Within the Himalayan collisional belt, granotoids occur along two sub-parallel belts, the Northern Himalayan Gneiss Domes (NHGD) and the High Himalayan Crystalline Series (HHCS). In the Yardoi area ...of NHGD, two-mica granite, a new type granite occurs in the core of the Yardoi gneiss dome (YGD), Dala and Quedang from north to south, and extends at least 50km long. These granites have similar mineral composition, elemental and radiogenic isotope geochemistry, and age of formation. SHRIMP zircon U/Pb dating indicates that the Yardoi and the Quedang two-mica granites formed at 42.6±1.1Ma and 42.8±0.6Ma, respectively, similar to the Dala pluton. These two-mica granites have (1) high SiO2 (>68wt.%), Al2O3 (>15wt.%), and A/CNK(>1.0); (2) relatively high Sr and LREE, but low Y(<10ppm) and Yb (<1ppm); (3) high Sr/Y (>40 and up to 250) and La/Yb (>30); (4) very weak or no Eu anomalies; and (5) as compared with those in the Himalayan leucogranites, low initial Sr (87Sr/86Sr(i)<0.7120) and similarly unradiogenic Nd (εNd(i)=−8.9–−15.0) isotopic compositions. These granites have initial Sr and Nd isotope compositions similar to those in the amphibolites but significantly different from those in the metapelite and granitic gneiss. Two-mica granites from the Yardoi area are of peraluminous granite with relatively high Na/K and Sr/Y ratios. Such features are distinct from those in the younger leucogranites along the HHCS as well as in the NHGD, and require melting of source consisting dominantly of amphibolite at thickened crustal conditions. This is also supported by the presence of amphibolites with similar Sr and Nd isotope compositions, and similar ages of metamorphism. Two-mica granites of similar age also occur in the other NHGD gneiss domes and along the HHCS belt, implying that Mid-Eocene melting of thickened crustal materials was widespread and might be a primary factor that led to the formation of high density materials (e.g. eclogitic rocks) beneath the Tethyan Himalaya.
► The Yardoi two-mica granites in the NHGD formed at ~43-44 Ma. ► Amphibolite experienced high grade metamorphism and partial melting at ~43 Ma. ► These high Sr/Y granites were derived from high pressure melting of amphibolite.
Late Mesozoic extension in NE Asia resulted in the development of a large extensional province. Metamorphic core complexes (MCCs) are the major features in this province and have 40Ar/39Ar ages of ...130–110 Ma for the mylonites and U‐Pb zircon ages of 150–110 Ma for the integral granitic intrusions. Based on this and previous studies, this paper summarizes major characteristics of these MCCs and recognizes a regional kinematic shear sense. Most MCCs in the Transbaikalia region, Sino‐Mongolia border tract, and the northwest‐central portion of the North China craton (NCC) show a top‐to‐the‐southeast (SE) shear, whereas those in the eastern and southern NCC locally underwent top‐to‐the‐northwest (NW) shear. The three largest basins (Songliao, Huabei and Ordos) in North China are located in the transitional zone between domains of opposing shear sense. We interpret the extension in the Transbaikalia, Sino‐Mongolia tract and northwestern part of the NCC to reflect late‐orogenic collapse of thickened crust following Middle‐Late Jurassic collision along the Okhotsk suture. The southeastward extension is probably controlled by crustal‐scale top‐to‐the‐SE tangential shear. The transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage (ecc, i.e., C′) in sub‐horizontal ductile shear zones late in orogenic crustal thickening. The combined effect of gravitational loading and thermal‐uplifting is considered to be the origin of the late‐or post‐orogenic collapse. The top‐to‐the‐NW extension in the NE of the NCC might reflect antithetic sub‐extensional zones or Mesozoic back‐arc extension as a far‐field effect of Cretaceous Pacific plate subduction.
