Linzhou Basin Research Articles

Sedimentary Record of the middle Cretaceous uplift across the Gangdese magmatic arc system in Southern Tibet

AbstractSedimentary basins adjacent to subduction‐related continental arcs provide important archives for deciphering the intricate history of convergent plate margins. The east‐west trending Gangdese magmatic arc was one of the most predominant topographic features located at the southern margin of Tibet before the arrival of the Indian plate. However, the detailed Cretaceous growth and evolution across the arc system remains ambiguous. Stratigraphy of the adjacent Xigaze forearc basin provides a well‐preserved and well‐exposed record of the tectonic and magmatic evolution of the arc throughout the Cretaceous period. We report new stratigraphic, sedimentological, geochronological, and provenance analyses of the Quarry Ridge sandstone in the Xigaze forearc basin along with compiled zircon U‐Pb ages (n = 9674) and Lu‐Hf isotopic signatures (n = 3389) from the Gangdese arc, the Xigaze forearc basin, and the Linzhou retroarc foreland basin to reconstruct the Early to middle Cretaceous magmatism and uplift of the Gangdese arc and concurrent sedimentary responses within both basins. Exhumation of the arc initiates at around 113 Ma suggested by arc detritus first arriving in both basins. Another episode of inferred uplift occurs at around 108 Ma, which resulted in coarse‐grained sedimentation in adjacent basins, preventing Central Lhasa detritus from reaching the Xigaze forearc basin further south and a facies and provenance change within the Linzhou basin. Finally, a third episode at around 101 Ma is reflected by deposition of the progradational Quarry Ridge clastic succession and marks the initiation of a substantial coarse‐grained depositional stage in the Xigaze forearc basin. Our study emphasizes the connection between coarse‐grained deposition in the forearc basin and arc magmatism and uplift. This study also provides an orogen‐scale assessment of the history of arc magmatism, uplift, and sedimentation across the Gangdese magmatic arc system, which supports interpretations that Tibet was already characterized by complex and substantial topographic relief during the Cretaceous before the collision between the Indian and Eurasian plates.

Remagnetization Under Hydrothermal Alteration of South Tibetan Paleocene Lavas: Maghemitization, Hematization, and Grain Size Reduction of (Titano)magnetite

AbstractThe Paleocene lavas from Dianzhong Formation (E1d) in Linzhou basin of southern Lhasa terrane are a key target for paleomagnetic investigations into the timing and paleolatitude of the initial India‐Asia collision. Controversy exists, however, on whether these rocks preserve a primary remanent magnetization. Here we reanalyze previously published thermal demagnetization data and report detailed rock magnetic results and petrographic observations of these rocks. We find that the original magnetic carrier, a magmatic multidomain Ti‐poor titanomagnetite, underwent significant grain size reduction and was variably reacted to single‐domain maghemite and nano‐hematite. Such strong alteration may have resulted from successive hydrothermal events: a first event related to the ∼52 Ma dike intrusions into the E1d that accompanied a massive ignimbrite eruption deposited above the E1d producing heating up to 300°C; and a secondary event related to the 42–27 Ma southward overthrusting of the basin, heating the E1d up to 130–145°C. Unblocking/inversion temperature spectra of the authigenic maghemite and nano‐hematite overlap with those of the titanomagnetite, implying that the primary remanence of the E1d lavas has been contaminated or replaced by thermoviscous and chemical remanent magnetizations. Thus the isolated characteristic remanent magnetization from these rocks, whether slightly or completely altered, cannot be considered primary and should not be used for paleolatitudinal determination. Our study confirms that hydrothermal alteration can seriously jeopardize the remanence carried by titanomagnetite and thus should be tested for paleomagnetic investigations of rock units from tectonically active areas.

Late Cretaceous−early Paleogene rise of the Gangdese magmatic arc (south Tibet) from sea level to high mountains

Recognition of the existence of an Andean-type continental margin in southern Tibet prior to its collision with India has provided crucial constraints on the formation of the Tibetan Plateau and South Asian climate evolution. Here, we focused on well-dated Late Cretaceous successions in the Linzhou Basin and determined the elevation changes from sea level to high mountains in the Gangdese magmatic arc in southern Tibet. Our results show that the Linzhou Basin was still submerged in the Tethyan Sea at around 92 Ma when it accumulated Cenomanian (ca. 105−92 Ma) shallow-marine orbitolinid-bearing limestones (Takena Formation); these limestones are unconformably overlain by Campanian (83−78 Ma) fluvial-lacustrine deposits (Shexing Formation) after an ∼9 m.y. depositional hiatus. A prominent unconformity between the tightly folded Shexing Formation and gently tilted overlying Paleocene−Eocene Linzizong successions represents the formation of the regional Lhasaplano ca. 70 Ma, which may have been linked to the rise of an Andean-type Gangdese mountain range along the southern margin of the Lhasa terrane. Using carbonate oxygen isotope and clumped isotope thermometry data from the Shexing Formation paleosols, we quantitatively documented the rise of the Andean-type Gangdese Mountains with a peneplain surface at an elevation of 2.7 +0.5/−0.9 km above sea level prior to onset of the India-Asia collision ca. 65−63 Ma. This scenario of surface gain may have been an isostatic compensation response to the crust thickening to over 50 km during the Late Cretaceous. Subsequently, the surface isostatically rose to its near-present elevation of ∼4.6 km due to the removal of overthickened mantle lithosphere and revival of intense Gangdese magmatism by 56 Ma.

