No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
... The Shuanggou ophiolite is the best preserved part of the Ailaoshan ophiolitic belt which formed by closure of a branch ocean of the Paleo-Tethys (Figure 1) (Mo et al., 1998;Zhong, 1998). The mantle sequence of the ophiolite is dominated by well-exposed plagioclase lherzolites with a smaller portion of harzburgites (Jian et al., 2009a(Jian et al., , 2009bZhang et al., 1995;Zhong, 1998). Previous studies on the mantle sequence indicated a role of melt impregnation in the formation of the lherzolites (Mo et al., 1998;Zhang et al., 1995;Zhou et al., 1995). ...
... The mantle sequence of the ophiolite is dominated by well-exposed plagioclase lherzolites with a smaller portion of harzburgites (Jian et al., 2009a(Jian et al., , 2009bZhang et al., 1995;Zhong, 1998). Previous studies on the mantle sequence indicated a role of melt impregnation in the formation of the lherzolites (Mo et al., 1998;Zhang et al., 1995;Zhou et al., 1995). Specially, mafic minerals (e.g., clinopyroxenes) in the lherzolites have isotope compositions (ε Nd (t) = −1 to +3) different from that of diabases in the crustal sequence (ε Nd (t) = +9.7 to +11.6) Hu et al., 2014), which is not common in studies on plagioclase peridotites in the magma-poor rifted margins (e.g., Müntener et al., 2004). ...
... The Re-Os isotope system has shown great potential in studies of mantle peridotites (Carlson et al., 2008;O'Driscoll et al., 2012O'Driscoll et al., , 2015Rampone & Hofmann, 2012;. Because Re (incompatible) and Os (compatible) behave differently during melting (e.g., Fonseca et al., 2011;Mallmann & O'Neill, 2007), the Zi et al., 2012) and (b) geological map of the Shuanggou ophiolite (after Zhang et al., 1995) melting process can create significant Re-Os fractionation between residues and melts, which, through time, will lead to distinct Os isotope compositions between the two members. Therefore, the Re-Os isotope system is efficient in tracking mantle-melt interactions and distinguishing different mantle domains (Ackerman et al., 2009(Ackerman et al., , 2013Carlson et al., 2008;Harvey et al., 2010Harvey et al., , 2011Luguet et al., 2007;Marchesi et al., 2014;O'Driscoll et al., 2012O'Driscoll et al., , 2015. ...
Petrology, geochemistry, platinum group elements (PGEs: Os, Ir, Ru, Rh, Pt, and Pd), and Re‐Os isotope data of harzburgites and plagioclase lherzolites from the Shuanggou ophiolite (southwest China) are presented in this study, in order to determine whether ancient refertilization process can be preserved in ophiolitic plagioclase peridotites. The harzburgites in the Shuanggou ophiolite are divided into two groups: (1) the Group‐1 harzburgites have ¹⁸⁷Os/¹⁸⁸Os ratios (0.12297–0.12727) and PGE abundances similar to those of oceanic peridotites, thereby representing the convecting upper mantle; (2) the Group‐2 harzburgites have extremely low ¹⁸⁷Os/¹⁸⁸Os ratios (0.11307–0.11651) with considerable fractionation in iridium group PGEs (Os, Ir, and Ru), indicative of an ancient refractory mantle in the oceanic lithosphere. The plagioclase lherzolites were formed by melt impregnation of the Group‐2 harzburgites, according to the PGE compositions and the petrographic features. However, the large Os isotope difference between the lherzolites (¹⁸⁷Os/¹⁸⁸Os: 0.12470–0.12666) and the Group‐2 harzburgites indicates that the refertilization process took place much earlier than the exhumation of the mantle at 0.4 Ga. In addition, clinopyroxene composition suggests that the percolating melt is different from the Shuanggou diabases and typical mid‐ocean ridge basalts. This study therefore demonstrates that some plagioclase lherzolites possibly were not related to mantle exhumation beneath the spreading center but formed by much older melt impregnation processes in the mantle instead.
... The Yuanjiang deposit is located in the southwestern portion of Yuanjiang county, Yunnan Province, Southwest China (Figure 1a). Tectonically, it is within the Ailaoshan suture, which marks the closure of the Ailaoshan ocean basin and subsequent late Triassic collision of the Yangtze plate in the northeast and the Simao-Indosinian plate in the southwest [30,31]. Ultramafic bodies occur as isolated exposures in this tectonic belt, and they have been interpreted to be a part of the Ailaoshan ophiolite complex that extends for >300 km along a NW-SE strike [32]. ...
... Thus, the climatic background and evolutional history responsible for the formation of the Yuanjiang lateritic Ni deposit can only be inferred from some regional paleoclimatic proxies. The complete history of the Yuanjiang deposit can be traced to the late Triassic, when the ophiolites (the parent rock of the Yuanjiang deposit) were tectonically emplaced [30,31]. Unfortunately, the available paleoclimatic materials involving in the study site and regional areas are limited to a period from early Jurassic to present. ...
... They are considered to be derived from the break-up of the Ailaoshan ocean basin in the early Carboniferous [32]. With the subduction and closure of the Ailaoshan ocean basin in late Triassic, fragments of the ophiolites were tectonically emplaced into the middle to late Triassic forming a mélange along the Ailaoshan orogenic belt [30,31]. From the late Triassic to Cretaceous, the ophiolite remnants were greatly impacted by the lndosinan-Yanshanian orogenic thrust, and the ultramafic rocks within the ophiolite belt experienced multiple stages of deformation and metamorphism [32]. ...
