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Migration of the Tibetan Cenozoic potassic volcanism and its transition to eastern basaltic province: Implications for crustal and mantle flow
... Current studies are investigating the existence of mantle-scale material flow and its front edge locations (Fig. 1). Mo et al. (2006Mo et al. ( , 2007 proposed the existence of low-density asthenospheric flows beneath the Tibetan Plateau based on spatiotemporal variations of postcollisional magmatism in the Tibetan Plateau and its surrounding areas. Subsequent validation of this concept has been provided by high-resolution geophysical data (e.g., Lei et al., 2019;Wang et al., 2008;Wu et al., 2015). ...
... However, the front edge of the Tibetan mantle flow is controversial. On one hand, Hou et al. (2021Hou et al. ( , 2024 and Mo et al. (2006Mo et al. ( , 2007 proposed that part of the Tibetan mantle is likely to flow eastward into the Sanjiang region in southwestern China due to compression from the west, south, and north, and ultimately upwelling in the structurally weaker Pingbian-Maguan area. The latest geophysical data suggest that this kind of mantle upwelling may originate from the continuous subduction of the Indian plate (Hou et al., 2023a(Hou et al., , 2023b(Hou et al., , 2024. ...
... The intracontinental alkali basalts in the SE Tibetan Plateau erupted in a post-collisional intracontinental extensional setting and are thought to be related to mantle upwelling and decompression melting (Kang et al., 2022;Mo et al., 2009;Lee et al., 2016;Zhang et al., 2020b). There are several potential dynamical mechanisms that may have triggered the erup-tion of regional alkali basalts, including the edge-driven convection caused by lithospheric thickness/structure variations (Tappe et al., 2016;Zhou et al., 2023), the Hainan mantle plume (Wu et al., 2023;Yu et al., 2020), the slab rollback (Lee et al., 2016;Maury et al., 2004) or tearing (Sano et al., 2022;Zhang et al., 2020b) of the subducted Indian slab, and the Indian-Eurasian collision-induced Tibetan asthenospheric mantle flow (Hou et al., 2021;Mo et al., 2006Mo et al., , 2007. ...
Tibetan lateral mantle flow could help to decipher the material movement mechanisms within global plate convergence zones. However, the front edge of this mantle flow is unclear. We conducted petrological, geochronological, mineralogical, geochemical, and Sr-Nd-Pb isotopic investigations of Quaternary intracontinental alkali basalts from southwestern Yunnan (south of 27°N) to determine the petrogenesis of the Quaternary alkali basalts in the southeastern Tibetan Plateau in particular and to trace the recent Tibetan lateral mantle flow. Alkali basalts in the region are mainly basanite and trachybasalt that erupted during the Pleistocene epoch. They possess highly incompatible elemental and radiogenic Sr-Nd-Pb isotopic compositions similar to those of the oceanic-island basalts, consistent with melts derived from asthenospheric mantle with a low degree of partial melting. Calculated magma water contents of regional alkali basalts range from 1.32 ± 0.48 wt% to 2.23 ± 0.18 wt%, which corresponds to water content of their mantle source comprising 269 ppm to 3591 ppm, which is significantly higher than that of the normal upper mantle (i.e., 50−250 ppm). Quantitative trace-element modeling and dramatic variations in oceanic crust−sensitive indicators such as Eu/Eu*, Sr/Sr*, Ce/Pb, (Nb/Th)N-PM, and (Ta/U)N-PM indicate variable contributions of upper and lower oceanic crust to magma sources. Systematic examinations of petrological, geochemical, and geophysical evidence reveal that the temporary small-volume Quaternary volcanism in the southeastern Tibetan Plateau is unrelated to Tibetan southeastward mantle flow but is primarily attributed to stagnant Neo-Tethyan slab in the mantle transition zone. Our study offers a distinctive perspective for reconciling the geochemical features of intracontinental alkali basalts and highlights the potential role of alkali basalts in tracing the front edge of recent Tibetan lateral mantle flow.
... Seismic imaging studies (e.g., Jiang et al., 2016;Guo and Chen, 2017;Song et al., 2018;Luo et al., 2020) revealed relatively high shearwave velocities in the mid-to-lower crust beneath the QOB and DBS, suggesting the absence of crustal flow beneath these regions. According to the temporal and spatial distribution of the Cenozoic volcanic rocks and other geological and geophysical data, Mo et al. (2007) inferred the existence of two asthenospheric mantle flow channels beneath the eastern TP: one channel flows southeastward to the southeastern TP along the Sanjiang Orogenic Belt, and the other flows eastward to east China along the Qinling-Dabie Orogenic Belt, which might be related to the uplifting of the HNMC, DBS and SNHL. However, these possible relationships requires further investigation. ...
