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Are the peridotitic xenoliths entrained in Late Mesozoic intermediate-mafic intrusive complexes on the North China Craton the direct samples of lithospheric mantle?
Abstract
Late Mesozoic intermediate-mafic intrusive complexes on the eastern North China Craton (such as Tietonggou complex in Laiwu region of Shandong province; Jingling-Hutian complex in Weifang region of Shandong province; Fushan complex in Hanxing region of Hebei province) often contain peridotitic xenoliths. The origin and petrogenesis for these peridotites remain controversial, of which two opinions: accumulative origin/representative of old oceanic ophiolite or direct samples from lithospheric mantle are prevalent. This paper, on the basis of the results from the study on the peridotitic xenoliths entrained in Jingling-Hutian intrusive complex in Weifang region of Shandong province and compilation and re-recognition of previously-published data realized that these peridotitic xenoliths were all accumulative origin or represent ancient oceanic ophiolites, rather than direct samples from the lithospheric mantle through careful petrological, mineralogical and geochemical studies. Therefore, these peridotitic xenoliths cannot be used to probe the compositional features of the Late Mesozoic lithospheric mantle beneath the North China craton.
... Some dunite bodies show evidence of magma-wall rock reactions in the upper mantle, and their formation would have changed the compositions of both the melt and the host peridotite (Kelemen, 1990;Kelemen et al., 1995b;Allan et al., 1996). The origins of dunites include: (1) as a residue formed by a high degree of partial melting of peridotite (Bernstein et al., 2007); (2) as a cumulate formed by the fractionation of olivine from a mafic melt (Zhang et al., 2005); and (3) as a replacement product formed during melt-peridotite reactions (Kelemen et al., 1998;Garrido et al., 2007). The Mg # of an olivine is usually used as a measure of the degree of partial melting and melt extraction in the mantle (Dick et al., 1984;Arai, 1994). ...
Experiments of the melt-peridotite reaction at pressures of 1 and 2 GPa and temperatures of 1250–1400°C have been carried out to understand the nature of the peridotite xenoliths in the Mesozoic high-Mg diorites and basalts of the North China Craton, and further to elucidate the processes in which the Mesozoic lithospheric mantle in this region was transformed. We used Fuxin alkali basalt, Feixian alkali basalt, and Xu-Huai hornblende-garnet pyroxenite as starting materials for the reacting melts, and lherzolite xenoliths and synthesized harzburgite as starting materials for the lithospheric mantle. The experimental results indicate that: (1) the reactions between basaltic melts and lherzolite and harzburgite at 1–2 GPa and 1300–1400°C tended to dissolve pyroxene and precipitate low-Mg# olivine (Mg#=83.6-89.3), forming sequences of dunite-lherzolite (D-L) and duniteharzburgite (D-H), respectively; (2) reactions between hornblende-garnet pyroxenite and lherzolite at 1 GPa and 1250°C formed a D-H sequence, whereas reactions at 2 GPa and 1350°C formed orthopyroxenite layers and lherzolite; and (3) the reaction between a partial melt of hornblende-garnet pyroxenite and harzburgite resulted in a layer of orthopyroxenite at the boundary of the pyroxenite and harzburgite. The reacted melts have higher MgO abundances than the starting melts, demonstrating that the melt-peridotite reactions are responsible for the high-Mg# signatures of andesites or adakitic rocks. Our experimental results support the proposition that the abundant peridotite and pyroxenite xenoliths in western Shandong and the southern Taihang Mountains might have experienced multiple modifications in reaction to a variety of melts. We suggest that melt-peridotite reactions played important roles in transforming the nature of the Mesozoic lithospheric mantle in the region of the North China Craton.
... Dunites may form via three different processes: accumulation from mafic magma (Zhang et al., 2005), a high degree of melting of the mantle (Bernstein et al., 2007), or melt-peridotite interaction (Garrido et al., 2007;Kelemen et al., 1998;Kelemen, 1990;Quick, 1981). ...
