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... The Lhasa terrane with a width of 100-300 km and a length of ca. 2000 km, is located between the Qiangtang and Tethyan Himalayan terranes, bounded by the Bangong-Nujiang suture zone to the north and the Indus-Yarlung Zangbo suture zone to the south (e.g., Yin and Harrison, 2000;Yin et al., 2006). Previous studies identified the Lhasa terrane as being composed dominantly of Precambrian metamorphic basement, Paleozoic to Mesozoic sedimentary rocks and Mesozoic and Cenozoic igneous rocks (e.g., Yin and Harrison, 2000;Pan et al., 2004). ...
... The IYSZ is a continuous zone, 2-10 km wide, consisting of highly deformed metasedimentary and ultramafic-mafic rocks, which represent relict slices of the Neo-Tethyan oceanic slab (e.g., Yin and Harrison, 2000;Geng et al., 2006). The eastern segment of Lhasa terrane is composed mainly of metamorphic rocks (termed the Nyingchi Complex), Paleozoic-Mesozoic sedimentary rocks, and abundant Mesozoic-Cenozoic granitoids (Booth et al., 2004;Geng et al., 2006Geng et al., , 2008Yin et al., 2006;Zhang et al., 2008Zhang et al., , 2010a. The Nyingchi Complex, consisting mainly of gneiss, schists, marble, migmatite and minor granulite, was previously suggested as the Precambrian basement of the Lhasa terrane (Xu et al., 1985;YIGS, 2005;Geng et al., 2006;Yin et al., 2006). ...
... The eastern segment of Lhasa terrane is composed mainly of metamorphic rocks (termed the Nyingchi Complex), Paleozoic-Mesozoic sedimentary rocks, and abundant Mesozoic-Cenozoic granitoids (Booth et al., 2004;Geng et al., 2006Geng et al., , 2008Yin et al., 2006;Zhang et al., 2008Zhang et al., , 2010a. The Nyingchi Complex, consisting mainly of gneiss, schists, marble, migmatite and minor granulite, was previously suggested as the Precambrian basement of the Lhasa terrane (Xu et al., 1985;YIGS, 2005;Geng et al., 2006;Yin et al., 2006). However, the zircon dating results have revealed that some metasedimentary rocks from the Nyingchi Complex were deposited after the Paleozoic Dong et al., 2009Dong et al., , 2010, and some orthogneisses have protolith ages from early Paleozoic to early Cenozoic (Dong et al., 2010;Zhang et al., 2010b;Guo et al., 2012). ...
The Lhasa terrane in south Tibet was traditionally considered as the northern margin of the Gondwana Indian continent. However, the Precambrian tectonic evolution of the Lhasa terrane remains poorly constrained. In this paper, we report zircon geochronology and Ti-in-zircon thermometer for the metamorphic rocks from the eastern segment of the Lhasa terrane. The studied three gneisses underwent the amphibolite-facies and granulite-facies metamorphism. Most zircon grains from two gneisses have a structure of core-mantle-rim. The magmatic cores of the zircon yielded the protolith ages of 1784 ± 25 Ma and 1782 ± 31 Ma; the overgrowth mantles of the zircon yielded the metamorphic ages of 618 ± 3 Ma and 604 ± 69 Ma; the metamorphic rims of the zircon from one sample yielded an age of 23.2 ± 0.3 Ma. Zircon grains from other gneiss have a structure of core-mantle-rim. The mantles of the zircon yielded a metamorphic age of 1117 ± 29 Ma. These data show that the Lhasa terrane witnessed Late Paleoproterozoic magmatism, and three episodes of metamorphism in the Late Mesoproterozoic, Late Neoproterozoic and Cenozoic. This results show that the Lhasa terrane and the Indian continent have similar Precambrian tectonic evolution, which was related to the amalgamation of supercontinents Columbia, Rodinia and Gondwana.
... Considering that the Nyingchi or Nyainqentanglha Group is a mixture of metasedimentry and granitic rocks, and some intrusions even emplaced during early Cenozoic (Geng et al., 2006;Zhang et al., 2008Zhang et al., , 2010aDong et al., 2009Dong et al., , 2010, we use Nyingchi Complex to term this lithological assemblage. Previous workers considered that the Nyingchi Complex represents the Precambrian crystalline basement of the Lhasa terrane based on its medium-to high-grade metamorphic assemblages (YIGS, 2005;Geng et al., 2006;Yin et al., 2006). However, detrital zircon dating suggests that the maximum depositional age of the Nyingchi Complex is no older than 490 Ma Dong et al., 2009Dong et al., , 2010. ...
