Karakoram Block Research Articles

Rapid India–Asia Initial Collision Between 50 and 48 Ma Along the Western Margin of the Indian Plate: Detrital Zircon Provenance Evidence

Constraining the collision timing of India and Asia requires reliable information from the coeval geological record along the ~2400 km long collisional margin. This study provides insights into the India–Asia collision at the westernmost margin of the Indian Plate using combined U-Pb geochronological data and sandstone petrography. The study area is situated in the vicinity of Fort Munro, Pakistan, along the western margin of the Indian Plate, and consists of the Paleocene Dunghan Formation and Eocene Ghazij Formation. The U-Pb ages of detrital zircons from the Dunghan Formation are mainly clustered between ~453 and 1100 Ma with a second minor cluster between ~1600 and 2600 Ma. These ages suggest that the major source contributing to the Dunghan Formation was likely derived from basement rocks and the cover sequence exposed mainly in Tethyan Himalaya (TH), Lesser Himalaya (LH), and Higher Himalayan (HH). Petrographic results suggest that the quartz-rich samples from the Dunghan Formation are mineralogically mature and have likely experienced log-distance transportation, which is possible in the case of an already established and well-developed river system delivering the sediments from the Craton Interior provenance. Samples of the overlying Ghazij Formation show a major detrital zircon age clustered at ~272–600 Ma in the lower part of the formation, comparable to the TH. In the middle part, the major cluster is at ~400–1100 Ma, and a minor cluster at ~1600–2600 Ma similar to the age patterns of TH, LH, and HH. However, in the uppermost part of the Ghazij Formation, ages of <100 Ma are recorded along with 110–166 Ma, ~400–1100 Ma, and ~1600–2600 Ma clusters. The <100 Ma ages were mainly attributed to the northern source, which was the Kohistan-Ladakh arc (KLA). The ~110–166 Ma ages are possibly associated with the TH volcanic rocks, ophiolitic source, and Karakoram block (KB). The Paleozoic to Archean-aged zircons in the Ghazij Formation represent an Indian source. This contrasting provenance shift from India to Asia is also reflected in the sandstone petrography, where the sample KZ-09 is plotted in a dissected arc field. By combining the U-Pb ages of the detrital zircons with sandstone petrography, we attribute this provenance change to the Asia–India collision that caused the provenance shift from the southern (Indian Craton) provenance to the northern (KLA and KB) provenance. In view of the upper age limit of the Ghazij Formation, we suggest the onset of Asian–Indian collision along its western part occurred at ca. 50–48 Ma, which is younger than the collision ages reported from central and northwestern segments of the Indian plate margin with 70–59 Ma and 56 Ma, respectively.

Geochronology and petrogenesis of the Miocene S-type Sumayar granite in the Karakoram Block, north Pakistan: Implications for post-collisional magmatism

The genesis and tectonic controls on the emplacement of post-collisional granites in the Karakoram Block (KB) in northern Pakistan are unclear due to lack of detailed studies. Zircon U-Pb dating indicates that the Sumayar pluton was emplaced at ∼ 9.6 Ma and represented one post-collisional magmatism. The Sumayar granite samples belong to strong peraluminum granites with aluminum saturation indexes of 1.1–1.2. Zircons from the granite exhibit strong negative εHf(t) values mostly of between −30 and −25, indicating a supracrustal source. Whole-rock geochemical data display enrichment in light rare earth elements and large ion lithophile elements, depletion in high field strength elements, and strong negative Eu anomalies. The low CaO/Na2O ratios and high Rb/Sr ratios together with the presence of tourmaline, suggest that the primary magma was formed from dehydrating melting of B-rich metapelites at moderate temperature conditions. In comparison with other post-collisional S-type granites in the KB, two groups were recognized. The first group represented by the Baltoro granite and other granites of ca. 36–15 Ma, is volatile-depleted and B-deficient, and formed from partial melting of greywacke source. The second group represented by the Sumayar granite and other small plutons of 9.6–3.0 Ma, formed from dehydration melting of B-rich metapelitic source. The emplacements of these S-type granitoids are attributed to the episodic partial melting of accretionary wedge rocks and associated crustal anatexes, due to the heat generation during the post-collisional stage at the hanging wall along the collisional boundary between the Kohistan Island Arc and the Asian Plate.