Hydrothermal events in the Linzizong Group: Implications for Paleogene exhumation and paleoaltimetry of the southern Tibetan Plateau

Knowledge of the thermal history of the Linzizong Group (69–47 Ma) within the Gangdese arc is critical for interpreting the geologic evolution and isotope-based paleoaltimetric results of the southern Lhasa terrane of the Tibetan Plateau. Here, we combine results from geochronologic and thermochronologic studies of this group (divided into Dianzhong, Nianbo, and Pana formations upsection) and the structurally overlying Qianggeren granite (∼52 Ma) in the Linzhou basin. Whole rock 40Ar/39Ar ages of volcanic rocks from the stratigraphically lower Dianzhong and Nianbo formations are ∼10 Myr younger than their corresponding zircon U-Pb ages, suggesting a thermal disturbance of the argon system. Zircon (U-Th)/He ages (ZHe, 63 dates) range from 54 to 24 Ma, and apatite (U-Th-Sm)/He ages (AHe, 43 dates) range from 27 to 4 Ma. Inverse modeling of the thermochronologic data from the Qianggeren granite indicates rapid cooling between 42 and 26 Ma, possibly induced by movement of the Gulu-Hamu thrust. Positive correlations between ZHe ages and effective uranium and other geologic observations provide evidence that the Dianzhong and Nianbo formations were heated 300°C at 54–50 Ma, and that the entire Linzizong Group was variably heated to 130–170°C at 42–27 Ma. These findings, together with published geochronologic and thermochronologic data, suggest that abundant 50–45 Ma zircon fission track and ZHe ages from the Gangdese arc likely reflect conductive cooling of the Gangdese arc after a ∼52 Ma magmatic flare-up episode rather than rapid regional exhumation. Areas far from Cenozoic faults and deeply incised river valleys in southern Tibet have experienced only ∼3 km of exhumation since 45 Ma, consistent with the establishment of a low-relief, plateau-like physiography by Eocene time. The (hydro)thermal events are also manifested by widespread calcite recrystallization and δ18O and Δ47 alteration in most carbonates from the Linzizong Group, which must be taken into account in past and future carbonate-based paleoaltimetric studies.

Origin of the volcanic rocks in Dianzhong Formation, central Lhasa Terrane, Tibet: implication for the genesis of syn-collisional magmatism and Neo-Tethyan slab roll-back

ABSTRACT The origin and formation of the continental collision-related magmas remain elusive. The volcanic rocks erupted during India-Asia continental collision offer an ideal opportunity to explore their genesis and geodynamic process. Here, we report new zircon U-Pb dating results and Hf isotope, whole-rock element and Sr-Nd isotope data of volcanic rocks in Linzhou Basin, central Lhasa Terrane, southern Tibet. These volcanic rocks are mainly comprised of andesites and belong to Dianzhong Formation. The timing of their formation is ca. 63–66 Ma, coeval with the India-Asia continental initial collision in the central part of southern Tibet. All these rocks show an arc-like geochemical affinity and they have more depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.705006–0.705963, εNd(t) = −1.78 to 3.52, zircon εHf(t) = 1.2–7.0) than the Cretaceous pre-collisional andesites and Eocene ancient lithospheric mantle-derived melts beneath the central Lhasa Terrane. The correlation between Mg# and CaO, TiO2, Al2O3, Sr/Y, 87Sr/86Sr, εNd(t) suggests that they were likely hybrid production between an isotopically depleted end-member and the enriched lithospheric mantle (ELM). The oceanic crust-derived melts would be the best candidate for the former end-member according to their depleted Sr-Nd isotopic compositions. The andesitic rocks of Dianzhong Formation in the central Lhasa Terrane were most likely stemmed from partial melts of altered Neo-Tethyan crust and then mixed with the ELM-derived melts. Given the Cretaceous-Tertiary upper crustal shortening, back-arc extension, and voluminous volcanism in the Lhasa Terrane, along with abrupt increasing in the magma temperature and the convergence rate between India and Asia during Palaeocene, the occurrence of the Dianzhong Formation volcanic rocks can be well explained by the Neo-Tethyan slab roll-back. The mixing between the oceanic crust- and the continental lithospheric mantle-derived melts induced by the oceanic slab roll-back would be a complimentary scenario for the formation of the syn-collisional magmatism in collisional orogeny belts.