... Geological map of the eastern Himalayan syntaxis, showing the major lithotectonic units and the locations of the studied samples. 2006Yin et al., 2006;Zhang et al., 2008Zhang et al., , 2010a. In this unit, 120-69 Ma granitoids are attributed to the northward subduction of the Neo-Tethyan oceanic slab (Booth et al., 2004;Geng et al., 2006), whereas 50-16 Ma granitoids are thought to be caused by partial melting of crustal materials during the collision and/or postcollisional stage of the India-Asia collision (Geng et al., 2008;Zhang et al., 2008Zhang et al., , 2010a. ...
... In this unit, 120-69 Ma granitoids are attributed to the northward subduction of the Neo-Tethyan oceanic slab (Booth et al., 2004;Geng et al., 2006), whereas 50-16 Ma granitoids are thought to be caused by partial melting of crustal materials during the collision and/or postcollisional stage of the India-Asia collision (Geng et al., 2008;Zhang et al., 2008Zhang et al., , 2010a. The Nyingchi Complex comprises amphibolitic gneiss, banded migmatite, biotite felsic gneiss, augen granitic gneiss, kyanite-sillimanite-garnet two-mica schist, biotite schist, leptynite, marble and mafic granulite (Geng et al., 2006;Yin et al., 2006;Zhang et al., 2008). The Nyingchi Complex was previously suggested as to be the Precambrian basement of the Lhasa terrane, based on medium-to high-grade metamorphic assemblages (YIGS, 2005;Geng et al., 2006;Yin et al., 2006). ...
In the eastern Himalayan syntaxis, the southern Lhasa terrane is dominated by middle- to high-grade metamorphic rocks (Nyingchi Complex), which are intruded by felsic melts. U–Pb zircon dating and zircon Hf isotopic composition of these metamorphic and magmatic rocks provide important constraints on the tectono-thermal evolution of the Lhasa terrane during convergent process between Indian and Asian continents. U–Pb zircon data for an orthogneiss intruding the Nyingchi Complex yield a protolith magma crystallization age of 83.4±1.2Ma, with metamorphic ages of 65–46Ma. This orthogneiss is characterized by positive εHf (t) values of +8.3 and young Hf model ages of ~0.6Ga, indicating a derivation primarily from a depleted-mantle or juvenile crustal source. Zircons from a quartz diorite yield a magma crystallization age of 63.1±0.6Ma, with εHf (t) values of −8.2 to −2.7, suggesting that this magma was sourced from partial melting of older crustal materials. Zircon cores from a foliated biotite granite show ages ranging from 347 to 2690Ma, with age peaks at 347–403Ma, 461–648Ma and 1013–1183Ma; their zircon εHf (t) values range from −30.6 to +6.9. Both the U–Pb ages and Hf isotopic composition of the zircon cores are similar to those of detrital zircons from the Nyingchi Complex paragneiss, implying that the granite was derived from anatexis of the Nyingchi Complex metasediments. The zircon rims from the granite indicate crustal anatexis at 64.4±0.7Ma and subsequent metamorphism at 55.1±1.3 and 41.4±2.3Ma. Our results suggest that the late Cretaceous magmatism in the southern Lhasa terrane resulted from Neo-Tethys oceanic slab subduction and we infer that Paleocene crustal anatexis and metamorphism were related to the thermal perturbation caused by rollback of the northward subducted Neo-Tethyan oceanic slab.
... Due to the adequate age lacking and intensively reworking, the existence of the Precambrian basement beneath the SLT has been controversial. As mentioned above, the Nyingchi complex has been considered as the Precambrian basement of the Lhasa terrane (e.g., Pan et al., 2004Pan et al., , 2006Yin et al., 2006). However, more recent studies suggested that most high-grade metamorphic rocks in the Nyingchi complex underwent Mesozoic to Cenozoic metamorphism and their protoliths included both sedimentary and magmatic rocks with various protolith ages (Dong et al., , 2012(Dong et al., , 2014Zhang et al., 2010Zhang et al., , 2013Zhang et al., , 2014bZhang et al., , 2015Zhang et al., , 2020Guo et al., 2011Guo et al., , 2012Guo et al., , 2017Xu et al., 2013b;Palin et al., 2014). ...