Exhumation of the Higher Himalaya: Insights from Detrital Zircon U–Pb Ages of the Oligocene–Miocene Chitarwatta Formation, Sulaiman Fold–Thrust Belt, Pakistan

This study reports the detrital zircon U–Pb ages of the post collisional Chitarwatta Formation, exposed along the western margin of the Indian plate at the Sulaiman fold–thrust belt (SFB), Pakistan. The Chitarwatta Formation overlies the shallow marine carbonate sequence of the Kirthar Formation and represents an Oligocene–Miocene transitional marine sequence. The sequence consists of sandstone, siltstone, and mudstone. The sandstone consists predominantly (79–82%) of quartz grains. The framework grains are sub-angular to sub-rounded and show recycled orogenic provenance. The detrital zircon U–Pb age data show the dominant population between 390 Ma and ~1100 Ma, which is ~70% of the total population. In addition to this, a significant percentage of the younger detrital ages exist between ~40 Ma and ~120 Ma. This younger age cluster indicates the northern sources, including the Kohistan–Ladakh arc (KLA) and Karakoram block (KB), whereas the provenance for the 390–1100 Ma detrital zircon is likely the Higher Himalaya (HH), with contribution from Tethyan Himalaya (TH). This post-collisional scenario suggests that the Chitarwatta Formation received detritus from the northern sources through a drainage system, named as the Indus drainage system. A comparison with the coeval units in the north (Murree Formation, Dagshai Formation, and Dumre Formation) suggests that the sediments may have been delivered through the same drainage system that shares similar detritus. Relying on the contribution of the HH detritus, we propose that the HH uplifted during the Oligocene–Miocene along the Main Central Thrust (MCT) and provided detritus to the foreland basin.

Characteristics, provenance, ore genesis and exploration of the Shimshal Valley placer gold deposits in the Karakoram Block, North Pakistan

The Karakoram Block in North Pakistan formed during the late Permian-Triassic on the southern margin of Asian Plate with a topography characterized by glaciated high mountains. Several rivers and streams draining from these mountain peaks carry detritus with precious and base metals. However, the primary source deposits and their ore genesis remain unknown due to the rugged glaciated topography and inaccessibility for prospecting and in situ sampling. Here, we present a multi-parametric analytical approach including major and trace element characteristics, thermoelectric analysis, EPMA elemental mapping, XRD analysis, and sulfur and lead isotopic studies on the detrital auriferous pyrite from Shimshal Valley placer gold deposits in the Karakoram Block of the north Pakistan to gain insights into the ore genesis, provenance and current erosion or denudation rate of the source primary deposits found in the hinterlands of the Shimshal Valley. The pyrite with varying forms such as {100} + {111} + {210} followed by {210} + {111} and {100} + {210}, which are highly enriched in gold (up to 1700 ppm) as revealed through EPMA analysis. The low to moderate temperature estimated for their formation (100 to 360 °C), pyrite geochemical characteristics, geochemical discrimination diagrams (Fe/S-As, Fe + S-As, As-Co-Ni) and elements contents (Au, Ag, Cu) of the placer gold grains, as well as the high (58.2 %) and low (41.8 %) thermoelectric coefficients of N-type and P-type, suggest that the auriferous pyrite belong to the epithermal type of magmatic-hydrothermal systems. The XnP value (average −48.7 %), γ value (average 62.8 %) in conjunction with the n(S)/n(Fe) values of 1.97–2.11 of pyrite indicate that the placer gold deposits are derived from the middle-upper part of the ore body. The high Mo/Ni (average 5.4) and Co/Ni (average 2.7) ratio combined with detrital zircon UPb geochronology, zircon fertility indicators such as Eu/Eu*, Ce/Nd, (Ce/Nd)/Y, Dy/Yb, (Eu/Eu*)/Y, texture and REE patterns correlate the ore mineralization with the Hunza Plutonic Unit emplaced on the catchment of the valley. The narrow variations in δ34S values, ranging from −5.3 to 3.8 ‰ with an average of −4.7 ‰, suggest that sulfur has a homogeneous magmatic source. The Pb isotopes values range 18.487 to 18.692, 15.713 to 15.745, and 38.964 to 39.416 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively. Our results suggest that metals for ore-forming fluid involved considerable contribution of crustal-derived materials.

