaissance exploration and as such little is known about its tectonometamorphic history and whether that history is similar to its neighbouring areas. Analysis of new specimens collected along the Yasin Valley within the Hindu Raj region outline mid-to-Late Cretaceous pluton emplacement (ca. 105 and 95 Ma). Some of those plutonic rocks were metamorphosed to ∼750 ± 30 °C and 0.65 ± 0.05 GPa during the ca. 80–75 Ma docking of the Kohistan arc. A record of this collisional event is well-preserved to the west in the Hindu Kush and variably so to the east in the Hunza Karakoram. A subsequent, ca. 61 Ma, thermal event is partially preserved in Rb–Sr geochronology from the Hindu Raj, which overlaps with sillimanite-grade metamorphism in the Hunza portion of the Karakoram region to the east. Finally, apatite U–Pb and in situ Rb–Sr both record a late Eocene thermal/fluid event likely related to the India-Asia collision. These new data outline a complex geological history within the Hindu Raj, one that shares similarities with both adjacent regions. The information about the tectonometamorphic development of the Hindu Raj is important to gaining a detailed view of the geological characteristics of the southern Asian margin prior to the India-Asia collision.

Mobilization of Cu in the continental lower crust: A perspective from Cu isotopes

Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust. Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits. To test this hypothesis and further explore the behavior of Cu in the lower crust, we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains. The collected samples include hornblendites from the Kohistan arc, granulite xenoliths and hornblendites from the Gangdese arc, hornblendites and gabbros from the Laiyuan complex in the North China Craton, and hornblendite xenoliths from the western margin of the Yangtze Craton. These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation, with significantly varied Cu contents (11.2 to 145 ppm) and δ65Cu (−1.05‰ to 1.40‰). Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks, and based on this, they can be further divided into three groups: Group I includes the Gangdese granulites and Yunnan hornblendites, which perhaps experienced significant metasomatism. This suite of rocks shows enrichment of 65Cu (δ65Cu = 0.01‰ to 1.40‰), positively correlated with metasomatism (δ65Cu vs. Ce/Pb). We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks, so the feature of heavy isotope enrichment is inherited. Group II includes Laiyuan hornblendites and gabbros, derived from the same parental magma and emplaced at different depths (hornblendites, 23.3 – 28.1 km; gabbros 8.4 – 11.1 km). The Cu isotopic compositions are strongly fractionated between these two kinds of rocks, with low δ65Cu in the hornblendites (0.00‰ to 0.28‰) and highly polarized δ65Cu in the gabbros (−1.05‰ to 0.81‰). Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature. Primary sulfides were decomposed by fluids and reprecipitated at shallower depths. Since this process involves multiple redox reactions, the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated. Group III includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction. The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition (−0.09‰ to 0.18‰) and Cu content (83.4 to 128 ppm), suggesting insignificant Cu migration and isotope fractionation. In contrast, the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated (−0.36‰ to 1.27‰). Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation. The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust, resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues. Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.

Fluid source and physicochemical conditions of the polymetallic mineralization in Gawuch Formation, Kohistan Island Arc, NW Pakistan