The Yuanjiang Ni deposit in southwestern margin of the Yunnan Plateau is the only economically important lateritic Ni deposit in China. It contains 21.2 Mt ore with an average grade of 1.05 wt % Ni and has been recognized as the second largest Ni producer in China following the Jinchuan super-large magmatic Ni–Cu deposit. This Ni deposit is hosted within the lateritic regolith derived from serpentinite within the regional Paleo-Tethyan Ophiolite remnants. Local landscape controls the distribution of the Ni mineralized regolith, and spatially it is characterized by developing on several stepped planation surfaces. Three types of lateritic Ni ores are identified based on Ni-hosting minerals, namely oxide ore, oxide-silicate mixed ore and silicate ore. In the dominant silicate ore, two phyllosilicate minerals (serpentine and talc) are the Ni-host minerals. Their Ni compositions, however, are remarkably different. Serpentine (0.34–1.2 wt % Ni) has a higher Ni concentration than talc (0.18–0.26 wt % Ni), indicating that the serpentine is more significantly enriched in Ni during weathering process compared to talc. This explains why talc veining reduces Ni grade. The geochemical index (S/SAF value = 0.33–0.81, UMIA values = 17–60) indicates that the serpentinite-derived regolith has experienced, at least, weak to moderate lateritization. Based on several lines of paleoclimate evidence, the history of lateritization at Yuanjiang area probably dates to the Oligocene-Miocene boundary and has extended to the present. With a hydrology-controlled lateritization process ongoing, continuous operation of Ni migration from the serpentinite-forming minerals to weathered minerals (goethite and serpentine) gave rise to the development of three types of Ni ore in the regolith. Notably, the formation and preservation of the Yuanjiang lateritic Ni deposit has been strongly impacted by regional multi-staged tectonic uplift during the development of Yunnan Plateau. This active tectonic setting has promoted weathering of serpentinite and supergene Ni enrichment, but is also responsible for its partial erosion.
... Previous studies have divided the basaltic rocks (i.e. microgabbros and basalts) of the Ailaoshan ophiolite into two types (Zhang, Zhou & Li, 1995;Zhong, 1998): high-alumina type with >16 % Al 2 O 3 and lowalumina type with <16 % Al 2 O 3 . However, the genesis of and relationship between these two types are still unclear. ...
... However, the genesis of and relationship between these two types are still unclear. For instance, Mo et al. (1998) and Shen et al. (1998a, b) interpreted the two different types as having been generated by two series of magmas which derived from different-degree partial melting of a common source, while Zhang, Zhou & Li (1995) suggested that their mantle sources were different. ...
... The Ailaoshan suture zone is over 1000 km long, extending from northern Vietnam to northern Yunnan, SW China ( Fig. 1) (Mo et al. 1998;Zhong, 1998;IP address: 170.140.26.180 Geochemistry of the Shuanggou ophiolite 3 Figure 2. Geological map for the Shuanggou ophiolite (after Zhang, Zhou & Li, 1995). Yumul et al. 2008;Lai et al. 2013a, b). ...
The Early Palaeozoic Shuanggou ophiolite is the best-preserved part of the Ailaoshan ophiolite belt. The microgabbros (basaltic dykes) and basalts (basaltic lavas) show distinct characteristics in geochemistry, implying that their genetic mechanisms are different. With Al2O3 contents ranging from 14.7% to 17.0%, the microgabbros belong to low-alumina type. They exhibit normal mid-ocean-ridge basalt (N-MORB) -like trace elemental characteristics with positive ε
Nd(t) values (9.7–11.6) and slightly variable (87Sr/86Sr)i ratios (0.7036–0.7046). In contrast, the basalts have high Al2O3 contents (19.5–23.2%), therefore belonging to high-alumina type. A plagioclase-accumulation model is used to account for the high alumina contents. Moreover, the basalts have enriched MORB (E-MORB) -like trace element characteristics with lower ε
Nd(t) values (6.4–8.0) and (87Sr/86Sr)i ratios (0.7032–0.7036). Their incompatible element ratios exhibit linear correlation with the isotopic data, which is probably related to the contribution of a mixed lithosphere–asthenosphere source. In summary, a two-stage model is proposed to explain the formation of the Shuanggou ophiolite: (1) at the continent–ocean transition stage, the basalts were generated by low-degree partial melting of the mixed mantle near a slow-spreading embryonic centre; and (2) at the mature stage of the Ailaoshan Ocean, the microgabbros were produced by moderate-degree partial melting of the depleted asthenospheric mantle.
... The Dupal anomaly in the East Asian Continental Margin and Pacific Ocean, divided by the subduction zone, is quite different, while the Dupal anomaly in the Indian Ocean and the Southern Atlantic Ocean seems likely to be a mixture of EMI and EMII, which is quite different to the others. Lei et al., 2008;Li, 2008;Li et al., 2007;Liu et al., 2013a, b;Peng et al., 2013;Wang et al., 2006;Wei et al., 2003;Wu et al., 2014;Zhang et al., 1995Zhang et al., , 2005Zhang et al., , 2015Zhou et al., 1995aZhou et al., , 2001Zhu et al., 2006). WPB, within-plate basalt; MORB, mid-oceanic ridge basalt; OIB, oceanic island basalt; CZ, Cenozoic; Mz, Mesozoic; Pz, Palaeozoic; Pt, Proterozoic. ...
... The Dupal anomaly in the East Asian Continental Margin and Pacific Ocean, divided by the subduction zone, is quite different, while the Dupal anomaly in the Indian Ocean and the Southern Atlantic Ocean seems likely to be a mixture of EMI and EMII, which is quite different to the others. Lei et al., 2008;Li, 2008;Li et al., 2007;Liu et al., 2013a, b;Peng et al., 2013;Wang et al., 2006;Wei et al., 2003;Wu et al., 2014;Zhang et al., 1995Zhang et al., , 2005Zhang et al., , 2015Zhou et al., 1995aZhou et al., , 2001Zhu et al., 2006). WPB, within-plate basalt; MORB, mid-oceanic ridge basalt; OIB, oceanic island basalt; CZ, Cenozoic; Mz, Mesozoic; Pz, Palaeozoic; Pt, Proterozoic. ...
The Dupal anomaly has been a frequently discussed feature since it was first proposed three decades ago. We here limit the distribution of the Dupal anomaly based on more than 10 000 Sr–Nd–Pb isotopic composition analyses and classify the anomaly into three types, located in three different oceans and with different mechanisms of formation. The Dupal anomaly in the East Asian Continental Margin subduction zone is related to enriched mantle II, which may originate from the recycling of the subducting plate or continental mantle. The high μ and enriched mantle I are the reason for the Dupal anomaly in the Pacific Ocean, which is related to the superplume. However, the source of the Dupal anomaly in the Indian Ocean and the Southern Atlantic Ocean is a mixture of enriched mantle I and enriched mantle II, and they come from the African superplume and the recycling of subcontinental mantle or continental crust of Gondwana. In consideration of all the above factors, we suggest that the superplume from the core–mantle boundary, the recycling subducting plate and continental mantle are the main characteristics of the global Dupal anomaly. Copyright
... Wang et al., 1995;Y.J. Wang et al., 1995;Dilek and Ahmed, 2003;Bazylev et al., 2009). REE totals from serpentinized dunite are slightly higher than the mantle rocks, and have slightly enriched REE, which is due to the later result of strongly serpentinized altered absorption of light rare earth elements (LREE) possibly through melt-rock reaction (Frey, 1984;Huang et al., 1995;X.B. Wang et al., 1995;Y.J. Wang et al., 1995;Qiu et al., 2005;Bazylev et al., 2009). ...