The Qinling Orogenic Belt (QOB) is an important tectonic belt in eastern Asia, which is believed to have been formed by the intracontinental collision between the North China Craton and South China Block. By analyzing radial P-wave receiver functions recorded by 239 broadband stations, we investigate the crustal thickness and Vp/Vs ratio maps under the QOB and its adjacent regions using the H-k and common-conversion-point (CCP) stacking methods. The crustal thickness (H) and Vp/Vs ratios (k) showed significant lateral variations. The assessment of crustal isostatic compensation reveals that the topography of the Dabashan and central QOB are compensated by crustal thickness, although the later may not be fully compensated. The crust of the western and eastern QOB are possibly in a non-Airy-type isostasy state, and the low-density uppermost mantle may play an important role in compensating their elevations. The Dabashan and the region to the south have thick crustal thickness and high Vp/Vs ratios, while the east QOB have thin crustal thickness and low Vp/Vs ratios. The CCP images reveal two strong positive amplitudes at depths consistent with the Moho interface near the Ankang fault. These observations indicate complex lower crustal structure and suggest the existence of eclogitized lower crust beneath Dabashan and lower crust delamination beneath east QOB. We also observe a continuous area with thick crust and high Vp/Vs ratios that covers the Dabashan, Hannan–Micang Dome, Shennong–Huangling Dome and their surrounding regions. This area shows consistency in location with a rapidly uplifted area since approximately 15 Ma, which—with other geological studies—implies the asthenospheric flux from the Tibetan Plateau. The asthenosphere flow may have eroded the lithosphere beneath the Dabashan and promoted the lower crustal eclogization.
... Volcanic fields within the Tibetan Plateau can be divided into three zones, all of which are controlled by ongoing collision (Deng 2003;Hou et al. 2006;Guo & Wilson 2019). Accounting for evolution of the plateau, Mo et al. (2007) argued that the location of volcanism followed the progression of the uplifted asthenosphere to the northwest, east-northeast, and southeast, into the uplifted margins where the historical eruptions of Tengchong and Ashikule (Kunlun mountains) occurred. ...
China ishas a rich record of Holocene volcanism that is relatively little known outside the country. It is encountered in home to a number of volcanoes that have erupted in the Holocene. These range from large stratovolcanoes in the northeast, linked to subduction of the pPacific plate (e.g., Changbaishan); in , more diffuse volcanismsmaller volcanoes on the edges of the TibetTibetan margin, linked toassociated with the collision of India and AEurasia (e.g., Tengchong, Ashishan), and more isolated regions of volcanismcentres possibly linked topossibly resulting from mantle upwelling (e.g., volcanoes in Hainan island). This makes China a natural laboratory for studyingstudies of intraplate volcanism, yet the study of volcanology in China is young, with a significant increase in research only over the last 25 yearsand significant progress in understanding its nature and origins has been made over the past quarter century. To highlight recent advances and the current state of knowledge, thisHere, we introduce the first publication in English to provide a comprehensive survey of the state of knowledge and research highlights. special volume presents the first compilation of research on the active volcanoes of China in English. This first paper introduces the book, which coversAccordingly, we provide an overview of the dynamics, geology, geochemistry, volcanic histories and geophysical studies of the 14 volcanoesvolcanic areas that have erupted in theassociated with Holocene documented thus far. Our hope is that this special publication acts as The special publication represents a benchmark reference on the topic but, as importantly, we hope it will stimulatea resource to allow new, international collaborations to be developed to help understandaimed at deepening our understanding of the origins, history, hazards and associated risks from future eruptions of China's volcanoes.
... 结合前人在青 藏高原东缘的地震层析成像和接收函数观测研究成 果 [25,27,34] [41] . Mo等人 [42,43] . Bao等人 [44] [45,46] 都显示秦岭地块下方除了其西部岩石圈地幔在青藏 图 5 青藏高原及其邻区新生代火山岩的时空分布叠合于 95 km深度剪切波速度分布图(据Bao等人 [4] 和Mo等人 [42] 修改) Figure 5 Shear-wave velocity map at 95 km depth of the Tibetan Plateau and its adjacent area, overlain by spatial and temporal distribution of the Cenozoic volcanisms (compiled from Bao et al. [4] and Mo et al. [42] ) [5,28,46,47] , 这个低速带可能成为祁连造山带下方地 壳中的一个物理软弱层 [46] , 虽然这一中下地壳低速 [49,50] , 因而可能有更高的黏度 [5,46] [1] . ...
... Cenozoic agpatitic basanite and potassic basalt with abundant mantle inclusion exposed in Maguan in Yunnan (Fig. 6) are also characterized by a right-leaning REE pattern with no Eu anomaly (δEu¼ 0.99-1.01) (Xia and Xu, 2005;Yu et al., 2006;Mo et al., 2007). Similar rightleaning REE records without Eu anomaly (δEu¼ 0.984) are also reported by Zhang and Schärer (1999) for the Jingping (Phan Si Pang) alkali granite in northern Vietnam. ...