The Cu-Sb-Pb polymetallic vein deposit is hosted by metavolcanics rocks of the Gawuch Formation at the Kaldom Gol area of the northwest Kohistan arc terrain in northern Pakistan. The mineralization is closely associated with the dioritic to granodioritic rocks of the Lowari pluton, which was intruded into the Gawuch metavolcanics. Details of ore characterization and processes of ore genesis of this evidently hydrothermal mineralization are not well documented. Integrating petrographic, mineral-chemical and isotopic investigations, this study aims to comprehend the source of hydrothermal fluids, geochemical evolution, mineral inclusions and physicochemical conditions of the Cu-Sb-Pb polymetallic vein deposit in Gawuch metavolcanics in the Kohistan arc terrain in northern Pakistan. The mineralization is distinguished into three types of ore-gangue associations: Type Ia, Type Ib, and Type II. The textural study revealed two pyrite generations: (i) Py1 displaying euhedral to subhedral habits and containing scarce inclusions, and (ii) Py2 occurring as anhedral grains hosting abundant inclusions. Type Ia is characterized by Py1 associated with abundant quartz (Qz) showing comb texture, sericite (Ser), and minor chlorite (Chl). Type Ib comprises Qz + Ser + Chl and Py2, chalcopyrite (Ccp), and magnetite (Mag). Type II is represented by mosaic quartz, rhombic adularia, and bladed calcite, and the ore minerals fahlore and galena. Alteration zones composed of Qz-Ser ± Chl and Qz-Ser-Chl, surround Type I (a, b) and Type II veins, respectively. Fahlore and galena mostly replace pyrite of Type Ia and chalcopyrite of Type Ib. In addition, malachite, azurite, hematite and covellite occur as secondary (supergene) minerals. The Co/Ni ratios (>1) of Kaldom Gol pyrites suggest that the ore-forming fluids were hydrothermal in origin and Py1 and Py2 solidified at 221–304 °C and 225–261 °C, respectively. The LA-ICP-MS time-resolved depth profiles confirm the existence of sphalerite, and chalcopyrite inclusions in pyrite (Py1 and Py2) and millerite, bravoite, vaesite, Au-tellurides, native Au and galena inclusions in chalcopyrite and fahlore. Sulfur isotope compositions of pyrites (δ34S = Py1, −0.58 to +2 ‰; δ34S = Py2, −0.24 to +2.04 ‰) indicate that the ore-forming fluids were derived from magmatic source (s). The mineral assemblage, hydrothermal alterations, textures, temperature and δ34S of pyrites suggest that the Cu-Sb-Pb polymetallic mineralization at Kaldom Gol represents an intermediate-sulfidation type of epithermal deposit.

Petrogenetic, geochemical, and geochronological constraints on magmatic evolution of the Chilas Complex gabbros, Kohistan arc, NW Himalaya

The Kohistan Arc preserves an exquisite and ideal petrogenetic record for studying mafic magmatism in an arc environment. Here we report the geochronological, mineralogical, and geochemical data of the gabbros from the Chilas Complex of Kohistan in order to understand their genesis, timing, role in crustal thickening, and tectonic evolution in Neo‐Tethys. The mineral composition of orthopyroxene (Opx) in gabbros displays transitional to metamorphic behaviour with high amount of Al2O3 (0.59–2.66 wt.%), suggesting the influence of subsolidus re‐equilibration during cooling or magmatic crystallization. Thermobarometric constraints propose the crystallization of Chilas gabbros around 892–985°C and ≤1.5 GPa pressure coupled with high H2O content (5.6%–7.7%). Geochemical modelling and isotopic peculiarities such as (87Sr/86Sr)i (0.000018–0.704140) and 206Pb/204Pb (18.526–18.554) suggest that the source of gabbros was depleted, likely controlled by peridotite melts and experienced insignificant crustal contamination. Relatively lower ratios of Nb/Yb (0.89–3.32) and TiO2/Yb (0.14–0.79) imply that the garnet was not a residual phase during the partial melting of a mantle source. The Chilas gabbros with depletion in Nb and Ta, enrichment in light rare earth element (LREE)s, the lower Ce/Pb ratios, plot of Th/La versus Th, and negative anomalies of Eu provide a consistent clue regarding the negligible participation of recycled components. Zircon U–Pb concordant ages of 83–86 Ma indicate the timing of emplacement and a Cretaceous magmatic event in the Chilas Complex. We infer that the Cretaceous magmatic flare‐up is a significant crustal growth event of Kohistan and consider the underplating of mantle‐derived magmas as one of the primary reasons for the crustal thickening based on zircon Hf isotopic signatures. The average chemical composition of Kohistan Complex provides support for the generation of andesitic magmas in arcs and the “andesite model” of continental crustal growth. We propose the similar petrogenesis for the Nidar, Kargil, and Chilas gabbros linking the formation of these variants within Neo‐Tethys based on identical magmatic records.