The Precambrian basement of the Lhasa terrane provides important information for understanding the formation and evolution of the Tibetan Plateau. However, due to the paucity of exposure, the properties of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon U–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses in the Milin area of the southeastern Lhasa terrane, southern Tibet. Inherited magmatic zircon cores of these gneisses yield protolith crystalline ages of 1556–1517 Ma, whereas the zircon rims give metamorphic ages of 1197–1192 Ma, both could be local provenance for the Mesoproterozoic detrital zircons in the Neoproterozoic–Paleozoic strata in the Lhasa terrane. Geochemical data indicate that the protoliths of the Mesoproterozoic rocks are continental arc-like granitoids with bulk-rock εNd(t) values of −3.7 to +0.7 (TDM2 = 1.99–2.34 Ga) and zircon core εHf(t) values of +1.8 to +8.6 (TDM2 = 1.71–2.17 Ga), suggesting their most likely derivation from partial melting of Paleoproterozoic crustal materials. We propose that the Mesoproterozoic gneisses in the Milin area represent the Precambrian metamorphic basement of the Lhasa terrane, likely related to the growth of Columbia supercontinent. Local exposure of the Precambrian metamorphic basement may be caused by crustal rapid uplift and erosion in the late Cenozoic and their inhomogeneous distribution.
... Traditionally, the metamorphic rocks on the east side of the EHS were termed the Bomi Group and those on the west the Nyingchi Group, with a general correlation with the Precambrian metamorphic basement of the Lhasa terrane (e.g. Pan et al., 2004Pan et al., , 2006Yin et al., 2006). However, recent laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) zircon U^Pb in situ dating has revealed that these rocks experienced Late Mesozoic to early Cenozoic metamorphism Zhang et al., 2010b;Guo et al., 2011Guo et al., , 2012Zhang & Wu, 2012;Xu et al., 2013). ...
The Nyingchi complex, forming the eastern segment of the Gangdese magmatic arc, occurs within the southern Lhasa terrane in
south Tibet, and is composed dominantly of plutons and their metamorphosed equivalents. Together with some metasedimentary
units these rocks record multiple Mesozoic and Cenozoic magmatic and metamorphic events during northward subduction of Neo-Tethyan
oceanic lithosphere beneath the Eurasian continent. Petrological and geochronological studies reveal that the Nyingchi complex
experienced intense Paleocene subduction-related magmatism and almost synchronous granulite-facies metamorphism accompanied
by the formation of S-type granites. Subduction-related I-type granitoids show geochemical features typical of continental
magmatic arcs; zircon separates from them yield 206Pb/238U ages from c. 65 to c. 56 Ma, and commonly display positive εHf(t) values ranging from −1·7 to +13·0. The occurrence of magmatic epidote, as well as the syn-intrusion high-grade metamorphism,
indicates that the plutons were emplaced at middle to lower crustal depths within the Lhasa terrane. Associated S-type granitoids
are peraluminous and contain garnet and muscovite; their zircons yield 206Pb/238U ages ranging from c. 66 to c. 55 Ma, and these have distinct but mostly negative εHf(t) values from −18·4 to +2·1. The zircons from the associated metasedimentary rocks include both detrital and metamorphic
types; the detrital zircons yielded variable inherited 206Pb/238U ages ranging from c. 2910 to c. 235 Ma, constraining the maximum depositional age to the Triassic. The metamorphic zircons from the metaplutonic and metasedimentary
rocks yielded ages from c. 67 to 52 Ma. Phase equilibria modeling shows that the Nyingchi complex experienced peak granulite-facies metamorphism and
partial melting under conditions of 800–830°C and 9–10·5 kbar, and then cooled isobarically to c. 700°C in the lower crust at depths of >30 km. We argue that rollback of the flat-subducted Neo-Tethyan oceanic slab during
Early Paleogene times resulted in a contractional orogeny and intrusion of voluminous mantle-derived magmas, which caused
large-scale crustal heating, partial melting and granulite-facies metamorphism within the deep crust of the Gangdese arc.
The Nyingchi complex represents the exposed lower crust of the Gangdese magmatic arc, and links the granulite-facies metamorphism
with silicic magmatism and crustal growth during Paleocene arc accretion.
... These rocks have been considered to be the Precambrian basement of the Lhasa terrane (e.g. Pan et al., 2004Pan et al., , 2006Yin et al., 2006). However, recent studies have revealed that these rocks experienced Late Mesozoic to Cenozoic metamorphism, and their protoliths include both sedimentary and magmatic rocks with various protolith ages (Dong et al., ,b, 2012Zhang et al., 2010cZhang et al., , 2013Zhang et al., , 2014aGuo et al., 2011Guo et al., , 2012Zhang & Wu, 2012;Xu et al., 2013;Zhang et al., 2014b). ...
