Spatiotemporal Variation of the Cretaceous‐Eocene Arc Magmatism in Lhasa‐Tengchong Terrane

Abstract It was recognized that two magmatic belts in the Lhasa‐Tengchong terrane formed due to the Mesozoic‐Cenozoic Tethyan evolution. Still, their spatiotemporal variations of magmatic flare‐ups/lulls are rarely discussed. Here we use the new U‐Pb and Lu‐Hf isotopic data of captured zircons and a comprehensive data set to show that the flare‐up of northern magmatic belt has peak ages of 110 Ma in central and northern Lhasa and 120 Ma in eastern Tengchong, possibly related to the tectonic transition from Meso‐ and Neo‐Tethyan double subduction to Neo‐Tethyan single subduction. For the southern magmatic belt, the flare‐ups at 100–85 Ma and 65–45 Ma in eastern southern Lhasa indicate obvious juvenile crustal growth, while flare‐ups at 75–45 Ma in western southern Lhasa and Tengchong record ancient crustal reworking. Such flare‐up variations in the southern magmatic belt possibly resulted from asynchronous changes in the Neo‐Tethyan slab dip

Discovery of Cenozoic magnetite volcanic rock in High Himalaya and its tectonic significance

Late Cenozoic ultra-mafic magnetite-rich volcanic rocks in the High Himalaya metamorphic belt, southern Tibet, are firstly reported in this study. The rocks are mainly composed of Fe-olivine, Fe oxides, Fe-garnet and potassic-rich glasses. A few euhedral Fe-spinel inclusions are discovered in the Fe-olivine grains. Vesicular structure, vitro basic porphyritic texture and typical spinifex texture are normally observed in the rocks. The bulk rock composition of the rock has a feature of strong Si-unsaturation (18. 8% similar to 29. 7% SiO2) and extreme Fe-richness (56. 2% 74. 2% Fe2O3T). Geochemical analyses show that it is strong rich in LILE of Th and U, relative depletion in high field strong elements such as Nb, Ta and Ti, and obvious negative anomalies for Sr element, indicating a geochemical process related to subduction. The volcanic lava cuts across the regional gneissic foliation, implying that it was probably formed in a post-collisional intracontinental extensional environment. Further, the Fe-rich and Si-unsaturated feature also indicates that it might be formed as a result of eruption along extentional fault and melting of Fe-rich protolith from a deep source. The volcano probably erupted during Pliocene-Pleistocene (2 similar to 4Ma) , based on its K/Ar ages between 4. 76 similar to 7. 25Ma, and surrounding rocks wrapped by lava with an apatite fission track ( AFT) age of 2. 04 +/- 0. 21Ma. Generally, the volcanic rock provides first volcanic evidence for post-collisional extension in the High Himalaya tectonic belt during Late Cenozoic, which is critical to further understand the tectonic framework of the southern Tibet Plateau and its evolutionary processes

dx.doi.org/10.6084/m9.figshare.24464134

Supporting information: Detailed methods, and Figure S1-S2.Table S1: U-Pb Age of captured zircons from Tengchong Cenozoic volcanics.Table S2: Lu-Hf isotope of captured zircons from Tengchong Cenozoic volcanics.Table S3: Lhasa-Tengchong magmatic database.Table S4: Compiled magmatic zircon Hf isotopic values and ages from the Lhasa-Tengchong terrane.</p

Multiple events in the Neo-Tethyan Oceanic upper mantle: Ru-Os-Ir alloys in the Luobusa and Dongqiao ophiolitic podiform chromitites, Tibet

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Melt/mantle mixing produces podiform chromite deposits in ophiolites : implications of Re-Os systematics in the Dongqiao Neo-tethyan ophiolite, northern Tibet

Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re-Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong-Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships.The ¹⁸⁷Os/¹⁸⁸8Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported ¹⁸⁷Os/¹⁸⁸Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645±0.00004 (2s; n=145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower ¹⁸⁷Os/¹⁸⁸Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (¹⁸⁷Os/¹⁸⁸Os: 0.12337±0.00001) is low compared with the carbonaceous chondrite value (¹⁸⁷Os/¹⁸⁸Os: 0.1260±0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809±0.00085). In contrast, the basalts have higher ¹⁸⁷Os/¹⁸⁸Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of ¹⁸⁷Os/¹⁸⁸Os (0.12979~0.14206). Correspondingly, the basalts have the highest ¹⁸⁷Re/¹⁸⁸Os ratios, up to 45.4±3.2, and the chromites have the lowest ¹⁸⁷Re/¹⁸⁸Os ratios, down to 0.00113±0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/¹⁸⁸Os: 45.4, ¹⁸⁷Os/¹⁸⁸Os: 0.34484; mantle peridotite: ¹⁸⁷Re/¹⁸⁸Os: 0.0029, ¹⁸⁷Os/¹⁸⁸Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the ¹⁸⁷Os/¹⁸⁸Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes.13 page(s

Archean mantle contributes to the genesis of chromitite in the Palaeozoic Sartohay ophiolite, Asiatic Orogenic Belt, northwestern China