Can Post-Subduction Porphyry Cu Magmas Form by Partial Melting of Typical Lower Crustal Amphibole-Rich Cumulates? Petrographic and Experimental Constraints from Samples of the Kohistan and Gangdese Arc Roots

Abstract Sulfide-rich hydrous arc cumulates in the lower crust are considered a likely source for porphyry Cu-forming magmas in post-subduction settings. However, it remains controversial whether or not post-subduction porphyry Cu magmas can be produced by partial melting of lower crustal arc cumulates. Here we performed detailed petrographic and geochemical studies on exhumed lower to middle crustal cumulates from the Gangdese and Kohistan arc sections, and then conducted partial melting experiments of the Gangdese sulfide-rich lithologies at 1.5 GPa and 800°C–1150°C using a piston cylinder press. Both the Gangdese and the Kohistan arc-root samples formed as cumulates in the lower to middle crust, but the Kohistan samples were subsequently brought into the garnet stability field during the India–Eurasia continental collision. Most of the Gangdese samples contain relatively high abundances of magmatic sulfides preserved in the form of Cu-poor monosulfide solid solution. In contrast, most of the Kohistan samples contain very low abundances of Cu-rich sulfides preserved as inclusions in garnet, probably due to intense metamorphic dehydration and up to 2 wt% partial melting in the garnet stability field. The original magmatic sulfides at Kohistan appear to have been removed mainly by metamorphic fluids rather than by partial melts, as the arc-root samples could have experienced only very low degree of partial melting and the sulfur solubility in experimentally produced partial melts is low (≤0.1 wt%) even at oxygen fugacities two log-units above the fayalite-magnetite-quartz buffer. Furthermore, the experimental partial melts contain much less K2O, MgO, Cl and ±S than typical post-subduction porphyry Cu-forming magmas. Hence, we conclude that the role of sulfide-rich arc cumulates in the formation of post-subduction porphyry Cu magmas has been severely overestimated.

A Test of the Hypothesis That Syn‐Collisional Felsic Magmatism Contributes to Continental Crustal Growth Via Deep Learning Modeling and Principal Component Analysis of Big Geochemical Datasets

AbstractThe origin, way of growth, and compositional transition (from basaltic to andesitic) of continental crust remain enigmatic. To better understand the evolution of the Earth's crust, geoscientists have hypothesized two competing models, one is the widely accepted island‐arc model, the other is the newly proposed collision‐zone model that continental collision produces and preserves syn‐collisional Mantle‐derived Bulk‐continental‐crust‐like Granitoids (MBGs), and hence maintains net continental crust growth. Here, we tested the collision‐zone model by investigating the existence, temporal‐spatial distribution, geochemical signatures, and possible sources of the syn‐collisional MBGs. We applied deep learning (DL) algorithm and principal component analysis (PCA) to the database GEOROC and Tibetan Magmatism Database. DL successfully built a regression model of whole‐rock element compositional data and mean zircon εHf(t) data of igneous rocks. This can not only assign values to the missing Hf data, but statistically unveil the potential relations between the compositions (both isotopic and geochemical) and the possible sources of the igneous rocks. The DL and PCA enabled to recognize the MBGs and define their geochemical and isotopic fingerprints differing noticeably from arc magmas (e.g., Kohistan arc type and Tibetan adakite‐like type). Besides, our observations suggest that MBGs are common in collisional settings as a response to known collision events. Moreover, the MBGs' distinct geochemical and isotopic signatures indicate that they are likely sourced from subducted ocean crust. Our results therefore generally support evident contribution of syn‐collisional felsic magmatism to net continental crust growth. However, further refinement of the petrogenesis and estimation of the (relative) volume are critically needed.

