Weiming Fan

Palaeozoic and Cenozoic lithoprobes and the loss of >120 km of Archaean lithosphere, Sino-Korean craton, China

Abstract In eastern China Palaeozoic kimberlites and Cenozoic basalts have been erupted through the same Archaean crust, thus providing deep probes of the cratonic lower lithosphere over a period of 400 Ma. While Palaeozoic diamondiferous kimberlites point to the existence of thick, refractory lower lithosphere in the east, Cenozoic basalt-borne xenoliths reveal the presence of hot, thin, less refractory lower lithosphere. Remnants of the Archaean lithosphere may have survived as harzburgites which are chemically similar to those from the Kaapvaal craton but very different from recently accreted lherzolites. In the absence of convincing evidence for supra-subduction or intraplate processes it is believed that the dramatic change of lithosphere architecture in the Phanerozoic was caused by indentor tectonics resulting from the collision of India and Eurasia. Passive reactivation and remobilization of the Archaean lower lithosphere, in particular metasome horizons, contributed to Cenozoic magmatism aligned along major lithospheric faults.

Secular evolution of the lithosphere beneath the eastern North China Craton: Evidence from Mesozoic basalts and high-Mg andesites

Abstract Geochemical and isotopic data from Mesozoic lavas from the Jianguo, Niutoushan, Wulahada, and Guancaishan volcanic fields on the northern margin of the North China Craton provide evidence for secular lithospheric evolution of the region. Jianguo lavas are alkaline basalts with LILE- and LREE-enrichment ((La/Yb)N=12.2–13.2) and MORB-like Sr-Nd-Pb isotopic ratios ((87Sr/86Sr)i (Zhang et al., 2002) , manifests a vast secular evolution of the lithospheric mantle beneath the eastern NCC from the Paleozoic refractory continental lithosphere to this Mesozoic modified lithosphere. Compared with the cratonic margin, the lithospheric mantle beneath the center of the craton was less extensively modified, implying the secular evolution was related to the subduction processes surrounding the NCC. Therefore, we suggest that the interaction of the slab-derived silicic melt with the old refractory lithospheric mantle converted the Paleozoic cratonic lithospheric mantle into the late Mesozoic fertile mantle, which was also different from the Cenozoic counterpart. A geodynamic model is proposed to illustrate such a secular lithosphere evolution.

Late Mesozoic calc-alkaline volcanism of post-orogenic extension in the northern Da Hinggan Mountains, northeastern China

Abstract Late Mesozoic calc-alkaline volcanism in the northern Da Hinggan Mountains (NDHM), NE China, exhibits geochemical and Sr–Nd isotopic characteristics similar to those of Cenozoic calc-alkaline volcanism in the Basin and Range Province, USA. Whole-rock K–Ar dating results show that these volcanic sequences were erupted during 138–116 Ma, composed of basaltic andesites/trachyandesites (Group 1), hornblende andesites/trachytes (Group 2) and rhyolite lavas (Group 3). They are characterized by low MgO contents (≤4.20%), LILE, LREE enrichment and significant Nb–Ta depletion, as well as a little depleted to slightly enriched Nd and weakly enriched Sr isotopic ratios (Group 1: initial 87Sr/86Sr=0.70502–0.70572; ϵNd(t)=−0.78 to +0.91; Group 2: initial 87Sr/86Sr=0.70497–0.70518; ϵNd(t)=+0.86 to +1.26; Group 3: initial 87Sr/86Sr=0.70510–0.70635; ϵNd(t)=−0.41 to +0.25). The systematic variations in major and trace elements, homogeneous Sr–Nd isotope data and temporal consistency among three volcanic groups, indicate that they were derived from a similar mantle source metasomatized by fluids related to the closure of the paleo-Asian and/or Mongolia–Okhotsk Oceans, and were produced through different degrees of fractional crystallization of the primary melts. Group 1 basaltic rocks were formed through removal of olivine and pyroxene of the primary melts, while Group 2 trachytes, which contain the lowest LREE contents (e.g., La=24–28 ppm) and relatively less enriched Sr and higher Nd isotope ratios, were generated after removal of a few percent of LREE-rich minerals such as hornblende, clinopyroxene and apatite of melts like Group 1. Group 3 rhyolite lavas exhibiting the highest abundances of strongly incompatible elements such as Rb and K, moderate LREE contents (e.g., La=28–53 ppm) as well as apparently negative Eu and Sr anomalies, represent the final crystallized products following a plagioclase-predominant fractionation of melts like Group 2. The low MgO contents and evolved affinities of the volcanic rocks imply that beneath the NDHM there existed many crustal magma reservoirs throughout the eruption episodes, in which mantle-derived primary melts had experienced intense differentiation. These facts, in combination with the contemporaneous basin and range tectonic regime, suggest that the extensive calc-alkaline volcanism in the NDHM was attributed to post-orogenic diffuse extension rather than either an upwelling mantle plume or Mesozoic oceanic plate subduction.