The Gangdese magmatic arc, southeastern Tibet, was built by mantle-derived magma accretion and juvenile crustal growth during the Mesozoic to Early Cenozoic northward subduction of the Neo-Tethyan oceanic slab beneath the Eurasian continent. The petrological and geochronological data reveal that the lower crust of the southeastern Gangdese arc experienced Oligocene reworking by metamorphism, anatexis and magmatism after the India and Asia collision. The post-collisional metamorphic and migmatitic rocks formed at 34–26 Ma and 28–26 Ma, respectively. Meta-granitoids have protolith ages of 65–38 Ma. Inherited detrital zircon from metasedimentary rocks has highly variable ages ranging from 2708 to 37 Ma. These rocks underwent post-collisional amphibolite-facies metamorphism and coeval anatexis under P–T conditions of ~710–760 °C and ~12 kbar with geothermal gradients of 18–20 °C km−1, indicating a distinct crustal thickening process. Crustal shortening, thickening and possible subduction erosion due to the continental collision and ongoing convergence resulted in high-pressure metamorphic and anatectic reworking of the magmatic and sedimentary rocks of the deep Gangdese arc. This study provides a typical example of the reworking of juvenile and ancient continental crust during active collisional orogeny.This article is protected by copyright. All rights reserved.
... Traditionally, the metamorphic rocks on the east side of the EHS were termed the Bomi Group and those on the west the Nyingchi Group, with a general correlation with the Precambrian metamorphic basement of the Lhasa terrane (e.g. Pan et al., 2004Pan et al., , 2006Yin et al., 2006). However, recent laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and sensitive high-resolution ion microprobe (SHRIMP) zircon U^Pb in situ dating has revealed that these rocks experienced Late Mesozoic to early Cenozoic metamorphism Zhang et al., 2010b;Guo et al., 2011Guo et al., , 2012Zhang & Wu, 2012;Xu et al., 2013). ...
The Nyingchi complex, forming the eastern segment of the Gangdese
magmatic arc, occurs within the southern Lhasa terrane in south
Tibet, and is composed dominantly of plutons and their metamorphosed
equivalents.Together with some metasedimentary units these
rocks record multiple Mesozoic and Cenozoic magmatic and metamorphic
events during northward subduction of Neo-Tethyan oceanic
lithosphere beneath the Eurasian continent. Petrological and geochronological
studies reveal that the Nyingchi complex experienced
intense Paleocene subduction-related magmatism and almost
synchronous granulite-facies metamorphism accompanied by the formation
of S-type granites. Subduction-related I-type granitoids
show geochemical features typical of continental magmatic arcs;
zircon separates from them yield 206Pb/238U ages from c. 65 to c.
56 Ma, and commonly display positive eHf(t) values ranging from
�1·7 to þ13·0.The occurrence of magmatic epidote, as well as the
syn-intrusion high-grade metamorphism, indicates that the plutons
were emplaced at middle to lower crustal depths within the Lhasa
terrane. Associated S-type granitoids are peraluminous and contain
garnet and muscovite; their zircons yield 206Pb/238U ages ranging
from c. 66 to c. 55 Ma, and these have distinct but mostly negative
eHf(t) values from �18·4 to þ2·1.The zircons from the associated
metasedimentary rocks include both detrital and metamorphic types;
the detrital zircons yielded variable inherited 206Pb/238U ages ranging
from c. 2910 to c. 235 Ma, constraining the maximum depositional
age to the Triassic. The metamorphic zircons from the
metaplutonic and metasedimentary rocks yielded ages from c. 67 to
52 Ma. Phase equilibria modeling shows that the Nyingchi complex
experienced peak granulite-facies metamorphism and partial melting
under conditions of 800^8308C and 9^10·5 kbar, and then cooled
isobarically to c. 7008C in the lower crust at depths of430 km.We
argue that rollback of the flat-subducted Neo-Tethyan oceanic slab
during Early Paleogene times resulted in a contractional orogeny
and intrusion of voluminous mantle-derived magmas, which caused
large-scale crustal heating, partial melting and granulite-facies
metamorphism within the deep crust of the Gangdese arc. The
Nyingchi complex represents the exposed lower crust of the Gangdese
magmatic arc, and links the granulite-facies metamorphism with silicic
magmatism and crustal growth during Paleocene arc accretion.