This article reports the petrography and mineral chemistry of dunite xenoliths and olivine xenocrysts entrained by the Early
Cretaceous Xi’anli (西安里) hornblende (Hb)-gabbros from the southern Taihang (太行) Mountains, with the aim of constraining the
nature of the Mesozoic lithospheric mantle in Central China. Rounded dunite xenoliths are 1–3 cm3 in size and display porphyroclastic, tabular, and protogranular textures. Chromite with Cr#=60–89 is common in the xenoliths. Olivine xenocrysts of 4–6 mm in size are also found in the Hb-gabbros. Orthopyroxene reaction
rims are commonly observed around olivine xenocrysts or between dunite xenoliths and host rocks. The porphyroclastic olivines
within dunite xenoliths and olivine xenocrysts have kink bands and Mg#=83–94. The Mg# of olivine cores and rims are 89–94 (average, 90) and 83–86 (average, 84.4), respectively. The CaO contents of all olivines
from the xenoliths and xenocrysts are less than 0.1 wt.%, suggesting a lithospheric mantle origin. The Ca content (214 ppm–818
ppm) and Ti content (15 ppm–137 ppm) in the xenoliths and xenocrysts are similar to those of olivines from the dunite xenoliths,
but are much higher than those of olivines from harzburgite and lherzolite xenoliths in the Fushan (符山) intrusion. This finding
implies that the xenoliths and xenocrysts may have originated from harzburgites or lherzolites that were intensively modified
by silica-rich melts. This result, combined with high Mg# (92–94) of olivine cores from the dunite xenoliths and xenocrysts, indicates that these olivine xenocrysts and dunite xenoliths
could represent the residue of ancient (Archean or Paleoproterozoic) lithospheric mantle and might have experienced the same
intensive modification by silica-rich melts as the host magma, resulting in enrichment in MgO and SiO2.
Key WordsNorth China craton-Xi’anli-Hb-gabbro-dunite-xenocryst-lithospheric mantle
Two types of complex peridotitic xenoliths exist in the earth mantle and/or lower crust: polymict peridotitic xenoliths and composite peridotitic xenoliths. Composite peridotitic xenoliths usually comprise two or more components of peridotite and pyroxenite/granulite or of peridotite with pyroxenite veins and can be easily identified in petrography, sometimes in hand samples. But the polymict peridotitic xenoliths are tectonites composed of minerals from various peridotites and/or pyroxenites and melts and brought together by mantle shear zone and difficult to identify in petrography, they must be classified by compositional analyses of constituent minerals. Polymict peridotitic xenoliths have only been found in kimberlite pipes from the Kaapvaal craton, South Africa so far. These polymict peridotitic xenoliths originated from the mantle shear zone and recorded the whole evolutionary processes of fluid/melt assisted mantle deformation and metasomatism. Composite peridotitic xenoliths are relatively common and occur in global basalts and kimberlites of all kinds. Composite peridotitic xenoliths have a variety of origins such as mantle deformation, crust mantle transitional zone sampling, metasomatism and accumulates, etc. These composite peridotitic xenoliths also record the intermediate stage of metasomatism and modification processes of the lithospheric mantle and/or lower crust. Therefore, these rare and important complex peridotitic xenoliths provide the most direct samples for the geodynamic evolution of the continental lithosphere.
A suite of high-Mg sunukites occurred in the uppermost of Early Cretaceous Yixian Formation in western Liaoning Province, China, which show a series of disequilibrium phenomena in textures and their constituent minerals. Major and trace elemental compositions of the orthopyroxenes and glass (hyaline) in the sunukites reveal existence of mixing feature between magma of a crustal origin and that of a mantle derivation. This paper interpreted the existence of orthopyroxenes and the formation of sunukites in a point view of magma mixing between melt from the crust and that from the depleted mantle. According to the fact that rapid eruption of sunukitic magma followed by basaltic magma of depleted mantle origin in the studied area, we believed that the occurrence of these sunukites was a sigh of magma source sift from the enriched lithospheric mande to the depleted asthenosphere in eastern China in the end of Early Cretaceous.