Tectonic evolution of the Caledonian orogeny in Scotland: a review based on the timing of magmatism, metamorphism and deformation

AbstractClassic tectonic models for the Caledonian orogeny in Scotland involve Ordovician collision of Laurentia–Midland Valley arc (Grampian orogeny), followed by middle Silurian collision of Laurentia–Baltica (Scandian orogeny) and 500–700 km of sinistral displacement along the Great Glen fault separating the Northern Highlands (Moine Supergroup) from the Grampian Highlands (Dalradian Supergroup). A review of the timing of magmatic and metamorphic rocks across Scotland allows a simpler explanation that fits with a classic Himalayan-style continent–island arc–continent collision. Late Cambrian – Early Ordovician NW-directed ophiolite obduction (Highland Border complex) coincided with the ending of stable continental shelf sedimentation along the eastern margin of Laurentia. Following collision between Laurentia and the Midland Valley arc–microcontinent in Early Ordovician time, crustal thickening and shortening led to almost continuous regional metamorphism from c. 470 to 420 Ma, rather than two discrete ‘orogenies’ (Grampian, Scandian). U–Pb monazite and garnet growth ages indicating prograde metamorphism, and S-type granites related to melting of crustal protoliths are coeval in the Grampian and Northern Highlands terranes. There is no evidence that the Great Glen fault was a terrane boundary, and strike-slip shearing post-dated emplacement of Silurian – Early Devonian granites. Late orogenic alkaline granites (c. 430–405 Ma) in both Moine and Dalradian terranes are not associated with subduction. They are instead closely related to regional alkaline appinite–lamprophyric magmatism resulting from simultaneous melting of lower crust and enriched lithospheric mantle. Caledonian deformation and metamorphism in northern Scotland, with continuous SE-directed subduction, show geometry and time scales that are comparable to the Cenozoic India–Kohistan arc–Asia collisional Himalayan orogeny.

Amphibole fractionation and its potential redox effect on arc crust: Evidence from the Kohistan arc cumulates

Abstract Arc magmas, a major contributor to continental crust growth, are thought to be more oxidized than mid-ocean ridge basalts as reflected by an enrichment in ferric iron relative to ferrous iron. But how arc magmas become oxidized is hotly debated. It is acknowledged that the fractionation of common Fe-rich phases (e.g., amphibole) may change the Fe valence of the derivative melt. Amphibole has Fe-rich compositions commonly found in arc systems. We present high-precision (±0.01%) Fe valence data of amphibole and cumulates from the Kohistan arc determined by Mössbauer spectroscopy and bulk cumulate Fe3+/ΣFe ratios by wet chemistry. We evaluate the Fe3+/ΣFe trend of Fe-rich amphibole during arc magma fractionation. Our results show that bulk Fe3+/ΣFe ratio of cumulates in mature island arc settings is mainly controlled by amphibole due to its abundance and decreases (from 0.4 to 0.2) with decreasing Mg# as Fe3+/ΣFe ratios decrease in amphibole (from 0.35 to 0.2). Our modeling suggests that amphibole fractionation from parental arc magmas to Fe3+/ΣFe above 0.2 may lead to an increase in Fe3+/ΣFe ratios of middle–late stage residual melts by 0.1–0.3, and the partitioning of ferrous Fe into amphibole increases with the evolution of arc magmas. Our findings highlight the importance of petrological processes in the magma that contribute to the production of fertile arc crust.