... Abundant multi-stage felsic veins, including an early sequence of granitic and granodoritic gneiss, leucogranite, pegmatitic and quartz veins, indicate widespread migmatization (Guo et al., in press;Zhang et al., 2010a,c). The Nyingchi Complex has previously been considered as the Precambrian basement of the Lhasa terrane based on the middle-and high-grade metamorphic assemblages (Geng et al., 2006;Yin et al., 2006). However, recent detrital zircon U-Pb dating of the metasedimentary rocks from Nyingchi Complex indicated that the Nyingchi Complex is no older than 490 or 340 Ma (Dong et al., 2010;Guo et al., in press;Zhang et al., 2008). ...
This integrated study of whole rock geochemistry, zircon U–Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U–Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25Ma. The Jurassic granitic gneiss (165Ma) exhibits εHf(t) values of +1.4 to +3.5. The late Cretaceous granite (81Ma) shows variable εHf(t) values from −0.9 to +6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61Ma) has εHf(t) values of +5.4 to +8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25Ma) shows adakitic geochemical characteristics, with Sr/Y=49.3–56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene–Eocene magmatism, metamorphism and anatexis are interpreted to result from roll-back and break-off of the subducted Neo-Tethyan slab that occurred in the early stage of the India–Asian collision, respectively. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at ~25Ma.
... Previously, the Nyingchi Group was considered to be a Precambrian metamorphic series in the Gangdise terrane (RGS-Tibet, 1993;YIGS, 2005;Yin et al., 2006). Our detrital zircon data from samples T525 and T527 and inherited zircon data from sample T519 can be used to constrain the maximum depositional age of the Nyingchi Group protoliths. ...
The Eastern Himalayan Syntaxis of Namche Barwa carries critical information for understanding the geodynamics of the Indian-Asian collision. In this syntaxis, the Nyingchi Group is a sequence of medium- to high-grade metasediments, located on the north side of the Indus-Yarlung suture zone. Results from zircon U-Pb ages coupled with their Hf isotope systematics for the Nyingchi Group and a two-mica granite intruding the paragneisses reveal strong contrasts in provenance and depositional age within the sequence. Detrital zircons from a garnet paragneiss of the lower Nyingchi Group display 206Pb/238U ages from 493 Ma to 2466 Ma, with peaks at 506 Ma, 821 Ma, and 1032 Ma. The minimum detrital zircon age of 493 Ma constrains the maximum depositional age of the lower Nyingchi Group. Detrital zircons from a paragneiss of the middle Nyingchi Group yield 206Pb/238U ages ranging from 55 Ma to 1362 Ma, with peaks at 60 Ma, 95 Ma, 138 Ma, and 171 Ma. This indicates that the maximum depositional age is less than 55 Ma. Zircons from a two-mica granite, intruding the upper Nyingchi Group, yield a magma crystallization age of 22 ± 1 Ma and inherited zircon ages of 52 Ma to 911 Ma. Because the Hf isotopic ratios together with the ages of detrital zircons suggest an origin from anatexis of the middle Nyingchi Group sediments, these data reduce the maximum depositional age of the middle Nyingchi Group to less than 52 Ma. Results preclude the Nyingchi Group from representing Precambrian basement of the Gangdise terrane. The detrital zircon and inherited zircon age distributions, combined with their Hf isotopic compositions, reveal that whereas the lower Nyingchi Group may be correlated with crystalline units from the Himalayan terrane, the middle-upper Nyingchi Group is a much younger deposit derived from the Gangdise magmatic arc. In order to explain the medium- to high-grade metamorphism of the middle-upper Nyingchi Group, we suggest that following the Indian-Asian collision, a thrust slice from the leading edge of the Asian continental slab (the middle-upper Nyingchi Group) became detached and subducted, together with Himalayan lithologies from the Indian continental slab. These results provide new insight into the tectonic evolution of the Eastern Himalayan Syntaxis.
