Rising as “the roof of the world” the Tibetan plateau is now underlain with the thickest continental crust on Earth. How and when was this crust formed, which would have exerted pivotal controls to ...the formation of the plateau, has long been an issue of hot debates. This paper reports zircon U–Pb ages and Hf isotope data for postcollisional (~
30–9
Ma) adakites in the southern Lhasa terrane, southern Tibet. A comparative analysis of whole-rock rare earth element geochemistry and zircon Hf isotopes between the adakites and associated Gangdese igneous rocks suggests that the Tibetan crust underwent a major phase of tectonic thickening between ca. 45 and 30
Ma in the region. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the Late Cretaceous and Eocene time related to the Neotethyan subduction processes including breakoff of the subducted slab at ca. 50
Ma in the early stage of the India–Asia collision. These processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern Lhasa terrane. The indentation of India, consequently, caused distributed lithospheric thickening with formation of an orogenic root beneath southern Tibet. Root foundering during the Oligocene gave rise to the adakitic magmatism, regional topographic uplift, and onset of northward underthrusting of the Indian plate that has since played a key role in forming the entire Tibetan plateau.
The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic ...(Carboniferous to Permian) strata and a north–south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called “Gaoligong Group”). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64
Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~
22
Ma. Leucosome and S-type granite of 22–53
Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages. This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore, the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~
53–22
Ma) that has brought the Paleogene sediments to depths of greater than 25
km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent.
►Gaoligong Group is Paleocene sediments with Archean to Cretaceous detrital zircons. ►The Gongshan block has undergone high-grade metamorphism and anatexis at 53–22
Ma. ►Crustal thickening and rotation was the major teconism before 22
Ma. ►The Indochina continent is the east extension of the Lhasa Block.
•Cona rift is the easternmost rift in the Himalayas.•Multi-thermochronometers suggest 3.0–2.3 Ma rift initiation.•Initiation of Himalayan rifts youngs monotonically eastward.•Lithospheric flow driven ...by asynchronous GPE gradients caused E–W extension.
Several competing hypotheses have been proposed to explain east-west extension observed across the Himalayan orogen, based primarily on observations from the western and central Himalaya. They make predictions for the temporal and spatial patterns of deformation in the eastern Himalaya. These tectonic models include radial spreading or oroclinal bending of the Himalayan arc, oblique convergence of the Indian continent, tearing or lateral detachment of the Indian slab and eastward flow of lithosphere. Here, for the first time we constrain the activity history of the Cona rift, the only north-trending rift currently recognized in the eastern Himalaya, based on biotite and K-feldspar 40Ar/39Ar and zircon and apatite (U–Th)/He thermochronology, to evaluate these proposed rifting mechanisms. Low-temperature thermochronological results suggest that the Cona rift is the youngest rift system in the Himalayas: after a slow phase of exhumation since ∼14 Ma (∼0.2–0.13 mm/yr), normal faulting initiated here at ∼3.0–2.3 Ma with a fault slip rate of ∼3.8–1.6 mm/yr and a horizontal extension magnitude of ∼2–5 km. Analysis of rifting across the Himalayas shows that rift initiation ages young eastward, which is matched by eastward decreasing rift-extension magnitudes. Monotonically eastward younging rift development is consistent with the tectonic model involving eastward lithospheric flow driven by Indian slab dynamics and coupled asynchronous gravitational potential energy gradients.
The Gangdese batholith emplaced from the Cretaceous to Eocene in southern Tibet has been widely regarded as the major constituent of an Andean-type convergent margin resulting from northward ...subduction of the Neo-Tethyan oceanic lithosphere under Asia. While the Gangdese batholith consists predominantly of calc-alkaline rocks, we identify from the eastern part of the batholith a suite of epidote-bearing granodiorites that shows adakitic geochemical characteristics, marked with apparently higher La/Yb and Sr/Y, and lower Y and HREE, than other Gangdese rocks and common arc magmas. SHRIMP zircon U–Pb analyses of two of the samples yielded
206Pb/
238U dates of 80.4
±
1.1 and 82.7
±
1.6 (2
σ) Ma, which constrain the emplacement ages of the adakitic rocks. Trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that, on the basis of the Sr and Nd isotope data I
Sr
=
0.7044–0.7048;
ε
Nd(
T)
=
+
3.2 to +
0.9, we interpret to be a newly underplated, mafic lower crust, rather than the subducted Neo-Tethyan oceanic crust. This juvenile crust was produced by Cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the Neo-Tethyan subduction, a process that also led to the adakitic magmatism. Our interpretation involving a Late Cretaceous flat-slab subduction and related orogenesis in southern Tibet is consistent with petrographic data, such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites, which indicate deep-seated emplacement followed by rapid tectonic exhumation.