Location of the Lhasa terrane in the Late Cretaceous and its implications for crustal deformation

The Cenozoic crustal deformation of the Asian continent is constrained by the location of its southern margin in the Late Cretaceous, which is, however, affected by potential remagnetization and inclination shallowing of redbeds in the Linzhou Basin. Our paleomagnetic results from the Shexing Formation redbeds in the Late Cretaceous yield a mean paleo-inclination of ~21.5°, consistent with the recalculation of previous data. Rock-magnetic, paleomagnetic and petrographic analyses indicate that the redbeds have recorded the original paleomagnetic field and have not suffered from significant inclination shallowing. The paleopole from 70 redbeds sites is at 71.8°N, 281.5°E with A 95 = 2.6°, yielding a paleolatitude of ~12.0 ± 2.6°N at ~70–91 Ma for the Linzhou Basin. This result indicates a latitudinal convergence of 1264 ± 477 km north of the sampling area within Asia since the Late Cretaceous. Combined with previous data, our results show that the southern margin of Asia was at ~11–15°N in the Late Cretaceous, aligning in a WNW-ESE direction, which is similar to its present-day orientation. A review of available data shows that the sharp bending of the southern margin of Asia around the eastern Himalayan syntaxis (EHS) is probably due to different crustal movement patterns around the EHS. The shape of the southern margin of Asia determines the arc-shaped orientation of Himalayan orogen. • The Shexing Formation redbeds have recorded the prefold magnetizatioin. • The southern margin of Asia was oriented in a WNW-ESE direction at ~12°N. • Different movement around the eastern Himalayan syntaxis make a sharp turn. • The southern margin of Asian plays a primary role in shaping the Himalayan orogen.

An Initial Collision of India and Asia in the Equatorial Humid Belt

AbstractPaleocene lavas of the Dianzhong Formation preserved in the Linzhou Basin of South Tibet provide a unique opportunity to constrain the initial geometry of the India‐Asia collision; however, earlier studies argued a complex magnetic signature resulting from thermal and/or chemical remagnetizations. To better characterize the remanences obtained from the Dianzhong lavas, we carried out an intraformational conglomerate test on a previously‐studied section in the Linzhou Basin. The positive conglomerate test suggests that the characteristic remanences reported from the Dianzhong Formation are primary. The updated Paleocene pole confirms a paleolatitude of 6.7° ± 4.4°N for the Lhasa terrane and positions the southern margin of Asia in the equatorial humid belt. An initial collision, between India, Asia and an intra‐oceanic arc in the equatorial humid belt, may have intensified silicate weathering and resulted in an extra consummation of carbon dioxide, which contributes to a long‐term cooling of the Earth during the Cenozoic.

From extension to tectonic inversion: Mid-Cretaceous onset of Andean-type orogeny in the Lhasa block and early topographic growth of Tibet

AbstractRecent studies have indicated that an Andean-type orogen (Lhasaplano) developed on the Lhasa block in the Cretaceous during northward subduction of Neo-Tethyan oceanic lithosphere. When and how uplift of the Lhasaplano began, however, has remained controversial. This article integrates stratigraphic, sedimentological, tectonic, and provenance data from the latest marine to nonmarine strata in the Linzhou Basin to pinpoint the early topographic growth in southern Tibet. The Takena Formation mainly consists of lagoonal carbonates and mudstones yielding foraminiferal assemblages of Early Aptian age (ca. 123–119.5 Ma). The conformably overlying lower member of the Shexing Formation, mainly deposited in fluvial environments, was fed by volcanic and sedimentary rock fragments from the north Lhasa terrane. Clasts of the Gangdese arc to the south firstly appeared in the middle member and became dominant in the upper member of the Shexing Formation. By contrast, coarse grained, braided river facies occur in the uppermost part of the Shexing Formation, where detritus was mostly recycled from Paleozoic strata of north Lhasa, with minor volcaniclastic contribution from the Gangdese arc. Basin analysis indicates accelerating subsidence and sedimentation rates during deposition of Takena to middle Shexing strata (ca. 125–108 Ma), followed by steady subsidence during deposition of upper Shexing strata (ca. 108–96 Ma). Given this regional tectonic and sedimentary evidence, such an evolution is interpreted to reflect tectonic extension followed by thermal subsidence. Basin inversion and regional compression initiated during deposition of the uppermost Shexing strata (ca. 96 Ma), as indicated by active thrust faults and widespread accumulation of syntectonic conglomerates in the western part of the Lhasa block. This event marked the beginning of the Andean-type orogeny in southern Tibet. Such a paleotectonic evolution, from extension to tectonic inversion, is also documented in the Andes mountain range. It may be typical of the early stage growth of Andean-type active continental margins.

