Jianping Zheng

Widespread Archean basement beneath the Yangtze craton

The age distribution of the crust is a fundamental parameter in modeling continental evolution and the rate of crustal accretion through Earth9s history, but this is usually estimated from surface exposures. The exposed Yangtze craton in eastern China consists mainly of Proterozoic rocks with rare Archean outcrops. However, the U-Pb ages and Hf isotope systematics of xenocrystic zircons brought to the surface in lamproite diatremes from three Proterozoic outcrop areas of the craton suggest the widespread presence of unexposed Archean basement, with zircon age populations of 2900–2800 Ma and 2600– 2500 Ma and Hf model ages of 2.6 to ca. 3.5 Ga or older. The zircons also record thermal events reworked on the craton ca. 2020 Ma (remelting of older crust) and 1000–850 Ma (addition of juvenile mantle material). The observation of deep crust significantly older than the upper crust will require revision of models for the rates of crustal generation through time.

Reappraisal of the ages of Neoproterozoic strata in South China: No connection with the Grenvillian orogeny

The Jiangnan fold belt separates the Yangtze and Cathaysia blocks in South China and has long been considered Grenvillian in age in order to place South China in central Rodinia. It consists of deformed Early Neoproterozoic strata that are unconformably overlain by undeformed Late Neoproterozoic strata and intruded by undeformed and unmetamorphosed granitic plutons. Zircons from the Early Neoproterozoic strata yield U-Pb ages as young as 830 Ma, and one granitic pluton has a zircon U-Pb age of ca. 827 Ma. The ≥830 Ma mafic rocks along the southeastern margin of the Yangtze block have arc-affinity geochemical characters, whereas mafic rocks younger than 830 Ma have typical ocean island basalt (OIB)—like compositions. Thus, we suggest that the Early Neoproterozoic strata were deposited on an active continental margin prior to amalgamation of the Yangtze and Cathaysia blocks at ca. 830 Ma. The overlying Late Neoproterozoic strata were deposited in the intracontinental rifted Nanhua Basin at 820–730 Ma and probably reflect backarc spreading above the long-lived (950–735 Ma) oceanic subduction zone along the northern and western margin of the Yangtze block. This model is consistent with the secular tectonic evolution of South China during the Neoproterozoic. The Jiangnan fold belt is therefore not a Grenvillian feature as previously suggested, and there is no evidence to place South China in central Rodinia. Instead, we believe that South China was located in a marginal position relative to this supercontinent.

Relict refractory mantle beneath the eastern North China block: significance for lithosphere evolution

Xenolith-bearing Neogene basalts occur in Hebi county, at the southern end of the Taihangshan–Luliangshan paleo-rift zone in the North China block of the Sino–Korean craton. This locality lies on the North–South Gravity Lineament, which divides the craton into two geophysical zones. The spinel peridotite xenoliths hosted by the basalts can be divided into two groups based on the Mg# values of olivine. The whole-rock compositions of the low-Mg (Fo<91) xenoliths have high Al2O3+CaO (average 3.06 wt.%) and Na2O (average 0.19 wt.%), and low Mg/Si; they are similar to xenoliths from many localities in eastern China and other Phanerozoic volcanic areas. The dominant high-Mg (Fo≥92) group consists of harzburgites (66%) and depleted lherzolites (34%) with coarse-grained (mainly) and porphyroclastic microstructures, and high-Cr spinels (mean Cr#=0.51). The high-Mg xenoliths have low Al2O3+CaO (average 1.36 wt.%) and high Mg/Si, are in general strongly depleted in HREE, Ti, Zr and Y, and are compositionally similar to xenoliths in kimberlites from Archean cratons. The Archean lithospheric root beneath the eastern part of the Sino–Korean craton, which was sampled by Paleozoic kimberlites, was largely replaced by fertile Phanerozoic mantle during Mesozoic extension and subduction events. The high-Mg xenoliths are interpreted as relics of the Archean lithosphere, preserved locally at relatively shallow levels, and re-equilibrated to spinel facies in a regime of high heat flow caused by advective heat transport during extension. Their calculated mean room-temperature density (3.36 g/cm3) and Vp (8.39 km/s) are consistent with this interpretation and with geophysical data for the Hebi area. Regional geophysical data suggest that similar material may be widespread in the uppermost mantle west of the North–South Gravity Lineament, and more locally in the eastern part of the former craton.