... The young basalts cropping out near Hangzhou Bay are located at the eastward extension of the Qinling-Dabie suture (Fig.3), which was mainly formed in the Late Triassic (Meng and Zhang, 2000) with a broad 'n'-shape LAB boundary across the suture Li and van der Hilst, 2010). The suture was proposed as a channel (Mo et al., 2007) for the north-eastward extrusion of the Tibetan asthenosphere passing through the Sichuan basin and the Ordos block, both of which could have deep lithospheric roots. Consequently, we propose that the post-mid-Miocene basalts near Hangzhou Bay could have a similar origin to those in the western and central zones of SCCB, that is, they were the production of the local decompressional melting and originated from the lateral asthenospheric flow from Tibet along the Qinling-Dabie suture (Fig. 2). ...
South China Cenozoic basalts (SCCB) are regionally distributed at the south-east coastal area and can be grouped into three zones: a western zone (>38 Ma), a central zone (17–8 Ma) and an eastern zone (<8 Ma), leaving a temporal gap at 38–17 Ma between the western and central zones. An eastward migration of the SCCB could therefore be identified by the systematic decrease in the eruption age from the western inland to the eastern coast. We propose that most SCCB were associated with the lateral asthenospheric flows moving along the lithosphere/asthenosphere boundary (LAB) beneath South China and the subsequent decompressional partial melting. The lateral asthenospheric flows had been pulled by the eastward retreat of the pan-Pacific plate subduction before 38 Ma and have been pushed by the northward indentation of the Indian plate into the Eurasian plate since 17 Ma.
The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike‐slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India‐Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike‐slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk‐rock Sr‐Nd‐Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low ⁸⁷Sr/⁸⁶Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ²⁶Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo‐Tethyan seafloor subduction. Based on ⁴⁰Ar/³⁹Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike‐slip faulting. These observations suggest that the India‐Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike‐slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low‐volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike‐slip fault systems.
The Sanjiang orogenic belt, located in southwestern China and the southeastern Tibetan Plateau, includes a variety of economically important metal deposits. Previous studies have focused on Lu-Hf isotopic mapping to suggest its lithospheric architecture and mineralization. In this study, we provide the results of Nd isotopic mapping and compare them with the results of Hf isotopic mapping based on the similarity of Sm-Nd and Lu-Hf isotope systems, which indicate three juvenile domains with high εNd(t) and young Nd model ages within the Eastern Qiangtang-Simao terrane, while presenting negative εNd(t) values over the entire horizon. The very negative εNd(t) and old Nd model ages found in the Tengchong-Baoshan terrane and Changning-Menglian suture suggest that these terranes are old and might be reworked. The Nd isotopic mapping of the Sanjiang orogenic belt also suggests a relationship between different lithospheric architectures and the locations of distinct ore deposits. Porphyry-skarn Cu–Mo–(Au) deposits occur in the juvenile crust, which has relatively high εNd(t) (−3.3–5.1) and young TDM ages, whereas skarn and hydrothermal vein-type W–Sn deposits and Pb‒Zn‒Cu‒Ag deposits are located in the low-εNd(t) area.
A large number of Eocene–Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau. In this study, we present zircon U–Pb ages, Hf isotope data, and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block. Zircon U–Pb ages were measured via LA–ICP–MS to be 39.6, 37.6 and 32.0 Ma. The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents. The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O, but deficient in MgO. All of the samples belong to the alkaline rocks. These rocks display enrichment in REE, LREE, and LILE, depletion in HFSE, and no obvious Eu anomalies. Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region: (1) the removed mafic lower crust dropped into the asthenosphere, forming the mix magma (Nariniya trachyte); (2) the upwelling asthenosphere triggered the crustal melting (Nariniya and Zamaqu orthophyre). The Eocene–Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic–magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination. This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region (~ 92°E) from Banda Co (~ 82°E).
The eastern margin of the Qinghai–Tibet Plateau (QTP) is the focus of studies on eastward lateral extrusion of the latter’s crustal material. This study aims to explore the structural response of the QTP’s eastern crust–mantle to the extrusion, and the basis for the latter’s geological structure. Data on long-period magnetotelluric sounding of cross-tectonic units and Bouguer gravity were used to determine the physical structure of the crust–mantle at the plateau’s eastern margin. The findings are as follows: (i) the apparent density structure indicates extensive presence of a low-density material in the middle–lower crusts of the Songpan and Sichuan–Yunnan blocks at the QTP’s eastern margin. On the other hand, the Yangtze cratonic block (Sichuan Basin) contains a material with a significantly higher density. To the west of the Longmenshan–Panxi tectonic zones, and along the lower crust at 40–50 km depth, is an obvious low-density zone aligned in a northeast–southwest orientation; (ii) the electrical structural model spanning Songpan block–Longmenshan tectonic zone–Yangtze block reveals three distinct electrical structural units along the cross-section bounded by the Longmenshan tectonic zone. The first is the Songpan block, which has high and low resistivity at the shallow layer and middle–lower crusts, respectively. Next is the Yangtze craton, which has low and relatively higher resistivity at the shallow layer and middle–lower crusts, respectively. Third is the Longmenshan transitional tectonic zone, whose shallow layer and deep structure are characterized by an electrical structure with a thrust nappe towards the east, and a high-conductivity material extending to the lithospheric mantle, respectively; (iii) the apparent density and electrical structures indicate that the Panxi tectonic zone has a weakened structure in the lower crust; and (iv) physical properties of the QTP’s deep structure indicate that its eastern margin may contain a middle–lower crustal fluid material with the attributes of high conductivity and low density. Its distribution is closely related to the uplift mechanism and deep seismogenic activities at the QTP’s eastern margin.