Podiform chromitites in ophiolite usually are interpreted as the crystallization products of mafic magmas, contemporaneous with the generation of the ophiolite at mid-oceanic ridges or back-arc spreading centres. However, the real ages of the chromitites can rarely be determined directly, because their chemistry provides few opportunities for geochronology. Here we employ the ¹⁸⁷Re-Os¹⁸⁸ decay system (N-TIMS) to date the chromitite and a cross-cutting troctolite in the Sartohay ophiolite, northwestern China, and we have dated zircons separated from the troctolite by SHRIMP U-Pb methods. Inherited zircons from the troctolite yield a lower intercept age of 0.48 ± 0.08 Ga and an upper intercept age of 2.28 ± 0.11 Ga; two concordant grains give ages >2.4 Ga. Whole-rock Re-Os data for the troctolites and chromites plot between 2.45 Ga and 0.43 Ga reference isochrons. Plots of T-MA VS ¹⁸⁷Os/¹⁸⁸Os suggest mixing between ultra-depleted chromitite and suprachondritic troctolite, mainly affecting the ¹⁸⁷Os/¹⁸⁸Os of the troctolites; subsets of troctolite samples scatter around reference isochrons with ages of 0.4-0.5 Ga. The zircon data and the Re/Os data are consistent with published Sm-Nd evidence that the troctolites, and hence the Sartohay ophiolite, were formed in Palaeozoic time. However, the ¹⁸⁷Os/¹⁸⁸Os compositions of the chromites (0.1109 ± 4 to 0.1249 ± 5), give Neo-Proterozoic to Paleo-Archean model ages (T-MA = 0.8-3.5 Ga), indicating extraction from the primitive mantle as early as 3.5 Ga, some 3 Ga before the formation of the Sartohay ophiolite. A Re-Os apparent isochron age (2.45 Ga), the old T-MA model ages and the ancient zircon ages (>2.4 Ga) all are consistent with part formation of the Sartohay chromitite in Archean time, and then reworked in the Palaeozoic time. We suggest that a volume of early Archean depleted mantle remained within the Siberian lithospheric mantle for 2.5-3.0 Ga; it later became portions of the mantle wedge above the Paleozoic subduction zone of the Paleo-Asian Ocean, and ultimately was incorporated into the overthrust ophiolite.8 page(s

Distinctive melt activity and chromite mineralization in Luobusa and Purang ophiolites, southern Tibet: constraints from trace element compositions of chromite and olivine

To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites. In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements (e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements (e.g., Nb, Zr). Such melts resemble MORB-like melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition, the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively, and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels

Tibetan chromitites : excavating the slab graveyard

Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (>400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages (TRD) of 234 ± 3 Ma, while TRD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 ± 7 Ma, εHF = 9.7 ± 4.6, and δ¹⁸O = 4.8‰-8.2‰. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.4 page(s

New Paleomagnetic and Chronological Constraints on the Late Triassic Position of the Eastern Qiangtang Terrane: Implications for the Closure of the Paleo-Jinshajiang Ocean

The tectonic evolution of the Paleo-Jinshajiang Ocean, and in particular the time of its closure, is debated. Here we present new constraints on the evolution of this region from an integrated paleomagnetic and geochronologic study of the Late Triassic Batang Group volcanic rocks in Qamdo, Eastern Qiangtang Terrane (EQT). Two zircon U-Pb ages indicate that the volcanic rocks erupted at ∼227–222 Ma. New paleomagnetic results yield a robust Late Triassic paleopole of 57.6°N, 176.4°E (A95 = 7.8°), corresponding to a paleolatitude of 32.5 ± 7.8°N for the study area. Integrated with other lines of paleomagnetic and geological evidence, we show that the northward drift of the EQT placed it at the same paleolatitude as the Tarim Block by ∼227–222 Ma, which suggests that the Paleo-Jinshajiang Ocean in the Qamdo region closed before that time, and likely around ∼230 Ma

New Paleomagnetic and Chronological Constraints on the Late Triassic Position of the Eastern Qiangtang Terrane: Implications for the Closure of the Paleo-Jinshajiang Ocean

The tectonic evolution of the Paleo-Jinshajiang Ocean, and in particular the time of its closure, is debated. Here we present new constraints on the evolution of this region from an integrated paleomagnetic and geochronologic study of the Late Triassic Batang Group volcanic rocks in Qamdo, Eastern Qiangtang Terrane (EQT). Two zircon U-Pb ages indicate that the volcanic rocks erupted at ∼227–222 Ma. New paleomagnetic results yield a robust Late Triassic paleopole of 57.6°N, 176.4°E (A95 = 7.8°), corresponding to a paleolatitude of 32.5 ± 7.8°N for the study area. Integrated with other lines of paleomagnetic and geological evidence, we show that the northward drift of the EQT placed it at the same paleolatitude as the Tarim Block by ∼227–222 Ma, which suggests that the Paleo-Jinshajiang Ocean in the Qamdo region closed before that time, and likely around ∼230 Ma