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.

3.6 Ga lower crust in central China: New evidence on the assembly of the North China craton

U-Pb and Hf isotope analyses of zircons from felsic granulite xenoliths in Mesozoic volcanics reveal Early Archean (≥3.6 Ga) lower crust beneath the younger (<2.85 Ga) southern margin of the North China craton, and suggest that the eastern part of the craton formed a coherent block by 3.6 Ga. Hf model ages indicate extraction of protoliths from the mantle ca. 4 Ga or earlier, followed by remelting at 3.6–3.7 Ga. Hf isotope data require both recrystallization of magmatic zircons, and growth of new zircon, up to ca. 1.9 Ga. One sample records 2.1–1.9 Ga remelting of a 2.5 Ga protolith. If large parts of the exposed upper continental crust elsewhere also are underlain by older lower crust, estimates of crustal growth rates through time will require revision.

Nature and Evolution of Cenozoic Lithospheric Mantle beneath Shandong Peninsula, Sino-Korean Craton, Eastern China

The Shanwang and Qixia basalts lie within the North China block and were erupted in Miocene to Pliocene time (18.1 to 4.3 Ma) and Pliocene time (6.4 to 5.9 Ma), respectively. The Shanwang area lies astride the Tancheng-Lujiang (Tanlu) fault zone, a major lithospheric fault, whereas the Qixia area lies east of the fault zone. The basaltic rocks (alkali olivine basalts, basanites, nephelinites) carry abundant deep-seated xenoliths including spinel lherzolite (dominant), dunite, and pyroxenite, and a megacryst suite including augite, anorthoclase, phlogopite, ilmenite, and garnet. Xenoliths with coarse-grained microstructures are common in the Qixia xenolith suite, but are absent in Shanwang. Reconstructed bulk compositions of the lherzolites range from relatively depleted ( 12% modal diopside). Equilibration temperatures of 850° to 1020°C indicate entrainment of these lherzolites from depths ≤45 km, within the lithosphere; the geotherm may have been higher beneath Shanwang. T...

A refractory mantle protolith in younger continental crust, east-central China: Age and composition of zircon in the Sulu ultrahigh-pressure peridotite

Zircons have been extracted from garnet peridotite and its wall rock (gneiss), which was intersected by the pre–pilot hole of the Chinese Continental Scientific Drilling project (CCSD-PP1) in the Sulu ultrahigh-pressure (UHP) metamorphic belt. The peridotitic zircons record early Mesozoic UHP metamorphism ( 206 Pb/ 238 U age of 223.5 ± 7.5 Ma), but their Hf isotope compositions indicate that the protolith of the peridotite is at least Mesoproterozoic in age (minimum depleted-mantle ages [ T DM ] of 1.4 Ga). Zircons from the gneiss also reflect the Mesozoic metamorphism, with a cluster of 206 Pb/ 238 U ages at 224.5 ± 11.5 Ma; a trail of discordant grains indicates a protolith age older than 800 Ma, consistent with T DM model ages of younger than 1.2 Ga. The peridotitic zircons have trace-element patterns similar to kimberlitic and/or carbonatitic zircons, while those from the gneiss have affinities with zircons from syenites/monzonites. The differences suggest that the metasomatic agents that affected the peridotitic zircons were derived from the asthenospheric mantle rather than from subducted continental crust. The strong depletion of the CCSD-PP1 peridotite in basaltic components, and the relatively unradiogenic Hf isotopic compositions (e.g., −16.3 to ∼−13.8 e Hf ) of the peridotitic zircons, indicate that the peridotitic body is a fragment of refractory Archean mantle that experienced Mesoproterozoic metasomatism and represents a tectonic intrusion into younger crust.