Detrital Zircon Provenance of the Cenozoic Sequence, Kotli, Northwestern Himalaya, Pakistan; Implications for India–Asia Collision

This study reported the detrital zircon U-Pb geochronology of the Cenozoic sequence exposed in Kotli, northwestern Himalaya, Pakistan, which forms part of the Kashmir foreland basin. The U-Pb detrital age patterns of the Paleocene Patala Formation show a major age cluster between ~130–290 Ma, ~500–1000 Ma and ~1000–1500 Ma, which mainly resembles the lesser and higher Himalayan sequence. However, the younger age pattern (~130–290 Ma) can be matched to the ages of the ophiolites exposed along the Indus–Tsangpo suture zone. In addition, two younger grains with 57 Ma and 55 Ma ages may indicate a contribution from the Kohistan-Ladakh arc. The detrital zircons in the upper Tertiary sequence show the increased input of younger detrital ages <100 Ma, with more pronounced peaks at ~36–58 Ma, ~72–94 Ma and ~102–166 Ma, indicating the strong resemblance to the Asian sources including the Kohistan–Ladakh arc, Karakoram block and Gangdese batholith. This provenance shift, recorded in the upper portion of Patala Formation and becoming more visible in the upper Tertiary clastic sequence (Kuldana and Murree formations), is related to the collision of the Indian and Asian plates in the northwestern Himalayas. Considering the age of the Patala Formation, we suggest that the Indian and Asian plates collided during 57–55 Ma in the northwestern Himalayas, Pakistan.

Late Jurassic adakite˗like granodiorite along the southern Karakoram block, NE Pakistan: New evidence for subduction initiation of the Neo-Tethys Ocean

The Neo-Tethys Ocean separated the India and the Karakoram block of southern Asia during the late Mesozoic. Most native records have been significantly obscured by subsequent deformation and metamorphism during the early Tertiary India-Asia collision, the consecutively opening and closure of the Neo-Tethys is least understood. This paper presents new petrologic, bulk rock major and trace elements, and zircon U-Pb age and Hf isotopic investigation on the recently identified Karimabad meta˗granodiorites in the southern Karakoram block, N Pakistan. The results indicated these meta˗granodiorites experienced one stage of crystallization (150 ± 0.8 Ma, n = 28, MSWD = 1.5) and at least one stage of metamorphism (59 to 63 Ma). The Late Jurassic meta˗granodiorites display some adakite-like geochemical features, such as high SiO2(67.8–69.3 wt%), Al2O3(16.4–17.7 wt%), Sr (386–476 ppm), low Y (6.5–13.8 ppm) and Yb (0.49–0.98 ppm) contents, and thus high Sr/Y (32.8–72.0) ratios. In addition, they have low MgO (0.67–1.12 wt%), Mg# (31.14–43.08) and K2O/Na2O (0.32–0.46) with positive zircons ԐHf(t) values from +7.1 to +14.0. Few zircon grains from the same meta-granodiorite yielded values around 59–63 Ma, respectively, that may be associated with the metamorphic events associated with the crustal thickening soon before or during the later continental collision. Combining with previous researches, we suggest that these igneous rocks are related to Neo-Tethys oceanic lithosphere break-off beneath the Karakoram block, and the northward subduction initiated at the Late Jurassic (~150 Ma).

EVOLUTION OF KARAKORAM SEQUENCE, HUNZA VALLEY NE PAKISTAN: EVIDENCES FROM FIELD MAPPING AND MICRO-ANALYTICAL WORK

The Karakoram metamorphic Complex (KMC) in the southern Karakoram block is one of the best examples of Barrovian type metamorphism that comprises numerous exhumed metapelite units where a series of low to high grade (green schist to sillimanite facie) rocks are exposed. This sequence shows a complex polyphase history of metamorphism and deformation which offer deeper understanding of collision orogeny. Karakoram metamorphic Complex contains metapelites, meta-carbonates, meta-igneous and amphibolite layers, cross-cut by granite sheets in the northern part. This complex is bounded to the north by the Hunza plutonic unit which is the central portion of the massive east-west trending Karakoram axial batholith and to the southwest by the Main Karakoram thrust (MKT). In this contribution, we provide detail geological mapping, petrography, geochemistry and micro-analytical work using Electron Prob-micro analysis in the central Hunza Valley.

Heavy metal contamination in water and fish of the Hunza River and its tributaries in Gilgit–Baltistan: Evaluation of potential risks and provenance

This study focused on heavy metal (HM) in the water and fish of the Hunza River and its tributaries, located in Gilgit–Baltistan, Pakistan. Water and fish samples were collected and the concentrations of Cd, Ni, Co, Fe, Pb, Cr, Cu, and Zn were analyzed. The maximum HM concentration observed was 87.2 μ g/L for Fe in water at the Kohistan Block (downstream); the minimum HM value was found for Cd at 1.02 μ g/L from the Karakoram Block (midstream). Physicochemical parameters (basic parameters, anions, and HM) were observed to be within the drinking water guidelines set by the World Health Organization, except for Ni. Most physicochemical parameters showed an increasing trend from the Wakhan Block (upstream) to downstream. The concentration of HM in water and fish muscle was analyzed to evaluate potential health risks including chronic daily ingestion (CDI) and hazard quotient (HQ). Children showed the highest CDI of 53.2 μ g/kg-day of Fe through water intake and 48.1 μ g/kg-day of Ni via fish consumption. The HQ values of HM through water and fish consumption were <1. Provenance analyses of HM suggest that geogenic sources were the main source of water contamination.