We report the presence of a Grenvillian ophiolite on the northern margin of the Yangtze craton, drastically changing current ideas about South China's role in plate reconstructions of the Rodinia supercontinent. Strongly deformed amphibolites that locally show relict pillow lavas, isotropic and layered metagabbro, diabase dikes, serpentinized dunite and harzburgite with podiform chromite are dated at circa 1100–985Ma (U–Pb zircon). The ophiolite is structurally dismembered and thrust over the Proterozoic shelf sequence that covers the north margin of the Yangtze craton, and overrode a flysch to conglomerate-wildflysch unit shed from the ophiolite and a magmatic arc terrane and deposited on the older Yangtze carbonate platform. The youngest clasts in the conglomerate are circa 861–813Ma (U–Pb zircon), giving a maximum age for ophiolite emplacement. Fine-grained layered amphibolites exhibit slightly depleted-flat type REE curves with no obvious Eu anomalies, and are N-MORB type tholeiites. Metagabbro has typical cumulate textures, flat REE distributions and obvious positive Eu anomalies. The REE characteristics of serpentinized dunites show a U-shape of slight loss of middle REE, representing cumulates metasomatized by LREE slightly enriched mantle. All these features indicate that the metamafic–ultramafic rocks from the Proterozoic Miaowan Formation form a structurally dismembered ophiolite resting above an ophiolitic wildflysch, sitting on top of the Proterozoic shelf sequence on the Yangtze craton. The ophiolite is contemporaneous with an arc sequence preserved to the north on the edge of the Yangtze craton, suggesting that the entire ophiolitic forearc–arc was accreted to the Yangtze craton between 1000 and 850Ma. Xenocrystic zircons in granite clasts in the basal wildflysch unit have ages consistent with Australian affinity, and detrital zircons in the arc sequence also show derivation from Australia, suggesting that the arc formed on the Australian segment of Rodinia before collision with the Yangtze craton. The discovery of the Proterozoic Miaowan ophiolite supplies important evidence for the existence of a Neoproterozoic oceanic basin on the north margin of the Yangtze craton, and demonstrates that the Yangtze craton first collided with Rodinia on its northern margin, with subsequent accretion of the Cathaysian block on the southern margin of the craton.
... The preliminary study of isotopic characteristics of the Babu opluolite shows features comparable with the southern hemisphere DUPAL anomaly. Such features were reported from the Shuanggou ophiolites, representing a branch ocean of Paleotethys in westYunnan (Zhang et al., 1995). The Babu-Phu Ngu ocean should be an eastward elongation of the Shuanggou ocean, and indicate a geosuture zone between the Huanan (South China) Subcontinent and the Indosinian Microcontinent in the region bordering China and Vietnam. ...
... To enhance the representativeness of this study, our sampling targeted rocks within and beyond the Shuanggou area, which is the main area of previous studies (Zhang et al., 1988(Zhang et al., , 1995Yumul et al., 2008). Samples were largely collected from boulders/blocks (2 m ≤ 10 km in dimension) in the CAL Ophiolitic mélange around the towns of Shuanggou, Bainadu, Pingzhang, Lanitang and Heping, and included ultramafics, gabbros, dolerite, basalt, and plagiogranite. ...
The Central Ailaoshan (CAL) ophiolite represents an important tectonic component of the Jinshajiang– Ailaoshan–Song Ma suture zone separating the South China and Indochina blocks in the mainland SE Asia. The CAL ophiolite occurs as a complex tectonic mélange, and preserves the history of the opening and closure of the once vast Jinshajiang–Ailaoshan–Song Ma branch of the Paleotethys. New and existing geological data indicate that the CAL ophiolite contains magmatic rocks generated by: (1) L. Devonian–E. Carboniferous (ca. 380–330 Ma) volcanic passive margin-breakup development in the NW Gondwana margin; (2) L. Permian (ca. 258 Ma) Emeishan large igneous province-related continental rift magmatism, together with (and intruded by) (3) earliest M. Triassic (ca. 244 Ma) continent–continent syn-collisional S-type granitoids. The Devono-Carboniferous suites of the CAL ophiolite are highly comparable with many continental margin-type Alpine Tethyan ophiolites. In addition, the various CAL magmatic suites have strong South China block-affinities with coeval magmatism particularly in the western South China block, Jinshajiang-, Song Ma-, and Song Da terranes.
... (La/Sm) N 0.61-0.78, which are consistent with those of N-MORB [27] and ophiolites from Babu [28] , Shuanggou [29] and Tongchangjie [30] in the Sanjiang area. In addition, the contents of LREE from other three samples (1627b2, GL03, GL06) of rocks are higher, and (La) N is 13.87-18.15, ...
N-MORB-type metabsites are discovered in the Guoganjianian area, central Qiangtang, Tibet, which are mainly metagabbro with
cumulate structure and metabasalt. The rocks are distributed nearly from west to east unconformably underlying the Wanghuling
Group of upper Triassic. On the basis of geochemical analysis, we find that the content of SiO2 is 43.03%–53.42%, and TiO2 1%–2.67%, Al2O3 16.75%–21.52%, CaO 7.03%–11.13%, K2O 0.05%–0.38%; the REE pattern is slight depletion or flat, and the trace spider diagram is like that of N-MORB, so we consider
that the metabasite was formed under the setting of mid-ocean ridge or adult back-arc basin, and it is the fragment of Paleo-Tethys
ophiolite.