Geochemistry of Mesozoic mafic rocks adjacent to the Chenzhou-Linwu fault, South China : implications for the lithospheric boundary between the Yangtze and Cathaysia Blocks

To constrain the Mesozoic tectonic evolution and the lithospheric boundary between the Yangtze and Cathaysia blocks in South China, we present geochronological and geochemical data for Mesozoic basaltic lavas and related mafic dikes west (Group 1) and east (Group 2) of the Chenzhou-Linwu fault. Three episodes of mafic magmatism around the Chenzhou-Linwu fault were identified: ca.175 Ma, 125-150 Ma, and 80-95 Ma, respectively. Group 1 rocks (alkaline basanite and trachybasalt), with ages of >125 Ma, have a wide range of 87Sr/86Sr(t) values (0.7035-0.7069), and εNd(t) values (-3.75 to + 6.10). In contrast, Group 2 rocks (subalkaline basalt and basaltic andesite), with ages of > 125 Ma, exhibit 87Sr/86Sr(t) values of 0.7075-0.7087 and εNd(t) values of -2.04 to + 1.05. Both groups are strongly enriched in incompatible elements, with variable negative Nb-Ta anomalies. However, Group 1 rocks commonly have higher LREE and Ba/Nb, Rb/Nb, Ba/Th, and Ba/La ratios and lower Th/Nb, Th/La, and Zr/Nb ratios than Group 2 rocks. Rocks with ages of 80-95 Ma from both groups have very similar elemental and isotopic compositions (87Sr/86Sr(t) = 0.7033-0.7052, εNd(t) = +3.99 to + 8.00), consistent with those of OIB. Strong coupling between incompatible elemental ratios and isotopes suggests that Group 1 rocks might have been derived from an EMI-like continental lithospheric mantle with an OIB source. In contrast, Group 2 rocks come from an EMII-like mantle source contaminated by an OIB component. We conclude that Mesozoic mafic rocks with ages of >125 Ma originated chiefly from an enriched lithospheric mantle heated by ascending asthenosphere, whereas the mafic rocks with ages of ca. 80-95 Ma were derived from upwelling asthenospheric mantle in response to intra-continental lithospheric extension in the South China interior. The spatial variations of EMI-and EMII-like source signatures for Mesozoic mafic rocks around the Chenzhou-Linwu fault suggest that the fault represents the Mesozoic lithospheric boundary between the Yangtze and Cathaysia blocks. The Jinxian-Anhua fault was only a near-surface boundary between the sutured blocks. The crust of the Cathaysia block might have been thrust westward over the Yangtze block with a displacement of >400 km at a time no later than ca. 175 Ma. A model for crustal detachment collision (>ca. 175 Ma) and subsequent intra-continental lithospheric extension (175-80 Ma) is proposed for the Mesozoic tectonic evolution of South China.

Post-orogenic bimodal volcanism along the Sulu Orogenic Belt in eastern China

Abstract The early Cretaceous volcanism occurring along the Sulu orogenic belt, east Shandong Province, demonstrates a bimodal characteristic. The volcanic rocks belong to high-K alkaline series, dominated by alkali basalt, basaltic trachyandesite, latite and trachyte with LILE (e.g. K, Sr and Ba) and LREE enrichment but HFSE depletion (especially for Nb and P) in the primitive mantle-normalized spidergrams and steeply right-declined REE patterns. The enriched initial Sr-Nd isotopic ratios ( 87 Sr/ 86 Sr = 0.70724 to 0.70750 and e Nd (t) = −17.0 to −15.9) of the basaltic samples suggest their origin from an enriched lithospheric mantle, which might had undergone a fluid metasomatism or source mixing by the continental crust during or shortly after the Triassic continental subduction. Significantly negative Nb anomalies observed in the spidergrams and other “crustal” signatures of these rocks suggest an important role of continental material in their petrogenesis. The felsic rocks demonstrate geochemical and Sr-Nd isotopic features (initial 87 Sr/ 86 Sr = 0.70814 to 0.70961 and e Nd (t) = −18.9 to −17.0) similar to those of the post-collisional granitic plutons, probably derived from the anatexis of lower/middle crust in response to basaltic magma underplating. The widespread melting of the metasomatized mantle was probably attributed to the thermal perturbation or lithospheric extension induced by the mega-large displacement along the Tan-Lu wrench fault system when northward strike-slipping movement of the Izanagi Plate occurred during the late Mesozoic.