Estructura profunda e implicaciones geotectónicas del margen oriental del altiplano Qinghai-Tíbet
Resumen
El margen oriental del altiplano Qinghai-Tíbet (QTP, del inglés Qinghai-Tibet Plateau) es el área de la extrusión lateral hacia el Este de material cortical. Este trabajo se enfoca en explorar la respuesta estructural de las capas superiores en el altiplano y las bases para su estructuración geológica. Se utilizó información magnetotelúrica y anomalías de Bouguer para determinar la respuesta geofísica de las capas superiores en el margen occidental del altiplano. Dentro de los principales resultados se tiene: (i) la distribución de la densidad aparente indica la presencia de material de baja densidad en las capas medias y bajas de los bloques Songpan y Sichuan-Yunnan en el Este del QTP. Por otro lado, el bloque cratónico Yangtze (en la cuenca Sichuan) contiene material con una mayor densidad. Al oeste de las zonas tectónicas Longmeshan-Panxi, y a lo largo de las capas inferiores, entre 40 y 50 kilómetros de profundidad, hay una zona de baja densidad con orientación noreste-suroeste. (ii) El modelo eléctrico que abarca el bloque Songpan, la zona tectónica Longmeshan y el bloque Yangtze, revela tres unidades a lo largo de la sección cruzada subordinada a la zona tectónica Longmenshan. La primera unidad está en el bloque Songpan, con alta resistividad en la capa superficial y baja en las capas media e inferiores. Luego aparece el cratón Yangtze, con baja resistividad en la superficie y resistividad media en las capas media e inferiores. La tercera unidad es la zona tectónica transicional de Longmenshan, cuya estructura superficial y profunda está caracterizada por una estructura eléctrica asociada a una falla de cabalgamiento hacia el Este y alta conductividad de material que se extiende hacia el manto litosférico. (iii) La densidad aparente y las estructuras eléctricas indican que la zona tectónica de Panxi está debilitada en las capas inferiores. (iv) las propiedades geofísicas de la estructura profunda del altiplano Qinghai-Tíbet muestran que su margen oriental puede contener un fluido de material en las capas bajas y medias con características de alta conductividad y baja densidad. Su distribución está interrelacionada con el mecanismo de elevación y las actividades sismogénicas profundas en el margen oriental del altiplano.
Affected by both N-S and E-W directed compression, the lithosphere of the northeastern Tibetan plateau, located in the junction region of the youngest plateau and ancient blocks,has experienced active tectonic motions and intense deformation. Such dynamic processes should have left signatures in the gravity field of this region. To understand the gravity field and its relation to deep structure of the northeastern Tibetan plateau, this work made a multi-scale wavelet to the data from the Earth Gravitation Model 2008 (EGM2008) of this region. Our analysis separated the Bouguer gravity anomalies into different parts of shallow to deep and then estimated the corresponding source depths by logarithmic power spectrum technique. Moreover, we calculated the crust thickness and described the Moho topography. The two lithosphere density models respectively across the Longmen Shan orogenic belt and the western Qinling orogenic belt have been constructed, which permit to reveal the distribution features of the different materials in the crust and uppermost mantle beneath the study area. The results show that the crust-mantle structure in northeastern Tibetan plateau is different from that of the peripheral areas. The Sichuan basin, the Ordos basin and the Qilian block have homogeneous and rigid crust-mantle structure on the whole. While the crust-mantle materials in the Tibetan plateau are heterogeneous and plastic, where gravity anomalies strike dominantly in EW and SSE. It is inferred that the lithosphere materials extrude towards east in the interior of the Tibetan plateau,and then turn to south due to resistance of the Yangtze and the North China platforms in the margin of the plateau. The image of the crust thickness indicates that the northeastern Tibetan plateau gradually thins from west to east. However, there is no trace of "mountain roots" under the peripheral orogenic belts. Instead, the Moho below these areas uplifts. Combined with the low gravity anomalies, this study suggests that the upwelling mantle heat-flow interacts with the substances in upmost mantle and lower crust, uplifting the Moho or forming the new shallow Moho. We infer that under the influence of the northward motion of the Indian plate, thermal activity may have occurred in the Qiangtang block lithosphere, and then induced thermal mantle flow. This mantle flow shifts toward east along the asthenospheric channel. Due to the obstruction of the rigid lithosphere in the Yangtze platform and the North China platform, the mantle flow is forced to rise up in the western Qinling-Songpan tectonic node. The uplift of the Longmen Shan and the western Qinling as well as the generation mechanism of earthquakes in the western Sichuan and Gansu province may be all related to this dynamic process.