In this contribution, we present new structural, microstructural, fabrics, and geochronological data from the southern Chong Shan complex, one of the metamorphic complexes in the southeastern Tibetan Plateau that were sheared and exhumed during the India-Eurasia convergence. The NW-SE−striking complex is comprised of a central high-grade metamorphic zone (Unit I) flanked by two low-grade metamorphic zones (units II and III) on the northeastern and southwestern sides, respectively. High-grade metamorphic rocks (e.g., amphibolites, sillimanite-mica schists) of up to amphibolite facies, of the Proterozoic Chong Shan group and granitic intrusions of Permo-Triassic to Cenozoic in age in Unit I are characterized by high-temperature deformation. Units II (i.e., the Wuliangshan group) and III (i.e., the Lancang group) on both sides of the high-grade Unit I consist of metamorphic rocks of low greenschist facies (e.g., phyllites) with low-temperature deformation. The high- and low-grade units possess consistent kinematics, i.e., northwestward motion of the core rocks relative to the two limbs, and they are separated by large scale shear discontinuities. Thereby, the high- and low-grade units are kinematically linked but mechanically decoupled. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of syn-shearing granitic dikes reveals that ductile shearing occurred from 29 to 19 Ma.
Structural analysis reveals that these units constitute an A-type dome that has long axis parallel to the stretching lineations and fold axes of outcrop-scale A-type folds. It is shown that three stages of deformation contributed to the formation of the southern Chong Shan dome, during which subhorizontal shearing were in connection with regional doming. The events occurred as the consequence of middle to lower flow that led to lateral crustal flow and vertical exhumation of crustal masses. Therefore, the lateral crustal flow was not only limited along the boundary high strain zones of the Sundaland block, but distributed within the southeastern Tibetan Plateau. We would argue that the tectonic extrusion of the Sundaland block occurred through ductile crustal flow of a viscous middle and lower crust in the plate interior combined with concurring channel flow along the block margins.
The Paleozoic Jinshajiang ophiolitic melange in southwest China marks an important branch ocean (i.e., the Jinshajiang Ocean) of the Paleo-Tethys. Basic-intermediate rocks are widespread features in the melange; their formation age is well known, but the petrogenesis has not been well studied, which means that the evolutionary history of the Jinshajiang Ocean is not well constrained. To understand the nature of the melange and the ocean, we present a set of elemental and isotopic data from two typical crustal sequences in two areas of the Jinshajiang ophiolitic melange, Zhiyong and Baimaxueshan. The basalts in the ca. 343 Ma Zhiyong crustal sequence show mid-oceanridge basalt–like geochemical compositions with Nb/La ratios of 0.98–1.15 and εNd(t) values of +6.5 to +7.7, indicating that the basalts formed in the spreading ridge of the ocean. In contrast, the 283 Ma Baimaxueshan crustal sequence consists of gabbros and basaltic-andesitic lavas, which have an arc affinity with Nb/La ratios of 0.54–0.67 and εNd(t) values of +5.1 to +6.5. The geochemical differences were not caused by crustal assimilation but reflect mantle metasomatism by fluids dehydrated from the subducting slab. Therefore, we propose that the Zhiyong and Baimaxueshan crustal sequences formed in seafloor spreading and subduction settings, which were related to the opening and closure of the ocean, respectively.
To reveal the petrological characteristics, metamorphic evolution history and tectonic setting of the pelitic granulites from Ailaoshan Orogen, West Yunnan, China, a comprehensive study in mineral chemistry, petrogeochemistry and geochronology studies is presented in this paper. Two metamorphic stages of the granulites can be established: (1) the peak metamorphism recorded by the mineral assemblage of garnet, kyanite, K-feldspar and rutile, and the initial retrograde metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, spinel, K-feldspar, plagioclase and biotite; (2) the superimposed metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and ilmenite. Zircon LA-ICP-MS U-Pb dating indicates that the protolith of the granulite was deposited after 337 Ma. The initial retrograde metamorphism occurred at P-T conditions of 8.6–12 kbar at 850–920 °C estimated by mineral assemblages, the low pressure limit of kyanite stability and GBPQ geothermobarometer in Indosinian (about 235 Ma), and the late superimposed metamorphism occurred at the P-T condition of 3.5–3.9 kbar at 572–576 °C estimated by GBPQ geothermobarometer since 33 Ma. The first stage was related to the amalgamation of the South China and Indochina blocks during the Triassic, and the second stage was possibly related with the large scale sinistral slip-shearing since the Oligocene. It is inferred that the upper continental crust was subducted/underthrusted to the lower continental crust (deeper than 30 km) and underwent granulite-facies metamorphism and then quickly exhumed to the middle-upper crust (10–12 km) and initial retrograde metamorphism occurred due to the collision of the Indochina and South China blocks during Indosinian, which was followed by superimposition of the second stage of metamorphism since the Oligocene.
It is usually considered that an integrated ophiolite section is represented by the Troodos ophiolite. Recent study shows that ophiolite sections vary a lot and can be assorted at least into two types. The first type, represented by the Troodos ophiolite, is a thick ocean crust with stratigraphic sequence and is characterized by sheeted dike swarms and thick cumulate unit. The second type, represented by Shuanggou ophiolite, is a thinner ocean crust only with mafic extrusive and intrusive rocks and is characterized by lack of sheeted dike swarms and ultramafic cumulates which may sometimes be poorly developed. Different types of ophiolite sections reflect different dynamic processes under the ocean ridge.
The Yao Shan complex, a massif near the southern segment of the Ailao Shan–Red River (ASRR) shear zone, bears important information on the structural framework of the massif and the kinematics of ductile shearing along the ASRR shear zone. In this contribution, structural, microstructural, quartz c-axis fabric, magnetic fabric, and geochronologic data are used to determine the structural framework of the Yao Shan massif and its tectonic implications for the ASRR shear zone. The Yao Shan complex is characterized by an overall linear A-type antiform that contains a core of high-grade metamorphic rocks with Palaeoproterozoic to Mesozoic protoliths and a mantle of Permo-Triassic low-grade rocks. Both the high-grade metamorphic core and low-grade Permo-Triassic rocks have experienced progressive ductile shearing. Anisotropy of magnetic susceptibility (AMS) results from 17 samples collected along the Xinjie–Pingbian section across the complex show that magnetic lineation (Kmax) and foliation (Kmax–Kint) are generally subparallel to the corresponding structural elements in the sheared rocks. The shape parameter E values of the magnetic ellipsoids are indicative of dominantly oblate and plane strain, but vary with protolith type and degree of strain among the various rock types. In agreement with the field and microstructural observations, the corrected degree of anisotropy (Pj) values reflect high shear strain in the core rocks and relatively low shear strain in the low-grade strata. A kinematic analysis based on structural and magnetic fabric data shows that both left- and right-lateral shear occurred during the deformation of the Yao Shan complex. Therefore, instead of being an element of the ASRR shear zone, the Yao Shan complex constitutes a crustal-scale inharmonic A-type fold with a fold axis parallel to the stretching lineation. Geochronologic data reveal that the folding occurred coevally with ductile shearing of the middle to lower crust between ca. 30 and 21 Ma.