Phase equilibria and microstructural constraints on the high‐T building of the Kohistan island arc: The Jijal garnet granulites, northern Pakistan

AbstractThe Kohistan island arc represents a >50 km thick and completely exposed Late Cretaceous–Eocene Neo‐Tethyan island arc sequence between the Indian and Eurasian plates in the western Himalaya. At the base of the Kohistan Arc, the Jijal Complex exposes a continuous sequence from ultramafic upper mantle rocks to mafic rocks of the lower crust across the Moho transition zone. In the shallower mafic section of the Jijal Complex, widespread garnet–clinopyroxene gabbros are uniform in assemblage but show a spectrum of microstructural variation involving two distinguishable styles of garnet growth. Uncertainty surrounds the genetic origin of these equilibria across the sequence which have been variably proposed to derive either from (a) dehydration melting of amphibole‐bearing precursor rocks, (b) subsolidus recrystallization of gabbronorite, or (c) primary magmatic crystallization. Owing to extensive high‐T re‐equilibration, compositional records of magmatic precursors and the pre‐peak evolution across the complex have been erased. However, quantitative textural analysis evidences a progressively increasing departure from microstructural randomness from deep to shallow levels of the upper Jijal Complex. This is interpreted to reflect the increased propensity for shallow garnet–clinopyroxene gabbros to have been derived from solid‐state recrystallization of a cumulate gabbronorite precursor, resulting in the development of coronitic garnet microstructures. In the deepest portions of the mafic sequence, both cumulate textures and thermodynamic modelling attest to the basal levels of the Kohistan arc crust at ~10–14 kbar crystallizing primary magmatic garnet and clinopyroxene from low H2O (<6 wt%) basaltic–andesitic melts. Granulite facies metamorphism and re‐equilibration across the whole upper Jijal Complex—shown via phase equilibrium modelling to be associated with over‐accretion in the mid‐arc crust and a pressure increase of 2–3 kbar to 850–1,050℃ and 11–14 kbar—was followed by a period of approximately isobaric cooling across ~200℃ and amphibolitization. Compositions of low‐volume leucosome‐hosted garnet indicate melting occurred prior to, or during, this re‐equilibration period and melt reintegration simulations suggest that dehydration melting of amphibole‐bearing precursors played only a minor role in the genesis of the Jijal garnet–clinopyroxene gabbros. By combining field relationships, petrological and textural analysis, interpretations of mineral compositions, and phase equilibrium modelling, we therefore provide detailed constraints on crustal thickening and the thermal evolution of the lowermost arc crust.

Nb-Ta systematics of Kohistan and Gangdese arc lower crust: Implications for continental crust formation

• Kohistan and Gangdese (garnet) hornblendites and/or garnet gabbros are of cumulate origins. • Amphiboles in these lower crust cumulates have high Nb/Ta but low Nb and Ta concentrations. • Amphibole fractionation will not change Nb/Ta ratio of arc magmas significantly. • Rutile likely plays an important role in Nb/Ta fractionation in continental crust formation. Earth’s continental crust has a lower Nb/Ta ratio than that of its building blocks –basaltic melts from the mantle. This Nb/Ta mass imbalance implies the existence of a complementary high Nb/Ta reservoir in deep crust. Amphibole-rich arc cumulates have been proposed to be one such candidate due to the preferential partitioning of Nb over Ta into amphibole. To test this proposal, we carried out in-situ analyses of amphibole and rutile in the lower crustal rocks from Kohistan arc and Gangdese arc. Petrological observations and geochemical results suggest that these rocks in both Kohistan arc and Gangdese arc are of magmatic origins and formed by crystal accumulation, i.e. of cumulate. The Kohistan garnet-bearing hornblendite cumulates were formed at 952–1009 ℃; the garnet-free hornblendite cumulates in Kohistan arc and Gangdese arc were formed at 941–992 ℃ and 955–1027 ℃, respectively. We find that, although the amphibole of the Kohistan and Gangdese cumulates have high Nb/Ta ratios (average 18.2 and 17.0, respectively), their Nb and Ta concentrations are far lower than mantle-derived volcanics (Nb < 1.1 ppm, Ta < 0.11 ppm). We show that, with such low concentrations of Nb and Ta, amphibole fractionation will not change arc magma Nb/Ta ratios significantly. Instead, the rutile in Kohistan cumulates has high Nb/Ta ratios and high Nb-Ta concentrations. The calculation shows that rutile may play a more important role in Nb/Ta fractionation during arc magma differentiation in thickened arc crust. Our study provides a mineralogical perspective to understand the formation of continental crust, and improves our understanding how Nb or Ta becomes enriched in arc setting.