It has been accepted that the India-Asia continental collision started from about 65 Ma and completed at ∼45-40 Ma, followed by continued post-collisional convergence to the present. During the collision between India and Asia (∼65-40 Ma), the flatly subducted Neo-Tethyan oceanic slab might have undergone roll-back to steeper angles. This roll-back would not only account for the increase in the convergence rate but also induce the upwelling and decompression melting of the convective asthenosphere under Tibetan lithosphere, producing the collisional (or syn-collisional) volcanism. The Paleocene-Early Eocene (∼65-40 Ma) volcanic rocks in central and southern Tibet are the products of such collisional (or syn-collisional) volcanism. These collisional (or syn-collisional ) volcanic successions have a large compositional range from basaltic to rhyolitic, rather than simplex felsic and are interpreted to be the result from an asthenosphere source with component of εNd (t) ≈ + 3, 87Sr/86Sr (t) ≈ 0.705 and La/Nb ≈ 0.8. On the basis of petrogeochcmical data, the Paleocene-Early Eocene basic lavas can be classified into high-Ti/Y (HT, Ti/Y 500) magma type and low-Ti/Y (LT, Ti/Y < 500) magma type that can be further divided into two subtypes; LT1 and LT2. The HT and LT1 lavas are uncontaminated basic lavas characterized by relative high Nb/La ratios (0- 88-1.53) and absence of negative anomalies of Nb, Ta, and Ti on mantle-normalized plots; whereas the LT2 lavas are crustally contaminated basic lavas with low Nb/La ratios (0.20-0.49) and obvious depletions in Nb, Ta, and Ti. The chemical evolution of the Lagala basalts and Bangdaco alkaline basalts (in central Tibet) is controlled by an olivine (ol) + clinopyroxene (cpx) fractionation, but gabbroic fractionation accounts for the chemical variation of the Linzizong volcanic succession from southern Tibet. Elemental and isotopic data suggest that the chemical variation of Tibetan plateau Paleocene-Early Eocene basic lavas cannot be explained by crystallization from a common parental magma. The Sr-Nd isotopic variation of the crustally contaminated LT2 lavas is related to the involvement of various lithosphere components in the ascending course of the asthenosphere-derived melts. The involvement of a lower crust component caused the LT2 lavas of the Dianzhong Formation, Pana Formation and the Lagala basalts to have a lower to negative value of εNd (t) ( + 1.3 to - 3.9) and lower value of 87Sr/86Sr (t) (0. 704 6 to 0. 706 5), whereas the LT2 lavas of the Dazi basic volcanics and the Nianbo Formation are characterized by higher 87Sr/86Sr (t) (0. 705 1 to 0. 708 4) and variable εNd (t) ( + 5-4 to - 4.0), which are related to the contamination of upper crust. Several samples of the LT2 lavas have high εNd (t) ( + 5.4 to +9.4) and low 87Sr/86Sr (t) (0. 704 6 to 0. 705 1), that are related to a pre-Cenozoic subduction modification of the lithospheric mantle source region. The earlier (65-44 Ma) erupted lavas were generated to a small degree (<10%) of partial melting in the garnet stability field at 3-4 GPa of the asthenosphere compared to the later (42-38 Ma) erupted lavas (10%-30%, in spinel-garnet transition zone). The extent of contamination by lithosphere was likely controlled by the ascending rate of asthenosphere-derived magmas. The parental magmas of the uncontaminated HT and LT1 lavas ascended rapidly and those of the strongly contaminated LT2 lavas ascended more slowly.
This paper discusses the composition and structure of the lithosphere of the Tibetan Plateau, especially of the main collision zone in southern Tibet upon the basis of petrological and geochemical studies, combining with geological and geophysical researches. The Indo-Asia main collision zone possess the thickest crust of the Tibetan Plateau, which consists of two different types of the crust, juvenile crust and recycled crust. The thicken crust formed by two mechanism, both structural thickening and the inputs of the mantle materials into the crust via magmatism. The lithospheric mantle underneath the Tibetan Plateau is inhomogeneous in petrology and geochemistry. At least three mantle isotopic reservoirs may be distinguished from the heterogeneity of Tibetan magmatic sources: (1) a Neo-Tethyan, Indian Ocean (DUPAL-like) component, (2) an EM2-rich Indian subcontinental lithospheric mantle component, and (3) a primordial Tibetan lithospheric mantle component generated prior to the India-Asia collision, which can also be considered the pre- (India-Asia) collisional Asian lithospheric mantle component. Also, some mantle-and lower crust-derived xenoliths carried by volcanics, and the outcrops of high pressure-ultrahigh pressure mineral assemblages have been found on the Tibetan Plateau. Three structural types of the lithosphere of the Tibetan Plateau can be distinguished, i.e., thickened lithosphere (Pamirs-type), thinned lithosphere (Gangdese-type) and thickened-thinned-rethickened lithosphere (Qiangtang-type). The temporal relations among these three structural types of the lithosphere, however, is unclear so far. The post-collisional potassic-ultrapotassic volcanism along the Gangdese was presumably related to slab break-off of the subducting Neo-Tethyan plate and the subduction of Indian continental lithosphere beneath the Lhasa Block, with different mechanism in western, middle and eastern segments, respectively. In the northern part of the plateau (the Qiangtang, the Hoh Xil, etc.), however, volcanism could be related to a wavelike outward propagation of upwelling asthenosphere. In the northern margin of the plateau (western Kunlun, Yumen, etc.), volcanism might be as a result of decompressive melting induced by large-scale strike-slip faulting. Migration of collisional and postcollisional volcanism with time shows a highly distinctive pattern. Initially, as an initial response to the India-Asia collision, igneous activity migrated northward between ca. 65 and 45 Ma, away from the Tsangpo collision suture. Between ca. 45 and 6 Ma, volcanic activity migrated outward from the plateau interior, implying wavelike outward propagation of upwelling asthenosphere. A third stage, still in progress, is marked by the migration of activity to northwestern, northeast-eastern, and southeastern peripheral regions of the plateau between 6 Ma and the present. Overall, such a highly distinctive pattern of activity can be interpreted to reflect lateral asthenospheric mantle flow or lower crust flow induced by the approach, and ensuing collision, of relatively thick (India and Eurasia) continental plates.