To understand the characteristics and evolution of REE during the process of the ultramafic laterization under different climate conditions, two outcrops Kolonodale in Indonesia and Yuanjiang in China are chosen for comparision. It is found that the contents of REE from the laterite crusts are higher than those from the bed rocks in both places (enrichment factor being 44.21 and 236.19 respectively). The indices of differentiation between the LREE and HREE decrease with profile downward toward, and the indice of Ce anomaly shows a shift from the positive Ce anomaly in the upper segment to negative Ce anomaly in the lower part. The difference between the two profiles lies in the distribution of the highest REE enriched segment. The laterite layer represent the most REE enriched for the Yuanjiang whereas the saprolite layer for the Kolonodale. The evaluation of the mass balance shows remarkable migration and differentiation of REE in the ultramafic laterization process, which were constrained effectively by the pH environmentand organic matter (O. M.). The results indicate that climate have had great influence on the geochemical evolution of REE during the ultramafic laterization. Under the rainforest climate condition, the REE from the Kolonodale originates mainly from the basal rocks and has experienced intensive redistribution during the laterization; whereas the REE from the Yuanjiang has a mixed source stemming from both the parent rock and aeolian sediment, and it has been through only slight redistribution during the laterization.
The Pu Sam Cap high-potassic alkaline rocks are located in western part of the southern segment of Ailao Shan-Red River shear zone, including high-potassic alkaline trachyte, lamprophyre, syenite and alkaline granite. There into, the trachyte lays unconformably upon the Upper Cretaceous sedimentary rocks. The syenite and alkaline granite intruded the Triassic terrigenous sedimentary rocks, Jurassic rhyolite and amphibole-bearing granitic intrusion of 40 ∼ 35Ma. The Pu Sam Cap potassic alkaline rocks are geochemically characterized by rdatively low Ti02 (<0. 8%), P205(<0.64%) and FeO* (<7.74%) and high Na2O (1.41% ∼ 4. 5%) and K2O (5. 22%-9. 4%), coupled with high contents of incompatible trace elements. They are ultrapotassic or shoshonitic enriched in LILE, LREE and compatible elements, and have marked Nb, Ta and Ti depletion, which is similar to a subduction related component and has the characteristics of post-collisional arc type potassic igneous rocks. Zircon LA-ICPMS U-Pb dating gives the emplacement age of 32. 70 ±0. 24Ma for high-potassic syenite and 35. 1 ±0. 06Ma for alkaline granite. Their εHf (t) values are-2. 8 ∼2. 5 and-4. 9 ∼ 0. 1 with two-stage Hf model ages of 950 ∼ 1286Ma and 1107 ∼ 1421 Ma, εNd(t) values of-5. 63 ∼-3. 26, high 87Sr/86Sr(0.706254-0.707273) and low 143Nd/144Nd (0.512336-0.512447). It is suggested that the Pu Sam Cap high-potassic alkaline rocks were resulted from mixing between mantle and crust-derived magmas and may be derived from an EM2 mantle. The remnant of the subduction of Yangtze plate beneath the Indochina plate during the closure of the Mojiang Paleotethyan oceanic basin, and the Cenozoic continental collision between Indian and Eurasian plates provide a broad tectonic background for the Pu Sam Cap potassic alkaline magmatism.
Lhaguo Tso ophiolite, which is geographically located in south of Gerze county, Tibet, is one of the most completed ophiolites preserved on the south side of the Bangong-Nujiang suture zone. It was probably generated during Jurassic to Cretaceous and mainly consists of mantle peridotites, cumulates, pillow lavas, dykes, plagiogranites and radiolarian siliceous rocks units. Trace elements analyses indicates that mafic-intermediate rocks in the ophiolite are enriched in LILE, such as Sr, Rb, depleted in HFSE, such as Nb, Ta, clearly displaying compositional characteristicsof island-arc volcanic rocks, and their rare earth elements also mainly show a flat pattern. We suggest that these rocks be produced by partial melting of a mantle wedge metasomatized by the fluid released in the slab dehydration. Lhaguo Tso ophiolite likely occurred and developed in an inter-arc basin, representing the product of arc-arc collision on the south side of the Bangong-Nujiang suture zone.
The high-pressure (HP) and ultrahigh-pressure (UHP) metabasites including eclogite and blueschist are exposed in garnet phengite schists mainly as rock slices or variable tectonic lenses in the Habutengsu area, southwestern Tianshan, Xinjiang. Detailed petrology, mineralogy and geochemisitry studies have been carried out to put constrains on their protolith and regional tectonic evolution. Geochemical analyses indicate that compositions of the metabasites are mainly tholeiites with minor alkali basalts. Based on trace element and REE features, the eclogites can be subdivided into three types: N-MORB type, E-MORB type, and OIB type, while all the blueschists are OIB type. The first type eclogites are characterized by low Ti (TiO2 = 1. 00% ∼1. 08%), P (P2O5 =0. 09% ∼0.15%) and EREE (35.98 ×10 -6 ∼43.51 ×10-6) contents, and low LREE/HREE (1.35 ∼1.43) and (La/Yb)N (0. 63 ∼0.71) ratios, which are similar to those of typical N-MORB basalts, and are considered to be products of magma derived from the depleted mantle sources in an mid-ocean ridge setting. The second type eclogites are characterized by high Ti (TiO2 = 1. 35% -2. 25%), P (P2O5=0.05% ∼0.20%) and Σ REE (60. 1 ×10-6 ∼77. 35 ×10-6) contents, and high LREE/HREE (1.25∼2.43) and (La/Yb)N(0. 52 ∼1. 52) ratios, which are similar to those of typical E-MORB basalts, and are considered to be products of magma from the enriched mantle sources. The third type eclogites are characterized by higher Ti (TiO2 = 1.16% ∼2. 86%), Zr/Y (5. 32 ∼7.78), ΣREE (79. 12 ×10-6 ∼192. 1 ×10 -6) contents, and higher LREE/HREE (4. 15 ∼6. 54) and (La/Yb)N (3. 77 ∼9.44) ratios, which are similar to those of typical OIB basalts, indicative of enriched mantle sources. Geochemical characters of the OIB type blueschists are similar to the OIB type eclogites, characterized by high Ti (TiO2 = 1. 39% ∼2. 86%), Σ REE (88. 18 ×10-6 ∼227. 8 ×10-6) contents and high (La/Yb)N (5. 03 ∼9. 84), Zr/Y (4. 93 ∼9. 55) ratios, indicating the same magmatic sources. Combined with the previous data from this region, the presence of such MORB and OIB rock assemblages suggests the existence of an ocean basin in the Early Paleozoic era from the Habutengsu area, southwestern Tianshan, Xinjiang. The evolution process may be also impacted by the crystallization differentiation and different sources magma mixing, which produced the element geochemistry affinity of MORB and OIB in the metabasites. It is suggested that protolith of the metabasites were formed under the tectonic setting of mid-ocean ridge and adjacent ocean island, and the formation of the blueschists and eclogites might be related to HP-UHP metamorphism and later tectonic exhumation.