Tectono‐geomorphic evolution of Harlik Mountain in the Eastern Tian Shan, insight from thermochronological data and geomorphic analysis

The Harlik Mountain is in the easternmost part of the Tian Shan and is a prime example of an intracontinental orogenic belt. Several studies have used low‐temperature thermochronology to understand the uplift history of this range. However, complicated structures have a profound impact on the study of the tectonic evolution of the Harlik Mountain during the Mesozoic‐Cenozoic. Here, we refine the structural characteristics of the Harlik Mountain and then acquire the detailed thermal histories in different parts of the range. Through field observations, we demonstrate that the brittle Harlik Fault experienced two activities, including early normal faulting and late right‐lateral strike‐slip normal faulting. In particular, geomorphic and river analyses indicate that the topography of the Harlik Mountain was significantly influenced by faults. Combined with the thermochronological data from previous studies, thermal history modelling based on our new data on the Harlik Mountain suggest three fast cooling phases in the 128–110 Ma, 70–55 Ma, and 50–35 Ma. The first and the third cooling phases were associated with fault activities and the second cooling phase was related to regional denudation. The first phase of faulting, in the late Early Cretaceous, may be caused by stress relaxation after the Cimmerian collision. The second phase of faulting was likely to relate to local stress adjustment from the India‐Eurasia collision during the Eocene to Oligocene. Moreover, Late Cretaceous to Palaeocene regional cooling was probably affected by the collision of the Karakoram Block with Eurasia.

Occurrence, source identification and potential risk evaluation of heavy metals in sediments of the Hunza River and its tributaries, Gilgit-Baltistan

Abstract The present study examined heavy metals concentrations in sediments of the Hunza River and its tributaries, Gilgit-Baltistan, Pakistan. For this purpose, surface sediments were sampled from the Hunza River and its tributaries along with the three main geological units i.e. Wakhan Block (upstream), Karakoram Block (midstream) and Kohistan Block (downstream). Heavy metals concentrations in each sample were analyzed using the atomic absorption spectrometry (AAnalyst 700, PerkinElmer, USA). Results showed the highest mean concentration for iron (Fe, 44400 mg/kg) and the lowest was observed for cadmium (Cd, 0.62 mg/kg). Heavy metals concentration showed an increasing trend in sediments with the downward flow and resulted in higher values along the Kohistan Block (downstream) and lowest in the Wakhan Block (upstream). Determined heavy metals concentration were evaluated for the pollution quantification factors. The values of contamination factor (CF) and pollution load index (PLI) showed heavy metals contaminations in the sediment of the Hunza River and its tributaries. The highest (175) values of ecological risk index (ERI) were observed for Cd and classified at a moderate-risk level to the aquatic ecosystem. Statistical analyses revealed that natural denudation of bedrock and mineralized zone led to heavy metal contamination in sediments of the Hunza River and its tributaries with lesser input from anthropogenic activities.

Early Cretaceous to Eocene magmatic records in Ladakh arc: Constraints from U–Pb ages of Deosai volcanics, northern Pakistan

This research provides a detailed U–Pb geochronological record of Deosai volcanics in the western Himalaya. The volcanics comprise basalt, andesite, dacite, and rhyolite and are part of the Kohistan–Ladakh arc, exposed east of the Nanga Parbat–Haramosh Massif. Based on the U–Pb ages, this volcanic suite is classified into three chronostratigraphic units: (a) Chota Pani volcanics, (b) Shatung Nala volcanics, and (c) Bara Pani volcanics, with ages ranging between 123–115, 86–63, and 48–46 Ma, respectively. This study suggests that the Chota Pani volcanics are arc‐related, as their ages coincide well with the timing of initiation of the Kohistan–Ladakh arc. The Shatung Nala volcanics may have formed as a result of a collision between Kohistan–Ladakh arc and Karakoram Block (Eurasian Plate). Moreover, the U–Pb cluster age of Bara Pani volcanics coincides with the initial collision time between Indian and Eurasian plates in the western Himalaya, which strongly suggests its emplacement associated with the continental–continental collision. In addition, the dated granitic samples, yielding 120.9 ± 4.5, 80.2 ± 1.4, 50.42 ± 0.55, and 45.24 ± 0.64 Ma ages, provide further support that the emplacement of Deosai volcanic rocks took place as a consequence of closure of the northern and southern sutures, respectively.