A suite of Cenozoic shoshonitic basalts bearing abundant mantle-derived xenoliths and xenocrysts outcropped in Maguan area, Yunnan Province. This study provides results of petrological and geochemical characteristics of the Cenozoic shoshonitic volcanic rocks, aiming to offer some food to thoughts related to volcanism and mantle-crust interation induced by to the continental collision between India and Asia plates. Results showed that Cenozoic shoshonitic basalts from this area have relatively high and variable alkali contents (2.94%-8.23%), and are rich in potassium (average K 2O/Na 2O of 21 samples is 1.26). They are classified as shoshonitic basalts or basanite. They are enriched in both light rare earth elements (LREE) and large ion lithophile elements (LILE), and distribution patterns in the chondrite-normalized diagram and primitive-normalized spidergram collectively resemble the pattern of OIB. The shoshonitic basalts contain abundant mantle-derived xenoliths and xenocrysts, and are poorly crystallized with very low contents of phenocrysts, and have considerably high abundance of compatible elements such as Ni, implying that the Cenozoic basaltic rocks of Maguan area are representatives of primary magma derived from mantle sources. The relatively low Mg # of rocks, ranging from 0.49 to 0.72, can either be ascribed to the intrinsic characteristics of the source region or to the mixing of crust and mantle materials in the source region, which needs to be further studied. The Cenozoic shoshonitic basalts formed in a within-plate tectonic setting, the petrogenesis of which is related to the lateral extrusion of asthenospheric mantle along the southeastern of Tibetan plateau induced by the Indo-Asia collision.
The REE analysis of the sediment from six wells in Yinggehai basin in northwestern South China Sea finds out that the sediment of N4, N5 and N6, located along the axis of the basin, shows positive Eu-anomalies from Late Oligocene to Pleistocene, suggesting that the parent rock should be the basic-ultrabasic metamorphic rocks and volcanic rocks within the Red river catchment; the sediment of N1 and N2 in the eastern basin are controlled by Hainan Island, which features negative Eu-anomalies; The sediment of N3 in southeastern basin had been influenced by mafic provenance since Late Oligocene, causing Eu positive anomalies, before being dominated by Hainan Island in Pliocene. Both the metamorphic rocks within the Red drainage and the Middle Paleozoic metamorphic rocks along the Song Ma Fault Zone in the central-northern Vietnam, provided a large amount of basic sediment to the basin, resulting in Eu positive-anomalies in the sediment from this area. We believe that the parent rock type within the Red River drainage stayed stable from Late Oligocene to Pleistocene without any abrupt change. Therefore the event must have taken place before Late Oligocene or after Pleistocene if the Red river had been captured at all.