The genetic association of adakites and Cu–Au ore deposits

Adakites may form by partial melting of either the subducting oceanic lithosphere or the lower part of the continental crust. These two magma types can be discriminated geochemically using a combination of La/Yb, Sr/Y ratios, MgO and Na2O contents, and Sr–Nd isotopes. Given that the basaltic crust has Cu concentrations more than two times higher than the lower continental crust and the mantle wedge, ‘primitive’ adakites produced by oceanic slab melting should contain significantly higher Cu contents than adakites derived from the continental crust, as well as normal arc andesites. A globally compiled dataset shows that Cu concentrations in adakites are generally lower than that in normal arc rocks. We attribute this low copper content to loss of magmatic fluids as a result of sulphate reduction during adakitic magma differentiation, in turn induced by the crystallization of Fe–Ti oxides, essential to mineralization. Therefore, the underflow of oceanic-slab-derived adakites that can release larger amounts of Cu (presumably Au as well) by crystal fractionation leads to higher potential for Cu–Au mineralization along convergent margins, usually associated with ridge subduction. Such basaltic slab melts initially have considerably higher Cu contents and thus play a crucial role particularly in the relatively closed magma system responsible for generating porphyry Cu deposits.

Mesozoic large magmatic events and mineralization in SE China: oblique subduction of the Pacific plate

SE China is well known for its Mesozoic large-scale granitoid plutons and ore deposits. In SE China, igneous rocks with intrusion ages between 180 and 125 Ma generally become progressively younger towards the NE. More specifically, 180–160 Ma igneous rocks are distributed throughout a broad area, with mineralization ranging from Cu–Au and Pb–Zn–Ag to W–Sn; 160–150 Ma plutons are present mainly in the Nanling region and are associated with the large-scale W–Sn mineralization; younger igneous rocks occur in the NE area that has many fewer deposits. These can be plausibly interpreted as reflecting a southwestward subduction followed by a northeastward rollback of a subducted oceanic slab, in rough agreement with contemporaneous drift of the Pacific plate. Consistent with this scenario, SE China contains three Jurassic metallogenic belts distributed systematically from NE to SW: (1) a Cu–(Au) metallogenic belt in the NE corner of the South China Block, represented by the Dexing porphyry Cu deposits; (2) a Pb–Zn–Ag metallogenic belt in the middle, represented by the Lengshuikeng Ag and Shuikoushan Pb–Zn deposits; and (3) the famous Nanling W–Sn metallogenic belt in the SW. The distribution of these metallogenic belts is analogous to those in South America where Fe deposits are distributed close to the subduction zone, followed by porphyry Cu–Au deposits and Pb–Zn–Ag deposits in a medial zone, and Sn–W deposits distant from the trench. Inasmuch as quite a few late Mesozoic Fe deposits occur in the Lower Yangtze River Belt to the NE of the Cu–Au deposits in SE China, the distribution of late Mesozoic deposit belts in SE China is identical to that in South America. Therefore, southwestward subduction of the Pacific plate and the corresponding slab rollback are proposed here to explain the distributions of the late Mesozoic (180–125 Ma) magmatism and the associated metallogenic belts in SE China.