Structure of crust and upper mantle beneath NW Himalayas, Pamir and Hindukush by multi-scale double-difference seismic tomography

The India-Asia collision and subsequent subduction initiated the evolution of major tectonic features in the Western Syntaxis. The complex tectonic structure and shallow to deep seismicity have attracted geoscientists over the past two decades. The present research is based on a 3D tomographic inversion of P-wave arrival time data to constrain the crustal and upper mantle structure beneath the NW Himalayas and Pamir-Hindukush region using the Double-difference tomography. We utilized a very large multi-scale dataset comprising 19,080 earthquakes recorded at 397 local and regional seismic stations from 1950 to 2017. The northward dipping seismic zone coinciding with the low velocity anomaly suggests the subduction of the Indian lower crust beneath the Hindukush. The extent of the northward advancing Indian slab increases from east to west in this region. We observed no signs of northward subduction of the Indian plate under the Hindukush beyond 71°E longitude. The Indian plate overturns due south after interacting with the Asian plate beneath the southern Pamir, which correlates with the counter-clockwise rotation of the Indian plate. The Asian plate is also imaged as a southward subducting seismic zone beneath the southern Pamir. In the NW Himalayas, the northward subducting Indian plate appears as a gently dipping low velocity anomaly beneath the Karakoram Block. The stresses caused by the collision and subduction along the Shyok Suture and Indus Suture are translated to the south. The crustal scale seismicity and high velocity anomalies indicate an intense deformation in the crust, which is manifested by syntaxial bends and thrust faults to the south of the Main Mantle Thrust.

Electrical resistivity structures and tectonic implications of Main Karakorum Thrust (MKT) in the western Himalayas: NNE Pakistan

We discovered a conductive zone along Main Karakoram Thrust which could be an indication of flat subduction of Kohistan island arc beneath the Eurasian plate. Kohistan island arc collided with the Karakoram Block of the Eurasian Plate in the Early Cretaceous. However, according to findings of many researchers, the subduction ceased about 75 Ma ago. The presence of the conductive zone is an indication of current magmatism or hydrothermal fluids. Maximum low-frequency band data from Fourteen sites with recording periods of 10−2–103 s was acquired along a profile crossing MKT. Our results reveal the existence of multiple low resistivity zones beneath the region extending from shallow to the depths of more than 100 km. These low-resistivity zones might be a signature of the ongoing magmatic activities or hydrothermal fluids along the Shyok Suture Zone. In addition, we discovered another large conductive body towards the south of the study area which could be a result of uprising magmatic plumes generated by the subducting Indian plate along the Indian suture zone and their entrapment in the overlying Kohistan block.

A Cretaceous forearc ophiolite in the Shyok suture zone, Ladakh, NW India: Implications for the tectonic evolution of the Northwest Himalaya

The northwestern Himalaya in India contains critical evidence of the convergent margin and collision tectonics between the Ladakh arc and the Karakoram block. Here we present new petrochemical and geochronological data from a forearc ophiolite at the Shyok‐Nubra river confluence. Whole-rock geochemical data show relatively high large-ion lithophile elements (LILE), light rare earth elements (LREE), and Ba, U and Pb anomalies, and depletions in La, Ce and Zr, particularly, the high-field strength elements (e.g. Nb, Ta); these geochemical characteristics are similar to those in modern ophiolites that formed in arc-related environments. Meta-volcanic greenschists that contain spinel layers have high MgO, Ni, Co, and Cr contents. In contrast, they have low contents of TiO2, very low Nb and Zr that are diagnostic of high‐Ca boninitic magmas in modern forearc settings, as in Izu–Bonin–Mariana (IBM) and Tonga. The spinels have high Cr# [Cr/(Cr+Al)] and Mg#[Mg/(Mg+Fe2+)], which are characteristic of spinels in forearc boninite‐type of melts. The spinel-free meta-volcanic samples have incompatible trace element abundance patterns similar to those of MORB. However, their enrichments in Cs, Rb, Pb, U and depletion in HFSE may reflect an input of subduction fluids that are different from MORB; these MORB-like basalts are suggestive of a forearc complex that erupted prior to the formation of boninitic rocks.Our data from the Shyok ophiolite indicate the existence of supra-subduction rocks on the southern Karakoram margin. Albite porphyroblasts in greenschists yield a K–Ar age of 104.4±5.6Ma that represents the time of early exhumation of the greenschists. The boninite‐type melts formed prior to 104Ma, thereby indicating that the time of initiation of subduction on the southern margin of the Karakoram block was before 104Ma. These geochemical signatures together with the spatial and temporal distribution of the arc rocks on the southern margin of Karakoram block suggest northward subduction of NeoTethys in the Early Cretaceous. The subsequent collision between the Ladakh arc and the Karakoram block thrusted/obducted the forearc ophiolite onto the southern edge of the Karakoram block probably between 74Ma and 97Ma.