Geochemistry and petrogenesis of sandstones and their basaltic interlayers of Shexing Formation from Linzhou basin, South Tibet

印度与亚洲大陆的碰撞是青藏高原演化的重要构造事件，碰撞过程被记录在拉萨地块南部的晚白垩世到古新世的沉积-岩浆作用中。林周盆地的晚白垩世设兴组及其之后不整合覆盖的林子宗火山岩，是解析碰撞过程的重要记录。本文对设兴组最高层位的砂岩和玄武岩夹层进行了岩石学、地球化学和年代学研究，探讨了岩石成因和构造意义。设兴组砂岩属于杂砂岩，碎屑物质主要来自中酸性岩浆岩源区；锆石Hf同位素指示设兴组大部分碎屑物质来源于盆地北面的中部拉萨地块，少部分来自盆地南部的冈底斯岩基；砂岩中最年轻的碎屑锆石年龄指示林周盆地设兴组是在98Ma之后接受沉积的。以夹层产出在设兴组顶部的玄武岩和玄武安山岩，富集轻稀土元素、亏损重稀土元素、弱负Eu异常，强烈富集Ba、Th、U、Pb等大离子亲石元素，显著亏损Nb、Ta等高场强元素，属于高钾钙碱性玄武岩系列，与典型安第斯型玄武岩特征吻合。玄武岩和玄武安山岩的锆石均为捕获锆石，其最年轻碎屑锆石年龄限定了设兴组玄武岩的喷发晚于110Ma。综合分析表明，林周盆地晚白垩世时期为夹持在冈底斯岩浆弧与中部拉萨地块之间的弧后盆地，新特提斯洋壳晚白垩世俯冲到冈底斯弧和弧后盆地之下，大约在98~110Ma之后喷发到林周盆地的很少量中基性岩浆构成了设兴组顶部的玄武岩和玄武安山岩夹层，是新特提斯洋俯冲相关的幔源岩浆作用。林周盆地设兴组（晚于98Ma）与上覆的林子宗火山岩（底部约为65Ma）之间呈大约33Myr的构造间断，可能代表了冈底斯弧的碰撞之前的隆升剥蚀过程。;The collision between India and Asia was the most important tectonic event in the evolution of the Tibetan Plateau, which was recorded in the Late Cretaceous to Paleogene sedimentary and magmatic rocks in the southern Lhasa terrane. The Late Cretaceous Shexing Formation strata and later eruption of the Linzizong volcanic succession (ca. 65~45Ma) is the best candidate to show the collisional processes. This study provides new petrological, geochemical and zircon U-Pb geochronological data on the sandstones and basaltic interlayers collected from the highest level of Shexing Formation in the Linzhou basin in southern Tibet. The materials of the Shexing greywacke were from the felsic magmatic source region. Hf isotope implies that the majority of the sediments came from the central Lhasa terrane to the north of the basin, and others from the Gangdese arc batholiths to the south of the basin. The youngest age of the detrital zircons in Shexing sandstone constrains the deposition were later than 98Ma in Linzhou basin. The interlayers close to the top of the Shexing Formation are basalts and basaltic andesites, with enriched light rare earth elements (REE), depleted heavy REE, weak negative Eu anomaly, strongly enriched large ionic lithophile elements (LILEs, such as Ba, Th, U, Pb), and depleted high-field strength elements (HFSEs, such as Nb, Ta). These basalts and basaltic andesites are high potassium calc-alkaline basalt series, with the affinity of the typical Andes-type basalts from continental arc. The detrital zircons from these interlayers define their eruption to be later than 110Ma. Our results suggest that the Linzhou basin is the back-arc tectonic setting, which located between the Gangdese arc and the central Lhasa terrane during the Late Cretaceous. The Neo-Tethys Ocean subducted beneath the Gangdese arc and the back-arc system and triggered the mantle partial melting to produce the eruption of the basalts and basaltic andesites after 98~110Ma. The interval of 33Myr between the upper most Shexing Formation (98Ma) and the Linzizong volcanics (65Ma), may represent the uplift and erosion processes of the Gangdese arc before the collision of India and Asia.