Key Points
Characteristics of late Mesozoic metamorphic core complexes (MCCs) in NE Asia
Kinematic pattern with shear‐sense polarity for MCCs and basins
Transition from the contraction to extension is marked by extensional detachment
Identifying the timing of formation and geochemical nature of the Cenozoic granites along the Himalayan orogen is essential to test or formulate models that link crustal anatexis with tectonic ...transition during the evolution of large-scale collisional orogenic belts. The Malashan gneiss dome, one of the prominent domes within the Tethyan Himalaya, experienced Barrovian-type metamorphism and partial melting of pelitic rocks at relatively deep levels during the collision between India and Eurasia. New LA-MC-ICP-MS zircon U–Pb analyses yielded that the Malashan two-mica granites formed at a time span of 17.6±0.1 to 16.9±0.1Ma. The Malashan two-mica granites are characterized by: (1) high SiO2 (>71.3wt.%), Al2O3 (>14.8wt.%), and relatively high CaO (>1.3wt.%); (2) relatively high Sr (>146ppm), but low Rb/Sr ratios (<1.3) which are nearly constant relative to large variations in Ba concentrations; (3) enrichment in LREE, depletion in HREE, and no or weak negative Eu anomalies (Eu/Eu∗=0.7–0.9); (4) as compared to granites in the other Northern Himalayan Gneiss Domes and High Himalayan Belt, relatively lower initial 87Sr/86Sr ratios (0.7391–0.7484) and similar unradiogenic Nd isotope compositions (εNd(t)=−13.7 to −14.4). These characteristics imply that the two-mica granites were derived from fluid-fluxing melting of metapelite, possibly triggered by the E–W extension. Our new data in combination with literature data indicate that there are three types of granites with diverse geochemical characteristics and distinct formation mechanisms along the Himalayan orogen since the Cenozoic India–Eurasia continental collision. Conceivably, our new results will provide new insights on how the partial melting behavior of relatively deeper crustal rocks evolved as the tectonic evolution of large orogenic belts.
The southeast margin of the Tibetan plateau is characterized by deeply incised river valleys separated by a perched low relief landscape that gently descends from the high Tibetan plateau towards the ...southeast. When and how this unique landscape formed is debated. The onset of increased river incision is often interpreted as a proxy for the timing of surface uplift. Here, apatite and zircon (U–Th)/He and apatite fission track thermochronometries are employed to map the spatial and temporal pattern of exhumation in the region. Vertical profiles of granitic rocks were collected near Deqin (∼28.5°N) and Weixi (∼27.5°N). The two transects share a similar exhumation history, with two episodes of relatively fast exhumation (∼100–300 m/Myr) in the Cenozoic: during the Paleocene to Eocene (60–40 Ma) and Miocene to present (20–0 Ma), separated by an intervening period of slow exhumation. A pulse of moderate to high exhumation (70–300 m/Myr) during the mid- to late-Cretaceous (120–80 Ma) is also present in the data. However, the rate and total amount of exhumation near Deqin is larger than at Weixi and is especially pronounced in the interval between 20 Ma to present. We interpret this difference as possibly related to differences in erosion rates between the Lancang (Deqin) and the Jinsha (Weixi) rivers. The Paleocene to Eocene episode of fast exhumation is likely in response to early Cenozoic deformation along tectonic boundary structures, related to the transpressional collision of the Indian plate with this region. Pre-Miocene episodes of fast exhumation corroborate recent paleoaltimetric studies, which show that the southeast margin of the Tibetan plateau was elevated prior to the Oligocene.
•Three episodes of fast exhumation during 120–80 Ma, 60–40 Ma and 20–0 Ma.•Diachroneous initiation of fast exhumation along tectonic boundary structures.•Spatial varying magnitude of denudation since 20 Ma.•Results support an elevated region prior to the Oligocene.