Textural Implications in Assessment of Physico-Mechanical behaviour of Metavolcanic Rocks from Dir Upper, north western Pakistan

The Dir-Utror meta-volcanics from the south western portion of the Kohistan arc in northern Pakistan areanalyzed in term of their petrography, physico-mechanical properties. Field observations and petrography show thecollected representative samples to be fine-grained meta-andesites (FMA), coarse-grained meta-andesites (CMA) andagglomerate (AG). The relationship between petrography and physico-mechanical properties has been investigatedwhich inferred the grain size to be the major factor, alongside grains’ shape, arrangement and size distribution as wellas degree of mineral alteration significantly affecting the mechanical behavior of rocks. The CMA yield more strength(98 MPa) than FMA (93 MPa) due to its lesser degree of mineral alteration, inequigranular texture, lack of preferredmineral alignment, relatively low porosity and water absorption. The lower strength of agglomerate (57 MPa)corresponds to abundance of soft minerals (calcite), exotic rock fragments and coarse-grained texture. Based onphysico-mechanical properties including specific gravity, bulk density, aggregate impact value, Los Angeles abrasionvalue and unconfined compressive strength (UCS), these rocks fall within permissible range to be utilized for multipleengineering purposes including dimension stones and foundation materials for other civil structures. However,petrographic investigations reveal excessive amount of reactive silica in these rocks making them prone to alkali-silicareactivity in concrete works with ordinary Portland cement (OPC). Hence these rocks are not recommended for use assole aggregate material or low-alkali cement is recommended, if used.

Identification and Origin of Jurassic (~182 Ma) Zircon Grains from Chromitite within the Peridotite of the Jijal Complex, Kohistan Arc in North Pakistan

The Jijal ultramafic–mafic complex in Pakistan probably preserves the most complete fragments of the petrological Moho. However, a few studies argue for multiple origins (including petrogenetic speculations and tectonic reconstructions) for different lithologies. One of the main reasons for this dispute is the lack of direct age information of the ultramafic rocks. Zircon grains, despite generally being exotic in ultramafic rocks, can provide significant insights into the petrogenetic process of the host ultramafic rocks. This study reports the first zircon U–Pb age and Lu–Hf and trace element data for zircon grains separated from chromitite lenses within the peridotite, which is commonly considered the lowermost part of the Jijal complex. These zircon grains yield concordant 206Pb/238U ages of ~182 ± 3 Ma, which is much older than the late Early Cretaceous age (&lt;120 Ma) of the Jijal complex, and lying above it, the other complexes of the Kohistan paleo-arc. Furthermore, these Jurassic zircon grains present radiogenic εHf(t) values (+9.7 to +6.0) which are obviously lower than the values for the Cretaceous zircon grains of the Kohistan arc. From integrated analysis of the zircon trace element signatures (e.g., high Th, U, Th/U, and U/Yb ratios) and regional geology, we speculate that these zircon grains came from a ‘missing’ Early Jurassic arc akin to the Gangdese belt to the east, and entered the mantle by oceanic subduction processes. Although these Jurassic zircon grains cannot actually constrain the formation age of the chromitite as well as the peridotite, it reminds us that some cryptic pre-Cretaceous complexes and geodynamic processes were incorporated in building the oceanic crust of the Jijal intra-oceanic arc, or the mantle section (at least part of it) should probably belong to the Indus ophiolite mélange. Further research, particularly chronological studies on mantle (or ultramafic) rocks, as well as detailed geological mapping, should be carried out in the future for solving this issue.