Observational analyses of topographic effects on convective systems in an extreme rainfall event in Northern China

An extreme rainfall event occurred in northern China between 18 and 21 July 2016, with the strongest precipitation of 783.4 mm occurring within the rainfall zone of Taihang Mountain. In this study, mesoscale convective systems (MCSs) resulting in heavy rainfall are studied using high-resolution surface observations, soundings and radar data. The topographic effect in addition to shear zone cyclonic circulation is mainly responsible for the heavy rainfall. The synoptic low-pressure environmental circulation promotes mesocyclone and convection development in northern China, where a strong and moist low-level jet is observed. The interaction between the mesocyclone and mesoscale topography produces two strong, separate precipitation centers, which are clearly the direct results of three MCSs. Orographic blocking plays a crucial role in the development of MCSs. Diabatic cooling due to raindrop evaporation in association with MCS1 forms a cold pool on the plain east of the central Taihang Mountain area. Convective cell initiation and development in MCS2 is due to the impingement of southeasterly flow near the surface of the hills east of Linzhou basin. Convective cells that are successively generated and shifted along a mesoscale shear line comprise the MCS2 linear distribution and a northwestward rain belt. The northern extreme rainfall center is caused by MCS3 along the southern slopes of an eastward-opening valley. Convective cells are triggered along a shear line, which is affected by a combination of topography, a cold pool in the boundary layer and mesocyclone outer flow. Together, the orographic forcing and cold pool enhance the Taihang Mountain precipitation event.

Gangdese magmatism in southern Tibet and India–Asia convergence since 120 Ma

Abstract A compilation of 290 zircon U–Pb ages of intrusive rocks indicates that the Gangdese Batholith in southern Tibet was emplaced from c. 210 Ma to c. 10 Ma. Two intense magmatic pulses within the batholith occur at: (1) 90 ± 5 Ma, which is restricted to 89–94° E in the eastern segment of the southern Lhasa subterrane; and (2) 50 ± 3 Ma, which is widespread across the entire southern Lhasa subterrane. The latter pulse was followed by a phase of widespread but volumetrically small, dominantly felsic adakitic intrusive rocks at 16 ± 2 Ma. The Linzizong volcanism in the Linzhou Basin was active from 60.2 to 52.3 Ma, rather than 69–44 Ma as previously estimated. During 120–75 Ma, Gangdese Batholith magmatism migrated from south to north, arguing against rollback of the downgoing, north-dipping Neo-Tethyan oceanic lithosphere for the generation of the 90 ± 5 Ma magmatic pulse. Petrological, geochemical and metamorphic data indicate that this pulse was likely to have been generated through subduction of the Neo-Tethyan oceanic ridge lithosphere. Subsequent Gangdese Batholith magmatism propagated both south and north during 70–45 Ma, and finally concentrated at the southern margin of the Lhasa Terrane at 45–30 Ma. The enhanced mafic magmatism since c. 70 Ma, magmatic flare-up with compositional diversity at c. 51 Ma and increased magmatic temperature at 52–50 Ma are interpreted as the consequences of slab rollback from c. 70 Ma and slab breakoff of the Neo-Tethyan oceanic lithosphere that began at c. 53 Ma. The India–Asia convergence was driven by Neo-Tethyan subduction with a normal rate of convergence at 120–95 Ma, ridge subduction at 95–85 Ma, then subduction of a young and buoyant oceanic lithosphere after ridge subduction with rate deceleration at 84–67 Ma, Deccan plume activity and slab rollback with rate acceleration at 67–51 Ma, slab breakoff for sudden drop of the convergence rate at c. 51 Ma, and finally the descent of the high-density Indian continental lithosphere beneath Asia since c. 50 Ma.

The newly-discovered Late Cretaceous igneous rocks in the Nuocang district: Products of ancient crust melting trigged by Neo–Tethyan slab rollback in the western Gangdese

The newly discovered polymetallic Nuocang Pb-Zn skarn deposit is located in the southern Lhasa subterrane, western Gangdese, Tibet. The orebodies occur primarily at the contact zone between the Angjie Formation and the Linzizong volcanic rocks of Dianzhong Formation (LDF) that are dominated by basaltic andesitic tuff and rhyolite. Zircon U–Pb dating for two granite porphyries yield ages of 72.4 ± 0.2 Ma and 73.4 ± 0.9 Ma, which are different from the ages (~69–60 Ma) of the LDF in the eastern Gangdese. The basaltic andesite tuff at Nuocang exhibits enrichment of MgO, TiO2, LILE, and LREE, with a relative depletion of SiO2, K2O, HFSE, and HREE, low Sr/Y ratios (32.9–38.0), and weak negative Eu anomalies (mean 0.86). They have 87Sr/86Sr(i) from 0.70695 to 0.70807 and εNd(t) values between −4.3 and −5.9. These features are similar to the Linzizong volcanic rocks of Dianzhong Formation in the Linzhou basin, indicating that they were associated with partial melting of mantle wedge mixing with 25–35% ancient Lhasa terrane basement. The rhyolite and granite porphyry show high SiO2 and K2O, and low Sr/Y ratio (1.2–9.9), enrichment of LILE and LREE and strong depletion of the HFSEs. They have pronounced negative Eu anomalies (mean 0.46), and εHf (t) values of the granite porphyry zircons range from −22.0 to −6.0. All these features suggest that they are the product of anatexis of ancient crustal materials heated by mantle-derived magma, the latter derived from Neo-Tethyan slab dehydration mechanisms. Combined with the previous geochronological and geochemical data, we proposed that the Nuocang district of western Gangdese in the southern Lhasa subterrane contains an ancient block, and the igneous rocks here were triggered by the Neo-Tethyan slab rollback starting at ~82 Ma. The western Gangdese contains more ancient continental crustal materials and Late Cretaceous–Eocene Linzizong volcanic rocks and coeval intrusions than in the eastern Gangdese. Thus the western Gangdese block displays greater potential for deposits associated with Late Cretaceous-Eocene magmatism including porphyry Mo, polymetallic skarn (W-Sn-Mo-Pb-Zn-Ag), base and precious metal epithermal (Pb-Zn-Ag-Au), cryptoexplosive breccia (Pb-Zn-Ag), and hydrothermal vein Pb-Zn-Ag deposits.