The Ailaoshan tectonic zone is the most significant lineament in the eastern Tibet (Southeast Asia), which separates the Yangtze-South China and the Indochina blocks. Information on multi-stage complex tectonic evolution is preserved in the rocks in the tectonic zone. Late Archean-Neoproterozoic high grade metamorphic rock series, Cenozoic tectono-magmatic assemblages (shearing deformation structures), Late Permian-Early Triassic Jinping-Song Da rifting rock sequences and Early Carboniferous-Early Triassic Ailaoshan tectonic mélange were well developed from east to west along the Ailaoshan tectonic zone. The various tectonic units of different characteristics are separated by fault structures mainly developed in Cenozoic. Granitic intrusions of various stages which were formed due to allochthonous emplacement or mingmatization are widespread along the tectonic belt. The Ailaoshan tectonic belt has multiple tectonic natures during different geologic history stages. On the whole, it has experienced three important tectonic stages, i.e. Pre-tethys, Tethys, and Cenozoic intracontinental stages. During the Pre-tethys evolution until Early Paleozoic, the major part of the belt (especially along the eastern zone) had affinity to the Yangtze block, and preserved records of Late Archean-Neoproterozoic crustal evolution. Since the Late Paleozoic-Early Mesozoic the tectonic belt became a part of the Tethyan domain with the opening of Paleotethys. The belt was evolved into a tectonic domain with different nature from that of the South China-Yangtze plate. Subsequent closure of the Ailaoshan Ocean in from Early Carboniferous and the Jinping-Song Da Ocean since Early Permian resulted in the formation of the paleo-Ailaoshan orogenic belt. The closure of the Tethyan oceans made Yangtze-South China block and Indochina block to become a unified continent. Interaction between the Indian and the Eurasian plates had a sound influence on the Ailaoshan belt. There are Early Cenozoic (Paleocene to Early Oligocene) orogenic contraction, Late Oligocence-Early Miocene post-orogenic extension and high-potassium alkaline magmatic activity, and Late Oligocence-Early Miocene large-scale southeasternward extrusion of the Indochina block, and large-scale left-lateral strike-slips shearing coeval calc-alkaline magmatic activities.
This paper studies the volcanic rocks of Yaxuanqiao, Maoheshan and Lvchun in Ailaoshan tectono-magmatic belt, Yunnan Province, SW China. Yaxuanqiao volcanic rocks mainly composed of Mugearite-basalts with minor andesites formed in Late Permian. In TAS diagram, they are plotted in both alkaline and sub-alkaline areas. These rocks are low-in K 2O ( < 1. 19% ) and belong to low-potassic to middle-potassic calc-alkaline series. They are mainly calc-alkaline in terms of Peacock's index. Their spider diagrams of trace elements show enrichment in Pb and depletion in Nb and Ta. The trace element spider diagrams from Zr to V are flat and overall lower compared with those of MORB. The REE patterns are similar to those of MORB, but slightly enriched in LREE and depleted in HREE. These rocks are all plotted in arc settings in tectonic discrimination diagrams. The volcanic rocks in Talanghebian of Yaxuanqiao area are sub-alkaline dacites and belong to middle-potassic calc-alkaline series. Besides, it is mainly calsic in terms of Peacock's index. The features of spider diagrams of trace elements, REE distribution pattern and tectonic discrimination diagrams indicate that they formed in an arc tectonic setting. Thus it is suggested that Yaxuanqiao igneous rocks belong to arc-type igneous rocks in Late Permian. Zircon SHRIMP U-Pb dating of Maoheshan basalt gave an age of 249 ± 1. 6Ma, i. e., Early Triassic. These rocks are sub-alkaline albite basalts, belonging to low-potassic calc-alkaline series. It is mainly calsic in terms of Peacock's index. Compared with MORB, the trace element spider diagrams and REE patterns show slightly enriched in LREE. In the diagrams for discrimination of tectonic settings, they are plotted in the transition area from E-MORB to island-arc. Zircon SHRIMP U-Pb dating of Lvchun rhyolite yielded an age of 247. 3 ± 1. 8 Ma, i. e., Early Traissic. Rhyolites are sub-alkaline, shoshonite series. It is alkaline-calsic in terms of Peacock's index. Trace elements spider diagrams, REE patterns and plots of tectonic setting discrimination all indicate the Lvchun rhyolites occur in a transition setting from matured arc to continent-continent collision. Combined with previous studies, the paper suggests that Ailaoshan Ocean had opened since Late Devonian and probably been spreading between Carboniferous and Early Permian (?). But in Late Permian the Ailaoshan ocean had begun to subduct to form initial arc of Yaxuanqiao to the west of the Ailaoshan. In Early Triassic (249 ± 1. 6Ma) , Maoheshan basalts, characterized by both arc and MORB, formed as a part of Ailaoshan ophiolites. It may suggest that the Ailaoshan ocean basin had been shrunk, or converted to intra-arc or back-arc basins of supra-subduction zones. Thus the evolution of the Ailaoshan ocean went into the late stage, and in some area, such as Lvchun area, it became a transitional setting from matured arc to continent-continent collision (247. 3 ± 1. 8Ma). All those mentioned above support the conclusion that the Ailaoshan ocean had closed in Late Triassic, which also coincides with the fact that Yiwanshui Group of Late Triassic unconformably overlied the Ailaoshan Shuanggou ophiolites.