Multiple garnet growth in garnet–kyanite–staurolite gneiss, Pangong metamorphic complex, Ladakh Himalaya: New constraints on tectonic setting

Garnet–kyanite–staurolite gneiss in the Pangong complex, Ladakh Himalaya, contains porphyroblastic euhedral garnets, blades of kyanite and resorbed staurolite surrounded by a fine-grained muscovite-biotite matrix associated with a leucogranite layer. Sillimanite is absent. The gneiss contains two generations of garnet in cores and rims that represent two stages of metamorphism. Garnet cores are extremely rich in Mn (XSps=0.35–0.38) and poor in Fe (XAlm=0.40–0.45), whereas rims are relatively Mn-poor (XSps=0.07–0.08), and rich in Fe (XAlm=0.75–0.77). We suggest that garnet cores formed during prograde metamorphism in a subduction zone followed by abrupt exhumation, during early collision of the Ladakh arc and Karakoram block. The subsequent India–Asia continental collision subducted the metamorphic rocks to a mid-crustal level, where the garnet rims overgrew the Mn-rich cores at ca. 680°C and ca. 8.5kbar. PT calculations were estimated from phase diagrams calculated using a calculated bulk chemical composition in the Mn–NCKFMASHT system for the garnet–kyanite–staurolite-bearing assemblage. Muscovites from the metamorphic rocks and associated leucogranites have consistent K–Ar ages (ca. 10Ma), closely related to activation of the Karakoram fault in the Pangong metamorphic complex. These ages indicate the contemporaneity of the exhumation of the metamorphic rocks and the cooling of the leucogranites.

Mineral chemistry and K–Ar ages of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh Himalaya, India

Petrographic analyses of plutons across the Karakoram fault in the Shyok-Nubra confluence of northern Ladakh area revealed that the rocks consist of diorite, granodiorite, granite and leucogranite. Pyroxene and biotite chemistries by EPMA indicate that most of the diorite, granodiorite and granite are calc-alkaline and formed in an arc-related tectonic setting. These calc-alkaline plutons yield K–Ar biotite ages from 49.3 ± 1.1 to 51.2 ± 1.1 for Ladakh batholith in Khardung La and from 60.8 ± 1.3 to 65.8 ± 1.4 Ma for granites and diorites in Hundar area. The K–Ar ages of the Hundar igneous complex are within the age range of 46–70 Ma of the Ladakh batholith near Leh town and the neighbor sites, indicating the Hundar igneous complex as a part of the Ladakh arc. Their ages are also coincident with the Khardung volcanics (67.4 and 60.5 Ma). The calc-alkaline granites from Panamik of Karakoram block yield K–Ar biotite ages of 95.7 ± 2.1 and 96.7 ± 2.1 Ma, suggesting that the granites are member of the plutons (90–120 Ma) occurring in northern Pakistan, Tangtse in Ladakh and western Tibet that emplaced during subduction of the Neo-Tethyan oceanic plate under the Karakoram block. The leucogranite and granodiorite in the Panamik region yield K–Ar biotite and muscovite ages ranging from 9.18 ± 0.21 to 9.45 ± 0.21 Ma. The leucogranites are the S-type related to post-collision tectonic setting and are considered to have emplaced in relating to activation of the Karakoram fault.

Ion microprobe U-Pb ages of the Khunjerab granodiorite and some granitoids from Karakoram, Pakistan