Early cretaceous topographic growth of the Lhasaplano, Tibetan plateau: Constraints from the Damxung conglomerate

AbstractConstraining the timing of early topographic growth on the Tibetan plateau is critical for any models of India‐Asia collision, Himalayan orogeny and subsequent plateau development in the Cenozoic. Stratigraphic, sedimentological and provenance analysis of the Lower Cretaceous red‐beds of the Damxung Conglomerate provide new key information to reconstruct the paleogeography and the tectonic evolution of the Lhasa terrane at the time. The over 700‐m‐thick Damxung Conglomerate documents distal alluvial fan to braidplain sedimentation passing upward to proximal alluvial fan sedimentation. Deposition began near sea level, as documented by limestone beds occurring at the base of the unit. Zircon U–Pb dating of interbedded tuff layers constrain deposition age at ca. 111 Ma. Abundance of volcanic clasts, Cretaceous U–Pb ages and Hf isotopes of detrital zircons yielding mainly negative εHf(t) values together with paleocurrent data indicate an active volcanic source located in the North Lhasa subterrane. Pre‐Mesozoic‐aged zircon, recycled quartz and (meta) sedimentary rock fragments increase up‐section, indicating progressive erosional exhumation of the Paleozoic sedimentary/metasedimentary basement. The Damxung Conglomerate thus records a significant uplift and unroofing stage in the source region, implying initial topographic growth on the Lhasa terrane at early Albian time. Early Cretaceous topographic growth on the Lhasa terrane is supported by the stratigraphic record in the Linzhou basin, the Xigaze forearc basin and the southern Nima basin. In contrast, marine strata in the central‐western Lhasa terrane lasted until the early Cenomanian (ca. 96 Ma), indicating diachronous marine regression on the Lhasa terrane from east to west.

New Late Cretaceous paleomagnetic data from volcanic rocks and red beds from the Lhasa terrane and its implications for the paleolatitude of the southern margin of Asia prior to the collision with India

In order to improve estimation of the paleolatitude of the southern margin of Asia during the Late Cretaceous, as well as the constraints on the intracontinental convergence within Asia due to the India–Asia collision, we performed a combined geochronological and paleomagnetic investigation of Late Cretaceous red bed sequences and intercalated volcanic rocks from the Linzhou Basin in the Lhasa terrane (29.9°N/91.1°E). The results of potassium–argon (K–Ar) dating indicated that the basalt sequences are of Late Cretaceous age (68–75Ma). Stepwise thermal demagnetization successfully isolated high unblocking-temperature characteristic directions from 10 red bed sites and 11 lava flow sites. Positive fold and reversal tests indicate a primary origin for the characteristic remanence of the red beds and basalt sites. The consistent inclination recorded in the red beds and basalt lava flows indicates that no significant compaction-related inclination shallowing has occurred in the red beds from the studied section. The tilt-corrected mean direction is D/I=0.5°/20.2° with α95=6.4° and N=21 sites, corresponding to a paleopole at 70.5°N, 269.6°E with A95=4.9°. The paleomagnetic results yield a paleolatitude of 10.4±4.9°N for the southern margin of Asia in the Late Cretaceous. Compared with expected paleomagnetic directions from the Tethyan Himalaya and Asia, significant crustal shortening of 17.7±5.5° (1960±610km) and 9.4±4.7° (1040±520km), respectively, may have occurred between the southern margin of Asia and the Tethyan Himalaya, and within Asia, since the Late Cretaceous. Based on previous Cretaceous and Paleocene data from the Lhasa terrane, the latitude of the southern margin of Asia was located at a stable paleolatitudinal position (10°–15°N) during the Cretaceous and Paleocene.