Ailaoshan gold-polymetal ore concentration region refers to a polymetal deposits concentration area which had traditional Ailaoshan metallogenic belt as the center, its adjacent mine fields, mine area as the details. It locates at the combining zone of ancient tectonic units with different character caused by Ailaoshan composite orogeny. The evolutional process of Ailaoshan composite orogeny includes a series of complex orogenic processes such as formation of Precambrian-Early Paleozoic orogenic belt, subduction orogenesis of Late Paleozoic era, collisional orogenesis of Late Hercynian-Indosinain, and extensional orogenesis of Yanshainan-Cenozoic. Different mineralization geological environment and corresponding metallogenic system are formed in different structural-mineralization units of Ailaoshan ore concentration area. It involves the continental margin rift setting and metallogenic system in Yangtze plate of Early-Middle Proterozoic, the Ailaoshan continental margin oceanic basin rift (little oceanic basin)-suture environment and convergent continental margin metallogenic system of Middle-Late Paleozoic, the Gejiu-Wenshan compound rift basin poly-cycle evolvement and compound metallogenic system, the Jinping "stabilization" continental margin blocks environmental and intracontinental metallogenic system, and the Mojiang-Lvchun multi-phase supeiposition arc-rift basin setting and metallogenic system, etc. Series super large-large tin, gold, iron, copper, lead, zinc, nickel and other polymetal deposits ore concentration area are formed in this area.
A small-sized meta-basic rock system is discovered in Qilongwuru Gully of Central Qiangtang’s Shuanghu region and contains a meta-basalt and garnet-bearing plagioclase amphibolite. The zircon U/Pb age of this meta-basalt by SHRIMP analysis is 463.3±4.7 Ma, suggesting that this lava formed in the Middle Ordovician, and is consistent with that of the meta-basic rocks in the Taoxing Lake and Guoganjianian Mountain ophiolite found in the Qiangtang plate. As this lava system bears similar geochemistry to N-MORB, it might be a component of ophiolite that represents the trail of the extinction of the Proto-Tethys, suggesting that the formation of Proto-Tethys oceanic basin in the Longmu Co-Shuanghu suture zone could date as far back as to the Middle Ordovician. Isotopic geochemical analysis indicates that the magma source area consists of both depleted mantle (DM) and enriched mantle (EMII) end members and bears Dupal anomaly, similar to that of the Paleo-Tethys in the Neo-Tethys represented by the Yarlung-Tsangpo suture zone, the Paleo-Tethys represented by the Changning-Menglian suture zone, and the Paleo-Tethys in Sanjiang region. This suggests that they have inherited the attribute of the Proto-Tethys mantle domain, and the Longmu Co-Shuanghu suture zone may be a representative of the northern boundary of Gondwana.
The Jinping-Fan Si Pan (JFP) Cenozoic magmatic and
Cu-Mo-Au metallogenic belt in the southeastern part of the
Ailao Shan shear zone host the Tongchang, Chang‧an, Habo, and
Chinh Sang Cu-Mo-Au deposits. These deposits form an
integrated epithermal-porphyry regional mineralization system associated
with 40-32 Ma high-K alkaline magmatism. The magmatic rocks in the
belt have relatively low TiO2 (<0.73 wt%),
P2O5 (<0.29 wt%), and FeO* (<4.99
wt%), and high Na2O (2.86-4.75 wt%) and K2O
(4.01-7.98 wt%). They also have high contents of incompatible
trace elements, and are enriched in LILE (Rb, Ba, K, Sr) and LREE. They
have marked Nb, Ta, Ti and P depletion in primitive mantle-normalized
spidergrams, and plot close to the EMII mantle field in the Sr-Nd
isotopic diagram. These characteristics are similar to those of the
Eocene high-K alkaline rocks along the northern Ailao Shan belt, eastern
Tibet plateau. The sulfur and lead isotope analyses of sulfide minerals
from both the ores and related magmatic rocks confirm the involvement of
a magmatic ore fluid. The Cenozoic alkaline intrusions and
Cu-Mo-Au mineralization in the JFP were formed prior to the
initiation of left-lateral shearing along the Ailao Shan shear zone. The
magmas appear to have been derived from enriched mantle, possibly with
mixing of materials from the buried Tethyan oceanic lithosphere, and/or
crust.
The Ailao Shan–Red River (ASRR) shear zone, one of the most prominent geologic strike-slip shear zones in Southeast Asia, consists of high-grade metamorphic complexes that provide a rare opportunity to sample the mid-crustal rocks along the western margin of the Yangtze Block of South China. Here we report combined, in-situ analyses of zircon U–Pb and Lu–Hf isotopes of eight gneisses from the Diancang Shan and Ailao Shan segments of the ASRR shear zone. Our zircon U–Pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 Ma, with peak ages at ca. 770, 350 and 240 Ma, suggesting three major periods of protolith formation. The 350-Ma zircons, observed only in an orthogneiss from the Diancang Shan, show uniform initial Hf isotopic ratios marked by high and positive εHf(T) values from + 16 to + 10. This is in contrast with the other two zircon populations that are more common, occurring in both the Diancang Shan and Ailao Shan, and overall delineate very heterogeneous Hf isotopic compositions, with εHf(T) values ranging from + 15 to − 16. Many zircons reveal distinctive core-rim age variations. Zircon rims, formed between ca. 34 and 26 Ma, show significant variations in Th/U ratios (5–0.01) and εHf(T) values (+ 14 to − 10) that suggest complicated magmatic and metamorphic zircon overgrowth during the Oligocene. The presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies, corresponding to mid-crustal level P–T conditions, as also suggested by structural and petrologic data. Our data furthermore suggest that the ASRR gneisses were not produced solely by shear heating during the Tertiary strike-slip faulting, but are uplifted, mid-crustal basement rocks that formed essentially during two major stages of magmatism, represented by the Neoproterozoic Kangding Complex and Late Permian Emeishan large igneous province. As a result of the India–Asia collision, these basement rocks underwent regional magmatic and metamorphic overprinting in the Oligocene that, based on our relevant work (Searle et al., 2010, Geosphere, 6, 1–23), predated the initial left-lateral movement along the ASRR shear zone. The 350-Ma protolith, which requires a dominant depleted-mantle input in the petrogenesis, cannot be linked to any major magmatic events in South China but may be interpreted as part of the Paleotethys remnants that were later incorporated into the ASRR shear zone.