The metamorphic belt in the Dongjiu area is located in the eastern segment of the Lhasa terrane in South Tibet. The Dongjiu metamorphic rocks are primarily composed of schist and gneiss, with minor amounts of marble, and the protoliths are sedimentary rocks with Precambrian and early Palaeozoic zircons probably deposited during the Palaeozoic or late Neoproterozoic. On the basis of petrology and phase equilibria modelling, this study shows that the Dongjiu metamorphic belt has experienced a kyanite-grade metamorphism, which is characterized by a decompressional vector with slight cooling from a peak of 9.6 kbar and 745°C to medium-pressure amphibolite-facies metamorphic overprinting at 5–6 kbar and 600–630°C. This P–T path was well recorded and recovered by garnet zoning profiles. Laser ablation inductively coupled plasma mass spectrometry in situ U–Pb analyses on metamorphic zircons and zircon rims yielded concordant ²⁰⁶Pb/²³⁸U ages of c. 194–192 Ma, suggesting that the Dongjiu metamorphic rocks were formed during the Early Jurassic. Therefore, the Dongjiu metamorphic belt, together with the western Nyainqentanglha, Basongco, and Zhala metamorphic belts, constitutes a nearly continuous tectonic unit with an E–W extension of at least 500 km between the northern and southern Lhasa terranes. The metamorphic ages of these belts, ranging from 230 to 192 Ma, show a younger trend from west to east, indicating that the central segment of the Lhasa terrane experienced an eastward asynchronous collisional orogeny during the Late Triassic to Early Jurassic.
There are 52.2 m deep-low lacustrine facies and littoral-low lacustrine facies sediments developed at the later stage of late Pleistocene distributed on the third terrace of Yarlung Zangbo river, Milin region, southeast Tibet, indicating that there was a large ancient dammed lake in the Milin region at that time. In order to discuss the paleoclimate and paleoenvironment of this area, we measured some magnetic parameters of 259 directed samples collected from the Milin airport section. The results indicated that most of the samples have original magnetic fabric, and their κ 1 axis indicated the direction of provenance of the Milin ancient dammed lake changed from south and north directions to west and northeast directions, and then to west direction, i.e. a clockwise changing, the changes probably are related to the change in denudation rate induced by differential uplifting of the studied region. The natural remanent magnetization (NRM) and bulk susceptibility (κ) of Milin section have close relationship with particle size and sedimentary facies, which revealed that there are at least four times obvious fluctuates of paleoclimate during later stage of late Pleistocene. The NRM and κ curves of Milin section correspond to IS1-IS6 and IS8 stages of Greenland GISP2 δ 18O ice record and also clearly record the Younger Dryas (YD) event and Heinrich events (H1, H2, H3), indicating that the climate of Milin region was influence by the global climate system.
Songduo rock group includes Chashagang rock formation, Mabuku rock formation and Leilongku rock formation, which takes on fault blocks locating in the eastern Lhasa block, and its age has long been argued. The high accurate LA-ICP-MS zircon U-Pb isotope dating of meta-basic volcanic rock from Chashagang rock formation in Gongbujiangda County, Tibet, yielded a U-Pb upper-intercept age of 2 450.3±9.9 Ma. It is suggested a rock-forming age and the oldest isotopic age of basement of Lhasa block, belonging to the Early Paleoproterozoic. According to field investigation and the new data of geochronology, combining with new results from regional geological survey, we propose that "the original Songduo rock group" should be disintergrated, and re-establish a Gongbujiangda rock group. The Songduo rock Group only remain Mabuku rock formation (AnOm) and Leilongku rock Formation (AnOl). It is great significance to redefinite the Precambrian basement framework, compare in region, subdivide tectonic units, and find minerals related with the Precambrian.
In this paper, geological characters, rock types, P-T path, metamorphic ages and tectonic setting of the granulites in Phanerozoic orogens in China are chiefly introduced. Phanerozoic orogens in China mainly include western Kunlun Orogen, eastern Kunlun Orogen, Altun Tagh-North Qaidam orogen, Altay Orogen, northern Qinling Orogen, Mianlue in southern Qinling Orogen, eastern Qinling-Tongbai-Dabie Orogen, southwestern Tianshan-southern Tianshan Orogen, Bangong Co-Nujiang Orogen in Tibet and mid-eastern segment of Himalaya Orogen. The country rocks of the granulites in Phanerozoic orogen are most of ophiolite suite or ophiolitic melange belt partly of paragneiss, granitic gneisses, and they experienced granulite facies metamorphism together. There is usually one type of high pressure granulite in an orogen and some of them are coexistence of eclogite. But in one or two orogens, there are several types of high pressure granulites. Such as in Altay Orogen, there are not only low-high pressure pelitic granulites, high pressure mafic granulites and felsic granulites, mid-low pressure mafic granulites, but also high pressure-ultra high pressure pelitic granulites. A few of the metamorphic ages of the granulites are Late Neoproterozoic and others are Caledonian, Hercynian, Indo-China, Yanshan and Himalayan. As for the P-T paths, most of them are clockwise paths and have isothermal decompressional characters that indicate an oceanic-continental subduction-collision tectonic setting except Muzart low pressure granulite in southwestern Tianshan orogen who has a counterclockwise path that indicate probably a continental arc collision tectonic setting. Further more, studies on granulites in Phanerozoic orogens are on their infancy at present and some of them have been detailly researched but most of them are not clear and waiting for deep study.