Geochronology and Geochemistry of the Late Cretaceous to Paleocene Intrusions in East Gangdese, Lhasa, Tibet and Their Tectonic Significances

AbstractIn this study, we determined the granite ages in the middle to east Gangdese batholith. Zircon ages from these granites are 57.6–68.7 Ma, indicating that intrusions were formed in the Late Cretaceous to early Paleocene. The large‐ion lithophile elements are highly enriched, whereas some high‐field‐strength elements are depleted. The Sr‐Nd‐Pb isotopic characteristics are similar to those of the Dianzhong volcanics in the Linzhou Basin, indicating the same origin and tectonic environment. The samples show positive εHf(t) values that are slightly lower than the values for the Linzizong volcanics, the Quxu intrusion, and other intrusions in middle Gangdese. We conclude that our samples, the Linzizong volcanics, and most main‐collisional intrusions are derived from the same source with different ratios of crust and mantle input. On the basis of geological, geochemical, geochronological, and isotopic information, we conclude that the Late Cretaceous to Paleogene evolution of Gangdese can be divided into three stages. First, the collision began at 70–60 Ma, the same time as rollback of the Tethys Ocean slab. Second, during 60–50 Ma, slab breakoff triggered upwelling of the asthenosphere. Third, after 50 Ma, the Tethys Ocean slab's effect disappeared and the interaction between Indian and Asian crusts began influencing magmatism in Gangdese.

What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou Basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-Pb zircon dates of ~52 Ma for the upper Linzizong, and 40Ar/39Ar dates (~51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of ~5°N, compared to the upper part (~20°N) and to underlying Cretaceous strata (~20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by ~52 Ma at ~20°N.

Paleomagnetic and 40Ar/39Ar geochronological results from the Linzizong Group, Linzhou Basin, Lhasa Terrane, Tibet: Implications to Paleogene paleolatitude and onset of the India–Asia collision

The Linzizong Group (64–44Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India–Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D=183.6°, I=−12.4° (α95=8.1°) for the Dianzhong (64–60Ma), D=1.0°, I=18.1° (α95=8.1°) for the Nianbo (60–50Ma), and D=12.4°, I=23.2° (α95=7.3°) for the Pana (50–44Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar–Ar age constraints for the Lhasa Terrane indicate that onset of the India–Asia collision occurred no later than ∼60.5±1.5Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.

The Andean-type Gangdese Mountains: Paleoelevation record from the Paleocene–Eocene Linzhou Basin

Paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the Cenozoic uplift of the Himalayas and the Tibetan Plateau. This paper presents new oxygen and carbon isotopic compositions from well dated Tertiary paleosols, lacustrine calcareous carbonates, and marls from the Nianbo (60–54 Ma) and upper Pana Formations (51–48 Ma) of the Linzizong Group in the Linzhou (Penbo) Basin. The sediments of the Nianbo Formation, which are >180 m-thick, were deposited in alluvial fans, braided rivers, fan deltas, and on nearshore to offshore lacustrine settings, whereas those of the upper Pana Formation are >100 m-thick and are comprised predominantly of proximal alluvial fan and braided river deposits. Correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment. It is confirmed that the δ18Oc and δ13Cc values from Nianbo and Pana Formations have not yet been reset by late-stage diagenesis based on petrographic examination, oxygen isotope of the fossil ostracodes, and tectonic deformation of strata. The paleoelevations are reconstructed using the corrected most negative paleosurface water δ18Opsw values. These imply that the Linzhou area had attained an elevation of 4500±400 m during the period of the Indo-Asian collision, i.e., achieved a near-present elevation, and may form an Andean-type mountain range stretching the Gangdese arc before collision. The Gangdese Mountains probably maintained high elevations since at least the Paleocene and could play a crucial role in the climate change in the interior of the Tibetan Plateau during the Early Cenozoic. The paleogeomorphic scenario of the Eocene Tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin.

Position of the Lhasa terrane prior to India-Asia collision derived from palaeomagnetic inclinations of 53 Ma old dykes of the Linzhou Basin: constraints on the age of collision and post-collisional shortening within the Tibetan Plateau