Cenozoic basaltic magmatism in Maguan and Pingbian area, southeastern Yunnan Province, is an important component of post-collisional magamatism of eastern Tibetan Plateau. Whole-rock 40Ar/39 Ar dating results reveal two pulses of magma eruption occured in Maguan area, in 12. 9 ±0. 2Ma and 21. 2 ±1. 2Ma, respectively. Basalts in Pingbian area are very young products of lately erupted magma, with age less than 1. 7Ma, which is well synchronous to the magmatism distributed in Tengchong, Vietnam, Hainan and South China Sea area. According to CIPW normalization and whole-rock chemistry, basalts from Maguan can be classified into olivine tholeiite, alkaline olivine basalts and basanite, which originate from depths of 57 ∼73km, 82km and 67.5 ∼87km, respectively. Basalts from Pingbian are exclusively basanite, with segregation depth ranging from 79km to 88km. Basalts show OIB-like geochemical characteristic, with enrichment in light rare earth elements, large ion lithophlie elements and high field strength elements (Nb, Ta), high Nb/U ratios and without Eu anomaly. Relatively high initial 143Nd/144 Nd and low 87Sr/86Sr ratios and positive εNd collectively ascribe the source region of basalts to a hybrid of depleted asthenospheric mantle and EM I (and/or EM II) type enriched mantle, which is akin to those of basalts from northern Vietnam and South China Sea area. Simulation results and Re-Os isotopic characteristics illustrate that the source region of basalts situates at the transition zone of spinel and garnet, and the basaltic melt is a mixture of partial melt of garnet-facies lherzolite and spinel-facies lherzolite, with melt degree of 1% and 2% ∼5%,respectively. As a response to the huge collision between India and Eurasian plate onset around 65Ma, significant block rotation, extrusion or escape took place along Jinshajiang-Ailao Shan-Red River strike-slip fault belt, which resulted in the migration of deep material beneath Tibetan Plateau toward surrounding area. The suction of Western Pacific subduction zone stimulates the flow and upwelling of asthenospheric mantle to migrate along southeast direction, and subsequently bring forth the Cenozoic magamatism in southeastern Yunnan Province. As a result, volcanism in this area is also a response to the collision between Indian and Asian plate, and exceptionally records the lateral extrusion of asthenospheric material beneath Tibetan Plateau and accompanying migration along the Ailao Shan-Red River strike-slip belt.
Beiya gold-polymetallic ore district found in northeastern Yunnan province in the last few years, located in the middle-southern section of the eastern Xizang-Jinsha River - Ailao Mountain alkali-rich porphyry metallogenetic belt, is one of the typical gold-polymetallic metallogenesis system related to alkali-rich intrusion during the Himalayan epoch. Based on the previous researches and the extensive field investigations from the recent scientific programs, this article systematically studies the petrogenesis, metallogenesis evolving sequence in the ore district and their responses to the Indo-Asian collisional processes. The main intrusion rock types including quartz-albite porphyry, quartz-K-feldspar porphyry and biotite-K-feldspar porphyry with adakitic magma affinity, which intruded during the period of 65 to 59 Ma, 36 to 32 Ma, 26 to 24 Ma and 3.8 to 3.6 Ma respectively. The magmas of ore-bearing alkali-rich porphyries were derived from mixed melting of the crust-mantle transitional layer in eastern margins of the Tibet plateau, and genetically related to the asthemosphere eastern-extruding and the large-scale strike-slip faulting due to the Himalayanian Indo-Asian collisional systems. The duration of the first episode and the second episode are consisted with the two concentrative petrogenesis episodes approximately. Within the ore district, three types and seven subclasses of gold-polymetallic deposits have been recognized, there are (1) porphyry copper-gold-polymetallic deposits related to the first and second stage of alkali-rich porphyry intrusion, including skarn-type gold-polymetallic deposits (including magma-type), porphyry-type copper-gold deposits and hydrothermal-type gold-polymetallic deposits, (II) porphyry-related hydrothermal-type gold-polymetallic deposits due to the third stage of alkali-rich porphyry intrusion, including cryptoexplosive breccia-type gold-ferrous-lead- zinc-polymetallic deposits and hydrothermal superimposed-type gold-polymetallic deposits, and (III) weathered-sedimentary-type deposits associated with the fluvial and lacustrine sedimentary rocks formed by the surface weathering and the laterization, including red clay-type gold deposits and palaeoplacer-type gold deposits. The I- and II-type deposits were controlled directly by the alkali-rich porphyry intrusion and NE-NNE-trending strike-slip faulting, and occur in the porphyry intrusions, the cryptoexplosive breccia pipes, the porphyry contacts and the porphyry outside contacts of the fractured zone and the intensive joint zone within middle Triassic limestone of Beiya Formation. The earlier deposits usually were overprinted by the later epithermal mineralization system associated with late magmatism, which formed either isolated, but spatially coexisted a high-grade giant deposit, and all provided the material sources for the palaeoplacer-type deposits. Correspondingly, ore-forming mineralization also changed from Cu-Au-(Mo) associations, Fe-Cu-Au-Pb-Zn associations to Au-Pb-Zn-Ag associations, the mineralization resources and fluid resources both come from the alkali-rich porphyries magmatism, and construct a porphyry-hydrothermal metallogenic system, the wall rocks only provide a space for the metal deposition. The secondary enrichment and supergenetic mineralization began at the time of the primary gold deposits occurrence, and brought up various weathered-sedimentary-type deposits. Among them, the palaeo red clay-type gold deposits formed during the period from Eocene to Oligocene, the palaeoplacer gold deposits formed during the period from 23 to 5Ma, and the modern red clay-type (including eluvium-type) gold deposits continued all along. Comparing to the regional evolution sequences, it is explained that the Beiya gold-polymetallic deposits be controlled by the evolution of the Indo-Asian collisional orogen and tectonomagmatism under the collision belt, which implies the same dynamic setting of paroxysmal mineralization of porphyry-type deposits to the eastern Xizang-Jinsha River-Ailao Mountain porphyry metallogenetic belt, displayed the long-distance effects in the structural transform zone of the main collisional orogenic setting since the Palaeocene. Episodically stress relaxation during tectonically transforming from transpressional (55-40 Ma) to transtensional (24-17 Ma) regimes probably caused multiple magmatic intrusions, which most likely result in the protraction of the hydrothermal system and superimprosed mineralization in the eastem Indo-Asian continental collision zone, and the duration from 36 to 32Ma is the main mineralization period of the porphyry-hydrothermal metallogenic system. There are great gold-polymetallic prospective reserves within the deep areas of ore district and surrounding regions. It is possible that the Yanyuan-Lijiang fault control a comparatively independent porphyry metallogenetic belt with close relationship to the Xizang-Jinsha River - Ailao Mountain porphyry metallogenetic belt.