The extensive production of Miocene high- to ultrahigh-K basaltic rocks and high-Sr/Y granites across southern Tibet might be related to mantle geodynamics, but there is a lack of direct petrological ...evidence for the exact mechanism involved in this igneous activity. This study reports a study of silicocarbonatite dikes that intrude sedimentary rocks of the Tethyan Himalayan Sequence, with the aim of elucidating the geodynamic mechanism that led to this magmatism. These dikes are composed mainly of ferro- and magnesio-dolomite (55–65 vol%) and quartz (25–30 vol%), with accessory minerals of anatase, titanite, chrome spinel, apatite, monazite, pyrite, and zircon. The carbon (δ13C = −6.3‰ to −6.0‰) and initial Nd εNd(18.4 Ma) = +0.2 to +0.4 isotopic compositions, as well as the relatively high whole-rock contents of Cr (218–956 ppm), Ni (84.1–974 ppm), and Co (28.1–96.3 ppm), indicate that these carbonatitic magmas were sourced from a mantle region. Sensitive high-resolution ion microprobe zircon UPb analysis results showed that these carbonatitic igneous rocks have inherited single-grain 206Pb/238U ages of 2394.2–28.5 Ma and a weighted mean 206Pb /238U age for the ten youngest zircon grains of 18.4 ± 0.2 Ma, which represents the age of magmatic crystallization. Low-viscosity carbonatitic magma is inferred to have ascended rapidly, mixing with high-viscosity silicate melts at middle–lower crustal levels, ultimately forming silicocarbonatitic (SiO2 = 30.17–37.67 wt%) dikes that intruded shales and sandstones of the Tethyan Himalayan Sequence. Tearing of subducted Indian lithosphere might have occurred beneath the eastern Tibet–Himalayan Orogen, allowing magma to upwell and generate linearly distributed Miocene igneous rocks along the N–S-trending Cona rift.
A detailed petrographic, geochemical and geochronological study reveals a suite of Miocene (~21–23 Ma) high Sr/Y and La/Yb volcanic rocks of dacite compositions and adjacent Late Cretaceous tonalite ...and granodiorite (~78–80 Ma) in the Qiewa of the southern Gangdese batholith. The analyses results show that (1) the dacites are characterized by Na-rich, peraluminous, high Sr/Y and La/Yb ratios, showing adakitic affinity, the granitic wall-rocks (tonalite and granodiorite) and enclave have the Na-rich and metaluminous features; and (2) the dacites have relatively radiogenic Sr (86Sr/87Sr(t) = 0.7049–0.7061), unradiogenic Nd (εNd(t) = −3.8 - +0.1), and positive Hf (εHf(t) = +3.0 − +9.5) isotope compositions, while its associated plutonic rocks have relatively lower Sr (86Sr/87Sr(t) = 0.7035–0.7039), but higher Nd (εNd(t) = +3.3 − +4.9) and Hf (εHf(t) = +11.1 − +14.7) isotope ratios. Data presented in this study demonstrates that Early Miocene dacites are generated by partial melting of thickened (~70 km) juvenile lower crust, whereas the Late Cretaceous granitic wall-rocks (tonalite and granodiorite) and enclave are derived from partial melting of juvenile crust with a normal crustal thickness (~40–44 km). Deposition of ~23 Ma dacitic volcanic rocks directly upon the ~80 Ma plutonic rocks suggests that the Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma, possibly triggered by the delamination of thickened continental lithosphere.
•The Qiewa dacites and the underlying plutonic rocks are formed at ~21–23 Ma and ~ 78–80 Ma, respectively.•The high Sr/Y dacites are derived from partial melting of a thickened (~70 km) enriched lower crust.•The plutonic rocks result from partial melting of a normal thickness (~40–44 km) of juvenile curst.•The Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma.