The Lhasa terrane comprises the southern Eurasian margin since at least mid-Cretaceous. Therefore paleomagnetic data from the Lhasa terrane are crucial to reconstruct the pre-collisional southern margin of Eurasia. Previous paleomagnetic data from the Cretaceous Takena Formation provide a rough constraint on the pre-collisional paleolatitudinal position of the southern Eurasian margin. However, Paleogene paleomagnetic data from the Lhasa terrane, which can provide more direct constraints, are very limited. In addition, reliability of the existing data is ambiguous because of a possible later overprint. In our new study 17 sites were drilled from mafic dykes intruded in the T1 unit of the Linzizong Formation of the Linzhou Basin near Lhasa for paleomagnetic studies (Figure 1). The dykes were Ar 40 /Ar 39 dated to c. 53 Ma by Yue and Ding (2006). From ten sites a higher coercivity component demagnetized between 20 and 100 mT could be isolated. Detailed rockmagnetic analyses reveal Ti-rich titanomagnetite as the remanence carrier, which suggests that the rock is not much altered and the remanent magnetization is likely of primary origin. This is supported by a positive fold test. The occurrence of normal and reverse polarity directions within single sites, the subvolcanic character of the dykes, and the within-site scatter, as well as the scatter between site mean directions, indicate that paleosecular variation is recorded within sites. Tilt correction was performed using the bedding of overlying fluvial- lacustrine sediments and tilt angles of ignimbrite columns postdating the dyke emplacement. Bedding-corrected directions give an overall mean direction of D/I = 12.3°/27.2° (α95 = 10.6°, k = 21.7, N = 10) corresponding to a paleolatitude of 13.2° ± 5.8°N at a reference location on the suture zone (20°N/90°E). Comparison with previous Cretaceous data mainly from the Takena Formation yields a stable position of the Lhasa terrane during Cretaceous and Early Eocene. This indicates that no large north-south crustal shortening occurred along the southern Eurasian margin between Late Cretaceous and Early Eocene. The difference between expected paleolatitudes determined from the APWP of Eurasia and observed paleolatitudes reveals relative northward movement of the Lhasa block of ca. 1847±763 km since Early Eocene. This is attributed to indentation of India into Asia and implies a considerable amount of north-south crustal shortening. Two models of “Greater India” were used to determine the extended northern margin of India: Ali and Aitchison (2005) proposed a Greater India based on bathymetric features in the Eastern Indian Ocean and Patzelt and others (1996) determined the paleolatitude of northern India using paleomagnetic investigations on Paleocene sediments of the Zongpu Formation at Gamba and Duela. Intersection of the pre-collisional southern Eurasian margin and the northern margin of India yields a collision age of the Indian and Eurasian continents of c. 5349 Ma. An initial collision within this time range is in accordance with previous studies based on paleomagnetic or geologic data (e.g. Patriat and Achache 1984, Klootwijk and others, 1992, Guillot and others, 2003). A significantly later collision age at the Eocene/Oligocene boundary as proposed by Aitchison and others (2007) is not in agreement with our new and previous paleomagnetic data from the Lhasa Block.

Contribution of syncollisional felsic magmatism to continental crust growth: A case study of the Paleogene Linzizong volcanic Succession in southern Tibet

The Linzizong volcanic succession (~ 65–45 Ma) and the coeval batholiths (~ 60−40 Ma) of andesitic to rhyolitic composition represent a magmatic response to the India–Asia continental collision that began at ~ 70–65 Ma and ended at ~ 45–40 Ma with convergence continuing to present. These syncollisional felsic magmatic rocks are widely distributed along much of the > 1500 km long Gangdese Belt immediately north of the India–Asia suture (Yarlung–Zangbo) in southern Tibet. Our study of the Linzizong volcanic rocks from the Linzhou Basin (near Lhasa) suggests that syncollisional felsic magmatism may in fact account for much of the net contribution to continental crust growth. These volcanic rocks show a first-order temporal change from the andesitic lower Dianzhong Formation (64.4–60.6 Ma), to the dacitic middle Nianbo Formation (~ 54 Ma), and to the rhyolitic upper Pana Formation (48.7–43.9 Ma). The three formations show no systematic but overlapping Nd–Sr isotope variations. The isotopically depleted samples with ε Nd( t) > 0 indicate that their primary sources are of mantle origin. The best source candidate in the broad context of Tethyan ocean closing and India–Asia collision is the remaining part of the Tethyan ocean crust. This ocean crust melts when reaching its hydrous solidus during and soon after the collision in the amphibolite facies, producing andesitic melts parental to the Linzizong volcanic succession (and the coeval batholiths) with inherited mantle isotopic signatures. Ilmenite as a residual phase (plus the effect of residual amphibole) of amphibolite melting accounts for the depletion of Nb, Ta and Ti in the melt. The effect of ocean crust alteration plus involvement of mature crustal materials (e.g., recycled terrigeneous sediments) enhances the abundances of Ba, Rb, Th, U, K and Pb in the melt, thus giving the rocks an “arc-like” geochemical signature. Residual amphibole that possesses super-chondritic Nb/Ta ratio explains the sub-chondritic Nb/Ta ratio in the melt; residual plagioclase explains the slightly depleted, not enriched, Sr (and Eu) in the melt, typical of continental crust. These observations and reasoning plus the remarkable compositional similarity between the andesitic lower Dianzhong Formation and the model bulk continental crust corroborates our proposal that continental collision zones may be sites of net crustal growth (juvenile crust) through process of syncollisional felsic magmatism. While these interpretations are reasonable in terms of straightforward petrology, geochemistry and tectonics, they require further testing.