In Northeast China, oceanic subduction between the Jiamusi and Songliao blocks remains topic of hot debate. The Heilongjiang Complex has been regarded as an accretionary belt resulting from the ...subduction of an intervening ocean between the two blocks. In this study, we carry out extensive geological, geochemical and geochronological investigations on the sedimentary rocks, amphibolites and blueschists from the Heilongjiang Complex. The detrital zircons from meta-sedimentary rocks yield U-Pb age spams ranging from 268 to 780Ma. Whereas the interlayered amphibolites show negative Nb-Ta-Ti and positive Pb anomalies and have a protolithic age of 188.2±1.0Ma, suggesting a subduction zone or magmatic arc origin in the Jurassic. The elevated initial Sr isotopic ratios (0.708–0.711) and negative εNd(t) values (−4.3 to −1.3) provide further evidence of the modification by upper continental crust during the magma ascending. LREE/HREE and MREE/HREE ratios suggest that the magma was likely derived from the mixing of lithospheric and asthenospheric melts. The presence of low Th/Ce (0.04–0.12) and Hf/Sm (0.35–0.55), but high Zr/Hf (35.4–43.4) and Pb/Ce (0.25–0.49) ratios strongly implies a contribution from subducted sediments. The identification of active continental margin type magmas of Early Jurassic age suggests that the subduction of the oceanic plate between the Jiamusi and Songliao blocks may have started around Early Jurassic time. In addition, the blueschists have a protolithic age of 186±1.1Ma and display geochemical affinities of oceanic island basalts, suggesting that the ocean between the Jiamusi and Songliao blocks closed sometime after 186Ma. Mineral 40Ar/39Ar dating results from the Heilongjiang Complex further indicate that blueschist- to greenschist-facies metamorphism occurred in the Middle to Late Jurassic (158–175Ma), marking the onset of termination of the oceanic subduction.
•Detrital zircons from siliciclastic rocks yield U-Pb ages spanning from 268 to 780 Ma.•The blueschists have a protolithic age of 186 ± 1.1 Ma and display OIB affinities.•The amphibolites have a protolithic age of 188±0.97 Ma and show subduction zone or magmatic arc origin.•The blueschist- to greenschist-facies metamorphism occurred in the Middle- to Late Jurassic (158-175 Ma).•The subduction beneath the Songliao Block may have started since Early Jurassic.
► New and comprehensive major and trace element and Sr–Nd isotope results are reported. ► Calc-alkaline rocks (ca. 69–43Ma) show typical arc-lava geochemical characteristics. ► Eocene “flare-ups” ...display significant geochemical variations in magma compositions. ► Our data stand the models of rollback and breakoff of the subducted Neotethyan slab.