Abstract
Partial melting in the continental crust may play a critical role on the behavior of continents during collision. However, the occurrence of partial melt in orogenic continental crust is not ...well understood. Since the temperature of the orogen is controlled by the thermal properties of constituent rocks, we measured the thermal conductivity and diffusivity of eclogite, the most important ultrahigh pressure metamorphic rocks, as a function of pressure, temperature, composition, and water content, and simulated the thermal structure of the Sulu and Himalaya-Tibet orogens in eastern and southwestern China, respectively. Our results show that the temperature at ~30-km depth beneath the orogens reaches the solidus of wet granite and phengite (~940 K), therefore, the partial melting in the orogenic continental crust is well explained. The melt may facilitate the exhumation of subducted crust, produce the low seismic-velocity zone, and cause the high-conductivity anomaly in the shallow depth of orogenic belts.
The voluminous Late Cretaceous calc-alkaline magmatic rocks in the Gangdese batholith provide an important record on the mechanisms of the thickening and growth of the continental crust in a ...continental arc environment. Here, we report data from zircon UPb geochronology and geochemical (whole-rock element and SrNd isotopes and zircon Hf isotope) analyses for the granodiorite and granite from Langxian to Lilong in the eastern Gangdese batholith. These data show that 1) the intermediate-felsic rocks formed at 82–70 Ma and are Na-rich peraluminous rocks. 2) The granodiorite commonly developed magmatic epidote with allanite-rich cores. 3) They are characterized by high Sr/Y and (La/Yb)N ratios, exhibiting an adakitic affinity. 4) Most of the samples have positive εHf(t) (+5.2 − +12.4) and bulk εNd(t) (+1.9 − +2.8) values, indicating that they were derived from partial melting of juvenile crust. 5) The crustal thickness in the eastern part of the Gangdese batholith locally thickened to ∼67 km. The data presented in this paper demonstrates that the Late cretaceous granodiorite and granite were generated by partial melting of the juvenile thickened lower crust (∼67 km) with deep seated emplacement, and then they experienced rapid exhumation. The magmatic flare-up (90–75 Ma) and ocean ridge subduction in the eastern Gangdese batholith are responsible for the magma inflation and local crustal thickening.
•The granodiorite and granite from Langxian to Lilong are formed at ∼82–70 Ma.•The felsic rocks have elevated Sr/Y and (La/Yb)N ratios, showing adakitic affinity.•Late cretaceous felsic rocks are derived from melting of the juvenile lower crust.•The eastern part of Gangdese batholith had been locally thickened to ∼67 km.
In this study, we link mineral inclusion data, trace element analyses, U–Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and ...multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons. Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx+Pl+Ap±Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe+Grt+Omp+Phe+Ap, and formed at T=732–839°C and P=3.0–4.0GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T=612–698°C and P=0.70–1.05GPa). Inherited zircons from both samples yield SHRIMP 206Pb/238U ages of 695–520Ma with an upper intercept age of 800±31Ma. The UHP rims yield consistent Triassic ages around 236–225 and 239–225Ma for G12 and G13 with weighted means of 229±3 and 231±3Ma, respectively. Small clear zircons from both samples give 206Pb/238U ages around 219–210Ma with a weighted mean of 214±3Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar–Ar ages of 209±0.7 and 207±0.7Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (∼230Ma) times and subsequent exhumation to an early HP (∼214Ma) and a late LP stage (∼208Ma) over a period of ∼16 and 6Myr, respectively. Thus, early exhumation from a mantle depth of 120–100km to about 60km occurred at an average rate of 0.3cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2–3cm/y).
Based on similar P–T estimates and trace element and Hf isotope compositions, Sulu amphibolites can be identified as retrograde UHP eclogites. The εHf(800) of +8 implies a significant input from the depleted mantle to the Sulu–Dabie terrane during the middle Neoproterozoic. Overgrown rims are characterized by a distinct trace element composition with low Lu/Hf and Th/U and significantly higher 176Hf/177Hf ratios than inherited cores, consistent with formation during/after garnet (re-)crystallization and fractionation of the Lu–Hf system during UHP metamorphism. The combined dataset suggests homogenization of the 176Hf/177Hf ratio within the metamorphic mineral assemblage and during protolith formation. Observed variations are explained by mixing of material from both domains during laser ablation, e.g., due to partial recrystallization of inherited cores.
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