The Karakoram block was located at the southern margin of Asian continent before the Late Cretaceous amalgamation of the Kohistan block to Asia. The northward subduction of the ocean under the Asian continent was considered to form Early Cretaceous continental arc magmatism in the Karakoram block. The Karakoram Batholith is a major granitic body in the Karakoram block, extending from Baltoro to Chitral regions. However, several smaller granitic bodies are present to north of the Karakoram Batholith, which comprise the Khunjerab-Tirich Mir granite belt. Ogasawara et al. (1992) investigated several granitoids of this belt in the Khunjerab valley. They reported hornblende and biotite K-Ar ages of the granitoids. As the Karakoram block received continued tectonic activity since the Cretaceous to Cenozoic, the K-Ar ages may not provide clear timing of the primary igneous events. Some U-Pb zircon ages are available for the Karakoram Batholith, however, the U-Pb ages for the granitic rocks in the Khunjerab-Tirich Mir granite belt are limited (Hildebrand et al. 2001). It is important to have precise U-Pb zircon ages of the granitic rocks to understand timing of igneous activities and related tectonism of the region. Thus objectives of this study are (1) to provide precise timing of the igneous activity and (2) to search for the inherited core of the zircon and its age to understand source rock characteristics of the granitoids. The Khunjerab granodiorite occurs around the Khunjerab Pass 60 km north of the Karakoram batholith, mainly along the northern side of the Khunjerab valley. Small outcrop of the granodiorite is found 10 km west of the Khunjerab Pass along the Karakoram Highway, and a sample was collected for the analysis from this outcrop. North Sost pluton is found in a 3.8 km section along the Karakoram Highway in the Khunjerab valley, 8 km north of Sost. Although outcrops of the granitoids are limited in the Khunjerab valley, the granitoids are exposed extensively to the north of the Misgar, west of the Khunjerab valley. This pluton is also evaluated in this study. However, the granitoids contain very small amount of zircon grains. A high resolution ion microprobe (Cameca ims 1270) at the Geological Survey of Japan was used to obtain U-Pb zircon ages. Primary ion beam was about 25 micron diameter with intensity of 2 nA. Mass resolution was set at about 5000. A standard used for the U/Pb calibration is AS-3 from Duluth complex. Zircons were separated with magnetic and heavy liquids methods. Abundant zircon grains were obtained from the Khunjerab granodiorite. Typical size of the zircons from the granodiorite ranges 200 to 400 micron meters in length. Those grains show weak oscillatory zoning, however, inherited core is not common. 206

New record of Bathonian–Callovian calcareous nannofossils in the eastern Karakoram block: a possible clue to understanding the dextral offset along the Karakoram Fault

AbstractWe report the first record of Bathonian–Callovian calcareous nannofossils from a marine sedimentary sequence of the eastern Karakoram block, in northern India. The calcareous mudstones and packstones, occasionally bearing red chert nodules, yielded calcareous nanofossils and Middle Jurassic Choffatia furcula ammonoids. Middle to Upper Jurassic nannofossil assemblage is dominated by representatives of the genus Watznaueria. The occurrence of Ansulasphaera helvetica whose range is Upper Bathonian–Upper Callovian, indicates a correlation with nannofossil zones NJ12–13. The occurrence of Cyclagelosphaera wiedmannii further infer an Upper Bathonian–Callovian age. These specimens show affinities with those found in a similar sedimentary formation exposed in north Karakoram. This suggests the existence of a narrow and elongated sedimentary basin, oriented in a NW–SE direction, at a latitude of c. 25°–30°N. At that time, the Karakoram block was situated near the already welded Qiangtang block of Asia. The northern and eastern Karakoram blocks were connected during Middle Jurassic. The activity and dextral offset of the Karakoram fault separated the Jurassic sedimentary formations of the northern and eastern Karakoram blocks by c. 150 km.

Assembly of the Pamirs: Age and origin of magmatic belts from the southern Tien Shan to the southern Pamirs and their relation to Tibet

Magmatic rocks and depositional setting of associated volcaniclastic strata along a north‐south traverse spanning the southern Tien Shan and eastern Pamirs of Kyrgyzstan and Tajikistan constrain the tectonics of the Pamirs and Tibet. The northern Pamirs and northwestern Tibet contain the north facing Kunlun suture, the south facing Jinsha suture, and the intervening Carboniferous to Triassic Karakul–Mazar subduction accretion system; the latter is correlated with the Songpan‐Garze–Hoh Xi system of Tibet. The Kunlun arc is a composite early Paleozoic to late Paleozoic‐Triassic arc. Arc formation in the Pamirs is characterized by ∼370–320 Ma volcanism that probably continued until the Triassic. The cryptic Tanymas suture of the southern northern Pamirs is part of the Jinsha suture. A massive ∼≤227 Ma batholith stitches the Karakul–Mazar complex in the Pamirs. There are striking similarities between the Qiangtang block in the Pamirs and Tibet. Like Tibet, the regional structure of the Pamirs is an anticlinorium that includes the Muskol and Sares domes. Like Tibet, the metamorphic rocks in these domes are equivalents to the Karakul–Mazar–Songpan‐Garze system. Granitoids intruding the Qiangtang block yield ∼200–230 Ma ages in the Pamirs and in central Tibet. The stratigraphy of the eastern Pshart area in the Pamirs is similar to the Bangong‐Nujiang suture zone in the Amdo region of eastern central Tibet, but a Triassic ocean basin sequence is preserved in the Pamirs. Arc‐type granitoids that intruded into the eastern Pshart oceanic‐basin–arc sequence (∼190–160 Ma) and granitoids that cut the southern Qiangtang block (∼170–160 Ma) constitute the Rushan‐Pshart arc. Cretaceous plutons that intruded the central and southern Pamirs record a long‐lasting magmatic history. Their zircons and those from late Miocene xenoliths show that the most distinct magmatic events were Cambro‐Ordovician (∼410–575 Ma), Triassic (∼210–250 Ma; likely due to subduction along the Jinsha suture), Middle Jurassic (∼147–195 Ma; subduction along Rushan‐Pshart suture), and mainly Cretaceous. Middle and Late Cretaceous magmatism may reflect arc activity in Asia prior to the accretion of the Karakoram block and flat‐slab subduction along the Shyok suture north of the Kohistan‐Ladakh arc, respectively. Before India and Asia collided, the Pamir region from the Indus‐Yarlung to the Jinsha suture was an Andean‐style plate margin. Our analysis suggests a relatively simple crustal structure for the Pamirs and Tibet. From the Kunlun arc in the north to the southern Qiangtang block in the south the Pamirs and Tibet likely have a dominantly sedimentary crust, characterized by Karakul–Mazar–Songpan‐Garze accretionary wedge rocks. The crust south of the southern Qiangtang block is likely of granodioritic composition, reflecting long‐lived subduction, arc formation, and Cretaceous‐Cenozoic underthrusting.