Abstract This study carried out petrological and geochemical research on the mantle-derived peridotitic xenoliths borne in Cenozoic akali-basalts in Maguan, Yunnan Province, southwestern China. The first suite of Re-Os isotopic data of the peridotitic xenoliths was reported. The peridotitic xenoliths can be classified as spinel lherzolite, which show fertile characteristics in major elements and are depleted in light rare earth elements (LREE) and some high field strength elements (HFSE) and large ion lithophile elements (LILE) , such as Nb, Zr, Ti and Ba The Sm-Nd isotopic results of the peridotitic xenoliths imply that the subcontinental lithospheric mande ( SCLM) beneath Maguan area represented by the xenoliths is depleted and heterogeneous in Nd isotopes. Results of five wholerock Re-Os isotopic analysis show that the Os concentrations of the peridotitic xenoliths are relatively high ( 3. 29 x 10 -9 - 3.78 x 10 -9 ) , comparable to those of peridotite massif from orogenic belts, and the Re concentrations change in a relatively wide variation, ranging from 0.24 x 10 -9 to 0.54 x 10 -9 The 187Os/ 188Os ratios of samples vary between 0.12295 and 0.12530. Neither 187Re/ 188Os ratios nor Al 2O 3 contents show correlation with 187OsZ 188Os ratios, indicating that the Re-Os isotopic system is not solely controlled by melt extraction process. The Re-depletion age of the peridotitic xenoliths range from 254Ma to 604Ma, suggesting that the formation time of SCLM beneath Maguan area is at least no later than Neoproterozoic. The SCLM beneath Maguan area possessed a relatively complex structure and was not directly formed by the cooling of upwelling asthenospheric mantle induced by decompressions] melting. Alternatively, ancient melt extraction process of primitive mande gave birth to the original SCLM, which underwent modification and metasomatism by the melt/fluid derived from later geological processes, leading to parts of originally depleted peridotites transformed into relatively fertile peridotites, and leaving the other parts of SCLM intact with depleted characteristics. The melt/fluid modification process are probably attributed to the activities of Late-Permian Emeishan mantle plume and the Cenozoic potassic volcanisms occurred in Maguan area, which showed litde effect on the Os isotopic systems due to their young eruption age.
: The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However, their exact source and peterogenesis are still debated. 40Ar-39Ar and LAM-ICPMS zircon U-Pb isotopic dating confirm that these rocks erupted in Eocene. In addition, the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios, similar with the adakite derived from partial melting of oceanic crust. They can be further classified as high Mg# (Mg#= 48–57) and low Mg# (Mg#= 33–42) subtypes. The Nadingcuo adakitic rocks have relatively low (87Sr/86Sr)i and high εNd(t), showing a trend of similarity to the Dongcuo ophiolite present in the Bangong-Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong-Nujiang Ocean with 10%–20% crustal material of Lhasa terrane. On these bases we suggest that the low Mg# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment, and the high Mg# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge. Therefore, the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India-Asia during Cenozoic.
: Many igneous rocks distribute in Gejiu tin polymetallic ore-field at Yunnan province, rocks including basalt, gabbro, mafic microgranular enclaves, granites (porphyritic granite and equigranular granite) and akaline rocks. The ages of the granites and akaline rocks which are considered to have genetic connecting with the mineralization have been comfirmed, but the gabbro-mafic microgranular enclaves-granite assemblage's ages are still unknown. By means of LA-ICP-MS zircon U-Pb dating, the data of Shenxianshui equigranular granite, the mafic microgranular enclave in Jiasha area, the host rock of the mafic microgranular enclaves and the Jiasha gabbro are around ∼80 Ma. Besides the above mentioned data, a group of new ages at ∼30 Ma were discovered in this study, which is from gabbro and mafic microgranular enclaves. Based on the previous data and the new data gained this time, we suggest the major geochronology framework of the magmatism and mineralization events in Gejiu area is ∼80 Ma, which is consistent with the Late Cretaceous magmatism and mineralization events in the whole southeast Yunnan and west Guangxi area and they were suggested to belong to the same geotectonic setting in late Yenshannian. And the new ages of the ∼30 Ma obtained in this study is considered to represent a responding to the complicate tectonic evolution history of the Tibetan orogenic events in Cenozoic.
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