The Babu ophiolite in Malipo County, SE Yunnan, consists of three units: ultramafic, mafic and basaltic rocks. Studies of
geological mapping, petrology and geochemistry reveal that the gabbro is similar to that in the Troodos ophiolite, and the
diabase and basalt belong to a normal MORB-type, analogous to the Shuanggou ophiolite in west Yunnan. The ophiolite studied
as a thrust tectonic slab, was overthrust northwards onto the Late Paleozoic-Triassic deep marine deposits of continental
margin. It is inferred to be relics of Paleotethyan ocean; namely, a branch of Paleotethys occurs in South China, where the
tectonic nature and evolutional history of the area should be reconsidered.
A number of metamorphosed mafic rocks occurred within the Paleozoic strata in the Chenxing and Bangxi regions at the northern
side of the Changjiang-Qionghai Fault in Central Hainan Island. These metamorphosed mafic rocks are tholeiites in chemistry.
They are characterized by extreme depletion of Th, Nb, Ta and LREEs, resembling the depleted N-type mid-ocean ridge basalts
(MORB). Field relations suggest that the protolith of the metamorphosed mafic rocks were likely formed in Paleozoic. These
metamorphosed mafic rocks with N-type MORB geochemical features were probably the remnants of the Paleo-Tethys oceanic crust.
The metamophic peridotite of ophiolite belt in Mt. Ailao is mmposed of lhenolite and harzburgite. The former shows the charateristics
of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of Ihedite, two primary magmas:
tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbmdiabase-pyroxenic besalt rock series and
show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show
the characteristics of para-MORB.
The REE patterns of the basic volcanic rocks in Mangya area. Altun, are slight rich in LREE, with (La/Ya)N= 1.69–3.20, (La/Sm)N= 1.37–1.87, other trace element ratios of the rocks are Th/Ta≈l (for a few samples greater than 1.5). Nb/ Y=0.34–0.62, Ti/Y=310–443
(on the average: 381), Ti/V=37–62, Zr/Nb = 9.4–12.4, Sr/Rb= 12–80 (on the average: 371, and Nb/Th= 7.7–16.8. These features
are similar to that of E-MORE or OIB. TheεED (t) value, beiig 3.95–4.12, shows that the source of the volcanic rocks is derived from depleted asthenosphere mantle mixed
with materials from enriched mantle. These, together with the information of geological setting and rock assemblages, indicate
that the basic volcanic rocks arc of ophiolite. The Sm-Nd isotope ages for the eight basic volcanic rock samples construct
a suaight line with good correlation, and the calculated i.whmn age is (481.3 ± 53) Ma. Besides, the eight calculated εNd(t) and model ages are close to each other, which suggests that they are homologous, so the isochmn is not a mixed line. In
the meantime, the isachron age ((481.3 ± 53) Ma) is lower than the model ages (T
DM = 1 004–1 534 Ma) of thc samples. suggeting that the isochron age represents the formation age of the basic volcanic rocks
and the ophiolite belt in Mangya area, Altun is foxmed in the early Paleozoic (Cambrian-Ordovician). In spite of the greater
uncertainty of the age, it is still reliable because it is consistent with the age constrained by the regional strata.
In order to explore the disputed issue concerning the tectonic affinity of the ancient ocean mantle of North Qilian Mountains
(NQM), geochemical and Sr, Nd, Pb isotopic compositions of pillow basalts of the Yushigou Ophiolite (YSGO) suite from NQM
have been analyzed systematically. The pillow basalts exhibit tholeiitic characteristics, with flat chondrite-normalized REE
patterns ((La/Yb)N = 0.98–1.27). They display no Nb, Ta, Zr, Hf negative anomalies, and show MORB features in 2Nb-Zr/4-Y and Ti/100-Zr-Y × 3
tectonic discrimination diagrams. These results indicate that the Yushigou ophiolite is most likely to be formed in a mid-ocean
ridge or mature back-arc basin. Their isotopic data show a relatively broad and enriched 87Sr/86Sr (0.70509–0.70700), restricted 143Nd/144Nd (0.512955–0.512978). Pb isotopes are in the range of 206Pb/204Pb (18.054–20.562), 207Pb/204Pb (15.537–15.743) and 208Pb/204Pb (38.068–38.530). These isotopic data imply that the basalts originated from the depleted mantle (DMM), with the involvement
of enriched mantle components (mainly EMII). Geochemical comparisons between the basalts in YSGO and the MORB-type basalts
of ophiolite suites occurring in the known ancient Tethyan tectonic domain indicate that the ancient oceanic mantle represented
by YSGO suite forming in early Paleozoic in the North Qilian Mountains is very similar to the Tethyan mantle in both trace
elements and isotopic compositions. The North Qilian Mountains should be a part of the Tethyan tectonic domain in early Paleozoic.
This further implies that the Tethyan tectonic domain can be deduced to early Paleozoic in the study area, which will be helpful
to discussing the tectonic affinity and evolution of the North Qilian Mountains.
The ophiolites from the Yarlung Zangbo River (Tibet), Southwestern China, were analysed for the contents of helium and neon
and their isotopic compositions by stepwise heating. The serpentinites from Bainang showed a high 3He/4He value of 32.66R
a (R
a is referred to the 3He/4He ratio in the present air) in 700°C fraction. At lower temperature, all of the dolerites displayed as very high 3He/4He ratios as ones investigated for hotspots. It was clear that the high 3He/4He ratio was one of immanent characterics in the magma source formed the dolerites, suggesting that there was a large amount
of deep mantle fluids in these rocks. In the three-isotope diagram of neon, the data points from the ophiolites of the Yarlung
Zangbo River were arranged along the Loihi Line. This is in agreement with the characteristics of helium isotopes, revealing
that the high-3He plume from deep mantle had played an important role in the formation of the Neo-Tethyan Ocean. The helium isotopic compositions
in the basalts were far higher than atomospheric value but lower than the average value of MORB, although there were various
degrees of alteration. The possible reasons were that basaltic magmas had been contaminated by crust-derived fluids.
ResearchGate has not been able to resolve any references for this publication.