The Demala Group complex, located in Southeast Gangdise block of Tibetan Plateau, is a suite of amphibolite facies metamorphic rocks. Conventionally it is known as Precambrian metamorphic basement, but without credible geochronology evidences. In this paper, we focus on the samples biotite-hornblende schist and biotite-quartz schist we recently obtain in this area. Results of zircon U-Pb study display that the U-Pb age of magmatic zircons from biotite-hornblende schist is 217. 1Ma and detrital zircons from biotite-quartz schist is mainly 520 ∼600Ma and 900 ∼ llOOMa. The biotite Ar-Ar age of the biotite-hornblende schist is 22. 3Ma and the biotite-quartz schist is 16. 3Ma and 22. 3Ma, representing the age of their later metamorphism. These accurate geochronology evidences above at least suggests that the Demala Group complex is a suite of metamorphic complex which consists of Paleozoic sedimentary stratum and Mesozoic intrusive rocks modified by the Cenozoic magmatism and metamorphism, although we can not completely negate the existence of Precambrian metamorphic basement in Chayu area, Southeast Tibetan Plateau.
A petrological and geochronological study was produced on the Nyingchi Complex rock from the southeastern part of the Lhasa terrane, Tibet. The results show that the meta-sedimentary rocks of the Nyingchi Complex consist mainly of gneiss and schist, and metamorphosed under the medium-pressure amphibolite-facies. The zircons from these rocks contain an inherited detrital core and an overgrowth metamorphic rim. The detrital zircon core yielded variable ages ranging from Neo-Archean to Late Paleozoic, with main populations at ca. 1560Ma, ca. 1190Ma, ca. 620Ma and ca. 340Ma, respectively. The zircon rims of two samples yielded metamorphic ages of 53Ma and 27Ma, respectively. These indicate that the sedimentary rocks of the Nyingchi Complex deposited during Paleozoic time, and their material provenance recorded mainly the tectono-thermal events related to the Grenville and Pan-Africa orogenies, and that the Lhasa terrane derived from the north margin of the Gondwana continent. The Nyingchi Complex, representing the hinging wall of the subduction zone, experienced the Eocene and Oligocène metamorphism during the collision/subduction between the India and Eurasia continents.
The Lhasa terrane, located between the Bangonghu-Nujiang suture zone and the Indus-Yalung Tsangpo suture zone in the southern Tibetan Plateau, was considered previously as a Precambrian continental block. Mesozoic and Cenozoic tectonic evolution of the Lhasa terrane is closely related to the subduction of the Tethys ocean and the collision between the Indian and European continents; so it is one of the keys to reveal the formation and evolution of the Tibetan plateau. The garnet two-pyroxene granulite which was found at the Nyingtri rock group of the southeastern Lhasa terrene consists of garnet, clinopyroxene, orthopyroxene, labradorite, Ti-rich amphibolite and biotite, with a chemical composition of mafic rock. The metamorphic conditions were estimated to be at T = 747 ∼ 834°C and P = 0. 90 ∼ 1. 35GPa, suggesting a formation depth of 45km. The zircon U-Pb dating for the garnet amphibolite and marble associated with the granulite give a metamorphic age of 85 ∼ 90Ma. This granulite-facies metamorphic event together with a contemporaneous magmatism demonstrated that the southern Lhasa terrane has undergone an Andean-type orogeny at Late Mesozoic time.