Based on the teleseismic events recorded by seismic stations of Shandong, Hebei, Liaoning, Beijing, Tianjin regional seismograph networks, this paper investigates the crustal thickness and average Poisson's ratios beneath the stations by means of H-k receiver function stacking method. The results show that the crustal thickness in the studied region varies from 30 km to 46 km, and is thinning gradually from mountain to basin, suggesting a good correlation with surface terrain. On the other hand, distribution of the Poisson's ratio over the studied region is characterized by an obvious region segmentation. The low Poisson's ratios beneath the east part of Yanshan might relate to the delamination of North China craton. The region beneath east part of Tancheng-Lujiang fault zone exhibits a complex distribution of low Poisson's ratios mixed with high ones, which might be related to volcanic activity. The convergence zone of the western segment of Zhangjiakou-Bohai seismic active belt and the lift exhibits a little higher Poisson's ratios, which might be caused by both broken rocks and the partial fusion of rocks near the seismic active belt. The station Beihuangcheng located on the island chain in the Bohai Sea shows a thick crustal thickness and a low Poisson's ratio, but the reason behind the phenomenon waits for further investigation.
Plenty of ultramafic enclaves are included in the Mesozoic high-Mg diorites from the central and eastern parts of the North China block. The geochemical characteristics of the high-Mg diorites were formed through reactions between the ultramafic rocks and low-Mg, felsic magma produced by partial melting of lower crust. Residence timescales of the ultramafic rocks in the hot magma, however, are poorly known. In this work, we have investigated the residence timescales of dunites in hot (∼950°C) dioritic magmas from the Tietonggou pluton in western Shandong Province and the Han-Xing pluton in Hebei Province, based on numerical modeling of the diffusional Mg-Fe zonings preserved in the dunites. We show that the residence timescales of dunites in the dioritic magmas are 102 ∼ 103 y. This conclusion is meaningful for understanding the detailed processes of reaction between ultramafic rocks and intermediate-felsic magmas and formation of high-Mg diorites.
Ultramafic xenoliths are common in Tietonggou intrusion, Laiwu, Shandong Province. Peridotite xenoliths develop two-stage
metasomatism popularly: (i) The early metasomatism is characterized by intergranular clinopyroxene and phlogopite; (ii) The
later metasomatism is characterized by orthopyroxene veins with a few plagioclases and amphiboles. These ultramafic xenoliths
are thought to be mostly from the crust-mantle transitional zone on the basis of their combination, mineral chemistry, equilibrium
temperature, and metasomatic characteristics. Major elements, rare earth elements, and Sr-Nd isotopic composition of the ultramafic
xenoliths indicate that there is a petrogenetic relationship between the silica-enriched xenoliths and their host diorites.
We propose that such silica enrichment might be an indicator of melt-rock interactions in the Mesozoic lithosphere beneath
western Shandong.
Petrology, mineral chemistry and trace element geochemistry of the dunite xenoliths in gabbro-dioritic rocks from western Shandong indicate that the dunite xenoliths would be relicts of high degree partial melting of ancient (Archean) lithospheric mantle. Two types of the mantle metasomatisms occur in the dunite xenoliths, i. e. early CO2-H2O-rich anti-active liquid metasomatism by the representative of interstitial phlogopite and late silica-rich melt (liquid) metasomatism by the representatives of orthopyroxene veins and net orthopyroxene between olivine and chromite. The asthenosphere-derived silica-rich melt (liquid) metasomatism in the upper mantle is a chemical erosion to ancient lithospheric mantle. It is of important significance to realize the nature of Mesozoic lithospheric mantle and mechanism of lithospheric thinning.
The 40 Ar/39 Ar plateau age of biotite in pyroxene-diorite from Tietonggou intrusion of western Shandong is 132.79 ± 0.27 Ma. Petrographic, mineral chemical, petrochemical-geochemical and field data of xenoliths occurred in pyroxene-diorite indicate that they are subdivided into two groups. Group I is the mantle-derived dunite, harzbergite and spinel-lherzolite with metamorphic-deformation texture. The depletion of fusible elements such as Ca, Al, the high olivine Mg# s ( ave. Mg# = 92 ) and high olivine abundances for Group I xenoliths suggest that they could be the relics of high-degree partial melting of the ancient lithospheric mantle. However, the relative LREE-enrichment REE patterns, high 87Sr/86Sr ratios and the existence of secondary phlogopites in Group I xenoliths imply that they could be subjected to a late metasomatism. Group II is the mantle-derived dunite, harzbergite, olivine-pyroxenite and websterite with relic poikilitic, re-equilibrium and overprinted deformation textures. The relatively lower Mg# s and 87Sr/87Sr ratios, higher REE abundances for Group II xenoliths indicate that they could be the new accreted lithospheric mantle. The coexistence of two types of the mantle-derived xenoliths is of importance to realize the nature of Mesozoic lithospheric mantle and to reveal the evolutionary history of subcontinental mantle in eastern China.