Late Mesozoic-Eocene Mantle Replacement beneath the Eastern North China Craton: Evidence from the Paleozoic and Cenozoic Peridotite Xenoliths

Xenolith-bearing Paleozoic kimberlites and Cenozoic basalts from the eastern part of the North China craton provide unusual insights into intraplate processes and Phanerozoic lithospheric evolution. Paleozoic peridotite xenoliths represent samples of ancient cratonic mantle; P-T estimates show that a thick (˜230 km), cold (ca 40 mW/m2) lithosphere existed beneath the craton during mid- Ordovician time. However, xenoliths from Tertiary basalts sample a thin (> 90 km), hot (mean geotherm ca 80 mW/m2), compositionally heterogeneous lithosphere beneath the same area in Cenozoic time. Fertile, spinel-facies mantle makes up much of the Cenozoic lithosphere beneath the eastern North China craton, especially in regions along the translithospheric Tanlu fault. However, refractory spinel-facies xenoliths are found locally along the north-south gravity lineament in areas far away from the Tanlu fault. These refractory xenoliths are interpreted as derived from shallow relics of the cratonic mantle embedded in more fertile Cenozoic lithosphere. The increasing incidence of fine-grained, sheared microstructures in xenoliths from the north-south gravity lineament progressively toward the Tanlu fault suggests that the translithospheric fault system played an important role in the Mesozoic-Cenozoic replacement of pre-existing lithospheric mantle by more fertile material. Modification of cratonic mantle beneath the eastern North China craton involved irregular replacement of old lithosphere by cooling products of weakly depleted asthenosphere welling up along major shear systems. This lithosphere replacement was accompanied by an elevated geotherm and a shallower asthenosphere-lithosphere boundary.

Petrogenesis of the Yangkou layered garnet-peridotite complex, Sulu UHP terrane, China

Abstract The Yangkou garnet-peridotite complex in the Sulu UHP terrane, eastern China, consists of several blocks, tens of meters across, enclosed within gneiss. The blocks consist mainly of garnet lherzolite and garnet clinopyroxenite with small amounts of eclogite. The complex displays a finely layered structure from outcrop to thin-section scales. Lherzolitic garnet is generally homogenous in composition (Prp64-67Alm23-24Grs10-12Sps1). Garnet (Prp51-63) from clinopyroxenite contains more pyrope component than eclogitic garnets (Prp29-35). Porphyroblastic garnet from clinopyroxenite and eclogite contains abundant exsolved rods of rutile and apatite (±Cpx). Lherzolitic garnet exhibits an unusual sinusoidal REE pattern characterized by a decrease in abundance from La to Pr and an increase from Nd to the HREE, and shows Ba and Pb enrichment and negative Nb and Ni anomalies in primitive mantle-normalized trace-element plots. In contrast, diopside shows a normal REE pattern with decreasing abundances from La to Lu, a peak at Ba and Pb, and depletion in Co and Ni. Garnet from eclogite has a REE pattern with increasing abundances from La to Lu, or a maximum at Eu. Eclogitic clinopyroxene shows humped REE patterns with a peak at Nd. Except for slightly lower δ18OVSMOW of +4.40. for olivine, O-isotopic compositions (+5.03 to +5.53.) of analyzed garnets and clinopyroxenes from all three rock types lie in the range of mantle δ18O values of +5.1 to +6.6.. Small O-isotope fractionations (ΔGrt-Cpx: .0.16 to .0.35) in eclogite suggest isotopic equilibrium. On the basis of petrochemical and isotopic data, this garnet-peridotite complex was initially a mantle peridotite cut by mafic veins. These rocks experienced metasomatism and subduction-zone UHP metamorphism at 750 ± 50 °C and 40 to 45 kbar with a low thermal gradient. The fine-scale layering probably formed as a result of strong shear deformation during exhumation.

Anin situ zircon Hf isotopic, U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt: Tracking the early evolution of the lower crust in the North China craton

Backscattered electron images,in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes include the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842 ±40 Ma, except few ages of 3097–2824 Ma and 2489–2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5–2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εHf of the zircons with early Proterozoic U-Pb age vary around zero, but two have high εHfup to +9.2–+10.2, indicating mantle contribution during the collision and assembly between the Eastern and Western blocks in the early Proterozoic that resulted in the amalgamation of the North China craton.