The Lhasa terrane in southern Tibet is composed of Precambrian crystalline basement, Paleozoic to Mesozoic sedimentary strata and Paleozoic to Cenozoic magmatic rocks. This terrane has long been accepted as the last crustal block to be accreted with Eurasia prior to its collision with the northward drifting Indian continent in the Cenozoic. Thus, the Lhasa terrane is the key for revealing the origin and evolutionary history of the Himalayan–Tibetan orogen. Although previous models on the tectonic development of the orogen have much evidence from the Lhasa terrane, the metamorphic history of this terrane was rarely considered. This paper provides an overview of the temporal and spatial characteristics of metamorphism in the Lhasa terrane based mostly on the recent results from our group, and evaluates the geodynamic settings and tectonic significance. The Lhasa terrane experienced multistage metamorphism, including the Neoproterozoic and Late Paleozoic HP metamorphism in the oceanic subduction realm, the Early Paleozoic and Early Mesozoic MP metamorphism in the continent–continent collisional zone, the Late Cretaceous HT/MP metamorphism in the mid-oceanic ridge subduction zone, and two stages of Cenozoic MP metamorphism in the thickened crust above the continental subduction zone. These metamorphic and associated magmatic events reveal that the Lhasa terrane experienced a complex tectonic evolution from the Neoproterozoic to Cenozoic. The main conclusions arising from our synthesis are as follows: (1) The Lhasa block consists of the North and South Lhasa terranes, separated by the Paleo-Tethys Ocean and the subsequent Late Paleozoic suture zone. (2) The crystalline basement of the North Lhasa terrane includes Neoproterozoic oceanic crustal rocks, representing probably the remnants of the Mozambique Ocean derived from the break-up of the Rodinia supercontinent. (3) The oceanic crustal basement of North Lhasa witnessed a Late Cryogenian (~ 650 Ma) HP metamorphism and an Early Paleozoic (~ 485 Ma) MP metamorphism in the subduction realm associated with the closure of the Mozambique Ocean and the final amalgamation of Eastern and Western Gondwana, suggesting that the North Lhasa terrane might have been partly derived from the northern segment of the East African Orogen. (4) The northern margin of Indian continent, including the North and South Lhasa, and Qiangtang terranes, experienced Early Paleozoic magmatism, indicating an Andean-type orogeny that resulted from the subduction of the Proto-Tethys Ocean after the final amalgamation of Gondwana. (5) The Lhasa and Qiangtang terranes witnessed Middle Paleozoic (~ 360 Ma) magmatism, suggesting an Andean-type orogeny derived from the subduction of the Paleo-Tethys Ocean. (6) The closure of Paleo-Tethys Ocean between the North and South Lhasa terranes and subsequent terrane collision resulted in the formation of Late Permian (~ 260 Ma) HP metamorphic belt and Triassic (220 Ma) MP metamorphic belt. (7) The South Lhasa terrane experienced Late Cretaceous (~ 90 Ma) Andean-type orogeny, characterized by the regional HT/MP metamorphism and coeval intrusion of the voluminous Gangdese batholith during the northward subduction of the Neo-Tethyan Ocean. (8) During the Early Cenozoic (55–45 Ma), the continent–continent collisional orogeny has led to the thickened crust of the South Lhasa terrane experiencing MP amphibolite-facies metamorphism and syn-collisional magmatism. (9) Following the continuous continent convergence, the South Lhasa terrane also experienced MP metamorphism during Late Eocene (40–30 Ma). (10) During Mesozoic and Cenozoic, two different stages of paired metamorphic belts were formed in the oceanic or continental subduction zones and the middle and lower crust of the hanging wall of the subduction zone. The tectonic imprints from the Lhasa terrane provide excellent examples for understanding metamorphic processes and geodynamics at convergent plate boundaries.
The Gangdese batholith emplaced in southern Tibet in the Cretaceous has been widely regarded as the major component of an Andean-type convergent margin resulting from the northward subduction of the Neo-Tethyan oceanic lithosphere beneath Asia. While the Gangdese batholith consists predominantly of calcalkaline and adackitic ganodiorites, we have recognized a suite of chamockites in the eastern part of the Gangdese belt. They consist of andesine, microcline, enstatite, diopside and Ti-rich biotite, with FeO/(FeO + MgO) ratios of 0. 47 ∼ 0. 50, MALI of - 3. 97 ∼ + 0. 11 , and ASI of 0. 83 ∼ 0. 92, suggesting a typical characteristics of magnesian, alkali-calcic and metaluminous granitic rocks. The estimated temperature and pressure conditions for the crystallization of chamockites are 850 ∼950°C and >0. 4GPa, and the speculated formation depth is below 13 ∼ 15km. Zircons from the chamockites are typical of magmatic origin, and give the weighted mean ages of 87Ma. This demonstrates that the chamockites may have formed in a continental magmatic arc. The presence of chamockites and associated high-temperature granulites in the southern Gangdese indicates that the Gangdese belt has been subjected to an Andean-type orogenic event in the Late Cretaceous.
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