Petrogenesis, platinum-group element geochemistry and geodynamic evolution of the Cretaceous Chilas gabbros, Kohistan island arc, NE Pakistan

Kohistan, an intra-oceanic arc, formed due to the India-Asia collision-related tectonics. The arc is comprised of mafic-ultramafic and crustal lithologies. Chilas Complex, one of the major mafic-ultramafic body that lies at the base of the Kohistan arc, is dominantly composed of massive gabbronorites with local ultramafic-mafic intrusive bodies. The ultramafic rocks were considered as the products of fractional crystallization whereas gabbronorites were reported to have formed at the root zone of the magma chamber either from melts or due to fractional crystallization, based on major and trace element data. To understand the petrogenesis and origin of the Chilas gabbros, we conducted platinum group elements (PGEs) geochemistry combined with whole-rock major and trace element compositions. Geochemical data of the Chilas gabbros show tholeiite series, likely derived from a depleted lithospheric mantle source via peridotite melts. Presence of hornblende and relatively higher Al/Ti ratios of clinopyroxene in the Chilas gabbros suggest hydrous magma, ascribed to mantle wedge metasomatism by slab-derived fluids. Poor correlations between MgO vs. Ni, Cr, and V, Ni vs. Cr, Dy/Yb vs. SiO2, Sr/Y vs. Eu/Eu* and relatively low TiO2 indicate a minimal role of fractional crystallization. Low contents of LOI (0–3.24 wt%), Ce/Ce* (0.92–1.01), and positive relation of La, Ta, and Hf with Zr strongly support the insignificant impact of hydrothermal alteration. Enrichment of PGEs, such as Pt (0.63–13.75 ppb), Pd (0.43–13.87 ppb), and lower Pd/Ir ratios (0.76–40.04) suggest low degree partial melting of the parent magma. Higher Pd, Pd/Ir ratios compared to Pt and Pt/Pd, combined with enrichment of PPGE over IPGE, suggest the participation of slab-dehydrated fluids and mantle metasomatism however Cu/Ir vs. Ni/Pd, V-Pd/Ir and La-Pd/Ir ratios point towards olivine fractionation in the magma. Lower Cu/Pd ratios, relationships of Ru and Pt against Cr, and Os diminution corroborate S-undersaturation in parental magma. The PGEs contents and Pd/Pt ratios attest depleted mantle source for their production. In view of Cu/Zr and Cu/Pd ratios, it is substantiated that the sulphide segregation cannot be a promising reason for PGEs depletion. Therefore, we infer PGEs depletions in parental magma are due to PGEs-depleted mantle source, though sulfide retention in mantle source cannot be eliminated. The geochemical fingerprints in tectonic diagrams and negative anomalies of Nb and Zr in the studied samples provide credible facts that favour arc magmatism. Based on major and trace element data and PGEs values, we consider that the Chilas gabbros were formed from a lithospheric mantle that was metasomatized by fluids generated at the mantle wedge and derived from mafic melts in an arc setting. Geochemical data and their plotting on various tectonic discrimination diagrams indicate Chilas Complex as a remnant of the Neo-Tethyan oceanic arc system.

Late Jurassic Changmar Complex from the Shyok ophiolite, NW Himalaya: a prelude to the Ladakh Arc

AbstractThe Shyok Suture in western Himalaya preserves a record of the opening and closure of the Mesotethys Ocean between the Shyok ophiolite and Karakoram terrane prior to the India–Eurasia collision. The formation age of the Shyok ophiolite was unknown, which impeded correlation with similar rocks along the Shyok Suture in Pakistan and corresponding sutures in Tibet. We report the first zircon U–Pb ages of a newly documented suite, here named the Changmar Complex. The Changmar Complex gabbronorite and plagiogranite yielded SHRIMP U–Pb zircon Late Jurassic ages of 159.4 ± 0.9 Ma and 151.9 ± 1.5 Ma. Their highly positive initial εHf values (+14.9 to +16.9) indicate a juvenile mantle origin, without continental crust influence on the magma source. The Shyok ophiolite represents either: (1) a separate island arc that preceded formation of the Cretaceous–Eocene Ladakh Arc; or (2) the oldest magmatism and early stage of the Ladakh Arc. Intrusive and extrusive mafic rocks from the Shyok Suture analysed in this study have typical supra-subduction zone enrichment characteristics in their geochemistry and are classified as part of the volcanic-arc ophiolite. The U–Pb age and Hf isotopic signatures for the Shyok ophiolite are similar to the Late Jurassic Matum Das tonalite within the Kohistan Arc; we therefore suggest that they are part of the same intra-oceanic island-arc system that formed in the Mesotethys Ocean prior to Late Jurassic time.