Late mesozoic mafic intrusive complexes in North China Block: constraints on the nature of subcontinental lithospheric mantle

The nature of Mesozoic subcontinental lithospheric mantle (SCLM) has been rarely iscussed in detail even though it might be crucial to understanding of the lithospheric thinning process and mechanism in North China Block (NCB). Late Mesozoic ultramafic to mafic intrusive complexes occurring in west Shandong Province, eastern China, provide a possible approach to constrain the nature of the SCLM in NCB. These mafic rocks are characterized by LILE and LREE enrichment, HFSE depletion and apparently positive Eu anomalies with EM1-like Sr and Nd isotopic compositions. A long-term metasomatism by carbonatitic fluid can be inferred in accordance to their LREE enrichment, HFSE depletion and low 87Sr/86Sr ratios. Additionally, the strong LILE enrichment (such as Rb, Ba and K) and the relatively wider range of eNd(t) values indicate that the mantle source had undergone recent metasomatism by high-potassium OIB-like melt during or shortly before partial melting, resulting in mantle heterogeneity. The geochemical and Sr-Nd isotopic characteristics of the ultramafic to mafic intrusives suggest their origin from enriched lithospheric mantle, which was probably dominated by phlogopite-bearing harzburgites at 80 to 160km depths. A combination of the partial melting of such a heterogeneously enriched mantle with fractional crystallization of olivine and pyroxene during magma ascent can well interpret their petrogenesis. When comparing the nature of the Mesozoic SCLM with that in the early Paleozoic, lithospheric extension or thinning occurred in response to the widespread basaltic generation during the late Mesozoic. It is favorable that the Archaean lithospheric keel in NCB was removed by means of the decompressional melting of the preexistent metasome defined by Menzies (1993).

Geochemistry of late Mesozoic adakites from the Sulu belt, eastern China: magma genesis and implications for crustal recycling beneath continental collisional orogens

Both low-Al and high-Al adakitic andesites erupted at similar to 114 M a in the Sulu collisional belt, eastern China, provide evidence for recycling of continental crust into the mantle more than 100 million years after the Triassic (similar to 240 Ma) collision between the North China and Yangtze blocks. These rocks display similar normalized trace element patterns, with enrichments in large ion lithophile elements (LILE), light rare earth elements (LREE) and depletions in Nb, Ta and Ti, and have highly radiogenic Sr and non-radiogenic Nd isotopic compositions (high-Al: Sr-87/Sr-86(i) = 0.70645-0.70715 and epsilon(Nd)(t) = -20.1 to -19.1; low-Al: Sr-87/Sr-86(i) = 0.70593-0.70598 and epsilon(Nd)(t) = -17.1 to -15.8). The high-Al (Al2O3 > 15 %) adakitic andesites are compositionally comparable with experimental slab melts, whereas the low-Al series (Al2O3 similar to 13 %) have higher MgO, Cr and Ni, and higher Sr/Y ratios, and are compositionally comparable with slab melts hybridized by mantle peridotites. Combined major- and trace-element and Sr-Nd isotope data indicate that the two types of adakitic andesites have been derived from a LILE- and LREE-enriched eclogitic lower continental crust; in the case of the high-Al adakitic andesites, the melts underwent insignificant mantle contamination, whereas the low-Al magmas reacted with peridotites. Generation of the two types of late Mesozoic adakitic andesites favours a model of lithospheric delamination, leading to asthenospheric upwelling and extensive melting of lower continental crust, including a delaminated block, in the Sulu belt.

Emplacement age and tectonic implications of the Xilinhot A-type granite in Inner Mongolia,China

A new rock type of granite, approximate 45 km2 in area and located about 10 km south of Xilinhot, Inner Mongolia, was found in the Sunitezuoqi (or called Su-zuoqi)-Xilinhot tectonic belt and identified as an A-type miarolitic intrusion. The pluton has miarolitic structure and is composed chiefly of perthite, quartz, euhedral albite and potassium feldspar. Various types of textures occur in the pluton, such as perthitie, graphic and myrmekite textures. Only quartz is found in miarolitic cavity. This A-type granite with seagull-shaped REE patterns and obvious negative Eu anomalyδEu = 0.24–0.28) is high in SiO2 (76%–77%), K and Na (Na2O + K2O = 7.75%–8.15%) and low in Ca (CaO = 0.20%–0.22%), Fe and Mg. Both petrographical observations and chemical compositions indicate that it is an A-type granite. Zircon SHRIMP U-Pb analyses indicate that this A-type granite was emplaced at 276 ± 2 Ma and coeval with the same type of granites in the adjacent areas. Therefore, it suggests that this pluton was likely formed in a post-orogenic extensional setting and probably related to break-off of subducted slabs in Central Asian Orogenic Belt (CAOB), which indicate that the Sunitezuoqi-Xilinhot belt was tectonically evolved into post-orogenic stage since early Permian.