New field, structural and geochronological data from the Shyok and Nubra valleys, northern Ladakh: linking Kohistan to Tibet

Abstract The Nubra-Shyok confluence in northern Ladakh is a key area for understanding the tectonic evolution of NW Himalaya and provides the basis for linking the geology of Pakistan to that of Tibet. The geology of the confluence area has been the subject of much speculation centred mainly on the existence of ophiolites and their regional significance. These ophiolites are thought to represent the eastward extension of the Shyok Suture Zone (SSZ), which separates the Dras island arc from the southern margin of Eurasia, and which was overprinted by movement along the Khalsar Thrust (often thought to represent the eastern continuation of the Main Karakoram Thrust). The geology of the area is relatively complex and the little information available has hampered regional geological correlations. The Khalsar Thrust (KT) and the dextral Karakoram Fault (KF), two regional tectonic features of NW Himalaya, merge at the confluence defining a triple point and three blocks: the Ladakh block to the south, the Saltoro block to the northwest, and the Karakoram block to the northeast. Close to the triple point, the KF changes strike and movement direction. Movement vector analysis of the triple point indicates that the KT and the two parts of the KF could have moved contemporaneously, and allows prediction of the movement vectors across the faults. The KT and KF shear preferentially volcano-sedimentary rocks of the Shyok and Nubra formations, respectively. Contrary to previous interpretations, these sheared rocks do not represent disrupted ophiolites. Regional tectonic reconstructions, however, require suturing between the Ladakh block and Eurasia and the strike of the SSZ in Baltistan suggests that the suture zone might crop out north of the KT, either along the southern slopes of the Saltoro Range or further north along the Saltoro valley. In the few outcrops of the Saltoro block we were able to visit, we found no evidence of ophiolitic rocks. Instead we found outcrops of the calc-alkaline Tirit batholith. Although our observations do not confirm the presence of the suture-related rocks in the southern Saltoro block, this possibility cannot be ruled out. Zircons from a sample of Tirit granite (U-Pb ion-microprobe age) yielded an age centred at 68 ± 1 Ma. The similar range of modal composition and age of the Tirit and Ladakh batholiths suggest that they are part of the same magmatic event. This result and a number of other observations indicate that the post-75 Ma geology of the Ladakh and Saltoro blocks is similar. Thus, if there is a suture zone in the southern Saltoro block, suturing must have occurred before 75 Ma, as concluded by others along the same tectonic boundary to the west in Pakistan. The KF represents a much younger terrane boundary, juxtaposing rocks of the Ladakh and Saltoro blocks to those of the Karakoram terrane. Rocks related to suturing of continents were not found along the KF. Karakoram leucogranites cropping out in the southern part of Karakoram terrane yielded a U-Pb zircon age centred at 15.0 ± 0.4 Ma (2σ). Because these leucogranites were not found south of the KF, this fault must have initiated after leucogranite intrusion and must therefore be younger than 15 Ma old. At the confluence the KF cuts across the regional rock sequence than can be followed from Kohistan into Baltistan and into the confluence area. Movement on the fault displaces the sequence by approximately 150 km to southeastern Tibet where the regional rock sequence can be regained.