Mineralogy and geochemistry of peridotites and chromitites in the Jijal Complex ophiolite along the Main Mantle Thrust (MMT or Indus Suture Zone) North Pakistan

The Jijal Complex, one of the largest Neo-Tethyan ophiolites (ca. 150 km2) in Pakistan, occupies a deep-level section of the Cretaceous Kohistan Arc that was obducted along the Main Mantle Thrust or Indus Suture Zone. Peridotites of the Jijal Complex consist mainly of harzburgite and dunite with numerous podiform chromitites. In this paper, we present new mineral compositions, whole-rock major elements, trace elements, rare earth elements (REEs) and platinum group elements (PGEs) to evaluate the melt evolution and petrogenesis of these rocks. Chromian spinel in the chromitites is fairly uniform in composition with Cr#s ranging from 75 to 81, Mg#s from 59 to 63 and TiO2 < 0.2 wt%, which suggest crystallization from boninitic magmas. Chromian spinel in the peridotites has relatively lower Cr#s of 71–74, Mg#s of 38–48 and very low TiO2 contents (averaging 0.09 wt%). The Fe3+#s of chromian spinels both in the chromitites and peridotites are very low (maximum 0.02), reflecting crystallization under low oxygen fugacity (fO2). The harzburgites and dunites have low average CaO, Al2O3 and REE values and contain Al-poor clinopyroxene and high-Cr spinels. Mantle-normalized trace element patterns of the harzburgite, dunites and chromitites reflect high-degrees of partial melting (25%–30%). The high-Cr chromitites all show similar chondrite-normalized platinum group element patterns characterized by enrichment in Os, Ir, Ru and Rh relative to Pt and Pd. The mineral chemistry, whole-rock compositions and PGE geochemistry indicate that the chromitites in the Jijal Complex were generated from a boninitic parental magma generated under low to high fO2 in a supra-subduction zone environment. The calculated compositions of the parental melts of the Jijal chromitites are consistent with differentiation of arc-related magmas.

Zirconium in rutile thermometry from garnet granulites of the Jijal complex of Kohistan arc, NW Himalaya

Zirconium in rutile thermometry from garnet granulites of the Jijal complex of Kohistan arc, NW Himalaya

About this title - Himalayan Tectonics: A Modern Synthesis

The Himalaya–Karakoram–Tibet mountain belt resulted from Cenozoic collision of India and Asia and is frequently used as the type example of a continental collision orogenic belt. The last quarter of a century has seen the publication of a remarkably detailed dataset relevant to the evolution of this belt. Detailed fieldwork backed up by state-of-the-art structural analysis, geochemistry, mineral chemistry, igneous and metamorphic petrology, isotope chemistry, sedimentology and geophysics produced a wide-ranging archive of data-rich scientific papers. The rationale for this book is to provide a coherent overview of these datasets in addressing the evolution of the mountain ranges we see today. This volume comprises 21 specially invited review papers on the Himalaya, Kohistan arc, Tibet, the Karakoram and Pamir ranges. These papers span the history of Himalayan research, chronology of the collision, stratigraphy, magmatic and metamorphic processes, structural geology and tectonics, seismicity, geophysics, and the evolution of the Indian monsoon. This landmark set of papers should underpin the next 25 years of Himalayan research.