The Linzizong volcanic successions that crop out in the Lhasa terrane, southern Tibet have been conventionally regarded as the products of northward subduction of the Neotethyan oceanic slab beneath South Asia. This study reports geochemical data of 100+ volcanic rocks from the Lhasa terrane to better constrain the temporal–spatial distribution and petrogenesis of the Linzizong volcanism. The Linzizong volcanic successions consist dominantly of calc-alkaline rocks that erupted from ca. 69 to 43Ma and show typical arc-lava geochemical features marked with LILE enrichment and HFSE depletion in the spidergram. Their Sr and Nd isotope ratios εNd(T)=+3.3 to −2.4; ISr=0.7048–0.7072 are generally similar to those of the associated Gangdese I-type granitoids. The Linzizong volcanism is characterized by a flare-up period (ca. 50Ma) that shows significant geochemical variations, manifested by the coexistence of five types of volcanic rocks: (1) the main suite of calc-alkaline rocks SiO2=45–80wt.%; La=12–45ppm; εNd(T)=+3.8 to −4.9; ISr=0.7037–0.7105 that could be interpreted by partial melting of the mantle wedge followed by assimilation and fractional crystallization (AFC) processes with >10% crustal contamination and/or magma mixing; (2) the low-K, low-REE suite (SiO2=48–61wt.%; K2O=0.5–1.1wt.%; La=7–10ppm) that has the highest Nd isotope ratios εNd(T)=+5.9 to 3.5, suggesting a juvenile mantle origin possibly related to decompression melting of the asthenosphere; (3) the shoshonitic suite SiO2=53–71wt.%; K2O=3.8–6.7wt.%; εNd(T)=−2.8 to −6.1 from small-degree melting of the metasomatized lithospheric mantle; (4) the high-REE suite SiO2=61–75wt.%; La=36–87ppm; εNd(T)=−2.6 to −3.2 originating from remelting of a newly underplated basaltic lower crust; and (5) the evolved suite of rhyolitic flows and ignimbrites (SiO2>65wt.%) that have the lowest εNd(T) values of −14 to −18, representing remelting products of the basement or continental crust of the Lhasa terrane. Such geochemical heterogeneities are attributed to breakoff of the subducted Neotethyan slab under southern Tibet that occurred in the early stage of the India–Asia collision.
The driving forcing behind the secular evolution of the Earth system is controversial. Zircon, a common mineral in the continental crust, is a testimony of the growth and evolution of the continental ...crust and a recorder of Earth system evolution because of the time scale of the UPb isotopic chronology system zircon contains. Here, we compiled the largest known database of 2,042,944 zircon UPb ages derived by sampling the global continental crust. Comprehensive time series analysis techniques enable the identification of periodicities in the zircon production history and the growth and evolution of the continental crust over Earth's history. After the evaluation and exclusion of hot spot data impacts, we systematically obtained long-term cyclicities of zircon production of ca. 800, 360, 220, 160, 69, 57, 44, 30, 20, and 17 Myr, which are objective and statistically robust. These cycles are consistent with periodicities derived from multiple geological processes, i.e., mantle plumes, plate tectonics, orogenic events, large magmatic events, climate change, biological extinctions, and even meteorite impacts. The periodicities of these geological phenomena, including zircon production, indicate the periodicity of Earth system evolution in the past 3 billion years. Furthermore, these terrestrial periodicities also correspond to cyclic astronomical perturbations, such as the precession of the galactic warp, the galactic year, and the movement period of the solar system in the Milky Way, within error. Therefore, we suggest that the growth and evolution of the continental crust, and even Earth system evolution, probably originate from the periodic driving forcing of the Earth's astronomical environment.
To better understand Tibetan orogenesis, which involves complex tectonic processes, here we report the first integrated analysis of Hf isotopes of zircon separates and their host rocks from the ...Transhimalayan batholiths. The rocks studied include 19 granitoids from different parts of the Gangdese batholith, the largest Transhimalayan intrusive complex, and five postcollisional adakites that occurred as plugs or dikes cutting the Gangdese batholith, in the southern Lhasa terrane, southern Tibet. Published zircon Hf isotope data from other Transhimalayan granitoids and the Linzizong volcanic rocks are also synthesized. The magmatic zircons, crystallizing between ca. 200 and 15
Ma, show depleted mantle-type Hf isotopic characteristics throughout the Mesozoic but shift markedly in the Paleogene. The Hf isotopic shift is interpreted as tracking the evolving progress of Himalayan sediment subduction driven by the approaching Indian continent, and thus signals the initiation of the India–Asia collision that we infer to have occurred by 55
Ma. Our data furthermore indicate that southern Tibet underwent significant crustal thickening during ca. 45 and 30
Ma, before emplacement of the postcollisional adakites that exhibit Hf–Nd isotopic systematics suggesting binary mixing of melts from the juvenile Gangdese mafic crust and incorporated Himalayan sediments in the petrogenesis.
► Reporting the first integrated analysis of Hf isotopes of zircons and host rocks from Transhimalayan batholiths. ► Tracking the evolving progress of Himalayan sediment subduction driven by the approaching Indian continent. ► Offering new time constraint for initiation of the India-Asia collision, which we infer to have occurred by 55
Ma. ► Reporting Hf-Nd isotopes of postcollisional adakites for petrogenetic interpretation.
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