Isotopic geochronology, petrology and geochemistry of diorites containing mafic-ultramafic cumulate and basic granulite xenoliths from Harqin area of Inner-Mongolia have been studied. The results suggest that (1) K-Ar datings of whole rock and single mineral of diorites give ages of 218~223 Ma; (2) diorites have no genetic correlation with cumulate and granulite xenoliths; (3) these diorites in different lithologies are probably products of highly partial melting of Archean amphibolite; (4) the Early Mesozoic underplating lead to the formation of diorites, reflecting an extensional tectonic setting.
Oligocene ultrapotassic rocks from western Yunnan have abundant olivines of varying size and green-core clinopyroxenes. Olivine macrocrysts (1-5 mm) have very high forsterite contents (Fo(92-94)), and high CaO (0.15-0.30 wt%) and Cr2O3 (up to 0.23 wt%) contents. They are believed to be magmatic rather than disintegrated crystals from the upper mantle. These magnesian olivines are not in equilibrium with melts corresponding to the whole-rock compositions of host lavas and, as such, may represent xenocrysts entrained by ultrapotassic magmas during their ascent to the surface. While komaiites, boninites and olivine lamproites are potential sources of these xenocrysts, a genetic link to the late Permian Emeishan large igneous province (LIP) is favoured, because the Emeishan flood basalt is the only known geologic event in the studied area that could have produced highly magnesian olivines. The olivine xenocrysts may have been entrained from the high velocity lower crust (Vp = 7.1-7.8 km/s), which is believed to have formed from high Mg basalts (picrites?) that underplated and intruded the crust during the Emeishan flood basalt episode. Given the presence of olivine xenocrysts and reverse-zoned clinopyroxenes, it is apparent that some magnesian ultrapotassic lavas from Yunnan have a complex hybrid origin. While the former point to ultramafic/primary melts the latter indicate interaction between evolved melts and relatively primary melts thus indicating complex magma mixing processes.
Cenozoic (7.8 Ma) basalts from central Taihang Mountains contain obviously zoned high-Mg# olivines. The core is rich in MgO and NiO and poor in CaO and the rim has higher FeO, MnO, CaO and Al 2O3 contents. High-Mg# olivines have compositions in the core (Mg# = 90.1 ∼ 92.3) very similar to the high-Mg# ones from the mantle peridotitic xenoliths entrained in Cenozoic basalts from the North China Craton, and in the transitional zone (Mg# = 84.5 ∼ 89.7) slightly higher than those in the core of olivine phenocrysts of the host basalts (Mg# = 77.1 ∼ 81.7). The compositions in the rim of high-Mg# olivines (Mg# = 55.6 ∼ 73.6), identical to those of olivine phenocrysts of host basalts Mg# = 55.6 ∼ 73.6), are close to the olivines in groundmass ( Mg # = 54.8 ∼ 66.1). These compositional features as well as resorption borders and rounded crystal shapes demonstrate that these high-Mg# olivines are mantle xenocrysts, i. e. disaggregates of mantle peridotites. The core compositions of big grains can represent those of olivines of mantle peridotites, which indicate that old lithospheric mantle should be existent in the Central Taihang Mountains in Cenozoic. Comparative studies show that large temporal and spatial difference exists in the lithosphere thinning in the North China.
Mafic xenoliths occur in Mesozoic diorites from nine localities in eastern Inner Mongolia from the Da Hinggan Mts. to Huabei
craton. Their petrologic and geochemical features suggest that they are an intact suite of cumulate complex whose mineral
assemblages contain typical patent mantle metasomatism minerals such as pargosite, phlogopite and apatite. Their REEs and
trace element characteristics suggest a comagmatic relationship in these cumulates with different compositions. Isotopic dating
of the xenoliths indicates that they are products of the mantle magmatic underplating in the Early Mesozoic (237–224 Ma),
and they may provide direct evidence for the crust-mantle interaction and vertical accretion of continental crust under a
background of the mantle upwelling in the Early Mesozoic in Huabei region.